Project Manager Malcolm Drilling Company, Inc. Brier, WA
Abstract Description: The United States Bureau of Reclamation (USBR) implemented spillway modifications at the existing earthen dam at Bull Lake Reservoir near Riverton, Wyoming. The spillway modifications included deep excavation requiring specialized temporary construction dewatering to lower groundwater levels, maintain the stability of cut slopes, and prevent hydrostatic uplift of excavation subgrades.
The original dewatering design was developed based on soil and groundwater conditions provided by the USBR (geotechnical baseline conditions and previous subsurface investigations). Given the predominantly low-permeability hydrostratigraphy indicated by the available data (glacio-marine silt and clay with variable sand and an estimated mean hydraulic conductivity of 0.010 feet per day), a jet-eductor well system was selected for excavation dewatering. Jet-eductor wells rely on a jet nozzle and venturi at the bottom of each well to create a partial vacuum, lifting groundwater to the surface. Jet-eductor wells are generally suited for low-permeability conditions and excavation depths exceeding the vertical lift limitations of traditional vacuum wellpoint systems. Jet-eductor spacing, depth, and layout were evaluated using finite element numerical modeling methods. The modeling results indicated that jet-eductor wells should be installed to 60 feet below the excavation bench. However, early operation of the jet-eductor system generated about 15 feet of drawdown in the excavation area, significantly less than the required drawdown of about 30 feet.
The observed flow rates and associated time-drawdown relationships between the jet-eductor well system and piezometer measurements indicated hydraulic conductivity values up to ten times greater than the assumed design value of 0.010 feet per day. The actual hydrogeologic conditions differed significantly from those assumed for system design based on the available data. The hydraulic response from system operation indicated greater hydrogeologic complexity in the subsurface, including interconnected high-permeability sand zones within and below the planned excavation depths.
Achieving the required drawdown to complete excavation and construction ultimately entailed the installation of an expanded system of 315 deep dewatering wells and vacuum-assist deep wells up to 312 feet deep. Sonic drilling methods were used to install wells with continuous soil core sampling for more detailed hydrostratigraphic data to support dewatering system design augmentation. The success of the augmented dewatering system was measured and verified using a network of 36 piezometers. Safe and successful construction dewatering required continuous 24-hour dewatering system maintenance and monitoring. Completion of the spillway modifications occurred in February 2024.
Learning Objectives:
Learn about different temporary construction dewatering methods and their suitability to various hydrogeologic conditions.
Learn about effective collaboration between government agencies, general contractors, subcontractors, and consultants.
Learn about finite element numerical model applications to groundwater flow and construction dewatering.
