Method for predicting pile-side limit lateral resistance of cast-in-situ bored pile suitable for soft rock and hard soil

Abstract

The invention provides a bored pile side limit lateral resistance prediction method suitable for soft rock and hard soil, and belongs to the technical field of pile foundation construction. According to the method, firstly, the two-dimensional morphology of the hole wall after mechanical hole forming is measured through a hole diameter instrument, then the fluctuating two-dimensional morphology is reconstructed through a large number of right triangle elements, finally, the reconstructed two-dimensional morphology is decomposed into a large number of right triangle elements through linear approximation, and roughness parameters are extracted from the right triangle elements. Comprising independent relative height and inclination angle, analyzing a single right triangle element based on an energy conservation principle, and determining local side resistance which can be provided by the single right triangle element. And finally, estimating the macroscopic side resistance on the whole contact surface by accumulating and summing the local side resistance and the nominal contact area generated by all the right triangle elements. According to the method, a new technical means is provided for quantitatively predicting the limit lateral resistance of the cast-in-place pile considering the influence of the hole wall roughness, and the scientificity and reliability of pile foundation design can be improved.

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 suitable for soft rock and hard soil. Background Art

[0002] As a common choice for bridge foundations, bored piles often need to pass through some extremely hard clay layers or medium to strong weathered rock layers (such as shale and mudstone) until they enter the hard bearing layer. This type of stratum usually has strong cohesion and can be defined as a transition material from soil to rock in terms of its mechanical properties, 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 comes into play 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 the action of vertical load, the undulating contact surface will expand circumferentially, and its expansion is constrained by the stratum outside the hole wall. In other words, the 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. In addition, during the circumferential expansion process, the stratum constraint force outside the hole wall can be abstracted as the normal stress perpendicular to the contact surface. According to the basic principles of tribology, the exertion of pile side resistance is proportional to the normal stress. Obviously, both the roughness of the hole wall and the material properties of the soft rock and hard soil are key parameters that affect the exertion of pile side resistance.

[0004] At present, the current Technical Specification for Building Pile Foundations (JGJ 94-2008) in my country stipulates that when designing the ultimate bearing capacity of a single cast-in-place pile, the pile side resistance should be estimated based on the mechanical indicators of geotechnical materials. When cast-in-place piles placed in general soil lack on-site static penetration data, the ultimate side resistance of each layer can be estimated according to the soil properties, and then the standard values ​​of the ultimate bearing capacity in the strata at different depths can be linearly summed. As a simplified method, the physical meaning of each parameter of the method recommended by the specification is clear and convenient for engineering application; however, as a limit state, the standard value of the ultimate side resistance is difficult to fully reflect the mechanism of the side resistance of the surrounding rock before failure (or yielding), so this method does not consider the influence of the relative displacement of the pile-rock interface caused by the compression of the pile body and the settlement of the strata on the pile side on the degree of side resistance. Therefore, when the relative displacement of the pile-soil (rock) interface is small, the specification method may overestimate the pile side resistance. In summary, the specification method essentially only considers the influence of the mechanical properties of geotechnical materials on the side resistance, but does not consider the influence of the roughness of the hole wall. The essence of this is that the physical mechanism of the shear process at the contact surface between the pile side and the hole wall is ignored.

[0005] Related studies have shown that the ultimate bearing capacity can be increased nearly three times by simply increasing the surface roughness compared to the case of smooth pile sides. For soft rock formations, due to their strong plasticity, it is easier to form a stable serrated hole wall during the drilling process. However, existing studies basically obtain the qualitative relationship between hole wall roughness and pile side resistance through experimental research, and do not directly quantify the relationship between the two through geometric and mechanical models, which makes the prediction of pile side resistance lack a certain degree of universality.

