Over the last thirty years, the underground piping market in North America has seen tremendous growth in the use of thermoplastic materials. Benefits such as corrosion resistance, improved hydraulics, and reduced installation costs have been paying large dividends for owners of natural gas, water, and sanitary and storm sewer systems. The most widely used of this group of non-metallic polymers is polyvinyl chloride, also known as PVC. The second most prominent thermoplastic used in the underground pipe market is polyethylene (PE). This material was primarily used for gas piping and drainage tubing before its recent introduction into the water and sewage force-main markets. Understanding the similarities and differences of PVC and PE is essential to the proper selection and specification of these two thermoplastic products for pressure service. In-depth pipe-design considerations for both materials are beyond the scope of this book but are provided my many organizations such as the Plastic Pipe Institute.
While PVC pipe has been widely used for years, its use in the HDD construction industry is relatively new. The predominant plastic material of choice for HDD applications has been PE. PE, due to its fused joints, is ideal for pulling through bores. PE has been used in the gas industry for many years. Because of the advantages of trenchless installations PE pipe has found new markets in the water and sewer fields, mainly for trenchless projects. Fusible PVC pipe has recently been developed and is expected to compete directly against butt-fused PE pipe for the water and sewer trenchless construction market.
Underground Solutions introduced Fusible C900TM/C905TM PVC pipe to the HDD market. A first of its kind, this product uses a fusion process to join lengths of AWWA C900 and C905 pipe, ranging in diameters of 4 to 48 inches, in a continuous, jointless chain, similar to the way in which PE pipe is assembled. With the ability of PVC pipe to be fused in this manner, the product could potentially increase the use of PVC pipes installed by HDD construction methods. Overall, PVC has not been used as widely as other materials for HDD construction. Technological developments within the PVC pipe industry have resulted in the design and manufacture of some restrained joint products to enable trenchless installation of PVC water and sewer pipe via processes such as HDD.
Currently there are two types of PVC pressure-pipe products that are designed to withstand the pulling forces involved in HDD. The gasket joints in both products have been modified to provide the restraint necessary to hold the joints together during the pullback action.
Manufactured by IPEX, Inc., Terra Brute™ trenchless restraining joint pipe is AWWA C900 pressure pipe with a newly developed joint for pull-in-place installation of pipe in pressure piping systems. The modification made to a regular piece of AWWA C900 pipe consists of a set of small-diameter stainless-steel pins that are inserted through an external ring and the bell of the pipe into an internal steel ring placed in a groove cut into the spigot section. This bell-and-spigot joint modification increases the tensile-load capacity of the pipe joint by a factor of 36 to 60, depending on the diameter and the wall thickness of the pipe. The tensile capacity of the joint can be optimized by controlling the number of pins, pin diameter, depth of groove in the spigot, and wall thickness of the internal ring. Several successful installations of various lengths of Terra Brute™ have been completed with HDD. The design approach of the joint is universal and can be adapted to PVC pressure pipes of 4- through 42-inch diameters.
Certa-Lok C900/RJTM PVC restrained joint pipe, manufactured by Certain Teed Corporation, is another product conforming to AWWA C900 pipe, but again with a proprietary joint type that makes the product ideal for HDD installation. The Certa-Lok C900/RJ is used in new construction of water distribution/transmission lines or sewer force mains or for new gravity- sewer-main installation. The Certa-LokTM C900/RJ restrained joint pipe has a groove machined in the pipe and in the coupling to allow the insertion of a flexible thermoplastic spline that provides a full 360-degree restrained joint with evenly distributed loading. Although the wall thickness of the pipe is locally reduced at the groove area, positive reinforcement and stress control are provided at this location by the installed Certa-Lok coupling and spline. Finite-element structural analysis has verified that under internal pressure tensile hoop stress in the pipe groove area does not increase above the nominal predicted levels away from the groove. Available diameters currently include 4 inches through 12 inches in both 150 psi and 200 psi pressure classes.
