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®mQesign Considerations: Driscopipe performs ectively when buried because of its ability to eflect in the interacting pipe -soil system. The pipe actually gains strength from the surrounding soil and can support additional soil toads. The soil envelope surrounding the pipe will compress or deflect slightly under the additional loads of the dry weight of backfill above the pipe, the weight of the water table saturating the soil, the weight of nearby buildings or static structures, the weight of off -the -road trucks and tractors, railroad freight trains or any combination of all these loads. In a flexible "pipe -soil" system, the pipe deflection is the same as the soil deflection under the load of the secondary backfill and any surface loads. When the pipeline is laid in the trench, the soil envelope is shoveled, washed, flushed or dumped into place, in layers, to an elevation above the top of the pipe. It is usually compacted to a specified Proctor density by mechanical or hydraulic means. As additional layers of soil are backfilled into the trench, the weight of the soil over the primary, compacted backfill is increased. This increased weight slightly compresses (strains downwards) the soil envelope. The amount of soil compression is related to the strength of the soil at a given density and to the intensity of the vertical soil pressure. Since the soil is not an elastic material, this compression or strain of Afte soil envelope by the final backfill is permanent. As e soil envelope is compressed (strained), it becomes further compacted. In its denser state, the soil develops increased resistance to the vertical soil pressure until it reaches static equilibrium without further compression or strain. This is the reaction of the compacted soil envelope. r ` - • • - ri —,000 a iso k7jW7all a3M DenseWall Soli Budding hing Flexible Stiff Pipe Driscopipe IV'van . �.._ . R; Loose u a n9 Soil Deflection U Through mathematical computations presented later, the ring deflection of Driscopipe can be calculated based upon the known properties of the pipe and the measured compressibility of the soil envelopes. As the pipe deflects with the soil, it forms a very slight ellipse by decreasing in the vertical diameter an amount AY and by increasing in -the Horizontal diameter an almost equal (but slightly less) amount AX. The horizontal diametrical increase is beneficial because it further compacts the sidefill soil and thus develops lateral support in a fashion similar to the way soil abutments offer support to a masonry arch bridge. The compacted soil actually becomes a masonry arch even though the soil is a low grade masonry using soil particles for bricks. The vertical decrease in diameter is beneficial because it relieves the pipe of vertical soil pressure concentrations and thus forces the soil to support the major share of the vertical load by arching action over the pipe. Compressibility of the soil envelope is measured by the Soil Modulus, E', which -is the ratio of soil pressure (stress) to soil deflection (strain) at a given soil compaction density, as shown in Chart 26. The degree of compaction of the soil envelope can be determined from a family of curves developed from laboratory tests. Chart 26 is typical of such curves and provides a general guide if laboratory data are not available to plot a family of curves fora specific soil type. Deformation of buried flexible pipe becomes critical only when the pipe reaches that point of ring deflection beyond which it can no longer resist any increase in soil loading. By limiting ring deflection through proper soil compaction, the loading over a pipe is distributed through the soil and across the soil arch around the pipe. Thus the pipeline maintains its circular cross-section in stable soils and is protected against long term collapse. Trench Construction and Terminology Finish Grade ZT Haunch�.� Pipe • . .REM in suitable soils the construction of a bedding and/or a foundation may not be required- -Gravity now Driscopipe sewers may be accurately laid to grade using laser -beam scoping. 35