Design method of active compensation support for deep foundation pit in soil-rock combined stratum

Abstract

The application discloses a kind of deep foundation pit active compensation support design methods of soil and rock combination stratum, belong to geotechnical mechanics field, first carry out geological survey to the excavation foundation pit, then carry out support scheme design based on obtained geological exploration parameter, and according to whether using long and short anchor cable support and double gradient grouting technology etc., combined with engineering actual design monitoring scheme;And creatively adopt anchor cable construction mode, combined with high pre-tight NPR anchor cable, W steel band etc. Measure, complete the point-line-surface active support of high prestress compensation to foundation pit excavation face.This scheme abandons traditional passive support mode, but takes active compensation mode, according to the stress lost after actual stratum excavation, carries out support parameter design to high prestress NPR anchor cable, realizes the compensation measure to foundation pit, and according to fault, fracture zone etc. Geological condition encountered in actual foundation pit construction process, respectively proposed corresponding support measure, improved foundation pit compensation support design scheme.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of geotechnical mechanics, and particularly relates to a deep foundation pit active compensation support design method for soil and rock combined stratum. BACKGROUND

[0002] Today, underground engineering is developing rapidly, and various cities are vigorously building foundation pit projects. At present, the support method used in foundation pit engineering is mainly pile foundation support, which belongs to passive rigid support. That is, before the foundation pit is excavated, drilling is carried out, and then the pile is poured, the construction process is complex, and a large number of piles are required, with high construction cost. Such passive rigid support is difficult to mobilize the self-bearing capacity of the soil body.

[0003] Academician He Manchao proposed an excavation compensation theory, which believes that all damage in underground engineering is caused by human excavation, so compensation measures need to be taken for the project. The main method is to quickly apply high pre-stressed NPR anchor cables after the rock mass is excavated to compensate for the stress loss of the rock mass, and to compensate the stress state of the disturbed rock mass to close to the original state. This principle is different from the passive support method in the past, and is a kind of active support compensation support method. The invention patent with application publication number CN116575470A discloses a foundation pit excavation support method and complete equipment based on excavation compensation method, which divides the foundation pit longitudinally, sets up vertically, and constructs the support pile; places the lining along the inner contour of the foundation excavation; and internally fixes and installs the excavation support equipment, injects the excavation compensation agent while excavating when the first layer is excavated; after the first layer excavation is completed, the mud in the foundation pit is pumped out by the foundation pit excavation support equipment to complete the first layer slag removal work; the lining part is added on the lining part to complete the support of the second layer; the next step is cycled to excavate the second layer. Although the excavation compensation method is mentioned in the patent, the method does not emphasize the timely high pre-stress compensation technology after the foundation pit is excavated, and the stress level of the surrounding rock is not restored to the unexcavated state, which is different from the basic principle of the excavation compensation theory proposed by Academician He Manchao.

[0004] In the prior art, the excavation compensation theory is mainly applied to the design of tunnel excavation support, and the foundation pit support is different from the tunnel support. The surrounding rock in the tunnel support is mainly rock, and the stress received during the tunnel excavation process is basically uniform; while the foundation pit engineering is more complex, and is often a combination of soil on top and rock below, and the stress received during excavation is constantly increasing, so it is not easy to apply the excavation compensation theory to the foundation pit, and there is currently no successful case. Foundation pit engineering is also an excavation support project, and currently has large size and quantity, which is the development focus of various cities, but passive rigid support is mostly used, which not only has complex construction steps, but also has poor support effect. Therefore, an active support design method suitable for the excavation compensation theory of foundation pit needs to be proposed. SUMMARY

[0005] The application fills the blank of the existing excavation compensation theory foundation pit support design research, and provides a deep foundation pit active compensation support design method of soil and rock combined stratum.

