The proposed expansion of the BoDean Company Asphalt Plant, located on two acres in Santa Rosa, California is surrounded by residential neighborhoods. Three new 14′ diameter batch silos and conveyor equipment rise more than 85′ above the 40′ x 60′ foundation pad and can contain 900 tons of bituminous material batched at 300° F.
Driven piles were not an option in this neighborhood and drilled cast in place piles were rejected due to the high water table. Helical piles provided the flexibility to deal with the variable conditions.
RGH Geotechnical Consultants assigned the project to Site Class D with ground acceleration parameters s, = 1.85g ands,= 0.72g. Given the small footprint, a single boring indicated highly variable alluvial deposits and that “the site is underlain by interbedded soft to very stiff clay and loose to very dense gravel and sand”.
The initial foundation design, prepared by the silo manufacturers, was a 5′-6″ thick monolithic slab intended to counteract the high overturning reactions.
Differential settlement across the 40′ dimension of the pad was estimated to be as much as 5″ and was deemed unacceptable by the Santa Rosa Building Dept. for this seismic zone.