CN111206627B - Centrifugal model test device and method for influencing existing pile foundation by tunnel-foundation pit multiple excavation - Google Patents
Centrifugal model test device and method for influencing existing pile foundation by tunnel-foundation pit multiple excavation Download PDFInfo
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- CN111206627B CN111206627B CN202010134702.4A CN202010134702A CN111206627B CN 111206627 B CN111206627 B CN 111206627B CN 202010134702 A CN202010134702 A CN 202010134702A CN 111206627 B CN111206627 B CN 111206627B
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- JIAARYAFYJHUJI-UHFFFAOYSA-L zinc dichloride Chemical compound [Cl-].[Cl-].[Zn+2] JIAARYAFYJHUJI-UHFFFAOYSA-L 0.000 description 2
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D33/00—Testing foundations or foundation structures
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Abstract
The invention relates to a centrifugal model test device for influencing the existing pile foundation by multiple excavation of a tunnel-foundation pit, which comprises a model box, a model pile, a model tunnel, a model foundation pit, model soil, heavy liquid, a pile head restraint device, a transverse bracket, a loading system, a control system and a measuring system, wherein the model pile is arranged on the model box; the model box is divided into a main body test box and an auxiliary test box through a partition plate and a transverse support, the model pile comprises a pile body and a pile cap arranged at the top of the pile body, the model pile is positioned in a main influence area of the model tunnel and a model foundation pit, the model tunnel comprises a lining and an annular rubber bag arranged on the periphery of the lining in a sectioning mode, and the model foundation pit comprises a supporting structure and an open rubber bag. The beneficial effects of the invention are as follows: the pile head restraint device adopts the telescopic upright rod and the pile head clamp, can effectively restrain the lateral deformation of the pile head, flexibly adjust the restraint position of the pile head, simulate various pile foundation types such as a high-low bearing platform, a pile raft foundation and the like, and has wide application range.
Description
Technical Field
The invention relates to the field of geotechnical centrifugal model tests, in particular to a centrifugal model test device and a centrifugal model test method for influencing the existing pile foundation by multiple excavation of a tunnel-foundation pit.
Background
The problem of the interaction between soil and a structure is a complex subject in the field of geotechnical engineering research, and the complexity is mainly reflected in that the rock and soil to be researched belongs to a discontinuous medium consisting of multiple phases, and has the characteristics of gravity dependence, stress path dependence and the like. Thus, the interaction between the rock-soil mass and the building(s) is a complex nonlinear or dynamic structural mechanics problem. The tunnel and the foundation pit are excavated and unloaded in different modes, the factors causing the soil deformation are different, the soil deformation modes and the soil deformation sizes are also different, and the influence on the adjacent existing pile foundations is further different. The existing research on the influence of excavation unloading on a near pile foundation is mainly focused on the influence problem of single tunnel excavation on a pile or the influence problem of foundation pit excavation on the pile. However, with the improvement of the utilization rate of urban underground space, the working conditions that tunnels and foundation pits are excavated in sequence exist around the existing pile foundation also frequently occur. Moreover, along with the tendency of 'subway heat' and dense construction foundation pit, the situation is more and more common, but the influence research of multiple excavation unloading of tunnels and foundation pits on the adjacent existing pile foundations is more delayed.
Aiming at the interaction problem of soil and a structure, the most commonly adopted test means at present comprise field test, reduced scale model test, centrifugal machine model test and the like. The field test can embody the real stress characteristic of the soil body, but has the defects of large test workload, high cost and complex influencing factors, so that the internal mechanism of the test result is difficult to analyze. Although the cost of the reduced scale model test is relatively low, the reduced scale model test cannot or hardly reproduce the actual environmental conditions on site and cannot reflect the stress state of the prototype. As the most advanced and effective test method which is accepted at present, the geotechnical centrifugal model test can truly simulate the existence of a gravitational field, reproduce the actual stress state of engineering, and is more and more widely applied to the scientific research of geotechnical engineering. Therefore, aiming at the defects of field test and reduced scale model test, it is necessary to develop a centrifugal model test device and a centrifugal model test method for influencing the existing pile foundation by multiple excavation of tunnels and foundation pits.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, and provides a centrifugal model test device and a centrifugal model test method for influencing the existing pile foundation by multiple excavation of a tunnel and a foundation pit, which can restore a real stress state in an ultra-high gravity state by adopting a scale model, and the simulation result is more reliable.