[0006] The application is implemented by adopting the following technical scheme: a deep foundation pit active compensation support design method of soil and rock combined stratum, comprising the following steps:

[0007] Step A, parameter acquisition: geological survey is performed on the excavated foundation pit, and various stratum parameters, underground water distribution, broken zone distribution and fault distribution are investigated;

[0008] Step B, support scheme design:

[0009] (1) according to the stratum parameters, stratum lateral stress calculation is performed based on the Rankine earth pressure theory;

[0010] (2) according to the stratum lateral stress, a reasonable support system is designed in combination with actual construction requirements, and main parameters of the support system include: prestress value of NPR anchor cable, spacing and row spacing of NPR anchor cable, etc. According to the actual stratum condition, in combination with whether long and short anchor cable support and double gradient grouting technology are used, and in combination with the actual engineering, a monitoring scheme is designed.

[0011] Step C, foundation pit excavation support:

[0012] (1) excavate the first layer, first, spray and support the exposed foundation pit surface, the purpose is to prevent small stone peeling; second, lay flexible steel mesh; then install W steel belt; perform anchor cable drilling construction, then install NPR anchor cable, and apply designed prestress.

[0013] (2) arrange anchor cable axial force gauges and earth pressure gauges at the pre-designed monitoring points, and perform long-term real-time monitoring, after the monitoring data is stable, excavate the next layer, and perform support, until the construction is completed.

[0014] (3) according to the monitoring data change, real-time adjustment is performed on the support parameters, such as increasing the prestress, increasing the NPR anchor cable spacing and row spacing, etc.

[0015] Compared with the prior art, the application has the following advantages and positive effects:

[0016] The scheme discards the traditional passive supporting mode, but adopts an active compensation mode, according to the stress loss after the actual stratum is excavated, the supporting parameter design of the high pre-stressed NPR anchor cable is carried out, the compensation measures for the foundation pit are realized, and the previous drilling and piling supporting mode is cancelled, the anchor cable construction mode is creatively adopted, the high pre-stress compensation point-line-surface active support for the foundation pit excavation surface is completed by combining the high pre-stressed NPR anchor cable, W steel belt and other measures, the high pre-stressed NPR anchor cable extrudes the tray, the tray extrudes the W steel belt, and the steel belt extrudes the flexible steel mesh, and the corresponding supporting measures are respectively proposed according to the geological conditions such as faults and broken zones encountered in the actual foundation pit construction process, and the foundation pit compensation supporting design is improved. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a foundation pit supporting design flowchart of the embodiment of the present application;

[0018] Figure 2 It is a long and short anchor rod / cable supporting mode schematic diagram of the embodiment of the present application;

[0019] Figure 3 It is a NPR-point-line-surface supporting system schematic diagram formed by the embodiment of the present application;

[0020] Among them, 1, combined beam; 2, anchored to deep rock mass; 3, NPR short anchor cable; 4, NPR long anchor cable; 5, fault; 6, anchor cable; 7, tray; 8, W steel belt; 9, flexible steel mesh. DETAILED DESCRIPTION

[0021] In order to more clearly understand the above-mentioned purposes, features and advantages of the present application, the present application will be further described below in combination with the drawings and embodiments. In the following description, a large number of specific details are set forth in order to fully understand the present application, however, the present application can also be implemented in other ways different from those described herein, therefore, the present application is not limited to the specific embodiments disclosed below.

[0022] The embodiment proposes a deep foundation pit active compensation supporting design method for soil and rock combined stratum, as shown in Figure 1 The method comprises the following steps:

[0023] Step A, parameter acquisition: the geological survey of the excavated foundation pit is carried out, the stratum parameters, underground water distribution, broken zone distribution and fault distribution are investigated;

[0024] Step B, supporting scheme design:

[0025] (1) According to the stratum parameters, the stratum lateral stress is calculated based on the Rankine earth pressure theory, in order to calculate the stratum lateral stress under the most dangerous condition, the passive earth pressure is taken as the stratum lateral stress;

[0026] (2) According to the stratum lateral stress, combined with the actual construction demand, the support system is reasonably designed, and the main parameters of the support system include: the number of NPR anchor cables, the prestress value; the spacing and row distance of NPR anchor cables. According to the actual situation of the stratum, combined with whether the long-short anchor cable support and double-gradient grouting technology are used, and combined with the actual engineering, the support scheme is designed.

