Recommended Installation Procedures for Fusible PVC™, including Fusible C-900®, Fusible C-905®, and FPVC®
Introduction
In most cases Fusible PVC™ is assembled at existing grade on the project site in long lengths prior to installation. This is the most efficient method for the butt-fusion procedure. As a result, the pipe must be moved from the layout at-grade assembly, to the final alignment for installation, normally below grade.
The insertion point is normally in a pit that provides access to the exit point of a horizontal directional drill (HDD) or the end of an existing pipeline for sliplining or pipe bursting type installations. Movement of the at-grade assembled Fusible PVC™ pipe from the surface to the prepared sub-base is also required in direct bury type installations.
This document will review the basic procedures Underground Solutions recommends to perform these installations with Fusible PVC™.
Pulling Parameters and Requirements for Fusible PVC™
All of the general installation methods with which Fusible PVC™ can be installed include some means of moving the assembled Fusible PVC™ pipe into final alignment in the ground. These installation procedures all rely on the allowable tensile pull force of the plastic pipe to accomplish this task.
2.1 Allowable Bend Radius
The safe allowable bending radius for Fusible PVC™ is a very important factor in the installation of the assembled pipe. This parameter dictates how much the pipe may be bent, not only during the installation phase and handling, but in the final alignment as well, which has a limiting effect on installation methods such as HDD and direct bury.
Underground Solutions has a document which explains the basis for the allowable bending radius in detail, but for the purposes of this document, allowable bending radius is based on the tensile properties of the plastic as described below. This information for each type of Fusible PVC™ pipe, as well as the document which describes the basis for this parameter, is available by contacting your Underground Solutions representative.
2.2 Safe Pulling Force
The amount of tensile load to be put into the pipe during a pull-in is called the safe pulling force. This value is a function of the cross-sectional area of the PVC pipe times a safe tensile stress. A requirement of PVC plastic used for AWWA C900 and AWWA C905 piping is to have a minimum tensile stress capability of 7000 psi. A safety factor of 2.5 is applied to this to derive the safe pulling stress. This value is 2800 psi. This is multiplied by the pipe area to come up with the Safe Pulling Force. The most common way to calculate the pipe area is to calculate the areas of the circles defined by the OD of the pipe and the ID of the pipe. The ID is arrived at by subtracting twice the minimum wall thickness from the OD. This information for each type of Fusible PVC™ pipe is documented; please contact your Underground Solutions representative for more details.
During installation there are multiple potential stresses on the pipe beyond the straight axial pull. There can be frictional resistance over ground and in the alignment, there could be bending stress, drag on the pipe due to buoyancy considerations, and so on. There also is the element of the unknown within the alignment. These all are additive in their occurrence requiring this significant safety factor to account for the possibility of them.
2.3 Temperature Effects on Safe Pulling Force and Bend Radius
Fusible PVC™ pipe properties, like all PVC pipe properties are defined at a standard ambient temperature of 73.3°F. As temperature rises above this, two key properties relative to pull are affected. These are tensile strength and modulus of elasticity.
The tensile strength of thermoplastics drops as temperature rises. This means that on especially warm days with longer pulls, the temperature at the time of pull-in may lower the overall safe pull force. The temperature impacts are defined in the following Table 2.3.1:
Table 2.3.1 - Temperature Impact on Safe Pull Force
Temperature (°F) % of Recommended Pull Force 73.3 100 80 95 90 87 100 78 120 63 140 58 As temperature rises, the modulus of elasticity is also affected. It too decreases as the temperature goes up. What this means is that as the pipe warms it becomes more flexible. The change in modulus directly reflects the increase in flexibility. This can be applied to the bend radius used for bending limitations. The following Table 2.3.2 defines the change:
Table 2.3.2 - Temperature Impact on Modulus of Elasticity
Temperature (°F) % Change in Modulus 73.3 100 80 98 90 94 100 88 120 78 140 70 The change in modulus can be directly applied to the bend radius. For a given pipe size, the bend radius can be reduced by multiplying the % based on temperature times the base bend radius. At 100°F, for example, the bend radius would be reduced by 12%, meaning that it becomes more flexible.
