Simplified calculation method for deformation stress of deep foundation pit support structure in soil-rock composite stratum

Abstract

A simplified calculation method for deformation stress of a deep foundation pit support structure in a soil-rock composite stratum belongs to the technical field of foundation pit engineering structural mechanics and comprises the following steps: step 1, determining a bedrock surface form and a rock stratum camber structural surface condition; step 2, determining the lateral pressure distribution of the rock-soil body and calculating the lateral pressure of the rock-soil body; 3, soil layer excavation stage calculation, rock stratum excavation stage calculation and rock-socketed excavation stage calculation are carried out; and step 4, superposing the calculation result of the soil layer excavation stage, the calculation result of the rock layer excavation stage and the calculation result of the rock-socketed excavation stage to obtain the deformation stress and the supporting axial force of the whole enclosure structure after the excavation of the deep foundation pit with the shallower rock surface under the condition of the soil-rock composite stratum. According to the method, deformation stress analysis of the deep foundation pit support structure with the shallow rock surface is simplified, excavation stress analysis of the deep foundation pit in the soil-rock composite stratum is divided into soil layer and rock layer segmented excavation, and the deformation stress rule of the deep foundation pit support structure with the shallow rock surface in the soil-rock composite stratum can be known.

Description

Technical Field

[0001] The invention belongs to the technical field of foundation pit engineering structural mechanics, and in particular relates to a simplified calculation method for deformation stress of a deep foundation pit retaining structure in a soil-rock composite stratum. Background Art

[0002] With the expansion of urban scale and the construction of urban rail transit, the construction of underground space is developing towards a deeper level. In the process of foundation pit construction, the excavation depth exceeds the bedrock burial depth, which will become more frequent. The design and construction concept has put forward higher requirements for "cooperating with the surrounding environment and seeking a balance between practicality, safety, economy and beauty". However, in recent years, the casualties and economic losses caused by deep foundation pit accidents at home and abroad have been relatively serious. This shows that the effective control of the deformation and stress of deep foundation pits is of great significance to the development of urban construction.

[0003] The stability of the retaining structure of deep foundation pits is the focus of safety construction. The stability of the retaining structure directly affects the safety and function of deep foundation pits, and further affects adjacent buildings, public facilities and the surrounding environment. Although the retaining structure designed according to the current specifications is not easy to be damaged, the traditional design and construction of the retaining structure of the foundation pit mainly follows the method of the foundation pit in the soft soil area, and there is a lack of theory and experience for the retaining structure of deep foundation pits in soil-rock composite strata. There is a certain impact on the stability, construction safety and economy of such foundation pits.

[0004] Through the analysis of previous accident experience of deep foundation pit retaining structure damage under soil-rock composite strata, people gradually realized the importance of stress and deformation of retaining structure under soil-rock composite strata to the stability of foundation pit. Due to the huge differences in the physical and mechanical properties of rock and soil, it is difficult to determine whether the deformation and stress of retaining structure in deep foundation pit in soil-rock composite strata are related to the lateral pressure distribution of rock and soil, the embedded depth of retaining structure and other unique factors. The retaining structure forms are more diverse, and the failure mode changes with the conditions of soil, rock and soil and rock overall quality. The risk assessment system is more complicated to integrate soil and rock characteristics. The applicability of the original pure soil layer foundation pit research conclusions is poor. Many scholars have conducted extensive research on the deformation and stress law of retaining structure combined with the characteristics of deep foundation pit in soil-rock composite strata, but the equivalent beam method cannot calculate the deformation problem, and the accurate solution calculation of the shoulder gang male method is relatively complex, which is difficult to meet the requirements of engineering design. The plane elastic foundation beam method has long been based on the experience of pure soil layer foundation pit, and there is a certain difference with the embedded end of retaining structure of deep foundation pit in soil-rock composite strata. Summary of the invention

[0005] In view of the problems of the prior art, the present invention discloses a simplified calculation method for the deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum. The method simplifies the deformation and stress analysis of a deep foundation pit retaining structure with a shallow rock surface, and divides the stress analysis of deep foundation pit excavation in a soil-rock composite stratum into segmented excavation of soil layers and rock layers, which is conducive to understanding the deformation and stress laws of a deep foundation pit retaining structure in a soil-rock composite stratum with a shallow rock surface.