[0027] Step C, foundation pit excavation support:

[0028] (1) Excavate the first layer, first, spray the exposed foundation pit surface with a mixture support, the purpose is to prevent small stones from peeling off; second, lay a flexible steel mesh; then install a W steel belt; carry out anchor cable drilling construction, then install NPR anchor cables, and apply a designed prestress.

[0029] (2) The pre-designed monitoring points are arranged: anchor cable axial force meters, soil pressure gauges, until the completion of construction.

[0030] In order to more clearly understand the scheme of the present application, the implementation details of the scheme are described in detail as follows:

[0031] In step A, the excavated foundation pit is subjected to geological exploration, and the mechanical parameters of the soil and rock and the stratum thickness and other parameters can be obtained by the following methods:

[0032] ① On-site drilling: through on-site drilling investigation at the construction foundation pit, the thickness of the soil and rock layer, whether there is a fracture zone, and other conditions are obtained;

[0033] ② Engineering geophysical exploration: mainly for seismic exploration, which is a method of exploring geological structure by studying the propagation law of artificially excited elastic waves in the foundation pit construction area. After collation, analysis and interpretation, the occurrence and structure of faults and other parameters are calculated;

[0034] ③ Laboratory test: the samples obtained by drilling are subjected to laboratory test, the direct shear test is carried out on the soil to obtain its shear strength and other parameters, and the uniaxial and triaxial tests are carried out on the rock mass to obtain its mechanical parameters.

[0035] In step B, when the foundation pit support parameters are designed, the following principles are adopted:

[0036] According to the stratum parameters obtained in step A, based on the Rankine passive earth pressure theory, the following can be obtained:

[0037]

[0038] In the formula: P p is the passive earth pressure intensity of the calculation point, kPa; γ i is the specific gravity of the i-th layer of soil above the calculation point, kN / m 3 ; h iFor the thickness of the i-th layer of soil above the point, m 3 ; K p For the passive earth pressure coefficient at the calculation point, c, The standard value of the cohesion of the soil at the calculation point (kPa) and the standard value of the internal friction angle of the soil at the calculation point (°), respectively.

[0039] In traditional foundation pit design, the corresponding passive rigid support such as piles is usually designed based on the calculation of soil pressure. Such support design needs to consider the coupling of the pile stiffness and the stratum, and since it is passive support, the support member needs to be activated when the soil body produces displacement, although the support is achieved, but it cannot be actively supported to prevent problems from occurring. The active support design method proposed in the embodiment is to perform prestress design according to the passive earth pressure, directly compensate the soil pressure generated by the excavation through high prestressed anchor cables, and the method is to actively support the soil body, and does not need to wait for the soil body to produce displacement, and is a safer and more effective support method. According to the physical and mechanical parameters obtained by indoor test, combined with the calculation result P p of the passive earth pressure calculated in formula (1), the prestress and spacing of the anchor cable can be designed according to the size of the passive earth pressure, and the specific steps are as follows.

[0040]

[0041] In the formula, n is the number of anchor cables, F is the prestress applied by the anchor cable, kN, a is the spacing of the anchor cable, m, and b is the row spacing of the anchor cable, m.

[0042] The above four parameters (the number and prestress value of NPR anchor cables, and the spacing and row spacing of NPR anchor cables) are considered in the design, and the number of anchor cables is given priority in terms of construction economy, and the prestress is considered second. When the number of anchor cables is reduced, the prestress of the anchor cable needs to be increased, and the two can be continuously adjusted to achieve dynamic balance. In the spacing and row spacing of the anchor cable, the row spacing of the anchor cable is designed first, and the row spacing is determined according to the excavation speed, and the spacing is determined according to the difficulty of mechanized construction operation. It should be noted that the above is only the order of the four parameters, and the parameters are not fixed after being determined, and can be dynamically adjusted according to actual construction requirements.

