One of the challenges facing municipalities in the replacement of failing pipelines is that many of these pipelines are located in developed and populated areas.
Open-cut replacement can disrupt or damage other existing utilities and surrounding property. Horizontal directional drilling (HDD) techniques have offered ways to avoid some of these issues but these require special pipe and/or joining methods. Still, HDD is becoming more and more common because of the reduced damage to property, decreased inconveniences for local property owners and avoidance of other installed utilities.
A recent project for the city of Houston required replacement of failing eight-inch and 12-inch asbestos-cement pipes utilizing trenchless installation methods. There were a number of fitting connections that were required along the route and the contractor for the project chose to make use of standard AWWA C900 PVC pipe with a new product – the Mega-Stop bell protection system from EBAA Iron Inc. This product enables the installer to push standard bell and spigot PVC pipe through bored or cased holes without the danger associated with over-belling the joints. The device attaches very simply to the spigot of each piece of pipe and is comprised of a fixed stop ring and an expansion retention spring to maintain joint flexibility.
Utilizing the Mega-Stop product with standard PVC pipe enabled the contractor to save as much as 40 percent in material costs and provided a simple installation process.
The bid for this project, designed by Lockwood, Andrews & Newnam Inc. of Houston, was awarded to BRH-Garver, a Houston based contracting firm. This contractor specializes in complex civil projects for public and private clients.
The primary focus of this project involved the relocation of a large diameter water line and updating of a treatment plant facility. However, in the process, 2,500 feet of failing eight-inch and 580-feet of failing12-inch asbestos cement distribution lines in the vicinity were to be replaced as well. Because this project took place in a fully developed and mature residential neighborhood, the replacement of the distribution lines was designed to be performed with trenchless pipe installation. Currently there are several items listed in the city of Houston Approved Water Products List that are advertised for use in HDD projects. Included in this list is spline and groove PVC pipe, restrained integral bell PVC pipe and restrained integral bell ductile iron pipe.
BRH-Garver submitted conventional AWWA C900 PVC for the trenchless application. The design engineer cautioned that pushing the pipe through a reamed hole for a 200 to 250-feet could cause over-insertion of the pipe joint. It is rather straight forward to understand why over-insertion of the joints of a conventional pipeline in a pushed HDD installation would be difficult to prevent without a physical limitation of some sort. The installation of a gasketed pipe joint requires a certain amount of force to cause the gasket to deflect and affect a seal. That amount of force for eight and 12-inch pipe can be between 1,000 and 2,000 pounds force (lbf).
Once the gasket is deflected and the joint is sealed, a relatively small amount of force is required to continue inserting the spigot into the bell until the joint is assembled per instructions. The amount of force required to push the pipe through the HDD bore easily exceeds this lower force. As a result, the additional force required to push the pipe through the bore will also cause over-insertion and stress build-up in the assembled joints. This excessive stress can result in joint leaks or pipe bell failure.
Because all PVC pipe manufacturers provide cautions about over-insertion of the joints, a physical means of prevention was required. The contractor provided additional submittal information to the engineer on the use of the Mega-Stop product as that means of protection against over-insertion. BRH-Garver also included information about the maximum push capacity of the equipment anticipated to be used in the installation and pushing of the pipe. The Mega-Stop does not have, nor does it need, a pressure rating. Therefore, its use has no effect on the pressure rating of the pipe. As a result, component pressure ratings and system test pressures were not affected. The use of the standard C900 PVC pipe and the Mega-Stop was approved for this application and project.
There are many different materials used in waterworks piping. Each material has its own particular strengths and weaknesses; however, no material is perfect for every application. For over 35 years PVC pipe has proven to be a very reliable and durable piping material accounting for two-thirds by weight of plastic pipe and almost half of all installed pipe by length in 2004. The simple installation of PVC pipe has exposed one of its few and infrequent weaknesses – bell splits caused by over-insertion. There have been several industry papers presented that address this subject.
“When pipe joint failures occur, in most instances the pipe was inserted well beyond the manufacturer’s recommended mark. This is referred to as ‘“Over-Belled”’ pipe.” (Folkman 2010)
“Numerous pipeline failure analyses have been traced back to excessive stresses on the bell as a result of over-insertion.” (Rahman 2005)
“In the Root Cause Analysis ‘“Over insertion of the joint”’ was listed first as the cause of failure.” (Dueck 2010)
“Gasketed joints must be able to accommodate the changes in length.” (Rahman 2005) This accommodation is accomplished in a properly installed joint via the resulting gap between the end of the spigot and the back of the bell as shown in Figure 1. (EBAA Iron Sales 2011) This gap is a characteristic of conventional PVC pipe bells and a stop line that is provided on every spigot as an installation aid to properly locate the spigot relative to the bell of the joined pipes.
Over-insertion prevents a joint from absorbing changes in length or joint deflection. Failure to address over-insertion can result in the build-up in stresses in the pipe bell that can, over time, exceed the strength capability of the pipe material and result in a failure of the pipe.
Figure 1: Gap in a properly assembled pipe joint.
The Mega-Stop bell protection device is designed specifically to address this issue. The simple device is shown in Figure 2 and is comprised of two basic components. The first is a stop ring that is fixed on the spigot to provide the means for physically preventing pushing the spigot of a pipe joint too far into the bell. The second is an expansion retention spring (ERS) that provides a “cushion” between the stop ring and the lip of the bell to ensure the proper placement of the spigot inside the bell. The ERS, therefore, maintains the ability of the assembled PVC pipe joint to expand and contract in response to temperature changes, soil movement, and joint deflection. This feature is beneficial for both trenchless and open cut installations.
