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The collaborative construction of undercrossing tunneling of Gongchang Road and the adjacent Metro Line 6 extension station section in Shenzhen is difficult and of high risk. In view of these characteristics, this paper studied the deformation and stability of rock-like material retaining structures in the process of underground engineering collaboration by combining the measured deformation data and the circular slide theory based on the limit equilibrium method. The results show that due to the difference between the supporting systems of rock-like materials on both sides and other reasons, the upper part of the retaining structure and the limited soil in the adjacent area tilt greatly to one side at the same time, and the surface settlement in the limited soil area also increases with the increase of the excavation depth of the foundation pit. On the basis of measured deformation data analysis, the mechanical model for calculating the stability concerning the relationship between the adjacent distance

With the continuous acceleration of the urbanization process in China, the number of various urban infrastructure projects also increases, which leads to the continuous improvement of building density and the increase in the construction difficulty of new projects, especially in the construction of urban three-dimensional transportation [

In the study of the adjacent construction of underground engineering, numerical simulation [

To sum up, previous studies on concurrent construction of super-adjacent deep foundation excavation are very few. Instead, most of them focus on the influence of new construction on existing projects, which is the force transfer mechanism among new foundation pit-soil-existing projects. The undercrossing tunnel project of Gongchang Road in Shenzhen on which this paper relied is constructed in parallel with the super-adjacent extension of Metro Line 6 for a long distance. Its force transfer mechanism is among new foundation pit-soil-new foundation pit or tunnel, and the two projects used different forms of rock-like material retaining structures Since it involves the construction safety risk of two or more projects, the research on the deformation characteristics and stability of this kind of foundation pit construction is of great significance. Therefore, this paper studies the deformation characteristics and stability of the rock-like material retaining structures between super-adjacent road tunnels by open excavation and subway stations through the analysis of measured deformation data and the optimization of common deep foundation pit stability analysis theory, which can provide a reference for the retaining system design of similar projects.

Shenzhen is a coastal city in the south of Guangdong Province and a special economic zone in China. The coverage of the undercrossing tunnel project of Gongchang Road in Shenzhen starts from the west side of Guangqiao Road intersection (design starting point

Relative position relationship and surrounding environment between the undercrossing tunnel project of the Gongchang Road and Metro Line 6 extension.

The underground (tunnel) section is planned to be constructed by the open excavation method. The buried depth of the tunnel floor is about 3∼20 m below the existing ground. The importance level of foundation pit retaining structure is grade I, grade II, and grade III, respectively. The south side of the proposed site is close to the extension project of Metro Line 6, and the metro extension is set parallel to the main channel of the project. The main underground channel is close to Sun Yat-Sen University Station, shield tunnel section, and Science and Technology City Station from west to East. The

According to the on-site investigation and indoor geotechnical test results, the strata distributed along the site mainly include artificial fill layer (Qml), quaternary Holocene alluvial and diluvial layer (q4al + pl), quaternary Holocene swamp sedimentary layer (q4h), quaternary Holocene sloping and diluvial layer (q4dl + pl), quaternary upper Pleistocene alluvial and diluvial layer (q3al + pl), and quaternary eluvial layer (Qel). The bedrock underlying the site includes Jurassic sandy mudstone (

Longitudinal geological profile of

Main physical and mechanical parameters of strata.

Strata | ^{3}) | _{sat} (kN/m^{3}) | _{s} (MPa) | _{o} (MPa) | Natural condition | ||

Φ (°) | |||||||

Artificial fill layer (Qml) 1 | 18.3 | 18.6 | 5.0 | 8.0 | 18 | 15 | 0.5/0.2 |

Silty clay (Q4al + pl) 2 | 18.5 | 19.0 | 5.0 | 13 | 16 | 23 | 0.01 |

Organic clay (Q4h) 3_{-1} | 17.5 | 17.9 | 3.0 | 4.0 | 4 | 15 | 0.01 |

Sandy silty clay (Q4dl + pl) 4 | 18.5 | 18.8 | 7 | 22 | 18 | 24 | 0.01 |

Silty clay (Q3al + pl) 5_{-1} | 18.3 | 18.6 | 6 | 13 | 16 | 18 | 0.01 |

Medium sand (Q3al + pl) 5_{-2} | 19.5 | 20.0 | — | 30 | 32 | — | 10 |

Sandy clayey soil (Qel) 6_{-1} | 18.5 | 18.9 | 8 | 22 | 23 | 23 | 0.05 |

Completely weathered mixed granite (O1N) 9_{-1} | 19.0 | 19.3 | 12 | 60 | 28 | 22 | 0.1 |

