CN113742961A - Construction process for taking engineering pile as support pile - Google Patents
Construction process for taking engineering pile as support pile Download PDFInfo
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Abstract
A construction process using an engineering pile as a support pile comprises the steps of firstly collecting and sampling a soil stratum of a to-be-built object, inputting soil stratum collection data in PLAAXIS 2D/3D software to establish a soil constitutive model, further simulating bearing capacity of an embedded rock pile, analyzing influence of the engineering pile serving as the support pile on the bearing capacity, verifying stress and displacement of two adjacent engineering piles serving as the support pile, providing a reference basis for construction of the engineering pile serving as the support pile, excavating a soil foundation pit by combining actual measurement data according to simulated soil layer numerical values, and constructing a foundation pit support structure; and finally, selecting an engineering pile in the constructed foundation pit supporting structure according to the simulated rock-socketed pile bearing capacity and the factors which influence the bearing capacity of the engineering pile and are obtained by analysis, wherein the perpendicularity of the pile bottom and the pile body of the selected engineering pile meets the requirement of simulation data.
Description
Technical Field
The invention relates to the technical field of pile end construction, in particular to a construction process for taking an engineering pile as a support pile.
Background
The vertical ultimate bearing capacity of the single socketed pile specified in the building pile foundation technical specification, in which the pile end is arranged in complete and relatively complete bedrock, consists of the total ultimate side resistance of the soil around the pile and the total ultimate resistance of a socketed section, and the total ultimate resistance of the socketed section can be expressed as follows: qrk＝ξfrkAp，frkIs the saturated uniaxial compressive strength of the rock, ApThe area of the pile section is shown, xi is the comprehensive coefficient of the side resistance and the end resistance of the rock-socketed section, and xi is related to the rock-socketed depth, the hardness degree of the rock and the pile forming process. When the rock face inclines, the rock embedding depth is based on the rock embedding depth below the slope. According to the rock-socketed pile bearing capacity calculation formula, although the rock-socketed depth is based on the rock-socketed depth below the slope when the rock face is inclined, the influences on the pile bearing capacity on the hardness degree of rocks on the inclined rock stratum, the rock-socketed depth, the thickness of sediment at the bottom of the pile, the rock stratum inclination angle, the pile perpendicularity and the like are not involved in the formula.
When the engineering piles are arranged on the periphery of the foundation pit, the foundation pit support can consider that the engineering piles are used as support piles, namely, the pile has two purposes, and as the engineering piles mainly bear vertical loads, the reinforcement distribution rates of spiral stirrups and longitudinal reinforcements are generally low, the horizontal displacement and the anti-cracking check calculation of the engineering piles are ensured to be within the standard allowable range, and careful research and analysis are needed; in addition, the rock penetration depth of the engineering pile is possibly inconsistent with that of the supporting pile, so that a supporting mode of combining the engineering pile with the long pile and the short pile of the supporting pile is formed. However, when the enclosure row piles are designed, the embedding depth of the row piles simultaneously meets the requirement of checking and calculating the stability of the foundation pit (the overall stability, the anti-uplift stability, the anti-skirting stability and the like) and effectively controls the deformation, but for short piles with smaller embedding depth, part of stress is transmitted to adjacent lengthened protection piles by means of crown beams, and the short piles are connected with adjacent support piles to bear the action of soil pressure together.
At present, relevant personnel in the field simulate and research the numerical value of an engineering pile as a support pile by using a three-dimensional limit balance method, a variational limit balance method, a layer-by-layer excavation supporting force invariant method, an elastic foundation beam method, a three-dimensional space numerical analysis method, a three-dimensional finite element analysis model and other methods, but the influence of rock-embedded depth on the bearing capacity of the rock-embedded pile is large in specific construction engineering, and particularly when a bearing layer of the rock-embedded pile is not in a horizontal state, the settlement of the pile top under the same load action is correspondingly reduced along with the increase of the gradient of a rock stratum, so that the construction progress and the bearing capacity of the pile are influenced.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a construction process using an engineering pile as a support pile, so as to solve the problems in the background technology.
The technical problem solved by the invention is realized by adopting the following technical scheme:
a construction process for taking an engineering pile as a support pile comprises the following specific steps:
1. soil layer numerical simulation
1) Constructing soil constitutive model
Firstly, collecting and sampling a soil stratum of a to-be-built object, and inputting soil stratum collecting data in PLAAXIS 2D/3D software to construct a soil constitutive model;
2) simulating initial stress field
Simulating an initial stress field in the soil body constitutive model, wherein the initial stress is an existing ground stress field in a rock body before excavation of the foundation pit and is in a relatively stable and balanced state;
3) physical unit simulation
Simulating solid units in the soil constitutive model, wherein the solid units are triangular units with 6 nodes or 15 nodes when used for simulating soil layers and other blocks;
4) board unit simulation
Simulating plate units in a soil body constitutive model, wherein the plate units are formed by different nodes on the same plane, except for a soil and interface material data set, the material attribute and the model parameter of a plate are input through a separate material data set, the plate is used for simulating the properties of a thin wall, a plate or a thin shell, including the elastic property and the elastoplastic property, and the data set of the plate usually represents a certain type of plate material and can be assigned to the corresponding plate unit in a geometric model;
5) simulating excavation process
Simulating an excavation process in a soil body constitutive model, wherein step-by-step construction is the most important type in load input, various loads, constructions and excavation processes can be truly and accurately simulated by utilizing the characteristics of PLAAXIS 2D/3D software and changing the geometric or load arrangement by freezing or activating loads, class groups or structural objects, the step-by-step construction can be used for re-specifying material data groups or changing the water level distribution conditions, step-by-step construction calculation is executed, and a geometric model is firstly created and should contain all objects used in the calculation;
2. bearing capacity simulation of rock-socketed pile
I) Building rock-socketed pile bearing capacity model
The building of the rock-socketed pile bearing capacity model refers to a geotechnical engineering survey outline of a simulated building, comprehensively considers the peripheral situation of foundation pit engineering, and builds a simplified model, wherein in the model, a soil body adopts a Mohr-Coulomb material model, and the material parameters and the deformation modulus of the soil body are determined according to a geotechnical survey report and experience;
II) geometric model building and parameter setting
