Construction control method for weak compaction area shape of ventilation pipe embankment in plateau frozen soil area

Abstract

The application discloses a construction control method for a compacted weak area shape of a ventilation pipe subgrade in a plateau permafrost region, and belongs to the technical field of plateau permafrost subgrade construction. A three-dimensional finite element model is established to simulate the rolling process of a plate tamper and a road roller under different vibration frequencies and amplitudes, to obtain the distribution law of filler compactness and pipe body stress, to screen out an optimal construction parameter combination, to roll according to the parameter combination during on-site construction, and to synchronously monitor the filler compactness, the pipe body hoop strain and crack echo signals respectively, to dynamically adjust the rolling residence time and rolling times according to the monitoring results, and to integrate all parameters, adjustment records and detection data according to sections after construction to form a standardized pipe-stone collaborative compaction technical scheme. The application realizes accurate control of the compacted weak area around the ventilation pipe, effectively improves the uniformity of the filler compactness and the safety of the pipe body structure, and significantly improves the construction quality and efficiency of the ventilation pipe subgrade in the plateau permafrost region.

Description

Technical Field

[0001] This invention discloses a construction control method for the compaction weakness zone of ventilation pipe subgrade in plateau permafrost areas, belonging to the field of plateau permafrost subgrade construction technology. Background Technology

[0002] In highway and railway construction projects in high-altitude permafrost regions, ventilation duct subgrades are a widely adopted active cooling measure. This involves embedding concrete or metal ventilation ducts within the subgrade fill, utilizing winter air convection to remove heat from the fill and underlying permafrost layer, thus maintaining the thermal stability of the permafrost. However, the area surrounding the ventilation duct, especially the sidewalls and bottom of the duct, suffers from poor compaction due to the space occupied by the duct and its irregular geometric boundaries. This results in what are known as weak compaction zones. Traditional compaction methods typically employ the same rolling techniques used for ordinary subgrades, uniformly using a roller for layered compaction, lacking targeted local reinforcement measures around the ventilation duct. This extensive construction approach easily leads to insufficient compaction of the fill material on the sidewalls and bottom of the ventilation duct, resulting in uneven settlement of the fill material above the duct, local deformation or even cracking of the duct under eccentric loads, ultimately reducing the cooling efficiency of the ventilation duct subgrade and exacerbating subgrade damage under frost heave and thaw cycles.

[0003] To address the aforementioned issues, some existing research has attempted to improve the compaction quality around ventilation ducts by adjusting compaction parameters or changing the filler gradation. For example, some engineering practices use small plate compactors to supplement the compaction of the area around the duct, or conduct laboratory tests to obtain the optimal moisture content and maximum dry density for different filler types. However, these methods generally lack a systematic understanding of the interaction between the duct, soil, and frozen soil.

[0004] On the one hand, the vibration responses generated by plate compactors and road rollers differ significantly in different areas around the pipe. Directly applying compaction parameters for uniform subgrades can easily lead to excessive local stress in the pipe body or under-compaction of the fill material. On the other hand, there is a lack of real-time monitoring methods to determine whether the compaction process meets standards during construction. Usually, sampling inspections can only be conducted after all compaction is completed, and once compaction defects are discovered, they are often difficult to remedy. More importantly, existing technologies have failed to establish a closed-loop technical path from numerical simulation to field verification and then to dynamic parameter adjustment, and cannot provide differentiated construction control schemes for different frozen soil conditions, different pipe diameters, and different fill material characteristics. Summary of the Invention

[0005] According to a first aspect of the present invention, the present invention claims protection for a construction control method for the compaction-weak zone shape of ventilation pipeline subgrade in plateau permafrost regions, comprising: S1. Obtain the filler gradation parameters of the ventilation duct subgrade, the elastic modulus of the ventilation duct body, and the thickness of the seasonal thawing layer of the frozen soil layer. Establish a three-dimensional finite element model, set contact pair elements, and divide the mesh elements into different densities. S2. In the three-dimensional finite element model, the vibration frequency and displacement amplitude of the plate compactor simple harmonic load and the vibration frequency and amplitude of the road roller rectangular distributed load are adjusted respectively. The nodal displacement of the filling element and the nodal equivalent force of the pipe element are recorded under each combination of frequency and amplitude. S3, construct a density distribution cloud map to extract the minimum density values ​​of the ventilation duct sidewall area and the bottom area, obtain the vibration frequency value and displacement amplitude combination, and use it as the construction parameter combination for the plate compactor and road roller; S4. According to the selected combination of construction parameters, local compaction construction is carried out in the weak area around the ventilation pipe, and the top filler layer of the ventilation pipe is compacted as a whole using a road roller. S5, adjust the compaction dwell time of the plate compactor in the weak zone and the number of compaction passes of the roller in the filler layer according to the compaction deviation value, strain safety margin value and waveform distortion characteristic value respectively; S6. After completing all the compaction work, summarize the combination of construction parameters used, the record of compaction dwell time adjustment, the record of compaction pass adjustment, and the compaction test values, circumferential strain test values ​​and crack echo signals of all test points, and integrate them to form a pipe-stone co-compaction technology solution for the ventilation pipeline subgrade.

[0006] Furthermore, the establishment of the three-dimensional finite element model and setting of contact pair elements in S1 includes: The ventilation duct is modeled as a hollow cylindrical structure, and the duct body is meshed using eight-node hexahedral elements. The stone packing material is modeled as a continuous medium surrounding the ventilation duct, and the packing material is meshed using four-node tetrahedral elements. The frozen soil layer is modeled as a semi-infinite space, and a six-node triangular prism element is used to mesh the frozen soil layer. A face-to-face contact pair is established between the outer wall of the ventilation duct and the packing. The normal behavior of the contact pair is set to hard contact, and the tangential behavior is set to penalized frictional contact. A face-to-face contact pair is established between the bottom surface of the filler and the top surface of the frozen soil layer, and the normal behavior of the contact pair is set to a bonded contact that does not allow separation.

[0007] Furthermore, S2 includes: In the three-dimensional finite element model, the action area of ​​the plate compactor is obtained by extending one pipe diameter to both sides of the central axis of the ventilation pipe. A rectangular moving load belt is set in this action area, and a harmonic variable nodal force is applied to the load belt. By changing the frequency parameter value and amplitude parameter value of the harmonic nodal force, different vibration frequencies and different excitation force magnitudes of the plate compactor are respectively obtained. The load belt is loaded in the following order: first, it moves along the axis of the ventilation pipe from one end to the other, and then it moves perpendicular to the axis from the side of the pipe to the side away from the pipe.

[0008] Further, S3 includes: The nodal displacements of all filling elements calculated from the three-dimensional finite element model are classified. Based on the inverse proportional conversion relationship between displacement and density, the displacement value at the centroid of each filling element is converted into the density value of that element. The three-dimensional scalar field is then used to extract isosurfaces and generate a density distribution cloud map. Using the outer wall of the ventilation duct as a reference, extend the preset layer thickness outward along the radial direction of the duct body, mark all the packing units as the surrounding area of ​​the ventilation duct, separate the packing units to count the density value, and take the minimum density value in each sub-area as the density representative value of that sub-area. From the nodal equivalent forces of all pipe elements output from the three-dimensional finite element model, the nodes located in the arch region of the ventilation duct and the nodes located in the side region of the ventilation duct are selected, and the maximum value of the equivalent force of all nodes in the arch region and the maximum value of the equivalent force of all nodes in the side region are calculated respectively. By changing the combination of vibration frequency and displacement amplitude, and repeating the process, multiple sets of different representative values ​​of compaction and maximum equivalent stress are obtained. The preset compaction threshold is used as the lower limit condition, and the yield strength threshold of the pipe material is used as the upper limit condition. All combinations of vibration frequency and displacement amplitude that simultaneously satisfy the condition that the representative values ​​of compaction in the three sub-regions are not lower than the preset compaction threshold, and the maximum equivalent stress of the arch and the maximum equivalent stress of the pipe side are not higher than the yield strength threshold of the pipe material are selected. From all the selected combinations, the one with the largest sum of representative density values ​​and the smallest sum of maximum equivalent stress values ​​is chosen as the combination of construction parameters for the plate compactor and road roller.