Earthy strongly weathered mixed granite (O1N) 9_{-2-1} | 20.0 | 20.4 | 16 | 170 | 33 | 45 | 0.5 |

Massive strongly weathered mixed granite (O1N) 9_{-2-2} | 21.5 | 21.8 | — | 250 | 34 | 50 | 1.0 |

Cataclastic rock (F) | 21.0 | 21.5 | 18 | 180 | 34 | 45 | 1.0 |

The main aquifers of the site can be divided into three types: the first type is artificial fill layer with poor water permeability and bearing, which can form upper stagnant water locally. The second type is the quaternary sand layer with strong water bearing and permeability, the groundwater in which is pore phreatic water. The third type is strongly and moderately weathered bedrock zone, whose water content and water permeability are mainly controlled by the development degree of stratum fissures, and the groundwater in it is bedrock fissure water, which is also slightly pressured. Other strata are layers with weak water bearing and permeability or relative aquicludes.

The

Longitudinal section diagram of foundation pit excavation method in

The schematic diagram of the retaining structure and internal support form of the foundation pits on both sides is shown in Figure

Schematic diagram of foundation pit retaining system on both sides: (a) retaining structure; (b) internal support.

Basic parameters of foundation pit retaining system on both sides.

Project | Sun Yat-Sen University Station | |||
---|---|---|---|---|

Retaining system | Retaining structure | ① Φ1200@1400 mm drilled grouting pile | 800 mm diaphragm wall | |

② Φ800 mm high-pressure jet grouting pile | ||||

③ Φ800@600 double-pipe high-pressure jet grouting pile waterproof curtain | ||||

Internal support | First support | 800 mm × 800 mm eight-claw reinforced concrete support, horizontal spacing: 9 m | 700 mm × 1000 mm reinforced concrete support, horizontal spacing: 9 m | |

Second to third support | Φ806 ( | Φ609 ( |

Material parameters of rock-like material retaining structure.

Project | Material of retaining structure | Material of the first inner support | |
---|---|---|---|

Drilled grouting pile | Underwater concrete (C30) | Shrinkage compensating concrete (C30) | |

Pressure jet grouting pile | Ordinary portland cement (level P42.5) | ||

Sun Yat-Sen University Station | Diaphragm wall | Underwater concrete (C35) | Concrete (C30) |

It can be seen from the above project overview that both the undercrossing tunnel project of Gongchang Road and the adjacent subway station are constructed by open excavation method, with a large construction scale; and the project site is located in the urban construction intensive area, with tight construction period, complex surrounding environment, and narrow construction space. Due to the parallel layout, collaborative construction, and cross-interference between the project and adjacent metro lines, the deformation and stability of the project during construction are faced with high safety risks. Therefore, this paper takes the

The automatic monitoring system of the project is composed of parts including monitoring equipment, data acquisition and analysis, signal transmission, and receiving terminals. Among them, the deep horizontal displacement _{h} of the retaining structure and the surrounding ground subsidence _{h} and

Distribution of project monitoring sections and points of foundation pits on both sides: (a) general monitoring layout; (b) monitoring points of ground subsidence of foundation pits on both sides and naming of retaining structures.

In order to facilitate the analysis and explanation of the follow-up study, the retaining structures of foundation pits on both sides are named according to the relative orientation. The retaining structures on the north and south sides of the Gongchang Road site and the north and south sides of Sun Yat-Sen University Station are named as

Figures _{h-max}, _{h-min}, and _{h-avg} are the maximum, minimum, and average values of the deformation of the retaining structures toward the pit. This section starts from the overall deformation trend of the retaining structures of foundation pits on both sides and first analyzes the

The overall deformation of the _{h-max} is 36.96 mm, and the average maximum displacement is 18.8 mm. The maximum horizontal displacement depth of

The deformation trend of _{t-NS} of 10.82 mm; the horizontal displacement of the top of _{t-SN} of 14.06 mm. In the deep horizontal displacement of the retaining structure, the maximum horizontal displacement depth of the _{h-max} is 25.48 mm, while the average maximum displacement is 12.81 mm. The maximum horizontal displacement depth of

Deep horizontal displacement of the retaining structures: (a)

Comparison of characteristic values of deep horizontal displacement of retaining structures of foundation pits on both sides.

