Soil-Structure Interaction and Imperfect Trench Installations As Applied To Deeply Buried Conduits

Abstract

The imperfect trench installation method is used to reduce earth pressure on deeply buried conduits. Few quantitative refinements to the imperfect trench installation method, however, have been added since the fundamental mechanics of the reverse arching action was proposed by Marston and Spangler. There have been limited research results published regarding primarily qualitative aspects of earth load reduction for imperfect trench conditions. It was found during the course of this study that significant frictional forces develop along the sidewalls of buried conduits and adjacent sidefills in imperfect trench installations. Current American Association of State Highway and Transportation Officials provisions do not consider these frictional forces, but they cannot be neglected in imperfect trench installations as their effect is dominant. The objectives of this study were to study the soil-structure interaction for deeply buried roadway conduits (concrete pipes, corrugated polyvinyl chloride (PVC) pipes, corrugated steel pipes, and box culverts) and the efficiency of the imperfect trench method for their installations. This research identifies variables that significantly affect earth loads, as well as the effects of bedding and sidefill treatments. The soil-structure interaction was computed using the finite element method with soil response simulated with the Duncan soil model and Selig soil parameters. The geometry of the soft material zone that induces the reverse arching action was optimized to maximize the earth load reduction for imperfect trench installations. The optimization process was based on parametric studies of the geometry and location of the soft material zone, combined with bedding and sidefill treatments. Predictor equations for earth load, maximum wall stress, and deflection of the conduits were formulated that incorporate the proposed optimum soft material zone geometry and installation techniques. Parametric studies revealed that the optimum geometry of the soft material zone in the proposed imperfect trench installations could reduce the maximum wall stress or vertical earth load by 69-85%.