ARCHIVED REPORTS_2014_5 - Laserfiche WebLink

Home Browse Search

air Y 4. Based upon the pipe SDR and the value of the Design by Ring Deflection. Ring deflection is defined the as the ratio of the vertical change in diameter to the • polyethylene modulus of elasticity.E.calculate _-- pipe s hydrostatic,critical-collapse differential original diameter.It is often expressed-as a pressure.Pc: percentage.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 Driscopipe to accommodate,without structural external soft pressure.Pi,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.�gn by�g example problem on Chart 26. defection comprises a calddation of vertica/sof7strain 6. Calculate the critical buckling pressure at the top to ensure it will be fess than the affowable ling ddkc6on of the pipe by the formula: of the pipe.See Chart 27.The tabulation shows that P,b = 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 = PCB _ 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 Reflection 8. The above procedures could be reversed to derive the minimum pipe SDR required for a given 32.5 8.1% soil pressure and an estimated soil density. 26.0 6.5% However,this procedure should permit the 21.0 5.2% engineer to optimize the system design quickly by 19.0 4.7% examining several combinations. 17.0 4.2% 15.5 3.9% ^'4 13.5 3A% _ Chart 26 11.0 2.7%7% Plot of Vertical Stress-Strain Data for Typical Trench Backfill(Except Clay) The allowable ring deflection of tangential strain th a is from Actual Tests* a function of the allowable tangential strain in the outer surface of the pipe wall.A conservative limit of .oma► 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 3000 s comparing two pipes of the same diameter but 14' ` different wall thickness. Ilk • 2000► low► a d es vertical Soil Strain(Percent) EXAMPLE Find:E'@ 2000 PSF and 80%Density Formula:E=Plies Calculations:E'=2000PSF1.018=111111 PSF=771 pal D � Note:The curves shown on this chart are sample curves for a granular son. ye Ring DetleCtiOn= 1—=1 x i00°/" "other types of soil are used for backfill,such as day or clay loam.curves n9 Do should be developed from laboratory test data for the material used.Soil pressures greater than 4000 Pat may be examined by.extrapofating the NOTE:5%denection decreases flow area by VM 10%deflection slope of the curve or by generating curves by testing at those higher son decreases now-area by 116. pressures.Probable error of curves Is about half the distance between adjacent fines. Y • � 38