The centrifugal model test device for the tunnel-foundation pit multiple excavation influence existing pile foundations comprises a model box, model piles, model tunnels, model foundation pits, model soil, heavy liquid, pile head restraint devices, a transverse support, a loading system, a control system and a measuring system; the model box is divided into a main body test box and an auxiliary test box through a partition plate and a transverse support, the model pile comprises a pile body and a pile cap arranged at the top of the pile body, the model pile is positioned in a main influence area of a model tunnel and a model foundation pit, the model tunnel comprises a lining and an annular rubber bag arranged on the periphery of the lining in a sectioning mode, the model foundation pit comprises a supporting structure and an open rubber bag, heavy liquid is consistent with the soil density for the model, a pile head restraint device is arranged at the top end of the model pile, the pile head restraint device comprises a telescopic vertical rod fixed on a backing plate and a pile head clamp provided with a semicircular opening, the transverse support freely slides along a groove at the top of a box wall, the loading system comprises a hydraulic jack and an extension rod fixed on the backing plate, the control system comprises a conduit, a valve and a monitoring camera fixed at the top of the model box, and the measuring system comprises a data acquisition instrument hung on the side wall of the model box, a strain gauge for measuring the pile shaft force and a bending moment, a surface pile foundation sensor for measuring the subsidence, a pile head displacement sensor for measuring the pile head, a pile head sensor for measuring the pile head and a pile head sensor for measuring the pile head force sensor for the pile head, and a pore gauge for controlling the water pressure of the excavated model.
As preferable: the strain gauges are distributed at equal intervals along the vertical direction of the pile body, the strain gauges are symmetrically stuck along the cross section of the pile body, and the longitudinal and transverse directions of the cross structure are respectively parallel to and perpendicular to the excavation direction of the model tunnel.
As preferable: and the outer side of the pile body of the model pile is uniformly coated with a layer of epoxy resin adhesive for protecting the strain gauge.
As preferable: the guide pipe is connected to the bottoms of the annular rubber bag and the open rubber bag in a sealing mode, sequentially penetrates through the through holes in the bottoms of the partition plates and is connected with the valve in the auxiliary test box to form a parallel passage, and the guide pipe is connected with the heavy liquid collecting cylinder.
As preferable: the open rubber bag is tightly attached to the inside of the supporting structure of the model foundation pit, and the part of the open rubber bag higher than the ground surface is fixed through the clamp.
As preferable: the diameter of the semicircular opening of the pile head clamp is consistent with the diameter of the model pile.
As preferable: the waveguide tube of the earth surface displacement sensor is fixed on the transverse support through the clamp block, and the movable magnetic ring at the lower part of the waveguide tube is contacted with the earth surface of the model soil.
As preferable: the waveguide tube of the pile top displacement sensor is fixed on a backing plate on the transverse support through a clamp block, and the movable magnetic ring at the lower part of the waveguide tube is propped against the surface of the transverse rib plate on the extension rod of the hydraulic jack and synchronously moves along with the extension rod.
As preferable: the pressure sensor and the pore water pressure are respectively stuck on the surface of an extension rod of the hydraulic jack and placed at the bottom of the open rubber bag.
