Sliplining
and the Secrets of Annular Space Grouting
By
Michael Bank
Lamson Vylon Pipe
Presented at
BCWWA 27th Annual Conference
Vernon, B.C.
April 25 - 28, 1999
Technology Transfer Session 1
As the infrastructure around the world continues to age, more and more emphasis is being placed on its condition. Communities and other system owners are realizing that the underground infrastructure reflects a tremendous investment. Different governmental agencies are placing pressure on the system owners to update their pipeline infrastructure.
Two of the largest problems plaguing the sewer systems are infiltration / exfiltration and corrosion. Infiltration/exfiltration is usually associated with either bad joints or cracked pipe. Corrosion is most predominant in concrete pipe and is directly related to the generation of hydrogen sulfide in the pipeline. Many articles have been written about the corrosion cycle and its devastating effects on pipe.
Over the years, many different systems have been developed to repair pipe. The initial technique, and one that continues to be used, is to dig and replace. This allows the owner to gain the benefits of many new developments in pipe materials and hence improve the performance of their systems. Newer materials eliminate the concerns over corrosion of the pipe and can provide joints which are leak free.
Other techniques have also been developed to rehabilitate pipe. These techniques work on restoring the original pipe to its initial strength, reducing infiltration and stopping corrosion. The elimination of the problems with the old pipe are the focus of these systems.
This paper will focus on sliplining, a method of installing a new pipe without digging a trench. In addition, some discussion of grouting basics will be presented since grouting is part of the total new pipe installation.
Sliplining Basics
In sliplining, the existing pipe is used as a shell to allow for the insertion of the new pipe. It can be thought of as a Trenchless method of pipe installation, similar to microtunneling or directional drilling. Sliplining is most commonly performed on sanitary sewers, storm sewers and culverts.
Why would one choose to Slipline a pipe?
Sliplining is done for the same reasons most rehabilitation of pipelines is performed, elimination of Infiltration, stopping corrosion, and restoring hydraulics. Sliplining pipes have new pipe properties. They are made of plastic materials which are corrosion resistant and use flexible gaskets or butt fusion to join sections of pipes. Both of these joining systems allow for leak free systems for the future.
When sliplining is complete, the hydraulics of the system are often improved. The plastic pipe materials used for sliplining are very smooth producing Manning n values of .010 or less. This smooth interior will generally carry as much flow as the existing deteriorated line with an n value of .015 or greater. A gain in capacity will also occur because of the elimination of infiltration. Leaking joints can account for 25% or more of the flow in the pipe.
What types of projects are best suited to sliplining?
There are 5 items to consider early in the design to evaluate whether sliplining is the right solution. These items are:
1. Number of Curves
Since sliplining involves pushing or pulling a new pipe through the old pipe, the straighter the existing pipe, the easier the process. Curves can be sliplined, but they must be discussed with the manufacturer of the particular pipe material. In segmental slipliner systems, short lengths of pipe are used to negotiate curves. In continuous sliplining, the stress which will be put on the pipe walls must be evaluated.
2. Availability of a Location for an Access Pit
A need of sliplining is an access pit. The access pit must be above the pipeline. If the project cannot have a small pit, usually 20 feet long by 10 feet wide dug, then sliplining may not be the best solution. Sliplining can be done from existing access structures and manholes in some cases. These situations need to be evaluated on a project by project case.
3. Number of Service Laterals
A consideration when deciding whether to Slipline an existing pipe is the number of service laterals or other types of connections. Reconnecting service laterals will require access from the outside. This will require a small pit or shaft to be dug at the location of the service. Large numbers of services will make a dig and replace option more economical. Systems for internally locating and restoring laterals are under development. These systems will make sliplining a more Trenchless method.
4. Bypass Pumping
Segmental sliplining generally does not require bypass pumping as other rehabilitation systems do. Besides the practical aspects of having a bypass pumping system in operation, there are significant savings by its elimination. Practical considerations include potential spillage, pump failure, piping systems for the bypass, noise, and capacity. These items point to bypass pumping as unsafe and dangerous.
