7 - Soil reinforcement

Abstract

With the inclusion of reinforcing elements (in the form of natural or synthetic fibres, metal strips, soil nails and anchors, and micropiles) in the soil mass, its various engineering properties are improved. The basic principle behind ground improvement using geosynthetics is that the reinforcing elements absorb the tensile loads or shear stress within the structure and thus preventing its failure due to shear or excessive deformation. The improvement in the engineering properties happens due to the friction developed at the soil–reinforcement interface and the passive resistance that occurs through the development of bearing-type stresses on transverse reinforcement surfaces in a direction normal to the relative movement of soil reinforcement. The contribution of the reinforcing element towards ground improvement will depend upon the roughness of the surface (skin friction); normal effective stress; grid-opening dimensions; thickness of the transverse members; elongation characteristics of the reinforcement; and the level of soil–reinforcement interaction based on soil characteristics including grain morphology, grain size distribution, density, water content, cohesion, and stiffness. The concept of soil nailing in the form of metal bars, tubes, or rods involves the placement of reinforcing elements in situ at suitable distances to increase the shear strength of the soil and to restrain its displacements during and after excavation. Similarly, in the case of soil anchoring, prestressed soil anchors are installed in the ground to reinforce the soil and support vertical or inclined excavations. In micropiling, small-diameter (about 10–30cm) piles are installed vertically, or in a reticulated manner to support excavations, slopes, and foundations, or for the purpose of underpinning or retrofitting structures. In all the reinforcement techniques mentioned above, the stress transfer between the soil and the reinforcing elements, failure surface of the reinforcing elements, strain compatibility between the soil and the reinforcement, placement method of the reinforcing element, and the durability and long-term behaviour of the reinforcing elements are the important factors that decide the effectiveness of these methods for various structures. In this chapter, the ground improvements by using different methods of soil reinforcement techniques are presented.