[0009] Further, S4 includes: After the ventilation duct is installed, graded crushed stone filler is backfilled on both sides and below the duct to form a weak area. A plate compactor is used to locally compact this weak area according to the selected vibration frequency and displacement amplitude. The plate compactor starts from one end of the ventilation duct and compacts it segment by segment along the axial direction of the duct to the other end. After each axial segment is completed, the plate compactor moves away from the duct by half the width of the compactor plate and then compacts it back from the other end of the duct along the axial direction until the entire width of the weak area is covered. After compacting the weak areas, backfill the top of the ventilation pipe with graded crushed stone to the preset cover layer thickness. Use a road roller to compact the cover layer as a whole according to the selected vibration frequency and amplitude values. The road roller starts from one edge of the subgrade and compacts one strip at a time along the direction perpendicular to the axis of the ventilation pipe to the other edge. Each compaction strip overlaps the previous compaction strip by one-third of the width of the road roller wheel. After completing one layer of cover compaction, backfill the next layer of cover and repeat the compaction operation until the design filling elevation is reached.

[0010] Further, S5 includes: When the compaction deviation value is positive and the absolute value exceeds the first threshold range, keep the current compaction residence time unchanged and reduce the number of compaction passes by one; When the compaction deviation value is negative and the absolute value exceeds the second threshold range, the compaction residence time of the plate compactor in the current weak section is increased. The amount of time increase is positively correlated with the absolute value of the compaction deviation value. At the same time, the number of compaction passes of the roller in the current filler layer is increased by one. When the strain safety margin value is lower than the third threshold, immediately stop the compaction construction in the current area, reduce the displacement amplitude of the plate compactor by one level and start compaction again, and reduce the vibration value of the road roller by one level. When the waveform distortion characteristic value shows that there is a reflection peak in the crack echo signal that exceeds the fourth threshold, mark the pipe location as a suspected crack point, increase the detection frequency of the resistance strain gauge at that location, and at the same time reduce the vibration frequency value of the road roller by one level before continuing to compact.

[0011] Further, S6 includes: The entire construction area was divided into multiple consecutive sections according to the mileage of the ventilation pipes, and section numbers were assigned. For each section, construction record data is extracted from the construction records, and the construction record data of each section is cross-compared to form records of residence time-compaction degree relationship, number of compaction passes-strain relationship, and crack location-waveform characteristics. The baseline construction parameters, residence time-compaction degree relationship records, number of compaction passes-strain relationship records, and crack location-waveform characteristic records for each section are arranged sequentially, and then the statistical characteristic values ​​calculated from all compaction degree test values ​​and circumferential strain test values ​​of that section are appended to characterize the overall distribution of the test values. The arrangement results of all sections are compiled into a technical solution document in document format, including a cover, a summary table of section parameters, a detailed record table of each section, and a schematic diagram of the distribution of test points.

[0012] Furthermore, the adjustment range of vibration frequency and displacement amplitude of the plate compactor simple harmonic load and the adjustment range of vibration frequency and amplitude of the road roller rectangular distributed load in S2 are realized in the three-dimensional finite element model through parametric scanning. The vibration frequency value is set to change from low frequency limit to high frequency limit. For each vibration frequency value, the displacement amplitude or amplitude value is set to change from small amplitude limit to large amplitude limit in an arithmetic sequence. The displacement of the filling unit node and the equivalent force of the pipe unit node under each combination of frequency value and amplitude are calculated in sequence. The difference between two adjacent frequency values ​​is equal, and the difference between two adjacent amplitudes is equal.

[0013] Furthermore, S4 also includes: The sand filling method test points are arranged on the surface of the filler in the weak area around the ventilation duct and on the surface of the filler layer at the top of the ventilation duct. The test point is set at a cross section every one pipe diameter along the axis of the ventilation duct. On each cross section, a test point is set at the top of the pipe, at the oblique top of the pipe, and at the horizontal direction of the pipe. Resistance strain gauges are attached to the outer surface of the pipe wall along the axial direction of the ventilation pipe. The sensitive grid direction of the strain gauge is parallel to the axial direction of the pipe body. Strain gauges are arranged at the pipe end flange connection, the mid-span of the pipe body, and one-quarter of the length of the pipe body on each section of the ventilation pipe. The ultrasonic flaw detector probe is placed at the end of the ventilation duct and emits ultrasonic waves into the duct along the axial direction of the duct body. Receiver probes are set on the outer surface of the duct body at intervals of the duct diameter. The ultrasonic wave propagation time and waveform amplitude between the transmitting and receiving probes are used as the raw data of the crack echo signal.

[0014] Furthermore, before simulating the compaction action of the plate compactor and the road roller, the three-dimensional finite element model in S2 performs the following initialization operations: Vertical displacement constraints are applied to the top surface of the frozen soil layer, and horizontal displacement constraints are applied to the sides of the frozen soil layer. An equivalent nodal force that dynamically changes with the movement of the compaction load band is applied to the top surface of the filler layer. The area of ​​action completely coincides with the load band area of ​​the plate compactor or roller. The magnitude of the equivalent nodal force is calculated by the width and length of the load band and the pressure value of the peak value of the harmonic load or the rectangular distributed load. Symmetrical boundary conditions are applied to the cross-sections at both ends of the ventilation duct to restrict the rigid body rotation of the cross-sections at both ends of the duct; absorbing boundary conditions are applied to the lateral boundary of the packing layer to reduce the reflection of stress waves at the model boundary.

[0015] This invention discloses a construction control method for weak compaction zones in ventilation pipe subgrades in high-altitude permafrost regions, belonging to the field of high-altitude permafrost subgrade construction technology. A three-dimensional finite element model is established to simulate the compaction process of a plate compactor and a road roller under different vibration frequencies and amplitudes. The distribution patterns of filler density and pipe stress are obtained, and the optimal combination of construction parameters is selected. During on-site construction, compaction is carried out according to this parameter combination, while simultaneously monitoring filler density, pipe circumferential strain, and crack echo signals. The compaction dwell time and number of compaction passes are dynamically adjusted based on the monitoring results. After construction, all parameters, adjustment records, and test data are integrated by section to form a standardized pipe-stone co-compaction technology scheme. This invention achieves precise control of weak compaction zones around ventilation pipes, effectively improving the uniformity of filler density and the structural safety of the pipe, significantly enhancing the construction quality and efficiency of ventilation pipe subgrades in high-altitude permafrost regions. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the construction control method for the compaction-weak zone of a ventilation pipeline subgrade in a plateau permafrost region, for which the present invention seeks protection. Figure 2 This is the second flowchart of a construction control method for the compaction-weak zone of a ventilation pipeline subgrade in a plateau permafrost region, for which the present invention claims protection. Figure 3 This is the third flowchart of a construction control method for the compaction of weak areas in the foundation of ventilation pipelines in high-altitude permafrost regions, for which this invention seeks protection. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0018] According to the first embodiment of the present invention, referring to Figure 1 This invention claims protection for a construction control method for the compaction-weak zone of a ventilation pipeline subgrade in a high-altitude permafrost region, comprising: S1. Obtain the filler gradation parameters of the ventilation duct subgrade, the elastic modulus of the ventilation duct body, and the thickness of the seasonal thawing layer of the frozen soil layer. Establish a three-dimensional finite element model, set contact pair elements, and divide the mesh elements into different densities. S2. In the three-dimensional finite element model, the vibration frequency and displacement amplitude of the plate compactor simple harmonic load and the vibration frequency and amplitude of the road roller rectangular distributed load are adjusted respectively. The nodal displacement of the filling element and the nodal equivalent force of the pipe element are recorded under each combination of frequency and amplitude. S3, construct a density distribution cloud map to extract the minimum density values ​​of the ventilation duct sidewall area and the bottom area, obtain the vibration frequency value and displacement amplitude combination, and use it as the construction parameter combination for the plate compactor and road roller; S4. According to the selected combination of construction parameters, local compaction construction is carried out in the weak area around the ventilation pipe, and the top filler layer of the ventilation pipe is compacted as a whole using a road roller. S5, adjust the compaction dwell time of the plate compactor in the weak zone and the number of compaction passes of the roller in the filler layer according to the compaction deviation value, strain safety margin value and waveform distortion characteristic value respectively; S6. After completing all the compaction work, summarize the combination of construction parameters used, the record of compaction dwell time adjustment, the record of compaction pass adjustment, and the compaction test values, circumferential strain test values ​​and crack echo signals of all test points, and integrate them to form a pipe-stone co-compaction technology solution for the ventilation pipeline subgrade.

