The Mormon Island Auxiliary Dam Key Block project enhanced 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. The planned excavation was 900 feet long, 50 feet wide and 60 to 80 feet deep in order to penetrate 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 the 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 were drilled with 1 meter (39 inch) diameters and 24 to 27 inch center spacing. Piles were installed using a Bauer BG40 with oscillator attachment, with wide flange steel beams placed as vertical reinforcement. The excavation was internally braced. Dewatering systems within the excavation footprint were employed for control of groundwater within the rock strata.

Secant piles were constructed in dredged and re-compacted alluvium, comprised of layered gravel, cobbles, boulders and silt/clays overlying amphibolite schist bedrock. The bedrock ranges from decomposed to moderately fresh. Groundwater level was within 10 feet of working grade. Piles were advanced through stone columns and steel obstructions.