With the development of urban construction, a large number of high-rise buildings, underground shopping malls, underground passages and other projects have been built, and the corresponding foundation pit projects are also increasing, and the excavation depth is also increasing. People pay more and more attention to the adverse environmental problems caused by deep foundation pit excavation. Once there are problems in foundation pit excavation, it will often lead to ground subsidence, building inclination, cracking and even collapse, as well as the rupture of underground water pipelines, resulting in huge losses. This paper analyzes the adverse environmental impact caused by foundation pit excavation and the preventive methods and measures, in order to provide some guidance and reference for engineering practice.
1. Adverse environmental problems during foundation pit excavation
1. 1 groundwater seepage damage
Due to the excavation of the foundation pit, the soil on the side wall and bottom of the foundation pit is empty, and groundwater infiltrates into the foundation pit under the action of hydrostatic pressure and confined water pressure, and the change of groundwater seepage path destroys the original equilibrium state of the soil. In the process of foundation pit excavation, if the dewatering measures are unreasonable and the retaining structure and water-stop curtain do not play their due roles, serious seepage damage will occur in the foundation pit.
1. 1. 1 quicksand
Groundwater will be blocked by soil particles in the process of seepage, and soil particles will inevitably be reacted by seepage water, so the seepage force acting on soil particles in unit volume is called hydrodynamic pressure, which is expressed by J. When the soil below the foundation pit is loose sand layer and the hydrodynamic gradient of seepage water is greater than the limit hydrodynamic gradient of sand layer, sand particles will be suspended, and under the action of seepage force, a large number of fine sand will swell up and form quicksand.
1. 1.2 pipeline
When the water infiltrating into the foundation pit has a strong effect, the fine particles in the soil are washed away, and the pores in the soil are constantly expanding, so that the seepage speed of water is also increasing, and then the coarse particles are taken away by the water, which will gradually form a tubular seepage channel, leading to the collapse of the soil, that is, piping (mechanical). In addition, when the soluble salt or cement contained in the soil is dissolved by the water flow, the structure of the soil becomes loose, the porosity increases, the permeability of the water flow increases, and the piping formation is aggravated.
1. 1.3 foundation pit swell
If there is confined water below the foundation pit bottom, with the excavation, the thickness of the water-resisting layer at the bottom of the foundation pit decreases, resulting in the head pressure of confined water being greater than the self-weight stress of the upper water-resisting layer, and the head pressure of confined water will break through the foundation pit bottom plate, resulting in a large amount of sand gushing. Therefore, when the foundation pit is excavated to a certain depth, it should be ensured that the confined water head will not break through the foundation pit floor, and then the foundation pit is excavated to prevent sudden surge.
Once quicksand, piping and sudden gushing of foundation pit occur, the retaining structure of foundation pit often fails and the foundation pit collapses due to improper treatment, which leads to large-scale settlement of the ground around the foundation pit, cracking, inclination and even collapse of surrounding buildings, and fracture of underground pipelines, resulting in huge losses.
1.2 water stop curtain leakage
In deep foundation pit, it is generally necessary to set up closed water-stop curtain (horizontal water-stop curtain and vertical water-stop curtain). However, due to the defects of the foundation pit water-stop curtain itself, such as lax overlapping of deep mixing piles, mismatched joints of underground continuous wall, poor hydrogeological conditions of the site, or due to the deep excavation of the foundation pit, the surrounding hydrodynamic pressure and earth pressure increase relatively, the soil retaining structure is greatly displaced, which drives the water-stop curtain to bend or move sideways. In addition, the water-stop curtain is mostly rigid structure, which has weak ability to resist deformation, and is easy to crack when bending straight and moving sideways, which makes the water-stop curtain leak under the action of groundwater pressure.
When the water-stop curtain leaks, it often suddenly leaks a lot of water and sand, which leads to slope instability, collapse, dumping of piles and sharp sinking of nearby buildings and roads.
1.3 ground subsidence caused by groundwater level decline
In the process of foundation pit excavation, it is necessary to artificially lower the groundwater level in most cases. On the one hand, it is necessary to ensure that the foundation pit is carried out in a dry environment and provide good conditions for mechanized construction. On the other hand, it is necessary to prevent accidents such as quicksand, piping, sudden surge and slope instability in the hole. The principle of effective stress shows that the pore water pressure in the soil layer dissipates continuously due to the continuous pumping of foundation pit. When the total stress remains unchanged, the dissipated pore water pressure is converted into effective stress, and the soil layer produces new consolidation compression deformation under the action of increased effective stress, and at the same time produces settlement and horizontal displacement on the ground. If the design of dewatering scheme is unreasonable, the radius of dewatering funnel is too large or groundwater is over-exploited, which will cause serious settlement in a large area around the foundation pit.
1.4 Failure of retaining structure leads to foundation pit collapse and ground settlement.
With the increase of excavation depth, the height difference between the soil surface inside and outside the foundation pit is getting bigger and bigger, which will make the soil outside the retaining wall produce shear strain and increase its sliding trend and kinetic energy to the foundation pit. The passive earth pressure on the retaining structure is increasing, and the retaining structure will undergo different degrees of deformation and lateral displacement. The retaining structure is not deep into the soil, and the rigidity of retaining structure itself is insufficient, which will make the retaining structure have poor ability to resist passive earth pressure, and cause large lateral displacement or fracture at the stress concentration part, which will lead to instability of surrounding soil, collapse of into the pit, failure of foundation pit excavation and serious ground settlement. Generally speaking, the influence range of ground movement caused by foundation pit excavation is 2.0-3.0 times of excavation depth. In practical engineering, when the soil quality is poor and the depth of the retaining structure is not large, the lateral deformation of the retaining structure is large, which is close to the maximum deformation outside the pit. The maximum surface subsidence appears at the pit edge and decreases with the increase of the distance from the pit edge. When the soil layer at the bottom of the pit is good or the wall is deep, the lateral deformation of the retaining structure is small, and the maximum ground settlement often occurs at a certain distance from the pit edge, which is 0.7 times of the maximum lateral displacement of the retaining structure.
