A design method for strengthening foundation with CFG piles in karst areas

Abstract

The present invention discloses a design method for strengthening the foundation with CFG piles in karst areas. First, obtain the diameter range of the top pointed rock, determine the diameter of the CFG pile, calculate the compressive strength under the effective contact area, and calculate the allowable end resistance of the pile tip corrected foundation; calculate the bearing capacity of the CFG pile composite foundation according to the allowable bearing capacity of a single pile, and obtain the settlement amount. Compare the settlement amount with the designed settlement amount, and adjust the parameters to make the result meet the requirements, so as to obtain the optimal layout scheme for the construction of CFG piles. Through reasonable calculation, the design requirements and construction requirements are finally met, and the construction parameters of the CFG pile construction reach the optimal state with reasonable structure and the lowest cost, saving the construction cost. The method of the present invention fully considers the characteristics of the bedrock surface in karst areas, and the proposed design method is closer to the actual situation, greatly improving the reliability of the obtained foundation reinforcement scheme result. This method is convenient to operate and is conducive to popularization and application.

Description

Technical Field

[0001] The present invention relates to the technical field of geotechnical engineering, and particularly relates to a design method for reinforcing a foundation with CFG piles in a karst area. Background Art

[0002] Karst landforms are widely distributed. In the foundation of karst areas, soluble rocks are gradually dissolved and weathered under the repeated action of groundwater. The bedrock surface at the top of the vast majority of the dissolved rocks is in a pointed shape such as bamboo shoot shape and stone forest shape, forming a unique karst landform. For example, the famous Guilin Landscape Scenic Area and Stone Forest Scenic Area are formed after the exposed rocks are weathered and washed by rainwater. In karst areas, the shape of the bedrock surface below the soil foundation is very similar to the shape of the exposed rocks on the surface.

[0003] When building high-speed railways, especially ballastless track high-speed railways, in karst areas, due to the very strict requirements for deformation of high-speed railways, when building subgrade projects, it is usually necessary to reinforce the soil foundation overlying soluble rocks to solve the problem of settlement control of the soil foundation. When the soil foundation overlying soluble rocks is in a hard plastic state, CFG piles (Cement Fly-ash Gravel) are usually used for soil foundation reinforcement. When the depth of the soil foundation is not large or the filling height of the subgrade project on the foundation is relatively large, in order to effectively control the settlement of the soil foundation, the CFG piles generally need to penetrate completely through the soil layer to the bedrock surface. However, since the bedrock surface of the rock below the soil foundation is in a pointed shape, the bottom of the constructed CFG piles cannot form effective contact with the bedrock surface. The existing CFG pile foundation reinforcement method is designed based on the complete and effective contact between the bottom of the CFG piles and the bedrock surface. Therefore, the existing design method cannot adapt to this special foundation condition. At the same time, the pointed structure at the top of the bedrock surface will cause cutting damage to the bearing CFG pile body. The smaller the contact area and the greater the contact eccentricity, the greater the damage to the CFG pile body. In addition, the confining pressure received by the bottom of the CFG piles at different foundation depths is different, and the degree of damage to the bottom pile body of the CFG piles under the action of the pointed damage of the bedrock is also different, which is not considered in the existing design method. Therefore, using the existing design method will cause huge safety risks in the reinforcement project. Therefore, it is necessary to propose a suitable design method for reinforcing a foundation with CFG piles in a karst area. Summary of the Invention

[0004] The purpose of the present invention is to provide a design method for reinforcing a foundation with CFG piles in a karst area in view of the problems that the existing CFG pile foundation reinforcement design method does not consider that the bottom of the CFG piles in the karst area cannot form effective contact with the bedrock surface and the degree of damage to the pile body is different at different foundation depths. This method has important significance and has the characteristics of simple design, convenient operation, safe and controllable, economical and environmentally friendly, and conducive to popularization and application.

[0005] To achieve the above-mentioned invention purposes, the present invention provides the following technical solutions:

[0006] A design method for reinforcing the foundation with CFG piles in karst areas, comprising the following steps:

[0007] Step S1: Obtain the development state of the rocks on the bedrock surface at the top of the soluble rock foundation, obtain the diameter range of the top pointed rocks and the proportion of different diameters, and determine the diameter of the CFG piles;

[0008] Step S2: Calculate the effective contact area between the CFG piles and the top bedrock surface according to the diameter range of the top pointed rocks, the proportion of different diameters, and the diameter of the CFG piles, and obtain the corresponding compressive strength of the CFG piles according to the relationship between the effective contact area and the compressive strength;

[0009] Step S3: Calculate the allowable end resistance of the foundation corrected by the pile tip of the CFG piles according to the compressive strength obtained in Step S2;

[0010] Step S4: Calculate the allowable bearing capacity of a single CFG pile. The formula for the allowable bearing capacity of a single pile is as follows:

