1、 Pipe Stiffness and Flattening Tests in Coilable HDPE Conduit; and Its Relationship to Burial Depth in Conduit Applications TR-47/2011 Prepared by: George Zagorski, PE for The Plastics Pipe Institute 2 FOREWORD Pipe Stiffness and Flattening Tests in Coilable HDPE Conduit; and Its Relationship to Bur
2、ial Depth in Conduit Applications This report was developed and published with the technical help of the members of the PPI (Plastics Pipe Institute, Inc.). The members have shown their interest in quality products by assisting independent standards-making and user organizations in the development o
3、f standards, and also by developing reports on an industry-wide basis to help engineers, code officials, specifying groups, and users. The purpose of this technical report is to provide important information available to PPI on design factors and design coefficients recommended for thermoplastic pre
4、ssure piping applications. These recommendations are based on discussions with several internationally recognized technical experts in the plastic pipe industry. More detailed information on its purpose and use is provided in the document itself. This report has been prepared by PPI as a service of
5、the industry. The information in this report is offered in good faith and believed to be accurate at the time of its preparation, but is offered without any warranty, expressed or implied, including WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Consult the manufacturer for more
6、 detailed information about the particular joining procedures to be used with its piping products. Any reference to or testing of a particular proprietary product should not be construed as an endorsement by PPI, which does not endorse the proprietary products or processes of any manufacturer. The i
7、nformation in this report is offered for consideration by industry members in fulfilling their own compliance responsibilities. PPI assumes no responsibility for compliance with applicable laws and regulations. PPI intends to revise this report from time to time, in response to comments and suggesti
8、ons from users of the report. Please send suggestions of improvements to the address below. Information on other publications can be obtained by contacting PPI directly or visiting the web site. The Plastics Pipe Institute, Inc. http:/www.plasticpipe.org This Technical Report, TR-47, was first issue
9、d in May 2011. 3 Pipe Stiffness and Flattening Tests in Coilable HDPE Conduit; And Its Relationship to Burial Depth in Conduit Applications Introduction Continuous length, coilable HDPE conduit has enjoyed tremendous growth not only as a protection for electrical cable, but in large part due to the
10、huge expansion of the telecommunications industry in the US and abroad to protect fiber optic cable. Its availability in long lengths combined with HDPEs ductility, strength and durability make it ideally suitable for the “long haul” and trenchless installation technologies such as HDD (horizontal d
11、irectional drilling) installations in the power and telecom industries. Buried cables installed in conduit provide long term and improved protection from damage caused by storms and vandalism when compared to aerial cables. Conduit also extends the life and reliability of underground cables by provi
12、ding added protection from ground movement or poor soil conditions. These permanent raceways allow for easier and less costly future cable replacements. HDPE conduit has been used for decades with great success due to its low coefficient of friction, resistance to corrosion, excellent chemical resis
13、tance and ability to remain ductile (flexible) even at low temperatures. Because the properties of HDPE result in a wide range of flexibility, HDPE conduit can be provided on coils or on steel reels in long lengths specifically for trenchless or plowing technologies (i.e. a reel of 1 ” diameter HDPE
14、 conduit could hold as much as an 8,500 ft continuous length). Larger diameter HDPE conduit such as 8” or greater is available in sticks up to 50-ft. An alternative plastic used in the conduit industry is PVC where typical lengths are limited to 20-ft. When HDPE conduit is considered for underground
15、 installations, there have been cases where PS (pipe stiffness) and the term “Crush Strength” are used to determine strength relative to other materials regardless of their property differences. (In reality, “Crush Strength” is a term misapplied to thermoplastic, 4 flexible pipes (i.e. HDPE and PVC)
16、. Crush Strength implies a brittle failure, where flexible pipes can deflect in excess of 20% with no signs of wall buckling, cracking or splitting. Flattening is the correct and more appropriate engineering term to measure how much deflection a flexible pipe can take without damage.) When HDPE is c
17、ompared to PVC at similar diameters and wall thicknesses, PVC will inherently have a higher PS attributed to the modulus being more than twice that of standard HDPE conduit materials1. However, this in no way indicates how HDPE will ultimately perform compared to PVC conduit. PS has applicability in
