Fill Settlement Despite Vertical Sand Drains

A marsh deposit, stabilized by vertical sand drains and a surcharge loading, settled 4 ft after completion of the stabilization, preconsolidation period. Soil borings and laboratory tests made 4 yr after construction show that 6 ft of primary consolidation should have been anticipated, of which only 1.5 ft would occur during the specified 3-month stabilization period. The remainder would be virtually complete approximately 10 yr after construction, but some secondary compression would continue indefinitely. The predicted settlement 11 yr after construction was 7 ft, including 1 ft of secondary compression; this total was within 10% of the measured value.