[0019] In this embodiment, during the construction of a highway subgrade in a permafrost region, a 200-meter-long test section of the subgrade containing a ventilation pipe was selected. The construction personnel first obtained the gradation parameters of the graded crushed stone used in this section of the subgrade from the design drawings, including the particle size range, curvature coefficient, and non-uniformity coefficient. They then obtained the elastic modulus of the ventilation pipe from the pipe quality certificate; since the ventilation pipe is a reinforced concrete pipe, the elastic modulus was taken as the design value. Finally, they obtained the seasonal thaw layer thickness of the permafrost from the geological survey report; in this section, the seasonal thaw layer thickness was 2.5 meters. Based on these parameters, a three-dimensional finite element model containing the ventilation pipe, the stone filler, and the permafrost layer was established using finite element analysis software. During modeling, the ventilation duct is constructed as a hollow cylinder with an inner diameter of 1.2 meters, an outer diameter of 1.4 meters, and a length of 2.5 meters, based on actual dimensions. The filling area is a cubic region surrounding the ventilation duct, with a length of 20 meters, a width of 10 meters, and a height of 3 meters. The frozen soil layer is a semi-infinite space with a thickness of 10 meters. Face-to-face contact pairs are set at the contact surfaces of the ventilation duct and the filling material, and face-to-face contact pairs are also set at the contact surfaces of the filling material and the frozen soil layer. When meshing, the mesh size of the filling material within a 0.5-meter radius around the ventilation duct is set to 0.05 meters, and the mesh size of the area far away is set to 0.2 meters. The mesh size within the seasonal thawing layer of the frozen soil layer is set to 0.1 meters, and the mesh size of the permafrost layer is set to 0.3 meters.

[0020] Then, the operation of a plate compactor was simulated in the model: the effective area of ​​the plate compactor was defined within 0.5 meters outside the side wall of the ventilation duct, and a moving harmonic load was applied, with the load frequency varying from 20Hz to 50Hz in 5Hz intervals, and the displacement amplitude varying from 0.5mm to 2.0mm in 0.2mm intervals; at the same time, the operation of a road roller was simulated: a reciprocating rectangular distributed load was applied to the top filler layer of the ventilation duct, with the load frequency varying from 25Hz to 45Hz in 5Hz intervals, and the amplitude varying from 0.8mm to 2.5mm in 0.3mm intervals; calculations were performed for each combination of frequency and amplitude, and the nodal displacement of the filler unit and the nodal equivalent force of the pipe unit were recorded.

[0021] For each set of calculated packing node displacements, the construction personnel converted the displacement value at the centroid of each packing unit into a density value according to empirical formulas. For example, units with small displacements were converted to high density values. The density values ​​of all units were input into the post-processing module to generate a density distribution cloud map. From the cloud map, the density values ​​of all packing units in the left side wall region, right side wall region, and lower bottom region of the ventilation duct were extracted, and the minimum value in each region was found. At the same time, the equivalent stress of all nodes in the ventilation duct arch region with an angle between 0° and 30° with the vertical direction and the pipe side region with an angle between 0° and 45° with the horizontal direction were extracted from the equivalent stress of the pipe unit, and the maximum value in each region was calculated.

[0022] Construction personnel preset a compaction threshold of 93% and a yield strength threshold of 25 MPa for the pipe material. They screened out all frequency and amplitude combinations that satisfied the minimum compaction values ​​in the three regions—left sidewall, right sidewall, and bottom of the pipe—all not less than 93%, and the maximum equivalent stress at the arch crown and the pipe side not exceeding 25 MPa. For example, in a combination with a frequency of 35 Hz and a displacement amplitude of 1.2 mm, the three minimum compaction values ​​were 94.1%, 93.8%, and 93.5%, respectively, and the maximum equivalent stresses were 22.3 MPa and 23.1 MPa, respectively, meeting the requirements. Among all the compliant combinations, the one with the largest sum of minimum compaction values ​​and the smallest sum of maximum equivalent stresses was selected as the construction parameters for the plate compactor and road roller.

[0023] Next, on-site construction was carried out according to the selected parameters; a plate compactor with a vibration frequency of 35Hz and a displacement amplitude corresponding to the excitation force of 1.2mm was used to locally compact the weak areas on both sides and below the ventilation duct; a road roller with a vibration frequency of 35Hz and an amplitude of 1.5mm was used to compact the top filler layer of the ventilation duct as a whole; during construction, the compaction degree test value was obtained at the test points on the surface of the filler layer using the sand cone method, with a test point set every 2 meters along the axis of the ventilation duct, and one point each at the top of the duct, diagonally above the side of the duct, and horizontally on the side of the duct; resistance strain gauges were used to obtain circumferential strain test values ​​on the outer wall of the ventilation duct, with one gauge attached at the flange connection at the end of each duct section, at the mid-span of the duct body, and at one-quarter of the length of the duct body. An ultrasonic flaw detector was used to obtain crack echo signals at both ends of the ventilation duct and every 2 meters along the duct body.

[0024] The compaction degree value obtained from on-site testing is compared with the compaction threshold simulated at the same location in the model. If the compaction degree value is lower than the simulated threshold, the residence time of the plate compactor in the weak section is increased; if it is higher, the residence time is reduced. The circumferential strain value is compared with the ultimate strain value of the pipe material. If the strain margin is too small, the amplitude of the roller is reduced. The ultrasonic echo signal is compared with the crack-free reference signal. If an abnormal reflection wave appears, the location is marked and the roller frequency is adjusted. After construction is completed, all construction parameters, adjustment records, and test data are integrated into a complete pipe-rock co-compaction technology scheme in mileage chain order. This scheme includes the reference parameters for each section, a list of adjustment events, and a test point data table.

[0025] Furthermore, referring to Figure 2 The establishment of a three-dimensional finite element model and setting of contact pair elements in S1 includes: The ventilation duct is modeled as a hollow cylindrical structure, and the duct body is meshed using eight-node hexahedral elements. The stone packing material is modeled as a continuous medium surrounding the ventilation duct, and the packing material is meshed using four-node tetrahedral elements. The frozen soil layer is modeled as a semi-infinite space, and a six-node triangular prism element is used to mesh the frozen soil layer. A face-to-face contact pair is established between the outer wall of the ventilation duct and the packing. The normal behavior of the contact pair is set to hard contact, and the tangential behavior is set to penalized frictional contact. A face-to-face contact pair is established between the bottom surface of the filler and the top surface of the frozen soil layer, and the normal behavior of the contact pair is set to a bonded contact that does not allow separation.

[0026] In this embodiment, a geometric model of the ventilation duct is created in the finite element preprocessing module. The ventilation duct is modeled as a hollow cylinder with an inner diameter of 1.0 meter, an outer diameter of 1.2 meters, and a length of 2.0 meters. Eight-node hexahedral elements are used to mesh the duct. Three layers of elements are used in the wall thickness direction from the inner wall to the outer wall, each layer being 0.0667 meters thick. 36 elements are used in the circumferential direction of the duct, one element every 10 degrees. 20 elements are used in the axial direction of the duct, one element every 0.1 meters. Then, a geometric model of the stone filling material is created. The filling material model is a cuboid 10 meters long, 8 meters wide, and 2.5 meters high. The ventilation duct is located slightly below the center of this cuboid, with the top of the duct 1.5 meters from the top surface of the filling material. Four-node tetrahedral elements are used to mesh the filling material. Within a 1.2-meter radius around the ventilation duct, the element size is set to 0.04 meters. In areas far from the ventilation duct, the element size is set to 0.15 meters. The two sizes of elements are smoothly connected through transition elements. Next, a geometric model of the permafrost layer is created. The permafrost layer model is a cuboid 20 meters long, 16 meters wide, and 8 meters high, with the filler model placed at the center of its top surface. The permafrost layer is divided into two layers: the upper layer is the seasonal thawing layer with a thickness of 1.8 meters; the lower layer is the permafrost layer with a thickness of 6.2 meters. The permafrost layer is meshed using six-node triangular prism elements, with the element size in the seasonal thawing layer set to 0.08 meters and the element size in the permafrost layer set to 0.25 meters.

[0027] In terms of contact setup, the construction personnel established face-to-face contact pairs between the outer wall of the ventilation duct and the filling material; all outer surface unit faces of the outer wall of the ventilation duct were selected as master faces, and the unit faces of the filling material adjacent to the outer wall of the ventilation duct were selected as slave faces; the normal behavior of the contact pairs was set to hard contact, that is, the two surfaces are not allowed to penetrate each other, and once contact is made, the normal pressure can be transmitted arbitrarily; the tangential behavior was set to penalized friction contact, and the friction coefficient was set to 0.5; a face-to-face contact pair was also established between the bottom surface of the filling material and the top surface of the frozen soil layer, with all unit faces of the bottom surface of the filling material selected as master faces and all unit faces of the top surface of the frozen soil layer selected as slave faces, and the normal behavior was set to bonded contact that does not allow separation, that is, the bottom surface of the filling material and the top surface of the frozen soil layer always remain in close contact throughout the entire construction simulation process, without separation or relative sliding.