[0011]

[0012] θ = 1 - h / H

[0013] In the formula, [P]' is the allowable bearing capacity of a single pile, kN; U is the perimeter of the pile body cross-section, m; q i is the allowable side resistance of the i-th layer of soil around the pile, kPa; l i is the thickness of the i-th layer of soil around the pile, m; l n is the thickness of the n-th layer of soil around the pile, m; q n is the allowable side resistance of the n-th layer of soil around the pile, kPa; A' p is the effective contact area of the CFG pile, m 2 ; q' p is the allowable end resistance of the foundation corrected by the pile tip, kPa; θ is the thickness correction coefficient of the bottommost layer of soil of the CFG pile considering the pointed bedrock surface; h is the average height of the top pointed rocks, m;

[0014] Step S5: Calculate the bearing capacity of the CFG pile composite foundation according to the allowable bearing capacity of a single pile. The formula for the bearing capacity of the composite foundation is as follows:

[0015]

[0016] In the formula, σ sp ' is the bearing capacity of the composite foundation, kPa; η is the contact influence coefficient for non-central contact, m s is the area replacement ratio, and the area replacement ratio is related to the layout method of the CFG piles; A p is the cross-sectional area of the CFG pile body, m2 ; β is the reduction coefficient of the bearing capacity of the soil between piles, and β takes values from 0.75 to 0.95; σ s is the allowable bearing capacity of the soil between piles after treatment, in kPa;

[0017] Step S6: Calculate the composite modulus of the CFG pile reinforcement area;

[0018] Step S7: Calculate the settlement of the soil foundation after CFG pile reinforcement according to the composite modulus;

[0019] Step S8: Obtain the design settlement according to the requirements of the construction soil foundation. Compare and analyze the settlement in Step S7 with the design settlement. If the analysis result meets the requirements, obtain the optimal layout plan for CFG pile construction; if not, return to Step S5 to adjust the layout method of CFG piles, and loop through Steps S5 - S7 until the analysis result meets the design technical requirements and the minimum economic requirements.

[0020] In the process of calculating the allowable bearing capacity of a single pile in the present invention, the soil quality of the soil foundation around the CFG pile is stratified, and different stratifications are distinguished according to the lithology of the soil quality. The stratification situation can be obtained based on geological survey data. The non - central contact influence coefficient refers to the deviation degree between the center of the top pointed rock and the CFG pile. The non - central contact influence coefficient is the eccentricity coefficient, which is obtained from numerical simulation after experiments. The obtained optimal layout plan for CFG pile construction includes the number of CFG piles, pile spacing, and pile arrangement method. When the requirements are not met in Step S8, return to Step S5 to adjust the layout method of CFG piles, mainly by adjusting the pile spacing parameter.

[0021] Furthermore, according to the obtained diameter range of the top pointed rock, the diameter range is divided. When the diameter d of the top pointed rock j is divided into 5 levels, that is, when j = 1, 2, 3, 4, 5 in d j , the corresponding classifications are d 1 <0.1m, 0.1m ≤ d 2 <0.2m, 0.2m ≤ d 3 <0.3m, 0.3m ≤ d 4 <0.4m, 0.4m ≤ d 5 , the CFG pile diameter is set to 0.4m; when the diameter d of the top pointed rock j is divided into 6 levels, that is, when j = 1, 2, 3, 4, 5, 6 in d j , the corresponding classifications are d 1 <0.1m, 0.1m ≤ d 2 <0.2m, 0.2m ≤ d 3 <0.3m, 0.3m ≤ d 4 <0.4m, 0.4m ≤ d 5 <0.5m, 0.5m ≤ d6 , the diameter of the CFG pile is set to 0.5 m.

[0022] Furthermore, the detailed steps of step S2 are as follows:

[0023] Step S201: Calculate the effective contact area based on the diameter range and different diameter ratios of the top pointed rock and the CFG pile diameter. The formula for the effective contact area is as follows:

[0024] A' p = αA p

[0025]

[0026] In the formula, α is the area contact influence coefficient; R is the CFG pile diameter, m, d j is the diameter of the top pointed rock, m; ρ j is the proportion when the diameter of the top pointed rock is d j ;

[0027] Step S202: Obtain the compressive strength under different effective contact areas through triaxial tests, numerically simulate the relationship between the effective contact area and the compressive strength, and obtain the compressive strength corresponding to the effective contact area of the CFG pile according to the relationship between the effective contact area and the compressive strength.

