Mormon Island Auxiliary Dam

Malcolm’s Secant Pile Expertise at Mormon Island Auxiliary Dam

The Mormon Island Auxiliary Dam Key Block project, where Malcolm served as a subcontractor for the secant pile cut-off wall, aimed to enhance the seismic stability of an earth dam on the east side of Folsom Lake, California. The scheme involved staged excavation and lean concrete backfilling along the downstream dam toe, with a planned excavation 900 feet long, 50 feet wide, and 60 to 80 feet deep, penetrating up to 8 feet into bedrock.

A test cell, 50 feet square and excavated 65 feet deep, was constructed to verify proposed construction procedures. After a successful test program, Malcolm performed secant pile installation to construct 2,200-foot-long walls around the excavation cell perimeter, extending 18 feet into schist bedrock. Secant piles, drilled with 1-meter (39-inch) diameters and 24 to 27-inch center spacing, were installed using a Bauer BG40 with an oscillator attachment, with wide-flange steel beams placed as vertical reinforcement. The excavation was internally braced, and dewatering systems within the excavation footprint controlled groundwater within the rock strata.

Secant piles were constructed in dredged and re-compacted alluvium, consisting of layered gravel, cobbles, boulders, and silt/clays overlying amphibolite schist bedrock. The bedrock ranges from decomposed to moderately fresh, and the groundwater level was within 10 feet of working grade. Piles were advanced through stone columns and steel obstructions, playing a crucial role as part of a key-block for seismic improvements on the existing dam. The general scope of the construction project included constructing the secant pile cut-off wall using cased rotary drilling methods.

Malcolm Drilling