Strip-shaped foundation pit bottom inrush-resistant reinforcement thickness determination method

Abstract

The invention provides a strip-shaped foundation pit bottom inrush-resistant reinforcement thickness determination method, a strip-shaped foundation pit bottom is provided with a cement soil bottom plate, and the method comprises the steps that the shear-resistant critical reinforcement thickness # imgabs0 # of the cement soil bottom plate is calculated; calculating the critical reinforcement thickness # imgabs 1 # of contact damage of the cement soil bottom plate and the supporting structure; the bending rigidity damage critical reinforcement thickness # imgabs2 # of the cement soil bottom plate is calculated; calculating the critical reinforcement thickness # imgabs3 # of the pre-stress flexural-tensile failure of the cement soil belt; calculating the critical reinforcement thickness # imgabs4 # of the non-prestressed bending and tensile failure of the cement soil; and determining the reinforcing thickness # imgabs5 # of the strip-shaped foundation pit bottom inrush resistance. And selecting the maximum critical reinforcement thickness under the five stress damage forms, and determining the maximum critical reinforcement thickness as the pit bottom reinforcement thickness. Compared with a pressure balance method given in the specification for checking the inrush stability, the method is more reasonable and accurate, the thickness of a pit bottom reinforcing cement soil layer can be greatly reduced, and the manufacturing cost is saved.

Description

Technical Field

[0001] The invention relates to the technical field of municipal construction, and in particular to a method for determining the reinforcement thickness of a strip foundation pit bottom to resist sudden surges. Background Art

[0002] In the construction of deep strip foundation pits for municipal pipelines with high pressure water levels, sudden surges at the bottom of the foundation pit often occur, making it impossible to carry out construction. Figure 1 As shown in the figure, the calculation formula for the stability of the foundation pit bottom against sudden surge in the current specifications and textbooks is as follows: ; In the formula, It is the thickness of the soil layer from the top of the confined water aquifer to the bottom of the pit; It is the natural weight of the soil layer from the top of the confined water aquifer to the bottom of the pit; is the pressure head height of the top surface of the confined water aquifer; K is the density of water. h For the surge stability safety factor, the specification requires that it should not be less than 1.1.

[0003] At present, the commonly used treatment method for pit surge is to install cement mixing piles at the bottom of the pit to strengthen the pit bottom strength and waterproof, and prevent the pit from surge. However, if the stability of pit bottom surge is calculated according to formula 1, only the influence of the water-proof soil weight at the bottom of the pit is considered, while the shear strength of the soil layer is ignored, the thickness of the pit bottom treatment will be greatly increased, resulting in great waste. Summary of the invention

[0004] The purpose of the present invention is to provide a method for determining the reinforcement thickness of the bottom of a strip foundation pit to resist sudden surges, so as to solve the above-mentioned technical problems.

[0005] The present invention provides a method for determining the reinforcement thickness of a strip foundation pit bottom to resist sudden surge, wherein the strip foundation pit bottom is provided with a cement soil bottom plate, and the method comprises: Calculation of critical shear reinforcement thickness of cement soil slab ; Calculation of critical reinforcement thickness for contact failure between cement soil base plate and supporting structure ; Calculation of critical reinforcement thickness for flexural stiffness failure of cement-soil slab ; Calculation of critical reinforcement thickness for prestressed flexural failure of cement-soil belts ; Calculation of critical reinforcement thickness for flexural failure of cement-soil without prestressing ; Determine the reinforcement thickness of strip foundation pit bottom to resist sudden surge .

[0006] Furthermore, the calculation of the critical shear reinforcement thickness of the cement soil bottom plate is include: The calculation formula for the shear bearing capacity of the end section of the cement soil bottom plate is obtained as follows: Formula 1; The calculation formula for obtaining the maximum shear design value of the end section of the cement soil bottom plate is: Formula 2: The shear strength τ of cement soil satisfies: Formula 3; By combining Formula 1, Formula 2 and Formula 3, the critical shear reinforcement thickness of the cement soil bottom plate can be calculated: satisfy: Formula 4; in, is the load partial factor, the value is 1.35, is the weight of water, The cement soil base plate is heavy. is the design value of axial compressive strength of cement soil at the construction age, is the head height of the pressurized water, l is the width of the strip foundation pit, is the thickness of the soil layer from the top of the confined water aquifer to the bottom of the pit, p The uniformly distributed net load on the cement soil slab is upward. .

