The undrained pullout capacity of reinforced soil

Hong-Sung Lee, Purdue University

Abstract

The current design method for MSE walls, based on limit state analysis, may not be appropriate under undrained conditions. Laboratory and numerical pullout tests are performed to determine drained and undrained pullout capacities for different soil types (clean sand, 5, 10, 15 and 35% silty sand) and overburden pressures (30, 100 and 200 kPa). Pullout experiments are conducted on a large pullout box with dimension of 1.5 m long, 0.5 m high, and 0.4 m wide, where a single steel inclusion is placed with a mass of soil compacted at 95% Proctor. The results of the pullout tests show that both drained and undrained pullout capacities depend on the silt content and increase with the internal friction angle of the soil and with the overburden pressure. Undrained conditions may reduce the pullout capacity. This reduction is caused by the generation of excess pore pressures in the soil under rapid loading which in turn decrease the effective normal stress at the soil-reinforcement interface. The ratio of undrained to drained pullout capacity changes with silt content and overburden pressure; for 100 and 200 kPa, the ratio is 1.0 for clean sand (i.e. no reduction), 0.67∼0.69 for 5%, 0.77∼0.78 for 10%, 0.72∼0.73 for 15%, and 0.57∼0.59 for 35% silty sand. For a 30 kPa, the ratio is 1.0 for clean sand, 0.5 for 5%, 0.67 for 10%, 0.78 for 15%, and 0.72 for 35% silty sand. The magnitude of the pullout reduction is also dependent on the permeability of the soil and pullout rate. For large permeabilities, the dissipation of excess pore pressures is very rapid and no reduction is produced. In contrast, for low permeabilities, excess pore pressures are generated faster than they dissipate. Numerical simulations with a FEM with an elasto-plastic soil model reproduce well the experiment observations, with difference within experimental error. The simulations show that the deformations of the soil in contact with the reinforcement are very small, which indicates that yielding in the soil does not occur. Because of this the more important factor for pullout capacity is the coefficient of friction at the interface.

Degree

Ph.D.

Advisors

Bobet, Purdue University.

Subject Area

Civil engineering

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