Construction method for cast-in-situ bored pile in collapsible loess stratum

Abstract

The invention discloses a construction method for a cast-in-situ bored pile in a collapsible loess stratum, and relates to the technical field of cast-in-situ pile construction.The construction method comprises the following steps that a construction site is cleaned and leveled; pile hole position coordinate points are determined through pile foundation positioning and in cooperation with survey lofting; a pile casing is placed through excavation and burying construction; configuring a drilling machine to be stable in position, and adjusting a drilling center point of the drilling machine based on the pile casing; carrying out rotary drilling to form a hole based on a hard soil layer; reverse circulation drilling and hole forming are carried out based on the sand soil stratum; primary hole cleaning treatment is carried out on the rotary excavating and reverse circulation drill holes which are checked to meet the indexes; manufacturing a reinforcement cage and lowering and positioning the reinforcement cage corresponding to the pile position; a guide pipe for concrete pouring is installed based on the pile hole; a gas lift reverse circulation slurry channel is constructed for secondary hole cleaning; configuring a guide pipe and pouring concrete to form a pile. The problem that in the prior art, safety and stability are not high when a pile is drilled in a collapsible loess stratum is solved.

Description

Technical Field

[0001] The invention relates to the technical field of bored cast-in-place piles, and in particular to a construction method for bored cast-in-place piles in collapsible loess strata. Background Art

[0002] At present, bored cast-in-place piles refer to piles that are made by forming pile holes in the foundation soil at the construction site through mechanical drilling, steel pipe squeezing or manual excavation, and placing steel cages and pouring concrete inside. Depending on the hole-forming method, cast-in-place piles can be divided into sunken pipe cast-in-place piles, bored cast-in-place piles and excavated cast-in-place piles.

[0003] Collapsible loess refers to soil that undergoes significant additional deformation due to structural damage after immersion in water under the action of the self-weight stress of the overlying soil layer, or under the combined action of self-weight stress and additional stress. It is a special soil. Due to its high water content and strong viscosity, it is easy to cause hole wall collapse, diameter shrinkage, inclined holes, etc. during drilling construction, which poses a safety hazard and affects the quality of the project. Therefore, how to avoid the collapse of the hole wall of the collapsible loess layer during drilling and pouring piles while ensuring the high efficiency of construction, and ensure the safety and quality of the hole, has become a technical problem that needs to be solved urgently and a focus of research for technicians in this field. Summary of the invention

[0004] To this end, the present invention provides a construction method for bored cast-in-place piles in collapsible loess strata to solve the problem of low safety and stability of bored cast-in-place piles in collapsible loess strata in the prior art.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A construction method for bored cast-in-place piles in collapsible loess strata, comprising the following steps:

[0007] Clean and level the construction site;

[0008] Determine the coordinates of the pile hole through pile foundation positioning and measurement and setting out;

[0009] The casing is placed by the cut and bury method;

[0010] Configure the drilling rig to be in a stable position and adjust the drilling center point of the drilling rig based on the casing;

[0011] Based on rotary drilling in hard soil layer;

[0012] Reverse circulation drilling based on sandy soil formation;

[0013] Carry out initial hole cleaning for rotary drilling and reverse circulation drilling holes that meet the inspection standards;

[0014] The steel cage is manufactured and lowered to the corresponding pile position;

[0015] Install the conduit for concrete pouring based on the pile hole;

[0016] Construct gas lift reverse circulation mud channel for secondary hole cleaning;

[0017] Configure the conduit to pour concrete into piles.

[0018] On the basis of the above technical solution, the present invention is further described as follows:

[0019] As a further embodiment of the present invention,

[0020] The cleaning and leveling of the construction site specifically includes:

[0021] After clearing the surface weeds and loose soil of the bored pile site, the site is leveled and compacted. At the same time, the surface collapsible loess layer and the area with developed pits are treated with strong tamping in advance to ensure that the foundation has sufficient bearing capacity and prevent the site from collapsing and the surface soil cracks of the pile hole from leaking during the drilling process.

[0022] The method of determining the pile hole coordinate points by positioning the pile foundation and coordinating with the measurement and setting out specifically includes:

[0023] According to the distance and angle between the pile foundation coordinate points and each control point, the on-site measurement and layout are carried out to determine the pile hole coordinate points. After that, the total station is used to check the pile hole coordinate points, and the positioning error is controlled within 5mm. Guard piles are set in the four directions corresponding to the pile hole coordinate points.

[0024] The center point of the pile position is determined by drawing cross lines based on the guard piles set in all directions, and the circular path of the pile hole determined according to the pile diameter value at the center point of the pile position and its outer side is pre-marked with white lime.

