Design of the flexible structural facing in soilnailing

Alberto Grimod, Barth Peirone, Giorgio Giacchetti

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Landslides II

ABSTRACT: The goal of soil nailing is to improve the slope stability when there are unfavorable circumstances. Nails mobilize friction forces along the entire length and contribute to improve the stability conditions when there are displacements in the soil. The protection of the exposed surface between the nails is obtained by the facing, which can contain the soil, prevent erosion phenomena and contribute to the aesthetics. Steel mesh is becoming a common flexible structural facing, instead of the classical shotcrete. This paper describes the general behaviour of the mesh facing and it gives the theoretical concepts of a new design approach based on laboratory and full-scale tests and in-situ evidence.

RÉSUMÉ: Le but du soil nailing est celui d™augmenter la stabilité d™un talus qui est exposé à des conditions défavorables. Les clous (nails) mobilisent leurs forces de frottement sur toutes leurs longueurs en augmentant le facteur de sécurité du talus, quand les mouvements du terrain commencent. La protection superficielle, qui a la fonction de contenir les mouvements du terrain, est garantie par un revêtement, qui normalement a une fonction anti-érosion et esthétique aussi. Depuis quelques années, les grillages (treillis) métalliques sont fréquemment utilisés comme revêtements structuraux flexibles, à la place du béton projeté. Cet article montre le comportement général des revêtements en grillage et donne les principes théoriques pour leurs conceptions. Cette nouvelle approche de calcul est fondée sur les essais en laboratoire et en vrai échelle et sur les évidences en site. 1 SOIL NAILING Soil nailing is a technique used to stabilize slopes with low cohesion or purely frictional behavior. It is recommended for soil with no groundwater presence, partially saturated, with a miscellaneous granular matrix (from sandy gravel to clayey silt) and with a low to medium cementation grade, or for altered rocks (Cola et al., 2011). It consists of anchors (nails), which increase the stability of the slope, and a facing, which contains the movement of the soil between the nails. From a geotechnical point of view, both nails and facing are passive systems, because they start to work only if soil movements occur. Section 1.1 and 1.2 describe the function and the contribution of these two elements in the system. Soil nailing is usually used for temporary applications, but nowadays it is becoming popular for permanent applications as well. It is normally easy and fast to install, it does not require excessive excavations and it can be environmentally friendly. This stabilization system can be applied on new excavated slopes/walls or on existing slopes. In the first case, the placement of the nails starts from the top of the slope, and then as the excavation proceeds subsequent rows (1 m to 2.5 m spacing) are placed (this installation method is called fitop-downfl, as described in several soil nailing manuals: FHWA, 2003; Ciria, 2005; Geoguide 7, 2008; HA 68/94, 1994). During the installation a fishielding effectfl is notable: in terms of tensile resistance a nail can collaborate only with 3 (maximum 4) nails placed under it. For artificial soil slopes reinforced with nails (Kim et al., 1995; Nishida et al., 1996; etc.), the highest deformation and stress occur at the top of the system. In the second case, the installation can start from the top (suggested) or the bottom, depending on the site conditions and the access available. In a natural slope it is always recommended to remove the vegetation (if possible) and re-profile the slope. In this way, both nails and facing can be installed in the proper way in order to maximize their performance. Figure 1. Typical soil nailing applications (upper-left: stabilization of excavation; upper-right: excavated slope with load at the top; lower-left: slope stabilization by infrastructures; lower-right: land slide stabilization). 1.1 Nail The stability of the soil can be attained by inserting soil anchors as reinforcement. These systems are normally steel bars, but for temporary works, and short anchoring lengths fiber glass bars can be adopted. The anchors mobilize the frictional forces along their entire length and they contribute to stabilize the slope only if soil

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