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V- 7esign Considerations: Driscopipe performs Through mathematical computations presented later, ffectively when buried because of its ability to the ring deflection of Driscopipe can h calculated ipeand t ---deflect in the interacting pipe-soil system.The pipe based upon the known properties of the pipe and the actually gains strength from the surrounding soil and measured compressibility of the soil envelopes.As can support additional soil loads.The soil envelope the pipe deflects with the soil,it forms a very slight surrounding the pipe will compress or deflect slightly ellipse by decreasing in the vertical diameter an under the additional loads of the dry weight of backfill amount AY and by increasing in-the borizontal above the pipe,the weight of the water table diameter an almost equal(but slightly less)amount saturating the soil,the weight of nearby buildings or BX.The horizontal diametrical increase is beneficial static structures,the weight of off-the-road trucks and because it further compacts a he slid similll ar and thus way tractors,railroad freight trains or any combination of develops lateral support all these loads.In a flexible"pipe-soil"system.the soil abutments offer support to a masonry arch pipe deflection is the same as the soil'deflection bridge.The compacted soil actually becomes a under the load of the secondary backfill and any masonry arch even though the soil is a low grade surface loads. masonry using soil particles for bricks.The vertical decrease in diameter is beneficial because it relieves, When the pipeline is laid in the trench,the soil envelope is shoveled,washed,flushed or dumped the pipe of vertical sat pressure concentrations and thus forces the soil to support the major share of the into place,in layers,to an elevation above the top of vertical load by arching action over the pipe. the pipe.It is usually compacted to a specified Proctor density by mechanical or hydraulic means.As Compressibility of the soil envelope is measured by additional layers of soil are backfifled into the trench, the Soil Modulus,E',which•is the ratio of soil pressure the weight of the soil over the primary,compacted (stress)to soil deflection(strain)at a given soil backfill is increased.This increased weight slightly compaction density,as shown in Chart 26.The compresses(strains downwards)the soil envelope. degree of compaction of the soil envelope can be The amount of soil compression is related to the determined from a family of curves developed from strength of the soil at a given density and to the laboratory tests.Chart 26 is typical of such curves intensity of the vertical sod pressure.Since the soil is and provides a general guide if laboratory data are not an elastic material,this compression or strain of not available to plot a family of curves fora specific %e soil envelope by the final backfill is permanent.As soil type. ie sal envelope is compressed(strained),it Deformation of buried flexible pipe becomes critical becomes further compacted.In its denser state,the only when the pipe reaches that point of ring soil develops increased resistance to the vertical soil' deflection beyond which it can no longer resist any pressure until it reaches static equilibrium without increase in sal loading.By limiting ring deflection further compression or strain.This is the reaction of through proper soil compaction,the loading over a the compacted soil envelope. 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 Typical Performance Limits of Buried against long term collapse. Pipes Due to External Soil Pressure Trench Construction and Terminology Frush Grade p 0 .Vo& yip � Dense W wan Soli tushing Son Spring Suckling tine Flexible- .Stiff Pipe �_4 m �. �, Pipe Driscopipe 0 O 1-- Embedment W a t4 00 Materials E CL Loose '^'y' y Bedding ,�:• Defl'ection A.Y Soil f 4 - :;:;:�" + "- •In suitable softs the construction of a bedding and/or ;' y a foundation may not be required. Gravity Now Drlscopipe sewers may be accurately laid to grade using laser-beam scoping. 35