A method for predicting the ultimate side resistance of a cast-in-place pile suitable for soft rock and hard soil

Abstract

The present application provides a kind of bored pile side limit side resistance prediction method suitable for soft rock hard soil, belong to pile foundation construction technical field.The present application first uses aperture instrument to measure the two-dimensional topography of hole wall after mechanical hole forming, then reconstructs the fluctuant two-dimensional topography using a large number of right triangle elements, finally linear approximation is used to decompose the reconstructed two-dimensional topography into a large number of right triangle elements and extract roughness parameters from it, including independent relative height and inclination, based on the principle of energy conservation, analyze a single right triangle element to determine the local side resistance it can provide.Finally, by cumulatively summing the local side resistance and nominal contact area generated by all right triangle elements, the macroscopic side resistance on the entire contact surface is estimated.The method of the present application provides a new technical means for quantitatively predicting the limit side resistance of bored pile considering the influence of hole wall roughness, which helps to improve the scientificity and reliability of pile foundation design.

Description

Technical Field

[0001] The invention belongs to the technical field of pile foundation construction, and in particular relates to a method for predicting the ultimate side resistance of bored cast-in-place piles applicable to soft rock and hard soil. Background Art

[0002] Bored cast-in-place piles, a common choice for bridge foundations, often need to penetrate extremely hard clay layers or moderately to strongly weathered rock formations (such as shale and mudstone) until they reach a hard bearing stratum. These formations typically have strong cohesive properties and, based on their mechanical properties, can be defined as a transitional material from soil to rock, collectively referred to as soft rock hard soil.

[0003] Due to the long depth of bored piles (pile length), the side resistance of the pile under vertical load usually takes effect before the end resistance. Therefore, the degree of side resistance will greatly affect the settlement of the pile top, which is also one of the most important control indicators for the normal service of highways and high-speed railways. A large number of engineering practices have also shown that when mechanical drilling is carried out in soft rock and hard soil layers, the rotary drilling of the tool will cause the hole wall to form undulating serrations, and form an undulating contact surface with the poured concrete. Figure 1 and Figure 2 As shown in the figure, under vertical load, the undulating contact surface will expand circumferentially, while this expansion is simultaneously constrained by the strata outside the borehole wall. In other words, the amount of circumferential expansion of the contact surface is determined by the stiffness of the soft rock and hard soil itself; the greater the stiffness, the smaller the expansion. Furthermore, during this circumferential expansion, the constraining force of the strata outside the borehole wall can be abstracted as a normal stress perpendicular to the contact surface. According to the basic principles of tribology, the exertion of pile side resistance is proportional to this normal stress. Clearly, both the roughness of the borehole wall and the material properties of the soft rock and hard soil are key parameters influencing the extent of pile side resistance.

[0004] At present, the current "Technical Code for Building Pile Foundation" (JGJ 94-2008) stipulates that in the design of the ultimate bearing capacity of single pile of cast-in-place pile, the pile side resistance should be estimated based on the mechanical indexes of rock and soil materials. When the cast-in-place pile placed in general soil lacks the data of static sounding, the ultimate side resistance of each layer can be estimated according to the soil stratification, and then the linear summation of the ultimate bearing capacity standard value in the stratum at different depths is obtained. As a simplified method, the parameters of the standard method have clear physical meaning and are convenient for engineering application; however, the ultimate side resistance standard value as a kind of ultimate state is difficult to completely reflect the side resistance development mechanism of the surrounding rock before failure (or yield), so the method does not consider the influence of the relative displacement of the pile body compression and the pile side stratum settlement on the side resistance development degree. Therefore, when the relative displacement of the pile-soil (rock) interface is small, the standard method may overestimate the pile side resistance. In summary, the standard method essentially only considers the influence of the mechanical properties of rock and soil materials on the side resistance, and the influence of the hole wall roughness is not considered. The essence is to ignore the physical mechanism in the shearing process of the pile side and the hole wall contact surface.

[0005] Relevant studies show that compared with the case of smooth pile side, the ultimate bearing capacity can be increased by nearly three times by increasing the surface roughness only. For soft rock stratum, due to the strong plasticity, it is easier to form a stable jagged hole wall during the drilling process. However, the existing research basically obtains the qualitative relationship between the hole wall roughness and the pile side resistance through experimental research, and does not directly quantify the relationship between the two through geometric and mechanical models, so that the prediction of the pile side resistance lacks a certain universality.

