Subway station excavation technology for determining foundation width based on foundation bearing capacity

Abstract

The present invention discloses a subway station excavation process for determining the foundation width based on the foundation bearing capacity, comprising the following steps: (1) excavating a pilot tunnel and applying initial support; (2) constructing a bottom longitudinal beam strip foundation in the lower pilot tunnel according to the corrected calculated bottom longitudinal beam strip foundation width, and then applying a frame support system on the bottom longitudinal beam strip foundation; (3) excavating the main soil body on the basis of the frame support system, excavating the earth from top to bottom to the middle floor slab elevation. The present invention analyzes the subway station structure as a whole by analyzing and studying the bearing capacity of the bottom longitudinal beam foundation of the tunnel column method station, taking the bearing capacity of the bottom longitudinal beam foundation of the tunnel column method station as the object, and using the Moore-Coulomb theory to obtain the plastic zone boundary equation to derive the value range of the total width of the strip foundation. After determining the total width of the bottom longitudinal beam foundation based on the condition that the bearing capacity meets the design requirements, it plays a guiding role in the subsequent design and construction of the subway station.

Description

Technical Field

[0001] The invention relates to a subway station excavation process for determining a foundation width based on foundation bearing capacity, belonging to a subway construction method. Background Art

[0002] With the rapid development of urbanization, urban ground transportation is increasingly unable to meet people's travel requirements, and people's demand for underground transportation is increasing. The development of subways has become an inevitable trend in urban transportation construction. The cave column method has the advantages of flexible construction methods, less influence by the number of structural layers and spans, and good adaptability to complex surrounding environments. It has become the mainstream construction method for subway underground excavation stations at this stage. However, the soil environment around subway stations constructed by the cave column method has a large thickness of station roof cover, and the strip foundations under the middle column and side piles are located in silt and silty clay. The foundation bearing capacity determination method provided by the specification is conservative, resulting in many situations in design practice where the foundation bearing capacity does not meet the design requirements and a transverse strip foundation needs to be added. This not only greatly increases the project cost, but also brings group tunnel construction risks to the project construction.

[0003] The width and depth of the strip foundation both increase the bearing capacity of the foundation, but the buried depth of the strip foundation of the hole-column method is determined after the upper and lower pilot holes are excavated, and it is difficult to change the buried depth of the strip foundation under the most unfavorable working condition (excavation to the bottom plate but no bottom plate is constructed). Therefore, only the foundation width factor needs to be considered to ensure that the bearing capacity of the foundation under the most unfavorable working condition meets the design requirements, and there will be no engineering accidents such as excessive vertical load during excavation, which will cause overall sliding damage in the deep foundation.

[0004] The existing hole-column method for calculating the bottom longitudinal beam strip foundation takes into account the lateral limit of the unexcavated soil inside and outside the foundation pit, as well as the different buried depths of the middle strip foundation and the side strip foundation due to the different degrees of influence of the overburden load, but does not take into account the effect of the adjacent strip foundation on the strip foundation foundation bearing capacity. It is conservative, which leads to the situation that the foundation bearing capacity does not meet the design requirements in this design practice. At present, in response to the situation that the foundation bearing capacity does not meet the requirements in the hole-column method, measures such as adding transverse guide holes are often taken. A transverse guide hole is set at a certain distance between the longitudinal lower guide holes, and a transverse strip foundation is constructed in the transverse guide hole to improve the foundation bearing capacity. However, the transverse guide hole will bring about related problems such as increased construction links, increased construction joints of the bottom plate structure, small working space, and increased construction costs. In particular, it is very uneconomical to still use transverse guide holes when the bearing capacity of the longitudinal strip foundation foundation can meet the requirements.

[0005] At present, the research contents of tunnel-column method for subway station construction are mainly about construction methods, load structure model calculation and numerical simulation of stratum structure model. Among them, the construction methods are discussed the most, and the research on foundation bearing capacity is very rare. In short, the theoretical research and design specifications of foundation bearing capacity are based on the foundation structure constructed under open excavation conditions. There is still a blank in the research and experiment of foundation bearing capacity in tunnels with tunnel-column method or other shallow buried tunnel methods, which needs further research. Summary of the invention

[0006] The technical problem solved by the present invention is: in view of the problem that the bearing capacity of the strip foundation cannot meet the design requirements when the subway station is excavated by the hole column method, a subway station excavation process is provided in which the foundation width is determined based on the foundation bearing capacity.