When designing any pressurized piping system, consideration must be given to the stress created in the pipe wall due to the internal operating pressure. Metallic materials used in pressure pipes are elastic—i.e., the relations between stress and strain are linear and independent of loading time. However, plastics are different. Their strain is not proportional to stress or independent of loading time. Even though plastics (such as PVC and PE) do not behave elastically, most of the design equations that have been derived on the assumption of elastic behavior can still be used, provided the strength values used are appropriately established. The use of elastic equations requires the selection of strength values that account for long-term loading response. For PVC and PE pipe materials such values are determined from long-term pressure tests conducted on pipe specimens made from the material under evaluation. Pipe testing is performed in accordance with ASTM D1598, Time to Failure of Plastic Pipe Under Constant Internal Pressure. Sufficient pressures versus time-to-failure points are obtained to plot a line on log stress versus log time-to-failure coordinates. In order for a thermoplastic material to qualify for pressure piping, the data must plot along a nearly straight line. That straight line is defined mathematically and extrapolated to the 100,000- hour intercept. This extrapolation procedure is detailed in ASTM D2837, Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials. The resulting long-term (extrapolated 100,000-hour) tensile strength values are categorized into hydrostatic design basis (HDB) values. The HDB values are appropriate for use in the elastic design equations for determining either the pipe wall thickness or selecting the pipe pressure rating/class needed.
External Load Considerations. Plastic pipe has been installed for gas, water, and sewer mains; electrical conduits; and a variety of chemical lines. These projects involved river crossings, highway crossings, right-of-ways through developed driveways, and business entrances. As mentioned in Chapter 2, the condition of the bore hole is an important factor in determining the loads on the product pipe. The primary area of concern is whether the bore hole stays open or collapses. The types of ground, drilling techniques, and the presence of drilling mud all influence the borehole condition. If the bore hole remains intact after drilling and reaming, the earth loads arch around the hole, resulting in little soil pressure being transmitted to the pipe. For intact bore holes the pipe experiences pressure from the hydrostatic forces caused by the drilling slurry or any groundwater present. If the bore hole does not remain intact and collapses, the earth pressure will be experienced on the pipe. Earth pressure resulting from a bore-hole collapse could exceed the slurry pressure unless considerable tunnel arching occurs above the hole. If tunnel arching does not occur, the external pressure is a combination of the earth, groundwa-ter, and live-load pressure. Arching is not usually experienced during river crossings because of the unconsolidated and saturated soil conditions. In this case the pressure on the pipe likely equals the geostatic stress or prism load. Arching may occur in consolidated soils. Under this condition the applied pressure is often less than the prism load.
The net external pressure is the difference between the inside and outside pressure acting on the pipe. The external pressure operating on the pipe may be reduced or eliminated by the internal pressure. Product pipes operating under an internal vacuum will experience an increase in the external pressure because of the absence of atmospheric pressure inside the pipe. For a collapsed or deformed bore hole Equation 4-6 calculates the net external pressure. For an intact or open bore hole Equation 4-7 calculates the net external pressure.
net ext water lxve int 1
Pr essurenet = Purry - Pmt Equation 4-7
Net external pressure, psi External pressure due to earth pressure, psi Groundwater pressure (including the height of river water), psi Live loads, psi
Internal pressure, psi (negative in the event of vacuum) Hydrostatic pressure of slurry or groundwater pressure, psi
Earth loads may be experienced on the pipe when the bore hole contacts the pipe. When bore-hole deformation places the soil above the hole in a plastic state, arching may result. Under these conditions the deformation of the bore hole is limited and may result in no soil actually making contact with the pipe. If the soil does not contact the pipe, it will not experience any earth load. However, the bore hole may deform enough to allow earth loads to be transmitted to the pipe. Under these conditions it is a challenge to determine the earth loading experienced by the pipe. The where: Pr essurenet =
i water Plive Pint = Pslurry reader should refer to materials published by plastic-pipe manufacturers and the Plastic Pipe Institute for detailed coverage of this type of earth loading. If arching in the soil above the pipe does break down, the pipe may experience considerable earth loading. Under this condition the earth load is the weight of the soil above the pipe. This type of prism load is more common in shallow HDD bore paths that experience live loads and in unconsolidated sediments such as river crossings. Equation 4-8 is used to calculate the external earth pressure load under this condition.