[0043] After the spacing and row spacing of the anchor cable are determined, excavation and support are performed, and the specific steps are shown in Figure 2 and Figure 3 , and the specific steps are as follows.

[0044] (1) First, when excavating the first layer, the exposed soil after excavation is sprayed with concrete to prevent the surface of the soil from peeling, falling off, and other conditions. Then, a flexible steel mesh 9 is laid on the surface of the concrete to cover the excavation surface. Then, a W steel belt 8 is laid, and an NPR anchor cable 6 is made in the reserved cavity of the steel belt, and high pre-tightening force is applied to the anchor cable. Then, the above steps are repeated to excavate and support the next layer of the foundation pit until the foundation pit construction is completed.

[0045] The key point of the anchor cable construction this time is the time of anchor cable construction. After the excavation of the foundation pit, the traditional New Austrian method support design concept is to resist first and then yield, that is, after the soil deforms to a certain extent, prestressed anchor cable support is performed on it. This support will cause damage to the soil and reduce its own strength. The traditional support method does not realize the importance of support time, so although anchor cable support is performed, it does not play its effect. The excavation compensation theory high prestressed anchor cable used in this embodiment is to resist first and then yield, that is, high prestressed support is completed within half an hour after excavation. This method compensates for the soil first to maintain its high strength, so the safety factor is higher.

[0046] In addition, in the traditional foundation pit support, the anchor cable can only be point supported, and the support effect is poor. The support measure proposed in this embodiment is point-line-surface support, which is as follows:

[0047] The high pre-tightening force NPR anchor cable 6 can compensate for the stress of the soil after excavation, so that it returns to or approaches the stress state before excavation. This is point support. The NPR anchor cable 6 is drilled through the cavity reserved by the W steel belt 8 to apply prestress. The NPR anchor cable, through high prestress, extrudes the tray 7, extrudes the W steel belt 8 through the tray, and tightly combines the anchor cable and the W steel belt together to actively compensate and support the soil. This is the conversion of point support to line support. The W steel belt is connected together and acts on the flexible steel mesh 9, which changes from line support to surface support. Through the above method, the high prestressed NPR anchor cable 6 extrudes the tray 7 to complete the point support; the tray 7 extrudes the W steel belt 8 to complete the line support; and the W steel belt 8 extrudes the flexible steel mesh 9 to complete the surface support, thereby completing the point-line-surface active support of the high prestressed compensation of the excavation surface of the foundation pit.

[0048] In particular, when the excavated rock mass is relatively broken or the soil is relatively loose, a double-gradient grouting method can be adopted:

[0049] The double gradient grouting includes cement slurry of coarse particle size grading and fine particle size grading. The coarse particle size utilizes the pressure transmission characteristics of the slurry under low pressure, the slurry overcomes the initial stress and tensile strength of the stratum, and makes the original pores or cracks in the stratum expand or form new cracks or pores, and then high pressure secondary grouting of fine particle size is carried out, the fine particle size slurry has good flowability, under the driving of high pressure, the fine particle size particles are filled into the expanded pores and cracks of the surrounding rock, which can fundamentally change the lithology and structure of the deep broken rock mass of the surrounding rock and improve the strength. At the same time, the bearing capacity of the deep surrounding rock is further improved, and a stable anchoring point is provided for the anchoring section of the NPR anchor cable.