Based on the bearing capacity model of the rock-socketed pile in the step I), taking a single pile as a reference, according to the Saint-Venn principle, the denaturation result beyond three times of the edge is very little, the denaturation result can be ignored, the length and width of the model are selected to be 20-25 times of the pile diameter, the height of the model is 1.5-5 times of the pile length, the height of the model is optimal, the requirements of calculation precision and calculation speed can be met, and the scale of the geometric model is as follows: the length and width are 20m, the height is 21.5m, as shown in figure 3, then the geometric model is divided into grids, the model soil body is divided into 5 layers from top to bottom, and the 5 layers are respectively filled with miscellaneous earth, fine medium sand, coarse gravel sand, strongly weathered granite and medium weathered granite;
setting boundary conditions and loads: setting model boundary conditions: the x and y directions are restricted, and the z direction is free; setting a load to a z-direction negative surface on the pile top;
III) calculating and analyzing the simulation conditions
The calculation in the PLAAXIS 2D/3D software is to realize numerical simulation through an activation unit, during simulation analysis, firstly establishing a soil layer initial stage, applying constraint to the periphery of a soil layer, then activating each initial soil layer according to specific construction steps, activating a pile unit, and then performing next load application until the PLAAXIS 2D/3D software shows that the soil layer is about to collapse;
the PLAAXIS 2D/3D software has a powerful post-processing function, can output result isolines, color cloud pictures, isosurface and vector distribution maps, can output internal force of a structural unit, internal force of an entity unit and pore pressure change of each stage, can add annotations to an output view, draw a change curve (curve manager) of a monitoring point, automatically generate a calculation result report and animation, can preview a calculation result in the calculation process so as to check and correct a model in time, and compares a pile static load test result in the monitoring report with a simulation value;
3. influence of engineering pile and supporting pile on bearing capacity is simulated and analyzed
In the actual foundation pit engineering, the surrounding environment and address conditions are complex, the geometric model is built again to analyze the influence of the engineering pile serving as a support pile on the bearing capacity, the planar strain model is adopted for building the geometric model, and the basic assumption is as follows:
(1) assuming that all materials in the model are isotropic materials and the materials are uniform;
(2) before construction, a precipitation measure is assumed, the influence of seepage action on a foundation pit is not considered, and consolidation change of a soil body in a precipitation process is not considered;
(3) in order to realize displacement coordination, the soil body is supposed to be in close contact with the building enclosure, and the disturbance condition of the construction to the soil body is not considered;
geometric model building and parameter setting
Soil layer parameters
A soil property parameter simulation object investigation report selects constitutive models as a Mohr-Coulomb and small strain reinforced soil model, and the model parameters are as follows:
the pile is cast by c30 concrete, a linear elastic model is adopted, and empirical values and specific indexes are taken as numerical valuesThe following were used: the volume weight gamma is 25kN/m3The elastic modulus E is 30000MPa, the Poisson ratio mu is 0.2, the supporting condition is that the section A2-A3 of the foundation pit is taken, the length of a supporting pile is 12.35m, the diameter of the pile is 1.0m, the distance between the supporting piles is 0.2m, the length of an engineering pile is 12.0m, and the diameter of the pile is 0.7 m;
in order to simplify calculation, when a geometric model is established, the triple-pipe high-pressure jet grouting piles around the foundation pit are subjected to equivalence, the triple-pipe high-pressure jet grouting piles are regarded as plates with equal thickness, the diameter of the triple-pipe high-pressure jet grouting pile is 0.7m, the length of the triple-pipe high-pressure jet grouting pile is 9.4m, solid units are adopted for modeling, the thickness of the equivalent rear plate is 0.7m, and according to the Saint Vietnam principle, the denaturation result beyond triple edges is very little, and the equivalent rear plate can be regarded as being ignored; the length of the geometric model is more than 20 times of the pile diameter, and the height of the geometric model is more than 1.5 times of the pile length, so that the requirements on calculation precision and calculation speed can be met; the geometric model scale is as follows: the length is 20m, the width is 20.3m, and the height is 25 m;
mesh partitioning
The geometric model soil body is divided into 5 layers from top to bottom, namely miscellaneous fill, fine medium sand, coarse gravel sand, strongly weathered granite and medium weathered granite, and the PLAAXIS 2D/3D software divided meshes adopt hexahedral units;
boundary conditions and load settings
Constraining two sides of the geometric model in the non-foundation pit direction to enable the geometric model to be fixed in the y direction and free in other directions;
simulation results
The soil body outside the foundation pit has a sedimentation effect due to the excavation unloading effect, the final maximum accumulated sedimentation is 15.36mm along with the deepening of the excavation, the soil pressure of the soil body acting on the supporting surface is increased along with the deepening of the foundation pit, the displacement of the enclosure structure is increased, and the displacement of the supporting wall body is increased; the maximum horizontal displacement of the enclosure structure is at the top, the first part is excavated, the lateral displacement of the wall body is not large, the horizontal displacement of the wall body is very obvious in the second part of the excavation, and the accumulated maximum horizontal displacement of the enclosure body is 15.09mm until the pit bottom is excavated;
4. verifying stress and displacement of engineering pile serving as support pile
In order to further explore the stress and displacement conditions of the engineering piles serving as the support piles, a verification model is established when two adjacent engineering piles serve as the support piles, and the dimension of the verification model is unchanged and is the same as the geometric model in the step 3; when two adjacent engineering piles are used as supporting piles, the pile top displacement is slightly increased compared with that of a single pile;
5. excavation soil layer foundation pit
Excavating a soil foundation pit according to the soil layer numerical value simulated in the step 1 and in combination with the actually measured data, and monitoring soil layer change data in the foundation pit in real time in the excavation process;
6. building foundation pit supporting structure
In the process of excavation of a foundation pit, a foundation pit supporting structure is built, a foundation pit supporting structure system comprises a cast-in-situ bored pile, a triple-pipe high-pressure jet grouting pile, a net jet release slope and a steel pipe soil nail wall, the triple-pipe high-pressure jet grouting pile is used for stopping water in an area with larger excavation depth of the foundation pit engineering, a water interception ditch is arranged outside the foundation pit, and a drainage well and a water collection well are used for draining water in the foundation pit;
7. setting engineering pile as support pile
And (3) selecting the engineering pile in the foundation pit supporting structure built in the step (6) according to the bearing capacity of the rock-socketed pile simulated in the step (2) and the influence of the engineering pile obtained by analyzing in the step (3) and serving as a factor of the supporting pile on the bearing capacity, taking the selected engineering pile as the supporting pile, and detecting when the perpendicularity of the pile bottom and the pile body of the engineering pile cannot meet the requirement of the simulation data so as to find out the reason influencing the requirement of the simulation data.
