Failure of Pile Foundations Driven into Bedrock through Hydrocollapsible Soils

A case history is presented where a three story wood frame condominium structure was supported on piles that were driven through fill and hydrocollapsible alluvium into bedrock. The depth of fill ranged from 10.7 to 16.8 m (35–55 ft). The fill was placed over native, hydrocollapsible alluvium that ranged in depth from 0 to 7.2 m (0–23.5 ft). Design records indicated the piles were up to 29.9 m (98 ft) in length. The piles failed to prevent settlement of the structure that reached 216 mm (8.5 in.) over the last 37 years. An angular distortion ratio of 1:127 was measured, which had caused the structure to be tilted. However, the superstructure is currently free of defects that are normally associated with significant differential settlements. The original subsurface investigation report indicated the alluvium was subject to hydrocollapse potential ranging from 1.6 to 8.6%. Recent drilling indicates the alluvium is unsaturated and that the hydrocollapse process is not complete. Laboratory tests indicate that the remaining hydrocollapse potential ranges from 0.5 to 3.4%. The piles consisted of steel rails commonly used in railroad industry, resulting in long, and slender piles. These were substituted for cast-in-drilled-hole (CIDH) piles during the original design to minimize downdrag. Reasons and contributing factors to the pile failure are examined. Various modes of pile failure are analyzed and discussed. Remedial measures are considered, and pros and cons are discussed. Lessons learned from this case history are presented.