[0025] As a further embodiment of the present invention,

[0026] The method of placing casing by digging and burying specifically includes:

[0027] The casing is made of 12mm thick steel plate, with a 20cm hole and double-layer reinforcement. The inner diameter is 30cm larger than the pile diameter, and the single section is 4.5-5m long. The casing welds ensure sealing and water isolation.

[0028] Since the hole mouth of the collapsible loess stratum is prone to collapse, the casing is placed by the excavation method, that is, the soil body corresponding to the buried depth of the casing at the pile hole coordinate point is first excavated by the rotary drilling bit. The diameter of the rotary drilling bit is 0.2m smaller than the diameter of the casing. Then the casing is hoisted into place and directly pressed into the excavation position.

[0029] The casing is buried by the rotary drilling rig static pressure method, that is, the clay layer and / or silt layer is used as the bearing layer at the end of the casing, the casing is hoisted by a crane, and the center point of the casing is positioned based on the cross pile, and the center point of the casing is coincident with the center point of the pile position. After confirming that the positions coincide, the horizontal ruler and vertical ruler are used to check until the casing is in place. At this time, the top of the casing is 0.3-0.5m above the ground, the position deviation of the top and bottom centers of the casing is less than 5mm, and the vertical inclination of the casing is less than 1%;

[0030] The gaps around the casing are further backfilled and compacted with clay;

[0031] When encountering further adverse soil conditions such as construction waste, long casing follow-up measures are adopted to extend the casing to maintain the mud head height in the casing to form a high hydrostatic pressure, thereby maintaining the stability of the hole wall.

[0032] As a further embodiment of the present invention,

[0033] The configuration of the drilling rig being stably positioned and adjusting the drilling center point of the drilling rig based on the casing specifically includes:

[0034] The ground corresponding to the drilling rig shall be leveled and compacted, and steel plates shall be laid. When the drilling rig is in place, the foundation under the drilling rig shall be verified and adjusted until no subsidence occurs. The drill rod of the drilling rig shall be adjusted vertically, and then the center point of the corresponding casing after the casing is buried shall be checked to see whether it coincides with the center point of the borehole formed by the drill rod of the drilling rig. If the deviation between the two is large, the position of the drilling rig shall be readjusted to ensure that the center point of the borehole meets the coincidence requirements.

[0035] As a further embodiment of the present invention,

[0036] The method of forming a hole by rotary drilling based on a hard soil layer specifically includes:

[0037] First, use a drilling rig to perform low-speed rotary drilling to form a hole, circulate mud in the borehole, and the mud is required to have a relative density ≥1.06, a viscosity range of 20-22Pa·s, a sand content of less than 4%, and a colloid rate ≥95%, and repeatedly sweep the hole to ensure that the drilling rig and drilling tools are unobstructed; after drilling through the pebble layer of the hard soil layer, adjust the mud relative density to ≥1.10, the viscosity range to 25-28Pa·s, the sand content of less than 2%, and the colloid rate of ≥95%, slowly rotate and lift the rotary drilling bit, and the lifting speed is ≤5mm / s, and follow-up monitoring and real-time replenishment of new mud to maintain the mud head height in the casing to avoid excessively fast rising speed to form negative pressure and cause the hole wall to collapse;

[0038] After the rotary drill bit is lifted away from the casing, slowly move the rotary drill rig to outside the construction impact range, and then review the center point of the hole drilled by the rotary drill rig to control its deviation from the center point of the pile position to be less than 20mm.

[0039] As a further embodiment of the present invention,

[0040] The reverse circulation drilling based on the sandy soil layer specifically includes:

[0041] Continue to perform reverse circulation drilling based on the sandy soil layer, slowly move the reverse circulation drilling rig to the previous rotary drilling rig position, adjust the drill bit to the center of the pile position, check the mechanical horizontality and the verticality of the drill rod, slowly rotate and lower the reverse circulation drill bit, the lowering speed of the reverse circulation drill bit is ≤5mm / s, maintain the fluidity and uniformity of the mud in the borehole, and prevent a large amount of sediment from affecting the mud weight;

[0042] During the drilling process, observe the changes in the mud head height in the borehole and the flatness of the drilling rig, and replenish the mud in time. At the same time, reduce the rotation speed, increase the mud viscosity, and prevent the sandy soil layer from shrinking, collapsing, and tilting the hole.

[0043] As a further embodiment of the present invention,

[0044] The initial hole cleaning process for rotary drilling and reverse circulation drilling holes that meet the inspection indicators specifically includes:

[0045] After the reverse circulation drilling is completed, the hole inspection device is moved by crane to check the hole position, hole diameter, hole depth and inclination of the pile hole, and the first hole cleaning treatment is carried out for the pile holes that meet the indicators;

[0046] When inspection reveals inclined holes and / or shrinkage holes and / or plum blossom holes, backfilling method shall be adopted immediately.

