Characterization of Pile Capacity with Time in the Cooper Marl: Study of Applicability of a Past Approach To Predict Long-Term Pile Capacity

The Cooper Marl is a massive geological group, consisting of stiff, cohesive calcareous soils, which serves as the bearing stratum for the large majority of deep foundations in the Charleston, South Carolina, area. It has long been recognized that piles driven into the material experience tremendous setup, or capacity gain with time. Numerous driven piles that bear within the Cooper Marl have been statically or dynamically tested at various times after installation. This pile-testing database has been used to backcalculate the coefficients of a logarithmic expression (Q/Q₀ - 1 = Alogt/t₀) that predicts pile capacity gain with time. For the largest data set, composed of 305-mm (12-in.) prestressed concrete (PSC) piles, the calculated coefficients are fairly similar, which seems to support the viability of the method and use of the equation. By using the calculated coefficients, long-term pile capacity of 305-mm (12-in.) PSC piles driven into the Cooper Marl should be able to be predicted based on the results of short-term restrike or load test data. The data also indicate that the coefficients of the expression are dependent on the pile size and that the rate of the capacity gain with time generally decreases as the pile size increases.