[0028] Even further, obtaining the relationship between the effective contact area and the compressive strength through triaxial tests includes the following steps:

[0029] Step S211: Calculate the horizontal stress of the soil foundation within the range of h~yh at the bottom of the CFG pile. The formula for the horizontal stress of the soil foundation is as follows:

[0030]

[0031] where h is the height of the top pointed rock, y is a constant, δ s is the horizontal stress, kPa; k 0 is the horizontal stress coefficient; δ z is the self-weight stress of the soil foundation, kPa; γ is the average unit weight of the soil foundation, kN / m 3 ; H is the total depth of the soil foundation, m. Among them, k 0 ranges from 0.4 to 0.7. When the soil is soft, k 0 takes a small value. When the soil is hard, k 0Take the larger value. In the above, the range of h to yh at the bottom of the CFG pile refers to the horizontal stress of the soil foundation within the range of h to yh upward calculated based on the bottom of the CFG pile. The greater the horizontal stress, the better the protective effect on the CFG pile, and the less likely the bottom of the CFG pile is to be damaged under the action of the bamboo-shaped bedrock. Therefore, different soil layer thicknesses and strengths have different degrees of protection for the CFG pile.

[0032] Step S212: Use the horizontal stress of the soil foundation as the confining pressure, and obtain the compressive strength of the CFG pile test block under different contact areas at the bottom center through triaxial tests. After fitting, obtain the relationship between the effective contact area and the compressive strength.

[0033] Furthermore, the range of h to yh is 0.5 - 2.0 m, and more preferably, the range of h to yh is 0.5 - 1.0 m. This step considers the damage effect of the pointed bedrock surface on the bottom of the CFG pile. This damage effect is directly related to the magnitude of the confining pressure on the bottom of the CFG pile, and the confining pressure is related to the thickness of the soil foundation above the bedrock surface. Therefore, use the horizontal stress of the soil foundation within the range of h to yh at the bottom of the CFG pile as the confining pressure, so as to obtain the relationship between the effective contact area and the compressive strength. The larger the contact area, the higher the compressive strength of the test block; δ s The greater the confining pressure, the greater the compressive strength of the test block.

[0034] Furthermore, in step S3, the formula for calculating the allowable end resistance of the pile tip corrected foundation is as follows:

[0035] q' p =λq p

[0036] λ=P fi / P f10

[0037] In the formula, λ is the correction coefficient of the allowable end resistance of the pile tip foundation; q p is the allowable pile tip resistance of the rock foundation, kPa; P fi where i = 1, 2, 3,... 10 are the compressive strengths when the contact area ratios are 10%, 20%, 30%,..., 100% respectively, and P f10 is the compressive strength when the area ratio is 100%.

[0038] Furthermore, η takes 0.6 - 0.9.

[0039] Furthermore, when the CFG pile is arranged in a plum blossom shape, m s =R 2 / (1.05D 2 ), when the CFG pile is arranged in a square shape, m s =R 2 / (1.13D 2), where D is the pile spacing of the CFG piles, in m.

[0040] Further, in step S6, the calculation formula for the composite modulus is as follows:

[0041] E' csi = ξE si

[0042] ξ = σ' sp / σ 0

[0043] In the formula, E' csi is the composite modulus of the i-th layer of soil, in MPa; ξ is the bearing capacity and compression modulus improvement coefficient; E si is the compression modulus of the natural soil of the i-th layer, in MPa; σ 0 is the allowable bearing capacity of the natural foundation soil, in kPa.

[0044] Further, in step S7, the calculation formula for the settlement amount is as follows:

[0045]

[0046] In the formula, s is the settlement amount of the composite foundation, in m; △p i is the additional stress increment on the i-th layer of composite soil, in kPa.

[0047] Compared with the prior art, the beneficial effects of the present invention are:

[0048] The design method of the present invention fully considers the uneven effective contact caused by the top pointed shape of the CFG pile and the soluble rock foundation surface, the damage effect of the top pointed rock on the bottom of the CFG pile, the confining pressure protection effect of the horizontal stress of the soil foundation at different depths on the bottom of the CFG pile, and the eccentric contact between the bottom of the CFG pile and the top pointed rock. By step S1, the diameter of the pointed body on the uneven surface of the soluble rock in the karst area is obtained, and then the diameter of the CFG pile is determined, providing a basis for calculating the effective contact area of the CFG pile to reinforce the foundation. In step S2, the calculation of the effective contact area fully considers the influence of the top pointed rocks with different diameters and their proportions, and calculates the compressive strength under the effective contact area. It also considers the contact area between the CFG pile and the bedrock surface and the damage effect at the pile end, that is, calculates the allowable end resistance of the pile end corrected foundation. Step S5 considers the influence of the non-centered contact between the CFG pile and the pointed body on the bedrock surface, calculates the bearing capacity and settlement of the CFG pile composite foundation, compares the settlement with the designed settlement, and adjusts the parameters to make the result meet the requirements, obtaining the optimal layout scheme for the construction of the CFG pile, including the number of CFG piles, pile spacing, and pile arrangement method. By reasonably calculating and determining the optimal layout scheme for the construction of the CFG pile, the obtained foundation reinforcement scheme is the most economical and reasonable one, and the construction parameters of the CFG pile construction reach the optimal state of reasonable structure and lowest cost, saving construction costs. The method of the present invention fully considers the characteristics of the bedrock surface in the karst area, and the proposed design method is closer to the actual situation, greatly improving the reliability of the results of the obtained foundation reinforcement scheme. This method is easy to operate and is conducive to popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a schematic flow chart of the design method for the CFG pile to reinforce the foundation in the karst area;