[0006] Therefore, it is necessary to provide a method for predicting the ultimate side resistance of the pile side of a cast-in-place pile suitable for soft rock and hard soil to solve the above problems. SUMMARY

[0007] The present application provides a method for predicting the ultimate side resistance of the pile side of a cast-in-place pile suitable for soft rock and hard soil, which measures the hole diameter size of the cross section at any depth after hole forming by a hole diameter instrument, and obtains the simplified two-dimensional rough morphology and roughness parameters of the hole wall according to some approximate analysis, and estimates the pile side resistance through mechanical modeling, so that the prediction of the pile side resistance can have universality, and at least one technical problem in the background art can be effectively solved.

[0008] In order to solve the above technical problems, the present application is implemented as follows: a method for predicting the ultimate side resistance of the pile side of a cast-in-place pile suitable for soft rock and hard soil, comprising the following steps:

[0009] Step S1, a plurality of reference cross sections are selected along the drilling depth direction, the hole diameter of the drilling at each reference cross section is measured, the drilling axis is taken as a first coordinate axis, the ground horizontal line is taken as a second coordinate axis, a plane rectangular coordinate system is established, each depth-hole diameter combination is marked as a coordinate point in the plane rectangular coordinate system, and a continuous broken line is formed by sequentially connecting a plurality of discrete points in the coordinate system in the increasing direction of the reference cross section depth, so as to represent the two-dimensional morphology of the drilling hole wall by the broken line;

[0010] Step S2, a straight line between two adjacent discrete points is taken as a hypotenuse, a projection of the straight line between the two adjacent discrete points in the horizontal direction is taken as a first right angle side, and a projection of the straight line between the two adjacent discrete points in the vertical direction is taken as a second right angle side, to construct a right triangle element, and based on the principle of continuous medium mechanics, the sum of the local side resistance generated by all the right triangle elements is used to equivalently represent the side resistance of the cast-in-place pile;

[0011] Step S3, the side resistance is averaged through the nominal contact area of the pile soil to obtain a unit side resistance, and an extreme value of the unit side resistance is obtained, and a prediction model of the maximum side resistance is constructed based on the extreme value of the unit side resistance;

[0012] Step S4, for any once soft rock hard soil drilling cast-in-place pile construction, the prediction model is used to predict the maximum side resistance of the cast-in-place pile.

[0013] As a preferred improvement, step S1 further includes the following steps:

[0014] Step S0, the conventional mechanical parameters of the rock-soil body in the target area are obtained by indoor test and in-situ test, wherein the rock-soil body in the target area is soft rock hard soil.

[0015] As a preferred improvement, the conventional mechanical parameters of the rock-soil body include elastic modulus, uniaxial compressive strength, internal friction angle, cohesion and Poisson's ratio.

[0016] As a preferred improvement, in step S1, "a plurality of reference cross sections are selected along the drilling depth direction, and the hole diameter of the drilling at each reference cross section is measured", which specifically includes the following process:

[0017] A unit depth is selected, a plurality of reference cross sections are selected along the drilling depth direction, a reference cross section is added every time the drilling increases by a unit depth, a hole diameter measuring instrument is used to measure the hole diameter value in 8 uniformly distributed directions at each reference cross section, and the mathematical average of the 8 direction measurement values is taken as the average hole diameter at the depth.

[0018] As a preferred improvement, in step S2, "based on the principle of continuous medium mechanics, the sum of the local side resistance generated by all the right triangle elements is used to equivalently represent the side resistance of the cast-in-place pile", which specifically includes the following process:

[0019] From the microcosmic point of view, taking the i-th right triangle element and the i-th pile body unit as a system, when the contact surface of the right triangle element and the pile body unit slides upward along the inclination angle of the right triangle element, the total side resistance generated by the system is denoted as S i , the normal stress is denoted as N i , dx i represents the settlement of the i-th pile body unit; dy i represents the lateral expansion of the i-th pile body unit; during the relative sliding of the contact surface, the total side resistance S i needs to overcome three friction resistance components, namely: the friction resistance component that does work on the normal stress N i caused by the lateral expansion dy i of the pile body unit, denoted as the surface friction resistance component that overcomes the sliding along the hypotenuse of the right triangle element, denoted as the plane friction resistance component that overcomes the horizontal plane sliding, denoted as

