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4. Based upon the pipe SDR and the value of the Design by Ring Deflection: Ring deflection is defined polyethylene modulus of elasticity.E.calculate the as the ratio of the vertical change in diameter to the pipes hydrostatic,critical-collapse differential original diameter.It is often expressed as a pressure,PC: percdntage.Ring deflection for buried Driscopipe is 2.32(E) conservatively the-same as(no more than)the vertical PC = 3 compression of the soil envelope around the pipe. (SDR) Design by ring deflection matches the ability of 5. Calculate the soil modulus.E',by plotting the total priscopipe to accommodate,without structural external soil pressure,P,,against a specified soil distress,the vertical compression of the soil• density to derive the soil strain as shown in the enveloping the buried pipeline.Design by ring example problem on Chart 26. deflection comprises a cafddation of vertical soil strain 6. Calculate the critical buckling pressure at the top to ensure it will be less than the allowable ring deflection of the pipe by the formula: of the pipe.See Chart 27.The tabulation shows that Ppb = 0.8 E'x P� with lower values of SDR,the allowable deflection is less.For installations which require this thicker wall to 7. Calculate the Safety Factor:S.F. = PCs _ P, resist the external soil pressure,actual ring deflection in burial applications,a safety factor of 1.0 may be can easily be limited to the tabular values by proper considered a minimum because of the margin of compaction of the backfill around the pipe.The safety provided by the arching action of the soil. recommended allowable deflection for the various However.Driscopipe endorses using a more SDR s are: conservative value approaching or exceeding a chart 27 Allowable 2.0 safety factor. Ring Deflection 8. The above procedures could be reversed to SDR derive the minimum pipe SDR required for a given 32.5 6. soil pressure and an estimated soil density. 26.0 .55%% 5 However,this procedure should permit the 21.0 5.2% 19.0 4.7% engineer to optimize the system design quickly by 1 0 4.2% examining several combinations. 15.5 3.9% 13.5 3.4% Chart 26 11.0 2.7% plot of Vertical Stress-Strain Data for The allowable ring deflection of polyethylene pipe is Typical Trench Backfill(Except Clay) from Actual Tests* a function of the allowable tangential strain in the outer surface of the pipe wall.A conservative limit of 1-1 th%tangential strain in the outer surface of the 4000 10- pipe wall due to vertical deflection of the pipe"ring" by soil compression can be understood by 3000 10- comparing two pipes of the same diameter but w different wall thickness. I4 a �V 2000 It► : E � DIAI 0 to a u m 1000 P. of If a oil Dmim 0 1 2 3 4 5 6 Do es Vertical soil strain(percent) EXAMPLE Find:E'@ 2000 PSF and 80%Density Formula:E'-Poles Calculations:E'=2000 PSF 1.016=111111 PSF=771 psi Due, Mote:The curves shown on this chart are sample curves for a granular soil. %R' Deflection= 1——I x 100% If other types of soil are used for backMi.such as day or clay loam,curves m9 ` Do should be developed from laboratory test data for the material used.Soil pressures greater than 400.0 psi may be examined by-extrapolating the NOTE-5%deflection decreases flaw area by Y.%.10%deflection `t slope of the curve or by generating curves by testing at those higher$ail decreases flow-area by 170 F^ pressures.Probable error of curves is about half the distance between adjacent lines. 38