[0050] Figure 2 It is a schematic cross-sectional view of the CFG pile to reinforce the foundation in the karst area;

[0051] Markings in the figure: 1 - soil foundation, 2 - soluble rock foundation, 3 - CFG pile, 4 - subgrade engineering, A - pointed rock. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] The present invention will be further described in detail below in conjunction with test examples and specific embodiments. However, it should not be understood that the scope of the above-mentioned subject matter of the present invention is limited to the following embodiments. All technologies implemented based on the content of the present invention belong to the scope of the present invention.

[0053] Example 1

[0054] A design method for the CFG pile to reinforce the foundation in the karst area, as Figure 1 shown, includes the following steps:

[0055] Step S1: Obtain the development status of the rock on the bedrock surface at the top of the erodible rock foundation, get the diameter range of the top pointed rocks and the proportion of different diameters, and determine the diameter of the CFG pile.

[0056] The development status of the bedrock surface at the top of the erodible rock foundation is obtained by investigating the ground and excavation-related engineering data in the karst area. The top pointed rocks refer to the rocks whose tops are higher than the top bedrock surface, such as bamboo shoot-shaped rocks and stone forest-shaped rocks. For example, in Figure 1 A, when conducting data investigation on the top pointed rocks, the diameter and height of the top pointed rocks are obtained. Let d j be the diameter of the top pointed rock, and ρ j be the proportion of the top pointed rock with diameter d j , and h be the average height of the top pointed rock.

[0057] According to the obtained diameter range of the top pointed rocks, divide the diameter range. When the diameter d of the top pointed rock j is divided into 5 levels, that is, when j = 1, 2, 3, 4, 5 in j , the corresponding grading is d 1 <0.1m, 0.1m ≤ d 2 <0.2m, 0.2m ≤ d 3 <0.3m, 0.3m ≤ d 4 <0.4m, 0.4m ≤ d 5 , the proportion under different graded diameters is obtained, and the diameter R of the CFG pile is set to 0.4m; when the diameter d of the top pointed rock j is divided into 6 levels, that is, when j = 1, 2, 3, 4, 5, 6 in j , the corresponding grading is d 1 <0.1m, 0.1m ≤ d 2 <0.2m, 0.2m ≤ d 3 <0.3m, 0.3m ≤ d 4 <0.4m, 0.4m ≤ d 5 <0.5m, 0.5m ≤ d 6 , the proportion under different graded diameters is obtained, and the diameter R of the CFG pile is set to 0.5m.

[0058] In engineering, the CFG pile generally adopts diameters of 0.4m and 0.5m. In the actual application process, the diameter of the CFG pile is determined according to the diameter range and proportion of the top pointed rocks. When the diameter of the top pointed rocks is large and the proportion is high, the CFG pile and the bedrock can form better contact, and vice versa.

[0059] Step S2: Calculate the effective contact area between the CFG pile and the top bedrock surface based on the diameter range and different diameter ratios of the top pointed rock and the CFG pile diameter, and obtain the corresponding compressive strength of the CFG pile according to the relationship between the effective contact area and the compressive strength.

[0060] The detailed steps of Step S2 are as follows:

[0061] Step S201: Calculate the effective contact area based on the diameter range and different diameter ratios of the top pointed rock and the CFG pile diameter. The formula for the effective contact area is as follows:

[0062] A' p = αA p

[0063]

[0064] In the formula, A' p is the effective contact area of the CFG pile, m 2 ; A p is the cross-sectional area of the CFG pile, m 2 ; α is the area contact influence coefficient; R is the diameter of the CFG pile, m, d j is the diameter of the top pointed rock, m; ρ j is the proportion when the diameter of the top pointed rock is d j at that time.

[0065] Step S202: Obtain the compressive strength under different effective contact areas through triaxial tests, numerically simulate the relationship between the effective contact area and the compressive strength, and obtain the compressive strength corresponding to the effective contact area of the CFG pile according to the relationship between the effective contact area and the compressive strength.

[0066] The steps to obtain the relationship between the effective contact area and the compressive strength through triaxial tests include the following steps:

[0067] Step S211: Calculate the horizontal stress of the soil foundation within the range of h~yh at the bottom of the CFG pile. The formula for the horizontal stress of the soil foundation is as follows:

[0068]

[0069] where h is the height of the top pointed rock, y is a constant, δ s is the horizontal stress, kPa; k 0 is the horizontal stress coefficient; δ z is the self-weight stress of the soil foundation, kPa; γ is the average unit weight of the soil foundation, kN / m 3 ; H is the total depth of the soil foundation, m. Among them, k 0 ranges from 0.4 to 0.7. When the soil is soft, k 0Take the smaller value. When the soil is hard, k 0 Take the larger value.