[0020]

[0021] In the formula, φ represents the external friction angle between the pile and the soil; ω i represents the inclination angle value of the i-th right triangle element; the above formula can be arranged as:

[0022] S i =N i tan(φ+ω i );

[0023] Divide both sides of the equation by the nominal shear area of a single right triangle element to convert it into the stress relationship of the right triangle element, denoted as:

[0024] τ i =σ i tan(φ+ω i );

[0025] In the formula, τ i represents the unit side resistance that the i-th right triangle element can generate; σ i represents the lateral constraint force on the i-th pile body unit;

[0026] From the macroscopic point of view, the lateral constraint force caused by the expansion of the pile body of the cast-in-place pile on the hole wall of the drill hole can be represented by the normal stiffness model, in which the lateral constraint force is provided by a horizontally placed spring, and the normal stiffness model is denoted as:

[0027] σ=σ0+K·y;

[0028] In the formula, σ represents the lateral restraint force of the hole wall of the borehole to the pile body expansion amount y of the cast-in-place pile; K represents the shear stiffness of the spring; σ0 represents the initial lateral restraint force, which is determined by the stress release degree in the excavation process;

[0029] The lateral expansion phenomenon of the cast-in-place pile and the borehole contact surface can be approximately equivalent to the cylindrical expansion model in the field of elasticity, and the lateral restraint force increment Δσ is obtained according to the cylindrical expansion solution, which is represented as:

[0030]

[0031] In the formula, E represents the elastic modulus of the rock-soil body; v represents the Poisson's ratio of the rock-soil body; r2 represents the pile diameter of the cast-in-place pile, r2 = r1 = r, r1 represents the hole diameter, and the value is r; y = x tan φ, x represents the pile body settlement amount;

[0032] Then:

[0033]

[0034] At the same position, σ i = σ, therefore, the unit lateral resistance that the i-th right triangle element can generate can also be represented as:

[0035]

[0036] The total lateral resistance that the i-th right triangle element can provide can be obtained by multiplying the unit lateral resistance τ i that the right triangle element can provide by the actual contact area a i that the right triangle element has in the horizontal direction in the process of shear sliding, and the actual contact area a i is continuously reduced with the increase of the pile body settlement amount x, and the relationship between them is represented as:

[0037] a i = ΔH - x;

[0038] In the formula, ΔH represents the unit depth;

[0039] The total lateral resistance S i that the i-th right triangle element can provide is represented as:

[0040]

[0041] Therefore, the total lateral resistance S of the system is represented as:

[0042]

[0043] In the formula, n represents the number of right triangle elements.

[0044] As a preferred improvement, the unit lateral resistance τ is represented as:

[0045]

[0046] As a preferred improvement, the prediction model of the maximum pile side resistance is represented as:

[0047] Q = u∑τ jmax l j ;

[0048] In the formula, u represents the perimeter of the cast-in-place pile; τ jmax represents the extreme value of the unit side resistance of the jth soil layer; l j is the depth corresponding to the soil layer.

[0049] The beneficial effects of the present application are:

[0050] (1) The measured hole wall morphology is reconstructed by using a large number of right triangle elements, the measured continuous undulating profile curve is finely segmented, and the slope of the secant line of each segment is used to establish a continuous polyline to approximate the curvature change of the curve. This approximation method effectively reduces the error caused by direct linear approximation, and improves the accuracy and reliability of the profile reconstruction.

[0051] (2) A macro-micro combined method is proposed, which decomposes the overall side resistance generated by the macro two-dimensional contact surface into the sum of the micro side resistance generated by a large number of right triangle elements in the shearing process. The micro side resistance generated by a single right triangle element in the shearing process is derived based on the principle of energy conservation, and a simple and clear friction coefficient expression is determined according to the different motion directions (oblique up or oblique down) of the element body. Then, the overall average shear stress of the pile side is obtained by using the averaging method according to the total nominal contact area.

