Keywords

1 Introduction

The research and application of the theory of underground space support (including tunnel lining support and deep foundation pit support) has a history of nearly 100 years [1]. Since the twentieth century, the development of support nursing theory has mainly gone through three stages: classical pressure theory, bulk pressure theory and modern theory of elastoplastic deformation pressure, and the more representative theories are: new Austrian support technology theory, axonal transformation theory three law theory, loose circle branch nursing theory and composite branch nursing theory and many other theories [2]. In recent years, China has made great achievements in underground space support technology and research theory, but due to the uncertainty and complexity of engineering geological conditions, along with the large-scale development and construction of urban underground space, various foundation pit projects continue to emerge, and the current foundation pit engineering support technology is still difficult to meet the actual needs, and further innovation and development are urgently needed.

This paper adopts the research patented technology “tunnel and underground space support components” [3], which belongs to the research and development of original basic theory and key technology application, which can be assembled into a “honeycomb” structure type and expanded to the application of deep foundation pit support engineering in underground space and the application of this technology fully reflects the combination of “industry-university-research” theoretical research and engineering practical application. This support technology can optimize the setting of lateral support components in the deep foundation pit support process of underground space, improve the current conventional deep foundation pit support construction method and technical problems, and has good research and industrialization promotion and application value. In this paper [4], the stress and strain analysis of each working condition of foundation pit excavation is carried out through numerical simulation, which further verifies the practicability and feasibility of this innovative technology.

2 Key Technologies and Applications of Deep Foundation Pit Support

2.1 Support Key Technical Principles

At present, the representative application technologies of deep foundation pit support mainly include new support technologies and groundwater control technologies such as composite soil nail support, cement-soil retaining wall, underground continuous wall and joint support [5]. According to the actual needs of deep foundation pit engineering and the application progress of new technologies, this paper summarizes the development trend of deep foundation pit engineering technology, studies the technology of combining supporting structure and main structure, and uses invention patent components to improve the construction technology of commonly used support structure, shorten the construction period and save resources according to the construction technology of partial or all permanent underground structure as temporary support structure of foundation pit.

The support technology process is to assemble the “honeycomb” prefabricated support members into a row of support structural units according to the requirements [6], and the two sides of the unit are fixed on the I-shaped steel sheet piles driven vertically into the deep foundation pit to form an overall supporting thin-walled wall structure. The earth pressure acting on the back of the supporting member on the component can be partially converted into horizontal thrust that can cancel each other between the components, so that the contact between the components is closer and the structural force mode is greatly optimized [7]. At the same time, the use of stiffener ribs increases the overall rigidity of the supporting wall and the ability to withstand loads. The support technology greatly optimizes the requirements for setting transverse support members usually required for deep foundation pit support measures, thereby solving the problem of affecting the overall construction due to the interference of transverse support members after deep foundation pit excavation.

The key technology of this support divides the length according to the standard required for the construction of deep foundation pits, assembles each unit separately, and then assembles into a whole structure. When assembling, it is necessary to first try to calculate the depth and width that can be safely carried in the longitudinal and horizontal directions of the excavation foundation pit, and then according to the calculation results, the assembled supporting members and vertical I-beam steel sheet piles are assembled into an integral wall structure to achieve the purpose of common force and support, and the supporting wall structure is shown in Fig. 1.

Fig. 1
3 illustrations. A, rectangular structure with wave-like curved design along the inside walls of the rectangle. B, 2 curved structures with I shaped frame. C, a horizontal frame with arch supports beneath it. I shaped frames at either end.