The test method of the centrifugal model test device for influencing the existing pile foundation by multiple excavation of the tunnel and the foundation pit comprises the following steps:
step one, preparation in the early stage: installing a partition plate and a transverse support, and dividing the model box into a main body test box and an auxiliary test box, wherein gaps among the partition plate, the box wall and the box bottom are sealed by glass cement;
step two, model making: manufacturing a model pile, a model tunnel and a model foundation pit, preparing heavy liquid with the same density as the model soil, and injecting the heavy liquid with corresponding volume into the annular rubber bag of the model tunnel according to the designed value of soil loss;
step three, model installation: the method comprises the steps of paving model soil in layers in a main body test box, sequentially installing a positioning model pile, a model tunnel and a model foundation pit during the process, sliding a transverse bracket to be right above the model pile, then anchoring, and installing a pile head restraint device and a loading system; wherein the contact surface of the pile head clamp of the pile head restraint device and the model pile is coated with a lubricant;
step four, laying a catheter: in the auxiliary test box, connecting a conduit which passes through a through hole at the bottom of the partition plate in advance with the valve correspondingly to form a parallel passage, and finally connecting the conduit with the heavy liquid collecting cylinder; injecting heavy liquid into the open rubber bag of the model foundation pit until the heavy liquid is flush with the ground surface of the model soil, so that the valve is in a closed state;
step five, installing a measuring system: moving a model box to a basket hanging platform of a geotechnical centrifuge, installing a monitoring camera at the top of the model box, adjusting the angle of the camera to a picture covering main body test box, hanging a data acquisition instrument at the outer side of the model box, calibrating and installing a ground surface displacement sensor, a pile top displacement sensor, a pressure sensor and a pore water pressure gauge, and sequentially connecting wires of the monitoring camera and various sensors to a channel of the data acquisition instrument;
step six, pile top working load application: calculating and installing a counterweight, debugging a loading system, a control system and a measuring system, starting a geotechnical centrifuge, and gradually accelerating rotation to required gravitational acceleration; the reading of the system to be measured is stable, a loading system is controlled by a remote computer, a hydraulic jack is started, an extension rod moves downwards until contacting with a pile cap, the pressure sensor data is read to be pressurized step by step to the design value of the pile top working load, and the readings of various sensors in the measuring system are recorded as initial values;
seventh, multiple excavation simulation: when a simulated tunnel is excavated, a valve connected with a first section of annular rubber bag of the model tunnel is controlled to be opened through a remote computer, after heavy liquid is completely discharged, after the reading of a measuring system is stable, the valve of the next section of annular rubber bag is continuously opened until the section excavation of the model tunnel is completed; when the foundation pit excavation is simulated, a valve connected with the open rubber bag is controlled to be opened through a remote computer, and heavy liquid is gradually discharged according to the reading of the pore water pressure gauge until the layered excavation of the model foundation pit is completed;
step eight, ending the test: after the reading of the measuring system is stable, closing the geotechnical centrifuge, reducing the speed to a heavy state, and removing the cleaning model box.
The beneficial effects of the invention are as follows:
1. the invention can truly reproduce the stress characteristics and deformation rules of the existing pile foundation under the multiple excavation actions of the near tunnel and the foundation pit, acquire data such as earth surface subsidence, pile foundation additional subsidence, pile body bending moment, axial force and the like, and simultaneously consider the influence of various factors such as pile top working load, pile head constraint conditions, tunnel volume loss, foundation pit supporting structure rigidity, tunnel-foundation pit-pile relative position, excavation sequence and the like on pile foundation response, thereby providing scientific and accurate test data for protecting pile foundation construction (structure) structures.
2. The pile head restraint device adopts the telescopic upright rod and the pile head clamp, can effectively restrain the lateral deformation of the pile head, flexibly adjust the restraint position of the pile head, simulate various pile foundation types such as a high-low bearing platform, a pile raft foundation and the like, and has wide application range.
3. The multiple excavation unloading is simulated by adopting a liquid discharge method, wherein the model tunnel adopts the annular rubber bag to remove a certain volume of heavy liquid to simulate soil loss caused by tunnel excavation, the model foundation pit adopts the method of removing the heavy liquid and controlling the pressure by adopting the open rubber bag to simulate layered excavation, and the operability and the accuracy of a centrifugal model test are effectively ensured by considering key influencing factors of the excavation unloading.
Drawings
Fig. 1 is a front view of a centrifugal model test apparatus.
Fig. 2 is a plan view of the centrifugal model test apparatus.
Fig. 3 is a schematic structural view of a model pile (where fig. a is a front view of the model pile and fig. b is a sectional view A-A of fig. a).