[0052] (3) The limit value of the cast-in-place pile side resistance under different soil mechanical properties, different pile body geometrical and material properties, and different hole wall roughness can be predicted, which has universal application value and can provide certain theoretical reference for the preliminary design of pile foundation and the optimization of pile length and pile diameter. BRIEF DESCRIPTION OF DRAWINGS

[0053] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0054] Figure 1 Fig. 1 shows a schematic diagram of the pile-soil contact surface when the pile is formed;

[0055] Figure 2 Fig. 2 shows a schematic diagram of the pile-soil contact surface after service;

[0056] Figure 3 A coarse two-dimensional profile map representing the borehole wall roughness;

[0057] Figure 4 A quantitative diagram representing the borehole wall roughness parameters;

[0058] Figure 5 A force analysis diagram of a right triangle element at a positive inclination angle;

[0059] Figure 6 A force analysis diagram of a right triangle element at a negative inclination angle;

[0060] Figure 7 A diagram representing a normal stiffness model;

[0061] Figure 8 A diagram representing the actual contact area change of a right triangle element during the sliding process of the pile-soil contact surface;

[0062] Figure 9 A two-dimensional profile map of the borehole wall after reconstruction in Embodiment 1;

[0063] Figure 10 A diagram representing the relationship between the pile body settlement x and the pile side shear stress τ of a cast-in-place pile. DETAILED DESCRIPTION

[0064] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without any creative work fall within the scope of protection of the present application.

[0065] Please refer to Figures 1-10 The present embodiment provides a method for predicting the ultimate lateral resistance of a cast-in-place pile in soft rock and hard soil, which comprises the following steps:

[0066] Step S0: Obtain the conventional mechanical parameters of the rock-soil mass in the target area through indoor tests and in-situ tests, wherein the rock-soil mass in the target area is soft rock and hard soil.

[0067] The conventional mechanical parameters of the rock-soil mass include the elastic modulus, uniaxial compressive strength, internal friction angle, cohesion and Poisson's ratio.

[0068] Soft rock hard soil refers to a special geological material with mechanical properties between soft rock and hard soil. Since soil and rock can be regarded as engineering geological materials with continuous variation in mechanical properties, the novel classification idea of soft rock hard soil can overcome the shortcomings of independent research objects of soil mechanics and rock mechanics. This geological material is mainly formed by Upper Tertiary System deposition, and part of it is formed by cementation or weathering of Quaternary System. According to the stratum properties in the range of pile body burial depth, it can be divided into two categories: (1) hard soil, generally including gravelly clay, silty clay, hard clay and loess, etc., and its unconfined compressive strength is between 0.6-2 MPa; (2) soft rock, generally including strongly weathered granite, mudstone, shale and sandstone, etc., and its unconfined compressive strength is between 2-6 MPa.

[0069] Step S1, a plurality of reference cross sections are selected along the drilling depth direction in the target area, the hole diameter of the drilling at each reference cross section is measured, the drilling axis is taken as a first coordinate axis, the ground horizontal line is taken as a second coordinate axis, a plane rectangular coordinate system is established, each depth-hole diameter combination is marked as a coordinate point in the plane rectangular coordinate system, and a continuous broken line is formed by sequentially connecting a plurality of discrete points in the coordinate system in the increasing direction of the reference cross section depth, so as to represent the two-dimensional morphology of the drilling hole wall.

[0070] In order to ensure the uniformity and accuracy of the hole diameter measurement, a unit depth ΔH is selected, a reference cross section is added every time the drilling increases by a unit depth ΔH, and a hole diameter measuring instrument is used to measure the hole diameter values in 8 uniformly distributed directions (each direction is separated by 45°) at each reference cross section. The mathematical average of the 8 direction measurement values is taken as the average hole diameter at this depth. If the range of the 8 direction measurement values is greater than 30%, the value is excluded and the remaining measurement values are then averaged. The hole diameter measuring instrument can adopt a conventional structure in the art, and the present embodiment does not limit this. By this method, the influence of the hole wall collapse in individual directions caused by the possible errors of the drilling equipment or the non-uniformity of the soil layer on the measurement results can be effectively reduced, thereby ensuring the accuracy and reliability of the measurement results.

[0071] In a theoretical case, the hole diameter of the drilling is equal everywhere, but in an actual construction scene, due to the constraints of the drilling equipment, the hole wall of the drilling often has a wavy sawtooth shape, so that there is a certain deviation between the drilling at different depths and the standard hole diameter. Therefore, the present application measures the hole diameter at different depths to restore the deviation of the drilling hole diameter in the actual construction scene, and sequentially connects the different depth-hole diameter points, so as to obtain an approximate two-dimensional morphology diagram of the drilling hole wall.