[0028] Furthermore, S2 includes: In the three-dimensional finite element model, the action area of ​​the plate compactor is obtained by extending one pipe diameter to both sides of the central axis of the ventilation pipe. A rectangular moving load belt is set in this action area, and a harmonic variable nodal force is applied to the load belt. By changing the frequency parameter value and amplitude parameter value of the harmonic nodal force, different vibration frequencies and different excitation force magnitudes of the plate compactor are respectively obtained. The load belt is loaded in the following order: first, it moves along the axis of the ventilation pipe from one end to the other, and then it moves perpendicular to the axis from the side of the pipe to the side away from the pipe.

[0029] In this embodiment, when simulating the local compaction effect of a plate compactor on the weak area around the ventilation duct, the construction personnel made the following specific settings in the three-dimensional finite element model: First, using the central axis of the ventilation duct as a reference, extend a distance of 1.2 meters in diameter to both the left and right sides, that is, from 1.2 meters to 1.2 meters on the left and right sides of the ventilation duct, defining this strip area with a width of 2.4 meters as the plate compactor's action area; within this action area, create a rectangular moving load band, the width of which is equal to the actual width of the plate compactor plate (0.6 meters), and the length of which is equal to the outer diameter of the ventilation duct (1.2 meters). The position of the load band changes over time; a harmonicly varying nodal force is applied to the load band. The magnitude of this harmonic nodal force changes sinusoidally over time and is controlled by two parameters: a frequency parameter value, adjustable from 20Hz to 50Hz; and an amplitude parameter value, adjustable from 0.5mm to 2.0mm, corresponding to the vibration frequency and excitation force of the plate compactor, respectively.

[0030] The load strip's movement path is divided into two stages: In the first stage, the load strip moves from one end to the other along the ventilation duct's axis. Assuming the ventilation duct's axis is along the longitudinal X direction of the roadbed, the load strip starts at X=0 and moves towards X=20 meters at a speed of 0.2 meters per second, with each 0.1-meter movement constituting a loading step. The first stage ends when the load strip reaches X=20 meters. In the second stage, the load strip moves perpendicular to the axis from the side of the duct away from it. Specifically, the load strip starts at the left edge of the ventilation duct at Y=-1.2 meters, moves 0.6 meters to the left to Y=-1.8 meters, and then starts at the right edge of the ventilation duct at Y=1.2 meters, moving 0.6 meters to the right to Y=1.8 meters. After each movement, repeat the first phase of axial movement from one end to the other; repeat this process until the entire 2.4-meter-wide area of ​​action is covered by the load strip at least once; throughout the entire loading process, the harmonic nodal force of the load strip is always applied to all nodes where the current load strip is located, and the loads on previously loaded nodes are removed.

[0031] Furthermore, referring to Figure 3 S3 includes: The nodal displacements of all filling elements calculated from the three-dimensional finite element model are classified. Based on the inverse proportional conversion relationship between displacement and density, the displacement value at the centroid of each filling element is converted into the density value of that element. The three-dimensional scalar field is then used to extract isosurfaces and generate a density distribution cloud map. Using the outer wall of the ventilation duct as a reference, extend the preset layer thickness outward along the radial direction of the duct body, mark all the packing units as the surrounding area of ​​the ventilation duct, separate the packing units to count the density value, and take the minimum density value in each sub-area as the density representative value of that sub-area. From the nodal equivalent forces of all pipe elements output from the three-dimensional finite element model, the nodes located in the arch region of the ventilation duct and the nodes located in the side region of the ventilation duct are selected, and the maximum value of the equivalent force of all nodes in the arch region and the maximum value of the equivalent force of all nodes in the side region are calculated respectively. By changing the combination of vibration frequency and displacement amplitude, and repeating the process, multiple sets of different representative values ​​of compaction and maximum equivalent stress are obtained. The preset compaction threshold is used as the lower limit condition, and the yield strength threshold of the pipe material is used as the upper limit condition. All combinations of vibration frequency and displacement amplitude that simultaneously satisfy the condition that the representative values ​​of compaction in the three sub-regions are not lower than the preset compaction threshold, and the maximum equivalent stress of the arch and the maximum equivalent stress of the pipe side are not higher than the yield strength threshold of the pipe material are selected. From all the selected combinations, the one with the largest sum of representative density values ​​and the smallest sum of maximum equivalent stress values ​​is chosen as the combination of construction parameters for the plate compactor and road roller.

[0032] In this embodiment, during the specific process of converting the nodal displacement under each combination of vibration frequency value and displacement amplitude into a density distribution cloud map and selecting the combination of construction parameters, the construction technicians performed the following detailed operations: The first step involves retrieving the nodal displacements of all packing elements from a specific set of vibration frequencies, such as 35Hz, and displacement amplitudes, such as 1.2mm, from the calculation results file. Each packing element has four tetrahedral nodes, and each node has displacement components in the X, Y, and Z directions. The average value of the four nodal displacement components for each element is taken as the representative displacement value at the centroid of that element. Then, using a relationship established beforehand through indoor compaction tests, the displacement value at the centroid of each element is converted into the density value of that element. The conversion relationship is as follows: when the displacement is less than 0.5mm, the density corresponds to 98%; when the displacement is between 0.5mm and 1.0mm, the density decreases linearly from 95% to 98%; when the displacement is between 1.0mm and 2.0mm, the density decreases linearly from 90% to 95%; and when the displacement is greater than 2.0mm, the density is below 90%. This conversion is performed on all packing elements in the model to obtain a three-dimensional scalar field, where the centroid coordinates of each spatially located element correspond to a density value. Next, in this three-dimensional scalar field, an isosurface extraction algorithm is used to connect adjacent units with the same density value to form continuous isosurfaces. For example, extracting an isosurface with a density of 93% allows for a direct visualization of areas in the filler with a density lower than 93%.

[0033] The second step involves using the outer wall of the ventilation duct as a reference, extending radially outwards by 0.3 meters, and marking all packing units within this range as the perimeter of the ventilation duct. Then, three sub-regions are separated from this perimeter: the left sidewall region (0.3 meters outside the left outer wall of the ventilation duct), the right sidewall region (0.3 meters outside the right outer wall of the ventilation duct), and the bottom region (0.3 meters below the bottom outer wall of the ventilation duct). The density values ​​of all packing units within each of these three sub-regions are then calculated, and the minimum value within each sub-region is determined. For example, assuming the minimum value for the left side region is 93.2%, the right side region is 93.5%, and the bottom region is 93.0%, these three values ​​are recorded.

[0034] Third, under the same set of vibration frequency values ​​and displacement amplitude values, read the nodal equivalent stress of all pipe elements from the calculation result file; filter the nodes located in the arch region: on the cross-section of the ventilation pipe, with the vertical upward direction as 0°, select the pipe wall nodes with an included angle in the range of -30° to +30°; filter the nodes located in the pipe side region: select the left side of the pipe wall nodes with an included angle in the range of 60° to 120° and the right side of the pipe wall nodes with an included angle in the range of 240° to 300°; calculate the maximum value of the equivalent stress of all nodes in the arch region and the maximum value of the equivalent stress of all nodes in the pipe side region respectively; assume that the maximum equivalent stress of the arch is 22.3MPa and the maximum equivalent stress of the pipe side is 23.1MPa.

[0035] The fourth step involves changing the combination of vibration frequency and displacement amplitude, repeating steps one through three. For example, dozens of combinations can be created, such as 30Hz frequency / 1.0mm amplitude, 35Hz frequency / 1.2mm amplitude, and 40Hz frequency / 1.5mm amplitude. Each combination yields three minimum compaction values ​​and two maximum equivalent stress values.

[0036] Step 5: Set the preset density threshold to 93% and the yield strength threshold of the pipe material to 25 MPa. Filter out all combinations that satisfy the following conditions: minimum density on the left side ≥ 93%, minimum density on the right side ≥ 93%, minimum density at the bottom of the pipe ≥ 93%, maximum equivalent stress at the crown ≤ 25 MPa, and maximum equivalent stress on the sides of the pipe ≤ 25 MPa. Assume there are 5 combinations that meet the conditions. Then, for each combination, calculate the sum of the three minimum density values ​​and the sum of the two maximum equivalent stress values. For example, combination A: sum of density = 93.2 + 93.5 + 93.0 = 279.7, sum of stress = 22.3 + 23.1 = 45.4; combination B: sum of density = 94.0 + 93.8 + 93.3 = 281.1, sum of stress = 23.5 + 24.0 = 47.5. Select the combination with the maximum sum of density and the minimum sum of stress. If the sum of density and stress is not the minimum at the same time as the sum of density and stress, then the sum of density and stress is taken first, and then the sum of stress is selected from the combinations with similar density and stress. The final selected combination is the construction parameter combination of the plate compactor and the road roller. The parameters of the plate compactor include the vibration frequency value and the displacement amplitude, and the parameters of the road roller include the vibration frequency value and the amplitude value. The vibration frequency value of the road roller and the vibration frequency value of the plate compactor can be the same or different. In this embodiment, the optimal parameter combination of the road roller is obtained by independently scanning the road roller load in the model, and then combined with the parameter combination of the plate compactor as the final output.

