Report No. : GEO Report No. 197

Report Title : Effects of Inclination, Length Pattern and Bending Stiffness of Soil Nails on Behaviour of Nailed Structures (2006), 116 p.

Author : Y.K. Shiu & G.W.K Chang


The soil nailing concept involves providing a stable block of composite material by reinforcing the in situ ground with soil nails. The reinforcing action is achieved through two fundamental mechanisms of soil-nail interaction. They are the soil-nail friction that leads to axial tension or compression; and the bearing failure of soil nails that leads to development of shear forces and bending moments in nails.

A study has been carried out to review these mechanisms and examine the mechanical behaviour of soil nailed structures in respect of the following aspects:

(a) the effect of nail inclination on development of nail axial force distribution and stability improvement of slopes and excavations;

(b) the influence of nail length patterns on facing deformation in a staged-excavation;

(c) the contribution from bending stiffness of soil nails to stability improvements of slopes and excavations.

In the study, numerical simulations have been conducted to investigate the effects of nail inclination using finite difference and finite element methods with strength reduction technique. The results are compared with outcomes from limit equilibrium methods.

From the results of the study, inclinations of soil nails can affect the reinforcing action of the nails. Increase in soil nail inclination would decrease the reinforcing forces in the nails and in turn reduce the stabilizing effect. For steeply inclined soil nails, axial compressive forces may be mobilized in the nails.The compressive forces would reduce the stability of the nailed structure.

Nail length patterns influence the displacement characteristics of nailed excavations. Soil nails placed in the upper part of a nailed structure in a staged-excavation contributes more towards reducing the horizontal displacement. Those installed in the lower part are more effective in improving stability of the structure.

Under working conditions, shear and bending resistance of soil nails have little contributions to the system of forces maintaining stability within the soil nailed structure. Even when failure conditions are approached, the contribution of shear and bending action may be more significant but is still small. Soil nail used in axial tension is much more efficient than that used in shear and bending.

This report presents the details and results of the study. It also provides guidance on some design aspects of soil nails.

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