[0047] As a further embodiment of the present invention,

[0048] The steel cage is manufactured and positioned corresponding to the pile position, specifically including:

[0049] In order to shorten the hoisting time of the steel cage after the pile foundation is bored, the segmented steel cage is processed and manufactured by the long-line method, and the main reinforcement of the segmented steel cage is mechanically connected by a straight thread sleeve at the cage joint;

[0050] In order to ensure the connection accuracy of the steel cage, a rolling welding machine is used in conjunction with a table mold long line for trial assembly. After the trial assembly, the steel cage is numbered in sections, and the main bars connecting the upper and lower adjacent sections are uniformly marked;

[0051] In order to ensure that the steel cage does not twist or deform during transportation and hoisting, triangular support bars are set inside the reinforcing stirrups, and a 10cm L-shaped steel bar head support is welded above the support ring at the lifting point;

[0052] Before lowering the steel cage, recheck the pile foundation coordinates and reversely calculate the length of the steel cage positioning bars at the top of the pile through the center point of the pile position to ensure that the center of the steel cage coincides with the center point of the pile position when the steel cage is installed.

[0053] As a further embodiment of the present invention,

[0054] The concrete pouring conduit installed based on the pile hole specifically includes:

[0055] For the water tightness test of the pre-installed conduit, ensure that the conduit has good water tightness;

[0056] Several guide tubes are installed based on the pile hole, and the distance between the bottom end of the several guide tubes and the predetermined pile bottom is 0.4 cm to ensure the compactness of the first poured concrete;

[0057] The construction of the gas lift reverse circulation mud channel for secondary hole cleaning specifically includes:

[0058] Since it takes a long time to lower the steel cage and install the guide tube, new slag is generated at the bottom of the pile hole, so a secondary hole cleaning operation is carried out immediately;

[0059] During the secondary hole cleaning, air lift reverse circulation is used to clean the hole, which mainly relies on the mud head pressure to maintain the stability of the hole wall. Therefore, the mud liquid level in the pile hole is controlled to be no less than 50cm below the top of the casing.

[0060] Air lift reverse circulation hole cleaning uses an air compressor to send compressed air to the bottom of the duct through an air inlet pipe installed in the duct. When the compressed air is discharged from the bottom of the air duct, it forms a slurry-gas mixture with the mud. Due to the low density of the slurry-gas mixture, it rises along the inside of the duct, forming a negative pressure at the bottom of the duct. Under the action of negative pressure, the mud at the bottom of the pile base continues to carry the sediment and rise along the inside of the duct until it is turned over from the water outlet of the duct and flows into the mud pool. The sediment is settled in the sedimentation tank, and the mud filtered through the mud pool flows back into the pile hole again. The cycle is repeated until the sediment thickness in the hole meets the design requirements.

[0061] As a further embodiment of the present invention,

[0062] The method of configuring a conduit to pour concrete into piles specifically includes:

[0063] After the secondary hole cleaning is completed, underwater concrete pouring is carried out to complete the pouring of the entire pile within the initial setting time. The volume of the first batch of poured concrete meets the initial burial depth of ≥1m and fills the bottom of the conduit;

[0064] The specific calculation formula for the first batch of injection volume is as follows:

[0065]

[0066] Where V is the volume of concrete required to pour the first batch, m 3; D is the pile hole diameter, m; H1 is the distance between the bottom of the pile hole and the bottom end of the conduit, which is 0.4m; H2 is the initial burial depth of the conduit, m; d is the inner diameter of the conduit, m; h1 is the height required for the concrete column in the conduit to balance the pressure outside the conduit (or mud) when the concrete in the pile hole reaches the burial depth H2, m;

[0067] To ensure the first pouring volume, two tank trucks are used to pour concrete at the same time. For example, the discharge speed of a single tank truck is 4m 3 / min; According to field practice, when pouring for 1 minute, the concrete in the hopper no longer covers the bottom of the hopper, and the hopper size is selected as 5m 3 ; According to calculation, when two tank trucks are used for pouring at the same time and the concrete in the hopper no longer covers the bottom of the hopper, the total volume poured is (5+4×2)=13m 3 , meet the requirements of the first injection volume;

[0068] After the first pouring is completed, continue pouring concrete according to the requirement of 2 to 6m buried depth of the conduit to ensure uninterrupted, rapid and continuous pouring of concrete until the pouring exceeds the pile top elevation by 0.5 to 1m, at which time the pile construction is completed.