[0007] Furthermore, the critical reinforcement thickness for calculating the contact failure between the cement soil bottom plate and the supporting structure is include: When the foundation pit is excavated for support, there is friction contact between the cement soil bottom plate and the support structure, and the limit equilibrium equation is: Formula 5; Formula 6; The critical reinforcement thickness of the cement soil base plate and the supporting structure for contact failure can be calculated by formula 5 and formula 6: satisfy: Formula 7; in, , is the passive earth pressure, is the friction angle between cement soil and structure, is the cohesion of cement soil, is the internal friction angle of cement soil, e p1 is the passive earth pressure on the top surfaces of the cement soil slab at both ends, e p2It is the passive earth pressure at the bottom of both ends of the cement soil slab.

[0008] Furthermore, the calculation of the critical reinforcement thickness of the flexural stiffness of the cement soil bottom plate is include: According to the material mechanics and elasticity theory, the maximum deflection occurs in the mid-span of the cement soil base plate, and the maximum deflection value satisfies: Formula 8; in , EI is the bending stiffness of the cement soil slab, is the elastic modulus of cement soil, and the critical reinforcement thickness of the cement soil bottom plate for flexural stiffness failure is ,but The thickness should be able to bear the uplift deformation of the bottom of the foundation pit, that is, the maximum deflection does not exceed the allowable deflection, and we get: Formula 9; Calculated from formula 8, formula 9 and formula 10, we can get: Formula 10; in, , It is the ratio of span to allowable deflection.

[0009] Furthermore, the calculation of the critical reinforcement thickness of the prestressed bending-tension failure of the cement soil belt include: When the foundation pit is excavated for support, there is passive earth pressure at both ends. The stress at any point in the cement soil bottom plate of unit width is equal to the sum of the stress generated by the bending moment and the earth pressure. The dangerous point is at the top of the mid-span section of the cement soil bottom plate. The limit equilibrium equation is: Formula XI; In the formula The value is 0.15* According to formula 11, the critical reinforcement thickness of cement soil belt prestressed bending and tension failure can be obtained satisfy: Formula 12; Where Mmax is the maximum bending moment in the span of beam AB, W is the resistance moment of beam AB, is the cohesion of cement soil, is the internal friction angle of cement soil, is the tensile strength of cement soil at the excavation age, It is the design value of the axial compressive strength of cement soil at the construction age.

[0010] Furthermore, the calculation of the critical reinforcement thickness of cement soil without prestressed bending and tension failure include: When the foundation pit is excavated without support, there is no passive earth pressure at both ends of the unit width cement soil bottom plate or only static earth pressure exists, the limit equilibrium equation of bending and tension failure of the dangerous section at the top of the mid-span of the cement soil bottom plate is: Formula XIII; In the formula The value is 0.15* According to Formula 13, the critical reinforcement thickness of cement soil without prestressed bending and tension failure can be obtained: satisfy: Formula 14; Where Mmax is the maximum bending moment in the mid-span of beam AB, W is the resistance moment of beam AB, is the tensile strength of cement soil at the excavation age, It is the design value of the axial compressive strength of cement soil at the construction age.

[0011] The beneficial effects of this program are: This scheme rationally utilizes the shear strength of the cement soil reinforced at the bottom of the foundation pit, uses the one-way plate theory, analyzes five stress failure forms, selects the maximum critical reinforcement thickness under the five stress failure forms, and determines it as the pit bottom reinforcement thickness. Compared with the pressure balance method given in the specification to verify the surge stability, the present invention is more reasonable and accurate, can greatly reduce the thickness of the cement soil layer reinforced at the bottom of the pit, and saves construction costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a schematic diagram of the stability of foundation pit sudden surge in the background technology; Figure 2 The figure is a schematic diagram of the reinforcement structure of the bottom of a strip foundation pit; Figure 3 It is the force diagram of the foundation pit cement soil bottom slab beam AB; Figure 4 It is the shear force diagram of the foundation pit cement soil bottom slab beam AB; Figure 5 It is the bending moment diagram of the cement soil bottom plate beam AB of the foundation pit. DETAILED DESCRIPTION

[0013] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0014] Aiming at the design conditions of using cement-soil waterproofing and reinforced base such as cement mixing piles at the bottom of the foundation pit, a comprehensive stress analysis and reasonable material strength parameter selection are carried out. This scheme provides a more practical calculation method for the thickness of the reinforcement treatment of the bottom of the foundation pit, which improves the design efficiency and accuracy and avoids large construction waste.

