Keywords: pullout resistance, nail, sand, surface roughness factor, cavity expansion theory.

Soil-nailing techniques have been widely used in soil excavation and slope stabilization. Closely spaced nails are buried or drilled into the soil to form a reinforced structure. Because the nail and surrounding soil have different mechanical properties, relative displacement occurs when the reinforced structure is loaded. In this structure, frictional resistance at the soil-nail interface is the major contribution to soil mass stabilization.

Soil particle rolling and slippage accompanies the advancement of nail pullout, which induces particle rearrangement and thus volumetric change. Some literature attributed the shear resistance mobilized along the reinforcing member to the restrained dilatancy in the shear zone between the nail and soil. For a nail embedded in confined granular soil, volumetric dilation in turn provides additional confining constraint to the soil particles in the nail vicinity. The increase in normal pressure usually induces an increase in frictional force at the nail-soil interface. Incurred normal stress increment as high as two to ten times the initial stress has been reported [1,2]. Nevertheless, the magnitude of volumetric dilatancy depends on the normal pressure acting on the nail surface. Results from experimental tests indicate that the apparent friction coefficient decreases with the increase in normal pressure. This may be attributed to the declination in dilative potential with the increase in confining pressure, thus diminishing the normal stress increment. In short, the increase in nail pullout resistance is not proportional to the increase with confining stress.

This study presented a theoretical analysis using the cavity expansion theory to evaluate cavity radial displacement around a ragged nail with normal pressure acting on the nail surface. The results obtained from this approach were compared with those acquired from pullout tests conducted in a laboratory. Good agreement was observed for nails tested under various conditions such as nail surface roughness, nail diameter and soil particle size. The apparent friction coefficient and stress increment ratio increased with the increase in the screw pitch, but this increase trend subsides for greater screw pitches. The particle size has a significant effect on the radial displacement and thus the pullout resistance. The experimental and theoretical analysis results show that nails embedded in sands with greater particles produce a higher apparent friction coefficient, which can be deduced from the higher radial displacement and normal stress increment ratio values. These results reveal that an increase in the surface roughness factor increases the radial displacement and normal stress increment ratio.

1

K. Wernick, "Stress and Strain on the Surface of Anchors", Proceedings of 9th International Conference on Soil Mechanics and Foundation Engineering, Session 4, Tokyo, 113-119, 1977.

2

P.P. Xanthakos, "Ground Anchors and Anchored Structures", John Wiley, New York, 1991.

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