[0069] The present invention has the following beneficial effects:

[0070] This method uses a rotary drilling rig combined with a reverse circulation drilling rig to drill holes. A rotary drilling rig is used to drill holes in hard soil layers, and a reverse circulation drilling rig is used to drill in sandy soil layers. This method can not only effectively increase the drilling speed, prevent hole wall collapse and ensure the verticality of the pile body, but also use pump suction reverse circulation to clean the bottom of the hole. While ensuring construction efficiency, it improves the quality and safety of the hole and enhances the overall adaptability and practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] In order to more clearly illustrate the implementation mode of the present invention or the technical solution in the prior art, the drawings required for the implementation mode or the description of the prior art will be briefly introduced below. The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read. Any structural modification, change in proportional relationship or adjustment of size should still fall within the scope of the technical contents disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0072] Figure 1 A schematic diagram of the overall process of a construction method for bored cast-in-place piles in collapsible loess strata provided in an embodiment of the present invention.

[0073] Figure 2 A schematic diagram of an air lift reverse circulation hole cleaning framework in a construction method for bored cast-in-place piles in collapsible loess strata provided in an embodiment of the present invention.

[0074] Explanation of the reference numerals: 1 - air compressor; 2 - conduit; 3 - air inlet pipe; 4 - mud; 5 - mud pool; 6 - sedimentation tank; 7 - sediment; 8 - conduit outlet. DETAILED DESCRIPTION

[0075] The following is a description of the implementation of the present invention by specific embodiments. People familiar with the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] The terms such as "upper", "lower", "left", "right", and "middle" used in this specification are only for the convenience of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships should be regarded as within the scope of the invention without substantially changing the technical content.

[0077] like Figure 1 As shown, the embodiment of the present invention provides a construction method for bored piles in collapsible loess strata, which adopts a rotary drilling rig combined with a reverse circulation drilling rig to drill holes in hard soil strata using a rotary drilling rig and a reverse circulation drilling rig to drill in sandy soil strata. It can not only effectively improve the drilling speed, prevent the hole wall from collapsing and ensure the verticality of the pile body, but also use pump suction reverse circulation to clean the bottom of the hole, thereby improving the quality and safety of the hole while ensuring the construction efficiency. It specifically includes the following steps:

[0078] S1: Clean and level the construction site;

[0079] The specific process is as follows: after clearing the surface weeds and loose soil on the bored pile site, the site is leveled and compacted. At the same time, the surface collapsible loess layer and the area with developed pits are treated with strong tamping in advance to ensure that the foundation has sufficient bearing capacity and prevent the site from collapsing and the surface soil cracks of the pile hole from leaking during the drilling process;

[0080] S2: Determine the coordinates of the pile hole through pile foundation positioning and measurement and setting out;

[0081] The specific process is: according to the distance and angle between the pile foundation coordinate points and each control point, the on-site measurement and layout are carried out to determine the pile hole coordinate points, and then the total station is used to verify the pile hole coordinate points, the positioning error is controlled within 5mm, and guard piles are set up in the four directions corresponding to the pile hole coordinate points;

[0082] The center point of the pile position is determined by drawing cross lines based on the guard piles set in all directions, and the circular path of the pile hole determined by the pile diameter value at the center point of the pile position and its outer side is pre-marked with white lime;

[0083] S3: Place casing by digging and burying method;

[0084] The specific process is as follows: configure the casing, which is made of 12mm thick steel plate, with a 20cm hole and double-layer reinforcement, an inner diameter 30cm larger than the pile diameter, a single section length of 4.5 to 5m, and the casing welds to ensure sealing and water isolation;

[0085] Since the hole mouth of the collapsible loess stratum is prone to collapse, the casing is placed by the excavation method, that is, the soil body corresponding to the buried depth of the casing at the pile hole coordinate point is first excavated by the rotary drilling bit. The diameter of the rotary drilling bit is 0.2m smaller than the diameter of the casing. Then the casing is hoisted into place and directly pressed into the excavation position.

[0086] The casing is buried by the rotary drilling rig static pressure method, that is, the clay layer and / or silt layer is used as the bearing layer at the end of the casing, the casing is hoisted by a crane, and the center point of the casing is positioned based on the cross pile, and the center point of the casing is coincident with the center point of the pile position. After confirming that the positions coincide, the horizontal ruler and vertical ruler are used to check until the casing is in place. At this time, the top of the casing is 0.3-0.5m above the ground, the position deviation of the top and bottom centers of the casing is less than 5mm, and the vertical inclination of the casing is less than 1%;

[0087] The gaps around the casing are further backfilled and compacted with clay;

[0088] When encountering further unfavorable soil conditions such as construction waste, long casing follow-up measures are adopted to extend the casing to maintain the mud head height in the casing to form a high hydrostatic pressure, thereby maintaining the stability of the hole wall;

