Site soil conditions play a vital role in Oil & Gas projects and it has high influence on project viability and cost. Adopting appropriate strategies by evaluating & implementing ideal geotechnical schemes, the construction cost/time can be significantly optimized. This paper presents a case study of recently executed project with similar concerns and discusses the challenges encountered and fit-for-purpose solutions offered catering to various soil types, which resulted in cost optimization and early project completion.
Geotechnical investigation was carried out at site to establish soil strata/characteristics by drilling bore holes at strategic locations. Results revealed presence of varying subsoil conditions across the plot with high ground water table. At some locations, extremely loose soil was encountered at intermediate depths and at other locations soil was highly loose for top 5m depth.
Initially it was proposed to remove top soil for entire area and replace with compacted fill, however was not pursued due to huge cost. By ‘Thinking differently’, a unique approach was devised and ‘fit for purpose’ solutions offering optimum bearing capacity/settlement were provided based on anticipated loads in respective zones.
The plot was divided into distinct zones based on soil profile, type of structures, anticipated loads, allowable settlement, etc., and cost benefit analysis was performed for different geotechnical solutions. Subsoil in large storage tank areas was investigated for settlement concerns under heavy loads and liquefaction potential during seismic event. Due to weak soil, improvement using ‘vibro-replacement’ technique was finalized to preclude excessive settlement.
In areas supporting heavy structures such as tall vessels/rotary equipment/piperack, etc, special pile foundations with optimum depth were decided to limit differential settlement and safely transfer huge loads to firm strata.
State-of-the-art soil stabilization technique using geo-synthetics, which is proven to modestly increase the stiffness of weak soil was employed in areas intended to support moderately loaded facilities. Removal of loose top soil and replacement by ‘Engineering Fill’ supplemented with adequate compaction was deemed sufficient in certain areas bearing lightly loaded facilities.
By tailoring geotechnical solutions to befit diverse soil conditions, unwarranted/expensive soil improvements & large/raft foundations were eliminated. As a result, foundation footprint/quantities were optimized which resulted in substantial savings in construction cost. In addition, the project was completed ahead of schedule due to elimination of laborious activities.