Has the advantages that:
1) the invention collects and samples the soil stratum of a to-be-built object, and inputs soil stratum collection data in PLAAXIS 2D/3D software to establish a soil constitutive model, so as to simulate the bearing capacity of a rock-socketed pile, analyze the influence of the engineering pile serving as a support pile on the bearing capacity, verify the stress and displacement of two adjacent engineering piles serving as the support pile, and provide a reference basis for the construction of the engineering pile serving as the support pile;
2) the construction progress of the engineering pile constructed by the method is greatly improved, the bearing capacity of the pile body is obviously improved, and meanwhile, the investment of manpower and materials is reduced;
3) after the on-site static load test of the engineering pile foundation constructed and poured by the invention, the bearing capacity of a single pile meets the design requirement.
Drawings
Fig. 1 is a schematic diagram showing the comparison between the static load test result and the simulation value of the rock-socketed pile bearing capacity pile in the preferred embodiment of the invention.
Fig. 2 is a Q-S curve diagram of the load on the pile top and the settlement of the pile top in the influence of the rock-socketed depth on the bearing capacity in the preferred embodiment of the present invention.
FIG. 3 is a graph showing the variation of the bearing capacity of the rock-socketed depth on the bearing capacity according to the preferred embodiment of the present invention.
Fig. 4 is a Q-S curve diagram of the pile top load and the pile top settlement in the influence of the rock stratum inclination on the bearing capacity in the preferred embodiment of the invention.
FIG. 5 is a graph illustrating the variation of bearing capacity due to the inclination of rock formation versus bearing capacity in the preferred embodiment of the present invention.
Fig. 6 is a Q-S curve of pile top loading and pile top settlement in the effect of rock formation softness on bearing capacity in a preferred embodiment of the invention.
FIG. 7 is a graph illustrating the variation of bearing capacity due to the influence of formation softness on bearing capacity in a preferred embodiment of the present invention.
Fig. 8 is a Q-S curve diagram of the pile top load and the pile top settlement in the influence of the pile verticality on the bearing capacity in the preferred embodiment of the present invention.
Fig. 9 is a schematic view of the load bearing capacity variation curve in the influence of pile verticality on the load bearing capacity in the preferred embodiment of the present invention.
Fig. 10 is a Q-S curve diagram illustrating the effect of pile top load and pile top settlement on bearing capacity due to the thickness of the pile bottom sediment in the preferred embodiment of the present invention.
Fig. 11 is a schematic view of a load-bearing capacity variation curve in the influence of the thickness of the pile bottom sediment on the load-bearing capacity in the preferred embodiment of the invention.
Fig. 12 is a schematic diagram of the displacement of the soil around the foundation pit and the displacement of the standard section of the enclosure structure in the preferred embodiment of the invention.
Fig. 13 is a diagram illustrating simulation values of horizontal displacement of the pile in the depth direction according to the preferred embodiment of the present invention.
Fig. 14 is a diagram illustrating simulation values of horizontal displacement of the pile in the depth direction according to the preferred embodiment of the present invention.
Fig. 15 is a diagram illustrating simulation values of horizontal displacement of two adjacent engineering piles in the depth direction when the two adjacent engineering piles are used as support piles in the preferred embodiment of the present invention.
Detailed Description
In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further explained below by combining the specific drawings.