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

[0007] The present invention provides a method for predicting the ultimate side resistance of bored cast-in-place piles in soft rock and hard soil. The aperture size of the cross section at any depth after the hole is formed is measured by a boremeter, and based on some approximate analysis, a simplified two-dimensional rough morphology and roughness parameters of the hole wall are obtained, and mechanical modeling is used to estimate the pile side resistance, so that the prediction of the pile side resistance can be universal, which can effectively solve at least one technical problem in the background technology.

[0008] In order to solve the above technical problems, the present invention is implemented as follows: a method for predicting the ultimate side resistance of bored piles suitable for soft rock and hard soil, comprising the following steps:

[0009] Step S1, selecting multiple reference cross sections along the depth direction of the borehole, measuring the borehole diameter at each reference cross section, establishing a plane rectangular coordinate system with the borehole axis as the first coordinate axis and the ground horizontal line as the second coordinate axis, marking the coordinate point formed by each depth-diameter combination in the plane rectangular coordinate system, and sequentially connecting multiple discrete points in the coordinate system along the increasing direction of the reference cross section depth to form a continuous broken line, and using the broken line to represent the two-dimensional morphology of the borehole wall;

[0010] Step S2, using the line connecting two adjacent discrete points as the hypotenuse, the projection of the line connecting two adjacent discrete points in the horizontal direction as the first right-angled side, and the projection of the line connecting two adjacent discrete points in the vertical direction as the second right-angled side, constructing a right-angled triangle element, and based on the principle of continuum mechanics, using the sum of the local lateral resistances generated by all right-angled triangle elements to equivalently characterize the lateral resistance of the cast-in-place pile;

[0011] Step S3, averaging the nominal contact area of ​​the pile-soil through the lateral resistance to obtain the unit lateral resistance, obtaining the extreme value of the unit lateral resistance, and constructing a prediction model of the maximum lateral resistance based on the extreme value of the unit lateral resistance;

[0012] Step S4: for any bored pile construction in soft rock and hard soil, the prediction model is used to predict the maximum side resistance of the bored pile.

[0013] As a preferred improvement, the following steps are also included before step S1:

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

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

[0016] As a preferred improvement, in step S1, "selecting multiple reference cross sections along the depth direction of the borehole and determining the borehole diameter at each reference cross section" specifically includes the following process:

[0017] A unit depth is selected and multiple reference cross sections are chosen along the depth direction of the borehole. A reference cross section is added for each additional unit depth of the borehole. The aperture values ​​of each reference cross section in eight evenly distributed directions are 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.

[0018] As a preferred improvement, in step S2, "based on the principle of continuum mechanics, the lateral resistance of the bored pile is equivalently characterized by the sum of the local lateral resistances generated by all right-angled triangle elements" specifically includes the following process:

[0019] From a microscopic perspective, the i-th right-angled triangle element and the i-th pile unit are considered as a system. When the contact surface between the right-angled triangle element and the pile unit slides upward along the inclination of the right-angled triangle element, 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; during the relative sliding of the contact surface, the total lateral resistance S i There are three frictional resistance components to overcome, namely: the lateral expansion of the pile unit dy i The normal stress N caused i The frictional resistance component of the work done is denoted by Overcoming the surface friction of sliding upward along the hypotenuse of the right triangle element is denoted by The frictional force that overcomes the sliding of the horizontal plane is expressed as

[0020]

[0021] Where φ is the external friction angle between pile and soil; ω i Represents the inclination angle value of the i-th right triangle element; rearranging the above formula, we can get:

[0022] S i =N i tan(φ+ω i );

[0023] 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 converted into:

[0024] τ i =σ i tan(φ+ω i );

[0025] In the formula, τ i represents the unit lateral resistance that can be generated by the i-th right triangle element; σ i represents the lateral restraint force on the i-th pile unit;

[0026] From a macroscopic perspective, the lateral restraint force caused by the borehole wall on the pile expansion of the cast-in-place pile can be expressed by the 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:

[0027] σ=σ0+K·y;