Peann =rsoil * Hsoil Equation 4-8
where:
Jso il = effective weight of the soil, pounds/feet3 Hsoil = soil height above the pipe, feet
Groundwater loads will be experienced on the pipe regardless of the bore-hole condition. The primary concern is determining whether the slurry head or ground-water head is higher and whether they will control the design requirements for external loading. If present in the soil conditions, the external pressure caused by groundwater must be taken into account during the design. Equation 4-9 calculates external pressure due to the slurry head, and Equation 4-10 calculates the external pressure due to the ground or surface water head.
Pslurry gslurry*H borehole Equation 4-9
Pwater gwater*H water Equation 4-10
where:
Pslurry = unit weight of the slurry and soil cuttings gwater = unit weight of the water H borehole = elevation difference between the lowest part of the bore hole and the entry or exit point H water = height of the water above the pipe
Plastic-Pipe Resistance to Loads. A key factor in the design of a HDD crossing using plastic pipe is to select a pipe that can withstand the external loads without incurring damage that can affect the operation or life of the pipe. Most product pipelines experience several operational cycles during their life. The different operational cycles result in various net external pressures that have to be considered during the design phase. In addition to determining the various loads for the operational cycles, consideration must be given to the duration of each load. Most plastic-pipe products react to loads with time-dependent properties. For example, a plastic conduit resists constant groundwater and soil pressure with its long-term stiffness. A plastic-pipe force main may be subjected to pressure surges resulting in cavitation. During cavitation the net external pressure is the sum of the external pressure plus the vacuum. Cavitation is instantaneous, so it is resisted by the pipe's short-term stiffness, which can be four times higher than the long-term stiffness.
The tensile-strength limits for plastic pipes may have a significant impact on HDD designs. Tensile properties are time-dependent, and the pipe's resistance to a newly applied load decreases with time. This causes a higher resistance to short-term than to long-term loading. The HDD design should take into account the duration and frequency of each load so the performance limit can be calculated using the appropriate pipe-material properties. Under HDD pullback operations the pipe's tensile yield strength decreases with pulling time. As a result, the allowable pulling stress is a function of time. In addition, for viscoelastic materials, the ratio of the applied stress to strain is called the apparent modulus of elasticity, because the ratio varies with the load rate. See Table 4-2 for typical values for HDPE and PE pipe.
Deflection and Bending-Load Considerations. The design process consists of calculating the loads applied to the product pipe, selecting a preliminary pipe dimension ratio (DR)—the outside diameter divided by the wall thickness—and then calculating the safety factor for that DR. If the safety factor is adequate, the design is complete. If not, a lower DR is used and the process repeated. Safety factors are established for the various performance limits of the pipe. Safety factors are the ratio of the pipe's ultimate strength or resistance applied to the load. During HDD construction plastic pipes are typically installed in a bore hole with a diameter that is approximately 1.5 times larger than the product pipe. The drilling mud and hole cuttings fill the annular space. This material is similar to very soft clay and does not provide soil support for the pipe. Due to these conditions, the design normally does not account for any support from the annular-space mixture. A pipe should be selected that has adequate ring stiffness to resist the net external pressure without the support of the surrounding soil. External pressure applied to the plastic pipe
Typical Apparent Modulus of Elasticity |
Typical Safe Pull Stress | ||||
Duration |
HDPE |
MDPE |
Duration |
HDPE |
MDPE |
Short term 10 hours 100 hours 50 years |
110,000 psi 57,500 psi 51,200 psi 28,200 psi |
87,000 psi 43,500 psi 36,200 psi 21,700 psi |
30 min 60 min 12 hours 24 hours |
1300 psi 1200 psi 1150 psi 1100 psi |
1000 psi 900 psi 850 psi 800 psi |
produces a compressive ring thrust in the pipe wall and may cause ring bending deflection. Ring buckling or collapse is the performance limit for plastic pipes that are subjected to compressive thrust, and ring deflection is the performance limit related to ring bending.
