Keywords: soil nail, pullout, length/diameter ratio, roughness, three-dimensional numerical analysis.

Nails have been used in geotechnical practice to improve the stability of an existing slope or to secure an excavation. Most of the nails used in these systems are constructed by inserting steel bars into pre-drilled holes or by driving pipes or bars into the soil. Nail inclination with respect to the horizon allows cement-nail gravity grouting and accomplishes a better reinforcing effect. The potential slip surface divides the nailed structure into active and resistant zones. The nails provide resistance against slope slip through nail tensile strength activation or by a frictional force at the soil-nail interface on the nail portion protruding from the active zone into the resistant zone. Experimental tests and analytical analyses claim that in routine excavations or steep slope stabilization applications, nails are subjected to primary axial forces. Nail resistance against shear force and bending moment were found insignificant. Therefore, the frictional resistance at the soil-nail interface is the major contribution to soil mass stabilization [1,2,3]. Nail tensile strength activation depends on the relative stiffness between the nail and adjacent soil mass. Frictional resistance along the nail surface is developed when the nail and adjacent soil mass move relatively. Understanding the frictional resistance at the soil-nail interface and the pullout force-displacement behaviour of a nail embedded in soil are the groundwork for a successful nailed structure design.

A pullout test for a nail embedded in a soil box is one of the methods for determining the interface frictional characteristics between the soil and nail. The pullout behaviour of nails embedded in sand depends on many factors, such as the nail length, the overburden pressure, the nail surface roughness, etc. Determining the interfacial behaviour calls for extensive test conditions, various nail arrangements and other laborious work. This paper presents an analytical method that facilitates the test setup and nail arrangement. The experimental work included pullout tests on single- smooth, roughed and double-nail systems embedded in dry sand. The interactive behaviour at the nail-sand interface during the pullout process is numerically investigated. Predictions made using the proposed approach are compared with the experimental measurements.

Field nail arrangements and in the sand box were subjected to three dimensional stress conditions. The simulating nail pullout behaviour is complicated by the highly nonlinear behaviour of the soil and the interaction between the soil and nail. However, nail pullout behaviour can be understood to some extent by simplifying the soil properties and the interaction characteristics at the soil-nail interface. Hence, the finite difference program (FLAC) combined with simplified interaction characteristics is used in this study.

A 3-D numerical model for soil-nail pullout behaviour elicited the following conclusions:

1. Numerical predictions of pullout force-displacement behaviour for smooth nails agree well with model tests for nails with various aspect ratios.
2. Surface roughness at the front wall of the sand box has an insignificant effect on the pullout behaviour of nails and the soil stress distribution. The numerical results preclude box surface smoothness anxiety.
3. Soil stress exhibited profound variations in the vicinity of the rigid front wall and along the nail. Nail portions 15 cm (about 16 times of the nail diameter) away from the front wall were subjected to nearly the same level of stress. Sleeves longer than 16 times the nail diameter delete stress concentration.
4. The nail pullout force obtained from tests in pairs increased with the increase in the spacing/diameter ratio and reached that of the single nail test result when the spacing between the pair became greater than 17.77 times the nail diameter.
5. Different degrees of surface roughness could not be directly simulated in the numerical model. The apparent diameter, D_a, corresponding to the degree of surface roughness was proposed in the numerical analysis.

1

Juran, I., Baudrand, G., Farrag, K., Elias, V., "Design of Soil Nailed Retaining Structure", ASCE Geotechnical Special Publication, No. 25, New York, 644-659, 1990.

2

Shewbridge, S.E. and Sitar, N., "Deformation-based Model for Reinforced Sand", Journal of Geotechnical Engineering, ASCE, 116(7), 1153-1170, 1990. doi:10.1061/(ASCE)0733-9410(1990)116:7(1153)

3

Jewell, R.A. and Pedley, M.J., "Analysis for Soil Reinforcement with Bending Stiffness", Journal of Geotechnical Engineering, ASCE, 118(10), 1505-1528, 1992. doi:10.1061/(ASCE)0733-9410(1992)118:10(1505)

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