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AND 00 / D Ring Deflection: Ring deflection is defined 4. Based upon the pipe SDR and the value of the 9n�' in elasticity, E. calculate the asthehe rginal diameter.it rt often expressed s ser to the polyethylene modulus of elas pipes hydrosveical r. tatic.critical-cotlaps percentage.Ring deflection for buried Driscopipe is pressure.Pc: conservatively the-same as(no more than)the vertical __ 2.32(E) compression-of the soil envelope around the pipe. Pc (SDR 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.Pt,against a specified soil distress,the vertical compresses of the soil density to derive the soil strain as shown in the enveloping the buried pipeline.Design by ting example problem on Chart 26. deflection ca9l bea aak Atfon of vertical sort strain es deltection 6. Calculate the critical buckling pressure at the top to ensure ithart 27 . tabulation aft m a shows that of the pipe by the formula: of the laps 0.8 E'x P with tower values of SDR.the allowable deflection is P eb = e less.For installations which require this thicker wall to P resist the external soil pressure.actual ring deflection 7. Calculate the Safety Factor.S.F. = Pical 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 thfpipe.various safety provided by the arching action of the soil. recommended allowable dem However.Driscopipe endorses or excemore a SDR s are: conservative value app 9 Chart27 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 17.0 4.2% examining several combinations. 15.5 3. 13.5 3.44%% 2.7% Chart 26 11.0 plot of Vertical Stti ess Straub Data for The allowable ring deflection of po�vethylene 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 V2%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 comparing two pipes of the same d 300 ® iameter but different wall thickness. .rr F 2W)® D„X Iwo I. e: +0►Q ♦ 2 ® 0 ® g Do 8g vertical soft strain(percent) EXAMPLE Find:E'@ 2000 PSF and 80%Density Formula:E'=PI/8s Calculations:E'=2000 PSF 1.018=111111 PSF=771 pal DMS, a _ X 1A0% Note:The curves shown on this chart arc sample curves for a granular soft. %Ring petlectiOn o if other types or soil are used for backlitl,such as day or slay loam,turves b Ys%tOK detlecdon should be developed from laboratory test data for the material used.Solt on decreases flow-area Y pressures greater than 4000 Psf may be examined by.at trapolatln9 the NOTE:S%deflects ®# Slope of the Curve or by generating curves by testing et those higher soil decreases flow-arca by t% pressures.Probable error of curves fs about hall the distance between adjacent lines. 38