Schematic diagram of the structure of the overall support wall of the deep foundation pit (In the picture: 1 - The inside of a deep foundation pit, 2 - poured filled waterproof concrete, 3 - arch support “member”, 4 - I-beam steel sheet pile column, 5 - fixed bolt, 6 - fixed nut, 7 - bolt hole, 8 - stiffener “member”, 9 - “member” reserved bolt hole)

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2.2 Support the Application of Key Technologies

The key technology of deep foundation pit support in the underground space first needs to calculate the safety and stability coefficient of foundation excavation, lateral earth pressure, and the depth, length and stability time of each excavation according to the depth and width of deep foundation pit excavation, and carry out design and construction guidance according to the data, and support the lining in time within the effective time to achieve the purpose of guiding construction safety. The specific implementation steps combined with a practical application engineering implementation plan are detailed as follows:

Application example: A grade IV surrounding rock geological deep foundation pit needs to be excavated at a depth \(H = 15.0\;{\text{m}}\), a foundation pit length \(L = 150.0\;{\text{m}}\), width \(B = 80.0\;{\text{m}}\). The soil is medium groundwater, leakage, construction vibration is medium, cohesion reduction coefficient \(K_{c} = 0.6\), pile height around the foundation pit \(H_{0} = 1.0\;{\text{m}}\), soil severity \(\gamma = 17.0\;{\text{KN/m}}^{3}\), cohesion \(c = 155KP_{a}\), cohesion change coefficient 0.6, internal friction angle \(\varphi = 23^{^\circ }\). According to the above conditions, the implementation plan of foundation pit excavation is designed.

According to the above data and requirements, the steps of excavation and support process of foundation pit construction are as follows:

Step 1: Release the line according to the length of the foundation pit L = 150 m and the width B = 80 m and determine the I-beam driving position, first drive the 17 m long I-beam sheet pile vertically at the four corner points, and then drive 31 and 14 I-beam sheet piles of the same specification along both sides of the length and width of the foundation pit at the determined position.

Step 2: Calculate and analyze a reasonable excavation plan, assuming that the excavation plan is as follows:

Scheme 1: Assume that the length of one excavation is L = 75 m, and the excavation depth is H1 = 7.5 m; The second excavation depth H2 = 7.5 m, the calculation results are shown in Table 1:

Table 1 Scheme 1 Calculation table of deep foundation pit excavation stability
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It can be seen from the comparison of Table 1 that the deep foundation pit excavation and support are carried out by scheme 1, the stability coefficient = 1.40, the total lateral horizontal force F = 34700KN, the maximum self-stabilizing depth Hz = 10.15 m, and the length without support Lmax = 75 m.

Calculation result: At the first excavation at a depth of 7.5 m, the foundation pit was stable. After taking the overall support measures, when the second excavation is carried out to a depth of 15 m, the stability coefficient is reduced to 1.13. At this time, the length of each excavation cannot exceed 20 m, and the foundation pit can be stable for 1 day. After taking timely support measures, excavation and support in the next cycle process can be carried out until the excavation reaches the required depth, which ensures construction safety.

Scheme 2: Assuming the excavation length L = 150 m, the excavation is divided into three depth directions: the first excavation depth H1 = 5 m, the second excavation depth H2 = 5 m, and the third excavation depth H3 = 5.0 m, the calculation results are shown in Table 2:

Table 2 Scheme 2 Calculation table of deep foundation pit excavation stability
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It can be seen from the comparison in Table 2 that the first excavation stability coefficient = 1.72, the total horizontal force F = 18100KN, the maximum self-stabilizing depth Hz = 10.15 m, and the length without support Lmax = 150 m.

Calculation result: At the first excavation of 5 m depth, the foundation pit was stable. After the completion of the support, the second excavation is carried out to a depth of 10.0 m, the safety factor is reduced to 1.2, and the total horizontal force is increased to 56300KN, at this time, the excavation length of each section cannot exceed 50 m, the foundation pit can be stable for 1 day, and the excavation of the next cycle section of 20 m length can be carried out after timely support measures; When the third excavation reaches a depth of 15 m, the safety factor is reduced to 1.13, and the total horizontal force is increased to 114600KN, at this time, the excavation length is required to not exceed 10 m long, and timely support is required after excavation before the excavation of the next cycle of construction section can be carried out until the excavation is completed.

Step 3: According to the comparative analysis of Table 1 and Table 2, according to the requirements of construction safety and time saving, it is recommended to adopt the excavation method of Option 2, which can meet the purpose of construction safety and investment saving.

As shown in Fig. 2, the first layer of soil in the deep foundation pit is excavated, the construction excavation depth is H1 = 5 m, the length is 150 m, and the excavation sequence is first the middle and then the four weeks. After the completion of each stage of construction and excavation, the excavated soil is cleared and transported in time, and the excavation of each layer of soil and the splicing and installation of the corresponding arch member support are completed within the required time to ensure that the foundation pit is stable and does not collapse.