A construction process for taking an engineering pile as a support pile comprises the following specific steps:
1. soil layer numerical simulation
1) Constructing soil constitutive model
Firstly, collecting and sampling a soil stratum of a to-be-built object, and inputting soil stratum collection data in PLAAXIS 2D/3D software to establish a soil constitutive model; the pseudo-building is a connected 4+1 frame structure building and is positioned in the north of a sand washing road in Fengxian county and in the east of a east lake park; the total occupied area is about 14000 square meters, the building area is about 36000 square meters, the building is planned to adopt a pile foundation, the foundation burial depth is about 2.00-3.00 m, the site leveling elevation is 39.00-42.20 m, the bottom elevation of a foundation pit is 36.95-38.30 m, the bottom elevation of the foundation pit in a local platform bin area is 31.95m, the excavation depth of the foundation pit is 1.40-5.25 m, the excavation depth of the foundation pit in the local platform bin area is 7.05-8.45 m, and 755 piles are arranged in the foundation pit engineering project at this time;
the stratum has: ploughing soil, fine medium sand, coarse gravel sand, strongly weathered fine granite, weathered fine granite and slightly weathered fine granite, wherein the following steps are sequentially performed:
the ploughing soil (Q4) is brown-gray black, loose, soft and plastic saturated, contains plant roots, mainly comprises cohesive soil and organic matters, and is a high-compressibility soil layer, the thickness of the layer is 0.3-2.6 m, the average thickness is 1.52 m, and the holes drilled in all the fields are distributed in the layer;
② fine medium sand (Q4al), which is ash yellow-grey brown, wet and slightly dense, and mainly consists of fine silt with a small amount of medium sand. The thickness of the layer is 3.1-5.3 m, the average thickness is 3.78 m, and the layer distribution is observed in all the drill holes in the field;
③ coarse gravel sand (Q4al): the gravel stone is flat and sub-round, and has a diameter of 0.5-5 cm in general; the thickness of the layer is 2.2-4.9 m, the average thickness is 3.38 m, and the layer distribution is observed in all the field drill holes;
the strongly weathered fine-grained granite (r24) is a middle-age Yanshan stage invaded granite, is positioned at the top of an invaded body, is formed by long-term erosion and weathering of water, ice, wind and the like, is off-white, softer, fine-grained structure and blocky structure, and consists of diagenetic minerals such as plagioclase feldspar, orthoclase feldspar, quartz and the like; weathering cracks and primary cracks are developed, the cracks show oxide films, the rock core is in short column shape and broken block shape, part of the feldspar minerals are weathered into clay, and the rock structure is basically reserved; the rock mass is broken, the basic quality grade is IV, and the layer can be seen in each drill hole in the exploration; the highest elevation of the bedding surface is-7.0 m, which is seen in zk-23; the lowest elevation of the bedding surface is minus 9.8 meters, which is found in zk-3 holes, and the average elevation is minus 8.68 meters;
weathered fine granite (r24): the strong weathering layer is positioned at the lower part of the strong weathering layer, is in a transition relation, is in medium weathering degree, is grey white, is hard, has a fine grain structure and a blocky structure due to the permeation of underground water, and consists of diagonalite, orthoclase, quartz and other rock-making minerals; the core is mainly columnar, a small amount of short columnar, a small amount of feldspar minerals are weathered into white powder, and the rock structure is complete. The rock mass is relatively complete, the basic mass is class III, the rock stratum is basically distributed in the whole field, the elevation of the highest position of the stratum is-8.3 meters, and the elevation is seen in zk-23; the elevation of the lowest part of the bedding surface is 18.3 meters below zero, which is seen in zk-18 holes, the average elevation is 13.57 meters below zero, and most of the drill holes are not drilled through;
sixthly, slightly weathered fine granite (r24), which is grayish, hard, compact, complete in core, complete in rock structure, and is drilled with zk-24 holes and not drilled with the layer, and other holes are not drilled with the layer;
wherein, the miscellaneous fill is the soil which is not consolidated and is uneven geological layer; secondly, the cultivated soil layer is not compacted and has poor properties and is an uneven geological layer; thirdly, the components of the silt clay layer are simple, and the uniformity of the soil is general; fine sand powder, fine medium sand and sand soil layer are simple in composition and general in soil uniformity; sixthly, the composition of the gravel sand layer is relatively mixed, and the soil uniformity is poor; the weathered granite layer, the stroke weathered granite layer and the stroke weathered granite layer have difference due to the weathered degree, the soil homogeneity is general, so the rock-socketed pile adopts the stroke weathered granite as a bearing layer, the top elevation of the bearing layer has fluctuation, the rock-socketed pile calculation model is established by taking a test pile with the pile number of 100 as a prototype, the position of the pile is approximately between ZK 57-ZK 58, obviously, the bearing layer is not in a horizontal state, and the bearing characteristic values of each rock-soil layer and the standard values of the limit end resistance and the limit side resistance of the pile are corrected as shown in table 1:
TABLE 1 adjusted rock-soil layer parameter table
The elasto-plastic Mohr-Coulomb model includes five input parameters, namely: e and v representing elasticity of soil body and E and v representing plasticity of soil body
And c, and the shear-expansion angle psi, the Mohr-Coulomb model describing a 'first order' approximation to the behaviour of the rock, is recommended for preliminary analysis of the problem, for each soil layer an average stiffness constant can be estimated, since this stiffness is constant, calculation is relatively fast, a preliminary impression of deformation can be obtained, in addition to the five model parameters mentioned above, the initial soil conditions play a critical role in many soil deformation problems, selecting the appropriate K value to generate the initial horizontal soil stress, the Mohr-Coulomb yield condition is a generalization of the Coulomb friction law under normal stress conditions, which in fact guarantees thatAny plane within a material element will obey the Coulomb law of friction, if described in terms of principal stress, the complete Mohr-Coulomb yield condition consists of (3.1-3.6) six yield functions:
the two plastic model parameters appearing in the above (3.1-3.6) yield function are the friction angle and cohesion c, which can collectively represent a hexagonal pyramid in the principal stress space, and when the Mohr-Coulomb model is used under normal stress conditions, special treatment is required if the two yield surfaces intersect, and part of the procedure uses a smooth transition from one yield surface to the other, i.e., the corner angle is polished, whereas in PLAXIS, the Mohr-Coulomb model uses the exact form, i.e., the exact change is used from one yield surface to the other;
2) simulating initial stress field