Fig. 4 is a schematic diagram of a strain gauge bonding method for measuring bending moment and axial force of a model pile (wherein fig. a is a schematic diagram of a strain gauge bonding method for measuring bending moment of a model pile, and fig. b is a schematic diagram of a strain gauge bonding method for measuring axial force of a model pile).
Fig. 5 is a schematic structural view of the model tunnel (where fig. a is a top view of the model tunnel and fig. B is a B-B sectional view of fig. a).
Fig. 6 is a schematic structural view of the model foundation pit (where fig. a is a top view of the model foundation pit, and fig. b is a C-C sectional view of fig. a).
Fig. 7 is a schematic structural view of the pile head restraint (where fig. a is a front view of the pile head restraint and fig. b is a D-D sectional view of fig. a).
FIG. 8 is a schematic view of the installation of the centrifugal model test apparatus on a geotechnical centrifuge.
Reference numerals illustrate: 1-model boxes, 2-model piles, 3-model tunnels, 4-model foundation pits, 5-model earth, 6-heavy liquid, 7-pile head restraint devices, 8-transverse brackets, 9-loading systems, 10-control systems, 11-measurement systems, 12-partition boards, 13-transverse supports, 14-main body test boxes, 15-auxiliary test boxes, 16-pile shafts, 17-pile caps, 18-linings, 19-annular rubber bags, 20-supporting structures, 21-open rubber bags, 22-telescopic vertical rods, 23-pile head clamps, 24-grooves, 25-backing plates, 26-hydraulic jacks, 27-extension rods, 28-guide pipes, 29-valves, 30-monitoring cameras, 31-data acquisition devices, 32-strain gauges, 33-surface displacement sensors, 34-pile head displacement sensors, 35-pressure sensors, 36-pore water pressure gauges, 37-epoxy resin gels, 38-through holes, 39-heavy liquid collection drums, 40-clamps, 41-waveguide pipes, 42-clamp blocks, 43-movable rib plates, 44-45-magnetic rings, a centrifuge basket, 47-suspended platform, 47-centrifuge, 47-suspended platform.
Detailed Description
The invention is further described below with reference to examples. The following examples are presented only to aid in the understanding of the invention. It should be noted that it will be apparent to those skilled in the art that various modifications and adaptations of the invention can be made without departing from the principles of the invention and these modifications and adaptations are intended to be within the scope of the invention as defined in the following claims.
The invention provides a centrifugal model test device and a method for influencing the existing pile foundation by multiple excavation of a tunnel-foundation pit, and the concrete implementation method of the embodiment comprises the following steps:
step one, preparing in the early stage.
As shown in fig. 1 and 2, a partition plate 12 is installed in a model box 1 to divide the model box into a main body test box 14 and an auxiliary test box 15, a plurality of transverse supports 13 are installed in the auxiliary test box 15 to ensure the vertical stability of the partition plate 12, and gaps among the partition plate 12, a box wall and a box bottom are sealed by glass cement. Preferably, the model box can be made of aluminum alloy materials, and aluminum plates on the peripheral side walls are grid-shaped, so that the overall rigidity of the model box is improved, and the model box is prevented from deforming in the high-speed operation process of the geotechnical centrifuge.
And step two, manufacturing a model.
And (3) manufacturing a model pile 2: as shown in fig. 3, the pile body 16 is made of standard aluminum alloy pipe, and the pile top anchors the circular pile cap 17 made of inherent aluminum alloy for uniformly transmitting the working load applied by the hydraulic jack 26. In order to study the internal force response of the existing pile foundation under the multiple excavation unloading effect, strain gauges are uniformly adhered to the pile body to measure the bending moment and axial force of the pile body. The strain gauges 32 are distributed vertically and equidistantly along the pile body 16, and are symmetrically stuck along the cross section of the pile body 16 in a cross shape. As shown in a section A-A of fig. 3, the corresponding strain gage 32 in the x-axis direction is perpendicular to the excavation direction of the model tunnel 3 for measuring pile body bending moment; the pair of strain gages 32 in the y-axis direction is parallel to the excavation direction of the model tunnel 3 for measuring pile body axial force. Preferably, in order to reduce the influence of temperature on the strain gauge measurement result, the strain gauge connection mode adopts a wheatstone bridge arrangement, each bridge is composed of four semiconductor strain gauges, and the circuit connection mode is respectively shown in fig. 4 (a) and fig. 4 (b). In order to protect the strain gauge from direct contact with the outside and prevent friction or damp failure, an epoxy resin adhesive 37 is uniformly coated on the outer side of the pile body 16 to cover the strain gauge 32.
