As utilities continue to diversify their energy mix and look for more efficient solutions to our growing energy needs, many are increasingly relying on liquified natural gas (LNG). In 2012, Puget Sound Energy (PSE), a private utility serving more than a million customers in Washington State, began preparing to build a LNG processing and storage facility in Tacoma, Washington. PSE contracted with GeoEngineers to provide extensive geotechnical, environmental and permitting services for this challenging project.

Use of LNG is continuing to grow, with a handful of processing facilities planned or established on the West Coast. Natural gas can be unwieldy to store and transport in its gaseous form. Liquefying natural gas requires cooling it to a brisk -260 degrees Fahrenheit. As a liquid, natural gas has 1/600th the volume, making it much more efficient to store and transport. The advantages of LNG over traditional fuel sources are numerous. Compared to bunker fuel or diesel, LNG reduces the emission of particulate matter by up to 90 percent, and greenhouse gases by up to 30 percent. If spilled, LNG will quickly evaporate, leaving little material to affect the environment.

PSE’s completed LNG facility will process and store 8 million gallons of LNG for use as marine transportation fuel by TOTE Maritime, a commercial shipping company running routes from Tacoma to Alaska. LNG will help TOTE reduce air emissions and poses less environmental risk if spilled. PSE can also load the stored LNG into trucks or convert LNG back into gas to supplement the regional energy supply during cold days and other times of high demand, a process known as peak shaving.

The facility’s location, on a tidal flat at the Port of Tacoma, posed a formidable geotechnical challenge. GeoEngineers’ team assessed foundation options and ultimately recommended one of the largest ground improvement programs west of the Rocky Mountains. To mitigate liquefaction risks and support the 8-million-gallon LNG tank and processing facility, the plan called for more than 2,500 36-inch auger-cast piles, extending up to 100 feet underground.

Engineering services also included seismic risk analysis and site-specific ground motion modeling by GeoEngineers’ performance-based engineering team. The team worked closely with PSE and the LNG designers to complete geotechnical investigation, engineering analysis, environmental analysis, permitting support, stormwater management and material management plans.

Approach

  • Geotechnical Investigation: GeoEngineers explored subsurface conditions at the site through a combination of strategies. A drill team completed boreholes and cone penetration test soundings, and engineers used multi-channel array surface wave (MASW) and microtremor (passive surface array) analysis to measure the shear wave velocity of soil across the site.
  • Seismic Risk Analysis: Regulators required the facility to withstand a 2,475-year seismic event—an extremely rigorous design standard. The facility’s location on a tidal flat characterized by lose soil vulnerable to liquefaction only made the challenge greater. The performance-based engineering team performed a site-specific seismic response analysis and liquefaction analysis to determine the design requirements. Traditional pile foundations were ruled out due to the depth of the liquefiable soils.
  • Ground Improvement: Based on the performance-based engineering team’s analysis, GeoEngineers proposed and implemented a massive and creative ground improvement program to meet the design requirements. Contractors drilled and grouted almost 2,500 36-inch auger-cast-in-place piles that extended between 80 and 100 feet below grade. These grout columns were cut off at the surface and then capped with crushed rock to act as a bearing layer.
  • Load Test Program: GeoEngineers developed a ground improvement load test program to evaluate the performance of the auger-cast piles. Testing demonstrated minimal settling—just 0.1 inches over the first 8 hours and less than 0.5 inches under the maximum design load.
  • Stormwater Permitting/Management: GeoEngineers’ environmental scientists guided PSE through the stormwater permitting process, and prepared and managed a detailed Stormwater Pollution Prevention Plan in accordance with the National Pollution Discharge Elimination System permit requirements. The plan laid out erosion and sediment control measures, pollution prevention measures, inspection/monitoring activities, and record-keeping for every anticipated stage of construction. The plan included excavating a large containment and infiltration pond. The plan successfully managed stormwater throughout construction—even during record rains late in the winter of 2017.
  • Construction Monitoring/Hydro Testing: The team performed construction monitoring services throughout the project and planned and implemented a hydro test of the 8 million-gallon LNG tank using an equivalent weight of water (5 million gallons). GeoEngineers evaluated the tank for settlement and performance of the ground improvements and found that the tank only settled half an inch under the weight of 41.75 million pounds of water—well within acceptable limits.
  • Permitted Soil Management: The project generated more than 50,000 cubic yards of soil cuttings, and GeoEngineers provided environmental support and materials management to ensure that soil was tested and disposed of as required by permit.
  • Geotechnical Recommendations for Pipeline: GeoEngineers also performed geotechnical and environmental investigation along the proposed alignment of a of a new four-mile-long, 16‑inch-diameter high pressure natural gas pipeline that PSE would use to transfer natural gas to the facility. Support included design recommendations, permitting, stormwater inspections and construction monitoring.

Results

The Tacoma LNG facility promises to provide a cleaner energy source and improve the reliability and efficiency of PSE’s energy grid. Throughout this high-profile regional project, GeoEngineers worked closely with PSE and partner firms to solve problems and push the facility toward completion. Thanks to the team’s seismic analyses and expansive ground improvement program, this LNG facility meets the highest standards for safety and earthquake resiliency.

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