Fig. 2
Step-by-step illustrations of the excavation process. 1, a hollow vertical bar. 2, an irregular embossed surface at the top left side. 3, the hollow bar is filled with curved frames. The left side of the bar has an uneven embossed surface. 4, the uneven surface is filled with solid structures.

Drawing of the excavation process of deep foundation pit

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Step 4, deep foundation pit support structure assembly: the supporting members are stitched horizontally according to one unit every 5 m, and the two ends of the unit are anchored and fixed on I-shaped steel sheet piles by bolts, from bottom to top, assembled row by row, until the first layer of soil is excavated and assembled. In the assembled components, the third row is the setting position of the stiffener component element layer. The stiffener member is set up to increase the overall bending stiffness of the supporting wall.

Step 5: The gap between the soil and the supporting structure after the support is completed is poured and compacted with 30 cm thick waterproof concrete, which plays the dual role of bearing earth pressure and stopping water.

Step 6: According to the requirements of the 2 plan, cycle the construction until the foundation pit is fully excavated to the required depth of construction and the support is completed.

After the construction of the deep foundation pit is completed, the prefabricated components can be used as permanent support walls of the foundation pit, or they can be dismantled and reused as wall support materials in underground parking lots or other projects to achieve the purpose of energy conservation and environmental protection.

3 Numerical Simulation of Key Technologies for Deep Foundation Pit Support

3.1 Numerical Models and Parameters

The model simulates the working conditions of the construction stage of the excavation process of the extended application of deep foundation pit, adopts the MC (Moore-Coulomb) constitutive model, and the supporting structure adopts prefabricated components and vertical I-beam fixed support form. The calculation model is formulated according to the long-span deep foundation pit, the size is 40 m × 20 m × 15 m (length × width × depth), divided into three excavations, each excavation depth is about 2.5 m, the support adopts the studied “honeycomb” prefabricated support members, the model is shown in Fig. 3.

Fig. 3
A 3 D image of a cuboid from which a small rectangular part is cut out and the internal filling is exposed. The enlarged part of the internal filling has 5 layers.

Schematic diagram of excavation and support model of foundation pit

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Initial Excavation.

According to the analysis of the initial excavation ground stress cloud 4, the effective stress of earth pressure in the X, Y and Z directions of the three axes is small, which are:

  • S-XXmax = 209 kPa, S-XXmin = 30 kPa; S-YYmax = 209 kPa, S-YYmin = 30 kPa;

  • S-ZZmax = 480 kPa, S-ZZmin = 40 kPa, Effective range S-ZZ = 40~120 kPa.

It shows that the in-situ stress in the horizontal X and Y directions of deep foundation pit excavation is small and balanced, and the in-situ stress in the vertical direction gradually increases with depth. After the I-beam is driven, because the step-by-step excavation of the foundation pit has not yet been carried out, the initial displacement is basically 0, and the supporting stress is the same as the in-situ stress (Fig. 4).

Fig. 4
a to c are sets of 3 heat maps of the excavation of the foundation pit present the distribution of in situ stress, displacement, and effective stress. The layers of the pit in a and c have increasing values from bottom to top. In b, the pit for T Z has highest value all over the block, except the pit.

Initial excavation stress cloud

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Excavation −1~5 Stages.

Only the displacement and effective stress cloud map in the Z-axis direction that has the main influence of the excavation −1~5 stage is analyzed, the step-by-step excavation depth is 2.5 m, and the prefabricated components are assembled and supported immediately after each excavation, and the displacement: effective stress in the Z-axis direction after excavation are analyzed as follows:

  • Displacement: main Z direction

excavation −1: T1-Zmax = 10.78 mm, T1-Zmin = −0.193 mm, effective range T1-Z = 0~2.5 mm.