Making a model tunnel 3: as shown in fig. 5, an annular rubber bag 19 with the inner diameter consistent with the outer diameter of the lining 18 is poured by adopting liquid silicone rubber and catalyst mixed liquid, a guide pipe 28 is installed in sealing connection with the bottom, and then the annular rubber bag 19 is closely nested on the periphery of the lining 18 made of aluminum alloy in turn in sections.
Making a model foundation pit 4: as shown in fig. 6, the supporting structure 20 is formed by splicing four aluminum alloy plates. The insertion ratio of the support structure is preferably between 0.5 and 2.0 according to typical foundation pit engineering design. In order to prevent the model soil 5 from falling into the excavation area of the foundation pit during the starting process of the centrifuge, the supporting structure is generally raised above the ground surface by a certain distance. An open rubber bag 21 matched with the excavation size of the model foundation pit 4 is manufactured, and a bottom sealing connection installation conduit 28 is arranged.
The zinc chloride powder is dissolved and stirred by adding water to prepare heavy liquid 6 with the same density as the model soil 5. And injecting heavy liquid 6 with corresponding volume into the annular rubber bag 19 according to the designed value of soil loss caused by the excavation of the model tunnel 3, and temporarily sealing the port of the guide pipe 28. Preferably, the soil loss caused by tunnel excavation is typically between 1% and 4%.
And thirdly, installing a model.
As shown in fig. 1 and 2, in this embodiment, the center buried depth of the model tunnel 3 is flush with the pile bottom of the model pile 2, and five sections of annular rubber bags 19 are symmetrically distributed on the x axis; the model foundation pit 4 is located on the other side of the model pile 2 and is also symmetrically distributed on the x-axis. As shown by the broken lines in fig. 2, the model piles 2 are located in the main influence areas of the model tunnel 3 and the model foundation pit 4 (in the figureIs the internal friction angle of the earth).
Firstly, according to the relative positions of the three, the size scale is marked on the inner wall of the main body test box 14 as a datum line, and the homogeneous model soil 5 is layered in the main body test box 14, and the positioning model pile 2, the model tunnel 3 and the model foundation pit 4 are sequentially installed during the process. When the model pile 2 is installed, the perpendicularity of the model pile is ensured by adopting the assistance of a level bar; when the model tunnel 3 is installed, the side with the guide pipe 28 is downward, and the guide pipe 28 sequentially passes through the through holes 38 at the bottom of the partition plate 12 to be reserved in the auxiliary test box 15; when the model foundation pit 4 is installed, the supporting structure 20 is positioned firstly, then the open rubber bag 21 is tightly attached to the inside of the supporting structure, the part higher than the ground surface is fixed by the clamp 40, and the conduit 28 at the bottom also passes through the through hole 38 of the partition plate 12 and then is reserved in the auxiliary test box 15.
And a transverse bracket 8 formed by welding steel plate trusses slides along a groove 24 at the top of the box wall to be anchored right above the model pile 2, and a backing plate 25 is installed. Before the pile head clamp 23 clamps the model pile 2, the semi-arc opening and pile body contact surface are uniformly coated with a layer of lubricant, preferably vaseline. Subsequently, four telescopic uprights 22 are adjusted and anchored on the backing plate 25 to fix the pile head clamp 23 in the design position, completing the installation of the pile head restraint 7 shown in fig. 7. Preferably, the invention can simulate various pile foundation types such as a high-low bearing platform, a pile raft foundation and the like by adjusting the telescopic upright rod 22 to change the position of the pile head clamp 23.