5. Competition
Sliplining should be included on projects where you want competition. Sliplining is an excellent alternative to cured in place pipe. Having competing equivalent systems bidding on a project will allow for the owner to maximize his dollars.
Existing Pipe Inspection
To get the information to determine whether or not to Slipline, inspection of the existing line must be done. Inspection can be performed in a variety of methods depending upon the conditions encountered and the type of pipe being inspected.
The most common method of inspecting sanitary sewers is closed circuit television, CCTV. CCTV has made many advances over the years to include color, and pan and tilt camera heads. With these advances, very detailed information about the existing pipe can be obtained. If the camera is pulled by a cable, by watching the location of the cable in relationship to the pipe wall, curves and other subtle changes in line may be detectable.
A second method of gathering data about the existing line is to simply jump in the manhole. Obviously, safety is a concern and proper procedures must be followed including, but not limited to, air quality testing and confined space procedures. Looking down a pipeline from a manhole can allow you to gather some of the basic information.
On highway or drainage projects, crawling through the pipe may be an option. Again, concerns about confined space and air quality must be addressed before personnel are allowed inside a pipe.
Pulling a mandrel through the line will help gather knowledge about the pipe's size. It is not uncommon for pipelines to change size in the middle of a run and return to the original size before entering the next manhole.
In performing any or all the above inspection methods, the following is a list of items to observe:
· Line and Grade
· Offset Joints
· Laterals
· Dropped Joints
· Cleanliness
· Hanging Gaskets
· Previous Point Repairs
· Changes in Flow Velocity
· Line and Grade Changes at Manholes
Cleaning of the Pipe
There are only three items to do to have a successful sliplining project, clean the line, clean the line, and clean the line. No single item can have a larger effect on the outcome of a project. A dirty line will lead to debris getting lodged below the pipe, high insertion forces and general failure of the process.
Today, one method is predominantly used, high pressure water cleaning. This method removes large as well as small items from the pipeline. Water jet cleaning will blast debris loose and thoroughly clean the line. It is important that the cleaning head be designed to primarily clean the bottom half of the pipe. Depending on the condition of the existing pipe, 360 degree cleaning may further deteriorate the pipe and cause local failures. In instances of severe corrosion, 360 degree cleaning may wash the surrounding soil into the pipe and create a cavity underground.
Bucket machines are still in use in some areas. These machines do a good job removing large debris from the bottom of the pipeline, but do not offer the same scouring action as high pressure water. Bucket machines also create environmental issues in the treatment of the material removed and handled.
Before final cleaning and after the insertion pit has been dug, a final sizing mandrel should be pulled through the pipe. This mandrel is sized larger than the slipliner. It will remove any obstructions and a final cleaning should be performed after it has been pulled. The sizing mandrel is discussed in more detail later.
Insertion Pit
The insertion pit is the location where the new pipe will be inserted into the existing host pipe. The location for the insertion pit is usually controlled by features of the pipe to be sliplined. If a dropped joint occurs in the middle of a run, this would be an excellent location. A balance between specific problems in the existing line, run lengths and above ground concerns are all factored into choosing the best location.
Insertion pit size will vary depending upon whether segmental or continuous length sliplining pipe is used. For continuous length pipe, ASTM F585 recommends a pit length of 12 times the diameter plus 2.5 times the depth should be used. For a segmental pipe, the insertion pit should be 5 feet longer than the length of the pipe segments. Regardless of the type of pipe, the pit should be 2 to 3 feet wider than the host pipe on each side.
For maximum worker safety, the pit should have a firm base. Depending on ground conditions, this may simply be natural ground, crushed rock or a concrete base. Trench boxes or other shields should also be used depending on ground conditions.
The insertion pit is usually excavated to the middle of the host pipe or slightly lower. Depending on ground conditions along with the existing pipe conditions, it may be necessary to excavate deeper.
After excavation of the insertion pit, the top half of the old pipe is removed. Typically, the pipe is cut on each side with an abrasive disk saw. The top is then removed and saved. It is best to cut the top half off rather than break it. By cutting, any chances of additional trash or debris getting introduced into the line is reduced. Also, by cutting and saving the top portions, the top can be replaced and the line grouted continuously thereby reducing the number of grouting stations.
