Keywords: soil nailing, excavation, numerical analysis, hyperbolic model.

Among the methods for designing a nailed soil retaining wall, the most commonly used is the limit equilibrium method. However, procedures based on the limit equilibrium method remain some differences in considering the tensile force, the shear force, and the moment mobilized in the nails. A numerical analysis method for investigating the behaviour of a nailed soil wall during and after excavation is presented in this paper. The analytical model consists of a non-linear constitutive model for the soil, and constitutive models for the nails and shotcrete. The validity of the model is verified through comparison between the analytical results and the experimental observations for a full-scale nailed soil wall built in France. From this study, the analytical results indicate that the tensile force mobilized in the nail contributes the most important reinforcing effect, while the shear force developed in the nail is negligible. Results from parametric studies regarding the inclination and the stiffness of the nails are also presented.

Soil nailing is a practical and a proven technique in excavated constructions and slope stabilizations. The mobilization of shear force and bending moment in the nail depend on many parameters, such as the stiffness of the nail, strain and displacement patterns in the reinforced soil mass and orientation of the nail [1].

Most of the previous numerical analyses used a linear elastic or linear elastic perfectly plastic model to describe the constitutive behaviour of the soil mass [2,3,4]. However, these models are appropriate only for a soil mass subjected to low level stresses. During excavation, the large soil mass region behind the facing reaches stress levels that are very close to or at the failure state. Therefore, a hyperbolic model with the Mohr-Coulomb failure criteria was used in this study to represent the stress-strain behaviour of the soil mass. The following subjects were addressed in this study: (1) the stress states of the soil elements inside and outside the reinforced zone; (2) the contribution of the induced tension, shear and moment of the nail to the retaining wall; (3) the influence of the nail inclination on the tensile distribution of the nail and deformation of the facing; and (4) the influence of the nail strength and nail inclination on the bearing capacity and failure mechanism in the wall. To verify the validity of this analytical model, results from a full-scale experimental test were compared with those obtained from the analytical prediction.

An explicit finite difference program was used to analyse the mechanical behaviour of nailed soil retaining walls. The soil matrix, reinforcing nails and facing constitute the excavated structure. A non-linear perfectly plastic constitutive model is introduced to describe the mechanical behaviour of the soil mass. The hyperbolic model is used in the stress-strain relation for stresses below yield and the Mohr-Coulomb yield criteria is adopted.

The conclusions are summarized below:

1. The stresses of some elements in the soil mass approach the failure stage during excavation. The behaviour of the soil after yielding must therefore be modeled properly. The analysis model predicted good agreement with observations of the experimental nailed walls.
2. The mobilization of shear force in the nail is very small in the working stress and limit states. Only the tensile force in the nail contributes an important reinforcing effect. Consequently, the authors recommend that the bending stiffness can be ignored for design purposes.
3. Increasing the nail inclination reduces the maximum tensile force in the nail and the bearing capacity of the excavated nailed structure for pullout failure.
4. In general practice, most nailed soil structures fail due to nail pullout. Increasing the axial nail stiffness can reduce the horizontal facing displacement and the bearing capacity of nailed walls.

1

Schlosser, F., "Behaviour and Design of Soil Nailing", International Symposium on Recent Development in Ground Improvement Techniques, Bangkok, 399-413, 1982.

2

Juran, I., Shafiee, S. and Schlosser, F., "Numerical Study of Nailed Soil Retaining Structures", Proceedings of the Eleventh International Conference on Soil Mechanics and Foundation Engineering, San Francisco, 1713-1716, 1985.

3

Kakurai, M. and Hori, J., "Soil-Reinforcement with Steel Bars on a Cut Slope", Performance of Reinforced Soil Structures, Proceedings of the International Reinforced Soil Conference, British Geotechnical Society, 213- 217, 1990.

4

Raju, G. V. R., "Behavior of Nailed Soil Retaining Structures," Ph.D. Thesis, Nanyang Technological University, Singapore, 1996.

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