[0028] In the formula, σ represents the lateral restraint force caused by the hole wall of the bored pile on the pile expansion y; K represents the shear stiffness of the spring; σ0 represents the initial lateral restraint force, and its magnitude is determined by the degree of stress release during the excavation process;

[0029] 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. The lateral constraint force increment Δσ is obtained according to the cylinder expansion solution and is expressed as:

[0030]

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

[0032] but:

[0033]

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

[0035]

[0036] The total lateral resistance provided by the i-th right triangle element can be expressed by the unit lateral resistance τ provided by the right triangle element i The actual contact area is obtained by calculating the horizontal projection area a of the actual contact area of ​​the right triangle element during shear sliding. i As the pile settlement x increases, it decreases continuously. The relationship between the two is expressed as:

[0037] a i =ΔH-x;

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

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

[0040]

[0041] The total lateral resistance S of the system is expressed as:

[0042]

[0043] Where n represents the number of right triangle elements.

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

[0045]

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

[0047] Q=u∑τ jmax l j ;

[0048] Where, u represents the circumference of the cast-in-place pile; τ jmax represents the extreme value of the unit lateral resistance of the jth soil layer; l j corresponds to the depth of the soil layer.

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

[0050] (1) A large number of right-angled triangle elements are used to reconstruct the measured hole wall morphology, and the measured continuous undulating contour curve is divided into small segments. The secant slope of each segment is then used to establish a continuous broken line to approximate the curvature change of the curve. This approximation method effectively reduces the error caused by direct straight line approximation and improves the accuracy and reliability of contour reconstruction;

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

[0052] (3) It can predict the limit value of the side resistance of cast-in-place piles under different stratum mechanical properties, different pile body geometries and material properties, and different hole wall roughness. It has universal application value and can provide a certain theoretical reference for the preliminary design of pile foundations and the optimization of pile length and diameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work, among which:

[0054] Figure 1 A schematic diagram showing the pile-soil contact surface when the pile is formed;

[0055] Figure 2 Schematic diagram showing the pile-soil contact surface after service;

[0056] Figure 3 A rough two-dimensional topography image showing the borehole wall;

[0057] Figure 4 Schematic diagram representing the quantification of hole wall roughness parameters;

[0058] Figure 5 The force analysis diagram of the right triangle element at positive inclination is shown;

[0059] Figure 6 The force analysis diagram of the right triangle element at negative inclination;

[0060] Figure 7 Schematic diagram of the constant normal stiffness model;

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

[0062] Fig. 9 represents a two-dimensional contour image of the borehole wall after reconstruction in implementation 1;

[0063] Fig.10 A graph showing the relationship between the pile settlement x and the pile side shear stress τ of a bored pile. DETAILED DESCRIPTION

[0064] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0065] Please refer to Figure 1-Figure 10 This embodiment provides a method for predicting the ultimate side resistance of bored piles applicable to soft rock and hard soil, comprising the following steps:

[0066] Step S0, obtaining conventional mechanical parameters of the rock and soil mass in the target area by indoor test and in-situ test methods, wherein the rock and soil mass in the target area is soft rock and hard soil.

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

[0068] Soft rock and 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 continuously changing mechanical properties, the novel classification idea of ​​soft rock and hard soil can overcome the disadvantage that the research objects of soil mechanics and rock mechanics are independent of each other. This geological material is mainly formed by the deposition of the Upper Tertiary System, and partly by the Quaternary System through cementation or weathering. According to the stratum properties within the buried depth of the pile body, it can be divided into two categories: (1) hard soil, generally including gravelly clay, silty clay, hard clay and loess, etc., with an unconfined compressive strength between 0.6 and 2 MPa; (2) soft rock, generally including strongly weathered granite, mudstone, shale and sandstone, etc., with an unconfined compressive strength between 2 and 6 MPa.