The hydraulic jack 26 with the extension rod 27 connected to the lower end of the anchor plate 25 is located on the same central axis as the model pile 2 to increase the displacement range of the hydraulic jack and prevent the offset load of the axial pressure, thus completing the installation of the loading system 9.
And step four, laying a catheter.
As shown in FIG. 2, in the auxiliary test chamber 15, the annular rubber bag 19 and the conduit 28 of the open rubber bag 21 which have previously passed through the through hole 38 at the bottom of the partition plate 12 are connected to the valves 29 one by one to form a parallel passage, and finally collected in a main passage to be connected to the heavy liquid collecting cylinder 39. The heavy liquid 6 is injected into the open rubber bag 21 of the model foundation pit 4 until the heavy liquid is flush with the ground surface of the model soil 5, so that all valves 29 are guaranteed to be in a closed state.
And fifthly, installing a measuring system.
As shown in fig. 1, 2 and 8, the model box 1 is moved to a basket hanging platform 46 of a geotechnical centrifuge 45, a monitoring camera 30 is installed at the top of the model box 1, the camera angle is adjusted to the picture covering main body test box 14, a data acquisition instrument 31 is hung and installed at the outer side of the model box 1, and measuring systems such as a ground surface displacement sensor 33, a pile top displacement sensor 34, a pressure sensor 35, a pore water pressure gauge 36 and the like are calibrated and installed, so that wires of the monitoring camera 30 and various sensors are sequentially connected to a channel of the data acquisition instrument 31. In order to prevent the sensor wire from breaking when rotating at high speed, a wire harness belt is adopted for binding and fixing.
The waveguide 41 of the earth surface displacement sensor 33 is fixed on the transverse bracket 8 through the clamp block 42, the upper section of the movable magnetic ring 43 is embedded into the waveguide 41 and can slide, the bottom is contacted with the earth surface of the model soil 5 and synchronously displaces along with the earth surface, at the moment, the embedded depth of the movable magnetic ring 43 changes, the sensor voltage also changes, and the earth surface subsidence can be calculated through the linear relation between the calibrated displacement and the voltage.
The waveguide 41 of the pile top displacement sensor 34 is fixed on the backing plate 25 of the transverse bracket 8 through a clamp block 42, and a movable magnetic ring 43 is abutted against the surface of a transverse rib plate 44 on the extension rod 27 of the hydraulic jack 26 and synchronously moves along with the extension rod 27.
The pressure sensor 35 is attached to the surface of the extension rod 27. The pore water pressure gauge 36 is placed at the bottom of the open rubber bag 21 after the vacuum is saturated.
And step six, pile top working load is applied.
As shown in fig. 8, a weight 47 is calculated according to the total weight and moment balance relation of the model box 1 and fixed on a basket platform 46 at the other side of the geotechnical centrifuge 45. And the loading system 9, the control system 10 and the measuring system 11 are debugged, after all the conditions are normal, the geotechnical centrifuge 45 is started, the geotechnical centrifuge gradually accelerates to rotate to the required gravitational acceleration, and the working state of the geotechnical centrifuge is monitored in real time. The reading of the system to be measured 11 is stable, the loading system 10 is controlled by the remote computer, the hydraulic jack 26 is activated, the extension rod 27 moves downwards until it comes into contact with the pile cap 17, at which point the reading of the pressure sensor 35 starts to change. And then, gradually pressurizing to the design value of the pile top working load by reading data of the pressure sensor 35, and recording readings of various sensors in the measuring system as initial values.
And seventhly, simulating multiple excavation.
And sequentially carrying out simulated excavation of the tunnel and the foundation pit according to the excavation sequence determined by the test scheme. When the tunnel is excavated, the valve 29 connected with the first annular rubber bag 19 of the model tunnel 3 is controlled by a remote computer to open, and the heavy liquid 6 is discharged under the action of dead weight and the action of earth covering pressure and finally flows into the heavy liquid collecting cylinder 39. After the reading of the measuring system is stable, the valve 29 of the next annular rubber bag 19 is continuously opened along the y-axis direction until the section excavation of the model tunnel 3 is completed. When the foundation pit excavation is simulated, the valve 29 connected with the open rubber bag 21 is controlled to be opened through a remote computer, the excavation progress is controlled through pressure change according to the reading of the pore water pressure gauge 36, and the heavy liquid 6 is gradually discharged until the layered excavation of the model foundation pit 4 is completed.
