Research on deep foundation excavation of rail transit station under water-rich soft soil

During the excavation of deep foundation, the deformation of surrounding structure usually occurs due to soil unloading, while in water-rich soft soil, the deformation becomes complex and diversified due to the increase of influencing factors. Therefore, the study of Influencing factors and control methods of excavation deformation should be carried out. Based on a rail transit station of Tianjin B1 Rail Line, considering the characteristics of water-rich soft soil, combining relevant monitoring data, the influence of deformation, stress and stability under various excavation stage will be analyzed, providing a guidance for similar project.


Project Profile
Xinjiayuan North Station is 305.08mlong, adopting a double-column box frame structure on the second floor underground.The width and height of the standard section of the station is 21.1m and 13.54m, and the width and height of the sinking section of the shield is 25.4m and 14.98m.The thickness of covering soil at the center of the platform is about 3.1m, which is constructed by open excavation.The station enclosure adopts 800mm thick ground connecting wall, concrete grade is C40P8, concave and convex locking pipe joint.The internal support forms are: reinforced concrete support (800 × 800mm) + two steel supports + one brace (standard section), reinforced concrete support (800×800mm) + three steel supports and one brace (shield section).The horizontal profile of the standard segment is shown in figure 1.

Geology
The stratum distribution of the proposed site of Xinjiayan North Railway Station is relatively stable.The subsoil within the depth of 65.00m in the site belongs to the sea-land interaction sedimentary layer of the Quaternary Neozoic series (Q4) and Upper Pleiocene Series (Q3), which is mainly composed of cohesive soil, silt and sandy soil, and is generally distributed horizontally.The composition of foundation soil of the site is shown in table 1.

Hydrology
The groundwater type in Tianjin area is mainly pore water of loose rock, which is divided into submersible aquifer and confined water aquifer according to the formation time, origin and hydrological characteristics.The aquifer closely related to the construction of this project is the submersible aquifer, the first confined aquifer of shallow layer ⑧2 and the second confined water aquifer composed of middle layer ⑩2, ⑪2, ⑪4 and ⑫2.
The phreatic water layer is generally distributed in the shallow soil layer, with the silty clay layer ⑦ and ⑧1 as the relative water-resistant bottom.The main source of phreatic water recharge is the infiltration of atmospheric precipitation.The water level is affected by seasonality obviously, the water level rises in high water period and drops in low water period.The stable buried depth of mixed groundwater measured in the drilling hole during the detailed survey is generally 1.80m ~3.80m, and its absolute elevation is generally between -1.42 ~0.67m.The initial water level is equivalent to the stable water level.
The confined water aquifers which have influence on this project are mainly the first confined water aquifer and the second confined water aquifer.The first confined water mainly lies in ⑧2 silty sand layer.⑧2 Layer is widely distributed in the site, the soil layer has strong permeability and rich water content.According to the detailed survey data, the buried depth of the first confined water level in this bid section is 2.56 ~3.87m below the ground, and the corresponding water level is about 0.27 -2.66m.
The second confined water exists in the ⑩2, ⑪2, ⑪4 and ⑫2 silty sand layer, which has strong permeability and rich water content.According to the detailed survey data, the buried depth of the first confined water level in this section is 11.0 ~14.3m below the ground, and the corresponding water level is about -9.

Excavation condition
According to the construction sequence, the foundation pit excavation of the standard section of the station is divided into the following three stages: Stage 1: The foundation pit is excavated to 500mm under the first support, the first support is applied, and the pre-application force is applied; Stage 2: After the prestressing force reaches the design standard, excavate step by step to 500mm under the second support, apply the second support, and apply the prestressing force; Stage 3: After the pre-loading reaches the design standard, excavate step by step to the bottom elevation of the design pit, and construct the foundation pit concrete cushion, bottom waterproofing layer and side wall waterproofing layer.