Final Preparation
Once the insertion pit has been dug and the top half of the pipe removed, a sizing mandrel is pulled through the line. This mandrel should be manufactured of steel and is approximately 1" larger in diameter than the outside diameter of the pipe. Additionally, for segmental pipe, the mandrel should be at least the same as the slipliner pipe sections. This sizing mandrel will check for joint offset and other irregularities. It will also knock off any debris that might impede the sliplining process. It is important to thoroughly clean the line one additional time after this final mandreling.
The final preparation stage is the installation of insertion equipment. The equipment which is used varies depending upon the type of pipe used. Continuous length pipe is pulled into the host pipe whereas segmental slipliner is pushed.
For continuous length sliplining, a nose piece with a pulling eye is attached to the lead end of the pipe. The pulling winch is then positioned at the downstream manhole. The pulling cable is strung through the existing pipe and pipe support rollers are positioned. The final step for continuous length pipe is the fusion of segments of pipe. Considerations for traffic and right of way for the continuous string of pipe must be made during the planning stage of the project.
For segmental pipe, various methods of pushing the pipe exist. They vary from unsophisticated to the very technical. The least sophisticated technique is to simply push with the backhoe bucket. Controlling this is difficult and can easily lead to damaging of the pipe, however with proper care, it can be done.
A more reliable system is the cable pulley system. In this technique, a pulley is located near the springline of the host pipe on each side at the near end of the pipe. At the far end, a block is placed across the end of the pipe. On each side of the block, a cable is attached which runs the length of the trench and through the pulley. The cables are then connected together. A backhoe or crane lifts the cable moving the block towards the pulley and pushing in the slipliner.
A highly sophisticated system may involve hydraulic pushing units. These systems are generally used on very large diameter pipe, 72" and larger, where large forces may be required for insertion.
For any of the segmental systems, a push plate or ring is recommended. The push ring helps in distributing the insertion load around the full diameter of the pipe. This should eliminate point loading and the potential problems which might occur.
Sliplining
For continuous length pipe, the bypass system should be started and a final cleaning of the pipe should occur. Once cleaned the sliplining proceeds. During the process, the coordination of the pulling of the pipe and the pushing of the pipe into the trench must occur. This is necessary to avoid stretching the pipe too much. The entire length of pipe should be pulled continuously. Additional footage of pipe should be pulled in to allow for relaxation of the pipe. Generally, an additional 1% should be pulled in to allow for the stretch during installation and for length change due to temperature variations.
For segmental pipe, it may be advantageous to lubricate the joint of all the pipe prior to inserting the first piece. Once all the pipe is lubricated, the first piece is placed into the trench and inserted. It should be inserted until approximately 2 -3 feet remain in the insertion pit area. This is necessary to allow for joining with the next section.
With many of the products available for sliplining, the previously installed sections of pipe must be restrained for joint assembly. Many different methods exist. The easiest is the air bag system. This system consists of two air lift bags and a valve system. The bags are Kevlar coated rubber and are highly abrasion resistant. The bags are placed in the annular space between the slipliner and the host pipe and inflated. The friction generated between the lift bag, the host pipe, and the slipliner pipe creates enough resistance to allow for joint assembly. After the joint is assembled, the pressure in the bag is released and the pipe sections pushed.
A key element to successful sliplining is not to pause for extended periods of time during the insertion process. Once started, the process should continue until the final anticipated point is reached. Long pauses can lead to material build up under and around the pipe and difficulty in starting the pull or push. It is therefore unwise to start a push at the end of the workday.
In many instances, it is possible to insert pipe in two directions from one insertion pit. This method helps to reduce the number of insertion pits required on the project. Closure pieces can be used to join the two different insertions into one continuous pipeline.
Grouting
The annular space between the sliplining pipe and the host pipe should be grouted. This grouting fills the annular space and eliminates the concern about potential point loading. Furthermore, it helps to stabilize the slipliner system from shifting due to a change in groundwater levels.