And step eight, ending the test.
After the multiple excavation is finished, after the reading of the measuring system 11 is stable, the geotechnical centrifuge 45 is closed, the speed is reduced to a heavy state, the cleaning model box 1 is removed, and test data are processed.
Claims (8)
1. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence existing pile foundations is characterized by comprising a model box (1), a model pile (2), a model tunnel (3), a model foundation pit (4), model soil (5), heavy liquid (6), a pile head restraint device (7), a transverse bracket (8), a loading system (9), a control system (10) and a measuring system (11); the model box (1) is divided into a main body test box (14) and an auxiliary test box (15) through a partition plate (12) and a transverse support (13), the model pile (2) comprises a pile body (16) and a pile cap (17) arranged at the top of the pile body, the model pile (2) is positioned in a main influence area of the model tunnel (3) and the model foundation pit (4), the model tunnel (3) comprises a lining (18) and an annular rubber bag (19) which is arranged on the periphery of the lining in a sectioned manner, the model foundation pit (4) comprises a supporting structure (20) and an open rubber bag (21), the heavy liquid (6) is consistent with the density of the model soil (5), the top end of the model pile (2) is provided with a pile head restraint device (7), the pile head restraint device (7) comprises a telescopic vertical rod (22) and a pile head clamp (23) with a semicircular opening, a backing plate (25) is fixed at the top of the telescopic vertical rod (22), the transverse support (8) freely slides along a groove (24) at the top of the box wall, the loading system (9) comprises a hydraulic pressure control jack (26) and a control jack (27) which are fixed at the top of the model box (27), the measuring system (11) comprises a data acquisition instrument (31) hung on the side wall of the model box, a strain gauge (32) for measuring the axial force and bending moment of the pile body, a ground surface displacement sensor (33) for measuring ground surface subsidence, a pile top displacement sensor (34) for measuring pile foundation subsidence, a pressure sensor (35) for measuring pile top load and a pore water pressure gauge (36) for controlling the excavation of the model foundation pit; the strain gauges (32) are distributed at equal intervals along the vertical direction of the pile body (16), the strain gauges (32) are symmetrically stuck in a cross shape along the cross section of the pile body (16), and the longitudinal and transverse directions of the cross structure are respectively parallel to and perpendicular to the excavation direction of the model tunnel (3); the guide pipe (28) is connected to the bottoms of the annular rubber bag (19) and the open rubber bag (21) in a sealing mode, the guide pipe (28) sequentially penetrates through the through hole (38) in the bottom of the partition plate (12) and is connected with the valve (29) in the auxiliary test box (15) to form a parallel passage, and the guide pipe (28) is connected with the heavy liquid collecting cylinder (39).
2. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence on the existing pile foundation according to claim 1, wherein the outer side of a pile body (16) of the model pile is also uniformly coated with a layer of epoxy resin glue (37) for protecting strain gauges (32).
3. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence on the existing pile foundation according to claim 1, wherein the open rubber bag (21) is tightly attached to the inside of a supporting structure (20) of the model foundation pit (4), and the part, higher than the ground surface, of the open rubber bag (21) is fixed through a clamp (40).
4. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence on the existing pile foundation according to claim 1, wherein the semi-circular arc opening diameter of the pile head clamp (23) is consistent with the diameter of the model pile (2).
5. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence on the existing pile foundation according to claim 1, wherein a waveguide tube (41) of the earth surface displacement sensor (33) is fixed on a transverse support (8) through a clamp block (42), and a movable magnetic ring (43) at the lower part of the waveguide tube (41) is in contact with the earth surface of the model soil (5).
6. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence on the existing pile foundation according to claim 1, wherein a waveguide tube (41) of the pile top displacement sensor (34) is fixed on a backing plate (25) on a transverse support (8) through a clamp block (42), and a movable magnetic ring (43) at the lower part of the waveguide tube (41) abuts against the surface of a transverse rib plate (44) on an extension rod (27) of a hydraulic jack (26) and synchronously moves along with the extension rod (27).