[0069] Step S1, select multiple reference cross sections along the depth direction of the borehole in the target area, measure the borehole diameter at each reference cross section, use the borehole axis as the first coordinate axis and the ground horizontal line as the second coordinate axis to establish a plane rectangular coordinate system, mark the coordinate points formed by each depth-diameter combination in the plane rectangular coordinate system, and connect multiple discrete points in the coordinate system in sequence along the increasing direction of the reference cross section depth to form a continuous broken line, which is used to characterize the two-dimensional morphology of the borehole wall.

[0070] In order to ensure the uniformity and accuracy of the aperture measurement, it is necessary to select a unit depth ΔH. For each additional unit depth ΔH of the borehole, a reference cross section is added. The aperture value of each reference cross section in 8 evenly distributed directions (each direction is 45° apart) is measured using an aperture measuring instrument, and the mathematical average of the measured values ​​in the 8 directions is taken as the average aperture at the depth. If there is a value with a range greater than 30% in the measured values ​​in the 8 directions, it will be eliminated and the remaining measured values ​​will be arithmetic averaged. The aperture measuring instrument can adopt the conventional structure in this field, and this embodiment does not limit this. Through this method, the influence of the hole wall collapse in individual directions caused by the possible errors of the drilling equipment or the unevenness in the soil layer on the measurement results can be effectively reduced, thereby ensuring the accuracy and reliability of the measurement results.

[0071] In theory, the borehole diameter is equal everywhere, but in actual construction scenarios, due to the limitations of drilling equipment, the borehole wall is often undulating and jagged, resulting in a certain deviation between the borehole diameter at different depths and the standard diameter. Therefore, this application measures the diameter at different depths to restore the deviation of the borehole diameter in actual construction scenarios, and connects different depth-diameter points in sequence to obtain an approximate two-dimensional topography of the borehole wall.

[0072] In order to make the rough two-dimensional topography of the borehole wall as close to the actual topography as possible, the value of the unit depth ΔH needs to be as small as possible, but too small a value of the unit depth ΔH will increase the measurement amount and affect the efficiency of the aperture measurement. After comprehensive consideration, the unit depth ΔH selected in this embodiment is 50 mm, that is, starting from the ground plane, a reference cross section is added for every 50 mm increase in the drilling depth.

[0073] Furthermore, after the aperture values ​​on all reference cross sections are measured, the point with the smallest aperture value among all discrete points is taken as the reference point, and the difference between the aperture values ​​of other discrete points and the reference point is taken as the relative height of the discrete point. The relative heights of all other discrete points are calculated. The relative height of the discrete point corresponding to the i-th reference cross section is expressed as h i , where i is a natural number from 1 to n, and n represents the total number of reference cross sections within the burial depth range.

[0074] The relative height value of the reference point is assumed to be zero, which serves as the reference point for the relative heights of all other discrete points. The relative heights of each discrete point on the two-dimensional profile relative to the reference point can be obtained. Through these relative height values, the roughness of each reference section can be more intuitively seen, and the secant slope of each line segment on the polyline can be quickly and accurately determined. This not only simplifies the data processing process, but also improves the measurement of the sensitivity of curve changes, so that we can more accurately capture the slight changes in the contact surface.

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

[0076] After determining the two-dimensional contour of the contact surface, we use a step-by-step approach to refine the analysis. The entire contact surface is divided into n continuous segments at equal intervals of unit depth, arranged from top to bottom. In this way, the contour curve of the contact surface intersects with these equidistant auxiliary lines to form n+1 intersections. Then, starting from a set reference point, these intersections are connected in sequence to form n right-angled triangle elements. In this way, we can measure the inclination of the hypotenuse and the height relative to the reference point from any right-angled triangle element. The core idea of ​​this method is to use the secant slope of the tiny segment of the contact surface contour 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 straight line approximation and improves the accuracy and reliability of contour reconstruction.