[0070] In the above, the range of h to yh at the bottom of the CFG pile refers to the horizontal stress of the soil foundation within the range of h to yh upward calculated based on the bottom of the CFG pile. The greater the horizontal stress, the better the protection effect on the CFG pile, and the less likely the bottom of the CFG pile is to be damaged under the action of the bamboo-shaped bedrock. Therefore, different soil layer thicknesses and strengths have different degrees of protection for the CFG.

[0071] Step S212: Use the horizontal stress of the soil foundation as the confining pressure, and obtain the compressive strength of the CFG pile test block under different contact areas at the bottom center through triaxial tests. After fitting, obtain the relationship between the effective contact area and the compressive strength.

[0072] Furthermore, the range of h to yh is 0.5 - 2.0 m. More preferably, the range of h to yh is 0.5 - 1.0 m. During the test, the CFG pile test block is a cube with a size of 30 * 30 * 30 cm. This step considers the damage effect of the pointed bedrock surface on the bottom of the CFG pile. This damage effect is directly related to the magnitude of the confining pressure on the bottom of the CFG pile, and the confining pressure is related to the thickness of the soil foundation above the bedrock surface. Therefore, use the horizontal stress of the soil foundation within the range of h to yh at the bottom of the CFG pile as the confining pressure to obtain the relationship between the effective contact area and the compressive strength.

[0073] The larger the contact area, the higher the compressive strength of the test block; δ s The greater the confining pressure, the greater the compressive strength of the test block.

[0074] Step S3: Calculate the allowable end resistance q' of the CFG pile tip corrected foundation according to the compressive strength obtained in step 2 p ; The calculation formula for the allowable end resistance of the pile tip corrected foundation is as follows:

[0075] q' p = λq p

[0076] λ = P fi / P f10

[0077] In the formula, q' p is the allowable end resistance of the pile tip corrected foundation, kPa; λ is the correction coefficient of the allowable end resistance of the pile tip foundation; q p is the allowable pile tip resistance of the rock foundation, kPa. P fi is the compressive strength, and P fi where i = 1, 2, 3,... 10 are the compressive strengths when the contact area ratios are 10%, 20%, 30%,..., 100% respectively.

[0078] Step S4: Calculate the allowable bearing capacity of a single CFG pile. The formula for the allowable bearing capacity of a single pile is as follows:

[0079]

[0080] θ = 1 - h / H

[0081] In the formula, [P]' is the allowable bearing capacity of a single pile, in kN; U is the perimeter of the pile cross-section, in m; q i is the allowable side resistance of the i-th layer of soil around the pile, in kPa; l i is the thickness of the i-th layer of soil around the pile, in m; l n is the thickness of the n-th layer of soil around the pile, in m; q n is the allowable side resistance of the n-th layer of soil around the pile, in kPa; θ is the thickness correction coefficient of the bottommost layer of soil around the CFG pile considering the pointed bedrock surface; h is the average height of the pointed rock at the top, in m. During the calculation of the allowable bearing capacity of a single pile, the soil foundation around the CFG pile is stratified according to the lithology of the soil, and the stratification situation can be obtained from geological survey data.

[0082] It can be seen from the formula for the allowable bearing capacity of a single pile that the greater the average height of the pointed rock at the top on the bedrock surface, the smaller the effective pile length of the CFG pile and the smaller the allowable bearing capacity of a single pile.

[0083] Step S5: Calculate the bearing capacity of the CFG pile composite foundation based on the allowable bearing capacity of a single pile. The formula for the bearing capacity of the composite foundation is as follows:

[0084]

[0085] In the formula, σ sp ' is the bearing capacity of the composite foundation, in kPa; η is the non-central contact influence coefficient, m s is the area replacement ratio; β is the reduction coefficient of the bearing capacity of the soil between piles, and β takes 0.75 - 0.95; σ s is the allowable bearing capacity of the soil between piles after treatment, in kPa. If there is no experience, the allowable bearing capacity of the natural foundation σ 0 can be taken. The non-central contact influence coefficient refers to the degree of deviation between the center of the pointed rock at the top and the CFG pile. The non-central contact influence coefficient is the eccentricity coefficient, which is obtained from numerical simulation after experiments. In this embodiment, η takes 0.6 - 0.9.

[0086] The area replacement ratio is related to the layout method of the CFG piles. Different numbers of CFG piles, pile spacings, and pile arrangement methods result in different area replacement ratios. This embodiment provides two pile arrangement methods. When the CFG piles are arranged in a plum blossom pattern, m s = R 2 / (1.05D 2), when the CFG piles are arranged in a square pattern, m s = R 2 / (1.13D 2 ), where D is the spacing between CFG piles, m.

