Foundation Design in Weathered Limestone - Where Will the Piles End?

A new pile-supported pier was designed to provide cruise ship access to an onshore resort in Labadee, on the north side of Haiti. The project consisted of an access trestle connecting to a pier with mooring dolphins extending offshore to support berthing and mooring. The structures were supported on steel pipe piles and designed to resist compression loads due to operations, horizontal loads from waves and seismic activity, and uplift loads from wave forces and as a result of the horizontal loads in multi-pile groups. The initial design was based on the results of a limited field exploration. However, during construction it was found that the site conditions varied from the initial field exploration, and a more detailed exploration was undertaken during construction to better evaluate the site conditions. The primary bearing layer for much of the structure was found to be a limerock (i.e., weathered limestone), which was noted to have voids and clay-filled cavities that provided minimal pile resistance. Pile Dynamic Analysis (PDA) testing was conducted along with CAPWAP analyses to estimate as-driven pile capacities. The results of the testing indicated unit skin resistance values much lower than values typically reported for materials with similar blowcounts. Though it was expected that this material would have low unit resistance values given the literature on pile behavior in calcareous soils, the resistance values were still much lower than anticipated. The final design consisted of using a combination of splicing and driving the piles deeper, as well as drilling out the inside of the piles and backfilling with concrete to force a plug in the pile as well as provide additional uplift resistance due to the dead weight of the concrete.