Lng Terminal in China – Seismic Design of Tanks on 100m Long Piles in Soft Soils


Abstract eng:
New 160,000m³ LNG tanks are under construction South of China. The Authors are in charge of geotechnical and earthquake engineering. European EN [1][2] and Chinese GB codes [3][4] have been followed. General offshore hydraulic backfill has been installed on a Chinese Sea island. To allow for site accessibility, soil improvement has been realized on the whole site resulting in a general surface settlement around 2.5m. According to soil investigations carried out from 2013 until 2015, the soil is made of 80 to 100m thick soft deposits, covering the Ignimbrite volcanic rocks. This very hard volcanic rock is outcropping at some places of the site. Detailed seismotectonic studies have been conducted for the region to verify the absence of active faults on the site, and to provide the bedrock seismic levels through a probabilistic study PSHA. The horizontal Peak Ground Accelerations at bedrock level are :  OBE PGA = 0.09g  SSE  PGA = 0.25g Elevated rafts on 319 large bored piles in triangular layout have been designed as tanks foundation. The piles have been socketed (embedded) by adequate length inside rock to provide the requested bearing capacities. An iterative process of Soil Structure Interactions (SSI) computations using SAP software has been conducted during some months. First SSI’s have been made using Spectrum as seismic input while the final SSI’s used Forced Response Analysis with accelerograms inputs. For preliminary SSI’s Spectrum, the piles have been considered as embedded in the raft without any isolator. Average pile length has been assumed from 2013 soil investigations. Bedrock seismic spectra have been used as earthquake input. Due to unacceptable bending moments at pile heads together with pile tractions impossible to be realistically taken by pile friction inside bedrock, use of isolators have been decided. Different types of isolators have been considered. Finally lead LRB isolators have been selected. During first SSI’s Spectrum, the resulting participating mass for 800 modes was horizontally admissible, but involved only 60% of the vertical mass. High traction loads have been computed at the pile heads of the two outer circles of piles. This computed traction, incompatible with isolator characteristics, was due to the too high stiffness of the isolators used for first trial and to the application of Square Root of Sum of Squares (SRSS) combination. Additional soil investigations made end of 2015 have provided more accurate variable pile length together with dynamic parameters to carry on nonlinear SSI Forced Response Analysis, using bedrock accelerograms as input. Due to short planning project, computations have been made only for selected nodes and elements. Full nonlinear computations were not possible. Anyway, no more traction and lower compression loads have been obtained at pile heads. By mid-2016, French and Chinese Third Party reviews have validated our design using ANSYS and stick models.

Contributors:
Conference Title:
Conference Title:
16th World Conference on Earthquake Engineering
Conference Venue:
Santiago (CL)
Conference Dates:
2017-01-09 / 2017-01-13
Rights:
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 Record created 2017-01-18, last modified 2017-01-18


Original version of the author's contribution as presented on USB, paper 4949.:
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