Deflection reduces the pipe's resistance to external loads. HDD installation may produce ring deflection from earth loads, bending loads, and buoyancy forces. Allowable deflection limits for pipes depend on many factors. Diametrical deflection is limited by geometric stability and by the bending strain induced in the pipe wall due to deflection. Geometric stability (collapse) will be covered next. The maximum deflection a pipe can handle before becoming unstable depends on a number of factors, but typically instability occurs above 20-percent deflection in ground above the water table and above 15-percent deflection in ground below the water table. Normally a safety factor is applied. ASTM F-894 gives long-term deflection for pressurized pipes between 7.5 and 3 percent depending on the pipe DR (See Table 4-3). Pipes operating under internal pressure are also subjected to additional strain due to internal pressure attempting to reround the pipe. Because of this it is typical to limit the deflection for pipes under internal pressure. Table 4-3 provides some design deflection limits for PE pipes.
To calculate the ring deflections, use the larger of the deflections resulting from soil loads assuming no side support or from buoyant deflection due to mud weight. The first equation is for ring bending deflection; the second, for buoyancy deformation:
_E__Equation 4-11
SDR |
Deflection Limit Non-pressure Pipe (% diameter) |
Deflection Limit Pressure Pipe (% diameter)1 |
21 |
7.5 |
7.5 |
17 |
7.5 |
6.0 |
15.5 |
7.5 |
6.0 |
13.5 |
7.5 |
6.0 |
11 |
7.5 |
5.0 |
9 |
7.5 |
4.0 |
7.3 |
7.5 |
3.0 |
1 Deflection limits for pressure pipe applications are equal to 1.5 times the short-term deflection limits given in ASTM F-714.
1 Deflection limits for pressure pipe applications are equal to 1.5 times the short-term deflection limits given in ASTM F-714.
where:
%ADb = percent of ring deflection dweight
D = pipe outside diameter, inches DR = pipe dimension ratio E = modulus of elasticity (usually long-term), psi
Uniform external pressure results in ring compressive forces around the circumference of the pipe. This force causes compressive stresses in the pipe wall that could cause the pipe to collapse. As mentioned above, for HDD applications the pipe is considered to have no side support from the soil. Equation 4-13, known as Levy's equation, is often used to determine the allowable net external collapse pressure for HDD installed pipe.
mudweigkt = weight of fluid in borehole, foot-pounds/inches3
collaspe 11 2 ^
Equation 4-13
where:
E = apparent modulus for the specified pipe u = Poisson's ratio (lon- term loading 0.45; short-term loading 0.35) DR = pipe dimension ratio fo = ovality compensation factor
N = safety factor (usually 2.0 or higher)
The ovality compensation factor is provided in Figure 4-4. The above equations are suitable to use for pullback calculations after applying a further reduction factor due to the pulling force applied to the product pipe. Chapter 7 covers the pullback forces in detail.
The critical loading concern for deep HDD crossings of PE and PVC pipe products is usually buckling loads caused by the pressure of the drilling fluid in the an-nulus around the product pipe. PVC offers a higher buckling pressure than PE, but
both are considerably lower than steel. Tables 4-4 and 4-5 show the critical buckling (collapse) pressure typically used for PE and PVC pipe.