[0006] To achieve the above purpose, the technical solution of the present invention is:

[0007] A simplified calculation method for deformation stress of a deep foundation pit retaining structure in a soil-rock composite stratum, the method is for stress analysis of a deep foundation pit retaining structure in a soil-rock composite stratum with a relatively shallow rock surface, and comprises the following steps:

[0008] Step 1: determine the bedrock surface morphology and the outward-dipping structural surface conditions of the rock layer;

[0009] Step 2, determine the lateral pressure distribution of the rock and soil body and calculate the lateral pressure of the rock and soil body;

[0010] Step 3: Calculate the stress analysis in stages: soil excavation stage calculation, rock excavation stage calculation, and rock-embedded excavation stage calculation;

[0011] Step 4: Superimpose the calculation results of the soil excavation stage, the rock excavation stage and the embedded rock excavation stage to obtain the deformation force and support axial force of the entire retaining structure under the soil-rock composite stratum conditions when the deep foundation pit excavation is completed with the rock surface relatively shallow.

[0012] Preferably, in step 1, the bedrock surface morphology and the outward-dipping structural surface conditions of the rock formation include: horizontal bedrock surface - no outward-dipping structural surface; horizontal bedrock surface - outward-dipping rock formation structural surface; inclined bedrock surface - no outward-dipping structural surface; inclined bedrock surface - outward-dipping rock formation structural surface.

[0013] Preferably, in step 3, the soil excavation stage calculation and the rock excavation stage calculation are simplified to the end of one excavation, that is, the soil excavation stage calculation and the rock excavation stage calculation only perform the calculation of the excavation end state, and ignore the calculation of the excavation process.

[0014] Preferably, in step 3, the following assumptions are made based on the plane elastic foundation beam method:

[0015] (1) The embedded rock part of the foundation pit retaining structure in each calculation stage is regarded as a fixed end.

[0016] (2) Treat the supports as elastically compressed rods and ignore vertical deformation.

[0017] (3) The surrounding rock and soil pressure action points and support positions do not change, and the deformation of the support and surrounding structures is coordinated.

[0018] Preferably, in the step 3, according to the position of the retaining structure in the soil-rock composite stratum, it is divided into three parts: the soil layer section, the rock layer section and the embedded rock section. During calculation, the retaining structure and the support force system are disassembled into "a cantilever beam subjected to rock and soil pressure and support axial force + a rod subjected to support axial force and fixed at one end", and an equilibrium equation is established through the displacement at the position where the retaining structure supports and the compression of the corresponding support, and the stress and deformation conditions of the part of the retaining structure located above the excavation surface in the soil layer excavation stage and the rock layer excavation stage are solved and calculated respectively, and then the stress condition of the top of the embedded section of the retaining structure is determined, and then the minimum embedded rock depth of the retaining structure and the deformation and stress law of the embedded end are solved.

[0019] Preferably, in step 4, based on the stress and deformation conditions of the part of the retaining structure above the excavation surface during the soil excavation stage and the rock excavation stage, the stress conditions at the top of the embedded section of the retaining structure, the minimum rock embedded depth of the retaining structure and the deformation and stress law of the embedded end, the overall lateral displacement, bending moment and shear force curves and support axial force of the deep foundation pit retaining structure in the soil-rock composite stratum with a shallow rock surface are finally obtained.

[0020] The beneficial effects of the simplified calculation method for deformation stress of a deep foundation pit retaining structure in a soil-rock composite stratum of the present invention are as follows:

[0021] 1. The calculation of the present invention is simple, and the calculation results are more in line with the stress deformation of the retaining structure of the deep foundation pit in the soil-rock composite stratum than the traditional method. The geotechnical pressure calculation formula in the present invention is obtained according to the specification, and the geotechnical lateral pressure distribution form is approximated to obtain the geotechnical pressure distribution behind the retaining structure under different bedrock surfaces and outward-inclined structural surfaces.

[0022] 2. The present invention is helpful to understand the deformation and stress law of the retaining structure of the deep foundation pit in the soil-rock composite stratum with shallow rock surface. In the process of segmented excavation, the assumption that the lower rock layer section is regarded as the embedded section is closer to the situation that the lower rock and soil of the soil-rock composite stratum has greater rigidity and stronger deformation resistance than the upper soil layer.