Monitoring program
In the process of deep foundation pit excavation, the soil inside and outside the foundation pit will change from the original static soil pressure state to the passive or active soil pressure state, and the change of stress state will cause the soil deformation, even if the support measures are adopted, a certain amount of deformation is difficult to avoid.In addition, the excavation of the Xinjiayuan North Station is deep and mostly silt soil with rich water content.Therefore, effective and feasible monitoring program should be conducted to guarantee the excavation process.
Combined with the actual situation of the geology, hydrology and surrounding environment of the construction section of the station, the monitoring objects of the foundation pit construction of Xinjiayuan North Station are the foundation pit envelope structure and the surrounding environment.The monitoring objects of foundation pit retaining structure include ground connecting wall and support; The monitoring objects of the surrounding environment are rock and soil bodies and groundwater in the main and secondary affected areas of foundation pit engineering.In this paper, the measuring points of wall horizontal displacement (ZQT-04, ZQT-06) and steel support measuring points ZCL-04 are selected as shown in figure 3.As shown in figure 4, the displacement and deformation of ZQT-04 and ZQT-06 inclinationmeasuring points are similar under different working conditions.It is shown that in the early stage of foundation pit excavation, the displacement of the ground wall is relatively small under working condition 1; under working condition 2 and working condition 3, the displacement begins to increase gradually with the continuous increase of excavation depth.
Comparing the horizontal displacement-depth curves of ZQT-04 and ZQT-06 inclinometer holes under different working conditions, it is found that: 1)The monitoring results of ZQT-04 inclinometer tube have a certain mutation, but no obvious failure conditions such as collapse and slip have occurred at the construction site.The monitoring results of ZQT-06 show that during the period from the excavation of the foundation pit to the beginning of the construction of the bottom plate, that is, under the condition of working condition 3, its value exceeds the prescribed limit value 40mm, and an orange warning occurs.The reasons are as follows: On the one hand, after the completion of basement excavation, the cushion pouring and floor construction were not carried out in time, resulting in a long exposure time.On the other hand, there is a large steel bar processing shed and lifting machinery near the measuring point, and its large local external load causes the maximum measured value at the measuring point to exceed the specified value, but with the concrete pouring of the bottom plate, its value gradually becomes stable.[1] 2)The deformation of ZQT-06 inclination hole under different working conditions is larger than that of ZQT-04.The reasons are as follows: On the one hand, the on-site steel bar processing shed is arranged near the measuring point, and the raw steel bars are stacked and processed in the shed [2]; On the other hand, there are large transportation machinery and material lifting machinery near the measuring point; The joint action of the two causes the ground connecting wall to be subjected to a large local external load, resulting in a large phenomenon of the maximum measured value at the measuring point.
From the perspective of the overall change, the closer the wall is to the ground, the greater the displacement of the diaphragm wall at the corresponding position, which is related to the soil excavation, the depth of the wall and the earth pressure.Therefore, the deeper the excavation, the greater the displacement of the upper wall.However, with the completion of excavation and the corresponding inhibition of support, the wall and surrounding soil eventually reach a stable state.

Axial force of support
The monitoring of the axial force of steel support is also an important part of the monitoring of the deep foundation pit of subway stations, which is closely related to the stability of the envelope structure.[3][4] There are 48 monitoring points for supporting axial force in the foundation pit of Xinjiayuan North Station.As shown in figure 5, the curve of supporting axial force of ZCL-04 measuring point in September, taking the change of this measuring point as an example to carry out specific analysis.With the excavation of earthwork, the unloading of soil mass, and the reduction of passive earth pressure, the horizontal displacement of the ground connecting wall tends to develop in the foundation pit.With the continuous erection of steel support, that is the orderly progress of working conditions 1 and 2, its role begins to be reflected.While counteracting part of the deformation of the ground connecting wall, the axial force gradually increases.[5] Before foundation pit excavation, the axial force of concrete support is large to resist the deformation of the ground wall.With the increase of excavation depth and the continuous erection of steel support, the axial force of concrete support begins to become smaller, indicating that steel support mainly plays a supporting role in the process of foundation pit excavation.By the end of September, when the foundation pit was excavated close to the foundation, the axial force of the support gradually became stable.The maximum axial force of the steel support at the measuring point ZCL-04 when excavated to the base is 1311.5kN,which does not exceed the design allowable value (3093 kN).The reasons why the axial force of the steel support fluctuates greatly in a short period of time are as follows: On the one hand, the soil layer at the adjacent position is being excavated, the construction work is being carried out near the position, and the bottom of the foundation pit is being leveled, which will inevitably have a partial impact on the steel support force; On the other hand, the exchange of large lifting and transportation machinery on the ground will produce a large local external load on the steel support.However, in general, the axial force of the steel support gradually increases with the increase of the excavation depth, and eventually tends to be stable, indicating that the excavation of the foundation pit is in a safe state.[4] 6. Conclusion 1)The geology of water-rich soft soil layer has a great influence on excavation construction and deformation control of foundation pit.Effective and feasible monitoring scheme is an indispensable work in foundation pit excavation project, which not only provides timely feedback on construction quality, but also provides foresight and guidance for problems encountered in construction.
2)During the excavation of the foundation pit, the inner support should be set up as soon as possible, and the concrete should be poured timely after the excavation to the base, reducing the exposure time and conducive to the deformation control of the envelope structure.
3)The horizontal displacement of the diaphragm wall has the same deformation under all working conditions, which is manifested as follows: the overall displacement will increase with the increase of excavation depth in the early stage of excavation, and the displacement shows an increasing trend

Figure 1 .
Figure 1.Transverse profile of the standard section of the station 2 ~-12.9m.The geological and hydrological diagram of Xinjiayuan North Station is shown in figure 2.

Figure 2 .
Figure 2. Geological and hydrological schematic map of Xinjiayuan North Railway Station

Figure 3 .Figure 4 .
Figure 3. Layout of station monitoring points 5. Analysis of monitoring result 5.1.Horizontal displacement of wall Deformation of enclosure wall is a key problem in deep foundation pit excavation, and its horizontal deformation is closely related to the depth of foundation pit excavation.The deformation of the wall can be obtained by measuring the horizontal displacement of the wall in different depth ranges with the inclinometer.The standard section ZQT-04 and ZQT-06 inclinometer holes were selected to analyze the monitoring results.

Figure 5 .
Figure 5. Change of support axial force of ZCL-04 measuring point in September

Table 1 .
Table captions should be placed above the tables.