[0015] Since the strength of cement soil is much higher than that of original soil, if the pressure balance method given in the specification is still used to verify the stability of sudden surge, it will inevitably cause great waste. According to the experimental research on the mechanical properties of cement soil, the critical thickness of cement soil for sudden surge prevention can be calculated by the homogeneous continuous beam and plate analysis method. Figure 2 According to the analysis method of one-way slab, the bottom plate has the same horizontal thickness. Since both ends form an integral whole with the surrounding structure, its rotation and displacement are restricted. Therefore, it can be regarded as a rectangular beam AB with fixed support at both ends. The bottom plate bears the uniformly distributed net load upward. .

[0016] According to the above analysis, the foundation pit cement soil bottom plate is simplified as follows: Figure 3-5 The beam AB shown is fixed at both ends and is subjected to a net uniformly distributed upward load p and horizontal passive earth pressures at both ends.

[0017] from Figure 3-5 It can be seen that there are two dangerous sections in the soil beam AB, one is the beam end section (point A or point B in the figure), where the shear force is the largest; the other is the mid-span section (the midpoint of beam AB), where the maximum positive bending moment exists. If the foundation pit has no supporting structure, it may cause shear failure of the cement soil layer; if the foundation pit has a supporting structure, it may cause contact failure with the supporting structure.

[0018] 1) Shear failure The calculation formula for the shear bearing capacity of the end section of the cement soil bottom plate is obtained as follows: Formula 1; The calculation formula for obtaining the maximum shear design value of the end section of the cement soil bottom plate is: Formula 2: The shear strength τ of cement soil satisfies: Formula 3; By combining Formula 1, Formula 2 and Formula 3, the critical shear reinforcement thickness of the cement soil bottom plate can be calculated: satisfy: Formula 4; in, is the load partial factor, the value is 1.35, is the weight of water, The cement soil base plate is heavy. is the design value of axial compressive strength of cement soil at the construction age, is the head height of the pressurized water, l is the width of the strip foundation pit, is the thickness of the soil layer from the top of the confined water aquifer to the bottom of the pit, pThe uniformly distributed net load on the cement soil slab is upward. .

[0019] 2) Contact damage When the foundation pit is excavated for support, there is friction contact between the cement soil bottom plate and the support structure, and the limit equilibrium equation is: Formula 5; Formula 6; The critical reinforcement thickness of the cement soil base plate and the supporting structure for contact failure can be calculated by formula 5 and formula 6: satisfy: Formula 7; in, , is the passive earth pressure, is the friction angle between cement soil and structure, is the cohesion of cement soil, is the internal friction angle of cement soil, e p1 is the passive earth pressure on the top surfaces of the cement soil slab at both ends, e p2 It is the passive earth pressure at the bottom of both ends of the cement soil slab.

[0020] 3) Failure of flexural rigidity According to the material mechanics and elasticity theory, the maximum deflection occurs in the mid-span of the cement soil base plate, and the maximum deflection value satisfies: Formula 8; in , EI is the bending stiffness of the cement soil slab, is the elastic modulus of cement soil, and the critical reinforcement thickness of the cement soil bottom plate for flexural stiffness failure is ,but The thickness should be able to bear the uplift deformation of the bottom of the foundation pit, that is, the maximum deflection does not exceed the allowable deflection, and we get: Formula 9; Calculated from formula 8, formula 9 and formula 10, we can get: Formula 10; in, , It is the ratio of span to allowable deflection.

[0021] 4) Prestressed bending and tensile failure When the foundation pit is excavated for support, there is passive earth pressure at both ends. The stress at any point in the cement soil bottom plate of unit width is equal to the sum of the stress generated by the bending moment and the earth pressure. The dangerous point is at the top of the mid-span section of the cement soil bottom plate. The limit equilibrium equation is: Formula XI; In the formula The value is 0.15* According to formula 11, the critical reinforcement thickness of cement soil belt prestressed bending and tension failure can be obtained satisfy: Formula 12; Where Mmax is the maximum bending moment in the mid-span of beam AB, W is the resistance moment of beam AB, is the cohesion of cement soil, is the internal friction angle of cement soil, is the tensile strength of cement soil at the excavation age, It is the design value of the axial compressive strength of cement soil at the construction age.