18、 calculations of burial depth but it should not be used to compare dissimilar materials. Also, too often, engineers consider “Crush Strength” or Flattening to determine allowable burial depth. Flattening does not relate to burial depth and should be considered a quality control test. Finally, the na
19、tional specifications calculate PS inconsistently. This technical note provides the reader with detailed information showing that HDPE conduits PS is more than adequate at typical burial depths. Furthermore, because of the inconsistencies in the national specifications, PS and Flattening should be c
20、onsidered only as quality control tests. Specifications The inconsistencies of existing national specifications referencing stiffness and flattening have created confusion even for comparable materials. PS in ASTM D-2412 titled, “Determination of External Loading Characteristics of Plastic Pipe by P
21、arallel-Plate Loading”, is defined as the force per unit length of the test specimen, loaded at a prescribed loading rate, at a prescribed percentage deflection (0.5” per minute, typically 5% deflection). The test measures the conduits resistance to ring deflection as it is being compressed between
22、two steel plates. This parallel plate test empirically determines PS where as an alternative “calculated” method uses the materials modulus and SDR (dimension ratio). This shows the usefulness of PS as a Quality Control Test. Requirements referenced in several applicable national specifications are
23、as 5 follows where PS and Flattening are both used as quality assurance tests. Note: “Crush Strength” does not have a defined formula in the ASTM standards. 1. HDPE conduit for power and telecom applications is manufactured in accordance with standard ASTM F-2160, a standard written specifically for
24、 solid wall conduit based on controlled outside diameter. ASTM F-2160 specifies the material, dimensional, manufacturing, and quality assurance testing requirements for HDPE Conduit. 2. ASTM F-512 is the Standard Specification for PVC Conduit for Underground Installation. It uses PS as a quality con
25、trol test at 5% deflection with a modulus of 500,000 psi (the minimum allowed by the material specifications). 3. ASTM D-1785 is the Standard Specification for PVC pipe in Schedule 40, 80 and 120. This is a pressure pipe specification. The test method only includes a flattening test with no load req
26、uirement. The requirement is a pass/fail on visual inspection. 4. NEMA (National Electrical Manufactures Association) are specifications for conduit used in electrical applications. The following are pipe stiffness requirements for HDPE and PVC: NEMA TC 7 for HDPE Electrical Conduit (Various Types)
27、Minimum load at 5% deflection Compression and recovery test to 50% with 85% diameter recovery NEMA TC 6 however, a general discussion will assist the reader in determining that PS and Flattening should not be the overriding variables in choosing which material is suitable for protection of fiber and
28、 power cable in conduit applications. Simplified, the four calculations for buried flexible pipe are as followswhich are applicable for HDPE conduit as well2: 1. Ring Deflection: The deflection of the pipe due to earth and live loads is calculated to determine if the deflection exceeds the recommend
29、ed values. The formula to calculate ring deflection, the Spangler Modified Iowa Formula does use a form of pipe stiffness 7 PS = EI/0.149r3Eq (1) where E = material modulus I = moment of inertia r = pipe radius 2. Compressive Ring Thrust: The radial compressive force around the circumference of the
30、pipe. Failure occurs when the compressive stress in the wall exceeds the yield stress of the material. PS does not have any direct application in compressive ring thrust. 3. Buckling: Excessive compressive stress along the pipe wall may cause the pipe wall to buckle. Although the buckling calculatio
31、n does involve the modulus and moment of inertia, it is not directly related to the pipe stiffness calculation. Buckling will rarely be a determining factor in burial depth for conduit applications, as HDPE conduit is heavier than SDR 21 and burial depths are much less than 25-ft(note 3). 4. Flexibi
32、lity Factor (FF): FF is the stiffness of the pipe required to adequately and properly backfill the structure. It is important in large diameter (above 12”), light walls and profile pipe in deep fill applications. It is not a controlling factor in conduit applications as it uses heavier walls compare
33、d to drainage pipe and it is buried at depths much less than 25-ft. Burial Depths for HDPE Conduit Although pipe stiffness plays an important factor in one of the design requirements of HDPE conduit, other variables often override it as the critical factor. For example, in the majority of telecom an
