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u. I I ISOM 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 nal diameter its oftertical n expreangessed as diameter to the pipe's hydrostatic,critical-collapse differential original pressure.Pc: percentage.Ring deflection for buried Driscopipe is 2.32(E) conservatively the-same as(no more than)the vertical __ Pc (SDR)3 compression of the soil envelope around the pipe. Design by ring deflection matches the ability of 5. Calculate the soil modulus.E',by plotting the total Driscopipe to accommodate,without structural external soil pressure.P,,against a specified soil distress,the vertical compression of the oan density to derive the soil strain as shown in the enveloping the buried pipeline.Designy g example problem on Chart 26. deflection comprises a calculation of vertical so#strain 6. Calculate the critical buckling pressure at the top to ensure it will be less than the allowable ring dellect►on of the pipe by the formula: of the pipe.See Chart 27.The tabulation shows that Pio = 0.8 E'x Pc 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. = PC8 _ P, resist the external sol 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. SDR Ring Deflection 8. The above procedures could be reversed to 8.1% derive the minimum pipe SDR required for a given 32.5 6.5% soil pressure and an estimated soil density. 26:0 5.2% However,this procedure should permit the 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) a function of the allowable tangential strain in the from Actual Tests* outer surface of the pipe wall.A conservative limit of 1-1 Y2%tangential strain in the outer surface of the pipe wall due to vertical deflection of the pipe"ring" by soil compression can be understood by moo 10,- comparing two pipes of the same diameter but e different wall thickness. 2=► Dim a� 4 0 m toxo► 11111;lilll!lllIJ 'j,■ 0 °r`A �► ® ® ® s s D® Dum es vertical Sou strain(percent) EXAMPLE Find:E'Ca 2000 PSF and 80%Density Formula:V=P,1 es calculations:E'=2000 PSF1.018=111111 PSF=771 pill D Note:she curves shown on this chart are ssmPre Curves for s sou. %R' Deflection= 1— —� x 100% N other types of sou are used for backfin.such as day or Clay loam.aures �9 Do should be developed horn Laboratory test data for the material used.Sou derawdort decnases flow was by y.%10%deflection ®! a"*Ipressures erecter than gen000 mating ybe currves by testined ng at the rel ttmil the de��w^area by l%' !°^ slew r s. curve le error or Of pressures,probable error of curves is about LhaM the distance between . adjacent fines. 38