The method comprises the following steps of simulating an initial stress field in a soil body constitutive model, wherein the initial stress is an existing ground stress field in a rock body before foundation pit excavation, the initial stress field is in a relatively stable and balanced state, the initial stress field of soil needs to be simulated before excavation, and finite element software mainly simulates the initial stress field in two modes, namely a first method: the boundary is called as a load, a dead load is applied to the boundary, and initial stress is obtained through calculation; the second method comprises the following steps: the method comprises the steps that initial stress is transmitted to a unit high-speed point according to a formula and a self-weight stress method to form a virtual stress field, the PLAAXIS 2D/3D software working condition definition used in the embodiment has a displacement zero clearing function, and numerical calculation is carried out on the stepwise excavation process after displacement zero clearing is finished by means of the displacement zero clearing function;
3) physical unit simulation
Simulating entity units in the soil body constitutive model, wherein triangular units with 6 nodes or 15 nodes are adopted when the entity units are used for simulating soil layers and other blocks, the default units are 15-node triangular units under general conditions, the units provide 4-order displacement interpolation, and numerical integration adopts 12 Gaussian points (stress points); the interpolation of the 6-node triangular unit is2 orders, the numerical integration adopts 3 Gaussian points, and the types of the structural unit and the interface unit are automatically matched with the types of the soil units;
4) board unit simulation
Simulating plate units in a soil body constitutive model, wherein the plate units are formed by different nodes on the same plane, except for a soil and interface material data set, the material attribute and the model parameter of a plate are input through a separate material data set, the plate is used for simulating the properties of a thin wall, a plate or a thin shell, including the elastic property and the elastoplastic property, and the data set of the plate usually represents a certain type of plate material and can be assigned to the corresponding plate unit in a geometric model;
5) simulating excavation process
Simulating an excavation process in a soil body constitutive model, wherein step-by-step construction is the most important type in load input, various loads, constructions and excavation processes can be truly and accurately simulated by utilizing the characteristics of PLAAXIS 2D/3D software and changing the geometric or load arrangement by freezing or activating loads, class groups or structural objects, the step-by-step construction can be used for re-specifying material data groups or changing the water level distribution conditions, step-by-step construction calculation is executed, and a geometric model is firstly created and should contain all objects used in the calculation;
2. bearing capacity simulation of rock-socketed pile
I) Building rock-socketed pile bearing capacity model
Establishing a reference Fengxian county culture artistic center geotechnical engineering reconnaissance schema of a rock-socketed pile bearing capacity model, comprehensively considering the peripheral situation of foundation pit engineering, establishing a simplified model for analysis, wherein in the model, a soil body adopts a Mohr-Coulomb material model, and the material parameters and the deformation modulus of the soil body are determined according to a geotechnical reconnaissance report and experience;
in the actual foundation pit engineering, the surrounding environment and address conditions are complex, the model is properly adjusted according to the actual situation, and the model is simplified;
to simplify the operation, the model building applies symmetry, assuming the following:
(1) assuming that all materials in the model are isotropic materials and the materials are uniform;
(2) the method is characterized in that precipitation measures are assumed before construction, and the influence of seepage on the foundation pit is not considered in the embodiment; the consolidation change of the soil body in the precipitation process is not considered;
(3) in order to realize displacement coordination, the soil body is supposed to be in close contact with the building enclosure, and the disturbance condition of the construction to the soil body is not considered;
selecting a constitutive model as Moire-Coulomb according to the soil property parameters according to the survey report of the Fengxin county cultural art center, wherein each index parameter is specifically shown in Table 1;
model parameters
The pile is cast by c30 concrete, a linear elastic model is adopted, numerical values are referred according to empirical values, and adjustment is carried out in the model building process, wherein the concrete parameters are as follows: the volume weight gamma is 25kN/m3The elastic modulus E is 30000MPa, and the Poisson ratio mu is 0.2;
II) geometric model building and parameter setting
With step I) socketed pile bearing capacity model is the basis, the study of taking the single root stake of stake number 100, stake footpath 0.6m, stake length 16.9m, according to saint winan principle, the degeneration result beyond the marginal triple is very little, can be for neglecting, the long wide size of model selects 20 ~ 25 times of stake footpath, 1.5 ~ 5 times of stake length is the best highly got to the model, can accomplish the requirement that satisfies computational accuracy and computational rate, the geometric model yardstick is: the length and width are 20m, the height is 21.5m, then the geometric model is subjected to grid division, the model soil body is divided into 5 layers from top to bottom, and the 5 layers are respectively filled with miscellaneous earth, fine medium sand, coarse gravel sand, strongly weathered granite and medium weathered granite, and the PLAXIS2D/3D software division grid adopts hexahedral units;
setting boundary conditions and loads: setting model boundary conditions: the x and y directions are restricted, and the z direction is free; setting a load to a z-direction negative surface on the pile top;
III) calculating and analyzing the simulation conditions
The calculation in the PLAAXIS 2D/3D software is to realize numerical simulation through an activation unit, during simulation analysis, firstly establishing a soil layer initial stage, applying constraint to the periphery of a soil layer, then activating each initial soil layer according to specific construction steps, activating a pile unit, and then applying load in the next step, as shown in table 2, until the PLAAXIS 2D/3D software shows that the soil layer is about to collapse;
TABLE 2 working condition load table
| Working conditions | Content providing method and apparatus | Remarks for |
| 1 | Creating an initial soil layer | ----- |
| 2 | Construction pile | ----- |
| 3 | First loading | Plus 820kN (820 kN total) |
| 4 | Second loading | Plus 410kN (1230 kN total) |
| 5 | Third time loading | Add 410kN (total 1640kN) |
| 6 | Fourth time loading | Plus 410kN (2050 kN total) |
| 7 | Fifth time loading | Plus 410kN (2460 kN total) |
| 8 | Sixth time of loading | Plus 410kN (2870 kN altogether) |
| 9 | Seventh time loading | Plus 410kN (total 3280kN) |
| 10 | Eighth time loading | Adding 410kN (3690 kN total) |
| 11 | Ninth time of loading | Plus 410kN (4100 kN total) |
The PLAAXIS 2D/3D software has a powerful post-processing function, can output result isolines, color cloud pictures, isosurface and vector distribution maps, can output internal force of a structural unit, internal force of an entity unit and pore pressure change of each stage, can add annotations to an output view, draw a monitoring point change curve (curve manager), automatically generate a calculation result report and animation, can preview the calculation result in the calculation process so as to check and correct the model in time, and compares the pile static load test result in the monitoring report with a simulation value, wherein the result is shown in FIG. 1;
