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Jim 4. Based upon the pipe SDR and the value of the Design by Ring Deflection: Ring deflection defined polyethylene modulus of elasticity.E.calculate the as the ratio of the vertical change in diameter to the inal diameter. essed as a pipe's hydrostatic,critical-collapse differential pe ce tage.Ring it forrburied Driscopipe is pressure. Pc: conservatively the.same as(no more than)the vertical .32(E) Pc __ 2compression of the soil envelope around the pipe. (SDRj3 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.Pl,against a specified soil distress,the vertical compression of t� soil. nl'n density to derive the soil strain as shown in the enveloping the buried pipeline.Design Y g example problem on Chart 26. deflection cornpdfses a cafcutabon of vertical sort strain 6. Calculate the critical buckling pressure at the top to ensure it will be less than lire alfowaWe ring deflection of the pipe by the formula: of the pipe.See Chart 27.The-tabulation shows that Peb _ 0 8 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 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. 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 a 4.7% engineer to optimize the system design quickly by 17 0 4.2% examining several combinations. 15.5 3.9% 13.5 3.4% Chart 26 11.0 2.7% Plot of Vertical ss-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,/2%tangential strain in the outer surface of the 4000► pipe watt due to vertical deflection of the pipe"ring" by soil compression can be understood by 3000®� comparing two pipes of the same diameter but LP different wall thickness. a .p a 2000► f__D IAI r9i a toxo► a d o®' ® ® ♦ .�_ Dwrov ® ® 2 3 4 5 a Do cs vertical soil Strain(percent) EXAMPLE Find:E'@ 2000 PSF and BO*/.Density Formula:E'-Pit Cs Calculations:E'=200pPSFr.018=tttttt PSF=TT1 psi DMW Note:The curves shown on this chart are sample Curves for a granular soil. %Rin Deflection= 1 x 100% N other types of soil are used for backfill,such as day or clay barn.curves 9 1 Do shoutd'be developed from laboratory test data for the material used.Soil pressures greater than 4000 pat may be examined by.extrapolaung the NOTE:5%deflection decreases flow area by V.%.10%deflection ®' • slope of the curve or by generating curves by testing at those higher soil decreases now-area by I'I- pressure I- f pressures.Probable error of curves is about half the distanCO between adjacent lines. 38