1.5 deformation and uplift of foundation pit bottom
In the process of foundation pit excavation, due to the excavation of soil above the foundation pit bottom, the vertical load of the basement is released and the force to resist the vertical deformation of the basement is lost. When the supporting structure is not deeply embedded in the soil, the basement will uplift under the combined action of water absorption and expansion of cohesive soil, extrusion of supporting structure and pushing of soil in front. The rebound of the bottom surface of excavation unloading pit is inverted basin in space, close to the supporting structure, and the curvature is the largest. The closer to the center of the foundation pit, the smaller the curvature, and the ground uplift will be a certain value after a certain distance from the center. The uplift of foundation pit is related to the properties of foundation soil, the depth and width of foundation pit excavation, exposure time, stiffness of supporting system, construction technology and construction quality. The uplift of the basement not only causes the deformation of the supporting system of the foundation pit itself, but also causes the settlement of the surrounding surface, which has adverse effects on the surrounding buildings.
2. Measures to prevent adverse environmental problems caused by foundation pit excavation
Pay attention to the investigation of the topography, geomorphology and engineering geological characteristics of the deep foundation pit excavation site, and it is important to evaluate the stability of the site soil. In the investigation, we should find out all kinds of factors that may lead to slope soil landslide in advance to ensure the reliability of important sections, key strata and important soil indexes that may threaten the stability and safety of supporting structures. Survey data that do not meet the technical requirements shall not be used for the design and construction of deep foundation pit support.
During the construction, excavation shall be carried out by sections, blocks and layers as much as possible, and the excavation principle of "slotting support, first support and then excavation, layered excavation and no overbreak" shall be followed, so as to shorten the exposure time of foundation pit as much as possible and adopt reasonable and efficient earthwork excavation construction organization design as much as possible.
Adopt foundation pit dewatering, reasonably select well point type, optimize filter screen, appropriately slow down the slope of dewatering funnel line, set up waterproof curtain and set up reinjection system in the protected area, on the one hand, control the groundwater level in the settlement area; On the other hand, it controls the overflow amount of well wall particles. Paper reference. At the same time, arrange enough logging outside the pit in time to monitor the change of water level outside the pit and use it as a recharge well if necessary.
Choose a reasonable supporting scheme for deep foundation pit. The supporting system is arranged reasonably, and the overall stress is reasonable; Sufficient burial depth, the root of the supporting structure should enter the good soil as far as possible; Soil reinforcement in the pit (chemical grouting, lime pile reinforcement, etc.). ), timely support, pre-loading axial force. The concrete strength of piles and walls must meet the design requirements, and there can be no honeycomb and exposed reinforcement; There can be no gap at the lap joint of water stop pile; We can't change the design intention at will, cancel the anchor rod and turn the anchor structure into a cantilever structure.
In the stage of foundation pit excavation, the monitoring unit should focus on monitoring the surrounding surface settlement and retaining wall displacement, optimize and rationally arrange monitoring points, pay close attention to the development of settlement and deformation, carefully analyze the data, and provide information tracking monitoring data in time so as to give an alarm in time when danger occurs.
When the deformation of foundation pit is too large, or the environmental conditions do not allow, measures such as block construction of bottom plate and additional diagonal bracing should be considered in combination with the design of upper structure. When the surrounding environment of the foundation pit allows, the soil can be unloaded behind the wall to reduce the active earth pressure. When the surrounding environment of the foundation pit is not allowed, straw bags in the passive area at the bottom of the pit can be used for weighting, sand filling and filling. In case of severe quicksand and emergency, the pit can be filled with water.
From the point of view of controllable measures, the deformation of the envelope is a precursor. If the envelope is deformed or suddenly changed, the surrounding ground surface will be obviously settled. At this time, in addition to strengthening the foundation pit construction measures immediately, reinforcement measures should be taken in time to reduce the impact on the surrounding environment as soon as possible.
3. Conclusion
Deep foundation pit engineering is a complex underground engineering with strong concealment and complicated situation, and the probability of engineering quality accidents is also relatively high. Once quality problems occur, they often cause major engineering accidents, which are difficult to correct and remedy afterwards, resulting in heavy losses. Such as quicksand, piping, water gushing, failure of foundation pit retaining structure, into the pit of soil collapse, etc. The uplift of foundation pit and the leakage of water-stop curtain will lead to different degrees of ground settlement, which belongs to uneven settlement, leading to road subsidence, cracking, even building collapse, underground water pipeline, gas pipeline fracture, overflow, underground cable damage and other engineering accidents. Therefore, a feasible construction scheme should be made, and the progress of the project should be tracked, and the construction scheme should be improved and optimized according to the actual situation. Construction and construction units should conscientiously implement the construction plan, strictly control the quality and try to avoid engineering accidents. Take practical measures to prevent the accident from getting worse.
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