Dynamic Load Tests at drilled shafts in sandy soil deposits to bedrock, Central Pacific, Costa Rica: strategic learnings for the construction methods.

The foundation of a new building located on the shore of Central Pacific of Costa Rica, a second phase of a complex with existing lower rise buildings, was designed to be supported on 1.2 m diameter short-drilled shafts. The soil profile is a fine sand deposit with high piezometric level due to sea tides, over bedrock. (4) dynamic load tests (DLT) were performed on existing piles not used for the first phase, which have about 2-3 m of rock socket, and new piles with 4 m sockets. However (3) of these tests, (2) existing and (1) new, presented insufficient capacity with null end bearing. Coring trough the centre of these piles and cross-hole logging testing on the new pile, denotated absence of concrete on the bottom. On the other side, the DLT of the last 4 m socket pile, displayed greater than required vertical capacity, with adequate end bearing response. Cross-hole-logging in this pile showed no significant defects. Further calculations with a finite element model (FEM), demonstrates that piles could achieve the required capacity and fixity with a shorter 3 m socket, instead of the designed 4.5 to 5.5 m, representing an important value engineering for the project.