A gas-to-liquids project involved construction of a new facility over a thick, soft clay site in West Africa's Niger Delta region. The facility consists of numerous large diameter tanks, processing facilities, and a wharf area. The project's original geotechnical consultant performed extensive site investigation and laboratory testing, and developed a complex site preparation approach involving tightly spaced wick drains (7,000 km of wick drains), placement of 3 million cubic meters of sand fill surcharge dredged from a source 135 km from the site, and groundwater drawdown for additional temporary surcharge. Given the complexity of the approach, the remoteness of the site, and the potential schedule and cost implications if the site preparation program did not progress as planned, the owner engaged Geosyntec to perform a geotechnical peer review on the owner's behalf.
Geosyntec's Scope of Services
Geosyntec received multiple assignments in support of this project, including:
- Peer Review of Site Preparation Approach - Geosyntec performed an independent evaluation of the available geotechnical data and analyses. We identified that much of the laboratory soil testing had been performed on disturbed specimens and we developed procedures to reinterpret the test results. We developed independent estimates of settlement magnitude and rate. We concluded that the original estimate of the rate of settlement was overly optimistic and that site preparation would likely take several months longer than had been predicted, with the potential for significant cost overruns.
- Cost Savings Review - With the project under way, Geosyntec was asked by the owner to review the project geotechnical consultant's foundation recommendations and to assess whether alternative, less expensive, foundations could be implemented. The primary goal was to reduce, if possible, the extent to which facility structures were supported by deep pile foundations. We performed calculations of primary and secondary consolidation settlement below critical structures, and concluded that: (a) with proper tank design and accommodations for future settlements, tanks did not need to be supported on piles; and (b) the more lightly loaded structures at the site could be supported on shallow foundations as opposed to piles.
- Dewatering / Settlement Troubleshooting Review - Settlements were found to be progressing slower than anticipated, and the pumps were not able to draw the water table down to the target depth. We undertook a "troubleshooting review" and concluded that: (a) the sand fill was acting as a reservoir to store both rainfall and consolidation water; (b) there was an apparent hydraulic connection to the adjacent river at depth; and (c) pumps were being operated in a less than optimal fashion. We recommended that the sand fill be covered to isolate it from rainfall, additional pumps be installed within the sand fill, and pump operations be modified to improve efficiency.
Based on Geosyntec recommendations, revised foundation strategies in the tankage areas are estimated to have saved more than$3 million in piling costs. Implementation of the recommendations from the troubleshooting review increased the rate of sand fill settlement. Additionally, Geosyntec provided on-site engineering support during installation of site foundations to assist the owner and the implementation contractor in achieving the design intent at this critical remote site.