Design and verification of the foundations of the Dahej LNG tanks

Abstract

The first two tanks for storage of liquefied natural gas (LNG) at Dahej in Gujarat are already in operation, with the next two currently under construction. Each tank, capable for 148,000 m3, rests on 578 concrete piles. The more demanding loads on the foundation arise from the hydraulic test and the safe shutdown earthquake (SSE). The former consists in filling the tank with water to achieve 25% greater weight than in operation; the latter, based on the code versions prevailing at the time, corresponds to an earthquake with a return period of 10,000 years. For axial loads, the pile capacity calculations are straight-forward and their results were verified in full scale tests as per IS 2911. For horizontal loads, however, test results cannot be used directly because the pileheads are rigidly fixed to the slab but are free to rotate in a test pile. The approach followed consisted in constructing p-y curves to describe pile-soil interaction at each depth, as well as moment-curvature relations to characterise pile bending; this allowed predicting the response under both tests and earthquakes. The experimental verification of the former was used to confirm the latter, which normally cannot be tested directly.