[0022] 5) No prestressed bending and tension failure When the foundation pit is excavated without support, there is no passive earth pressure at both ends of the unit width cement soil bottom plate or only static earth pressure exists, the limit equilibrium equation of bending and tension failure of the dangerous section at the top of the mid-span of the cement soil bottom plate is: Formula XIII; In the formula The value is 0.15* According to Formula 13, the critical reinforcement thickness of cement soil without prestressed bending and tension failure can be obtained: satisfy: Formula 14; is the tensile strength of cement soil at the excavation age, It is the design value of the axial compressive strength of cement soil at the construction age.

[0023] 6) Final critical reinforcement thickness of the foundation pit bottom .

[0024] The present invention rationally utilizes the shear strength of the cement soil reinforced at the bottom of the foundation pit, and uses the one-way plate theory to analyze five stress failure forms, thereby determining the thickness of the pit bottom reinforcement. Compared with the pressure balance method given in the specification to verify the surge stability, the present invention is more reasonable and accurate, can greatly reduce the thickness of the cement soil layer reinforced at the bottom of the pit, and save construction costs.

[0025] This scheme is mainly used for the stability analysis of the bottom of municipal strip foundation pit against sudden surge, and to reasonably determine the thickness of the cement-soil reinforced bottom of the pit.

[0026] Example: A new construction project needs to lay a sewage pipe with a diameter of 2200mm under the road on the west side of the road, and the buried depth exceeds 6.9m. The geology of a certain unfavorable section is 32.8-31.0m is plain fill, 31.0-30.3m is silty clay, 30.3-29.3m is silt, 29.3-21.8m is silt sand, and below 21.8m is round gravel. In the past five years, the highest water level of the pressurized water is 33.50m, and the lowest water level is 25.5m.

[0027] According to the design, the excavation elevation of the bottom of the foundation pit of the sewage pipe section is 24.4m. Considering the construction, the excavation width of the foundation pit is It is 4.2m. As the bottom elevation of the foundation pit is already located in the silt sand of the confined water aquifer, the design adopts the technical measure of Φ600 cement mixing piles to reinforce the bottom of the pit to ensure the stability of the foundation pit sudden surge. In addition, three rows of interlocking cement mixing piles are designed on both sides of the foundation pit to serve as water-cutting curtains and also as support.

[0028] Since cement mixing piles are also used for support, there is no contact failure. Due to the uncertainty caused by passive earth pressure at both ends of the bottom plate, or only static earth pressure, the design adopts non-prestressed bending and tension failure calculation for safety considerations. Therefore, the critical thickness calculation only includes shear failure, bending stiffness failure, and non-prestressed bending and tension failure. 20.4KN / m 3 The water level of the confined water is 33.5m, the highest water level in the past five years, and the design value of the axial compressive strength of the cement soil is 1.0MPa (28 days construction age), elastic modulus .

[0029] 3.2.1 Use of standard formula , where , , can be obtained .

[0030] 3.2.1 Considering the strength formula of cement soil (1) Shear failure , where , .

[0031] Using the trial method, we can get .

[0032] (2) Failure of flexural rigidity , using the trial algorithm, we can get

[0033] (3) No prestressed bending and tensile failure , using the trial algorithm, we can get

[0034]

[0035] This project uses cement mixing piles to reinforce the bottom of the foundation pit, with a reinforcement thickness of 1.8m. During the construction period, the foundation pit is dry and stable without sudden surges. The present invention can greatly reduce the thickness of the pit bottom reinforcement. From the above comparison, it can be seen that it is only 16.8% of the thickness calculated in the specification, which greatly saves the cost.

[0036] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for determining the thickness of the reinforcement for the bottom of a strip foundation pit to resist sudden surge, wherein the bottom of the strip foundation pit is provided with a cement soil bottom plate, It is characterized in that The method includes: Calculation of critical shear reinforcement thickness of cement soil slab ; Calculation of critical reinforcement thickness for contact failure between cement soil base plate and supporting structure ; Calculation of critical reinforcement thickness for flexural stiffness failure of cement-soil slab ; Calculation of critical reinforcement thickness for prestressed flexural failure of cement-soil belts ; Calculation of critical reinforcement thickness for flexural failure of cement-soil without prestressing ; Determine the reinforcement thickness of strip foundation pit bottom to resist sudden surge .