The product can be installed in typical open-cut applications at every joint as well. Doing so ensures that every joint will be installed correctly, can be inspected easily, and will provide assurance of long-term performance. Likewise it has proven valuable in casing/HDD applications where pushing conventional pipe would result in over-inserted joints.
Figure 2: Installed Mega-Stop at a pipe joint identifying components.
Drilling, pipe installation
Schwanke’s Direction Drilling of Hannibal, MO, was selected as the sub-contractor for the HDD portion of the project. Schwanke used a Vermeer Navigator D50x100 HDD rig, Figure 3, for each of the lines. A 17-inch hole was bored for the eight-inch pipeline and a 22-inch hole was bored for the 12-inch line. Native soil in this area of Houston is predominantly clay. As a result, Schwanke used a drilling fluid consisting of Baroid EZ-Mud polymer emulsion for clay stabilization, Penetrol wetting agent to control the sticking action of the clay and soda ash to moderate the pH level.
Figure 3: Vermeer Navigator D50x100.
The design plans called for the existing eight and 12-inch asbestos cement pipelines to be abandoned in place and, as is typical with rehab projects in populated areas, there were a number of additional utilities underground that were to be avoided including sewer, gas, electric and cable. This resulted in the need for the pipeline to be installed at a depth of eight feet.
The use of full lengths of standard C900 PVC pipe meant no requirement to assemble/fuse and stage lengths of pipe above ground with an extended entry pit. This enabled a standard construction box to be placed for pipe insertion. Bore hole lengths of 150 to 180 feet were utilized because of fitting and interconnection requirements.
With the limited amount of space at the project site, the Mega-Stop bell stops were assembled on the pipe spigots at an offsite location. That installation was quick and simple requiring less than five minutes per pipe joint. The required pipe lengths were then taken to the bore location for insertion/installation as shown in Figure 4.
Figure 4: Mega-Stop Bell assembled on pipe and ready for installation.
The spigot end of the first pipe length was fitted with a common bullet tip on the spigot. The pipe was placed in the box with the tip covered spigot placed at the bored hole. Schwanke used a block of wood on the face of the pipe bell so that the excavator bucket could be used to push the pipe through the bore without damaging the bell as can be seen in Figure 5. (On a subsequent 16-inch project, Schwanke fabricated a steel “cap” that fit over the bell for this purpose.) From that point it was a simple matter to lower the next piece of pipe into the box, place the wood block at the bell, and continue pushing the pipe until the tip was visible at the next pit location.
Figure 5: Pipe ready for pushing with the bell protected with wood block.
Most of the application involved straight runs of pipe. However a portion of the eight-inch line was required to negotiate a slight curve to parallel the street. A portion of the project drawing showing the curved section is shown in Figure 6.
Figure 6: Portion of project plans showing the curved trenchless installation requirement.
This was achieved utilizing standard drilling and bit location techniques. Once the bore was completed the pipe was pushed in as before and the piping easily followed the established curve. The ERS component of the Mega-Stop device is important to allow for and to be able to maintain the flexibility of the pipe joints needed to navigate the curve.
Figure 7: Inspection camera
As a means of demonstrating the effectiveness of the mega-Stop assembly, a video inspection was performed on a portion of the eight-inch section of the pipeline to verify that the pipe joints were properly assembled and not over-inserted. The camera, shown in Figure 7, was inserted in the pipe at an open location where a fitting was to be installed. Over 300 feet of installed piping was inspected and every one of the joints was found to be installed properly. A screen shot of the inspection video can be seen below in Figure 8.
Figure 8: Screen shot from inspection video showing proper gap in the pipe joint.
Aging infrastructure is an issue that any water and/or wastewater utility must deal with on an ever increasing frequency as pipelines start to fail or must be upgraded to meet new demand requirements. The use of trenchless installation techniques for replacing these lines is becoming more and more prevalent and minimizes the damage and disruption to adjacent property and utilities.
Frequently, this type of installation also requires limited excavation and entry and numerous fittings. Use of the Mega-Stop bell protection device with standard AWWA C900 and C905 PVC pipe simplifies material acquisition and allows for the close quarters and frequent fitting connections required in these applications.
This asbestos-cement pipeline replacement project in southwest Houston demonstrated the viability and benefits of incorporating the Mega-Stop device with standard bell and spigot pipe for short length bore installations and multiple fitting installation requirements. This combination of products provided for simple installation, reduced material costs, and increased pipe availability. Based on the simple and successful installation of the pipe that BRH-Garver Construction experienced for this project, plans are in place to incorporate the new Mega-Stop bell stop product in similar HDD projects in the future.
ABOUT THE AUTHORS:
Dennis Shumard, P.E., MASCE, is the product support manager for EBAA Iron Sales Inc., (713) 447-0707; firstname.lastname@example.org
Pat Moravits is a project manager for BRH-Garver Construction, Houston, (713) 921-2929; email@example.com
FOR MORE INFORMATION: BRH-Garver Construction L.P., (713) 921-2929, www.brhgarver.com
Schwanke’s Direction Drilling
Vermeer Corp., (888) 837-6337, www.vermeer.com
Baroid Industrial Drilling Products, (877) 379-7412, www.baroididp.com
EBAA Iron Sales, (254) 629.1731, www.ebaa.com