Designing and Installing a Dewatering System

Malcolm drilling was contracted to design and install a dewatering system that would allow the contractor to shore and excavate the new Santa Monica Beach club during the day. Malcolm contracted Middour Consulting LLC to design the dewatering system using the geotechnical information and structural drawings of the shoring. A three-dimensional model was developed based on the groundwater levels, aquifer properties and geometries presented in the geotechnical report. The model domain was 4,000 feet by 4,000 feet and consisted of two layers representing the granular soils of the unconfined aquifer. The model was discretized using a non – uniform grid with cell sizes ranging from 5 to 7 feet in the area around the excavation to a cell size of 80x 80 at the limit of the model. The groundwater flow field was established by assigning a general head boundary to the east side of the model and a constant head boundary along the west side representing the Pacific Ocean and running a steady state simulation. To achieve a day excavation Malcolm installed 11 dewatering wells inside the shoring. The wells were on average 25 feet deep or until the silt layer was encountered. The well screen and well filter pack were designed to allow the water to move through the screen while not allowing the fines/ soils to be mined during pumping. During the well development Malcolm properly adjusted the flows of each well to allow for the drawdown of the water without over dewatering the soils reducing the risk of dewatering induced settlement as well as reducing the cost of discharge/ treatment. Malcolm provided maintenance for the system to ensure that the pumps were operating properly and help with any sump pumping that was needed at the silt interface.