[0072] In order to make the rough two-dimensional topography of the borehole wall as close as possible to the actual topography, the value of the unit depth AH needs to be as small as possible, but the value of the unit depth AH is too small, which will increase the measurement and affect the efficiency of the aperture measurement. Considering comprehensively, the unit depth AH selected by the embodiment is 50 mm, that is, from the ground level, the reference cross section is increased by 50 mm for each increase of the drilling depth.

[0073] Further, after the aperture value measurement of all the reference cross sections is completed, the smallest point of the aperture value of all the discrete points is taken as the reference point, and the difference between the aperture values of the other discrete points and the reference point is taken as the relative height of the discrete point. The relative height of all the discrete points is calculated, and the relative height of the discrete point corresponding to the ith reference cross section is represented as hi. i Where i is a natural number from 1 to n, and n represents the total number of reference cross sections in the buried depth range.

[0074] The relative height value of the reference point is assumed to be zero, which is used as the reference point of the relative height of all the remaining discrete points. Thus, the relative height of each discrete point on the two-dimensional profile with respect to the reference point can be obtained. Through these relative height values, the roughness of each reference cross section can be more intuitively seen, and the slope of each line segment on the broken line can also be quickly and accurately determined. This not only simplifies the data processing process, but also improves the sensitivity of the measurement of the curve change, so that we can more accurately capture the small changes of the contact surface.

[0075] In step S2, the line connecting the two adjacent discrete points is taken as the hypotenuse, the projection of the line connecting the two adjacent discrete points in the horizontal direction is taken as the first right angle side, and the projection of the line connecting the two adjacent discrete points in the vertical direction is taken as the second right angle side, to construct a right triangle element. Based on the principle of continuum mechanics, the sum of the local side resistances generated by all the right triangle elements is used to equivalently represent the side resistance of the cast-in-place pile.

[0076] After determining the two-dimensional profile of the contact surface, we use a step-by-step method for detailed analysis. The entire contact surface is divided into n continuous segments according to the equidistant unit depth, arranged from top to bottom. In this way, the profile curve of the contact surface intersects with these equidistant auxiliary lines, forming n+1 intersection points. Then, taking a set reference point as the starting point, the intersection points are connected in turn to form n right triangle elements. In this way, we can measure the inclination of the hypotenuse and the height relative to the reference point from any right triangle element. The core idea of this method is to use the slope of the secant line of the small segment of the contact surface profile curve to approximate the curvature change of the curve, rather than simply using a straight line to approximate the curve. This approximation method effectively reduces the error caused by the straight line approximation, improves the accuracy and reliability of the profile reconstruction.

[0077] The surface of the borehole wall is the contact surface of the borehole and the cast-in-place pile, and is directly affected by the side resistance of the cast-in-place pile. Therefore, the stress analysis of the broken line obtained in step S2 is performed, and based on the principle of continuum mechanics, each segment of the broken line can be equivalent to a right triangle element. The local side resistance of each right triangle element can be used to represent the side resistance of the cast-in-place pile at the corresponding local position of the hypotenuse. Therefore, the side resistance of the cast-in-place pile is equivalent to the sum of the local side resistances of all right triangle elements.

[0078] The hole wall topography is reconstructed by a series of right triangle elements, which aims to simplify the two-dimensional form of the contact surface and make it easier to model the mechanics. The degree of friction exerted by a single element when the pile and soil slide relative to each other is determined by the numerical value of the triangle inclination angle, and a specific friction coefficient expression is derived based on the principle of energy conservation, which is closely related to the size and sign of the triangle inclination angle.

[0079] For each right triangle element, the angle between the hypotenuse and the second right angle is first determined, which is defined as the inclination angle. When the hypotenuse rotates counterclockwise relative to the second right angle to form the inclination angle, the inclination angle is defined as a positive inclination angle. When the hypotenuse rotates clockwise relative to the second right angle to form the inclination angle, the inclination angle is defined as a negative inclination angle. The inclination angle value is represented by ω.

[0080] The inclination angle value ω of the right triangle element corresponding to the i-th reference cross section i is represented as:

[0081]

[0082] where h i+1 represents the relative height of the discrete point corresponding to the i+1-th reference cross section.

[0083] To simplify the analysis, the right triangle element is analyzed without considering its self-weight, and the process is as follows:

[0084] (1) When the inclination angle is positive, the right triangle element is rotated by 90° for analysis, and the force analysis diagram during shearing is shown in Figure 5 .