[0077] The surface of the borehole wall serves as the contact surface between the borehole and the bored pile, and is directly affected by the side resistance of the bored pile. Therefore, the present application performs a force analysis on the broken line obtained in step S2. Based on the principle of continuum mechanics, each segment of the broken line can be equivalent to a right-angled triangle element, and the local side resistance of each right-angled triangle element can be used to characterize the side resistance of the bored pile at the local position corresponding to the hypotenuse. Therefore, the present application uses the sum of the local side resistances generated by all right-angled triangle elements to equivalently characterize the side resistance of the bored pile.

[0078] The hole wall morphology is reconstructed through a series of right-angled triangle elements, with the aim of simplifying the two-dimensional form of the contact surface to make it easier to carry out mechanical modeling. The degree of friction exerted by a single element during relative sliding between the pile and the soil is determined by the value of the triangle inclination angle, and the specific expression of the friction coefficient is derived based on the principle of energy conservation, which is closely related to the size and positive and negative of the triangle inclination angle.

[0079] For each right triangle element, to perform force analysis, we first need to determine the angle between the hypotenuse and the second right-angled side. This angle is defined as the inclination angle. When the hypotenuse rotates counterclockwise relative to the second right-angled side to form an inclination angle, the inclination angle is defined as a positive inclination angle; when the hypotenuse rotates clockwise relative to the second right-angled side to form an inclination angle, the inclination angle is defined as a negative inclination angle. The inclination angle value is represented by ω.

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

[0081]

[0082] In the formula, h i+1 Represents the relative height of the discrete point corresponding to the i+1th reference cross section.

[0083] In order to simplify the analysis, the force analysis of the right-angled triangle element is carried out without considering the self-weight of the right-angled triangle element. The process is as follows:

[0084] (1) When the inclination angle is positive, for the convenience of analysis, the right triangle element is rotated 90° and a force analysis diagram of the shear process is drawn as follows: Figure 5 shown.

[0085] From a microscopic perspective, the right-angled triangle element corresponding to the i-th reference cross section is defined as the i-th right-angled triangle element, and the pile body area that interacts with the i-th right-angled triangle element is defined as the i-th pile body unit. The i-th right-angled triangle element and the i-th pile body unit are analyzed as a system 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 shaft 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 of the pile unit dy i The normal stress N caused i The frictional resistance component of the work done is denoted by Overcoming the surface friction of sliding upward along the hypotenuse of the right triangle element is denoted by The frictional 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, therefore:

[0089]

[0090] The deformation can be obtained:

[0091]

[0092] also, YesS i The surface friction generated by the additional normal constraint during the upward sliding 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 pile and soil.

[0095] is the surface friction force of horizontal plane sliding 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] Rearranging the above formula yields:

[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 converted into:

[0102] τ i =σ i tan(φ+ω i );

[0103] In the formula, τ i represents the unit lateral resistance that can be generated by the i-th right triangle element; σ i represents the lateral constraint force on the i-th pile unit.

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

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

[0106] τ i =σ i tan(φ+ω i ).

[0107] From a macroscopic perspective, the lateral restraint force caused by the right-angled triangle element on the pile expansion can be expressed 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:

[0108] σ=σ0+K·y;

[0109] Where σ represents the lateral restraint force caused by the right triangle element on the pile expansion y of the bored pile; K represents the shear stiffness; σ0 represents the initial lateral restraint force, and its magnitude is determined by the degree of stress release during the excavation process.

[0110] 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. The lateral constraint force increment Δσ of the contact surface is obtained according to the cylinder expansion solution, which is expressed as:

[0111]

[0112] In the formula, E represents the elastic modulus of the rock and soil; v represents the Poisson's ratio of the rock and soil; r2 represents the pile diameter of the cast-in-place pile, r2=r1=r, r1 represents the diameter of the drilled hole, and its value is r;

[0113] And y = xtanφ, x represents the settlement of the pile body, then:

[0114]

[0115] At the same position, whether analyzed from a macroscopic perspective or from a microscopic perspective, the magnitude of the lateral restraint force remains consistent, that is, σ i =σ, therefore, the unit lateral resistance generated by the i-th right triangle element can also be expressed as:

[0116]