[0007] The present invention is implemented by the following technical solutions:

[0008] The subway station excavation process for determining the foundation width based on the bearing capacity of the foundation includes the following steps:

[0009] (1) Excavate a pilot tunnel and provide initial support;

[0010] (2) Construct a bottom longitudinal beam strip foundation in the lower pilot tunnel, and then construct a frame support system on the bottom longitudinal beam strip foundation, where the total width B of the bottom longitudinal beam strip foundation satisfies the following formula:

[0011]

[0012] Where G is the deadweight of the superstructure per unit cross section (in kN / m), c is the cohesion of the soil layer below the bottom surface of the bottom longitudinal beam strip foundation (in kPa), is the internal friction angle of the soil layer below the bottom surface of the bottom longitudinal beam strip foundation (in degrees), γ is the average gravity of the soil layer above the bottom surface of the bottom longitudinal beam strip foundation (in kN / m 3 ), H is the buried depth of the bottom longitudinal beam strip foundation from the ground elevation (in m), and h is the thickness of the soil layer on the top plate (in m);

[0013] (3) Excavating the main soil on the basis of the frame support system, excavating the soil from top to bottom to the middle floor slab elevation.

[0014] In the subway station excavation process of the present invention for determining the foundation width based on the bearing capacity of the foundation, further, in the step (1), the upper guide tunnel is excavated first and then the lower guide tunnel, and the side guide tunnel is excavated first and then the middle guide tunnel.

[0015] In the subway station excavation process of the present invention for determining the foundation width based on the foundation bearing capacity, further, in the step (2), when the shear strength of the soil is not considered, the total width B of the strip foundation satisfies the following formula:

[0016]

[0017] In the subway station excavation process of the present invention for determining the foundation width based on the bearing capacity of the foundation, further, in the step (2), constructing the frame support system includes constructing side piles and steel pipe columns, and then constructing the side spans and middle span arches to form a frame support system of piles, beams and arches.

[0018] In the subway station excavation process of the present invention for determining the foundation width based on the bearing capacity of the foundation, further, in the step (2), the stratum is reinforced by pre-grouting before constructing the arch super-support structure in the side span and the middle span.

[0019] In the subway station excavation process of the present invention for determining the foundation width based on the bearing capacity of the foundation, further, in the step (3), when the soil of the side span arch is symmetrically excavated, the side span initial lining buckle arch construction is carried out, lagging the side span initial lining buckle arch excavation surface by 10 to 15 meters.

[0020] In the subway station excavation process of the present invention for determining the foundation width based on the bearing capacity of the foundation, further, in the step (3), when excavating the arch soil, the middle span initial lining buckle arch construction is carried out, the initial support structure of the upper guide tunnel part is chiseled out, and the waterproof layer and the second lining buckle arch are constructed backward. After the second lining buckle arch reaches the designed strength, an integral arch structure is formed.

[0021] The present invention adopts the hole-column method as the main idea of ​​subway station excavation. During the excavation, the strip foundation and the side strip foundation bear all the cover soil load above the top plate and the self-weight of the top plate, beams and columns. The most unfavorable working condition can be studied from an overall perspective, and the entire subway station structure and foundation can be analyzed to obtain the value of the foundation width when the bottom longitudinal beam foundation is damaged.

[0022] Assume that the foundation is an elastoplastic semi-infinite body, and the buried depth of the strip foundation of the hole-column method is H. Before the excavation of the small pilot hole, the horizontal section with a depth of H below the ground surface (i.e., the bottom surface of the strip foundation to be constructed later) is evenly distributed with vertical pressure γH (γ is the weight of the soil). After excavating each pilot hole in turn and constructing the initial support, the strip foundation in the pilot hole is constructed. At this time, the vertical components have not been constructed, the strip foundation does not bear the upper load, and the base pressure generated by the self-weight of the foundation is relatively small. Construct the side piles and crown beams and backfill the concrete between the outer side of the side piles of the lower pilot hole and the pilot hole. At this time, the side strip foundation bears the self-weight of the side piles. Then construct the steel pipe columns and top longitudinal beams and backfill the upper pilot hole concrete. The middle strip foundation bears the weight of the steel pipe columns, top longitudinal beams and the upper soil within the width of the pilot hole. Then construct the side span arch super support structure, advance pre-grouting to reinforce the stratum, symmetrically excavate the side span arch soil, carry out the side span initial lining buckle arch construction, lag the side span initial lining buckle arch excavation surface by 10 to 15m, construct the arch super support structure within the middle span, advance pre-grouting to reinforce the stratum, excavate the arch soil, carry out the middle span initial lining buckle arch construction, remove part of the initial support structure of the upper guide tunnel, retreat to construct the waterproof layer and the second lining buckle arch, after the second lining buckle arch reaches the design strength, the overall arch structure is formed, the soil pressure above the arch is transmitted to the foundation through the vertical components and converted into base pressure, and the base pressure basically reaches the maximum value.

