A soil mass shifts load from its weaker components to its stronger components such that the load is distributed in proportion to the stiffness of the components. This characteristic often results in the shifting of load around weak zones or cavities in the soil. Such redistribution of stresses in a soil mass is called arching. Arching is usually thought of in regard to pipe and tunnels. However, arching is a consequence of a more general property of soil known as internal shearing resistance. Slope stability, the bearing capacity of shallow footings, and the pressure distribution on retaining walls depend on the internal shear resistance. This paper discusses internal shearing resistance, the movement required to mobilize it, its measurement, typical values for common trench soils, and how it relates to the load applied to a buried pipe.
Plastic pipes are widely used in trenchless applications for both rehabilitation and new construction. What makes a pipe suitable for trenchless installation? What material characteristics and pipe properties should a specifying engineer require from pipe in trenchless applications? Engineers often specify for the same project a number of piping products having significantly different properties. This paper assists the engineer in determining which properties and features are actually important and relevant to a trenchless application and thus which need to be addressed during design. Key properties and features include corrosion resistance, joint reliability, rapid crack propagation resistance, flexibility, static and surge pressure capacity, fatigue resistance, impact strength, pull-in strength, stiffness, safety and ease of tapping and transition to other piping materials, ease of repair, abrasion resistance, cold weather performance, and usage history. The relative importance of these factors to a trenchless application with respect to design, installation, and operation is discussed with an emphasis on the development and preparation of trenchless engineering specifications. In addition, this paper addresses the role that ASTM standards play in defining and quantifying many of these properties. Without standardization it would be hardly possible for a designer to properly select a piping product.
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