[0117] The total lateral resistance provided by the i-th right triangle element can be expressed by the unit lateral resistance τ provided by the right triangle element i The actual contact area is obtained by calculating the horizontal projection area a of the actual contact area during the shear sliding process of the right triangle element. i As the pile settlement x increases, it decreases continuously. Figure 8 It can be seen that the horizontal projection area a of the actual contact surface of the right triangle element i is a linear function of pile settlement, expressed as:

[0118] a i =ΔH-x;

[0119] The total lateral resistance S that the i-th right triangle element can provide is i It can be written as:

[0120]

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

[0122]

[0123] Step S3, averaging the lateral resistance through 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 of the maximum lateral resistance based on the extreme value of the unit lateral resistance.

[0124] The unit lateral resistance τ can be obtained by averaging the nominal contact area l (i.e. the depth of the pile in the soil layer):

[0125]

[0126] From the above expression, it can be seen that the unit lateral resistance τ is a quadratic function of the pile settlement x, and there must be an extreme point τ max , using the extreme point τ max To predict the maximum lateral resistance Q that the cast-in-place pile can exert: Q = u∑τ jmax l j ;

[0127] Where, u represents the circumference of the cast-in-place pile; τ jmax is the extreme value of the unit lateral resistance of a certain j soil layer; l j corresponds to the depth of the soil layer.

[0128] The above prediction formula can take into account the situation where there are multiple different soil layers within the buried depth of the pile body.

[0129] Step S4: for any bored pile construction in soft rock and hard soil, the prediction model is used to predict the maximum side resistance of the bored pile.

[0130] Example 1

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

[0132] After drilling, the basic roughness parameters of the hole wall after drilling were measured by the boremeter and are shown in Table 1. According to the relative height value, the reconstructed two-dimensional hole wall profile is as follows Figure 1 shown.

[0133] Table 1 Relative height and inclination of each counting point on the hole wall after drilling

[0134]

[0135]

[0136] Substitute the data in Table 1 into the expression of τ to estimate the unit lateral resistance when the settlement x = 0, and then gradually increase the value of x to obtain the corresponding unit lateral resistance under settlement, and draw a curve as follows Fig.10 shown.

[0137] from Fig.10 The maximum unit lateral resistance (shear stress) τ can be obtained from max =1505kPa, and the corresponding pile top settlement is reached when x = 24mm. The prediction formula is used to predict that the ultimate pile side resistance that the test pile can exert is Q = 28.35KN.

[0138] The embodiments of the present invention are described above, but the present invention is not limited to the above-mentioned specific implementation modes. The above-mentioned specific implementation modes are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the purpose of the present invention and the claims, all of which are protected by the present invention.

Claims

1. A method for predicting the ultimate side resistance of bored piles in soft rock and hard soil, characterized in that: The steps include: Step S1, selecting multiple reference cross sections along the depth direction of the borehole, measuring the borehole diameter at each reference cross section, establishing a plane rectangular coordinate system with the borehole axis as the first coordinate axis and the ground horizontal line as the second coordinate axis, marking the coordinate point formed by each depth-diameter combination in the plane rectangular coordinate system, and sequentially connecting multiple discrete points in the coordinate system along the increasing direction of the reference cross section depth to form a continuous broken line, and using the broken line to represent the two-dimensional morphology of the borehole wall; Step S2, using the line connecting two adjacent discrete points as the hypotenuse, the projection of the line connecting two adjacent discrete points in the horizontal direction as the first right-angled side, and the projection of the line connecting two adjacent discrete points in the vertical direction as the second right-angled side, constructing a right-angled triangle element, and based on the principle of continuum mechanics, using the sum of the local lateral resistances generated by all right-angled triangle elements to equivalently characterize the lateral resistance of the cast-in-place pile; Step S3, averaging the nominal contact area of ​​the pile-soil through the lateral resistance to obtain the unit lateral resistance, obtaining the extreme value of the unit lateral resistance, and constructing a prediction model of 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, the prediction model is used to predict the maximum side resistance of the bored pile.