[0089] S4: configure the drilling rig to be in a stable position and adjust the drilling center point of the drilling rig based on the casing;

[0090] The specific process is as follows: the ground corresponding to the drilling rig is leveled and compacted and steel plates are laid. When the drilling rig is in place, the foundation under the drilling rig is verified and adjusted until no sinking occurs; the drilling rod of the drilling rig is adjusted vertically, and then the center point of the corresponding casing after the casing is buried is checked to see if it coincides with the center point of the borehole formed by the drilling rig drill rod. If the deviation between the two is large, the position of the drilling rig is readjusted to ensure that the center point of the borehole meets the coincidence requirement;

[0091] S5: Rotary drilling based on hard soil layer;

[0092] The specific process is as follows: first, use a drilling rig to perform low-speed rotary drilling to form a hole, circulate mud in the borehole, and the mud is required to have a relative density ≥1.06, a viscosity range of 20-22Pa·s, a sand content of <4%, a colloid rate ≥95%, and repeatedly sweep the hole to ensure that the drilling rig and drilling tools are unobstructed; after drilling through the pebble layer of the hard soil layer, adjust the mud relative density to ≥1.10, the viscosity range to 25-28Pa·s, the sand content of <2%, the colloid rate ≥95%, slowly rotate and lift the rotary drilling bit, the lifting speed ≤5mm / s, and follow up monitoring and real-time replenishment of new mud to maintain the mud head height in the casing to avoid excessively fast rising speed to form negative pressure and cause the hole wall to collapse;

[0093] After the rotary drilling bit is lifted away from the casing, slowly move the rotary drilling rig to outside the construction impact range, and then recheck the center point of the hole drilled by the rotary drilling rig to control its deviation from the pile position center point to less than 20mm;

[0094] S6: Reverse circulation drilling based on sandy soil;

[0095] The specific process is: continue to perform reverse circulation drilling based on the sandy soil layer, slowly move the reverse circulation drilling rig to the previous rotary drilling rig position, adjust the drill bit to the center of the pile position, check the mechanical horizontality and the verticality of the drill rod, slowly rotate and lower the reverse circulation drill bit, the lowering speed of the reverse circulation drill bit is ≤5mm / s, maintain the fluidity and uniformity of the mud in the borehole, and prevent a large amount of sediment from affecting the mud weight;

[0096] During the drilling process, observe the changes in the mud head height in the borehole and the flatness of the drilling rig, and replenish the mud in time, while reducing the rotation speed and increasing the mud consistency to prevent the shrinkage of the sandy soil layer, the collapse of the hole and the inclination of the hole;

[0097] S7: Perform initial hole cleaning for rotary drilling and reverse circulation drilling holes that meet the inspection standards;

[0098] The specific process is as follows: after the reverse circulation drilling is completed, the hole inspection device is moved by the crane to check the hole position, hole diameter, hole depth and inclination of the pile hole, and the first hole cleaning treatment is carried out for the pile hole that meets the indicators;

[0099] When inspection finds inclined holes and / or shrinkage holes and / or plum blossom holes, backfilling method shall be adopted immediately;

[0100] S8: The steel cage is manufactured and lowered to the corresponding pile position;

[0101] The specific process is as follows: in order to shorten the hoisting time of the steel cage after the pile foundation is bored, the segmented steel cage is processed and manufactured by the long-line method, and the main reinforcement at the joint of the segmented steel cage is mechanically connected by a straight threaded sleeve;

[0102] In order to ensure the connection accuracy of the steel cage, a rolling welding machine is used in conjunction with a table mold long line for trial assembly. After the trial assembly, the steel cage is numbered in sections, and the main bars connecting the upper and lower adjacent sections are uniformly marked;

[0103] In order to ensure that the steel cage does not twist or deform during transportation and hoisting, triangular support bars are set inside the reinforcing stirrups, and a 10cm L-shaped steel bar head support is welded above the support ring at the lifting point;

[0104] Before lowering the steel cage, recheck the pile foundation coordinates and calculate the length of the steel cage positioning bar on the pile top through the pile center point to ensure that the center of the steel cage coincides with the center of the pile when the steel cage is installed;

[0105] S9: installing a conduit 2 for concrete pouring based on the pile hole;

[0106] The specific process is: for the water tightness test of the pre-installed conduit 2, ensure that the conduit 2 has good water tightness;

[0107] Several conduits 2 are installed based on the pile hole, and the distance between the bottom end of the several conduits 2 and the predetermined pile bottom is 0.4 cm to ensure the compactness of the first poured concrete;

[0108] S10: constructing a gas lift reverse circulation mud channel for secondary hole cleaning;

[0109] The specific process is as follows: since it takes a long time to lower the steel cage and install the guide tube 2, new slag is generated at the bottom of the pile hole, so a secondary hole cleaning operation is immediately followed up;

[0110] During the secondary hole cleaning, air lift reverse circulation is used to clean the hole, which mainly relies on the mud head pressure to maintain the stability of the hole wall. Therefore, the mud liquid level in the pile hole is controlled to be no less than 50cm below the top of the casing.