Project | Characteristic values of deep horizontal displacement of the retaining structure on the north side (mm) | Characteristic values of deep horizontal displacement of the retaining structure on the south side (mm) | ||||
---|---|---|---|---|---|---|

_{h-max} | _{h-min} | _{h-avg} | _{h-max} | _{h-min} | _{h-avg} | |

36.96 | 3.56 | 18.8 | 25.48 | 5.38 | 10.92 | |

Sun Yat-Sen University Station | 24.94 | 19.99 | 19.06 | 30.26 | 17.08 | 23.49 |

In the existing research on ground subsidence, most of the research objects are semi-infinite soil. The research results concerning measured deformation of ground subsidence include ground subsidence mode, main influence range, maximum subsidence value, and its location. In this paper, both the north side soil of the foundation pit in the

Prediction curve of ground subsidence and subsidence influence range in semi-infinite soil area: (a) north side of

As the adjacent area is relatively narrow, the horizontal distance between adjacent retaining structures is about 3.29 m in the

The data statistics of ground subsidence monitoring sections on the north side of the foundation pit in the

Characteristic values of ground subsidence in semi-infinite soil area.

Monitoring area | Characteristic values of ground subsidence in semi-infinite soil area (mm) | ||
---|---|---|---|

North side of | −32.1 | −17.5 | −25.62 |

South side of Sun Yat-Sen University Station | −23.1 | −11.7 | −19.62 |

Because the maximum excavation depth _{e} of the two foundation pits is close in value, the influence range of ground subsidence and the location of the maximum subsidence after excavation are also close, which are in the range of 10∼12 m and 36∼41 m, respectively. However, the fact that the excavation width of the two foundation pits is different and the stiffness _{w}^{4}_{avg} of the retaining system of the diaphragm wall is higher than that of the pile wall structure [

Figure

Longitudinal curve of ground subsidence and subsidence trend in finite soil area.

Characteristic values of ground subsidence in finite soil area.

Monitoring area | Characteristic values of ground subsidence in finite soil area (mm) | ||
---|---|---|---|

South side of | −18.24 | −6.06 | −11.77 |

North side of Sun Yat-Sen University Station | −23.20 | −4.90 | −14.29 |

It can be seen from the figure that with the increase of abscissa, the two longitudinal deformation curves of ground subsidence in finite soil area show a gradually increasing trend, and the abscissa of the increasing inflection point is 250∼300 m. The maximum values of the two curves are −18.24 mm and −23.2 mm, respectively, and the average values are −11.77 mm and −14.34 mm, respectively. The range of these inflection points is included in the

Through the above statistical analysis of the measured deformation data, it can be found that for different forms of rock-like material retaining structure, the lateral displacement law of the retaining structure with semi-infinite soil on one side of the super-adjacent deep foundation pit and the deformation law of ground subsidence on that side are basically consistent with the existing research conclusions of the deformation of a single foundation pit; the top of the retaining structure with finite soil on one side and a certain range below has a tendency of deformation to the north side at the same time, and the top displacement value is larger. At the same time, due to the narrow range of finite soil area, the ground subsidence deformation analysis is mainly based on the longitudinal deformation trend, which shows that the subsidence value increases with excavation depth.

In the collaborative construction process of super-adjacent deep foundation pits, the deformation form and degree of the adjacent area soil and retaining structures may directly affect the stability of foundation pits on both sides. Once the adjacent retaining structure has a large displacement to the same side, it is likely that the foundation pits on both sides will lose stability to different degrees at the same time, so special attention should be paid to the stability of finite soil and the rock-like material retaining structures on both sides. The against basal heave stability of deep foundation excavation is particularly important, which is related not only to the stability and safety of foundation pits but also to the deformation of foundation pits. Therefore, based on the measured deformation data analysis and the existing theoretical mechanical model of the against basal heave stability, the stability analysis method for super-adjacent soil and rock-like material retaining structure characteristics is further studied.

The common analysis method of the against basal heave stability of single foundation pit adopts the calculation theory of circular sliding mode based on limit equilibrium method, which can be divided into two circular sliding modes according to the different center positions of circular: one takes the position at the bottom of pile wall as the center, and the other takes the lowest retaining point of pile wall as the center. In Asia, many regions such as Japan, Taiwan (China), and Shanghai (China) usually adopt the second mode [

Schematic diagram of checking against basal heave stability of pit bottom based on circular arc sliding mode (homogeneous foundation) [

There are two main types of the sliding moment: ① the sliding moment

There are three types of antisliding moment: ① allowable moment _{s} of the retaining structure, ② antisliding moment

It can be seen from the figure that the sliding moment

However, for ultra-adjacent deep foundation excavation, the stability calculation method for the interlayer between the foundation pits on both sides is different from the traditional one. Combined with the actual situation, this paper takes the most unfavorable working condition where foundation pits on both sides are excavated to the bottom as an example and divides the mechanical model for the against basal heave stability calculation of super-adjacent area into the following two cases based on the relationship between the horizontal spacing

