A collapsible loess foundation soil compaction hammer hole expanding device and construction method

Abstract

The present application relates to the field of hammering reaming, and discloses a collapsible loess foundation soil compaction hammering reaming device and a construction method, the collapsible loess foundation soil compaction hammering reaming construction method comprises the following steps: S1: embedding a gas guide pipe in a hammer rod, and connecting one end of the gas guide pipe with a main gas cavity in a hammer head body, and connecting the other end of the gas guide pipe with a high-pressure gas source arranged on the ground through a rotary joint to form a gas conveying system; S2: in the process of impact compaction of the hammer head body. The present application is provided with a bottom center jet, and the negative pressure cavity is filled instantly when the hammer head is lifted, the rebound negative pressure adsorption force of the loess is offset, the hammer sticking is reduced, the lifting load is reduced, the disturbance of the soil body at the hole bottom is reduced, the soil body at the pile end is ensured to be compact and complete, a lateral air curtain is formed on the side of the hammer head, the lifting resistance is reduced through fluid lubrication, and the hole wall collapse is inhibited; the compaction pressure wave is generated through pulse modulation, the hole wall is secondarily compacted in a non-contact manner, and the shear strength of the soil body and the bearing capacity of the pile body are improved.

Description

Technical Field

[0001] This invention relates to the field of percussion reaming technology, and more specifically, to a percussion reaming device and construction method for compacting collapsible loess foundation soil. Background Technology

[0002] Collapsible loess is a special type of soil widely distributed in Northwest and North my country, and its foundation treatment is a key technical aspect of engineering construction. Due to its large pore structure and weak cementation, collapsible loess is prone to significant additional settlement deformation when exposed to water or subjected to large loads, thus endangering the safety of superstructures. To eliminate the collapsibility of loess foundations and improve their bearing capacity, compaction piles are commonly used in engineering for deep foundation reinforcement. This involves using specialized equipment to hammer holes in the foundation and compact the surrounding soil. Hammer-driven hole enlargement technology is widely used due to its simple equipment and direct compaction effect. In the specific implementation of hammer-driven hole enlargement, the enlargement device uses a free-fall hammer or mechanical drive to send a shaped hammer head to the bottom of the hole, using impact force to strongly expand the soil at the bottom and sidewalls of the pile hole. The hammer head is then lifted back to the ground, and this cycle is repeated to gradually form a pile hole with a specified diameter and depth. During this process, the enlargement hammer head maintains a close dynamic contact with the surrounding soil.

[0003] However, in existing hammer-driven borehole construction on collapsible loess foundations, after the hammer applies high-energy impact and compression to the soil, the high-pressure gas in the local area at the bottom of the hole forms an instantaneous negative pressure cavity the moment the hammer leaves. Coupled with the significant elastic recovery characteristics of loess itself, a strong negative pressure adsorption effect is generated in the initial stage of hammer lifting. This directly causes the hammer to be adhered to the soil at the bottom of the hole and unable to be smoothly removed. This phenomenon forces the lifting equipment to output a lifting force far greater than the weight of the hammer itself, which not only drastically increases construction energy consumption and equipment wear, but also causes irreversible loosening and heaving of the soil structure at the bottom of the hole due to strong pulling, seriously damaging the density and integrity of the bearing layer at the end of the pile hole and reducing the effect of compaction and reinforcement. Summary of the Invention

[0004] This invention provides a compaction hammer hole-expanding device and construction method for collapsible loess foundation soil, which solves the technical problem in related technologies where the hammer head adheres to the hole due to negative pressure adsorption at the bottom of the hole during lifting, requiring a lifting force far exceeding its own weight, which not only increases energy consumption and equipment wear, but also causes the soil at the bottom of the hole to loosen and bulge, destroying the compaction of the bearing layer and reducing the compaction effect.

[0005] This invention provides a method for compaction and borehole enlargement construction of collapsible loess foundation soil, comprising the following steps: S1: The air delivery pipe is built into the hammer rod, and one end of the air delivery pipe is connected to the main air chamber inside the hammer head body, while the other end is connected to a high-pressure air source set on the ground through a rotary joint to form an air delivery system. S2: During the impact compaction process of the hammerhead body, the ground air pressure control system controls the high-pressure air source to output high-pressure airflow, and the high-pressure airflow is injected into the main air chamber through the rotary joint and the air guide pipe. S3: Under pressure, the high-pressure airflow in the main air chamber pushes the one-way elastic sealing valve plate placed in the central air outlet at the bottom of the hammer body and the micro-hole jet array on the side wall, forming a central jet at the bottom and a side air curtain. S4: During the lifting process of the hammer body, the continuously sprayed bottom center jet directly acts on the negative pressure cavity between the bottom of the hammer body and the soil at the bottom of the hole to counteract the lifting negative pressure; at the same time, the continuously sprayed lateral air curtain forms an isolation lubrication layer between the side wall of the hammer body and the hole wall. S5: Repeat steps S2 to S4 until the pile hole of the specified depth and diameter is formed.