[0037] Further, S4 includes: After the ventilation duct is installed, graded crushed stone filler is backfilled on both sides and below the duct to form a weak area. A plate compactor is used to locally compact this weak area according to the selected vibration frequency and displacement amplitude. The plate compactor starts from one end of the ventilation duct and compacts it segment by segment along the axial direction of the duct to the other end. After each axial segment is completed, the plate compactor moves away from the duct by half the width of the compactor plate and then compacts it back from the other end of the duct along the axial direction until the entire width of the weak area is covered. After compacting the weak areas, backfill the top of the ventilation pipe with graded crushed stone to the preset cover layer thickness. Use a road roller to compact the cover layer as a whole according to the selected vibration frequency and amplitude values. The road roller starts from one edge of the subgrade and compacts one strip at a time along the direction perpendicular to the axis of the ventilation pipe to the other edge. Each compaction strip overlaps the previous compaction strip by one-third of the width of the road roller wheel. After completing one layer of cover compaction, backfill the next layer of cover and repeat the compaction operation until the design filling elevation is reached.

[0038] In this embodiment, the specific process of compaction construction on-site according to the selected combination of construction parameters is as follows: First, after the ventilation pipe is installed and the bottom bedding layer is laid, graded crushed stone filler is backfilled to both sides and below the ventilation pipe. Backfilling is done in layers, with each layer controlled to a thickness of 0.2 to 0.25 meters. After the first layer is backfilled, a plate compactor is used to locally compact the weak area at the selected vibration frequency of 35 Hz and displacement amplitude of 1.2 mm. The plate compactor starts from one end of the ventilation pipe, for example, at kilometer marker K0+000, placing the compactor plate in the weak area on the left side of the ventilation pipe, and compacts it segment by segment along the ventilation pipe axis towards the other end, K0+020. Each segment is 2 meters long, and the plate compactor stays on that segment for 10 seconds before moving to the next segment. When the plate compactor reaches K0+020, then... Return from K0+020 to K0+000 along the axis to complete the first round of compaction; then, move the plate compactor away from the ventilation pipe by half the width of the compactor plate (0.3 meters), and start compacting again from K0+000 towards K0+020. Repeat this process, moving half the width of the compactor plate each time a round trip is completed, until the entire width of the weak zone, from 1.2 meters on the left to 1.2 meters on the right side of the ventilation pipe, is completely compacted and covered; each layer of fill material in the weak zone is compacted in the same way until it is backfilled to the level of the top of the pipe.

[0039] After compacting the weak areas, graded crushed stone filler is backfilled onto the top of the ventilation pipe. The first layer of soil is 0.3 meters thick. After backfilling, a road roller is used to compact the entire structure at the selected vibration frequency of 35 Hz and amplitude of 1.5 mm. The road roller starts from 3 meters away from the left side of the ventilation pipe on one side of the roadbed edge and compacts the material in a transverse direction perpendicular to the ventilation pipe axis until it reaches 3 meters away from the right side of the ventilation pipe on the other side edge. The width of each compaction strip is equal to the width of the road roller wheel, which is 2.1 meters. The two adjacent compaction strips overlap by one-third of the wheel width, i.e., 0.7 meters. After the first pass of compaction is completed from the left edge to the right edge, the roller moves 1.4 meters (one roller width minus the overlap) towards the leading edge axis, then rolls back from the right edge to the left edge, repeating this process until the entire length of the top fill layer of the ventilation pipe is compacted and covered. After completing one layer of topsoil compaction, the next layer of topsoil is backfilled, each layer still 0.3 meters thick, and the above compaction operation is repeated until the design filling elevation is reached 1.5 meters above the top of the pipe. Throughout the compaction process, the compaction degree is tested using the sand cone method after each layer is completed to ensure that the compaction degree of each layer meets the requirements.

[0040] Further, S5 includes: When the compaction deviation value is positive and the absolute value exceeds the first threshold range, keep the current compaction residence time unchanged and reduce the number of compaction passes by one; When the compaction deviation value is negative and the absolute value exceeds the second threshold range, the compaction residence time of the plate compactor in the current weak section is increased. The amount of time increase is positively correlated with the absolute value of the compaction deviation value. At the same time, the number of compaction passes of the roller in the current filler layer is increased by one. When the strain safety margin value is lower than the third threshold, immediately stop the compaction construction in the current area, reduce the displacement amplitude of the plate compactor by one level and start compaction again, and reduce the vibration value of the road roller by one level. When the waveform distortion characteristic value shows that there is a reflection peak in the crack echo signal that exceeds the fourth threshold, mark the pipe location as a suspected crack point, increase the detection frequency of the resistance strain gauge at that location, and at the same time reduce the vibration frequency value of the road roller by one level before continuing to compact.

[0041] In this embodiment, the specific method for adjusting the compaction dwell time and number of compaction passes in real time during construction based on the compaction deviation value, strain safety margin value, and waveform distortion characteristic value is as follows: Construction personnel set up a real-time monitoring system; when the compaction value is measured at a certain detection point on site using the sand cone method, the system compares this value with the density threshold simulated at the same location in the three-dimensional finite element model, calculating the compaction deviation value by subtracting the simulated threshold from the actual measured value; assuming the simulated threshold is 93% and the actual measured value is 94.5%, then the compaction deviation value is +1.5%; the system sets a first threshold of ±1% and a second threshold of ±1% but in different directions; when the compaction deviation value is positive and the absolute value exceeds... When the compaction deviation exceeds 1%, for example, +1.5%, it indicates that the compaction degree of the area is already too high. Continuing to maintain the current compaction dwell time may cause over-compaction. Therefore, the system instructs to reduce the number of compaction passes by one, that is, the area that originally required 6 passes will be compacted by 5 passes instead. When the compaction degree deviation value is negative and the absolute value exceeds 1%, for example, -1.3%, it indicates that the compaction degree is insufficient. The system instructs to increase the compaction dwell time of the plate compactor in the current weak section. The amount of time increase is positively correlated with the absolute value of the compaction degree deviation value. For example, for every 0.5% increase in the absolute value of the deviation, the dwell time increases by 2 seconds, and the original dwell time of 10 seconds increases to 12 seconds. At the same time, the system instructs the roller to increase the number of compaction passes in the current fill layer by one, from 6 passes to 7 passes.

[0042] In addition, the system monitors the circumferential strain value measured by resistance strain gauges in real time and calculates the strain safety margin value: the ultimate strain value of the pipe material minus the measured strain value. A third threshold is set at 20% of the ultimate strain value. Assuming the ultimate strain is 0.003 and the measured strain is 0.0025, the safety margin is 0.0005, accounting for 16.7% of the ultimate strain, which is less than 20%. At this point, the system immediately issues an alarm and stops compaction in the current area. Construction personnel manually reduce the displacement amplitude of the plate compactor from 1.2mm to 1.0mm and the vibration amplitude of the roller from 1.5mm to 1.2mm, then restart compaction in the area. Simultaneously, this location is marked as a high-risk area, and the strain monitoring frequency is increased during subsequent compaction.

[0043] For the crack echo signal from the ultrasonic flaw detector, the system compares the real-time acquired waveform with the reference waveform in the crack-free state and calculates the waveform distortion characteristic value. If the distortion characteristic value exceeds the fourth threshold, for example, the amplitude of the reflected wave peak exceeds 50% of the amplitude of the reference wave peak, the pipe location is determined to be a suspected crack point. The system increases the detection frequency of the resistance strain gauge at this location from once every 10 minutes to once every 2 minutes, and at the same time reduces the vibration frequency of the road roller from 35Hz to 30Hz, and then continues rolling. All the above adjustment operations are performed in real time during the rolling construction. After each adjustment, the system automatically records the pipe mileage station number (e.g., K0+125) and the fill layer elevation (e.g., 0.6 meters above the top of the pipe) corresponding to the adjustment time, as well as the parameter values ​​before and after the adjustment, forming an adjustment log.