PE and PVC products have a much smaller bending radius than steel, and the bending radius of the drill rod will usually control the allowable bending radius of the bore path. The bending radius should be sufficiently large to allow for minimal bending strain and stress. The recommended minimum bending radius can be provided by the pipe manufacturer. Tables 4-6 and 4-7 show the allowable bend radius typically used for HDPE/PE and PVC pipe.
Pullback Load Considerations. HDD installation can exert significant pull forces on the product pipe. While Chapter 7 of this book covers HDD pulling loads in detail, a brief discussion of the topic is provided here. Determining what percentage of the total pullback force will be transmitted to the product pipeline is a challenge at best. The pulling force, which overcomes the combined frictional drag, capstan effect, and hydrokinetic drag, is applied to the pull head and first joint of product pipe, and the axial tensile stress grows in intensity over the length of the pull. The tensile forces on the pipe are caused by the fractional drag forces due to the weight or buoyancy forces as it is pulled into and through the bore hole.
TABLE 4-4 Typical HDPE Critical Buckling Pressure
Critical Buckling (collapse) Pressure, PE; psi
TABLE 4-4 Typical HDPE Critical Buckling Pressure
Critical Buckling (collapse) Pressure, PE; psi
SDR |
7.3 |
9 |
11 |
13.5 |
15.5 |
17 |
21 |
Short Term |
1,003 |
490 |
251 |
128 |
82 |
61 |
31 |
100-hours |
488 |
238 |
122 |
62 |
40 |
30 |
15 |
50-years |
283 |
138 |
71 |
36 |
23 |
17 |
9 |
4.1 HDD Design Basics 101 | |
TABLE 4-5 Typical PVC Critical Buckling Pressure | |
Dimension Ratio |
Pressure (psi) |
14 |
426 |
18 |
190 |
25 |
67 |
32.5 |
27 |
41 |
14.6 |
51 |
7.4 |
TABLE 4-6 Typical Minimum Bend Radius for PE Fusion Joined Pipes; Bend | |
radius is based on the pipe outside diameter. | |
SDR |
Allowable Bend Radius |
<= 13.5 |
R = 20 times pipe outside diameter |
> 13.5 to 21 |
R = 25 times pipe outside diameter |
> 21 |
R = 30 times pipe outside diameter |
Pipe with fittings or flanges in the bend R = 100 times pipe outside diameter | |
TABLE 4-7 Typical Minimum Bend Radius for PVC Fusion Joined Pipes; Bend | |
radius is based on the pipe outside diameter. | |
Nominal Pipe Size (inches) |
Minimum Allowable Bend Radius (feet) |
4 |
100 |
6 |
144 |
8 |
188 |
10 |
232 |
12 |
275 |
14 |
319 |
16 |
363 |
18 |
406 |
20 |
450 |
24 |
538 |
30 |
667 |
36 |
798 |
42 |
927 |
48 |
1058 |
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. |
The force is amplified by pulling the pipe through the curves and to the resistance due to the pipe stiffness. The required tensile force at the leading end of the product pipe will vary during the pullback. The amount of time that the pipe experiences the pulling stress is longest at the pull nose. The tail end of the pipe segment has zero applied tensile stress for zero time. The tensile stress should not exceed the allowable tensile stress for the pipe. Increasing the pipe wall thickness will allow for a greater total pull force, but the thicker wall also increases the weight per foot of the pipe in direct proportion. Selecting a thicker wall pipe may not necessarily reduce stresses but rather only increase the absolute value of the pull force.