[0050] In the embodiment, the following improvements are made according to the actual stratum during the specific operation. Since the foundation pit is an engineering on the ground surface, more soil bodies are involved instead of rock bodies. Therefore, the following adjustments are made during the grouting design: 1) The grouting particle size in the tunnel is determined by comparing with the rock cracks, and therefore it is not applicable to the shallow foundation pit. In the foundation pit support design method proposed in the embodiment, in order to better combine the loose soil bodies together by grouting, the soil body particle size of the excavated stratum is first investigated. The coarse particle in the grouting is taken as the maximum particle size in the soil body particle grading, and the fine particle is taken as the minimum particle size in the soil body particle grading. 2) The grouting in the tunnel is designed according to the ground stress test results, and the numerical value is uniform and will not change. The grouting pressure in the foundation pit support of the embodiment is different from the constant high pressure in the tunnel, but is designed to increase with the increasing of the foundation pit depth, which not only maintains good grouting effect, but also avoids the uneconomic behavior caused by continuous high pressure. The specific design is that the grouting high pressure is 3 times of the soil pressure calculated by formula (1), and the grouting low pressure is 1.5 times of the soil pressure.

[0051] In addition, when there is a fault in the foundation pit, NPR long and short anchor cables are used. Under the action of pre-tightening force, the short anchor cable forms a composite beam (composite beam theory) to extrude the thin plate into a thick plate to improve its bending stiffness. According to the suspension theory, the long anchor cable suspends the composite beam formed by the short anchor cable in the stable rock stratum, thereby avoiding the instability of the composite beam. Since the foundation pit project is located in the shallow surface, unlike the single stratum in the deep surrounding rock, the foundation pit project involves more and more complex strata, so the long and short design of NPR anchor cable needs to be better applicable. When the anchor cable is quickly anchored, a resin anchoring agent is usually used. Through multiple field tests, it is found that the optimal anchoring length of NPR anchor cable under the action of resin anchoring agent is 1.5m-1.8m. In the actual construction process, the long anchor cable needs to cross the fault, in order to ensure that the anchoring effect of the anchor cable is not affected by the fault, so the design length of the long anchor cable is at least 2m across the fault zone. Specifically, 1) if grouting is not required, the design length of the long anchor cable is 2m across the fault zone, to ensure that the anchor cable is anchored in the stable rock mass; 2) if grouting is required, the design length of the long anchor cable is the length of the grouting pipe increased by 2m, that is, the long anchor cable is anchored within the grouting range to better stabilize the composite beam. The length of the short anchor cable is determined according to the construction design requirements, and the short anchor cable is anchored in the stable rock mass to maintain the composite beam and improve the stiffness.

[0052] In addition, during construction, anchor cable shaft force gauges are installed on the NPR anchor cables, and pressure gauges are installed in the surrounding rock. Through multi-source information monitoring technology, real-time shaft force of the anchor cable and rock pressure are obtained to monitor the long-term safety of the foundation pit, and the monitoring data can be used to adjust the support parameters in real time, such as increasing the pre-tightening force, increasing the spacing and row spacing of NPR anchor cables, etc., to achieve better support effect.

[0053] The above is only a preferred embodiment of the present application, and is not intended to limit the present application in other forms. Any person skilled in the art can modify or change the above disclosed technical content to equivalent embodiments applied to other fields, but any simple modification, equivalent change and modification of the above embodiments made in accordance with the technical essence of the present application still falls within the protection scope of the present application.