[0111] The air lift reverse circulation hole cleaning is to use an air compressor 1 to send compressed air to the bottom of the conduit 2 through the air inlet pipe 3 installed in the conduit 2. When the compressed air is discharged from the bottom of the air duct, it forms a slurry-gas mixture with the mud 4. Due to the low density of the slurry-gas mixture, it rises along the inside of the conduit 2, forming a negative pressure at the bottom of the conduit 2. Under the action of the negative pressure, the mud at the bottom of the pile base continuously carries the sediment 7 and rises along the inside of the conduit 2 until it turns out from the conduit outlet 8 and flows into the mud pool 5. The sediment is precipitated in the sedimentation tank 6. The mud that has been precipitated and filtered through the mud pool 5 flows back into the pile hole again, and the cycle is repeated until the sediment thickness in the hole reaches the design requirement.

[0112] S11: Configure the conduit to pour concrete into piles;

[0113] The specific process is as follows: after the secondary hole cleaning is completed, underwater concrete pouring is carried out to complete the pouring of the entire pile within the initial setting time. The volume of the first batch of poured concrete meets the initial burial depth of ≥1m and fills the bottom of the conduit;

[0114] The specific calculation formula for the first batch of injection volume is as follows:

[0115]

[0116] Where V is the volume of concrete required to pour the first batch, m 3 ; D is the pile hole diameter, m; H1 is the distance between the bottom of the pile hole and the bottom end of the conduit, which is 0.4m; H2 is the initial burial depth of the conduit, m; d is the inner diameter of the conduit, m; h1 is the height required for the concrete column in the conduit to balance the pressure outside the conduit (or mud) when the concrete in the pile hole reaches the burial depth H2, m;

[0117] To ensure the first pouring volume, two tank trucks are used to pour concrete at the same time. For example, the discharge speed of a single tank truck is 4m 3 / min; According to field practice, when pouring for 1 minute, the concrete in the hopper no longer covers the bottom of the hopper, and the hopper size is selected as 5m 3 ; According to calculation, when two tank trucks are used for pouring at the same time and the concrete in the hopper no longer covers the bottom of the hopper, the total volume poured is (5+4×2)=13m 3 , meet the requirements of the first injection volume;

[0118] After the first pouring is completed, continue pouring concrete according to the requirement of 2 to 6m buried depth of the conduit to ensure uninterrupted, rapid and continuous pouring of concrete until the pouring exceeds the pile top elevation by 0.5 to 1m, at which time the pile construction is completed.

[0119] Although the present invention has been described in detail above by general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. A construction method for bored piles in collapsible loess strata, characterized in that: The steps include: Clean and level the construction site; Determine the coordinates of the pile hole through pile foundation positioning and measurement and setting out; The casing is placed by the cut and bury method; Configure the drilling rig to be in a stable position and adjust the drilling center point of the drilling rig based on the casing; Based on rotary drilling in hard soil layer; Reverse circulation drilling based on sandy soil formation; Carry out initial hole cleaning for rotary drilling and reverse circulation drilling holes that meet the inspection standards; The steel cage is manufactured and lowered to the corresponding pile position; Install the conduit for concrete pouring based on the pile hole; Construct gas lift reverse circulation mud channel for secondary hole cleaning; Configure the conduit to pour concrete into piles.

2. The construction method for bored piles in collapsible loess strata according to claim 1, characterized in that: The cleaning and leveling of the construction site specifically includes: After clearing the surface weeds and loose soil of the bored pile site, the site is leveled and compacted. At the same time, the surface collapsible loess layer and the area with developed pits are treated with strong tamping in advance to ensure that the foundation has sufficient bearing capacity and prevent the site from collapsing and the surface soil cracks of the pile hole from leaking during the drilling process. The method of determining the pile hole coordinate points by positioning the pile foundation and coordinating with the measurement and setting out specifically includes: According to the distance and angle between the pile foundation coordinate points and each control point, the on-site measurement and layout are carried out to determine the pile hole coordinate points. After that, the total station is used to check the pile hole coordinate points, and the positioning error is controlled within 5mm. Guard piles are set in the four directions corresponding to the pile hole coordinate points. The center point of the pile position is determined by drawing cross lines based on the guard piles set in all directions, and the circular path of the pile hole determined according to the pile diameter value at the center point of the pile position and its outer side is pre-marked with white lime.