geometric model, selection of calculation parameters: the diameter D of the socketed pile is 600mm, and the elastic modulus of the concrete of the pile body is measured to obtain Ep＝3×104Under the condition of MPa, taking v as 0.2 for Poisson ratio;
a. analyzing the influence of the rock-socketed depth on the bearing capacity, only adjusting the rock-socketed depth under the condition that other conditions are not changed, wherein the rock-socketed depth is changed to 0 d-0 mm, 0.5 d-300 mm, d-600 mm,2 d-1200 mm,3 d-1800 mm,4 d-2400 mm and 5 d-3000 mm, applying z-axis negative direction face load on the pile top, and gradually applying the load until the soil layer is damaged to obtain a Q-S curve of the pile top load and the pile top settlement as shown in figure 2, and the bearing capacity change curve as shown in figure 3; as is apparent from fig. 2 and 3, under the same load, the increase of the rock-socketed depth obviously reduces the settlement of the pile top, and the bearing capacity of the pile is also increased along with the increase of the rock-socketed depth;
b. analyzing the influence of rock stratum inclination on the bearing capacity, only adjusting the angle of the bearing layer under the condition that other conditions are not changed, wherein the rock stratum inclination is changed into 0 degree, 10 degrees, 20 degrees, 30 degrees, 40 degrees and 50 degrees, applying z-axis negative direction surface load on the pile top, and gradually loading until the soil layer is destroyed to obtain a Q-S curve of the pile top load and the pile top settlement as shown in figure 4, and a bearing capacity change curve as shown in figure 5; as shown in fig. 4 and 5, under the same load, the pile top settlement is reduced correspondingly with the increase of the rock stratum inclination, and the bearing capacity is gradually increased; however, when the inclination of the rock stratum exceeds 30 degrees, the pile top settlement is increased to some extent when the inclination is larger, and the bearing capacity is reduced;
c. analyzing the influence of rock stratum softness on bearing capacity, only adjusting the elastic modulus of a bearing layer under the condition that other conditions are not changed, changing the elastic modulus into 1 time, 0.8 time, 0.6 time, 0.4 time and 0.2 time of the initial elastic modulus, applying z-axis negative direction face load on the pile top, and gradually loading until a soil layer is destroyed to obtain a Q-S curve of the pile top load and the pile top settlement as shown in figure 6, wherein the bearing capacity change curve is shown in figure 7; as seen from fig. 6 and 7, as the elastic modulus of the bearing layer is reduced, the settlement of the pile top is increased sharply, and the bearing capacity of the pile is also reduced seriously;
d. analyzing the influence of pile perpendicularity on bearing capacity, under the condition that other conditions are not changed, adjusting the perpendicularity of a pile body, wherein the pile perpendicularity is changed into 0%, 1%, 2%, 3% and 4%, applying z-axis negative direction surface load on the pile top, and gradually loading until a soil layer is destroyed to obtain a Q-S curve of pile top load and pile top settlement as shown in figure 8, and a bearing capacity change curve as shown in figure 9; as seen from fig. 8 and 9, when the verticality variation range is small, the influence of the verticality of the pile body on the settlement amount of the pile top and the bearing capacity of the pile is small;
e. analyzing the influence of the thickness of the pile bottom sediment on the bearing capacity, under the condition that other conditions are not changed, adjusting the thickness of the pile bottom sediment, wherein the thickness of the pile bottom sediment is changed into 0mm,10mm,20mm,30mm and 40mm, applying z-axis negative direction surface load on the pile top, and gradually loading until the soil layer is destroyed to obtain a Q-S curve of the pile top load and the pile top settlement as shown in figure 10, and a bearing capacity change curve as shown in figure 11; as seen from fig. 10 and 11, when sediment is generated at the bottom of the pile, the sediment amount at the top of the pile is increased, and the bearing capacity of the pile is correspondingly reduced, but when the thickness of the sediment at the bottom of the pile is changed within 10-40 mm, the sediment amount at the top of the pile and the bearing capacity are changed slightly;
3. influence of engineering pile and supporting pile on bearing capacity is simulated and analyzed
In the actual foundation pit engineering, the surrounding environment and address conditions are complex, the geometric model is built again to build the geometric model to analyze the influence of the engineering pile and the support pile on the bearing capacity, the geometric model is built by adopting a plane strain model, and the basic assumption is as follows:
(1) assuming that all materials in the model are isotropic materials and the materials are uniform;
(2) before construction, a precipitation measure is assumed, the influence of seepage action on a foundation pit is not considered, and consolidation change of a soil body in a precipitation process is not considered;
(3) in order to realize displacement coordination, the soil body is supposed to be in close contact with the building envelope. The disturbance condition of the construction to the soil body is not considered;
geometric model establishment and parameter setting
Soil layer parameters
Soil property parameters refer to a survey report of the culture and art center of Fengxian county, a constitutive model is selected as a Mohr-Coulomb and small strain reinforced soil model, indexes of the constitutive model are shown in table 1, and changes of partial soil parameters are shown in table 3:
TABLE 3 partial soil parameters change table
Parameters of structural model
The pile is cast by c30 concrete, a linear elastic model is adopted, empirical values are taken as numerical values, and the concrete indexes are as follows: the volume weight gamma is 25kN/m3The elastic modulus E is 30000MPa, the Poisson ratio mu is 0.2, the supporting condition is that the section A2-A3 of the foundation pit is taken, the length of a supporting pile is 12.35m, the diameter of the pile is 1.0m, the distance between the supporting piles is 0.2m, the length of an engineering pile is 12.0m, and the diameter of the pile is 0.7 m;
in order to simplify calculation, when a geometric model is established, the triple-pipe high-pressure jet grouting piles around the foundation pit are subjected to equivalence, the triple-pipe high-pressure jet grouting piles are regarded as plates with equal thickness, the diameter of the triple-pipe high-pressure jet grouting pile is 0.7m, the length of the triple-pipe high-pressure jet grouting pile is 9.4m, solid units are adopted for modeling, the thickness of the equivalent rear plate is 0.7m, and according to the Saint Vietnam principle, the denaturation result beyond triple edges is very little, and the equivalent rear plate can be regarded as being ignored; the length of the geometric model is more than 20 times of the pile diameter, and the height of the geometric model is more than 1.5 times of the pile length, so that the requirements on calculation precision and calculation speed can be met; the geometric model scale is as follows: the length is 20m, the width is 20.3m, and the height is 25 m;
mesh partitioning
The geometric model soil body is divided into 5 layers from top to bottom, namely miscellaneous fill, fine medium sand, coarse gravel sand, strongly weathered granite and medium weathered granite, and the PLAAXIS 2D/3D software divided meshes adopt hexahedral units;
