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®_ Assume each of the pipes is equally deflected under ds required to achieve that result. The tangential rface strain developed in the thickwall pipe is much greater than the surface strain in the thinwall pipe. The tangential strain varies directly as the wall thickness (i.e.: distance from the neutral axis) and is proportional to the amount of ring deflection. For a given ring deflection, the thicker the wall, the higher the strain. Alternately, assume that each of the pipes is subjected to loads 'such that the tangential surface strain in the pipe's wall surface is equal for both pipes. For equal surface strain, the degree of vertical deflection of the pipe ring is different for the two pipes. Under these circumstances, the degree of deflection would be less for the thickwall pipe and greater for the thinwall pipe. The percentage ring deflection based upon strain for a given SDR pipe can be calculated as follows: AY = (0.25) (E)(D ) AY = (0.25) (E) (SDR) D AY = (.0025) (SDR) D Where: E = Tangential strain in the surface of the pipe ring due to deflection (conservatively 0.01 for Driscopipe) D = Pipe OD, inches t = Pipe wall thickness, inches SDR = Standard Dimension Ratio, D/t DY = Vertical deflection, inches Driscopipe recognizes a tangential surface strain value of 0.01 due to ring deflection as a conservative yet responsible design parameter. This value is based upon the following: • Most of the deflection of a flexible pipe occurs within a few hours or a few days after final backfilling and increases very little thereafter. This results in the development of the soil arch over the pipe which relieves the pipe of much of the vertical soil load by the arching action of the soil envelope and by the development of soil restraint at the sides of the pipe. 11 • With an allowable long term stress of 800 psi and a time of approximately four days (100 hours) to reach the maximum allowable ring deflection, the tangential strain would be 2% to 2112% (See Chart 25 for time and load dependent modulus of elasticity to calculate: strain = stress _ modulus.) Therefore, the use of 800 psi incorporates an additional margin of safety of over 2.0. • An allowable strain value of 0.01 will allow for reasonable additional deflection due to disturbance of the backfill by earthquake, fluctuations of the water table, etc. • An allowable design strain value of 0.01 allows for the normal deviation of temperature encountered during installation. In summary, a soil density can be specified for the bedding and initial backfill so that the vertical strain of the sidefill soil under the total soil pressure, P,, at the top of the pipe will be no greater than the maximum allowable ring deflection for a given SDR pipe' Test Performance of Direct Burial Driscopipe: The density of the bedding and soil envelope determines the performance capability of the pipe -soil system with regard to ring deflection. Tests conducted on Driscopipe at Utah State University by Dr. Reynold K. Watkins show that Driscopipe will not buckle under ordinary conditions if the soil envelope is compacted and is in full contact with the pipe. A virtual fail-safe installation can be assured if soil density is generally over 85% of Standard Proctor (AASHTO T-99) Density. With the backfill compacted to 900/o of standard density, the depth of laying for Driscopipe is almost unlimited. However, in the thinner wall series, due to the flexibility of the pipe, it is difficult to achieve the desired high soil density without buckling of the pipe cross-section, particularly when using backfill material other than sand or gravel. Soil densities less than 90% are certainly adequate if depth of soil cover is in the range of most installations. Compaction to 75% standard density is relatively easy to achieve even in poor soils. This would cover a large percentage of all installations. However, 85% Standard Proctor Density (AASHTO T-99) should be considered a conservative minimum. 39