3. The construction method for bored piles in collapsible loess strata according to claim 2, characterized in that: The method of placing casing by digging and burying specifically includes: The casing is made of 12mm thick steel plate, with a 20cm hole and double-layer reinforcement. The inner diameter is 30cm larger than the pile diameter, and the single section is 4.5-5m long. The casing welds ensure sealing and water isolation. Since the hole mouth of the collapsible loess stratum is prone to collapse, the casing is placed by the excavation method, that is, the soil body corresponding to the buried depth of the casing at the pile hole coordinate point is first excavated by the rotary drilling bit. The diameter of the rotary drilling bit is 0.2m smaller than the diameter of the casing. Then the casing is hoisted into place and directly pressed into the excavation position. The casing is buried by the rotary drilling rig static pressure method, that is, the clay layer and / or silt layer is used as the bearing layer at the end of the casing, the casing is hoisted by a crane, and the center point of the casing is positioned based on the cross pile, and the center point of the casing is coincident with the center point of the pile position. After confirming that the positions coincide, the horizontal ruler and vertical ruler are used to check until the casing is in place. At this time, the top of the casing is 0.3-0.5m above the ground, the position deviation of the top and bottom centers of the casing is less than 5mm, and the vertical inclination of the casing is less than 1%; The gaps around the casing are further backfilled and compacted with clay; When encountering further adverse soil conditions such as construction waste, long casing follow-up measures are adopted to extend the casing to maintain the mud head height in the casing to form a high hydrostatic pressure, thereby maintaining the stability of the hole wall.

4. The construction method for bored piles in collapsible loess strata according to claim 3, characterized in that: The configuration of the drilling rig being stably positioned and adjusting the drilling center point of the drilling rig based on the casing specifically includes: The ground corresponding to the drilling rig shall be leveled and compacted, and steel plates shall be laid. When the drilling rig is in place, the foundation under the drilling rig shall be verified and adjusted until no subsidence occurs. The drill rod of the drilling rig shall be adjusted vertically, and then the center point of the corresponding casing after the casing is buried shall be checked to see whether it coincides with the center point of the borehole formed by the drill rod of the drilling rig. If the deviation between the two is large, the position of the drilling rig shall be readjusted to ensure that the center point of the borehole meets the coincidence requirements.

5. The construction method for bored piles in collapsible loess strata according to claim 4, characterized in that: The method of forming a hole by rotary drilling based on a hard soil layer specifically includes: First, use a drilling rig to perform low-speed rotary drilling to form a hole, circulate mud in the borehole, and the mud is required to have a relative density ≥1.06, a viscosity range of 20-22Pa·s, a sand content of less than 4%, and a colloid rate ≥95%, and repeatedly sweep the hole to ensure that the drilling rig and drilling tools are unobstructed; after drilling through the pebble layer of the hard soil layer, adjust the mud relative density to ≥1.10, the viscosity range to 25-28Pa·s, the sand content of less than 2%, and the colloid rate of ≥95%, slowly rotate and lift the rotary drilling bit, and the lifting speed is ≤5mm / s, and follow-up monitoring and real-time replenishment of new mud to maintain the mud head height in the casing to avoid excessively fast rising speed to form negative pressure and cause the hole wall to collapse; After the rotary drill bit is lifted away from the casing, slowly move the rotary drill rig to outside the construction impact range, and then review the center point of the hole drilled by the rotary drill rig to control its deviation from the center point of the pile position to be less than 20mm.

6. The construction method for bored piles in collapsible loess strata according to claim 5, characterized in that: The reverse circulation drilling based on the sandy soil layer specifically includes: Continue to perform reverse circulation drilling based on the sandy soil layer, slowly move the reverse circulation drilling rig to the previous rotary drilling rig position, adjust the drill bit to the center of the pile position, check the mechanical horizontality and the verticality of the drill rod, slowly rotate and lower the reverse circulation drill bit, the lowering speed of the reverse circulation drill bit is ≤5mm / s, maintain the fluidity and uniformity of the mud in the borehole, and prevent a large amount of sediment from affecting the mud weight; During the drilling process, observe the changes in the mud head height in the borehole and the flatness of the drilling rig, and replenish the mud in time. At the same time, reduce the rotation speed, increase the mud viscosity, and prevent the sandy soil layer from shrinking, collapsing, and tilting the hole.