[0023] The completion of the overall arch structure construction means that a framework support system of piles, beams, and arches has been formed. The strip foundations jointly bear all the vertical loads on the upper part, including the self-weight G of the upper structure (including the weight of steel pipe columns, side walls, top arches, middle floor slabs, top longitudinal beams, backfill concrete in the guide tunnel, etc.) and the cover load. The middle strip foundation and the side strip foundation can be regarded as the overall strip foundation of the bottom longitudinal beam (with a total width of B), which is the sum of the widths of all strip foundations in the guide tunnel. The self-weight G of the upper structure and the cover load (thickness is h) are transferred to the overall strip foundation, and the uniformly distributed pressure is γh+G / B. Under the action of the additional strip distributed pressure γ(hH)+G / B and the self-weight γH of the soil, the principal stresses of a point N at a depth Z below the bottom surface of the strip foundation are:

[0024]

[0025] Where: ψ is the viewing angle (°), which is the angle between point N and the lines connecting the two end points of the overall foundation.

[0026] During the construction of the hole-column method, if the deadweight of the superstructure or the covering soil layer is too large, point N will reach the limit equilibrium state. According to the stress condition of limit equilibrium, the maximum principal stress σ1 and the minimum principal stress σ3 will satisfy:

[0027]

[0028] Combining equation (3) and equation (4) and setting It can be obtained that under the most unfavorable working condition, the maximum depth Z of the plastic deformation zone when the strip foundation bears the load of the superstructure and the covering soil, and the foundation bears the load transmitted from the foundation max for:

[0029]

[0030] When Z max =0, which means that the plastic zone shrinks to the two ends of the overall strip foundation. Only the soil at the two ends of the base of the entire foundation reaches the limit equilibrium and begins to produce plastic deformation. The ultimate plastic pressure should satisfy:

[0031]

[0032] As long as the uniformly distributed pressure γh+G / B transmitted to the overall strip foundation by the deadweight G of the superstructure and the cover load is not greater than the pressure on the right side of formula (6), it can be ensured that no plastic zone will appear in the foundation below the bottom longitudinal beam strip foundation during the excavation of the station structure, and no overall shear failure will occur, resulting in the destruction of the underground station structure.

[0033] The total width calculation formula (1) of the bottom longitudinal beam strip foundation of the present invention is obtained by transforming formula (6). When the plastic zone only appears at the two end points of the strip foundation, the foundation will not be damaged, thereby determining the value of the total width of the foundation.

[0034] The present invention systematically analyzes and studies the bearing capacity of the bottom longitudinal beam foundation of the tunnel column method station, and through overall analysis of the subway station structure, takes the bearing capacity of the bottom longitudinal beam foundation of the tunnel column method station as the object, obtains the maximum and minimum principal stresses of the foundation soil unit after excavation, uses the Moore-Coulomb theory to obtain the boundary equation of the plastic zone, and finally derives the value range of the total width of the modified strip foundation. After the total width of the bottom longitudinal beam foundation is determined based on the condition that the bearing capacity meets the design requirements, it plays a guiding role in the subsequent design and construction of the subway station.

[0035] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of the cross section of a subway station in the embodiment. DETAILED DESCRIPTION