Schematic diagram of the assumed sliding surface of foundation bottom with different adjacent pit spacing: (a)

Due to the complexity of working conditions and corresponding stress conditions of super-adjacent deep foundation pit in practical construction, further assumptions and simplification should be made in the study of its against basal heave stability. Taking the left foundation pit as the main research object, the basic assumptions are as follows:

The stratum of foundation pits on both sides is homogeneous stratum; that is to say, the weighted average strength index of layered soil is equivalent to the homogeneous soil layer

During the excavation of the right foundation pit, the unloading Earth pressure is balanced with the external force provided by the support of each layer

The antisliding moment of the vertical sliding surface above the lowest support and the ultimate flexural strength of rock-like material retaining structure are not considered [

When

Antisliding moment

Antisliding moment on sliding surface

Shear strength _{ce} [

where _{a} is the active earth pressure coefficient of the corresponding soil layer, and

Then, the antisliding moment of sliding surface

Antisliding moment on sliding surface

Shear strength on sliding surface

where _{p} is the passive Earth pressure coefficient of the soil layer,

where

Sliding moment

At this time, the sliding moment is produced by the self-weight of soil in

When ^{R} should be added in the calculation of antisliding moment.

Antisliding moment

Antisliding moment on sliding surface

Shear strength _{ne} on sliding surface

where

Antisliding moment on sliding surface

At this time, the antisliding moment on sliding surface ^{R}_{ef} in the first case. Then, the total antisliding moment is

Sliding moment

At this time, the sliding soil area is the rectangular area

Through the above division of the mechanical models for the against basal heave stability of rock-like material retaining structure with different adjacent degrees, the results can provide references for the stability calculation of similar projects. In practical application, the basic parameters of the retaining system of the foundation pits on both sides may be different, so the retaining system design should be carried out after calculating the parameters according to the above models, on the premise of satisfying the stability requirements of the foundation pits on both sides.

Based on the above study on the calculation of anti-uplift stability of rock-like material envelope in super-adjacent engineering, this paper verifies the applicability of the calculation method through calculating the stability of the typical excavation section of Shenzhen Gongchang Road undercrossing tunnel project and applying relevant prediction theories.

The typical section of Shenzhen Gongchang Road undercrossing tunnel adjacent to Sun Yat-Sen University Station and Science and Technology Town Station is selected for stability calculation, as shown in Figures _{1} = 3 m, _{1}' = 15 m; in Figure _{2} = 6 m and _{2}' = 14.3 m. Both of them belong to the second case of stability calculation; i.e.,

Typical section diagram of foundation pits in the adjacent area between Gongchang Road undercrossing tunnel and Metro Line 6 extension stations: (a)

According to the physical and mechanical parameters of the corresponding soil layer in Table _{S} of the deep foundation pits of Gongchang Road undercrossing tunnel is 1.87, and for the section in Figure _{s} = 2.03, which is basically consistent with the design scheme and meets the standards of the stability verification by circular sliding method for foundation pit with internal support required by the

The prediction method gives the diagram of the relationship between the stiffness of the retaining system, the maximum relative lateral displacement of the retaining structure, and the stability coefficient, as shown in Figure ^{4}_{w} is the stiffness of the retaining system, _{w} is the standard weight of water, and

Curves of the relationship between the maximum lateral displacement of retaining structure and stiffness of the retaining system.

Figure

Curves of the relationship between the maximum relative displacement of retaining structure and factor of safety of the against basal heave stability.

In the process of super-adjacent collaborative construction of road tunnels by open excavation and subway stations, due to the difference of rock-like material retaining systems on both sides, the upper part of the retaining structure and the finite soil in the adjacent area have inclined to one side at the same time, and the ground subsidence in the finite soil area also increased with the excavation depth of the foundation pit. Since the deformation of the adjacent area directly affects the stability of foundation pits on both sides, the deformation control should be fully considered in the design of rock-like material retaining system

Taking the calculation method of foundation stability of circular sliding theory based on limit equilibrium method, the mechanical models and formulas for rock-like material retaining structure stability calculation under the conditions of

In the process of theoretical research, this paper simplifies and assumes the research object. In the follow-up studies, the applicability of the theory can be optimized, and the mechanical models for rock-like material retaining structure stability analysis of different retaining system types and super-adjacent deep foundation pits with comprehensive retaining stiffness can also be further studied

The measured data used to support the findings of this study are included within the article.

The authors declare that they have no conflicts of interest.

This research was supported by the Science and Technology Project of China Communications First Bureau Group Co. Ltd, grant no. 4GS (J)-GUD-GCL-01-JS-016, and the National Natural Science Foundation of China, grant no. 51278233.