Finite Analysis For Unsteady Seepage In Excavation Engineering

During 30 years of Reform and Opening-up, China has achieved rapid development in economic and urban infrastructure construction. With the increasing of city amount and size, it is becoming a development direction to be active in exploiting underground space for solving a series of environmental concerns like the ubiquitously exiting city-population expansion and heavy traffic. All kinds of underground constructions emerged in many big cities, such as large-scale building foundation pit, underground transportation and station, garage, and underground shopping mall and so on, the depth and area of the foundation pit is showing the upward trend, and even the depth has come to more than 40m. Some areas'excavation deep have gone into the artesian aquifer though them varies around China, within the excavation deep, partly artesian aquifer has been unwatered, so foundation pit no longer is a shallow dewatering precipitation problems, in order to prevent the piping which is caused by the lower artesian aquifer destroys the foundation pit floor, we must pay serious attention to the depressurization and precipitation of the lower artesian aquifer section problems.This paper mainly does the following work with aiming at the precipitation (or depressurization) which appears in the foundation construction engineering: First, Through the seepage module SEEP/W, part of the large-scale geotechnical finite element software GEO-SLOPE, to simulate the unsteady seepage field of foundation pit, and in respectively, to make study that when dewatering in the phreatic aquifer, with the water level inside the pit constantly changing, how could those factors (like the seepage coefficient anisotropy, layered soil, groundwater flow, the flawed wall, the insertion depth of curtain or other factors) impact on the flow of the lower waterproof wall section and the largest flow velocity distribution.Second, Simulation on modeling with SEEP/W: When the foundation pit floor near the artesian aquifer and use relief well to reduce the head of artesian water, with which changes and the leakage recharge effection by the upper phreatic aquifer to the artesian aquifer, thereby to make study that how do the different factors (like seepage coefficient anisotropy, different artesian water head, soil seepage coefficient , insertion depth of waterproof wall and other factors)impact on the flow of the lower waterproof wall section and the largest flow velocity distribution.Conclusions obtained through the simulation analysis as followed:1. In the process of pit precipitation, waterproof wall can effectively decrease the amount of water into the pit, and as the waterproof wall increases with the depth of groundwater, the ground-water's free surface around the pit rises, thereby can effectively decrease the changes in stress which generated by the phreatic declined of the surrounding soil so as to minimize effects on surroundings.2. When there is no complete waterproof be finished, the anti-seepage body partially invalidated will cause seepage field mutation, in the waterproof defects point, the water flow velocity and hydraulic gradient are larger here, where is rather liable to lead to the soil body damage and then the developmental seepage destroy. In the foundation pit dewatering project, the importance of the curtain must be aware of to ensure the construction quality of the waterproof wall.3. In view of the anisotropy effect of seepage coefficient, while the soil horizontal seepage coefficient is larger, distribution of the flow velocity in the lower curtain section is more even, and the head equipotential line changes relatively strong vertically; and while soil vertical larger, the head equipotential line changes gently vertically, almost a straight line at far away from the pit, also with the vertical permeability increasing, the flow rate on both sides of the curtain gradually increasing.4. If there was circumstance about groundwater flows initially in the process of dewatering and the flow rate of the lower section of the upstream curtain be faster than the downstream, part of the water will outflow through downstream curtain in the early stage of dewatering while the water head is widely ranged from the upstream and downstream, Moreover, the outer groundwater of downstream waterproof wall declines more in height.5. In process of using relief well to reduce the head of artesian water, the flow of the lower waterproof wall section decreases gradually along with the waterproof wall insertion in the artesian aquifer increases, the large flow velocity is in the region between the bottom of waterproof wall and the relief well, and the increasing depth of waterproof wall insertion can not effect the free surface elevation of foundation pit outside's phreatic aquifer obviously.