The Use of Reliability Analyses in the Design of Deep Excavations in Soft Clay, Fargo, North Dakota, USA

Excavations for two deep lift stations in Fargo, North Dakota completed in 2009 extended 12.2 to 13.7 m (40 to 45 feet) below grade. Both excavations were entirely within deep deposits of high plasticity clay of relatively low undrained shear strength (16.8 to 52.7 kPa [350 to 1,100 psf]). The excavations were designed with an upper sloped cut 4.6 m (15 feet) deep, and a lower temporary excavation support system designed to retain 7.6 to 9.1 m (25 to 30 feet) of vertical cut. The excavation support systems consisted of circular sheet pile and soldier pile and lagging structures, 30.5 to 36.6 m (100 to 120 feet) in diameter. Lateral earth pressures were resisted by reinforced concrete ring wales. The project specifications dictated minimum safety factors for basal heave. In addition to evaluating safety factors for stability using conventional limit equilibrium methods, reliability analyses were performed to quantify the uncertainty in soil strength parameters. Although limited shear strength data were available, reliability analyses proved beneficial to the excavation analyses and design. The deep excavations were designed using a target probability of failure of 0.001 for global stability and basal heave, in addition to the minimum specified safety factors.