[0006] As a further optimization of the present invention, the output pressure of the high-pressure air source is 0.8 MPa to 1.5 MPa, the opening pressure of the one-way elastic sealing valve is 0.15 MPa to 0.35 MPa, and the opening pressure is less than the output pressure but greater than the soil pressure in the hole that the hammer body bears at the working depth.

[0007] As a further optimization of the present invention, in S2 to S4, the ground air pressure control system modulates the output high-pressure airflow by means of a pulse modulation valve set between the high-pressure air source and the rotary joint, so as to generate a pulsed high-pressure airflow with adjustable frequency and duty cycle. When the pulsed high-pressure airflow acts on the orifice wall through the lateral air curtain, it forms a periodic compaction pressure wave.

[0008] As a further optimization of the present invention, the modulation frequency of the pulse modulation valve is 2.5 Hz to 12 Hz, and the duty cycle is 25% to 65%.

[0009] As a further optimization of the present invention, in step S4, the bottom center jet is continuously sprayed during the lifting process of the hammer body, and its air replenishment rate is greater than the volume expansion rate caused by the elastic recovery of the soil, so that the bottom of the hammer body and the soil at the bottom of the hole maintain a slightly positive pressure state throughout the lifting process.

[0010] As a further optimization of the present invention, the central axis of each micro-hole in the micro-hole jet array forms an inclination angle of 10 to 25 degrees with the radial plane of the hammer body, so that the ejected lateral air curtain has a velocity component pointing obliquely downward.

[0011] As a further optimization of the present invention, the bottom of the hammer body is a centrally symmetrical parabolic structure, and the central air vent is located at the geometric center of the bottom structure.

[0012] As a further optimization of the present invention, the rotary joint is a high-pressure rotary fluid connector, which can maintain the sealed connection of the high-pressure air circuit while bearing the torsional torque and axial impact load generated by the hammer body and hammer rod during hammering and lifting.

[0013] As a further optimization of the present invention, the micro-hole jet array is composed of multiple micro-holes arranged in a ring at equal intervals, with 3 to 5 horizontal circumferential surfaces distributed along the axial direction of the hammer head body. Each circumferential surface is evenly distributed with 6 to 8 of the micro-holes, and the micro-holes on adjacent circumferential surfaces are staggered by a phase angle of 15 to 30 degrees in the circumferential direction.

[0014] A compaction and reaming device for collapsible loess foundation soil includes a reaming actuator placed inside the borehole and a ground air source control mechanism set on the ground surface. The reaming actuator includes a hammer head body, a hammer rod, and an air guide tube. The hammer head body has a main air chamber inside, and a central air outlet connected to the main air chamber is opened at the bottom of the hammer head body. The side wall of the hammer head body has a micro-hole jet array connected to the main air chamber. The hammer rod is a hollow rod, and the air guide tube is built into the hammer rod. One end of the air guide tube is connected to the main air chamber, and the other end of the air guide tube is connected to the high-pressure air source of the ground air source control mechanism through a rotary joint. One-way elastic sealing valves are installed in the central air outlet and the micro-hole jet array, respectively. The opening pressure of the one-way elastic sealing valves is configured to be less than the output pressure of the high-pressure air source and greater than the soil pressure in the hole that the hammer body bears at the working depth.

[0015] The beneficial effects of this invention are as follows: 1. This invention constructs a bottom center jet that sprays directly to the bottom of the hole, which can actively and instantly fill the negative pressure cavity formed when the hammer is lifted. This fundamentally counteracts the negative pressure adsorption force caused by the elastic rebound of loess, reduces the problem of hammer sticking, reduces the instantaneous lifting load of the lifting equipment, reduces the disturbance of the soil at the bottom of the hole by strong pulling, and improves the original density and integrity of the bearing layer at the end of the pile hole.

[0016] 2. This invention constructs a lateral air curtain covering the side wall of the hammer head, forming a uniform isolation and lubrication layer between the hammer head and the hole wall. This transforms the traditional non-uniform solid friction into uniform fluid friction, reducing lifting resistance and fundamentally solving the problems of hole wall collapse and shrinkage caused by water content fluctuations, thereby improving the integrity and regularity of the hole formation.

[0017] 3. This invention uses pulse modulation technology to transform the lateral air curtain into a periodic compaction pressure wave. This pressure wave performs non-contact secondary compaction of the soil around the borehole during the hammer lifting process, improving the shear strength of the soil around the borehole and further enhancing the overall bearing capacity of the compaction pile.