[0023] 3. The present invention has a wide range of applications. As the depth of rock excavation increases, during the excavation stage of the rock strata in the pit, the lateral force of the rock mass on the retaining structure cannot be ignored, and the integrity of the rock mass is also an important factor affecting the stress and deformation of the retaining structure. The present invention fully considers these factors and is in line with the current trend of increasing excavation depth of foundation pits year by year.

[0024] Instruction Manual

[0025] Figure 1 , a simplified calculation flow chart of the present invention;

[0026] Figure 2 , a schematic diagram of the enclosure structure of the present invention in sections;

[0027] Figure 3, a simplified calculation equivalent schematic diagram of the present invention;

[0028] Figure 4 , different types of geological models of soil-rock composite strata of the present invention;

[0029] Figure 5 , distribution diagrams of rock and soil pressure under different types of soil-rock composite strata of the present invention;

[0030] Figure 6 , schematic diagram of the forces acting on the soil layer section during excavation of the present invention;

[0031] Figure 7 , schematic diagram of the forces acting on the rock layer section during excavation of the present invention;

[0032] Figure 8 , schematic diagram of the lateral pressure distribution of the rock-embedded pile body of the present invention.

[0033] Fig. 9 , a cross-sectional view of the foundation pit strata and surrounding structure of a specific verification example of the present invention.

[0034] Fig.10 , schematic diagram of the forces during excavation of a soil layer section in a specific verification example of the present invention.

[0035] Fig.11 , schematic diagram of the stress on the retaining structure during the rock excavation stage of a specific verification example of the present invention.

[0036] Fig.12 , deformation internal force diagram of the enclosure structure of the specific verification example of the present invention. DETAILED DESCRIPTION

[0037] The following description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0038] The following embodiments may be understood as individually expressing a part of a local structure or method of the present invention, or may be understood as a combination of the embodiments to explain the connotation of a larger structure or method of the present invention.

[0039] Example 1

[0040] A simplified calculation method for deformation stress of deep foundation pit retaining structure in soil-rock composite strata, the method is aimed at stress analysis of deep foundation pit retaining structure in soil-rock composite strata with shallow rock surface, such as Figure 1 As shown, the following steps are included:

[0041] Step 1: determine the bedrock surface morphology and the outward-dipping structural surface conditions of the rock layer;

[0042] Step 2, determine the lateral pressure distribution of the rock and soil body and calculate the lateral pressure of the rock and soil body;

[0043] Step 3: Calculate the stress analysis in stages: soil excavation stage calculation, rock excavation stage calculation, and rock-embedded excavation stage calculation;

[0044] Step 4: Superimpose the calculation results of the soil excavation stage, the rock excavation stage and the embedded rock excavation stage to obtain the deformation force and support axial force of the entire retaining structure under the soil-rock composite stratum conditions when the deep foundation pit excavation is completed with the rock surface relatively shallow.

[0045] In this embodiment, the foundation pit with a relatively shallow rock surface refers to a foundation pit where the thickness of the rock layer in the foundation pit to be excavated accounts for more than half of the excavation depth of the foundation pit, that is, the foundation pit with a relatively shallow rock surface.

[0046] Example 2

[0047] Based on Example 1, this embodiment discloses: in the step 3, the calculation of the soil layer excavation stage and the calculation of the rock layer excavation stage are simplified to the end of one excavation, that is, the calculation of the soil layer excavation stage and the calculation of the rock layer excavation stage only perform the calculation of the excavation end state, ignoring the calculation of the excavation process.

[0048] Example 3

[0049] Based on Example 2, this embodiment discloses that: in step 3, the following assumptions are made based on the plane elastic foundation beam method:

[0050] (1) The embedded rock part of the foundation pit retaining structure in each calculation stage is regarded as a fixed end.

[0051] (2) Treat the supports as elastically compressed rods and ignore vertical deformation.

[0052] (3) The surrounding rock and soil pressure action points and support positions do not change, and the deformation of the support and surrounding structures is coordinated.