[0044] Further, S6 includes: The entire construction area was divided into multiple consecutive sections according to the mileage of the ventilation pipes, and section numbers were assigned. For each section, construction record data is extracted from the construction records, and the construction record data of each section is cross-compared to form records of residence time-compaction degree relationship, number of compaction passes-strain relationship, and crack location-waveform characteristics. The baseline construction parameters, residence time-compaction degree relationship records, number of compaction passes-strain relationship records, and crack location-waveform characteristic records for each section are arranged sequentially, and then the statistical characteristic values ​​calculated from all compaction degree test values ​​and circumferential strain test values ​​of that section are appended to characterize the overall distribution of the test values. The arrangement results of all sections are compiled into a technical solution document in document format, including a cover, a summary table of section parameters, a detailed record table of each section, and a schematic diagram of the distribution of test points.

[0045] In this embodiment, after construction is completed, the technician imports all construction record data into a spreadsheet. First, the entire 200-meter-long test section is divided into 20 sections according to the mileage of the ventilation pipe. Each section contains a 10-meter-long ventilation pipe and its surrounding filler area. In reality, the length of a single ventilation pipe section is 2.5 meters, but for ease of summarization, four consecutive ventilation pipe sections of the same type and their surrounding filler are divided into one section, with each section being 10 meters long. The sections are numbered S01, S02, S03...S20 sequentially from low mileage to high mileage.

[0046] For section S01, from kilometer markers K0+000 to K0+010, the following construction record data were extracted from the construction logs: The first type of data consists of the baseline construction parameters determined before construction began in this section: plate compactor vibration frequency 35Hz, plate compactor displacement amplitude 1.2mm, road roller vibration frequency 35Hz, and road roller amplitude 1.5mm. The second type of data consists of all rolling residence time adjustment events recorded during the construction process in this section. For example, at K0+003, at a fill layer elevation of +0.3 meters, the residence time before adjustment was 10 seconds, and the residence time after adjustment was 12 seconds, with a compaction deviation of -1.3% at the time of adjustment. The third type of data is the rolling pass adjustment event; for example, at K0+007, at the fill layer elevation +0.6 meters, the number of rolling passes before adjustment was 6, and the number of rolling passes after adjustment was 7. The compaction deviation at the time of adjustment was -1.1%, and the strain safety margin was 18%. The fourth type of data is the measured values ​​of all detection points in this section; including the compaction values ​​of 25 sand cone detection points, such as the compaction of 94.2% above the top of the pipe at K0+002; the circumferential strain values ​​of 12 resistance strain gauges, such as the strain of 0.0021 at the mid-span of the pipe at K0+005; and the echo signals of 8 ultrasonic flaw detection points, such as the reflected wave peak amplitude of 0.8mV at K0+008. The fifth type of data is the list of pipe locations marked as suspected crack points in this section, such as the left side of the pipe at K0+004.5, and the original ultrasonic waveform corresponding to this location stored in the form of a time-amplitude curve.

[0047] Then, the construction record data for each section were cross-referenced; the adjustment events in the second type of data were correlated with the compaction degree test values ​​at the same location in the fourth type of data. For example, the residence time adjustment event at K0+003 was correlated with the compaction degree value of 94.5% at the plane coordinates X=3m, Y=0.5m of the sand cone test point at that location, forming a residence time-compaction degree relationship record, specifying: location K0+003, adjusted residence time 12 seconds, corresponding compaction degree 94.5%; the adjustment events in the third type of data were correlated with the circumferential strain test values ​​at the same location in the fourth type of data, forming a rolling pass-strain relationship record; the suspected crack points in the fifth type of data were correlated with the crack echo signals at the same location in the fourth type of data, forming a crack location-waveform characteristic record, for example, at K0+004.5, the waveform shows obvious reflection peaks.

[0048] Next, following the order of section numbers S01 to S20, the baseline construction parameters, residence time-compaction degree relationship records, number of compaction passes-strain relationship records, and crack location-waveform characteristic records for each section are arranged sequentially. After each section record, a statistical characteristic value calculated from all compaction degree test values ​​and circumferential strain test values ​​for that section is appended. This statistical characteristic value does not contain specific data, but describes the overall distribution of the test values ​​in words. For example, the compaction degree test values ​​are mainly concentrated between 94% and 95%, showing a normal distribution; the circumferential strain values ​​are concentrated between 0.0020 and 0.0023, with a small degree of dispersion.

[0049] Finally, the arrangement results of all sections were compiled into a technical solution document. This document included a cover page with the project name, date, and compiling unit; a section parameter summary table listing the baseline construction parameters and statistical characteristic values ​​for each section from S01 to S20; a detailed record table for each section, with each section on a separate page, listing all adjustment events and suspected crack points within that section in chronological order; and a schematic diagram of the detection point distribution, marking the mileage, elevation, and corresponding compaction and circumferential strain values ​​of all detection points on the longitudinal profile, using different colors to mark the points where adjustment events occurred and suspected crack points. This technical solution document was archived as a guidance document for subsequent construction.

[0050] Furthermore, the adjustment range of vibration frequency and displacement amplitude of the plate compactor simple harmonic load and the adjustment range of vibration frequency and amplitude of the road roller rectangular distributed load in S2 are realized in the three-dimensional finite element model through parametric scanning. The vibration frequency value is set to change from low frequency limit to high frequency limit. For each vibration frequency value, the displacement amplitude or amplitude value is set to change from small amplitude limit to large amplitude limit in an arithmetic sequence. The displacement of the filling unit node and the equivalent force of the pipe unit node under each combination of frequency value and amplitude are calculated in sequence. The difference between two adjacent frequency values ​​is equal, and the difference between two adjacent amplitudes is equal.

[0051] In this embodiment, the construction personnel set two scanning dimensions in the parameter analysis module of the finite element software. The first dimension is the vibration frequency value, with the frequency set from 20Hz to 50Hz, and the step size of the arithmetic sequence being 5Hz, i.e., calculating a total of 7 frequency points: 20Hz, 25Hz, 30Hz, 35Hz, 40Hz, 45Hz, and 50Hz. For the displacement amplitude of the plate compactor's harmonic load and the amplitude of the road roller's rectangular distributed load, scanning ranges are set respectively. The displacement amplitude of the plate compactor starts from 0.5mm and ends at 2.0mm, with a step size of 0.2mm, i.e., 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, and 1.9, a total of 8 amplitude points. The amplitude value of the road roller starts from 0.8mm and ends at 2.5mm, with a step size of 0.3mm, i.e., 0.8, 1.1, 1.4, 1.7, 2.0, and 2.3, a total of 6 amplitude points.

[0052] During parametric scanning, the software performs calculations in a nested loop sequence: the outer loop represents the vibration frequency, and the inner loop represents the amplitude. For each fixed frequency value, the model response corresponding to all amplitudes at that frequency is calculated sequentially. The difference between any two adjacent frequency values ​​is 5 Hz, and the difference between any two adjacent amplitudes is 0.2 mm or 0.3 mm. After each calculation, the software automatically outputs the nodal displacement of the filling element and the equivalent stress of the pipe element at that frequency-amplitude combination. All calculation results are arranged in a result matrix according to the order of frequency and amplitude values ​​for easy subsequent processing. For example, at a frequency of 20 Hz, the results for amplitudes of 0.5 mm, 0.7 mm, 0.9 mm, etc., are calculated respectively; then at a frequency of 25 Hz, the results for all amplitudes are calculated in the same way, and so on. In this way, a total of 7 frequency × 8 amplitude = 56 plate compactor simulations and 7 × 6 = 42 road roller simulations are required. The model size and boundary conditions are exactly the same for each simulation, only the load parameters are different.

[0053] Furthermore, S4 also includes: The sand filling method test points are arranged on the surface of the filler in the weak area around the ventilation duct and on the surface of the filler layer at the top of the ventilation duct. The test point is set at a cross section every one pipe diameter along the axis of the ventilation duct. On each cross section, a test point is set at the top of the pipe, at the oblique top of the pipe, and at the horizontal direction of the pipe. Resistance strain gauges are attached to the outer surface of the pipe wall along the axial direction of the ventilation pipe. The sensitive grid direction of the strain gauge is parallel to the axial direction of the pipe body. Strain gauges are arranged at the pipe end flange connection, the mid-span of the pipe body, and one-quarter of the length of the pipe body on each section of the ventilation pipe. The ultrasonic flaw detector probe is placed at the end of the ventilation duct and emits ultrasonic waves into the duct along the axial direction of the duct body. Receiver probes are set on the outer surface of the duct body at intervals of the duct diameter. The ultrasonic wave propagation time and waveform amplitude between the transmitting and receiving probes are used as the raw data of the crack echo signal.