2.4 Pulling Mechanism Connections
All installation methods, including HDD, pipe bursting, sliplining, and direct bury, must utilize a pulling mechanism to accomplish the installation. There are several methods available to attach the pulling mechanism to the pipe as reviewed below:
2.4.1 Nylon Strap
For open cut or movement on the surface, if the length is short and a minimum force is needed, the pipe can be pulled with a nylon strap. The tensile capability of the strap needs to be known to compare to the relative force it may take to move the pipe.
2.4.2 Through-bolt with Linkage
In the case of sliplining short lengths in dry conditions, round head carriage bolts can be used to connect a chain link internally, to allow attachment of a cable for pull-in.
Care must be taken to make the bolted connection at least 12" from the end of pipe. This distance controls the amount of pipe wall in shear. If the connection is too close to the pipe end, it will rip out.
This method is only good for relatively short lengths in dry conditions. The connection, when under higher load, will tend to pull in toward the center of the pipe putting it into an oval shape. This also introduces pull out forces on the bolts and stress in the pipe wall that could lead to connection failure. In longer lengths a pull head must be used.
2.4.3 Pull Head
For HDD and long pulls in all other methods, pull heads have been designed and fabricated for Fusible PVC™. The pull head is sized to take and effectively transfer the recommended safe pulling force from the head to the pipe.
Because PVC is a relatively hard thermoplastic compared to other plastics used with the heads, serrated grip type heads don't bite into the PVC with enough force to effectively transfer the full safe pulling stress of the Fusible PVC™. As a result these types are not recommended for use with Fusible PVC™. The pulling heads for use with Fusible PVC™ are designed with through bolts located to accommodate the recommended safe pulling forces. The bolts are smooth shank to eliminate stress concentrations caused by the edges of a threaded bolt or all-thread. The bolts are countersunk to provide a smooth pulling surface on the outside of the head. The bolts are made to connect inside the pulling head to allow double shear loading of the bolts at the pipe wall.
The pulling heads are sized to accommodate a specific pipe OD. The pipe OD plus the out-of-round tolerance are used to set the ID of the pull head. This assures a tight fit and also accommodates the allowable variances in the pipe OD. With larger pull heads (greater than 12-inches in diameter) an additional gap allowance of approx .05" is added due to the stiffness of the pipe. It is recommended that the pull head be removed and re-installed on the Fusible PVC™ pipe after each pull-in.
Pull-heads are available for rent or purchase from Underground Solutions, Inc. Please contact your Regional Sales Manager or our main office at 858-679-9551 about rental rates or sell price.
Figure 2.4.1 - Pull Head for Use with Fusible PVC™
The pull head should also be used when fusing long lengths together prior to installation. As the pipe gets longer and heavier, a strap may slip. The pull head provides support of the wall and will prevent ovaling and folding of the pipe if a through connection is needed for the pulling.
In most cases it is recommended that a swivel be placed between the pull head and the pulling mechanism to eliminate any torsion forces being transferred to the pipe. In the case of pipe bursting, additional isolations are recommended to prevent any compression forces being transmitted to the pipe.
For HDD installations and any other methods requiring a sealed pull head, several steps can be taken. Rubber hose washers or "O" rings can be placed under the bolt heads to act as a sealing gasket at these locations. The back end of the pull head can be sealed with a silicone caulk. Another preventive step is to close the end of the PVC pipe prior to installation of the pull head. This can be done with plastic sheeting and/or tape. See Figure 2.4.2 below:
Figure 2.4.2 - Sealing of Pipe End Prior to Pull Head Installation
Installation by Sliplining or Pipe Bursting Methods
In sliplining installation the pipe is normally fused on the surface. The insertion point is at a removed section of the host pipe. The host pipe, in most cases, is generally at a horizontal grade.
In pulling a fused pipe from at-grade to below grade and the final alignment, the pipe must bend generally in an "S" configuration, with two separate, but equal bending radii. The recommended bend radius for the pipe being inserted (shown as 'RB' in Figure 3.1) is the parameter that defines the length of the insertion pit (shown as 'L' in Figure 3.1).
