Test For Inheret Strength Of Pipes

Rigid circular or elliptical pipes, usually precast such materials as plain or reinforced concrete, burned clay, asbestos cement, and cast iron, are not readily analyzed by principles of mechanics; and, since they are usually relatively small structures, their inheret supporting strength can be most easily determined by testing a representative group of specimens in the laboratory. Several methods of testing sections of pipes have been devised. Four methods of supporting and loading pipes are : the two-edge bearing, the three-edge bearing, the sand bearing, and the Minnesota bearing.
Of these tests, that with the three-edge bearing is the simplest and ost easily performed; and it also gives accurate and uniform results. For these reasons it is widely employed in pipe-strength determinations, altough some engineers prefer the sand bearing test because of the wider distribution of both the applied load and the reaction. See images below of three-edge bearing test.

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Load On Ditch Conduit

In the case of a ditch conduit, the backfilling material has a tendency to consolidate and settle downward. This action plus the settlement of the conduit into its soil foundation, causes the prism of soil within the ditch and above the pipe to move downward relative to the undisturbed soil at the sides. This relative movement mobilizes along the sides of the ditch certain shearing stresses or friction forces which act upward in direction and which, in association with horizontal forces, create an arch action that partially supports the soil backfill. The difference between the weight of the backfill and these upward shearing stresses is the load which must be supported by the conduit at the bottom of the ditch. If it assumed that cohesion between the backfill material and the sides of the ditch is negligible, then the magnitude of the vertical shearing stresses is equal to the active lateral pressure exerted by the soil backfill against the sides of the ditch multiplied by the tangent of the angle of friction between the two materials.
This assumption of negligible cohesion is justified on two accounts. Even when a ditch is backfilled with cohesive soil, considerable time must elapse before effective cohesion soil, considerable time must elapse before effective cohesion between the backfill and the sides of the ditch can develop after backfilling. Also, the assumption of no cohesion yields the maximum probable load on the conduit. This maximum load may develop at any time during the life of the conduit as a result of heavy rainfall or some other action which may eliminate or greatly reduce cohesion between the backfill and the sides of the ditch.

Underground Conduits

CLASSES OF UNDERGROUND CONDUITS. Underground conduits are divided into 2 major classes, known as ditch conduit and projecting conduit, the classification based on the construction or environmental conditions which influence the load. Projecting conduits are further subdivided into positive projecting conduits and negative projecting conduit.

DEFINITIONS. A ditch conduit is define as one which is installed in a relatively narrow ditch dug in passive or undisturbed soil and which is then covered with earth backfill. Ex : sewers, drains, water mains, and gas mains. A positive projecting conduit is one which is installed in shallow bedding with its top projecting above the surface of the natural ground and which is then covered with an embankment. Ex : Railway and highway culverts. A negative projecting conduit is one which is installed in a relatively narrow and shallow ditch with its top at an elevation below the natural ground surface and which is then covered with an embankment. This is a very favorable method of installing a railway or highway culvert, since the load produced by a given height of fill is generally less then it would be in the case of a positive projecting conduit. This method of construction is most effective in minimalizing the load if the ditch between the top of the conduit and the natural ground surface is backfilled with loose uncompacted soil.