[0018] 4. The present invention adopts a non-return structure with one-way opening characteristics. Its opening pressure is precisely limited between the output pressure of the high-pressure air source and the soil pressure in the hole. When air jet is needed, the air path is unobstructed. At the moment when the hammer impacts and squeezes the soil, the external mud and soil particles will not enter the air path system in reverse, which improves the long-term reliability and maintenance-free nature of the device. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a compaction and hole-expanding device for collapsible loess foundation soil proposed in this invention; Figure 2 This is a partial side cross-sectional view of a compaction hammer hole-expanding device for collapsible loess foundation soil proposed in this invention. Figure 3 This is a schematic diagram of the working principle framework of the bottom center jet and the lateral air curtain in this invention; Figure 4 This is a schematic diagram of the principle framework for forming a compaction pressure wave using a pulse-modulated lateral air curtain in this invention. Figure 5 This is a logical flowchart of the compaction and borehole enlargement construction method for collapsible loess foundation soil in this invention. Figure 6 This is a schematic diagram of the gas path interaction between the orifice expansion actuator and the ground gas source control mechanism in this invention.

[0020] In the picture: 1. Hammer head body; 101. Central air vent; 102. Micropores; 2. Air delivery tube; 3. Hammer rod; 4. One-way elastic sealing valve plate; 5. Rotary joint. Detailed Implementation

[0021] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, features described in some examples may be combined in other examples.

[0022] like Figures 1 to 6As shown, this embodiment is specifically applied to the foundation treatment project of a factory in the loess region of Northwest China. The site is a typical self-weight collapsible loess site with a collapsibility level of III to IV, a natural moisture content between 8% and 18%, a void ratio greater than 1.0, and a loose soil structure with high elastic recovery characteristics. Under these geological conditions, when using the traditional hammer-driven compaction and hole-expanding process to construct compaction piles with a depth of 12 meters and a pile hole diameter of 600 mm, the hammer sticking phenomenon caused by negative pressure adsorption during the lifting process of the hammer head, as well as hole wall collapse and hole shrinkage accidents caused by excessive hole wall friction and soil collapsibility sensitivity, the hole completion qualification rate has been lower than 70% for a long time. This embodiment discloses a hammer-driven hole-expanding device for compaction of collapsible loess foundation soil, which consists of two major subsystems: a hole-expanding execution mechanism placed in the borehole and a ground air source control mechanism set on the ground surface. The two are integrated and operated in a coordinated manner through an air pipeline system that integrates high-pressure pneumatic transmission, rotary sealing and control functions.

[0023] The core load-bearing and working element of the borehole reaming actuator is a hammer head body 1. This hammer head body 1 is integrally cast from high-strength, low-alloy, wear-resistant cast steel and then precision-machined using a CNC machine tool. Its yield strength is no less than 850 MPa, and its surface hardness reaches Rockwell hardness of 50 to 55, enabling it to withstand the impact kinetic energy of up to several hundred kilojoules during hammering and the intense abrasive wear of quartz particles in the loess. The hammer head body 1 is designed as an axisymmetric rotating body, with a centrally symmetrical parabolic structure at its bottom. The radius of curvature of the parabola's generatrix gradually increases from the center to the edge, resulting in a more uniform diffusion of impact energy in the soil at the bottom of the borehole and facilitating the forceful expansion of the soil to the sides and downwards. The maximum diameter of the hammer head body 1 is located at... A cylindrical section on the side wall, located approximately one-third of the total height from the bottom, has a length-to-maximum-diameter ratio of 0.8 to 1.2 to provide a compaction guide surface. Inside the hammer body 1, a main air chamber is cast. The volume of the main air chamber is determined proportionally to the maximum diameter of the hammer body 1, approximately 0.04% to 0.06% of the cube of the maximum diameter. Specifically, for a hammer body 1 with a maximum diameter of 600 mm, the main air chamber volume is 85 cubic centimeters to 130 cubic centimeters. The main air chamber functions as a high-pressure gas stabilization and distribution container, locally buffering and equalizing the pulsating or stable high-pressure airflow input from the air guide pipe 2, and then distributing it to the bottom center air outlet 101 and the jet array of side wall micropores 102.