Figure 3.1 - Sliplining or Pipe Bursting Insertion
The reality of the installation may not dictate such a straightforward 'S' curve of the Fusbile PVC. A portion of the bending could happen above grade by supporting the pipe on pipe stands. This allows the bending to start prior to reaching the start of the pit. If the grade of the host pipe is greater than 0 %, sloping downward away from the installation, a lesser bend to enter the host pipe could be required. Also, if the host pipe is substantially larger than the Fusible PVC™ to be inserted, a portion of the bending could be accomplished inside the host pipe. Factors such as the sharp lip of a host pipe entrance or host pipe fragments leftover from pipe bursting may scratch the surface of Fusible PVC™. If a scratch in the surface of the Fusible PVC™ pipe is made greater than 10% of the pipe wall thickness, then you should contact your Underground Solutions representative immediately to determine the next course of action. It is highly recommended that the installation of the new Fusible PVC™ pipeline be closely monitored. Other issues that can influence the length needed for making such a transition are the ambient temperature (higher temperatures reduced the modulus of elasticity making the pipe more flexible).
Contact your Underground Solutions representative for information regarding the parameters of your particular sliplining or pipe bursting installation.
Installation by Horizontal Directional Drill Methods
In HDD installations the pipe is normally joined at-grade. The insertion point is at the exit pit of the HDD bore.
In HDD applications, the entry point of the Fusible PVC™ is often defined by an entry angle. This is expressed in degrees from the horizontal. A typical entry angle is between 6° and 15°. The length of Fusible PVC™ required to make the transition from the horizontal fused alignment to the point of entry is shorter than for sliplining and pipe bursting insertions because there is a single curve involved and not a full 'S' curve. As in some slipline and pipe bursting applications, sometimes for an HDD where the drilled alignment is greater in diameter than the pipe being inserted, a portion of the bending can take place as the pipe enters the bore hole. As previously mentioned, it is highly recommended that the installation of the new Fusible PVC™ pipeline be closely monitored.
The following Table 4.1 provides a factor to be used times the Fusible PVC™'s bend radius to determine the length required to make the transition into the bore hole:
Table 4.1 - HDD Bend Radius Factors
Degree of entry point Factor 6 0.1 8 0.14 10 0.17 12 0.21 14 0.24 15 0.26 Contact your Underground Solutions representative for information regarding the parameters of your particular HDD installation.
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Installation by Direct Bury Methods
The final installation method for Fusible PVC™ reviewed here is direct bury, or "Open Cut" method. Direct bury installations can be completed primarily in two ways.
5.1 Pull-in Method
The first is to perform a pull-in from the end of the trench. The pipe is fused ahead of the trench excavation in approximate alignment with the trench. In this approach, the insertion trench is the transition from the at-grade pipe fusion location to the cut trench bottom. The insertion trench follows the same geometry shown in the insertion configuration section for sliplining or pipe bursting (Section 3) of this document, as well as Figures 3.1 and 3.2. This technique works well when there is not enough room in the trench easements for fusion and lay-down area next to the trench. In many cases the spoils from the trench take up available room adjacent to the trench. The governing factor on insertion of this type is the insertion trench, and making sure the allowable bend radius of the pipe is not exceeded from the at grade elevation down to the final alignment elevation.
5.2 Placement Method
The second method is moving the pipe from next to the trench, into the trench and final alignment. In this case, the bend radius of the pipe is used to determine the location of the bends required to relocate the pipe. The distance to be moved is determined from the initial at-grade alignment location, and the final installed alignment location of the pipe.
Figure 5.2.1 - Direct Bury Pipe Placement Installation Steps
The starting point for installation is the Fusible PVC™ laying next to an excavated trench (Step #1). The end of the pipe is lifted and placed in the trench and anchored or held for the initial pipe insertion (Step #2). After some length is placed the pipe will no longer need additional hold back from sliding. Two machines are then used to move the pipe into the trench (Step #3). The spacing of the machines are at the midpoint of the curve and the end as determined by the 'S' curve for the size of Fusible PVC™ being installed. The machines then leap frog down the alignment taking positions at the changes in the bend direction to move the pipe into the trench.
Figure 5.2.2 - Pipe Installation Cross Section with Theoretical 'S' Curve Determination
To determine the total length of the 'S' curve, the distance the pipe will move from the at-grade alignment to the trench and final installation alignment must be known. This can be determined from the depth of the trench and the distance from pipe to the center of the trench. Using the Pythagorean Theorem to determine the length of the hypotenuse of a right triangle, and using the 'depth of trench' and the 'horizontal offset distance' as the legs of the triangle (see Figure 5.2.2), provides this 'required offset distance' the pipe will be moved. This is then used to calculate the length of the 'S' curve for a particular size of Fusible PVC™.
Contact your Underground Solutions representative for specific information regarding your direct bury project and installation.