[0085] From a microscopic perspective, the right triangle element corresponding to the i-th reference cross section is defined as the i-th right triangle element, and the pile body region interacting with the i-th right triangle element is defined as the i-th pile body unit. The i-th right triangle element and the i-th pile body unit are analyzed as follows:

[0086] When the contact surface between the right triangle element and the pile unit slides upward along the inclination angle, the total lateral resistance generated by the system is recorded as S i , the normal stress is recorded as N i , dx i represents the settlement of the i-th pile unit; dy i Represents the lateral expansion of the i-th pile unit.

[0087] During the relative sliding of the contact surfaces, the total lateral resistance S i There are three frictional resistance components to overcome, namely: the lateral expansion dy of the pile unit i The normal stress N caused i The frictional resistance component of the work done is recorded as Overcoming the surface friction of the right triangle element sliding upward along the hypotenuse is denoted as The friction force that overcomes the sliding of the horizontal plane is expressed as

[0088] According to the principle of conservation of energy, The work done in the horizontal direction is equal to N i The work done in the vertical direction is:

[0089]

[0090] The deformation can be obtained:

[0091]

[0092] also, It's S i The surface friction generated by the additional normal constraint during the upward sliding process along the hypotenuse of the right triangle element is It can also be expressed as:

[0093]

[0094] Where φ is the external friction angle between the pile and the soil.

[0095] is the surface friction force when the horizontal plane slides without considering the unit lateral expansion dy, then It can also be expressed as:

[0096]

[0097] According to the balance of horizontal forces, we can know that:

[0098]

[0099] Arranging the above formula, we can get:

[0100] Si = N i tan(φ+ω i );

[0101] Dividing both sides of the equation by the nominal shear area of a single right triangle element, the stress relationship of the right triangle element is transformed into:

[0102] τ i = σ i tan(φ+ω i );

[0103] where τ i represents the unit lateral resistance that the i-th right triangle element can generate; σ i represents the lateral constraint force that the i-th pile element is subjected to.

[0104] (2) When the inclination angle is negative, the force analysis diagram is shown in Figure 6 .

[0105] The analysis process is the same as that for the positive inclination angle, and the obtained stress relationship is also expressed as:

[0106] τ i = σ i tan(φ+ω i ).

[0107] From the macroscopic perspective, the lateral constraint force of the right triangle element caused by the pile body expansion of the cast-in-place pile can be represented by the conventional normal stiffness model, in which the lateral constraint force is provided by a horizontally placed spring, and the conventional normal stiffness model is expressed as:

[0108] σ = σ0+ K·y;

[0109] where σ represents the lateral constraint force of the right triangle element caused by the pile body expansion amount y of the cast-in-place pile; K represents the shear stiffness; and σ0represents the initial lateral constraint force, the size of which is determined by the stress release degree in the excavation process.

[0110] The lateral expansion phenomenon of the contact surface between the cast-in-place pile and the drill hole can be approximately equivalent to the cylindrical expansion model in the field of elasticity, and the contact surface lateral constraint force increment Δσ is obtained according to the cylindrical expansion solution and is expressed as:

[0111]

[0112] where E represents the elastic modulus of the rock-soil mass; v represents the Poisson's ratio of the rock-soil mass; r2represents the pile diameter of the cast-in-place pile, r2= r1= r, r1represents the hole diameter of the drill hole, and the value is r;

[0113] And y = x tan φ, x represents the pile body settlement amount, then:

[0114]

[0115] At the same position, the lateral constraint force is consistent, whether analyzed from a macroscopic perspective or from a microscopic perspective, that is, σ i Therefore, the unit lateral resistance that the ith right triangle element can generate can also be expressed as:

[0116]

[0117] The total lateral resistance that the ith right triangle element can provide can be obtained by multiplying the unit lateral resistance τ i that the right triangle element can provide by the actual contact area A i The actual contact area A Figure 8 of the right triangle element in the horizontal direction is the projection area a i of the actual contact surface of the right triangle element in the horizontal direction, which is a function of the settlement amount x of the pile body, and is denoted as:

[0118] a i = ΔH - x

[0119] Therefore, the total lateral resistance S i that the ith right triangle element can provide can be written as:

[0120]

[0121] Therefore, the total lateral resistance S of the system can be expressed as:

[0122]

[0123] Step S3: Average the lateral resistance through the nominal contact area of the pile soil to obtain the unit lateral resistance, obtain the extreme value of the unit lateral resistance, and construct a prediction model of the maximum lateral resistance using the extreme value of the unit lateral resistance.