34、d power installations the burial depth is relatively shallow and the “installation stresses” frequently dictate the strength requirements rather than in-situ earth loading. HDPE conduit is generally installed using three distinct methods: open cut or continuous trench, plowing, or HDD (horizontal di
35、rectional drilling). 8 Open Cut/Continuous Trench: Open cut trenching is a commonly used method in the installation of telecom and power conduits. Since the physical application of the installation method does not change, we can apply the same principles in determining the anticipated earth loading
36、strength requirements of HDPE conduit when they are being installed using open cut, as for other common flexible pipe applications. The Plastic Pipe Institute (PPI) handbook for “PE Pipe-Design and Installation”, describes a design window3, where constrained buckling becomes the determining factor i
37、n choosing the pipe strength and depth of cover allowed. PE pipe is limited to SDR 21 or heavier and no calculations are required because critical buckling for this wall thickness passes with a safety factor of 2. Coiled HDPE conduit for power and telecom applications is rarely manufactured lighter
38、than SDR 15.5 in small diameters, and SDR 13.5 in larger diameters (4-6”) because of the potential for excessive ovality and kinking as it is coiled or wound on the reel during manufacturing. In this case, the manufacturing process dictates a heavier wall in order to properly wind the conduit. Nearl
39、y all conduit installations, even open cut, are significantly less than 20 depth which is well within the parameters of the allowable design window established in PPIs, handbook. Other design window parameters that can be found in the handbook are: HDPE Conduit like pressure rated HDPE pipe is manuf
40、actured from HDPE resin in accordance with ASTM D-3350. The modulus and tensile properties of the specified cell classification for conduit resin from ASTM F-2160 is designated as 4 or 5. Because these cell values are the same as pressure rated resin the resulting mechanical properties for the finis
41、hed conduit will be the same as those of pressure pipe. Note: Schedule 40 HDPE conduit through 3” has a wall thickness greater than SDR 15.5. Schedule 40 HDPE conduits in diameters above 3” and Schedule 80 in 6” have wall thickness ranges that are less than SDR 15.5, for this reason these types are
42、not recommended for coiling due to ovality and kinking concerns. 9 No surcharge loading Acceptable embedment materials are typically coarse-grained, compacted to a minimum of 85% proctor yielding an E of 1000 psi assuming prism loading. The native soil also must be stable. (These parameters are impo
43、rtant! It is assumed in an open cut application the conduit will be backfilled properly as it should be for drainage pipes. It should be noted these backfill conditions are easily achieved.) Unit weight of soil does not exceed 120-pcf. (Some soils and gravels can exceed 120-pcf) Installed in accorda
44、nce with ASTM D-2321 (Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications). For open cut, HDPE conduit typically has a wall thickness that is heavier than SDR 17 due to manufacturing considerations or “construction survivability” requir
45、ements, so the height of cover table shown below is extremely conservative. Table 1: Design Window Maximum therefore the earth loads will be minimal. On long HDD applications, the tensile forces of pullback are perhaps the most critical variable in choosing the pipe strength. Along with the friction
46、al and mud pressure forces, the pipe has bending forces (the pipe has deflected laterally to avoid obstructions, or from the depth of the bore to the ground surface) requiring high tensile strength, with flexibility. For HDD projects, selecting the proper strength HDPE conduit takes due consideratio
47、n and a full design (see ASTM F-1962 and PPI report TR-46 for detail design criteria related to very long difficult bores with larger diameter pipe7,8). PPI report TR-46, states that commonly used wall thicknesses DR 7.0 DR 17 would be sufficiently strong for depths to 15-ft. Conduit wall thicknesse
48、s lighter than DR 13.5 is rarely used in power and telecom applications. PS has minimal relevance in these cases. HDD installations are not conducive to PVCs short lengths, so a comparison to PVC pipe stiffness seems immaterial. HDD contractors are often the best judge of what DR pipe is required as
49、 the end performance of the pipe pull often is directly related to their boring expertise, the soil conditions, and contractors equipment capabilities. Conclusion PS and Flattening can be useful for manufacturing quality control and assurance. However, the values should be related to the specific material (HDPE or PVC) and wall thickness, not a single arbitrary target. PS testing for HDPE pipe should be at 5% deflection in accordance with ASTM D-2412, Standard Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel Plat
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