Control measures of subway tunnel displacement cau

Control measures for the displacement of the subway tunnel under the excavation of the foundation pit

the unloading of the excavation of the foundation pit will inevitably cause the displacement of the existing buildings below, which will affect the use function and safety of the buildings below, and even cause serious harm [1] Controlling the influence of the upper unloading on the existing buildings below and reasonably selecting the technology to control the displacement of underground buildings to ensure the normal use of the buildings below have become an urgent problem to be solved in the engineering community

the foundation pit excavation of Shanghai Dongfang Road overpass project is located on the metro tunnel line 2 that has been operated. Excavate a deep foundation pit with a width of 18m and a depth of 6.5m above the subway tunnel, and the nearest distance from the bottom of the foundation pit to the top of the tunnel is only 2.8m It is worth noting that the conventional large-area excavation can not meet the allowable deformation requirements of the subway tunnel, so the construction method considering the space-time effect is adopted for excavation. The excavation of foundation pit will inevitably cause the displacement of the buildings below. The displacement of the buildings below is related to many factors [2 ~ 4], such as the unloading amount of foundation pit (excavation depth), unloading modulus, excavation method (space-time effect), etc. However, the allowable displacement of the buildings below is very small. We analyzed the influence of excavation and unloading on the underlying tunnel from the perspective of construction technology, and put forward control measures, which achieved success

2. Project overview

the interchange project under Dongfang Road is located at the intersection of Shanghai Dongfang Road, Century Avenue and Zhangyang Road (see Figure 1). Under the lower interchange project, there are three rail transit lines that have been built and planned to be built. From north to south, there are Mingzhu line phase II, Metro Line 2 and the interval tunnel of planned Metro Line R4 (see Figure 2). The total length of the project is 600m N1 and N2 sections are located above the subway line 2 in operation, and the subway line must be protected during construction. The nearest place from the tunnel of Metro Line 2 to the tunnel floor is 2.8m, and most of the tunnels are located in ④ gray muddy clay. Engineering geological characteristics

3. Construction control measures to reduce tunnel displacement

3.1 foundation reinforcement

in order to ensure the construction safety of the lower interchange project and the safety of the running metro line 2, the foundation pit project adopts the reinforcement of cost cement mixing pile, triple pipe high-pressure rotary jet grouting pile and double liquid grouting. By strengthening the weak foundation, improve the strength of the soil and prevent the liquefaction of the soil, so as to increase the anti floating performance of the foundation pit, improve the stability of the foundation pit, and reduce the rebound of the pit bottom and the uplift deformation of the tunnel below

layer ③ -1 is gray muddy silty clay, saturated, with a water content of 50%, and the soil is uneven. The price of ③ battery grade lithium carbonate rises by 7%-2. Layer ③ -3 is silty soil and silty clay, and the soil layer is also saturated. This three-layer soil layer is just at the bottom of the lower interchange. During construction, if these three layers of soil are disturbed or encounter water, it is very easy to liquefy and complete the corresponding work, which will lead to foundation pit collapse and accidents. We also reinforced these three layers of soil by injecting a large amount of cement slurry, which improved the soil strength, density and resilience modulus of the soil layer

3.2 construction of mixing pile

during the construction of mixing pile above the tunnel, the unloading amount of mixing pile construction is also affected by the water cement ratio and grouting amount of mixing pile. The unloading amount can be adjusted by adjusting the grouting amount and controlling the water cement ratio. And according to the mechanical model of soil squeezing effect of mixing pile, the soil squeezing effect of deep mixing pile is related to the equivalent radius and pile length of the penetrated "mud pile". Controlling the amount of grouting and the water cement ratio can adjust the equivalent radius of "mud pile", so as to control the soil squeezing effect of mixing pile

two, six and 21 deep mixing piles have been continuously constructed on both sides of the downline tunnel, and the incremental value of tunnel uplift is shown in Figure 3 The incremental value of tunnel uplift shows an increasing trend with the increase of the number of continuous piles, but it does not increase linearly, but gradually slows down. It can be seen from Figure 3 that reducing the number of continuous piles each time and continuing the construction of deep mixing piles after the pore water pressure generated by pile driving is partially dissipated is an effective way to control the uplift value of the tunnel

when carrying out large-area deep mixing pile reinforcement, under different driving conditions, the comparison of the uplift values of the upper and lower line bottom is shown in Figure 4 The measured values of the lower and upper line tunnels are the measured uplift values of the lower (upper) line tunnels during the construction of deep mixing piles in zone N1 and N2 (as shown in Figure 2). The main reason why the measured uplift values of the up and down line tunnels differ so much (their relative tunnel position, pile length and equivalent pile number are the same) is that the following measures have been taken for the reinforcement of the side of the down line tunnel