SOIL NAILING FOR STABILIZATION OF SLOPES
After its development in early 1980’s it become widely used in method of providing permanent and temporary support for earth, and stabilization of slope on many engineering projects in world wide. In starting of this period soil nailing was only performed on that project where geotechnical contractor provide this as an alternative to other soil improvements techniques. Now a days soil nailing is accepted only because of its performance and effectiveness. But the question may rise that in which condition this method is applicable or where is this more efficient and why this technique is to be used. Means when, where and why this technique is been applied. Generally the detail of soil nail, installation procedure, its design and testing and also case studies give us information to use this technique or not. Introduction:
Soil nailing is technique in which retaining walls, soil slopes and excavation are reinforced passively by the help of steel bars or may be by inserting the relative slender element. The structural elements which transfer load to ground in excavation reinforcement are called nails. The concept of soil nailing was developed in early 1970 in Europe for repairing of earth wall, for stabilization of permanent and temporary slopes and for retaining walls. The first application of this method was done in 1972 in France. According to Chassie 1993 this technique are also applicable to basement excavation (temporary shoring) and for temporary supports for excavation related to tunnels and railroads. As Chassie also said that first record of this application was in United States in 1970’s mid to provide temporary and permanent support for building of hospital. The major use of this technique in US is in earth retention during construction of building and other structures.
Concepts of soil nailing:
Soil nailing is economical technique which enhances the shear strength of unstable in situ soil by installation of steel bars (closely spaced) in to rock or soil. Mainly it is not used alone. The grouting is done or it is used with concrete so to safe reinforcement from deterioration. It is misconception that the structural facing or concrete is major part of soil nailing for load transfer. In fact, the nail do this work. Soil nails are passive elements. They are different form tie back anchor. As tie back anchor are made pre-tensioned after installation of it. While soil nails are not pre-tensioned. While they become forced in tension when the surrounding soil deforms laterally as the excavation depth increases. If there is no soil movement then nail remain in passive state. As according to chassie, 1993 that to mobilize the soil nail the very small movement is required and this movement may corresponding to the movement may occur in bracing system. In some specification it is written that soil nail may be tensioned just like tie anchor to mobilize it but this is wrong. And also in some cases it may be pre-tensioned by application of small load to eliminate its deflection and to mobilize it. It is applicable and correct but if major pre-tension is done then this pre-tension eliminate the effect and purpose of soil nail.
As soil nailing provides the temporary supports but it will be more economical for using this technique as permanent retaining walls. This is a flexible technique so it is no need that wall is cut at 90’ or very refined and have specific geometry. This is easily applied at curves and bends to stable them. If the final product calls for such geometry, more than likely the permanent facing could be shot Crete (Figure 6). Once the temporary shot Crete wall is constructed, a permanent facing typically involves adding another 100 to 150 mm (4 to 6 in) of shot Crete to the existing 75 to 100 mm (3 to 4 in) of shot Crete. The permanent facing will be reinforced and a screed face finish can be provided.
References: Thomas J. Tuozzolo, P.E. “SOIL NAILING: WHERE, WHEN AND WHY A PRACTICAL GUIDE” Presented at the 20th Central Pennsylvania Geotechnical Conference Hershey, PA, 2003
William K. Petersen, P.E., “PRACTICAL SOIL NAIL WALL DESIGN AND CONSTRUCTABILITY ISSUES” URS Corporation, Ft. Washington, Pennsylvania, USA.
“Design and construction guideline for soil nail wall” Geotechnical engineering manual
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