Geotechnical structures: investigation of drilled shafts in highly expansive soils

Abstract

Construction over extremely expansive soils raises the risk of structural foundation failure and potential failure to the building itself. This is due to shrinkage and swell characteristics of expansive soils. This works presents an extensive case study of a distressed building built on drilled piers and expansive soil, and describes innovative practical ideas that can be used in the renovation of its foundation. The building is located west of San Antonio, Texas, USA. This building has experienced significant settlements and differential building movement resulting in widespread building distress. The following foundation based structural distresses were found in the building: 1) vertical movements of more than 300 mm, 2) bearing surface had completely spalled away and the beams were supported solely by bent and corroded anchor bolts which were not well confined in the surrounding concrete, 3) the beam rotations and lateral movement caused the steel stub columns supporting the floor framing to tilt sideways. This created eccentric support conditions that could result in sudden instability failure of either the beams or columns, and 4) under bathrooms in the northwest corner of the building, significant corrosion of steel framing was observed due to long term exposure to moisture leaking through cracks in the floor slab above. Drilled piers were studied using spot study, soil data obtained from boreholes and laboratory tests based on American standards. It is recommended that 0.5 m diameter piers of lengths 18.3 m with positive skin friction to prevent uplift, and a load carrying capacity of 1737 kN be used to rehabilitate the failing foundation. Also, new shafts are to be designed for a minimum factor of safety 2.5 and the rejection of an unacceptable pier required installation of one or more replacement piers at locations that would facilitate load transfer from the structure above.

Keyword : construction, foundation, building, drilled piers