[0124] The unit lateral resistance τ can be averaged through the nominal contact area l (i.e., the depth of the pile in the soil layer) to obtain:

[0125]

[0126] As can be seen from the above expression, the unit lateral resistance τ is a quadratic function of the settlement amount x of the pile body, and there must be an extreme value point τ max . The extreme value point τ max is used to predict the maximum lateral resistance Q that the cast-in-place pile can exert: Q = u∑τ jmax l j ;

[0127] In the formula, u represents the circumference of the cast-in-place pile; τ jmax is the maximum value of the unit side resistance of the jth soil layer; l j is the depth corresponding to the soil layer.

[0128] The above prediction formula can consider the case that there are multiple layers of different soil layers within the range of the pile body depth.

[0129] Step S4: For the construction of the cast-in-place pile in soft rock and hard soil, the prediction model is used to predict the maximum side resistance of the cast-in-place pile.

[0130] Embodiment 1

[0131] A concrete cast-in-place test pile of a construction site penetrates soft rock and hard soil from top to bottom, the thickness of the soft rock and hard soil layer l = 5m, the test pile diameter r = 0.6m, and the initial normal stress σ0 = 200kPa when the pile is formed. The basic mechanical parameters of the soft rock and hard soil are obtained through indoor tests, the Poisson's ratio v = 0.24, the basic friction angle φ = 30°, and the elastic modulus E = 232MPa.

[0132] After the drilling is completed, the basic roughness parameters of the hole wall after the hole is formed are measured by a hole diameter instrument, as shown in Table 1. According to the relative height value, the reconstructed two-dimensional hole wall profile is as shown in Figure 1 .

[0133] Table 1: The relative height and inclination angle of each counting point of the hole wall after the hole is formed

[0134]

[0135]

[0136] The data in Table 1 is substituted into the expression of τ, the unit side resistance size when the settlement x = 0 is estimated, and then the value of x is gradually increased to obtain the unit side resistance size under the corresponding settlement, and a curve is drawn, as shown in Figure 10 .

[0137] From Figure 10 , the maximum unit side resistance (shear stress) τ max = 1505kPa, which corresponds to the settlement of the pile top at x = 24mm. The prediction formula is used to predict the limit pile side resistance that can be exerted by the test pile, which is Q = 28.35KN.

[0138] The above describes the embodiments of the present application, but the present application is not limited to the above specific embodiments, and the above specific embodiments are only illustrative but not limiting, and those skilled in the art can make many forms under the inspiration of the present application without departing from the purpose of the present application and the scope protected by the claims, which all belong to the protection of the present application.

Claims

1. A method for predicting the ultimate side resistance of bored piles in soft rock and hard soil, characterized by: The steps include: Step S1: Select multiple reference cross sections along the depth direction of the borehole, measure the borehole diameter at each reference cross section, establish a plane rectangular coordinate system with the borehole axis as the first coordinate axis and the ground horizontal line as the second coordinate axis, mark the coordinate point formed by each depth-diameter combination in the plane rectangular coordinate system, and sequentially connect multiple discrete points in the coordinate system along the increasing direction of the reference cross section depth to form a continuous broken line, and use the broken line to represent the two-dimensional morphology of the borehole wall; Step S2: construct a right triangle element using the line connecting two adjacent discrete points as the hypotenuse, the horizontal projection of the line connecting the two adjacent discrete points as the first right-angled side, and the vertical projection of the line connecting the two adjacent discrete points as the second right-angled side. Based on the principles of continuum mechanics, the sum of the local lateral resistances generated by all right triangle elements is used to equivalently represent the lateral resistance of the cast-in-place pile. Step S3, averaging the lateral resistance by the nominal contact area of ​​the pile and soil to obtain the unit lateral resistance, obtaining the extreme value of the unit lateral resistance, and constructing a prediction model for the maximum lateral resistance based on the extreme value of the unit lateral resistance; Step S4, for any bored pile construction in soft rock and hard soil, using the prediction model to predict the maximum lateral resistance of the bored pile side; In step S1, "selecting multiple reference cross sections along the depth direction of the borehole and measuring the borehole diameter at each reference cross section" specifically includes the following process: A unit depth is selected, and multiple reference cross sections are selected along the depth direction of the borehole. A reference cross section is added for each additional unit depth of the borehole. The aperture value of each reference cross section in eight evenly distributed directions is measured using an aperture measuring instrument. The mathematical average of the measured values ​​in the eight directions is taken as the average aperture at that depth. Unit lateral resistance Expressed as: ； Where, It indicates the nominal contact area between pile and soil, i.e. the depth of pile in the soil layer; Indicates the total lateral resistance of the system; It represents the initial lateral restraint force, the magnitude of which is determined by the degree of stress release during the excavation process; Indicates the diameter of the cast-in-place pile and the diameter of the drilled hole The value of ; represents the Poisson's ratio of rock and soil; E represents the elastic modulus of rock and soil; Indicates the settlement of the pile; represents the external friction angle between pile and soil; Indicates the i The inclination angle value of the right triangle elements; Indicates unit depth; n Represents the number of right triangle elements; The prediction model of the maximum pile side resistance is expressed as: ； Where, Indicates the circumference of the bored pile; Indicates the j The extreme value of the unit lateral resistance of each soil layer; is the depth of the corresponding soil layer.