7. The centrifugal model test device for the tunnel-foundation pit multiple excavation influence on the existing pile foundation according to claim 1, wherein the pressure sensor (35) and the pore water pressure gauge (36) are respectively adhered to the surface of an extension rod (27) of a hydraulic jack (26) and placed at the bottom of an open rubber bag (21).
8. A method of testing a centrifugal model test apparatus for multiple excavation of a tunnel-foundation pit affecting an existing pile foundation as claimed in claim 1, comprising the steps of:
step one, preparation in the early stage: a partition plate (12) and a transverse support (13) are arranged to divide the model box (1) into a main body test box (14) and an auxiliary test box (15), and gaps among the partition plate, the box wall and the box bottom are sealed by glass cement;
step two, model making: manufacturing a model pile (2), a model tunnel (3) and a model foundation pit (4), preparing heavy liquid (6) with the same density as that of the model soil (5), and injecting the heavy liquid (6) with the corresponding volume into an annular rubber bag (19) of the model tunnel (3) according to a soil loss design value;
step three, model installation: the method comprises the steps of layering and paving model soil (5) in a main body test box (14), sequentially installing a positioning model pile (2), a model tunnel (3) and a model foundation pit (4) during the layering and paving process, sliding a transverse bracket (8) to be directly above the model pile (2), then anchoring, and installing a pile head restraint device (7) and a loading system (9); wherein the contact surface of the pile head clamp (23) of the pile head restraint device (7) and the model pile (2) is coated with lubricant;
step four, laying a catheter: in the auxiliary test box (15), a conduit (28) which passes through a through hole (38) at the bottom of the partition plate (12) in advance is correspondingly connected with a valve (29) to form a parallel passage, and the conduit (28) is finally connected with a heavy liquid collecting cylinder (39); injecting heavy liquid (6) into the open rubber bag (21) of the model foundation pit (4) until the heavy liquid is flush with the ground surface of the model soil (5), so that the valve (29) is in a closed state;
step five, installing a measuring system: moving a model box (1) to a basket hanging platform (46) of a geotechnical centrifuge (45), installing a monitoring camera (30) at the top of the model box (1) and adjusting the camera angle to a picture coverage main body test box (14), hanging a data acquisition instrument (31) at the outer side of the model box (1), calibrating and installing a ground surface displacement sensor (33), a pile top displacement sensor (34), a pressure sensor (35) and a pore water pressure gauge (36), and sequentially connecting wires of the monitoring camera (30) and various sensors to a channel of a data acquisition instrument (31);
step six, pile top working load application: calculating and installing a counterweight (47), debugging a loading system (9), a control system (10) and a measuring system (11), starting a geotechnical centrifuge (45), and gradually accelerating rotation to required gravitational acceleration; the reading of a system to be measured (11) is stable, a loading system (9) is controlled through a remote computer, a hydraulic jack (26) is started, an extension rod (27) moves downwards until contacting with a pile cap (17), the data of a pressure sensor (35) is read to be pressurized step by step to a pile top working load design value, and the readings of various sensors in the measuring system are recorded as initial values;
seventh, multiple excavation simulation: when a simulated tunnel is excavated, a valve (29) connected with a first section of annular rubber bag (19) of the model tunnel (3) is controlled to be opened by a remote computer, after the heavy liquid (6) is completely discharged and the reading of a measuring system is stable, the valve (29) of the next section of annular rubber bag (19) is continuously opened until the section excavation of the model tunnel (3) is completed; when the foundation pit excavation is simulated, a valve (29) connected with the open rubber bag (21) is controlled to be opened by a remote computer, and heavy liquid (6) is gradually discharged according to the reading of a pore water pressure gauge (36) until the layered excavation of the model foundation pit (4) is completed;
step eight, ending the test: after the reading of the system to be measured (11) is stable, the geotechnical centrifuge (45) is closed, the speed is reduced to a heavy state, and the cleaning model box (1) is removed.
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