A common practice to control this loading and to reduce the pullback loads is to fill the product pipe with water during the installation. The water will counteract the external buckling pressures and help provide neutral buoyancy for the product pipe. The critical load for shallow HDD crossings with PE and PVC products is usually the pullback load. Engineering judgment should be used in selecting the standard DR of the product pipe to resist the pullback load. Smaller values of DR will result in stiffer PE/PVC pipe. During the design the first step is normally to select the DR by determining the DR requirement for the internal pressure (or other hydraulic requirements). This DR is used in the preliminary calculations to determine if it is capable of withstanding the earth, live, and groundwater service loads. The next step is usually to check the DR against the installation (pullback) forces that are anticipated. Based on these steps, a DR is selected that will satisfy all three requirements. It is common to have some pipe wall stresses generated by the combination of internal pressure and wall bending. However, the internal pressure and external service-load stresses are usually treated independently. This is acceptable because plastic pipe is a ductile material and failure is usually caused by average stress rather than local maximums. The estimated pulling loads should not exceed the manufacturer's recommended maximum. Tables 4-8 and 4-9 provide typical maximum pulling loads for PE and PVC pipe.
Pipe resistance to pullback in the bore hole depends primarily on the frictional force created between the pipe and the hole or the pipe and the ground surface in the entry area, the frictional drag between pipe and drilling slurry, the capstan effect at bends, and the weight of the pipe. As mentioned previously, the buoyant force pushing the empty pipe to the bore-hole crown will cause the pipe to rub the crown. During pullback, the moving drill mud lubricates the contact zone. If the drilling stops or if the pipe or the mud flow stops, the pipe, slightly ring-deflected by the buoyant force, can push up and squeeze out the lubricating mud. The resultant startup friction is measurably increased. The pulling load to loosen the PE pipe from the mud can be very high. This situation is best avoided by using higher-ring-stiffness pipes, inserting full rather than empty pipe, and continuing nonstop drilling.
The maximum outer-fiber tensile stress should not exceed the safe pull stress. The maximum outer-fiber tensile stress is obtained by taking the sum of the tensile
TABLE 4-8 Typical PE Safe Pulling Loads
Safe Pull Load @ 24 hours; lbs
Safe Pull Load @ 24 hours; lbs
TABLE 4-8 Typical PE Safe Pulling Loads
Pipe |
PE |
HDPE |
2-inch SDR 11 |
1,525 |
1,875 |
4-inch SDR 11 |
5,470 |
6,732 |
6-inch SDR 11 |
11,855 |
14,590 |
8-inch SDR 11 |
20,093 |
24,729 |
8-inch SDR 13.5 |
16,675 |
20,524 |
10-inch SDR 11 |
31,213 |
38,416 |
12-inch SDR 11 |
43,908 |
54,040 |
12-inch SDR 13.5 |
36,440 |
44,848 |
24-inch SDR 11 |
155,577 |
191,480 |
24-inch SDR 17 |
104,220 |
128,271 |
36-inch SDR 11 |
350,048 |
430,829 |
36-inch SDR 17 |
234,496 |
288,610 |
stress in the pipe due to the pullback force, the hydrokinetic pulling force, and the tensile bending stress due to pipe curvature. During pullback it is advisable to monitor the pulling force and to use a weak link (such as a pipe of higher DR) or other failsafe method to prevent overstressing the pipe. The axial tensile stress due to the pulling forces should not exceed the safe pull load. Allowable safe pullback values for gas pipe are given in ASTM F-1804-97, Determining Allowable Tensile Load for Polyethylene (PE) Gas Pipe during Pull-In Installation. After pullback, pipe may take several hours (typically equal to the duration of the pull) to recover from the axial strain. When pulled from the reamed bore hole, the pull nose should be pulled out about 3 percent beyond the total length of the pull. The elastic strain will recover immediately, and the viscoelastic stretch will remember its original length and recover overnight. After the pull the pipe should be inspected. Significantly ovaled or flattened pipe is an indicator of collapse and that the pipe was over-stressed during the pull. Deep scratches or gouges can reduce the pressure rating of the pipe and increase stress. A common rule of thumb requires that pipes with scratches or gouges deeper than 10 percent of the pipe wall thickness should be replaced. In some instances this may be restrictive, and other engineering judgments may be necessary.
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