2. The method for determining the thickness of the reinforcement for the bottom of a strip foundation pit against sudden surge according to claim 1, It is characterized in that Calculation of critical shear reinforcement thickness of cement soil slab include: The calculation formula for the shear bearing capacity of the end section of the cement soil bottom plate is obtained as follows: Formula 1; The calculation formula for obtaining the maximum shear design value of the end section of the cement soil bottom plate is: Formula 2: The shear strength τ of cement soil satisfies: Formula 3; By combining Formula 1, Formula 2 and Formula 3, the critical shear reinforcement thickness of the cement soil bottom plate can be calculated: satisfy: Formula 4; in, is the load partial factor, the value is 1.35, is the weight of water, The cement soil base plate is heavy. is the design value of axial compressive strength of cement soil at the construction age, is the head height of the pressurized water, l is the width of the strip foundation pit, is the thickness of the soil layer from the top of the confined water aquifer to the bottom of the pit, p The uniformly distributed net load on the cement soil slab is upward. .

3. The method for determining the thickness of the reinforcement for the bottom of a strip foundation pit against sudden surge according to claim 2, It is characterized in that The critical reinforcement thickness for calculating the contact failure between the cement soil base plate and the supporting structure is include: When the foundation pit is excavated for support, there is friction contact between the cement soil bottom plate and the support structure, and the limit equilibrium equation is: Formula 5; Formula 6; The critical reinforcement thickness of the cement soil base plate and the supporting structure for contact failure can be calculated by formula 5 and formula 6: satisfy: Formula 7; in, , is the passive earth pressure, is the friction angle between cement soil and structure, is the cohesion of cement soil, is the internal friction angle of cement soil, e p1 is the passive earth pressure on the top surfaces of the cement soil slab at both ends, e p2 It is the passive earth pressure at the bottom of both ends of the cement soil slab.

4. The method for determining the thickness of the reinforcement for the bottom of a strip foundation pit against sudden surge according to claim 2, It is characterized in that The calculation of the critical reinforcement thickness of the cement soil base slab flexural stiffness failure include: According to the material mechanics and elasticity theory, the maximum deflection occurs in the mid-span of the cement soil base plate, and the maximum deflection value satisfies: Formula 8; in , EI is the bending stiffness of the cement soil slab, is the elastic modulus of cement soil, and the critical reinforcement thickness of the cement soil bottom plate for flexural stiffness failure is ,but The thickness should be able to bear the uplift deformation of the bottom of the foundation pit, that is, the maximum deflection does not exceed the allowable deflection, and we get: Formula 9; Calculated from formula 8, formula 9 and formula 10, we can get: Formula 10; in, , It is the ratio of span to allowable deflection.

5. The method for determining the thickness of the reinforcement for the bottom of a strip foundation pit against sudden surge according to claim 2, It is characterized in that The calculation of the critical reinforcement thickness of prestressed flexural tensile failure of cement soil belt include: When the foundation pit is excavated for support, there is passive earth pressure at both ends. The stress at any point in the cement soil bottom plate of unit width is equal to the sum of the stress generated by the bending moment and the earth pressure. The dangerous point is at the top of the mid-span section of the cement soil bottom plate. The limit equilibrium equation is: Formula XI; In the formula The value is 0.15* According to formula 11, the critical reinforcement thickness of cement soil belt prestressed bending and tension failure can be obtained satisfy: Formula twelve; Where Mmax is the maximum bending moment in the mid-span of beam AB, W is the resistance moment of beam AB, is the cohesion of cement soil, is the internal friction angle of cement soil, is the tensile strength of cement soil at the excavation age, It is the design value of the axial compressive strength of cement soil at the construction age.

6. The method for determining the thickness of the reinforcement for the bottom of a strip foundation pit against sudden surge according to claim 2, It is characterized in that The calculation of the critical reinforcement thickness of cement soil without prestressed bending and tension failure include: When the foundation pit is excavated without support, there is no passive earth pressure at both ends of the unit width cement soil bottom plate or only static earth pressure exists, the limit equilibrium equation of bending and tension failure of the dangerous section at the top of the mid-span of the cement soil bottom plate is: Formula XIII; In the formula The value is 0.15* According to Formula 13, the critical reinforcement thickness of cement soil without prestressed bending and tension failure can be obtained: satisfy: Formula 14; Where Mmax is the maximum bending moment in the mid-span of beam AB, W is the resistance moment of beam AB, is the tensile strength of cement soil at the excavation age, It is the design value of the axial compressive strength of cement soil at the construction age.

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