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

3. The method for predicting the ultimate side resistance of bored piles applicable to soft rock and hard soil according to claim 2 is characterized in that: The 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 is characterized in that: In step S1, "selecting multiple reference cross sections along the depth direction of the borehole and determining the borehole diameter at each reference cross section" specifically includes the following process: A unit depth is selected and multiple reference cross sections are chosen along the depth direction of the borehole. A reference cross section is added for each additional unit depth of the borehole. The aperture values ​​of each reference cross section in eight evenly distributed directions are 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.

5. 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 principle of continuum mechanics, the sum of the local lateral resistances generated by all right-angled triangle elements is used to equivalently characterize the lateral resistance of the cast-in-place pile" specifically includes the following process: From a microscopic perspective, the i-th right-angled triangle element and the i-th pile unit are considered as a system. When the contact surface between the right-angled triangle element and the pile unit slides upward along the inclination of the right-angled triangle element, 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; during the relative sliding of the contact surface, the total lateral resistance S i There are three frictional resistance components to overcome, namely: the lateral expansion of the pile unit dy i The normal stress N caused i The frictional resistance component of the work done is denoted by S i1 ; overcome the surface friction of sliding upward along the hypotenuse of the right triangle element, denoted as S i2 ; overcome the plane friction force of horizontal sliding, denoted as S i3 ; S i =S i1 +S i2 +S i3 =N i tanω i +S i tanω i tanφ+N i tanφ; Where φ is the external friction angle between pile and soil; ω i Represents the inclination angle value of the i-th right triangle element; rearranging the above formula, we can get: S i =N i tan(φ+ω) i ); 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 converted into: t i =s i tan(φ+ω) i ); In the formula, τ i represents the unit lateral resistance that can be generated by the i-th right triangle element; σ i represents the lateral restraint force on the i-th pile unit; From a macroscopic perspective, the lateral restraint force caused by the borehole wall on the pile expansion of the cast-in-place pile can be expressed by the 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: σ=σ0+K·y; In the formula, σ represents the lateral restraint force caused by the hole wall of the bored pile on the pile expansion y; K represents the shear stiffness of the spring; σ0 represents the initial lateral restraint force, and its magnitude is determined by the degree of stress release during the excavation process; 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. The lateral constraint force increment Δσ is obtained according to the cylinder expansion solution and is expressed as: Where, E represents the elastic modulus of the rock and soil; v represents the Poisson's ratio of the rock and soil; r2 represents the pile diameter of the cast-in-place pile, r2=r1=r, r1 represents the diameter of the drilled hole, and its value is r; y=xtanφ, x represents the settlement of the pile body; but: At the same position, σ i =σ, therefore, the unit lateral resistance generated by the i-th right triangle element can also be expressed as: The total lateral resistance provided by the i-th right triangle element can be expressed by the unit lateral resistance τ provided by the right triangle element i The actual contact area is obtained by calculating the horizontal projection area a of the actual contact area of ​​the right triangle element during shear sliding. i As the pile settlement x increases, it decreases continuously. The relationship between the two is expressed as: a i =ΔH-x; In the formula, ΔH represents unit depth; The total lateral resistance S that the i-th right triangle element can provide i Written as: The total lateral resistance S of the system is expressed as: Where n represents the number of right triangle elements.

6. The method for predicting the ultimate side resistance of bored piles applicable to soft rock and hard soil according to claim 5 is characterized in that: The unit lateral resistance τ is expressed as:

7. The method for predicting the ultimate side resistance of bored piles applicable to soft rock and hard soil according to claim 6, characterized in that: The prediction model of the maximum pile side resistance is expressed as: Q=u∑τ jmax l j ; Where, u represents the circumference of the cast-in-place pile; τ jmax represents the extreme value of the unit lateral resistance of the jth soil layer; l j corresponds to the depth of the soil layer.

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