MSE Walls BR-1 AND BR-2 Support At The Mall of San Juan

The new Mall of San Juan project required improvements to the existing state road PR-8 East bound by constructing an overpass bridge with two Mechanically Stabilized Earth Walls at each abutment, identified as BR-1 and BR-2.  The MSE walls had maximum height of 23 feet and width of 61 feet.  The main geotechnical concern was performance of the MSE walls if built over soft foundation soil encountered along the bridge alignment and over-stressing of the piles supporting the bridge foundations.

The wall alignment partially passed over a wide trench backfill for a 42-inch diameter sanitary pipe line.  The backfill soil was described as soft silty clay fill with undrained shear strength of 500 pounds per square foot.  The backfill layer extended to depths of 15 to 20 feet underlain by stiff to very stiff native clay.  Groundwater table was observed from 10 to 11 feet below existing grade.

A grid of Geopier elements was installed beneath the footprint of the wall affected by the soft backfill, fully penetrating through the compressible soil.  The pier spacing was selected by modeling the soil geometry and soil strata with a limit equilibrium approach until obtaining minimum factors of safety 1.5 against global stability for static condition, and 1.2 for pseudo-static condition. The spacing of the piers depended heavily on the wall height.  The closest pier separation was 4 feet center-to-center at the abutments and around the driven piles to stiffen the matrix soil and prevent downdrag. Two field modulus tests were performed to confirm pier stiffness and both tests results were favorable.