7. The construction method for bored piles in collapsible loess strata according to claim 6, characterized in that: The initial hole cleaning process for rotary drilling and reverse circulation drilling holes that meet the inspection indicators specifically includes: After the reverse circulation drilling is completed, the hole inspection device is moved by crane to check the hole position, hole diameter, hole depth and inclination of the pile hole, and the first hole cleaning treatment is carried out for the pile holes that meet the indicators; When inspection reveals inclined holes and / or shrinkage holes and / or plum blossom holes, backfilling method shall be adopted immediately.

8. The construction method for bored piles in collapsible loess strata according to claim 7, characterized in that: The steel cage is manufactured and positioned corresponding to the pile position, specifically including: In order to shorten the hoisting time of the steel cage after the pile foundation is bored, the segmented steel cage is processed and manufactured by the long-line method, and the main reinforcement of the segmented steel cage is mechanically connected by a straight thread sleeve at the cage joint; In order to ensure the connection accuracy of the steel cage, a rolling welding machine is used in conjunction with a table mold long line for trial assembly. After the trial assembly, the steel cage is numbered in sections, and the main bars connecting the upper and lower adjacent sections are uniformly marked; In order to ensure that the steel cage does not twist or deform during transportation and hoisting, triangular support bars are set inside the reinforcing stirrups, and a 10cm L-shaped steel bar head support is welded above the support ring at the lifting point; Before lowering the steel cage, recheck the pile foundation coordinates and reversely calculate the length of the steel cage positioning bars at the top of the pile through the center point of the pile position to ensure that the center of the steel cage coincides with the center point of the pile position when the steel cage is installed.

9. The construction method for bored piles in collapsible loess strata according to claim 8, characterized in that: The concrete pouring conduit installed based on the pile hole specifically includes: For the water tightness test of the pre-installed conduit, ensure that the conduit has good water tightness; Several guide tubes are installed based on the pile hole, and the distance between the bottom end of the several guide tubes and the predetermined pile bottom is 0.4 cm to ensure the compactness of the first poured concrete; The construction of the gas lift reverse circulation mud channel for secondary hole cleaning specifically includes: Since it takes a long time to lower the steel cage and install the guide tube, new slag is generated at the bottom of the pile hole, so a secondary hole cleaning operation is carried out immediately; During the secondary hole cleaning, air lift reverse circulation is used to clean the hole, which mainly relies on the mud head pressure to maintain the stability of the hole wall. Therefore, the mud liquid level in the pile hole is controlled to be no less than 50cm below the top of the casing. Air lift reverse circulation hole cleaning uses an air compressor to send compressed air to the bottom of the duct through an air inlet pipe installed in the duct. When the compressed air is discharged from the bottom of the air duct, it forms a slurry-gas mixture with the mud. Due to the low density of the slurry-gas mixture, it rises along the inside of the duct, forming a negative pressure at the bottom of the duct. Under the action of negative pressure, the mud at the bottom of the pile base continues to carry the sediment and rise along the inside of the duct until it is turned over from the water outlet of the duct and flows into the mud pool. The sediment is settled in the sedimentation tank, and the mud filtered through the mud pool flows back into the pile hole again. The cycle is repeated until the sediment thickness in the hole meets the design requirements.

10. The construction method for bored piles in collapsible loess strata according to claim 9, characterized in that: The method of configuring a conduit to pour concrete into piles specifically includes: After the secondary hole cleaning is completed, underwater concrete pouring is carried out to complete the pouring of the entire pile within the initial setting time. The volume of the first batch of poured concrete meets the initial burial depth of ≥1m and fills the bottom of the conduit; The specific calculation formula for the first batch of injection volume is as follows: Where V is the volume of concrete required to pour the first batch, m 3 ; D is the pile hole diameter, m; H1 is the distance between the bottom of the pile hole and the bottom end of the conduit, which is 0.4m; H2 is the initial burial depth of the conduit, m; d is the inner diameter of the conduit, m; h1 is the height required for the concrete column in the conduit to balance the pressure outside the conduit (or mud) when the concrete in the pile hole reaches the burial depth H2, m; To ensure the first pouring volume, two tank trucks are used to pour concrete at the same time. For example, the discharge speed of a single tank truck is 4m 3 / min; According to field practice, when pouring for 1 minute, the concrete in the hopper no longer covers the bottom of the hopper, and the hopper size is selected as 5m 3 ; According to calculation, when two tank trucks are used for pouring at the same time and the concrete in the hopper no longer covers the bottom of the hopper, the total volume poured is (5+4×2)=13m 3 , meet the requirements of the first injection volume; After the first pouring is completed, continue pouring concrete according to the requirement of 2 to 6m buried depth of the conduit to ensure uninterrupted, rapid and continuous pouring of concrete until the pouring exceeds the pile top elevation by 0.5 to 1m, at which time the pile construction is completed.

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