Diaphragm Wall Construction, Instrumentation, and Testing for a Hydroelectric Power Facility-Paper

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The paper describes construction, testing, and monitoring of a diaphragm wall for a hydroelectric facility being added to an existing USACE dam in Iowa. The T-shaped wall supports an intake platform and retains fill up to 21.3 m (70 ft) high.<br /><br />Construction took place on a severely constrained site, with little room for equipment or material storage. A bentonite slurry mix was designed to stabilize excavations up to 39.6 m (130 ft) deep while also supporting nearby equipment loads. Excavation used a mechanical clamshell through overburden and a trench cutter in rock; cutter sensors and ultrasonic measurements helped verify panel alignment and verticality.<br /><br />A major challenge was lifting and installing reinforcement cages weighing up to about 100 tons. Laboratory load tests showed the initial lifting connection did not meet the required safety factor, so it was redesigned with a rebar coupler as a backup load path. Concrete mix trials, including calorimetry, helped balance specification requirements with the workability and setting time needed for construction.<br /><br />The first production panel was treated as a test element. After curing, six full-depth core holes were drilled, and Cross Hole Sonic Logging was performed. Thirteen selected core samples averaged 58 MPa (8,500 psi), nearly twice the specified strength of 31 MPa (4,500 psi).<br /><br />Monitoring instruments included manual and in-place inclinometers, pressure cells, piezometers, and survey monuments. Most readings were collected automatically and displayed on a secure website, with alerts for displacement thresholds. Data tracked wall movement, water levels, and pressure changes, including responses to rainfall and pore-pressure dissipation.<br /><br />The project demonstrates the importance of careful planning, material and connection testing, quality assurance, and real-time instrumentation when constructing deep diaphragm walls under difficult site and operational constraints.

Keywords

diaphragm wall

hydroelectric facility

USACE dam

bentonite slurry

deep excavation

trench cutter

reinforcement cages

cross hole sonic logging

instrumentation monitoring

quality assurance