[0024] At the geometric center of the parabolic structure at the bottom of the hammerhead body 1, i.e., at the lowest point, a central air outlet 101 is provided. The diameter of the central air outlet 101 is 8 mm to 12 mm, and its inner wall surface is precision-reamed to accommodate the installation of a one-way elastic sealing valve plate 4. The axis of the central air outlet 101 is completely coincident with the central axis of the hammerhead body 1 to ensure that the ejected central jet can accurately cover the core area of ​​the negative pressure cavity formed during the lifting process. On the cylindrical section and transition arc section of the side circumferential wall of the hammerhead body 1, a micro-hole jet array composed of multiple micro-holes 102 is provided in a ring distribution. Each circumferential surface is evenly distributed with 6 to 8 micro-holes 102, and the micro-holes 102 on adjacent circumferential surfaces are staggered by a phase angle of 15 degrees to 30 degrees in the circumferential direction, thereby forming a jet coverage network on the outer circumference of the hammerhead body 1. The diameter of the micro-holes 102 is 2 mm to 4 mm. The total number of micro-holes 102 is related to the volume of the main air chamber and the surface area of ​​the hammer body 1, and is usually 48 to 72. The central axis of each micro-hole 102 is not along the pure radial direction of the hammer body 1, but forms a preset tilt angle with the radial plane of the hammer body 1. This tilt angle is between 10 degrees and 25 degrees, so that the axis of the micro-hole 102 points obliquely downward, that is, towards the bottom of the hole. When the lateral air curtain generated flows close to the side wall of the hammer body 1, its resultant velocity vector has an axial component pointing towards the bottom of the hole. This axial component guides the air curtain to migrate downward along the hole wall to the bottom area of ​​the hole, participates in balancing the overall pressure field at the bottom of the hammer, and avoids the formation of gas accumulation in the annular area above the hammer due to continuous upward spraying, thereby weakening the residence of the air curtain in the gap between the hammer and the hole wall.

[0025] Each of the central air outlet 101 and each micropore 102 is fitted with a one-way elastic sealing valve 4, forming a complete non-return function array. This one-way elastic sealing valve 4 (existing technology, not described in detail here) has an opening pressure greater than the sum of the soil lateral pressure and pore water pressure borne by the hammer body 1 at its maximum working depth of 12 to 15 meters. Specifically, for the working condition of the high-pressure air compressor output pressure range of 0.8 MPa to 1.2 MPa in this embodiment, the opening pressure of the one-way elastic sealing valve 4 is set to... The pressure ranges from 0.15 MPa to 0.25 MPa, while the lateral pressure of loess at a depth of 12 meters is approximately 0.08 MPa to 0.12 MPa. This creates a safe pressure window of over 0.12 MPa, allowing the one-way elastic sealing valve 4 to open when injection is required. At the moment when the hammer body 1 impacts and squeezes the soil, when external mud and soil particles attempt to backflow into the air passage due to inertial force and pressure gradient, the one-way elastic sealing valve 4 is tightly closed on the valve seat under the combined action of the soil pressure inside the hole and its own elastic restoring force, achieving a zero-leakage seal below 50 micrometers.

[0026] The force transmission and air supply composite component of the reaming actuator is the hammer rod 3. The hammer rod 3 is made of high-strength seamless alloy steel pipe with an outer diameter of 140 mm to 168 mm and a wall thickness of 16 mm to 22 mm. Its tensile strength is not less than 950 MPa. Inside the axially penetrating central channel inside the hammer rod 3, a guide pipe 2 is coaxially installed. The guide pipe 2 is made of seamless stainless steel pipe with an outer diameter of 25 mm to 32 mm and a wall thickness of 3 mm to 4 mm. The annular gap between the guide pipe 2 and the inner wall of the hammer rod 3 is filled with polyurethane foam. The heat insulation and damping material serves two purposes: firstly, it suppresses the fatigue damage to the joint of the air duct 2 caused by the high-frequency vibration of the hammer rod 3 during hammering; secondly, it isolates the influence of geothermal heat deep in the foundation on the temperature of the high-pressure airflow. The bottom end of the air duct 2 is connected to the air inlet at the top of the main air chamber of the hammer body 1 through a tapered pipe thread joint to achieve a metal-to-metal hard seal connection, and threaded anaerobic sealant is applied as a secondary seal protection. The top end of the air duct 2 extends upward to 150 mm above the top surface of the hammer rod 3 and is connected to the high-pressure air compressor on the ground through a rotary joint 5.

[0027] The rotary joint 5 serves as an air interface connecting the rotating and reciprocating motion components to the fixed pipeline.

[0028] The ground air source control mechanism is deployed on a hardened surface 5 to 10 meters away from the orifice. It consists of a high-pressure air compressor, a high-pressure air tank, a ground air pressure control system, and a central control unit. The high-pressure air compressor is a four-stage, air-cooled reciprocating piston compressor with a rated discharge pressure of 1.2 MPa and a rated discharge volume of 3.0 cubic meters per minute. The compressor outlet is connected to a high-pressure air tank with a volume of 2 cubic meters. The air tank acts as a buffer container, which smooths the airflow pulsation at the compressor outlet and provides air volume supplement for the instantaneous high-flow injection demand. The outlet pipeline of the air tank is connected in series with a precision filter and a refrigerated air dryer to ensure that the solid particulate matter content of the output high-pressure air is less than 5 mg / m³ and the pressure dew point is less than -20 degrees Celsius, so as to prevent dust and moisture from scaling and clogging in the main air chamber and micropores 102 of the hammerhead body 1.