2. The method for predicting the ultimate side resistance of bored piles in soft rock and hard soil according to claim 1, characterized in that: The following steps are also included before step S1: Step S0: obtaining conventional mechanical parameters of the rock and soil in the target area by indoor tests and in-situ tests, wherein the rock and soil in the target area is soft rock and hard soil.

3. The method for predicting the ultimate side resistance of bored piles in soft rock and hard soil according to claim 2, characterized in that: Conventional mechanical parameters of rock and soil include elastic modulus, uniaxial compressive strength, internal friction angle, cohesion and Poisson's ratio.

4. The method for predicting the ultimate side resistance of bored piles applicable to soft rock and hard soil according to claim 1, characterized in that: In step S2, "based on the principles of continuum mechanics, the lateral resistance of the bored pile is equivalently represented by the sum of the local lateral resistances generated by all right-angled triangle elements" specifically includes the following process: From a microscopic perspective, i The right triangle element and the i The pile body unit is a system. When the contact surface between the right triangle element and the pile body unit slides upward along the inclination of the right triangle element, the total lateral resistance generated by the system is recorded as , the normal stress is recorded as ， Indicates the i Settlement of each pile unit; Indicates the i The lateral expansion of each pile unit; During the relative sliding process of the contact surface, the total lateral resistance There are three frictional resistance components that need to be overcome, namely: the lateral expansion of the pile unit The normal stress caused The frictional resistance component of the work done is recorded as ; overcome the surface friction of the right triangle element sliding upward along the hypotenuse, denoted as ; overcome the plane friction force of sliding on the horizontal surface, denoted as ; ； Arranging the above formula, we can get: ； Dividing both sides of the equation by the nominal shear area of ​​a single right triangle element converts it into the stress relationship of the right triangle element, expressed as: ； Where, Indicates the i The unit lateral resistance generated by the right triangle elements; Indicates the i The lateral restraint force on each pile unit; From a macroscopic perspective, the lateral restraint force caused by the borehole wall on the pile expansion can be represented by a constant normal stiffness model. In the constant normal stiffness model, the lateral restraint force is provided by a horizontally placed spring. The constant normal stiffness model is expressed as: ； Where, Indicates the expansion of the bored hole wall on the pile body induced lateral restraint; represents the shear stiffness of the spring; the lateral expansion phenomenon of the contact surface between the cast-in-place pile and the drilled hole can be approximately equivalent to the cylinder expansion model in elastic mechanics, and the lateral constraint force increment is obtained based on the cylinder expansion solution , expressed as: ； Where, ; but: ； At the same location, , therefore, i The unit lateral resistance generated by the right triangle elements can also be expressed as: ； No. i The total lateral resistance provided by the right triangle elements can be calculated by the unit lateral resistance provided by the right triangle elements. The actual contact area is obtained by calculating the horizontal projection area of ​​the right triangle element during shear sliding. With the pile settlement The relationship between them is expressed as follows: ； No. i The total lateral resistance provided by the right triangle elements Written as: ； The total lateral resistance of the system is Expressed as: ； Where, n Represents the number of right triangle elements.

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