Many of the problems experienced with sliplining when it was first used, had to do with the grouting of the pipe. Many advances have occurred in grouting, eliminating many of the problems. In addition, contractors who specialize in the grouting of annular space are now available.
Prior to the grouting of the pipe, submittals should be made to the engineer or owner. These submittals should contain information as to grouting procedure, grout mixes and methods of preventing floatation. It is important that these details are well thought out to prevent a good project from becoming a problem project.
Types of Grout
Two general types of grout can be used for filling the annular space, neat cement grouts and cellular concrete. Each of these types have advantages and disadvantages which will be discussed in detail. Both of these types of grout contain no aggregate or sand. Their main purpose is to fill the space and therefore strength is of little concern.
Neat cement grouts are mixtures of cement, fly ash and water. The grout generally has a 28 day compressive strength of 1000 psi. These types of grout are heavy, approximately 100 pcf and therefore grouting must be done in stages to prevent floating the liner.
Neat cement grouts can be ordered from the local ready mix company, making it readily available and inexpensive. Since specialized equipment is not required, the overall cost of using neat cement grouts is low. They are generally used on smaller projects where the cost of mobilizing a specialty contractor is great in comparison to the cost of the overall project.
Cellular concrete is a mixture of cement, water and foam. This type of grout has a 28 day compressive strength of approximately 300 psi. The weight of these grouts can be controlled and typically run 35 to 50 pcf. At these weights, single stage grouting can be accomplished.
Specialty, licensed contractors generally install cellular concrete. The contractor will have specialized equipment for generating the foam. The foam is usually introduced into the grout as it is being pumped. This develops the most stable mixture. Other methods include metering the foam generating liquid into the ready mix truck. The mixture is turned and the cellular concrete created.
Bulkheads
Bulkheads are one of the key elements to a successful grouting operation. The bulkhead must be strong enough to resist the grouting pressure. A good bulkhead will contain three elements, vent pipes, chemical grout, and hydraulic cement.
Vent and grout pipes are usually installed in pairs, two at the upstream grout injection location and two vent pipes at the downstream end. If a large amount of water is in the annular space, a third pipe may be located at the downstream end in the invert.
Bulkhead Building
The upstream bulkheads are built first. This allows any water in the annular space or flow coming in from laterals to have a drainage path. The grout pipes are put in place and temporarily held. Next, virgin oakum is soaked in chemical grout, usually either an Avanti or 3M grout. The soaked oakum is placed around the pipe approximately 12 inches from the end. The grout will be activated by the flow and foam and form a seal. After the grout has been activated, hydraulic cement is packed around the pipe to complete the seal and build the structural part of the bulkhead.
The next step is reconnection of the laterals. Saddles and other mechanical connections are used. After the connections are made, the downstream bulkhead is built.
Grouting
Prior to the start of grouting, a weir or other device is placed in the line to fill the pipe with water. This water will serve two purposes, to weigh down the liner pipe to prevent floatation and to cool the pipe. Cooling is necessary because the hydration of cement may create enough heat to soften some plastic pipe materials. The pipe manufacturer should be consulted on this issue, especially if neat cement grouts are used.
Grouting pressures should be monitored and controlled. Grouting can be accomplished at low pressure, generally less than 5 psi. The 5 psi limit allows for a large factor of safety against buckling the liner pipe. The pressure should be monitored with a gage gradated from 0 to 15 psi in one pound increments.
The grout is pumped through both the upstream grout pipes until it exits both the downstream pipes. Once this occurs, the downstream grout pipes are capped and the grouting continues through one of the upstream pipes. At this point, the other upstream pipe serves as the vent pipe. When grout starts to exit from the upstream vent pipe, the line is full.
Conclusions
Sliplining can be a cost effective method to replace a pipe which has reached the end of its service life. By sliplining, a new pipe can be installed without digging a trench. The sliplining method allows the owner of a system to get all the benefits of a new pipe without the disruption of digging a trench. With the new advances in grouting, many of the concerns that previously existed are removed.