[0029] The core actuator in the ground air pressure control system is a pulse modulation valve, which is connected in series between the outlet of the air storage tank and the air supply pipeline leading to the rotary joint 5. The pulse modulation valve is a pilot-operated double-diaphragm high-speed pneumatic control valve with a valve core response time of less than 15 milliseconds, capable of withstanding more than 1200 continuous switching actions per minute. The electrical interface of the electromagnetic pilot valve of the pulse modulation valve is connected to the pulse width modulation output channel of the central control unit. The central control unit is an embedded system based on a programmable logic controller or an industrial microcontroller, which stores the pulse modulation parameter setting program. The operator can set the modulation frequency and duty cycle of the pulse modulation valve on the touch screen interface. In this embodiment, for the shallow loess section with low water content and relatively high structural strength, the modulation frequency is set to 5 Hz and the duty cycle is 60% to generate periodic compaction pressure waves; for the deep soft plastic loess section with high water content, the modulation frequency is reduced to 2.5 Hz and the duty cycle is maintained at 50% to reduce the erosion of the borehole wall by the airflow and maintain a stable isolation lubrication layer.

[0030] A method for compaction and borehole enlargement construction of collapsible loess foundation soil includes the following steps: Step S1: Assemble the entire device and connect the air circuit. The construction team hoists the aforementioned hole-reaming actuator, including the hammer body 1 and hammer rod 3, to the pile hole location. At this time, the hammer body 1 is placed at the pre-drilled guide hole opening. The guide hole has a diameter of 500 mm and a depth of 1.5 m. Technicians securely connect the central shaft end of the rotary joint to the top of the air guide pipe 2 using a compression fitting, and tighten it to the specified torque of 120 NM with a torque wrench. Subsequently, one end of the high-pressure rubber hose in the ground air source control mechanism is connected to the air inlet of the rotary joint 5 housing, and the other end is connected to the pulse modulation valve. Turn on the high-pressure air compressor at the output pipeline interface. After the air tank pressure reaches the rated pressure of 1.0 MPa, open the air tank outlet shut-off valve to deliver high-pressure air through the refrigerated dryer and precision filter to the inlet of the pulse modulation valve. At this time, the central control unit is in standby mode, the pulse modulation valve is in normally closed mode, and the entire air supply system is in pressure maintenance and leakage detection mode. The operator observes the value fed back by the pipeline pressure sensor through the touch screen of the central control unit. If the system pressure drop is less than 0.02 MPa within 5 minutes of pressure maintenance test, the air circuit sealing is deemed qualified, and the system enters the work-ready state.

[0031] In step S2, during the impact compaction process of the hammer body 1, the active intervention process of the ground air pressure control system is initiated. The lifting equipment releases the brake on the hammer rod 3, and the hammer body 1 accelerates down along the guide hole under the action of gravity and the self-weight of the hammer rod 3. The parabolic bottom of the hammer body 1 first impacts the soil at the bottom of the hole. The huge impact force causes the loess to compress and laterally extrude and deform. The duration of this impact moment is about 10 to 30 milliseconds. During the moment of impact and the very short time after the hammer body 1 is embedded in the soil, the central control unit does not immediately issue an air jet command. Instead, it monitors the attenuation process of the impact vibration signal through the acceleration sensor installed at the top of the hammer rod 3. When the acceleration amplitude attenuates to less than 20% of the peak value, it is determined that the hammer body 1 has made contact with the soil. Once the impact coupling is complete, the central control unit outputs an opening command to the electromagnetic pilot valve of the pulse modulation valve. If the pulse modulation mode is not used, the central control unit directly sends an opening signal to a normally closed electromagnetic switch valve. The valve core of the pulse modulation valve opens rapidly under the drive of electromagnetic force. The high-pressure air in the gas tank passes through the pulse modulation valve along the gas supply pipeline, through the inner hole of the central shaft of the rotary joint 4, and into the internal channel of the air guide pipe 2 to inject into the main air chamber inside the hammer body 1. This process transports the high-pressure airflow from the ground to the hammer body 1 at a depth of 12 meters underground. The total length of the gas transport path is about 15 meters, and the total pressure drop along the path is controlled within 0.05 MPa. The pressure inside the main air chamber rises to more than 0.9 MPa within 0.2 seconds.

