UNDERGROUND IMPERMEABLE DIAPHRAGMS AND STABILITY OF SLURRY TRENCHES IN -COHESIONLESS SOILS

Abstract

The history of the subject of '"Underground Impermeable Diaphragms and Stability of. Slurry Trenches in Cohesionless Soils" dates back to only two decades. The method of excavating trenches in the presence of bentonite suspensions for construc-tion of impermeable diaphragms had a somewhat complex origin, but no doubt the search for a new approach to the provision of posi-tive cutoffs below earth dams on gravel, sand and boulder deposits of large depth led to the development of this technique. The development of the science had advanced in two phases (.i) the field technique of construction of various types of diaphragms in the presence of bentonite suspensions with the aid of different types of trenching equipment and (ii) the , search for the knowledge of factors affecting the stability of slurry trenches'.' The development for the first phase comprises of t i separately developed practices (i) the European Practice an (ii) the American Practice f with regard to the carious commbnents of diaphragms, such as, "Size of Panels,. types of concrete ~ixes for concrete, Reinforcement etc. The former practice has been mor a widely adopted in the field, as large number of works were exlcuted in European countries in early stages of developme of this process. The details of the practices followed and eecommendation in that regard. have been dealt with in Chapter'; U4. "I.) - In India, a limited nuTbber of works has been executed, through the patents available with a solitary firm. However the construction technique needs three Important aspects to be watch-ed and developed as arts. They are (i) slurry control (ii) Trench-ing skill and (iii) Concreting. The problems connected with these arts have been dealt in Chapter M. The "reverse mud circulation system" in trenching is essential for the success of the work. Moreover, the Engineer must have a thorough working knowledge of the subject, with regard to the equipment available in the field its utility and problems met with during the cons-truction of impermeable diaphragms. This will help meet with the challenge of timely and planned construction. This dissertaa Lion aims to cover these aspects. In the last twenty one pears, very important works of construction of such diaphragms have come both Abroad and at Home. Typical works have been chosen for study to provide the details which led to their adoption in field against conventional methods and their performance. 1hese works have been described in Chapter IV. The second phase of the developments is also important, as a sound knowledge of the factors involved will help to design safe and economic slurry trenches without depending on proprietary techniques of private firms. Many noted Engineers have worked on the study of factors that dominate: the stability of slurry tren-ches in copes .onless soils. Laboratory tests have been carried out on small slurry trenches, and the behaviours has been analysed by measuring slip surfaces as well as failure of wedges (vJ by Coulomb's Wedge Theory. Eison has proved by these studies that stability of system depends largely on hydrostatic pressure of slurry, shearing resistance of the slurry saturated zone and passive resistance of the clay suspension. Other factors, like resistance to deformation of impermeable cake # Formed. by bentonite on trench facet and eieotro-osmotic faces also account to a small extent In stabilizing the slurry trench. The contribution of these factors in stability of slurry trench has been dealt with in Chapter V. However there are still gaps in the knowledge of mechan .zm of slurry wenches. Also there is scope of improve. merit in the construction methods of concrete diaphragms. These have been described in the Chapter VI on "Conclusions and Recommendations.