[0087] When the area of the soil foundation is determined, first determine the pile spacing, and then the number of piles can be determined.

[0088] Step S6: Calculate the composite modulus E' of the CFG reinforcement area according to the bearing capacity ratio method csi , and the composite modulus calculation formula is as follows:

[0089] E' csi = ξE si

[0090] ξ = σ' sp / σ 0

[0091] In the formula, E' csi is the composite modulus of the i-th layer of soil, MPa; ξ is the bearing capacity and compression modulus improvement coefficient; E si is the compression modulus of the natural soil of the i-th layer, MPa; σ 0 is the allowable bearing capacity of the natural foundation soil, kPa.

[0092] Step S7: Calculate the settlement of the soil foundation after CFG pile reinforcement according to the composite modulus, and the settlement calculation formula is as follows:

[0093]

[0094] In the formula, s is the composite foundation settlement, m; △p i is the additional stress increment on the i-th layer of composite soil, kPa.

[0095] Step S8: Obtain the design settlement according to the requirements of the construction soil foundation, compare and analyze the settlement in Step S7 with the design settlement. If the analysis result meets the requirements, obtain the optimal layout plan for CFG pile construction; if not, return to Step S5 to adjust the layout method of CFG piles, and loop through Steps S5 - S7 until the analysis result meets the design technical requirements and the minimum economic requirements. Return to Step S5 to adjust the layout method of CFG piles, which is to adjust the pile spacing parameter D and the pile arrangement method.

[0096] The design method of the present invention fully considers the uneven effective contact caused by the top tip of the CFG pile and the soluble rock bedrock surface, the damage effect of the top tip rock on the bottom of the CFG pile, the confining pressure protection effect of the horizontal stress of the soil foundation at different depths on the bottom of the CFG pile, and the eccentric contact between the bottom of the CFG pile and the top tip of the rock. By step S1, the diameter of the tip of the uneven surface of the soluble rock in the karst area is obtained, and thus the diameter of the CFG pile is determined, providing a basis for calculating the effective contact area of the CFG pile to reinforce the foundation; in step S2, the calculation of the effective contact area fully considers the influence of the top tip rocks with different diameters and their proportions, and calculates the compressive strength under the effective contact area. It also considers the contact area between the CFG pile and the bedrock surface and the damage effect at the pile end, that is, calculates the allowable end resistance of the pile end modified foundation; step S5 considers the influence of the non-centered contact between the CFG pile and the tip of the bedrock, calculates the bearing capacity and settlement of the CFG pile composite foundation, compares the settlement with the designed settlement, and adjusts the parameters to make the result meet the requirements, obtaining the optimal layout plan for CFG pile construction. By reasonably calculating to determine the optimal layout plan for CFG pile construction, the minimum number of CFG piles, pile spacing, and pile arrangement method required are obtained. Finally, meeting the requirements means meeting the design requirements and construction requirements, and the obtained foundation reinforcement plan is the most economical and reasonable plan. The construction parameters of CFG pile construction reach the optimal state of reasonable structure and lowest cost, saving construction costs. The method of the present invention fully considers the characteristics of the bedrock surface in the karst area, and the proposed design method is closer to the actual situation, greatly improving the reliability of the results of the obtained foundation reinforcement plan. This method is easy to operate and is conducive to popularization and application.

[0097] Example 2

[0098] As Figure 2 shown is a soil foundation in a karst area. Under the soil foundation is a soluble rock foundation. There is a 6m-thick silty clay layer covering the soluble rock foundation, forming the soil foundation. On the soil foundation is a subgrade project, such as a railway. The CFG pile is used for soil foundation reinforcement treatment to solve the problem of soil foundation settlement. The design of the CFG pile to reinforce the foundation adopts the method of Example 1, where the soil foundation is silty clay. According to the geological survey data, the soil foundation is not stratified. The bearing capacity of the natural foundation σ 0 is 150 kPa, the compression modulus of the natural soil E si is 7 MPa, the allowable side resistance q n around the pile is 60 kPa, the allowable pile end resistance q p of the rock foundation is 600 kPa, the filling height H of the high-speed railway subgrade is 5 m, the width of the subgrade surface is 13.0 m, the slope ratio of the filling slope is 1:1.5, the average unit weight γ of the soil foundation is 20 kN / m 3 , and the soil foundation is reinforced with CFG.

[0099] In step S1, the diameter d of the stalagmite is obtained through investigation. 1 The proportion ρ of <0.1m 1 = 5%, 0.1m ≤ d 2 The proportion ρ of <0.2m 2 = 20%, 0.2m ≤ d 3 The proportion ρ of <0.3m 3 = 25%, 0.3m ≤ d 4 The proportion ρ of <0.4m 4 = 20%, 0.4m ≤ d 5 The proportion ρ of <0.5m 5 = 20%, 0.5m ≤ d 6 The proportion ρ 6 = 10%, and it is determined that the diameter of the CFG pile is 0.5m.

