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Assume each of the pipes is equally deflected under With an allowable long term stress of 800 psi and a ads required to achieve that result.The tangential time of approximately four days(100 hours)to reach _,irface strain developed in the thickwall pipe is much the maximum allowable ring deflection,the greater than the surface strain in the thinwall pipe. tangential strain would be 2%to 2'/2%(See Chart The tangential strain varies directly as the wall 25 for time and load dependent modulus of thickness(i.e.:distance from the neutral axis)and is elasticity to calculate:strain = stress _ modulus.) proportional to the amount of ring deflection.For a Therefore,the use of 800 psi incorporates an given ring deflection,the thicker the wall,the higher additional margin of safety of over 2.0. the strain. • An allowable strain value of 0.01 will allow for Alternately,assume that each of the pipes is reasonable additional deflection due to disturbance subjected to loads'such that the tangential surface of the backfill by earthquake,fluctuations of the strain in the pipe's wall surface is equal for both pipes. water table,etc. For equal surface strain,the degree of vertical An allowable design strain value of 0.01 allows for deflection of the pipe ring is different for the two the normal deviation of temperature encountered. pipes.Under these circumstances,the degree of during installation. deflection would be less for the thickwall pipe and In summary,a soil density can be specified for the greater for the thinwall pipe. bedding and initial backfill so that the vertical strain of The percentage ring deflection based upon strain for the sidefill soil under the total soil pressure.P,,at the a given SDR pipe can be calculated as follows: top of the pipe will be no greater than the maximum AY Dallowable ring deflection for a given SDR pipe' = (0.25)(s)F.) Test Performance of Direct Burial Driscopipe: The D density of the bedding and soil envelope determines AY D = (025)(e)(SDR) the performance capability of the pipe-soil system with regard to ring deflection.Tests conducted on AY Driscopipe at Utah State University by Dr.Reynold K. D = (.0025)(SDR) Watkins show that Driscopipe will not buckle under Where: s =Tangential strain in the surface of the ordinary conditions if the soil envelope is compacted pipe ring due to deflection and is in full contact with the pipe.A virtual fail-safe (conservatively 0.01 for Driscopipe) installation can be assured if soil density is generally D =Pipe OD,inches over 85%of Standard Proctor(AASHTO T-99)Density. =Pipe wall thickness,inches With the backfill compacted to 90%of standard t t =Standard Dimension Ratio,DA unlimited. the depth of laying for Driscopipe is almost SDR =Vertical deflection,inches unlimited.However,in the thinner wall series,due to the flexibility of the pipe,it is difficult to achieve the Driscopipe recognizes a tangential surface strain desired high soil density without buckling of the pipe value of 0.01 due to ring deflection as a conservative cross-section,particularly when using backfill yet responsible design parameter.This value is based material other than sand or gravel. upon the following: Most of the deflection of a flexible � Soil densities less than 90%are certainly adequate if pipe occurs depth of soil cover is in the range of most installations. within a few hours or a few days after final Compaction to 75%standard density is relatively backfilling and increases very little thereafter.This easy to achieve even in poor soils.This would cover results in the development of the soil arch over the a large percentage of all installations.However,85% pipe which relieves the pipe of much of the vertical Standard Proctor Density(AASHTO T-99)should be soil load by the arching action of the soil envelope considered a conservative minimum. and by the development of soil restraint at the sides of the pipe. 39