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<br />.. <br /> <br />, <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />2.1 Trench IastallatioDS <br /> <br />Trench installations are made in relatively narrow excavations and <br />the pipeline covered with earth backfill which extends to the <br />original ground surface, The trench load theory is based upon <br />certain applied mechanics assumptions concerning the properties of <br />the materials involved, These assumptions are: <br /> <br />o Earth loads on the pipe develop as the backfill settles, <br /> <br />o The resulting earth load on the pipe is equal to the weight <br />of the material above the top of the pipe minus the shearing <br />(,frictional) forces on the sides of the trench. <br /> <br />o Cohesion is negligible because with cohesive soils, <br />considerable time must elapse before effective cohesion <br />between the backfill material and the sides of the trench can <br />develop, and with cohesionless soils, would never develop. <br />The assumption of no cohesion yields the maximum probable <br />load on the pipe. <br /> <br />o For a rigid pipe, the sidefills may be relatively <br />compressible and the pipe will carry a large portion of the <br />load developed over the entire width of the trench. <br /> <br />o Active lateral pressure against the pipe is usually <br />neglected, but it should be taken into account if trench <br />widths exceeds the defined narrow trench widths, <br /> <br />The type of bedding is one of the factors that determines the <br />supporting strength of buried pipe, Classes of bedding for the <br />trench condition and their corresponding fixed bedding factors are <br />as follows: <br /> <br />o Class a Bedding can be either a sbaped subgrade with granular <br />foundation or a granular foundation, A granular foundation <br />witbout a sbaped subgrade is used only with circular pipe. <br /> <br />o Class C Bedding can be either a abaped subgrade or a granular <br />foundation. A granular foundation witbout a shaped subgrade <br />is used only witb circular pipe, <br /> <br />o Class D Bedding is a flat aubgrade, and used only with <br />circular pipe. <br /> <br />o Otber Bedding can be selected wbich allowa for conditions <br />otber than those defined above. When this class is selected <br />the designer must specify the bedding factor to be used. <br /> <br />Both spangler and Schlick, in early Iowa Engineering Experiment <br />Stations publications, postulate thst some active lateral pressure <br />is developed in trench installations before the transition width is <br />reached, Experience indicates that tbe active lateral pressure <br />increases as the trench width increases from a very narrow widtb to <br /> <br />2 <br />