[0054] In this embodiment, the sand-filling test points are arranged as follows: a test section is set every 1.2 meters along the axis of the ventilation duct, for a test section of 200 meters, there are approximately 167 sections. Three test points are set on each section: the first test point is located directly above the top of the duct on the surface of the filler, at the intersection of the ventilation duct's central axis and the filler surface; the second test point is located diagonally above the duct side, shifted horizontally 0.8 meters to one side and simultaneously 0.2 meters downwards from the point directly above the top of the duct, essentially diagonally upwards from the outside of the duct sidewall; the third test point is located horizontally on the duct side, shifted horizontally 1.0 meter to the left or right from the ventilation duct's centerline, at a height level with the duct center. At each test point, workers dig a test pit with a diameter of 15cm and a depth of 15cm, collect and weigh all the filler material excavated from the pit, then pour standard sand into the pit. The volume of the test pit is calculated based on the sand's mass and density, the wet density is calculated based on the mass of the excavated filler material, and finally, the moisture content is obtained through a drying method, thereby calculating the dry density and compaction degree.

[0055] Arrangement of resistance strain gauges: Three strain gauges are attached to each 2.5-meter section of the ventilation duct along its axis. The first strain gauge is attached at the flange connection at the duct end, 0.1 meters from the duct end, circumferentially positioned horizontally on the side of the duct. The second strain gauge is attached at the mid-span of the duct, 1.25 meters from the duct end, also circumferentially positioned on the side. The third strain gauge is attached at one-quarter of the duct length, 0.625 meters from the duct end, circumferentially positioned directly above the duct top. The sensitive grid direction of each strain gauge is parallel to the duct axis. Before attaching the strain gauges, the duct wall surface is sanded, cleaned with alcohol, and then a special adhesive is applied. The strain gauges are then firmly attached and pressure-cured. The strain gauge leads are fixed along the duct wall with tape and connected to a dynamic strain acquisition instrument with a sampling frequency set to 100Hz.

[0056] Ultrasonic flaw detector setup: At the end of the ventilation duct, place the transmitting probe tightly against the center of the duct end face and apply coupling agent; the transmitting probe emits ultrasonic pulses at a frequency of 50kHz into the duct; on the outer surface of the duct, place a receiving probe every 1.2 meters of duct diameter; the receiving probe also contacts the duct wall through coupling agent to receive ultrasonic signals that penetrate or propagate along the duct wall; record the ultrasonic propagation time between the transmitting and receiving probes in microseconds and the waveform amplitude in millivolts; use the propagation time and waveform amplitude as the raw data of the crack echo signal; for each receiving point, simultaneously save a time-amplitude waveform diagram; when the ultrasonic wave encounters a crack, the waveform will show additional reflection peaks, its propagation time will be shortened because the reflection path is shorter, and the amplitude will increase or decrease; by comparing the waveforms at different receiving points, the location of the crack can be determined.

[0057] Furthermore, before simulating the compaction action of the plate compactor and the road roller, the three-dimensional finite element model in S2 performs the following initialization operations: Vertical displacement constraints are applied to the top surface of the frozen soil layer, and horizontal displacement constraints are applied to the sides of the frozen soil layer. An equivalent nodal force that dynamically changes with the movement of the compaction load band is applied to the top surface of the filler layer. The area of ​​action completely coincides with the load band area of ​​the plate compactor or roller. The magnitude of the equivalent nodal force is calculated by the width and length of the load band and the pressure value of the peak value of the harmonic load or the rectangular distributed load. Symmetrical boundary conditions are applied to the cross-sections at both ends of the ventilation duct to restrict the rigid body rotation of the cross-sections at both ends of the duct; absorbing boundary conditions are applied to the lateral boundary of the packing layer to reduce the reflection of stress waves at the model boundary.

[0058] In this embodiment, before simulating the compaction action of the plate compactor and road roller in the three-dimensional finite element model, the construction personnel perform the following initialization operations: First, apply vertical displacement constraints to the top surface of the permafrost layer. Select all top surface nodes of the permafrost layer model and set their vertical displacement in the Z direction to 0, meaning vertical settlement of the top surface of the permafrost layer is not allowed, but free horizontal movement is permitted. Apply horizontal displacement constraints to the four sides of the permafrost layer. Select all nodes on the four sides of the permafrost layer model and restrict their normal displacement respectively: restrict the X-direction displacement on the left and right sides, and restrict the Y-direction displacement on the front and rear sides; in this way, the permafrost layer is fixed in the horizontal direction, but vertical deformation is allowed.

[0059] Secondly, an equivalent nodal force that dynamically changes with the movement of the compaction load band is applied to the top surface of the filler layer. Specifically, a nodal force is applied to a rectangular area on the top surface of the filler layer, the position, width, and length of which completely coincide with the load band area of ​​the plate compactor or roller at the current moment. The width of the rectangular area is equal to the width of the plate compactor slab (0.6 meters) or the width of the roller wheel (2.1 meters), and the length is equal to the outer diameter of the ventilation pipe (1.2 meters) or the single compaction length of the roller (10 meters). The magnitude of the equivalent nodal force is calculated using the width and length of the load band and the peak value of the harmonic load or the pressure value of the rectangular distributed load. For example, for a plate compactor, the peak value of a simple harmonic load is 5000 N, and the effective area is 0.6 m × 1.2 m = 0.72 m², resulting in an average pressure of 6944 Pa. The total force of 5000 N is evenly distributed across all nodes within the effective area. For a road roller, the pressure of a rectangular distributed load is 0.3 MPa, the effective area is 2.1 m × 10 m = 21 m², and the total force is 6.3 × 10⁻⁶ Pa. 6 N is also evenly distributed to all nodes within the area of ​​action; when the load band moves, the nodal forces in the area of ​​action at the previous moment are removed, and the nodal forces in the area of ​​action at the new moment are applied.

[0060] Then, symmetrical boundary conditions are applied to the cross-sections at both ends of the ventilation duct; all nodes on the left and right end faces of the ventilation duct are selected, and the rotational degrees of freedom of these nodes around the X-axis and around the Y and Z axes are restricted, that is, the two ends of the duct are not allowed to twist, but the end faces are allowed to expand and contract along the axial direction and bend laterally; this can simulate the situation where the ventilation duct is constrained by adjacent pipe sections at both ends in the actual roadbed.

[0061] Finally, absorbing boundary conditions are applied to the lateral boundaries of the fill layer. All nodes on the left, right, front, and rear sides of the fill layer model are selected, and normal damper elements are placed on these nodes. The damping coefficient is set to reduce the reflected energy of the incident stress wave to below 5% of the incident energy. In this way, when the stress wave generated by the plate compactor or roller propagates to the fill boundary, most of the energy is absorbed by the damper, preventing strong reflected waves from interfering with the stress distribution inside the model. The absorbing boundary conditions are applied by adding a viscous damping element along the normal direction of the surface where the node is located at each boundary node. The damping force of this element is proportional to the normal velocity of the node. This method simulates the energy radiation effect of semi-infinite space soil.

[0062] The embodiments of this application have been described above with reference to the accompanying drawings. However, this application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of this application.

Claims

1. A construction control method for the compaction weakness zone of ventilation pipeline subgrade in plateau permafrost regions, characterized in that, include: S1. Obtain the filler gradation parameters of the ventilation duct subgrade, the elastic modulus of the ventilation duct body, and the thickness of the seasonal thawing layer of the frozen soil layer. Establish a three-dimensional finite element model, set contact pair elements, and divide the mesh elements into different densities. S2. In the three-dimensional finite element model, the vibration frequency and displacement amplitude of the plate compactor simple harmonic load and the vibration frequency and amplitude of the road roller rectangular distributed load are adjusted respectively. The nodal displacement of the filling element and the nodal equivalent force of the pipe element are recorded under each combination of frequency and amplitude. S3, construct a density distribution cloud map to extract the minimum density values ​​of the ventilation duct sidewall area and the bottom area, obtain the vibration frequency value and displacement amplitude combination, and use it as the construction parameter combination for the plate compactor and road roller; S4. According to the selected combination of construction parameters, local compaction construction is carried out in the weak area around the ventilation pipe, and the top filler layer of the ventilation pipe is compacted as a whole using a road roller. S5, adjust the compaction dwell time of the plate compactor in the weak zone and the number of compaction passes of the roller in the filler layer according to the compaction deviation value, strain safety margin value and waveform distortion characteristic value respectively; S6. After completing all the compaction work, summarize the combination of construction parameters used, the record of compaction dwell time adjustment, the record of compaction pass adjustment, and the compaction test values, circumferential strain test values ​​and crack echo signals of all test points, and integrate them to form a pipe-stone co-compaction technology solution for the ventilation pipeline subgrade.

