Ground improvement efficiency and back-analysis of settlements

The ground improvement works carried out for the construction of five oil tanks at the Chiriqui Grande Phase II Oil Terminal, Chiriqui Grande, Panama, and their results are presented herein. The site is underlain by some 28 m of loose/soft, poorly graded, silty sands to clayey sands and silts. Ground improvement was used to provide the required bearing capacity and reduce the expected total and differential settlements. To fulfil the technical specifications, two ground improvement techniques were used in association with preloading: wick drains to full depth of the soft layers and stone columns in the upper 18 m. Stone columns are commonly installed as partial replacement to improve a soft ground, increasing its bearing capacity and accelerating its primary consolidation. Wick drains are used to accelerate the consolidation of soft clayey soils. Particular emphasis was given to the behaviour of the treated ground under the preloading fill: a back-analysis was carried out on measured settlements and pore pressures; the improvement of the ground and the degrees of consolidation achieved at the end of the preloading are highlighted; some correlations were derived and they provide an interpretation of the overall behaviour of the improved ground; estimates of the settlements for the short and long term are presented, the short term being that of the hydrotest and the long term being the design life of the oil terminal. They are based on results of preloading for their primary consolidation part, and on reasonable assumptions for their long-term creep (secondary consolidation) part. The estimates are compared to the actual hydrotest survey results.