boundary conditions and load settings
Constraining two sides of the geometric model in the non-foundation pit direction to enable the geometric model to be fixed in the y direction and free in other directions;
working condition calculation
TABLE 4 operating condition analysis List
Thirdly simulation result
The settlement of the soil body outside the foundation pit occurs due to the unloading effect of excavation, the final maximum accumulated settlement is 15.36mm along with the deepening of excavation, the displacement of the soil body around the foundation pit and the displacement of the standard section of the enclosure structure are shown in figure 13, it can be seen that the soil pressure of the soil body acting on the supporting surface is increased along with the deepening of the excavation of the foundation pit, the displacement of the enclosure structure is increased, the displacement of the supporting wall body is increased, the maximum horizontal displacement of the enclosure structure is seen to be at the top in figure 12, the side displacement of the wall body is not large, the horizontal displacement of the wall body is very obvious in the second part of the excavation, and the maximum accumulated horizontal displacement of the enclosure body is 15.09mm until the bottom of the pit is excavated;
according to the monitoring report, the deep horizontal displacement monitoring report of the enclosure body has the maximum horizontal displacement of 15.19mm (positive value represents displacement towards the foundation pit direction, negative value represents displacement away from the foundation pit direction), the horizontal displacement simulation value of the enclosure body is 15.04mm, the simulation value is 0.15mm different from the monitoring report, the parameters used by the geometric model are considered to be approximately consistent with the actual value, the horizontal displacement rule of the enclosure structure is distributed in a parabolic shape by combining the monitoring data and the simulation data, the result obtained by professional computing software is slightly smaller than the actual value of a standard section, but the influence range is smaller than the actual measured value, the surface deformation is influenced by complex factors and is a very complex nonlinear process, the PLAXIS2D/3D software does not consider the influences, so that the deviation between the computing result and the actual measured data exists, the simulation data is slightly smaller than the actual deformation data, and the computing data plays a role in predicting the actual engineering, the design optimization of the foundation pit can be guided; the simulated value of the horizontal displacement of the pile along the depth direction is shown in fig. 13, and the comparison of the simulated value and the monitored value is shown in fig. 14;
4. verifying stress and displacement of engineering pile serving as support pile
In order to further explore the stress and displacement conditions of the engineering piles serving as the support piles, a verification model is established when two adjacent engineering piles serve as the support piles, and the size of the verification model is unchanged; when two adjacent engineering piles are used as supporting piles, the horizontal displacement of the piles in the depth direction is as shown in fig. 15, at the moment, the maximum horizontal displacement of the pile top reaches 15.59mm, and the displacement is increased by 0.40mm compared with the displacement of a single engineering pile which is used as a supporting pile;
through the simulation and verification of the steps 1 to 4, the following results are obtained:
(1) through the comparison direction of the numerical value and the measured data, the difference between the monitored data and the simulated data is not large, and the numerical simulation mode is shown, so that the applicability in foundation pit engineering is good;
(2) the rock-socketed depth has a large influence on the bearing capacity of the rock-socketed pile, when the rock-socketed depth is increased, the settlement of the pile top is obviously reduced, and the bearing capacity of the pile is obviously improved;
(3) when the bearing stratum of the rock-socketed pile is not in a horizontal state, along with the increase of the inclination of the rock stratum, the settlement of the pile top under the same load action is correspondingly reduced, the bearing capacity of the pile top is obviously improved, but when the inclination of the rock stratum exceeds 30 degrees, the settlement of the pile top is increased to some extent when the inclination is larger, and the bearing capacity is correspondingly reduced;
(4) the bearing capacity of the pile is greatly influenced by the softness of a bearing layer of the socketed pile, and when the elastic modulus of the bearing layer is reduced, the bearing capacity of the pile is obviously reduced;
(5) when the verticality variation range is small, the influence of the verticality of the pile body on the settlement amount of the pile top and the bearing capacity of the pile is small, and when the verticality variation of the pile body is within 1%, the bearing capacity is slightly reduced; when the verticality changes by 2-3%, the bearing capacity slightly rises; after the verticality changes by more than 3%, the bearing capacity begins to be reduced again;
(6) when sediment is generated at the bottom of the pile, the sediment amount at the top of the pile is obviously increased, the bearing capacity of the pile is correspondingly reduced, but when the thickness of the sediment at the bottom of the pile is changed within 10-40 mm, the sediment amount at the top of the pile and the bearing capacity are changed slightly;
(7) the maximum accumulated settlement of soil outside the foundation pit is 15.36mm along with the deepening of excavation, the maximum horizontal displacement of the enclosure structure is at the top, the horizontal displacement reaches 15.09mm, and when two adjacent engineering piles are used as supporting piles together, the pile top displacement is slightly increased compared with that of a single pile;
5. excavation soil layer foundation pit
Excavating a soil foundation pit according to the soil layer numerical value simulated in the step 1 and in combination with the actually measured data, and monitoring soil layer change data in the foundation pit in real time in the excavation process;
6. building foundation pit supporting structure
In the process of excavation of a foundation pit, a foundation pit supporting structure is built, a foundation pit supporting structure system comprises a cast-in-situ bored pile, a triple-pipe high-pressure jet grouting pile, a net jet release slope and a steel pipe soil nail wall, the triple-pipe high-pressure jet grouting pile is used for stopping water in an area with larger excavation depth of the foundation pit engineering, a water interception ditch is arranged outside the foundation pit, and a drainage well and a water collection well are used for draining water in the foundation pit;
7. setting engineering pile as support pile
And (3) selecting the engineering pile in the foundation pit supporting structure built in the step (6) according to the bearing capacity of the rock-socketed pile simulated in the step (2) and the influence of the engineering pile obtained by analyzing in the step (3) and serving as a factor of the supporting pile on the bearing capacity, taking the selected engineering pile as the supporting pile, and detecting when the perpendicularity of the pile bottom and the pile body of the engineering pile cannot meet the requirement of the simulation data so as to find out the reason influencing the requirement of the simulation data.