2. The method according to claim 1, characterized in that, The establishment of a three-dimensional finite element model and setting of contact pair elements in S1 includes: The ventilation duct is modeled as a hollow cylindrical structure, and the duct body is meshed using eight-node hexahedral elements. The stone packing material is modeled as a continuous medium surrounding the ventilation duct, and the packing material is meshed using four-node tetrahedral elements. The frozen soil layer is modeled as a semi-infinite space, and a six-node triangular prism element is used to mesh the frozen soil layer. A face-to-face contact pair is established between the outer wall of the ventilation duct and the packing. The normal behavior of the contact pair is set to hard contact, and the tangential behavior is set to penalized frictional contact. A face-to-face contact pair is established between the bottom surface of the filler and the top surface of the frozen soil layer, and the normal behavior of the contact pair is set to a bonded contact that does not allow separation.

3. The method according to claim 1, characterized in that, S2 includes: In the three-dimensional finite element model, the action area of ​​the plate compactor is obtained by extending one pipe diameter to both sides of the central axis of the ventilation pipe. A rectangular moving load belt is set in this action area, and a harmonic variable nodal force is applied to the load belt. By changing the frequency parameter value and amplitude parameter value of the harmonic nodal force, different vibration frequencies and different excitation force magnitudes of the plate compactor are respectively obtained. The load belt is loaded in the following order: first, it moves along the axis of the ventilation pipe from one end to the other, and then it moves perpendicular to the axis from the side of the pipe to the side away from the pipe.

4. The method according to claim 1, characterized in that, S3 includes: The nodal displacements of all filling elements calculated from the three-dimensional finite element model are classified. Based on the inverse proportional conversion relationship between displacement and density, the displacement value at the centroid of each filling element is converted into the density value of that element. The three-dimensional scalar field is then used to extract isosurfaces and generate a density distribution cloud map. Using the outer wall of the ventilation duct as a reference, extend the preset layer thickness outward along the radial direction of the duct body, mark all the packing units as the surrounding area of ​​the ventilation duct, separate the packing units to count the density value, and take the minimum density value in each sub-area as the density representative value of that sub-area. From the nodal equivalent forces of all pipe elements output from the three-dimensional finite element model, the nodes located in the arch region of the ventilation duct and the nodes located in the side region of the ventilation duct are selected, and the maximum value of the equivalent force of all nodes in the arch region and the maximum value of the equivalent force of all nodes in the side region are calculated respectively. By changing the combination of vibration frequency and displacement amplitude, and repeating the process, multiple sets of different representative values ​​of compaction and maximum equivalent stress are obtained. The preset compaction threshold is used as the lower limit condition, and the yield strength threshold of the pipe material is used as the upper limit condition. All combinations of vibration frequency and displacement amplitude that simultaneously satisfy the condition that the representative values ​​of compaction in the three sub-regions are not lower than the preset compaction threshold, and the maximum equivalent stress of the arch and the maximum equivalent stress of the pipe side are not higher than the yield strength threshold of the pipe material are selected. From all the selected combinations, the one with the largest sum of representative density values ​​and the smallest sum of maximum equivalent stress values ​​is chosen as the combination of construction parameters for the plate compactor and road roller.

5. The method according to claim 1, characterized in that, S4 includes: After the ventilation duct is installed, graded crushed stone filler is backfilled on both sides and below the duct to form a weak area. A plate compactor is used to locally compact this weak area according to the selected vibration frequency and displacement amplitude. The plate compactor starts from one end of the ventilation duct and compacts it segment by segment along the axial direction of the duct to the other end. After each axial segment is completed, the plate compactor moves away from the duct by half the width of the compactor plate and then compacts it back from the other end of the duct along the axial direction until the entire width of the weak area is covered. After compacting the weak areas, backfill the top of the ventilation pipe with graded crushed stone to the preset cover layer thickness. Use a road roller to compact the cover layer as a whole according to the selected vibration frequency and amplitude values. The road roller starts from one edge of the subgrade and compacts one strip at a time along the direction perpendicular to the axis of the ventilation pipe to the other edge. Each compaction strip overlaps the previous compaction strip by one-third of the width of the road roller wheel. After completing one layer of cover compaction, backfill the next layer of cover and repeat the compaction operation until the design filling elevation is reached.

6. The method according to claim 1, characterized in that, S5 includes: When the compaction deviation value is positive and the absolute value exceeds the first threshold range, keep the current compaction residence time unchanged and reduce the number of compaction passes by one; When the compaction deviation value is negative and the absolute value exceeds the second threshold range, the compaction residence time of the plate compactor in the current weak section is increased. The amount of time increase is positively correlated with the absolute value of the compaction deviation value. At the same time, the number of compaction passes of the roller in the current filler layer is increased by one. When the strain safety margin value is lower than the third threshold, immediately stop the compaction construction in the current area, reduce the displacement amplitude of the plate compactor by one level and start compaction again, and reduce the vibration value of the road roller by one level. When the waveform distortion characteristic value shows that there is a reflection peak in the crack echo signal that exceeds the fourth threshold, mark the pipe location as a suspected crack point, increase the detection frequency of the resistance strain gauge at that location, and at the same time reduce the vibration frequency value of the road roller by one level before continuing to compact.

7. The method according to claim 1, characterized in that, S6 includes: The entire construction area was divided into multiple consecutive sections according to the mileage of the ventilation pipes, and section numbers were assigned. For each section, construction record data is extracted from the construction records, and the construction record data of each section is cross-compared to form records of residence time-compaction degree relationship, number of compaction passes-strain relationship, and crack location-waveform characteristics. The baseline construction parameters, residence time-compaction degree relationship records, number of compaction passes-strain relationship records, and crack location-waveform characteristic records for each section are arranged sequentially, and then the statistical characteristic values ​​calculated from all compaction degree test values ​​and circumferential strain test values ​​of that section are appended to characterize the overall distribution of the test values. The arrangement results of all sections are compiled into a technical solution document in document format, including a cover, a summary table of section parameters, a detailed record table of each section, and a schematic diagram of the distribution of test points.

8. The method according to claim 1, characterized in that, The adjustment ranges of vibration frequency and displacement amplitude of the plate compactor simple harmonic load and the rectangular distributed load of the road roller in S2 are realized in the three-dimensional finite element model through parametric scanning. The vibration frequency value is set to change from low frequency limit to high frequency limit. For each vibration frequency value, the displacement amplitude or amplitude value is set to change from small amplitude limit to large amplitude limit in an arithmetic sequence. The displacement of the filling unit node and the equivalent force of the pipe unit node under each combination of frequency value and amplitude are calculated in turn. The difference between two adjacent frequency values ​​is equal, and the difference between two adjacent amplitudes is equal.

9. The method according to claim 1, characterized in that, S4 further includes: The sand filling method test points are arranged on the surface of the filler in the weak area around the ventilation duct and on the surface of the filler layer at the top of the ventilation duct. The test point is set at a cross section every one pipe diameter along the axis of the ventilation duct. On each cross section, a test point is set at the top of the pipe, at the oblique top of the pipe, and at the horizontal direction of the pipe. Resistance strain gauges are attached to the outer surface of the pipe wall along the axial direction of the ventilation pipe. The sensitive grid direction of the strain gauge is parallel to the axial direction of the pipe body. Strain gauges are arranged at the pipe end flange connection, the mid-span of the pipe body, and one-quarter of the length of the pipe body on each section of the ventilation pipe. The ultrasonic flaw detector probe is placed at the end of the ventilation duct and emits ultrasonic waves into the duct along the axial direction of the duct body. Receiver probes are set on the outer surface of the duct body at intervals of the duct diameter. The ultrasonic wave propagation time and waveform amplitude between the transmitting and receiving probes are used as the raw data of the crack echo signal.

10. The method according to claim 1, characterized in that, Before simulating the compaction action of a plate compactor and a road roller, the three-dimensional finite element model in S2 performs the following initialization operations: Vertical displacement constraints are applied to the top surface of the frozen soil layer, and horizontal displacement constraints are applied to the sides of the frozen soil layer. An equivalent nodal force that dynamically changes with the movement of the compaction load band is applied to the top surface of the filler layer. The area of ​​action completely coincides with the load band area of ​​the plate compactor or roller. The magnitude of the equivalent nodal force is calculated by the width and length of the load band and the pressure value of the peak value of the harmonic load or the rectangular distributed load. Symmetrical boundary conditions are applied to the cross-sections at both ends of the ventilation duct to restrict the rigid body rotation of the cross-sections at both ends of the duct; absorbing boundary conditions are applied to the lateral boundary of the packing layer to reduce the reflection of stress waves at the model boundary.

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