Reinforced soil foundation stabilizes diesel tank on weak clay 

In late 2015, Nichols Environmental was approached by a large multinational energy company to provide a geotechnical overview for a petroleum tank farm expansion in Sherwood Park, Alberta. The scope included recommending foundation strategies for a 129.6 ft, 600,349 ft³ diesel tank. The proposed location was underlain by a 16 ft (4.8 m) thick layer of high-plasticity lacustrine clay, resting above a silty sandy clay layer extending more than 32 ft (10 m) deep.

Challenge

Following the geotechnical investigation, Nichols Environmental initially recommended three different pile foundation solutions. However, VTEK Consultants Inc. – the engineering, procurement, and construction management (EPCM) consultant tasked with designing and building the tank – requested a more cost-effective alternative. Specifically, they asked Nichols to explore a Reinforced Soil Foundation (RSF) approach.

Having attended a Solmax technical seminar, one of Nichols Environmental’s project managers reached out to Solmax for assistance in preparing an RSF-based design. At the time, Nichols had been attempting to model an RSF using a biaxial geogrid, but the requirements called for something more robust.

After an initial review of the geotechnical data and project goals, Solmax confirmed that an RSF was a viable solution. Reinforced soil foundations incorporating geosynthetics have been successfully used for over two decades. Together, Nichols Environmental and Solmax refined the design parameters, completed settlement calculations, and carried out bearing capacity analyses. MIRAFI^® RS580i high-performance geotextile was selected for its high tensile modulus at low strain, superior flow capacity with a smaller average pore size, and enhanced soil confinement through an improved coefficient of interaction.

Solution

The foundation construction began with a 5.9 to 11.8 inches (150mm to 300 mm) thick layer of cement-stabilized subgrade to dehydrate and firm up the underlying soil. This was followed by the placement of eight layers of MIRAFI RS580i high-strength woven geotextile at 15-inch (375 mm) intervals, backfilled with 3 inches (80 mm) minus pit run material.

Nichols Environmental presented all four foundation options, including the RSF, in their geotechnical report to the EPCM consultant. Construction of the RSF began in August 2016, executed by a civil earthworks contractor with prior experience installing MIRAFI RSi products.

Settlement and quality control testing were conducted in April 2017, once the tank was filled with water. Based on the design, settlement was predicted to be 2 inches (40 mm) at the tank edge and 3 inches (74 mm) at the center. During hydrostatic testing, approximately 1 inch (25 mm) of differential settlement occurred, which was in line with the allowable limit.

Nichols Environmental attributed the observed differential settlement to two main factors:

• Subgrade variability: Roughly one-third of the excavated in situ foundation soils were in poorer condition than expected. This section was removed and replaced with more competent material, which led to increased resistance to settlement in that area, contributing to the differential behavior. For applications with high sensitivity to differential settlement, Nichols recommends that the entire foundation be uniformly reconstructed.

• Construction traffic: Access to the excavation was provided through two ramps. Repeated trafficking by construction equipment caused localized compaction in these zones, potentially increasing resistance to settlement in those areas. A mitigation strategy would be to rotate the access and egress ramps by 90 degrees with every second lift of RSF backfill.

Conclusion

The completed structure stands as a testament to Nichols Environmental’s commitment to innovation and cost-effective solutions. It now provides a safe, stable foundation for critical fuel infrastructure. Solmax is proud to have collaborated on this successful project and looks forward to future opportunities to support high-performance geotechnical designs.

References

Use of Reinforced Soil Foundation (RSF) to Support Shallow Foundations, Louisiana Transportation Research Center (LTRC), LTRC-04-2GT, 2008

Ochiai et al., Supporting Capability of Geogrid-Mattress Foundation, 1994

Shallow Foundations Reference Manual, FHWA NHI-01-023, June 2001