Development and Investigation of a Simultaneous Dewatering – Excavation Method for Reducing the Settlement in the Vicinity of Construction Pit

Steady population growth resulted in increasing the land prices which lead to construction of deep excavation. Therefore the use of underground spaces is economically acceptable. One of the most important issues in the design of underground spaces and deep foundation pits is the existence of groundwater and how to control it. The influence of groundwater on a structural project can be very extensive and it may affect the structural design, performance and the total cost of the project significantly. Many problems related to ground water have been observed in excavation sites recently. Those problems have made significant delays in construction process and sometimes lead to significant re-design of the entire operation process. Considering the negative influence of groundwater on the reinforcement system, (e.g. nailing and anchoring) as well as the difficulty of excavation process under wet condition, water must be pumped out from the excavation area. The process of ground water discharge from the construction area which results in lowering the ground water level in that area is called dewatering. Dewatering operation may lead to ground settlement in the vicinity of construction pit which may damage the building. Although the damage due to ground settlement may be negligible, the potential consequences, especially in connection with claims of third parties, requires sufficient attention. In recent years, dewatering and lowering groundwater levels along deep foundation pits subway tunnels in urban environments caused some significant ground subsidence. The mechanism of this type of ground subsidence is that due to water drainage from the soil layers, the hydraulic pressure decreases leading to an increase in the density of the soil and therefore, ground settlement. Dewatering can leads to ground subsidence in different ways where some of them are as: a) removing the finer soil particles from the ground during water pumpage due to a poor pumping performance, b) in the case of dewatering by pumping water from the floor of excavation pit, piping and boiling can reduce the soil strength properties, leading to a failure in the soil mass and c) compaction of the compressible soils or crushable sandy soils due to increasing the effective stress. Some methods such as cut off walls and artificial injection can be used to reduce the ground settlement due to dewatering, but these methods are very time consuming and costly. Furthermore, methods such as artificial injection may have low efficiency and cause environmental effects. Therefore providing a new technique for reducing the dewatering induced ground settlement in less time and cost is necessary. The strategy proposed in this study is to apply a step by step dewatering method, by setting pumping from drainage wells, coincides with the excavation process. For this purpose the PLAXIS2D software is employed as a finite element method to model the settlement due to dewatering. This operation could be applied by modeling two deep foundation pits with the same structures and different groundwater conditions. The proposed step by step method was modeled in this software by estimating the excavation time and coinciding it with water pumping. Step by step dewatering method has some advantages such as reducing the amount of settlement behind the wall and asymmetric ground settlement under adjacent foundation, reducing the amount of water pumping out of the ground, decreasing the Mohr-Coulomb full plastic points in the range of geogrids and reducing the time and cost of dewatering.