Excavation-2: T2-Zmax = 20.80 mm, T2-Zmin = -0.43 mm, effective range T2-Z = 1.34~6.65 mm. Excavation-3: T3-Zmax = 29.40 mm, T3-Zmin = −0.67 mm, effective range T3-Z = 1.84~9.85 mm. Excavation-4: T4-Zmax = 37.08 mm, T4-Zmin = −4.5 mm, effective range T4-Z = 2.42~12.82 mm. Excavation −5: T5-Zmax = 43.8 mm, T5-Zmin = −15.94 mm, effective range T5-Z = −6.0~8.96 mm.

  • Effective range of support stress

Support-1: S1-ZZ effective range = −480~0.25 kPa; Support-2, S2-ZZ effective range = −280~20 kPa; Support-3, S3-ZZ effective range = −340~50 kPa; Support-4, S4-ZZ effective range = −370~70 kPa; Support-5, S5-ZZ effective range = −400~120 kPa.

From the above data, it can be seen that after step-by-step excavation −1~5 stage, after step-by-step excavation at a depth of 2.5 m (total excavation depth 12.50 m, I-beam anchoring depth 5 m), the displacement deformation in the X and Y directions is small, and the maximum uplift displacement of the first excavation at the bottom of the vertical Z pit is 10.78 mm. After the timely support of the prefabricated supporting members is adopted, the prefabricated components can play the function of timely bearing and supporting, and the stress is 0.25~−480 kPa in the vertical change range and 0.45~−208 kPa in the horizontal direction, and the deformation and stress control in the excavation process are within the safe range.

By the completion of excavation, the maximum displacement of the excavation-5 pit bottom uplift reaches 43.8 mm, and the minimum displacement settlement is about 15.94 mm.

Figure 5 Cloud diagram shows that the stress of the excavation support member increases with the increase of excavation depth, and the vertical effective stress also gradually increases. After adopting prefabricated “honeycomb” supporting members in the direction of X and Y, the overall stress is effectively reduced. The Z-axis stress increases more with the increase of excavation depth, and the maximum stress range reaches −480~120 kPa, but it is far lower than the bearing capacity of prefabricated components, so the deformation and stress of the deep foundation pit of the entire excavation section are controlled within a safe and reasonable range

Fig. 5
2 parts. A, 5 heat maps of the pit with different depths. The visibility of the heat map in the pit decreases from 1 to 5. Higher displacement is present in the center of the pit. B, 5 heat maps of the excavation pit. Values of stress increase from bottom to top. S 1 Z Z has the layer of the highest value.

Cloud diagram of excavation −1 excavation excavation and supporting stress

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4 Conclusions

Combined with the key support technologies of the research topic, this paper introduces a new construction technology and construction method for the innovation of deep foundation pit support technology based on the application of engineering calculation examples and numerical simulation methods. The technology is green, low-carbon and environmentally friendly, using recyclable prefabricated support technology, which provides new ideas for solving the problems of high energy consumption, low utilization rate and waste of resources in conventional foundation pit support technology, has good research and industrialization promotion and application value, improves the commonly used support construction technology, broadens the new construction method of using prefabricated components in deep foundation pit, and concludes:

This paper adopts the key support technology, and provides a new construction idea and process method

  1. (1)

    The key technology of support of this research topic, using arched prefabricated support components, assembled into the best force-bearing support structure shaped like “honeycomb”, is applied to large-span deep foundation pit support engineering projects, simplifies the current commonly used deep foundation pit construction technology and technical treatment measures, solves the problem of common deep foundation pit excavation setting lateral support and affects a wide range of construction interference, and innovates the new construction method of using prefabricated assembly new material structure construction in deep and large foundation pit support.

  2. (2)

    Using numerical simulation, the deformation and stress–strain analysis of each working condition of foundation pit excavation were carried out, and it was concluded that the maximum uplift displacement and stress change of the pit bottom after excavation and timely support of the deep foundation pit were within the safe range of support control. However, considering the safety and stability factors of construction, it is necessary to take static pressure treatment measures on the bottom of the pit to reduce the stress at the bottom of the foundation pit; In the process of deep excavation of the foundation pit, the supporting stress is far lower than the bearing capacity of the prefabricated components, and the deformation and stress of the foundation pit are controlled within the safe range, which further verifies the practicality and reliability of this key innovative technology.