Design of MSE Walls for Fully Saturated Conditions

Abstract

The current design of mechanically stabilized earth (MSE) walls, which is based on limit state analysis, does not apply to undrained conditions. Laboratory and numerical pullout tests are performed to determine the relation between drained and undrained pullout capacities for different soil types (clean sand, 5, 10, 15 and 35% silty sand), overburden pressures (30, 100, and 200 kPa), and scale and permeability effects in the dissipation of excess pore pressures. The results of the pullout tests show that both drained and undrained pullout capacities change as silt content changes since the pullout capacity increases as the internal friction angle of the soil increases. It is also observed that the pullout capacity increases as the overburden pressure increases. Undrained conditions significantly reduce the pullout capacity as much as 50%. This is caused by the generation of excess pore pressures in the soil under rapid loading which decrease the effective stress at the sol-reinforcement interface. The magnitude of the pullout reduction is related to the permeability of the soil since for large permeabilities the dissipation of excess pore pressures is very rapid and no reduction in pullout is produced; in contrast for low permeabilities the dissipation of excess pore pressures is slower than the rate of pullout and thus a reduction occurs. This is confirmed by the experiments that show no reduction in pullout capacity for clean sand, and a large reduction for silty sands. The ratio of undrained to drained pullout capacity changes with silt content and overburden pressure; for 100 and 200 kPa overburden pressure, the ratio is 1.0 for clean sand, 0.67-0.69 for 5% silty sand, 0.77-0.78 for 10%, 0.72-0.73 for 15%, and 0.57-0.59 for 35% silty sand. For 30 kPa overburden pressure, the ratio is 1.0 for clean sand, 0.5 for 5% silty sand, 0.67 for 10%, 0.78 for 15%, and 0.72 for 35% silty sand. It is observed in the numerical analyses that the dissipation of pore pressures is very rapid for permeabilities larger than 0.01 cm/sec, and significantly slow for permeabilities smaller than 0.001 cm/sec. Scale effects are extremely important since as the length of the reinforcement increases the time for pore pressures to dissipate increases.