Claims

1. A design method for active compensation support of deep foundation pits in soil-rock composite strata, characterized in that, Includes the following steps: Step A, Parameter Acquisition: Conduct a geological survey of the excavated foundation pit, investigate the parameters of each stratum, the distribution of groundwater, the distribution of fracture zones, and the distribution of faults; Step B, Support Scheme Design: (1) Calculate the lateral stress of the formation based on the formation parameters; (2) Based on the lateral stress of the stratum and the actual construction needs, design the support system. The parameters of the support system include the number of NPR anchors, the prestress value, the spacing of NPR anchors, and the row spacing. Based on the actual stratum and the actual project, design the support scheme in combination with whether to use long and short anchor support and double gradient grouting technology. Step B is specifically implemented in the following way: (1) Based on Rankine's passive earth pressure theory, we obtain: In the formula: The passive earth pressure at the calculation point; To calculate the unit weight of the i-th soil layer above the calculation point; To calculate the thickness of the i-th soil layer above the calculation point; The passive earth pressure coefficient at the calculation point is... c, These are the standard values ​​of the soil cohesion and the internal friction angle at the calculation point, respectively. (2) Based on the passive earth pressure, perform prestressing design: Based on the magnitude of the passive earth pressure, according to the formula The design specifies the prestress and spacing of the anchor cables, where n is the number of anchor cables; F is the magnitude of the prestress applied to the anchor cables; a is the anchor cable spacing; and b is the anchor cable row spacing. Step C, Excavation and Support of the Foundation Pit: (1) Excavate the first layer, firstly spray concrete support is applied to the exposed foundation pit surface, then flexible steel mesh is laid to cover the excavation surface, then W steel strip is laid, and NPR anchor cables are installed in the reserved holes on the steel strip to apply the design prestress. (2) Install axial force and rock pressure monitoring sensors at the pre-designed monitoring points until construction is completed; In step C, when it is necessary to combine dual-gradient grouting technology for support: (1) First, investigate the soil particle size of the excavated stratum, and take the coarse particles in the grouting as the largest particle size in the soil particle size distribution, and the fine particles as the smallest particle size in the soil particle size distribution. (2) The grouting pressure in the foundation pit support is designed to increase as the foundation pit depth increases, and is dynamically adjusted according to the passive earth pressure.

2. The active compensation support design method for deep foundation pits in soil-rock composite strata according to claim 1, characterized in that: In step B, when designing the number of NPR anchors, the prestress value, and the spacing and row spacing of the NPR anchors, the order of design for the four parameters is as follows: First, consider the number of anchor cables, then consider the prestress. When the number of anchor cables decreases, the prestress of the anchor cables needs to be increased. The two are constantly adjusted to achieve a dynamic balance. As for the spacing and row spacing of the anchor cables, first design the row spacing of the anchor cables. The row spacing is determined based on the excavation speed, while the spacing is determined based on the ease of mechanized construction operations.

3. The active compensation support design method for deep foundation pits in soil-rock composite strata according to claim 1, characterized in that: In step B, when the excavated foundation pit encounters a fractured zone or relatively loose soil, a dual-gradient grouting technique is adopted for support; when the excavated foundation pit encounters a fault, long and short NPR anchor bolts / cables are used for support.

4. The active compensation support design method for deep foundation pits in soil-rock composite strata according to claim 1, characterized in that: In step C, when constructing the anchor cable, the prestressed support must be completed within half an hour after excavation.

5. The active compensation support design method for deep foundation pits in soil-rock composite strata according to claim 1, characterized in that: In step C, high prestress compensation is applied to the excavation surface of the foundation pit to form a point-line-surface active support system: Point support is achieved by using high-prestressed NPR anchor cables to compensate for soil stress after excavation, restoring it to or near the stress state before excavation. NPR anchor cables are drilled through holes reserved in the W steel strip and high prestress is applied. The NPR anchor cables are then compressed by the high prestressed tray, and the tray compresses the W steel strip, tightly bonding the anchor cables and the W steel strip together to provide active compensation support for the soil. This is a conversion of point support to line support. The W-shaped steel strips are connected together and act on the flexible steel mesh, changing the line support into surface support.

6. The active compensation support design method for deep foundation pits in soil-rock composite strata according to claim 1, characterized in that: In step C, when it is necessary to combine long and short anchor bolts / cables for support, the design length of the long anchor bolt / cable should be at least 2m across the fault zone; 1) If grouting is not required, the design length of the long anchor bolt / cable should be at least 2m across the fault zone; 2) If grouting is required, the design length of the long anchor rod / cable should be at least 2m longer than the length of the grouting pipe, that is, the long anchor rod / cable should be anchored within the grouting range.

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