[0053] Example 4

[0054] Based on Example 3, this embodiment discloses that in step 3, according to the position of the retaining structure in the soil-rock composite stratum, it is divided into three parts: the soil layer section, the rock layer section and the embedded rock section, such as Figure 2 As shown in the figure, the enclosure structure and support force system are decomposed into "cantilever beam under the action of geotechnical pressure and support axial force + rod under the action of support axial force and fixed at one end" during calculation. Figure 3 As shown, the location where the enclosure structure supports the Figure 3The displacement (as shown by the two vertical arrows on the right) and the compression of the corresponding support are used to establish a balance equation, and the stress and deformation of the part of the retaining structure above the excavation surface in the soil excavation stage and the rock excavation stage are calculated respectively, and then the stress condition at the top of the embedded section of the retaining structure is determined, and then the minimum embedded depth of the retaining structure and the deformation and stress law of the embedded end are solved.

[0055] Example 5

[0056] Based on Example 4, this embodiment discloses: in the step 4, according to the stress and deformation conditions of the part of the retaining structure located above the excavation surface during the soil excavation stage and the rock excavation stage, the stress conditions at the top of the embedded section of the retaining structure, the minimum rock embedded depth of the retaining structure and the deformation and stress law of the embedded end, the overall lateral displacement, bending moment and shear force curves and support axial force of the deep foundation pit retaining structure with a shallow rock surface in the soil-rock composite stratum are finally obtained.

[0057] Example 6

[0058] A simplified calculation method for deformation and stress of retaining structure of deep foundation pit in soil-rock composite stratum. Because soil-rock composite stratum is mainly composed of upper soil layer and lower rock layer with different properties, the different bedrock surface morphology and the conditions of rock layer outward structural surface lead to differences in soil pressure distribution and deformation and stress of retaining structure. The stress mechanism of retaining structure involves the position of soil-rock interface and deformation coordination of retaining structure. According to the different conditions of bedrock surface and structural surface, the geotechnical pressure geological models under four conditions are summarized, such as Figure 4 shown.

[0059] Taking the bedrock surface horizontal-rock layer structural surface outward inclination model as an example, the lateral pressure calculation formula of the rock and soil body is divided into soil layer lateral pressure formula (1-4) and rock layer lateral pressure formula (5-8) according to the Technical Code for Building Slope Engineering (GB50330-2013).

[0060]

[0061]

[0062]

[0063]

[0064] Where: E a1 — active earth pressure equivalent to the standard load combination (kN / m); K a1 —active earth pressure coefficient; —internal friction angle of soil; δ1—friction angle of soil on the back of retaining wall; β—angle between fill surface and horizontal plane; α—angle between the back of retaining structure wall and horizontal plane.

[0065]

[0066]

[0067]

[0068]

[0069] Where: θ—inclination angle of the slope outward structural surface (°); c s — Cohesion of the outward-inclined structural surface (kPa); K q -coefficient; —Internal friction angle of the outward-inclined structural surface (°); δ—Friction angle between rock and the back of the retaining wall (°), taken as (0.33~0.50).

[0070] After determining the value of the rock and soil lateral pressure in the previous step, the distribution of the rock and soil pressure on the retaining structure is further confirmed. When the sliding body is a stratum with large cohesion, the distribution form of its rock and soil pressure can be approximately regarded as a rectangle. When the sliding body is an accumulation body with internal friction angle as the main shear resistance characteristic, the distribution form of its rock and soil pressure can be approximately regarded as a triangle. If it is between the two, it can be distributed in a trapezoidal shape. For different bedrock surfaces and outward-inclined structural surface conditions, the distribution characteristics of the rock and soil pressure behind the retaining structure are different, which can be divided into Figure 5 Four types are shown.

[0071] After determining the value and distribution of the rock-soil lateral pressure in the previous step, combined with the special geological conditions of the soil-rock composite stratum, the lower rock mass is more rigid than the upper soil layer and is not easily disturbed. However, for deep foundation pits with deep excavation depths and shallow rock surfaces, it is necessary to consider the rock-soil lateral pressure under different bedrock surface shapes and outward-inclined structural surfaces of the rock layer. The deep foundation pit excavation is divided into two parts, the soil layer and the rock layer, for calculation. The excavation process is ignored and assumptions are made based on the plane elastic foundation beam method. When the soil layer section is excavated, the simplified force diagram is as follows: Figure 6 As shown, the support and the retaining structure deform in coordination, that is, the lateral displacement of the retaining structure at the support position is equal to the compression of the support. The lateral displacement of the retaining structure can be obtained by superimposing the deflection of the cantilever beam under the action of the soil pressure and the axial force of each support separately to obtain formula (9).