[0037] Comparative Example

[0038] Take the excavation construction of a subway station in Beijing as an example. Figure 1As shown, the subway station structure is an underground double-layer, double-column, three-span arched straight wall structure. The top plate covering layer is about 12m thick. The standard section structure is 18.18m high and 25.5m wide. The eight-guide tunnel PBA construction method is adopted. The conventional excavation process is to first excavate the upper guide tunnel and then the lower guide tunnel, first excavate the side guide tunnel and then the middle guide tunnel, and apply initial support; then construct the strip foundation in the lower guide tunnel, apply side piles and steel pipe columns; then construct the side span and middle span top arch to form a frame support system of piles, beams and arches; finally, on the basis of this system, a large area of ​​main soil excavation is carried out, and the earth is excavated from top to bottom to the middle floor slab elevation, and the side walls and middle floor slabs of the station hall are cast; after the middle slab structure reaches the design strength, the main soil is excavated longitudinally in layers and sections symmetrically to the bottom slab position. The buried depth of the strip foundation is continuously reduced, and the bearing capacity of the strip foundation foundation is weakened. The pressure of the upper soil layer and the structure will be transmitted to the strip foundation in the guide tunnel through the vertical components. When excavating to the bottom slab, according to the calculation results of the main structure, the maximum axial force value of the standard section side pile is N max =2085kN / m, maximum axial force of the middle column N max =2785kN / m, so the base pressures of the side guide tunnel and the middle guide tunnel are 463.3kPa and 618.9kPa respectively. According to the survey report, the standard value of foundation bearing capacity f ak =220 kPa, and the modified value of the bearing capacity of the side tunnel foundation f is calculated according to formula (7): a1 =726.11 kPa, correction value of foundation bearing capacity of side guide tunnel f a2 =495.65kPa, the bearing capacity of the foundation of the middle pilot tunnel does not meet the requirements. This indicates that the bearing capacity of the foundation under the middle column calculated according to the formula (Formula 7) in the "Beijing Area Building Foundation Investigation and Design Code" no longer meets the requirements, but the actual bearing capacity of the foundation soil layer obtained by the on-site in-situ load test is 625.65kPa, which meets the requirements.

[0039] f a =f ka +η b γ0(b-3)+η d γ m (d-1.5) (7);

[0040] In the formula, f ka is the standard value of foundation bearing capacity (unit: kPa), η b , η d is the bearing capacity correction factor for foundation width and depth, γ and γ0 are the average weights of soil above and below the foundation bottom, and the weight below the groundwater level is the floating weight (unit: kN / m 3), b is the width of the foundation bottom (unit: m, when it is less than 3m, it shall be considered as 3m, and when it is greater than 6m, it shall be considered as 6m), d is the foundation burial depth (unit: m, when it is less than 1.5m, it shall be considered as 1.5m), for the burial depth d of the strip foundation of the hole column method, the side pile strip foundation takes d=(d1+d2) / 2, and the middle strip foundation takes d=(3d1+d2) / 4, where d1 is calculated from the elevation inside the foundation pit and d2 is calculated from the ground mark.

[0041] This standard formula takes into account the lateral influence of the unexcavated soil inside and outside the foundation pit, as well as the different burial depths caused by the different degrees of influence of the overburden load on the middle strip foundation and the side strip foundation. However, it does not take into account the effect of the adjacent strip foundations on the strip foundation bearing capacity, and is biased towards conservatism. As a result, in this design practice, the foundation bearing capacity does not meet the design requirements.

[0042] Example

[0043] The same subway station structure is constructed by adopting the bottom longitudinal beam foundation width correction of the present invention. The structural deadweight G and the deadweight of the covering soil in the construction stage are determined according to the subway station structural drawings. The deadweight stress and additional stress of a certain point at a depth of Z in the foundation after the main structure of the station is excavated are obtained as γ(H+Z) and γ(hH)+G / B respectively. The maximum and minimum principal stresses of the soil unit are calculated by formula (3).

[0044]

[0045] The boundary equation of the plastic zone is obtained based on the equation that the principal stresses at a certain point in the foundation must satisfy when the ultimate equilibrium state is reached under the action of the superstructure and the overburden load. Find the maximum depth of the plastic zone:

[0046]

[0047] When Z max =0, which means that the plastic zone shrinks to the two ends of the strip foundation. Only the soil at the two ends of the base of the entire foundation reaches the limit equilibrium and begins to produce plastic deformation. Let the above formula equal to 0, and get the ultimate plastic pressure,

[0048]

[0049] The total width B of the bottom longitudinal beam foundation is determined by transforming equation (6). In this embodiment, the total width B is the sum of the widths B1, B2, B3 and B4 of the strip foundations in the four guide tunnels.

[0050]

[0051] In this embodiment, the self-weight of the steel pipe column, middle plate, side wall, top arch and top longitudinal beam of the most unfavorable section of the main structure is 646.5kN / m, 237kN / m, 1395kN / m, 573.8kN / m and 161kN / m respectively, and the self-weight of the backfill concrete in the pilot tunnel and the grouting reinforcement inside the initial support is about 800kN / m, so the total self-weight G of the structure is 3813.3kN / m. The soil bulk density γ is taken as 20kN / m 3 , H is 30.18m, h is 12m, and when the shear strength of the soil is not considered, the calculated total width of the foundation B is not less than 10.49m, and the bottom longitudinal beam foundation will not be damaged.

[0052] After calculating the revised bottom longitudinal beam strip foundation width, the subway station soil excavation is carried out according to the following construction method:

[0053] Step (1), excavating the pilot tunnel, first excavating the upper pilot tunnel and then the lower pilot tunnel, first excavating the side pilot tunnel and then the middle pilot tunnel, and applying initial support.