Claims (5)
1. A construction process for taking an engineering pile as a support pile is characterized by comprising the following specific steps:
1. soil layer numerical simulation
1) Constructing soil constitutive model
Firstly, collecting and sampling a soil stratum of a to-be-built object, and inputting soil stratum collecting data in PLAAXIS 2D/3D software to construct a soil constitutive model;
2) simulating initial stress field
Simulating an initial stress field in the soil body constitutive model, wherein the initial stress is an existing ground stress field in a rock body before excavation of the foundation pit;
3) physical unit simulation
Simulating entity units in the soil constitutive model;
4) board unit simulation
Simulating a plate unit in the soil body constitutive model, wherein the plate unit is formed by different nodes on the same plane;
5) simulating excavation process
Simulating an excavation process in the soil body constitutive model, and constructing step by step;
2. bearing capacity simulation of rock-socketed pile
I) Building rock-socketed pile bearing capacity model
Taking a proposed geotechnical engineering investigation outline as a reference, comprehensively considering the peripheral conditions of foundation pit engineering, and establishing a simplified rock-socketed pile bearing capacity model;
II) geometric model building and parameter setting
Taking the bearing capacity model of the socketed pile in the step I) as a basis, taking a single pile as a reference, establishing a geometric model according to the Saint-Vietnam principle, and setting boundary conditions and loads: the directions x and y are limited, the direction z is free, and the load is arranged on the pile top and faces to the direction z;
III) calculating and analyzing the simulation conditions
Carrying out working condition analysis after realizing numerical simulation in PLAAXIS 2D/3D software through an activation unit;
3. simulating and analyzing factors of influence of engineering pile and supporting pile on bearing capacity
Establishing the geometric model again and setting parameters, wherein the parameters of the geometric model are the same as those of the step II), and then setting boundary conditions and loads: constraining two sides of the geometric model in the non-foundation pit direction to enable the geometric model to be fixed in the y direction and free in other directions;
4. verifying stress and displacement of engineering pile serving as support pile
Establishing a verification model when two adjacent engineering piles are used as supporting piles, wherein the verification model is the same as the geometric model in the step 3 in size and has unchanged size, and when the two adjacent engineering piles are used as the supporting piles, the pile top displacement is slightly increased compared with that of a single pile;
5. excavation soil layer foundation pit
Excavating a soil foundation pit according to the soil layer numerical value simulated in the step 1 and in combination with the actually measured data, and monitoring soil layer change data in the foundation pit in real time in the excavation process;
6. building foundation pit supporting structure
In the process of excavation of the foundation pit, a foundation pit supporting structure is built;
7. setting engineering pile as support pile
And (3) selecting the engineering pile in the foundation pit supporting structure built in the step (6) according to the bearing capacity of the rock-socketed pile simulated in the step (2) and the influence on the bearing capacity of the engineering pile obtained by analyzing in the step (3), wherein the selected engineering pile is used as the supporting pile, and meanwhile, the perpendicularity of the pile bottom and the pile body of the selected engineering pile meets the requirement of simulation data.
2. A construction process using an engineering pile as a support pile according to claim 1, wherein in step 1, the ground stress field is in a relatively stable and balanced state.
3. A construction process for an engineering pile as a support pile according to claim 1, wherein in the step 1, the solid units are simulated by adopting triangular units with 6 nodes or 15 nodes.
4. The construction process of an engineering pile as a support pile according to claim 1, wherein in the step 2, during simulation analysis, an initial soil layer stage is established first, constraints are applied to the periphery of the soil layer, then each initial soil layer is activated according to specific construction steps, pile units are activated, and then next load application is performed until PLAAXIS 2D/3D software shows that the soil layer is about to collapse.
5. The construction process of an engineering pile as a support pile according to claim 1, wherein in the step 6, the foundation pit supporting structure system comprises a cast-in-situ bored pile, a triple-pipe high-pressure jet grouting pile, a net jet slope and a steel pipe soil nailing wall, the triple-pipe high-pressure jet grouting pile is adopted to stop water in the area with the larger excavation depth of the foundation pit engineering, a water intercepting ditch is arranged outside the foundation pit, and a drainage well and a water collecting well are adopted to drain water in the foundation pit.
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| CN114528687A (en) * | 2021-12-28 | 2022-05-24 | 中国地质大学（北京） | Method and device suitable for calculating rock-socketed depth of bridge pile in mining subsidence area |
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| JP2005290888A (en) * | 2004-04-01 | 2005-10-20 | Giken Seisakusho Co Ltd | Press fitting support pile as well as execution and design methods thereof |
| CN103255771A (en) * | 2013-05-24 | 2013-08-21 | 山西太行建设开发有限公司 | Method for excavating super-deep foundation pit engineering piles in graded mode and graded composite supporting of supporting piles |
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