[0072]

[0073] Where L is 1 / 2 of the support length, E is the elastic modulus of the support, A is the cross-sectional area of ​​the support, and E is am is the equivalent concentrated earth pressure of a single soil layer, N sm is the support axial force value at the soil layer segment when the soil layer segment is excavated, and the other parameters are as follows Figure 6 As shown, the N equations shown above are solved to obtain the axial force of the N supports located in the soil layer after the excavation of the soil layer is completed: Ns1 、N s2 、N s3 …N sn Substituting the obtained axial force into the above formula (9) can obtain the displacement curve along the depth of the retaining structure after the soil layer excavation is completed. According to the static equilibrium, the bending moment curve, shear force curve and the horizontal force F and bending moment M at the embedded end of the retaining structure within the excavation depth range of the soil layer can be obtained.

[0074] After the soil layer excavation calculation is determined in the previous step, in a further embodiment, the rock layer excavation stage calculation is performed. In this stage, it is considered that the retaining structure located in the soil layer above the rock layer is in a balanced state. The additional force and deformation caused by the rock layer excavation on the retaining structure and support are calculated based on the horizontal force F and bending moment M of the embedded end calculated in the soil layer excavation stage. The overall force of the foundation pit retaining structure is as follows: Figure 7 As shown in Figure 10, the solution ideas and calculation methods for the rock excavation stage are the same as those for the soil excavation stage. The specific formula is shown in Formula 10.

[0075]

[0076] Where E am+n is the equivalent concentrated soil pressure of a single soil layer, F sm is the additional value of the supporting axial force of the upper soil layer when excavating the rock layer, N sn It is the support axial force value located at the rock section when the rock section is excavated.

[0077] It should be noted that formulas (9) and (10) are applicable to the horizontal bedrock surface-outward inclination model of the rock layer structure surface. However, since the different geotechnical pressure geological models affect the distribution form of the lateral pressure of the rock and soil body and the calculation of the resultant force, it has no effect on the simplified calculation of the segmented force. Therefore, formulas (9) and (10) are also applicable to different geotechnical pressure geological models under other soil-rock composite strata, such as Figure 4 shown.

[0078] From the above listed n +r n +2 equations are solved to obtain the additional value of the axial force of the support structure located in the soil layer caused by rock excavation: F s1 ···F sm , Axial force in rock support: N r1 ···N rn and coefficients c1 and c2. The obtained axial force can be substituted into the left side of the above equation to obtain the displacement curve along the depth of the retaining structure during the rock excavation stage. The additional value of the axial force and displacement caused by the rock excavation and the axial force and displacement of the soil excavation can be superimposed to obtain the bending moment, shear force, displacement curve and the horizontal force F and bending moment M at the embedded end within the excavation depth of the retaining structure after the overall excavation is completed.

[0079] After the rock layer section excavation calculation is determined in the previous step, the rock-embedded section calculation is further performed. In this stage, the rock-embedded part of the retaining structure is regarded as a rock-embedded structure with bending moment and horizontal force on the top. According to the current "Highway Bridge and Culvert Foundation and Foundation Design Code", the minimum rock-embedded depth of the retaining structure can be calculated. The force distribution form of this part is as follows: Figure 8 As shown, the specific calculation formula can be obtained according to formula (11): the isobaric stress δ and width H R1 , and then the shear force and bending moment curves of the rock-embedded part of the retaining structure can be solved.

[0080]

[0081] Finally, superimposing the calculation results of the soil excavation stage, the rock excavation stage and the rock-embedded section can obtain the deformation stress and support axial force of the entire retaining structure under the soil-rock composite stratum conditions when the excavation of the deep foundation pit with a shallow rock surface is completed.

[0082] Example 7

[0083] Specific verification example:

[0084] In order to verify the correctness of the simplified deformation stress calculation method of the deep foundation pit retaining structure in the soil-rock composite stratum of the present invention, taking the standard section of a deep foundation pit in the soil-rock composite stratum as an example, the surrounding rock and soil mass is simplified into soil layer and rock layer according to the calculation assumptions, and one concrete support and three steel supports are set. The correctness of the simplified method is verified according to the derived calculation formula. It can be seen from the geological survey report that the rock mass in the area has good integrity. The failure form is weak plane shear failure, so the rock mass structural surface in actual engineering can be regarded as "bedrock surface horizontal-outward inclined structural surface". The parameters of the foundation pit and rock and soil mass are shown in Tables 1 and 2 below, and the cross-sectional view is as shown in Tables 1 and 2 below. Fig. 9 shown.

