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Assume under With an allowable long term stress of 800 psi and a each of the pipes is equally de ads required to achieve that result.The tangential time of approximately four days(100 hours)to reach ,arface 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'/21/6(See Chart The tangential strain varies directly as the watt 25 for time and load dependent modulus of thickness(i.e.:distance from the neutral axis)and is Therefore.elasticity to calculate:use of 800 psi inctress —orporates modulus.) proportional to the amount of ring deflection.For a of given ring deflectionan ,the thicker the watt,the higher • An alowable strain valargin of ue of 0.01 will allow for the strain. reasonable additional deflection due to disturbance Alternately,assume that each of the pipes is of the backfill by earthquake,fluctuations of the subjected to loads'such that the tangential surface water table,etc. strain in the pipe's wall surface is equal for both pipes. . An allowable design strain value of 0.01 allows for For equal surface strain,the degree of vertical the nominal deviation of temperature encountered deflection of the pipe ring is different for the two during installation. pipes.Under these circumstances,the degree of 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 il pressure,P`,at the the sidefill soil under the total so The percentage ring deflection based upon strain for top of the pipe will be no greater thanthe maximum a given SOR pipe can be calculated as follows: allowable ring deflection for a given SDR pipe' AY_ (025)(s)(D) Test Performance of Direct Burial Ddsoopipe: The D t density of the bedding and soil envelope determines AYthe performance capability of the pipe-soil system D = (0.25)(s)(SDR) with regard to ring deflection.Tests conducted on AY Driscopipe at Utah State University by Dr.Reynold K. - = (.0025)(SDR) Watkins show that Driscopipe will not buckle under ordinary conditions if the soil envelope is compacted Where: e =Tangential strain in the surface of the and is in full contact with the pipe.A virtual fail-safe pipe ring due to deflection installation can be-assured if soil density is generally (conservatively 0.01 for Driscopipe) over 85%of Standard Proctor(AASHTO T-99)Density. D =Pipe OD,inches With the backfill compacted to 90°6 of standard t =Pipe wall thickness,inches density,the depth of laying for Driscopipe is almost SOR =Standard Dimension Ratio,D/t unlimited.However,in the thinner wall series,due to AY =Vertical deflection,inches the flexibility of the pipe,it is difficult to achieve the Oriscopipe 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: Soil densities less than 900/6 are certainly adequate if • Most of the deflection of a flexible 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. K 39