ARCHIVED REPORTS_2006_4 - Laserfiche WebLink

Home Browse Search

DR/SCOWPE ] 4. Based upon the pipe SDR and the value of the polyethylene modulus of elasticity, E, calculate the pipes hydrostatic, critical -collapse differential pressure. Pc: _ 2.32 (E) Pc (SDR)3 5. Calculate the soil modulus, E', by plotting the total external soil pressure. Pt, against a specified soil density to derive the soil strain as shown in the example problem on Chart 26. 6. Calculate the critical buckling pressure at the top of the pipe by the formula: P,b = 0.8E x P 7. Calculate the Safety Factor: S.F. = PC8 _ P, In burial applications, a safety factor of 1.0 may be considered a minimum because of the margin of safety provided by the arching action of the soil. However, Driscopipe endorses using a more conservative value approaching or exceeding a 2.0 safety factor. 8. The above procedures could be reversed to derive the minimum pipe SDR required for a given soil pressure and an estimated soil density. However, this procedure should permit the engineer to optimize the system design quickly by examining several combinations. Chart 2� •; of Data fe Typical(Except •: from Actual Tests* 4000 PP.. e F2W) 4 v m 1000 a a es vertical Soil strain (percent) EXAMPLE Find: E'@ 2000 PSF and 80% Density Formula: E' = Piles Calculations: E'= 2000 PSF /.018 =111111 PSF = 771 pal Note: The curves shown on this chart are sample curves for a granular soil. If other types of soil are used for backfill, such as clay or clay loam, curves should be developed from laboratory test data for the material used. Soil pressures greater than 4000 psi may be examined by.extrapolating the slope of the curve or by generating curves by testing at those higher soil pressures. Probable error of curves is about halt the distance between adjacent lines. ICU Design by Ring Deflection: Ring deflection is defined as the ratio of the vertical change in diameter to the original diameter. It is often expressed as a percdntage. Ring deflection for buried Driscopipe is conservatively the.same as (no more than) the vertical compression of the soil envelope around the pipe. Design by ring deflection matches the ability of Driscopipe to accommodate, without structural distress, the vertical compression of the soil. enveloping the buried pipeline. Design by ring deflection comprises a calculation of vertical soil strain to ensure it will be less than the allowable ring deflection of the pipe. See Chart 27. The tabulation shows that with lower values of SDR, the allowable deflection is less. For installations which require this thicker wall to resist the external soil pressure, actual ring deflection can easily be limited to the tabular values by proper compaction of the backfill around the pipe. The recommended allowable deflection for the various SDR s are: Chart 27 Allowable SDR Ring Deflection 32.5 8.1% 26.0 6.5% 21.0 5.2% 19.0 4.7% 17.0 4.2% 15.5 3.9% 13.5 3.4% 11.0 2.7% The allowable ring deflection of polyethylene pipe is a function of the allowable tangential strain in the outer surface of the pipe wall. A conservative limit of 1-11/2% 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 diameter but different wall thickness. DMIN % Ring Deflection = { 1 - D o x 100% NOTE: 5%deflection decreases flow -area by y.%.10% deflection decreases flow -area by 1 %.