[0085] Table 1 Foundation pit parameters

[0086]

[0087] Table 2 Physical and mechanical parameters of rock and soil layers

[0088]

[0089] Simplified calculation method:

[0090] (1) Soil excavation stage:

[0091] When excavating the soil layer, the force diagram is as follows Fig.10 As shown:

[0092] The force calculation formula derived from the soil layer segment excavation is K a1 =0.46, E a1 =559.02kN / m, the specific formula is as follows:

[0093]

[0094] The solution is that the supporting axial force when the soil layer is excavated is N s1 =659.39kN, N s2 =168.59kN.

[0095] (2) Excavation of rock layer

[0096] When excavating the rock layer, the force diagram is as follows: Fig.11 As shown:

[0097] From the calculation formula of rock layer excavation force, K a1 =0.26, E a1 =832kN / m, and from the end of the soil excavation stage, it can be obtained that the bending moment at the soil-rock interface during the rock excavation stage is M=5949.94kN·m, F=2526.14kN, and the specific formula is as follows:

[0098]

[0099] Solving the above equations, we can get the coefficients c1 and c2 in the rock excavation stage as 0.023 and -0.185 respectively; the additional axial force F in the soil layer is s1 =210.89kN, F s2 =2568.40kN; Support axial force in rock layer N r1 =2268.80kN, N r2 =2582.33kN, then the supporting axial force at the end of foundation pit excavation is N1=N s1 +F s1 =870.28kN, N2=N s2 +F s2 =2736.99kN, N3=N r1 =2268.80kN, N4=N r2 =2582.33kN.

[0100] (3) Calculation of rock-embedded part

[0101] According to the calculation results of rock section excavation, the bending moment at the top of the rock-embedded part of the retaining structure is M = 522.67 kN·m, F = 112.28 kN. According to the formula of rock-embedded section, the equation group is solved to get δ = 29.46 kPa, H R1 =0.64m, and thus the bending moment curve of the rock-embedded section of the retaining structure is solved according to the force distribution.

[0102] Finally, by comparing with numerical simulation and elastic foundation beam method, a comparison is made Fig.12 Verify rationality.

[0103] Working principle of the present invention:

[0104] Before excavating a deep foundation pit project in a soil-rock composite stratum with a shallow rock surface, the deformation and stress of the retaining structure are simplified by using the corresponding theory of material mechanics based on the geological conditions of the specific bedrock surface and the outward-inclined structural surface according to the geological survey data. This can better predict and ensure the safety of the design and construction, reduce the stress deformation of the retaining structure and the excessive abundance of the supporting axial force, and improve the economic benefits. Compared with the calculation of the traditional elastic foundation beam excavation at one time, the calculation of the soil and rock layers in sections is more conducive to understanding the deformation and stress law of the retaining structure during the soil and rock excavation process. Compared with the more complex and accurate numerical simulation method, it is more convenient and simple, and more suitable for actual engineering.

Claims

1. A simplified calculation method for deformation stress of a deep foundation pit retaining structure in a soil-rock composite stratum, characterized in that: the method is aimed at analyzing the stress of a deep foundation pit retaining structure in a soil-rock composite stratum with a relatively shallow rock surface, and comprises the following steps: Step 1: determine the bedrock surface morphology and the outward-dipping structural surface conditions of the rock layer; Step 2, determine the lateral pressure distribution of the rock and soil body and calculate the lateral pressure of the rock and soil body; Step 3: Calculate the stress analysis in stages: soil excavation stage calculation, rock excavation stage calculation, and rock-embedded excavation stage calculation; Step 4: Superimpose the calculation results of the soil excavation stage, the rock excavation stage and the embedded rock excavation stage to obtain the deformation force and support axial force of the entire retaining structure under the soil-rock composite stratum conditions when the deep foundation pit excavation is completed with the rock surface relatively shallow.