[0032] In step S3, the high-pressure airflow in the main air chamber acts simultaneously on the inner surfaces of all one-way elastic sealing valve plates 4 under pressure. At this time, although the hammer body 1 is embedded in the soil, the soil at its bottom and sidewalls is in a state of high stress compression after impact. The soil pressure on the outside of the central air outlet 101 and the micropore array 102 has not fully recovered to the static lateral pressure value, but it is significantly higher than atmospheric pressure. However, since the opening pressure of the one-way elastic sealing valve plate 4 is set to 0.2 MPa, and the high-pressure airflow in the main air chamber... The flow pressure reaches over 0.9 MPa, and the pressure difference of up to 0.7 MPa between the two forces overcomes the elastic pre-tightening force of the one-way elastic sealing valve plate 4 itself, pushing the one-way elastic sealing valve plate 4 to open the airflow channel. At the central air outlet 101, the high-pressure airflow passes through the 8 mm diameter central air outlet 101 and is ejected at high speed in the form of a free jet, forming a bottom central jet with a core velocity of 150 m / s to 250 m / s. After being ejected, this jet quickly entrains the residual pores in the surrounding soil. The gas forms a jet stream pointing towards the bottom of the hole. At the same time, on the side of the hammer body 1, the one-way elastic sealing valves 4 in all the micro-holes 102 open synchronously. The high-pressure airflow is ejected radially from 48 to 72 micro-holes 102. Each micro-jet is a free jet structure in the initial stage of ejection. The length of the jet core is 8 to 12 times the diameter of the micro-hole 102. Then, the micro-jets intersect and merge at their boundaries to form a continuous lateral air curtain covering the entire circumference of the side wall of the hammer body 1. Since the central axis of the micro-hole 102 has an inclination angle of 10 to 25 degrees pointing downwards, the overall flow direction of the lateral air curtain is not vertically upwards, but has a clear longitudinal velocity component migrating towards the bottom of the hole. The bottom central jet and the lateral air curtain constitute a two-component pneumatic system working together. The main functional objective of the bottom central jet is to act directly on the central area of ​​the bottom of the hammer, which is the initial negative pressure core formed when the hammer is about to be lifted. The lateral air curtain establishes a complete fluid isolation layer between the side wall of the hammer and the hole wall.

[0033] Step S4: During the lifting process of the hammer body 1, how do the aforementioned dual-component jet and air curtain system work together to eliminate lifting resistance and protect the integrity of the borehole wall? When the lifting equipment begins to apply a vertically upward lifting force to the hammer rod 3, the bottom end face of the hammer body 1 begins to tend to separate from the soil at the bottom of the borehole. In the initial 10-micrometer to 50-micrometer micro-displacement stage of the separation between the bottom end face of the hammer body 1 and the soil, if there is no external gas supply, the rapid expansion of the volume in this micro-gap will cause the gas pressure to drop instantaneously, forming a negative pressure cavity lower than the pore pressure of the surrounding loess. However, in this device, the continuously sprayed bottom center jet directly acts on the core area of ​​this negative pressure cavity that is being generated, and the high-speed jet carries... A large amount of high-pressure air actively and instantly fills the expansion volume, dynamically compensating for and reconstructing the low-pressure space. The air replenishment rate is greater than the volume expansion rate caused by the elastic recovery of the soil, thus maintaining the air pressure in the central area of ​​the bottom end face of the hammer head at a positive pressure state of 0.05 MPa to 0.15 MPa. This fundamentally counteracts the physical conditions that generate negative pressure adsorption force. Traditional negative pressure adsorption force can usually reach 3 to 5 times the weight of the hammer head itself. That is, for a hammer head weighing 3 tons, a lifting force of up to 9 to 15 tons is required to detach it. However, with this device, the negative pressure component is eliminated, and the lifting equipment only needs to overcome the weight of the hammer head and hammer rod 3 as well as the frictional resistance of the side wall to lift the hammer head.

