STABILITY OF SLURRY TRENCHES

Abstract

The " slurry Trench Technique" for construction of deep water-tight cut-offs for dams and for construction of under-ground structural concrete walls is a recent development. The excavation is carried out by conventional draglines or clam-shells or by special trenching equipment and the excavated trench is kept filled with a thixotropic suspension which supports the vertical face of the cutting. In addition to bein a safe and economical method, the main advantage of the techni-que is that the problems of dewatering and timbering are eliminated. The technique is mainly based on field experience and the machanics of stabilization of the vertical trench face by the slurry was not clearly understood till recently. The density of the slurry actually being used for the trenches was quite low. Even if an impermeable boundary was assumed to be formed at the trench interface, calculations based on classical earth pressure theory indicated that caving in should occur. How this impermeable boundary was formed was another un-resolve problem. In contrast to the case of short trenches, the proble- of stability of vertical soil face in a long trench in cohesion. less saturated formation attracted particular attention, as in this case the advantage of arching action in soil mass did not accrue. In the decade 1961-70, many investigators conducted experiments to understand and explain the stabilizing mechanism in a slurry trench. These investigations have brought out a number of stabilizing factors besides the hydro-static pressure of the slurry. Chapter III of the dissertation gives a brief and consolidated account of these investigations. Chapter II contains relevent details about the properties of thixotropic suspensions being used in the slurry trench method, and the theoretical approach to analyse the stabilizy of the trench face has been discussed in Chapter IV. Mathematical expression have been obtained to calculate the factor of safety against sliding for vertical trench faces in different types of soils, viz, clay, dry cohesionless sand, and saturated cohesionless sand. A general equation ( based on Coulomb's Wedge Theory) has also been obtained to determine the required slurry density forgiven soil properties and desired factor of safety. While the derivation of equations is based on earlier work, an attemp has been made to put them in a more directly usable form for the designers. Illustrative examples have been included to elucidate the method of computations proposed by the author. Chapter V discusses how for the theory is supported by laborat-ory and field data. Stability analysis of three practical slurry trenches, viz., those at Wanapum (U.S.A.), Pierre-Benite (France) and Ukai (India) have been carried out to reflect the validity of the theory. An attempt has been made to correlate and compile, after a critical appraisal, the latest available information on the subject. In the last Chapter, after summarizing the main conclusions, a procedure for rational design of the " slurry" has been suggested. Although the field of application of this new technique is widening very rapidly, the theoretical study in this dissertation has been mainly oriented towards its use in construction of foundation seepage barriers under earth dams. To make the work more useful, an appendix on " Construction of impermeable cut-offs by slurry trench method" has been added in the end.