[0100] In step S2, the effective contact area between the CFG pile and the top bedrock surface is calculated. A' p = αA p = 0.441×3.14×0.5 2 / 4 = 0.086546m 2

[0101] The relationship between the effective contact area and the compressive strength is obtained through triaxial tests. First, the horizontal stress of the foundation soil in the range of 0.5 - 1.0m at the bottom of the CFG pile is calculated, where k 0 is taken as 0.6, and δ s = 0.6×20×5.5 = 66kPa.

[0102] The compressive strength P of the CFG pile test block under the confining pressure of 66kPa at the bottom center under different contact area conditions is obtained through triaxial tests. fi Average value. In this embodiment, the effective contact area is 0.086546m 2 , and the corresponding area contact influence coefficient is 0.441. To simplify the test, the relationship between the effective contact area and the compressive strength within the range of the area contact influence coefficient of 0.4 - 0.5 is considered the current relationship. Through the test, it is obtained that P f4 = 5.5MPa, P f5 = 8MPa, P f10 = 18MPa, so P f4.4 = 6.6MPa.

[0103] In step S3, the allowable end resistance q' of the CFG pile tip corrected foundation is calculated. p , q' p = λq p = 6.6 / 18×600 = 220kPa.

[0104] In step S4, calculate the allowable bearing capacity of a single CFG pile. Through investigation, the average height of the pointed rock at the top is 0.5 m.

[0105] θ = 1 - h / H = 1 - 0.5 / 6 = 0.917

[0106] [P]' = 3.14×0.5×60×6×0.917 + 0.086546×220 = 537.5 kPa

[0107] In step S5, calculate the bearing capacity of the CFG pile composite foundation.

[0108] First, it is given that the CFG piles are arranged in a square layout, and the initial value of the area replacement ratio m s is 0.06, η is taken as 0.8, and β is taken as 0.9.

[0109]

[0110] In step S6, calculate the composite modulus of the CFG reinforcement area, ξ = σ sp ' / σ 0 = 258.3 / 150 = 1.722

[0111]

[0112] In step S7, calculate the settlement of the CFG-reinforced foundation.

[0113] In step S8, the designed settlement of this subgrade project is 5 mm, and the settlement in step S7 is 7.6 mm, which is greater than the designed settlement and meets the design technical requirements and the minimum economic requirements. Finally, the construction layout plan of the CFG piles is obtained as: the diameter of the CFG piles is 0.5 m, the pile spacing is 1.6 m, and the square layout.

[0114] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A design method for reinforcing foundation with CFG piles in Karst area, Characterized in that, It includes the following steps: Step S1: Obtain the development state of the rocks on the bedrock surface at the top of the soluble rock foundation, get the diameter range of the top pointed rocks and the proportion of different diameters, and determine the diameter of the CFG piles; Step S2: Calculate the effective contact area between the CFG piles and the top bedrock surface according to the diameter range of the top pointed rocks, the proportion of different diameters and the diameter of the CFG piles, and obtain the corresponding compressive strength of the CFG piles according to the relational formula between the effective contact area and the compressive strength. The detailed steps are as follows: Step S201: Calculate the effective contact area according to the diameter range of the top pointed rocks, the proportion of different diameters and the diameter of the CFG piles. The calculation formula for the effective contact area is as follows: In the formula is the area contact influence coefficient; R is the diameter of the CFG pile, m, d j is the diameter of the top pointed rock, m; ρ j is the proportion occupied when the diameter of the top pointed rock is d j ; Step S202: Obtain the compressive strength under different effective contact areas through triaxial tests, numerically simulate the relational formula between the effective contact area and the compressive strength, and obtain the compressive strength corresponding to the effective contact area of the CFG piles according to the relational formula between the effective contact area and the compressive strength; Step S3: Calculate the allowable end resistance of the foundation corrected by the pile tip according to the compressive strength obtained in Step S2; Step S4: Calculate the allowable bearing capacity of a single CFG pile. The calculation formula for the allowable bearing capacity of a single pile is as follows: In the formula is the allowable bearing capacity of a single pile, in kN; is the perimeter of the pile cross-section, in m; is the allowable skin friction of the i th layer of soil around the pile, in kPa; is the thickness of the i th layer of soil around the pile, in m; is the thickness of the n th layer of soil around the pile, in m; is the allowable skin friction of the n th layer of soil around the pile, in kPa; is the effective contact area of the CFG pile, in m 2 ; is the allowable tip resistance of the foundation corrected for the pile tip, in kPa; is the thickness correction coefficient of the lowermost layer of soil at the bottom of the CFG pile considering the pointed bedrock surface; is the average height of the pointed rock at the top, in m; H is the total depth of the soil foundation, in m; Step S5: Calculate the bearing capacity of the CFG pile composite foundation according to the allowable bearing capacity of a single pile. The calculation formula for the bearing capacity of the composite foundation is as follows: where is the bearing capacity of the composite foundation, kPa; is the contact influence coefficient of non-centrality, is the area replacement ratio, which is related to the layout of CFG piles; is the cross-sectional area of the CFG pile, m 2 ; is the reduction coefficient of the bearing capacity of the soil between piles, taking 0.75 - 0.95; is the allowable bearing capacity of the soil between piles after treatment, kPa; Step S6: Calculate the composite modulus of the reinforced area of the CFG piles; Step S7: Calculate the settlement of the soil foundation after being reinforced by the CFG piles according to the composite modulus; Step S8: Obtain the designed settlement according to the requirements of the construction soil foundation, compare and analyze the settlement in Step S7 with the designed settlement. If the analysis result meets the requirements, obtain the optimal layout plan for the construction of the CFG piles; if not, return to Step S5 to adjust the layout method of the CFG piles, and cycle through Steps S5 - S7 until the analysis result meets the design technical requirements and the minimum economic requirements.