2. The simplified calculation method for deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum according to claim 1 is characterized by: In step 1, the bedrock surface morphology and the conditions of the outward-inclined structural surface of the rock formation include: horizontal bedrock surface - no outward-inclined structural surface; horizontal bedrock surface - outward-inclined rock formation structural surface; inclined bedrock surface - no outward-inclined structural surface; inclined bedrock surface - outward-inclined rock formation structural surface.

3. A simplified calculation method for deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum as claimed in claim 2, characterized in that: In the step 3, the calculation of the soil excavation stage and the calculation of the rock excavation stage are simplified to the end of one excavation, that is, the calculation of the soil excavation stage and the calculation of the rock excavation stage only perform the calculation of the excavation end state, ignoring the calculation of the excavation process.

4. A simplified calculation method for deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum as claimed in claim 3, characterized in that: In step 3, the following assumptions are made based on the plane elastic foundation beam method: (1) The embedded rock part of the foundation pit retaining structure in each calculation stage is regarded as a fixed end. (2) Treat the supports as elastically compressed rods and ignore vertical deformation. (3) The surrounding rock and soil pressure action points and support positions do not change, and the deformation of the support and surrounding structures is coordinated.

5. The simplified calculation method for deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum according to claim 4, characterized in that: In the step 3, according to the position of the retaining structure in the soil-rock composite stratum, it is divided into three parts: the soil layer section, the rock layer section and the rock-embedded section. When calculating, the retaining structure and the support force system are disassembled into "a cantilever beam subjected to rock and soil pressure and support axial force + a rod subjected to support axial force and fixed at one end", and an equilibrium equation is established through the displacement at the position where the retaining structure supports and the compression amount of the corresponding support, and the stress and deformation conditions of the part of the retaining structure located above the excavation surface in the soil layer excavation stage and the rock layer excavation stage are respectively solved and calculated, and then the stress condition of the top of the embedded section of the retaining structure is determined, and then the minimum rock-embedded depth of the retaining structure and the deformation and stress law of the embedded end are solved.

6. A simplified calculation method for deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum as claimed in claim 5, characterized in that: In the step 4, based on the stress and deformation conditions of the part of the retaining structure above the excavation surface during the soil excavation stage and the rock excavation stage, the stress conditions at the top of the embedded section of the retaining structure, the minimum rock embedded depth of the retaining structure and the deformation and stress law of the embedded end, the overall lateral displacement, bending moment and shear force curves and support axial force of the deep foundation pit retaining structure in the soil-rock composite stratum with a shallow rock surface are finally obtained.

7. A simplified calculation method for deformation and stress of a deep foundation pit retaining structure in a soil-rock composite stratum as claimed in claim 6, characterized in that: The step (3) comprises: Based on the plane elastic foundation beam method, it is assumed that when the soil layer is excavated, the support and the retaining structure deform in coordination, that is, the lateral displacement of the retaining structure at the support position is equal to the compression of the support. The lateral displacement of the retaining structure is obtained by superimposing the deflection of the cantilever beam under the soil pressure and the axial force of each support separately, formula (9): In formula (9), L is 1 / 2 of the support length, E is the elastic modulus of the support, and A is the cross-sectional area of ​​the support. The axial force of the N supports located in the soil layer at the end of soil excavation is solved by the N equations shown above: N s1 、N s2 、N s3 …N sn , Substitute the obtained axial force into formula (9) to obtain the displacement curve along the depth of the retaining structure after the soil layer excavation is completed, and calculate the bending moment curve, shear force curve and horizontal force F and bending moment M at the embedded end within the excavation depth range of the retaining structure soil layer according to static equilibrium; During the calculation of the rock layer excavation stage, it is assumed that the retaining structure located in the soil layer above the rock layer is in a state of equilibrium. The additional force and deformation caused by the rock layer excavation on the retaining structure and support are calculated based on the horizontal force F and bending moment M at the embedded end calculated in the soil layer excavation stage. The calculation formula for the rock layer excavation stage is shown in formula (10): According to formula (10), s n +r n +2 equations are solved to obtain the additional value of the axial force of the support structure located in the soil layer caused by rock excavation: F s1 ···F sm , Axial force in rock support: N r1 ···N rn and coefficients c1 and c2, and the obtained axial force is substituted into formula (10) to obtain the displacement curve along the depth of the retaining structure during the rock excavation stage.

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