[0054] Step (2), according to the bottom longitudinal beam strip foundation obtained by calculation and correction, the bottom longitudinal beam strip foundation is constructed in the lower pilot tunnel, and a frame support system is constructed on the bottom longitudinal beam strip foundation, including the construction of side piles and steel pipe columns, and then the side span and middle span arches are constructed to form a frame support system of piles, beams and arches, and at the same time, the stratum is reinforced by pre-grouting in advance, and the arch super support structure in the side span and middle span range is constructed.

[0055] Step (3), excavating the main soil on the basis of the frame support system, excavating the soil from top to bottom to the middle floor elevation, and when symmetrically excavating the side span arch soil, carrying out the side span primary lining buckle arch construction, lagging the side span primary lining buckle arch excavation surface by 10 to 15 meters; when excavating the arch soil, carrying out the middle span primary lining buckle arch construction, chiseling out the initial support structure of the upper guide tunnel part, retreating to construct the waterproof layer and the second lining buckle arch, and forming an integral arch structure after the second lining buckle arch reaches the designed strength.

[0056] The total width of the strip foundation of the bottom longitudinal beam of the subway station in this embodiment is 16m, and the design meets the design requirements of the foundation bearing capacity. The actual bearing capacity of the foundation soil layer obtained by the on-site load plate test is 625.65kPa, which is greater than the base pressure of the pilot tunnel and can also meet the requirements.

[0057] This embodiment aims to focus on the application of the present invention in the correction calculation of the width of the strip foundation of the bottom longitudinal beam of the subway station in the car body construction. The calculation method of the remaining structures during the construction of the subway station can still be calculated and constructed according to the existing specifications. This embodiment does not provide a specific description of these calculation processes.

[0058] The above is only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A subway station excavation method for determining foundation width based on foundation bearing capacity, characterized in that The steps include: (1) Excavate the pilot tunnel and provide initial support; (2) Construct a bottom longitudinal beam strip foundation in the lower pilot tunnel, and then construct a frame support system on the bottom longitudinal beam strip foundation, where the total width B of the bottom longitudinal beam strip foundation satisfies the following formula: Where G is the deadweight of the upper structure per unit cross section, in kN / m, c is the cohesion of the soil layer below the bottom surface of the bottom longitudinal beam strip foundation, in kPa, is the internal friction angle of the soil layer below the bottom surface of the bottom longitudinal beam strip foundation, in degrees, and γ is the average weight of the soil layer above the bottom surface of the bottom longitudinal beam strip foundation, in kN / m 3 , H is the buried depth of the bottom longitudinal beam strip foundation from the ground elevation, in m, and h is the thickness of the soil layer on the top plate, in m; (3) Excavating the main soil on the basis of the frame support system, excavating the soil from top to bottom to the middle floor slab elevation.

2. According to the subway station excavation method for determining the foundation width based on the bearing capacity of the foundation as described in claim 1, in the step (1), the upper guide tunnel is excavated first and then the lower guide tunnel, and the side guide tunnel is excavated first and then the middle guide tunnel.

3. According to the subway station excavation method for determining foundation width based on foundation bearing capacity as described in claim 1, in the step (2), when the shear strength of the soil is not considered, the total width B of the strip foundation satisfies the following formula:

4. According to the subway station excavation method for determining the foundation width based on the bearing capacity of the foundation as described in claim 3, in the step (2), constructing the framework support system includes constructing side piles and steel pipe columns, and then constructing the side span and the middle span arch to form a framework support system of piles, beams and arches.

5. According to the subway station excavation method for determining the foundation width based on the foundation bearing capacity of claim 4, in the step (2), before constructing the arch super support structure in the side span and the middle span range, pre-grouting is performed to reinforce the stratum.

6. According to the subway station excavation method for determining the foundation width based on the foundation bearing capacity of claim 5, in the step (3), when the soil of the side span arch is symmetrically excavated, the side span initial lining buckle arch construction is carried out, lagging the side span initial lining buckle arch excavation surface by 10 to 15 meters.

7. According to the subway station excavation method for determining the foundation width based on the bearing capacity of the foundation as described in claim 6, in the step (3), when excavating the arch soil, the middle span initial lining buckle arch construction is carried out, the initial support structure of the upper guide tunnel part is chiseled out, and the waterproof layer and the second lining buckle arch are constructed backward. After the second lining buckle arch reaches the designed strength, an integral arch structure is formed.

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