[0034] While the negative pressure is eliminated, the lateral air curtain performs a dual function of isolation lubrication and secondary compaction. The continuously ejected lateral air curtain, after exiting the micro-hole 102 jet array, forms a high-pressure gas film with a thickness of 0.5 mm to 3 mm within the narrow annular gap between the side wall of the hammerhead body 1 and the hole wall. This gas film completely transforms the dry friction or boundary friction state between the steel surface of the hammerhead and the hole wall of the collapsible loess in the traditional process into a hydrodynamic lubrication state. The direct solid contact between the collapsible loess particles and the hammerhead surface is severed, and the coefficient of friction is reduced. The pressure drop from 0.4 to 0.7 in the traditional direct contact state to below 0.01 in the air film lubrication state significantly reduces the impact of soil cohesion and internal friction angle changes on lifting resistance, even if the borehole wall softens or expands due to local changes in moisture content. This is because the metal and soil are not in direct contact, thus preventing soil tearing and collapse caused by local softening. More importantly, when the central control unit sets the pulse modulation valve to operate, the lateral air curtain is no longer a steady-state air film with constant pressure, but is transformed into an air film with a frequency of 2.5 Hz to 5 Hz and a duty cycle of 50. The pulsed compaction pressure wave, ranging from 60% to 80%, causes the lateral air curtain pressure to rapidly rise to over 0.9 MPa during one pulse cycle when the pulse modulation valve is in the open phase. This high-pressure gas acts directly on the surface of the borehole wall soil in the form of normal stress, applying instantaneous non-contact lateral compaction to the top 20 to 50 mm of soil. This compaction action compresses the large pores in the soil, causing particles to rearrange and become more compact. When the pulse modulation valve enters the closed phase, the air curtain pressure rapidly drops to near atmospheric pressure, and the borehole wall soil... A slight elastic rebound occurs, and the originally closed capillary channels reopen, preparing conditions for the compaction of the next pulse cycle. By periodically applying and releasing pressure, as the hammer head rises 20 to 50 millimeters along the hole depth, any point on the hole wall will undergo 3 to 8 complete compaction pressure wave cycles, forming a dynamic and gradually increasing secondary compaction effect. This increases the density and shear strength of the soil within a range of 1 to 1.5 times the hole diameter around the hole wall, and its static penetration resistance is 25% to 40% higher than that of traditional hole enlargement technology.

[0035] S5: Repeat steps S2 to S4 until the pile hole of the specified depth and diameter is formed. Each complete hammer compaction to air jet lifting cycle constitutes a compaction and hole expansion work cycle. A single work cycle begins with the impact stroke of the hammer body 1 driven by a free fall hammer or hydraulic hammer. During this stroke, the hammer body 1 impacts and compacts the bottom and sidewall soil along the guide hole, increasing the pile hole depth by 10 mm to 40 mm. The pile hole diameter expands by 3 mm to 8 mm under the lateral expansion action of the hammer body 1. After the impact is completed, the system automatically switches to the air jet lifting stroke under the trigger of the acceleration signal. The lifting equipment lifts the hammer body at a uniform speed of 0.2 m / s to 0.5 m / s. 1. The jet system works continuously. The bottom center jet and the side air curtain work together to eliminate lifting resistance and complete secondary compaction. When the hammer body 1 is completely lifted out of the hole, the jet system automatically shuts off. One working cycle takes about 15 to 30 seconds. The construction control system monitors the current hole depth and the cumulative number of hammer cycles in real time through the lifting encoder installed on the lifting equipment and the depth counting wheel on the hammer rod 3. When the hole depth reaches the designed depth of 12 meters and the hole diameter expansion increment in the last 3 cycles is less than 2 mm, the control system judges that the pile hole has reached the specified depth and diameter, and the hole compaction and expansion operation is completed. Then, the equipment is moved to the next pile position to start a new round of hole expansion operation.

[0036] Through the system architecture and workflow detailed in this embodiment, the following effects were achieved for collapsible loess geological conditions: the bottom center jet technology reduced the additional lifting force of 9 to 15 tons caused by negative pressure in the traditional process to zero, which reduced the total power requirement of the lifting equipment by about 45% and shortened the lifting operation cycle time by nearly 35%; the air film lubrication layer formed by the lateral air curtain reduced the frictional resistance of the hammer sidewall to 1 / 5 to 1 / 3 of that in the traditional process, reducing the collapse of the borehole wall caused by changes in moisture content; the pulse compaction pressure wave technology increased the unconfined compressive strength of the borehole wall soil by 30% to 50% compared with the untreated borehole wall; the check valve system composed of unidirectional elastic sealing valve 4 did not experience any air blockage or valve failure during 30 days of continuous construction.

[0037] The embodiments of the present invention have been described above, but the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the embodiments described above, all of which are within the protection scope of the embodiments described above.

Claims

1. A method for compaction and borehole enlargement construction of collapsible loess foundation soil, characterized in that, Includes the following steps: S1: The air guide pipe (2) is built inside the hammer rod (3), and one end of the air guide pipe (2) is connected to the main air chamber inside the hammer body (1), and the other end is connected to the high-pressure air source set on the ground through a rotary joint to form an air transmission system. S2: During the impact compaction process of the hammer body (1), the ground air pressure control system controls the high-pressure air source to output high-pressure airflow, and the high-pressure airflow is injected into the main air chamber through the rotary joint and the air guide pipe (2). S3: Under pressure, the high-pressure airflow in the main air chamber pushes the one-way elastic sealing valve (4) placed in the jet array of the bottom center air outlet (101) and the side wall micropores (102) of the hammer body (1), forming a bottom center jet and a side air curtain. S4: During the lifting process of the hammer body (1), the continuously sprayed bottom center jet directly acts on the negative pressure cavity between the bottom of the hammer body (1) and the soil at the bottom of the hole to counteract the lifting negative pressure; at the same time, the continuously sprayed lateral air curtain forms an isolation lubrication layer between the side wall of the hammer body (1) and the hole wall. S5: Repeat steps S2 to S4 until the pile hole of the specified depth and diameter is formed.

