Environmental effects and prevention measures caus

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Environmental effects caused by deep foundation pit excavation of subway and prevention measures

1. Overview

in recent years, with the development of discrete small railway construction in China, more and more cities have built subway. The subway deep foundation pit project has many problems, such as difficult excavation, long construction period, high cost and great impact on the surrounding environment. It has become a difficult problem to be solved urgently in urban construction. 4. The experimental speed problem of tensile testing machine, in which the environmental protection problem has become the top priority of many problems in foundation pit support. Therefore, the quality of subway foundation pit construction directly affects the cost and safety of foundation pit engineering. At the same time, protecting the safety of adjacent buildings (or pipelines) and ensuring their normal use has great economic and social benefits

2. Environmental effects caused by foundation pit excavation

deep foundation pit engineering of urban subway has the following characteristics:

(1) the construction environmental conditions of deep foundation pit engineering are relatively poor. As high-rise and super high-rise buildings are concentrated in the central area of the city and on both sides of the main streets, the construction density is large, the population is dense, the traffic is crowded, and the construction site is narrow, which restricts the hands and feet of the project construction

(2) the excavation of foundation pit is getting deeper and deeper, and almost all the paper industry is not doing well. In order to save land and make full use of the original base area and underground space, the owner has set up stations, civil air defense, machine rooms and fire-fighting facilities, so the depth and number of floors of the subway structure are increased accordingly

(3) a technically reliable and feasible support structure must be set to ensure safety, and the impact on the surrounding underground gas, water supply, drainage, telecommunications, cables and other pipelines must be taken into account to minimize the damaging impact on this series of buildings and facilities

(4) with the increase of competition mechanism, the owner has higher and higher requirements for cost, project progress and project quality, which correspondingly increases the difficulty of construction

2.1 reduce the environmental effects caused by groundwater. The environmental effects caused by reducing the groundwater level are as follows:

(1) land subsidence caused by reducing the groundwater level

(2) foundation pit collapse caused by groundwater seepage damage

(3) foundation soil cracking caused by foundation pit inrush

in the process of foundation pit excavation, well point dewatering is usually used to reduce water level, consolidate soil, stabilize slope and facilitate excavation. At the same time, the dewatering of foundation pit will cause ground settlement due to the falling of water level, which will correspondingly form a ground settlement deformation zone centered on the water level funnel, resulting in the fracture and inclination of buildings, roads, pipes and other facilities within the sub scope due to uneven settlement, affecting their normal use and safety

the mechanism of environmental change caused by lowering the groundwater level is:

(1) the lowering of the water level reduces the floating of groundwater in the soil to the above ground buildings, and the weak soil layer is compressed and subsided

(2) discharge pore water from soil. The consolidation deformation of soil itself is a process of compression settlement. In the process of precipitation, some fine soil particles in the soil layer are often taken away with the extracted water, causing the surrounding land settlement. Land subsidence is corresponding to groundwater level drop, and the curved distribution of groundwater level drop will inevitably cause uneven settlement of adjacent buildings. When the ground subsidence reaches a certain degree, buildings will crack, tilt and even collapse

(3) when the foundation pit is excavated, there is a certain head difference between the groundwater level in and around the foundation pit. Under the action of hydrodynamic pressure, the foundation pit soil will suffer from flow (soil) loss and potential erosion, resulting in the loosening and destruction of the soil structure and the collapse of the foundation pit

(4) when the water level difference between the inside and outside of the foundation pit is large, or there is confined water at the lower part of the foundation pit, and the foundation pit reduces the original earth pressure to a certain extent, the head pressure of the confined water is greater than the floating gravity of the soil at the bottom of the foundation pit, forming piping and side kick, resulting in significant cracking of the environmental protection benefit of the foundation soil by 10 points

2.2 environmental effects caused by deformation and displacement of support structure the environmental effects caused by deformation and displacement of support structure are as follows:

(1) slope instability caused by failure of support structure itself

(2) the overall failure of the supporting structure leads to the uplift of the foundation pit

(3) deformation and displacement of supporting structure cause damage to adjacent building facilities

The mechanism of the environmental effect caused by the deformation and displacement of the supporting structure is:

(1) the unloading of the foundation soil of the foundation pit changes the original stress state at the bottom of the pit. During the excavation of the foundation pit, the self weight pressure in the soil decreases, and the elastic effect of the soil causes a certain rebound deformation (uplift) at the bottom of the foundation pit. The bottom of the pit is characterized by elastic uplift, which is characterized by the highest uplift in the middle of the pit bottom, and the elastic uplift stops soon after the excavation of the foundation pit stops, Basically, it will not cause the soil outside the pit to move into the pit; With the increase of excavation depth, the loading caused by the height difference inside and outside the pit and various overloads on the ground make the soil outside the retaining wall move into the pit, causing upward plastic deformation at the bottom of the pit, which is characterized by the uplift state of large on both sides and small in the middle

(2) a large plastic zone is generated around the foundation pit and causes ground settlement

(3) the exposure time of the bottom of the foundation pit is too long, which makes the foundation pit ponding. On the one hand, the rheology of the cohesive soil will increase the displacement of the soil in the passive pressure area of the wall and the displacement of the soil outside the wall to the pit, thus increasing the settlement of the surface

(4) the bearing capacity of the support structure system is lost due to the destruction of the support or the insufficient pull-out capacity of the anchor rod system, the fracture of the tie rod itself or the loose connection between the tie rod and the anchor seat. The insufficient embedded depth of the support structure causes the uplift of the foundation pit and the reduction or loss of the foundation strength

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