2. The design method for reinforcing foundation with CFG piles in Karst area according to Claim 1, Characterized in that, According to the obtained diameter range of the top pointed rock, the diameter range is divided. When the diameter of the top pointed rock d j is divided into 5 levels, that is, d j in j = 1, 2, 3, 4, 5, the corresponding grading is <0.1 m, 0.1 m ≤ <0.2 m, 0.2 m ≤ <0.3 m, 0.3 m ≤ <0.4 m, 0.4 m ≤ , the CFG pile diameter is set to 0.4 m; when the diameter of the top pointed rock d j is divided into 6 levels, that is, d j in j = 1, 2, 3, 4, 5, 6, the corresponding grading is <0.1 m, 0.1 m ≤ <0.2 m, 0.2 m ≤ <0.3 m, 0.3 m ≤ <0.4 m, 0.4 m ≤ <0.5 m, 0.5 m ≤ , the CFG pile diameter is set to 0.5 m.

3. The design method for reinforcing foundation with CFG piles in Karst area according to Claim 1, Characterized in that, The steps for obtaining the relational formula between the effective contact area and the compressive strength through triaxial tests include the following: Step S211: Calculate the horizontal stress of the soil foundation within ~ The calculation formula for the horizontal stress of the soil foundation is as follows: Among them is the height of the pointed rock at the top is a constant is the horizontal stress, kPa is the horizontal stress coefficient is the self-weight stress of the soil foundation, kPa is the average unit weight of the soil foundation, kN / m 3 ; Step S212: Use the horizontal stress of the soil foundation as the confining pressure, obtain the compressive strength of the CFG pile test blocks under different contact areas at the bottom center through triaxial tests, and obtain the relational formula between the effective contact area and the compressive strength through fitting.

4. The design method for reinforcing foundation with CFG piles in Karst area according to Claim 3, Characterized in that, ~ The range is 0.5 - 2.0 m.

5. The design method for reinforcing foundation with CFG piles in Karst area according to Claim 1, Characterized in that, In Step S3, the calculation formula for the allowable end resistance of the foundation corrected by the pile tip is as follows: Where is the correction coefficient of the allowable tip resistance of the pile tip foundation; is the allowable pile tip resistance of the rock foundation, kPa; in i = 1, 2, 3,…10 are the compressive strengths when the contact area ratios are 10%, 20%, 30%,…, 100% respectively, is the compressive strength when the area ratio is 100%.

6. The design method for reinforcing foundation with CFG piles in Karst area according to Claim 1, Characterized in that, When the CFG piles are arranged in a plum blossom pattern, 2 / (1.05 D 2 ), when the CFG piles are arranged in a square pattern, 2 / (1.13 D 2 ), where D is the pile spacing of the CFG piles, in m.

7. The design method for reinforcing foundation with CFG piles in Karst area according to Claim 1, Characterized in that, Take 0.6 - 0.

9.

8. The design method for strengthening the foundation with CFG piles in karst areas according to any one of claims 1-7, characterized in that, in step S6, the calculation formula for the composite modulus is as follows: In the formula is the composite modulus of the i -layer soil, in MPa; is the improvement coefficient of bearing capacity and compression modulus; is the compression modulus of the natural soil of the i -layer, in MPa; is the allowable bearing capacity of the natural foundation soil, in kPa.

9. The design method for strengthening the foundation with CFG piles in karst areas according to claim 8, characterized in that, in step S7, the calculation formula for the settlement amount is as follows: In the formula s is the settlement of the composite foundation, in m; is the additional stress increment on the composite soil of the i th layer, in kPa.

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