2. The method for compaction and borehole enlargement construction of collapsible loess foundation soil according to claim 1, characterized in that, The output pressure of the high-pressure air source is 0.8 MPa to 1.5 MPa, and the opening pressure of the one-way elastic sealing valve plate (4) is 0.15 MPa to 0.35 MPa. The opening pressure is less than the output pressure and greater than the soil pressure in the hole that the hammer body (1) bears at the working depth.

3. The method for compaction and borehole enlargement construction of collapsible loess foundation soil according to claim 1, characterized in that: In S2 to S4, the ground air pressure control system modulates the output high-pressure airflow through a pulse modulation valve located between the high-pressure air source and the rotary joint, thereby generating a pulsed high-pressure airflow with adjustable frequency and duty cycle. When the pulsed high-pressure airflow acts on the orifice wall through the lateral air curtain, it forms a periodic compaction pressure wave.

4. The method for compaction and borehole enlargement construction of collapsible loess foundation soil according to claim 3, characterized in that: The modulation frequency of the pulse modulation valve is 2.5 Hz to 12 Hz, and the duty cycle is 25% to 65%.

5. The method for compaction and borehole enlargement construction of collapsible loess foundation soil according to claim 1, characterized in that: In step S4, the bottom center jet is continuously sprayed during the lifting process of the hammer body (1). Its air replenishment rate is greater than the volume expansion rate caused by the elastic recovery of the soil, so that the bottom of the hammer body (1) and the soil at the bottom of the hole maintain a slightly positive pressure state throughout the lifting process.

6. The method for compaction and borehole enlargement construction of collapsible loess foundation soil according to claim 1, characterized in that: The central axis of each micro-hole (102) in the micro-hole (102) jet array forms an inclination angle of 10 to 25 degrees with the radial plane of the hammer body (1) so that the ejected lateral air curtain has a velocity component pointing obliquely downward.

7. The method for compaction and borehole enlargement construction of collapsible loess foundation soil according to claim 1, characterized in that: The bottom of the hammer body (1) is a centrally symmetrical parabolic structure, and the central air outlet (101) is located at the geometric center of the bottom structure.

8. The compaction and borehole enlargement device and construction method for collapsible loess foundation soil according to claim 1, characterized in that: The rotary joint is a high-pressure rotary fluid connector, which can maintain the sealed connection of the high-pressure air circuit while bearing the torsional torque and axial impact load generated by the hammer body (1) and hammer rod (3) during hammering and lifting.

9. The compaction and borehole enlargement device and construction method for collapsible loess foundation soil according to claim 1, characterized in that: The micro-hole (102) jet array is composed of multiple micro-holes (102) arranged in a ring at equal intervals, and consists of 3 to 5 horizontal circumferential surfaces distributed along the axial direction of the hammer body (1). Each circumferential surface is evenly distributed with 6 to 8 micro-holes (102), and the micro-holes (102) on adjacent circumferential surfaces are staggered by a phase angle of 15 to 30 degrees in the circumferential direction.

10. A compaction and hole-expanding device for collapsible loess foundation soil, characterized in that, It includes a borehole enlargement actuator placed inside the borehole and a surface gas source control mechanism set on the ground surface; The hole-expanding actuator includes a hammer body (1), a hammer rod (3), and an air guide pipe (2). The hammer body (1) has a main air chamber inside. The bottom of the hammer body (1) is provided with a central air outlet (101) that communicates with the main air chamber. The side wall of the hammer body (1) is provided with a micro-hole (102) jet array that communicates with the main air chamber. The hammer rod (3) is a hollow rod. The air guide pipe (2) is built inside the hammer rod (3). One end of the air guide pipe (2) is connected to the main air chamber. The other end of the air guide pipe (2) is connected to the high-pressure air source of the ground air source control mechanism through a rotary joint. One-way elastic sealing valve (4) is provided in the central air outlet (101) and micro-hole (102) air jet array respectively. The opening pressure of the one-way elastic sealing valve (4) is configured to be less than the output pressure of the high pressure air source and greater than the soil pressure in the hole borne by the hammer body (1) at the working depth.

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