The benefits of pipe jacking are well established. By choosing pipe jacking over open-cut construction, owners can have a precisely installed pipeline with less disruption to the public – and at a competitive price, with less noise and lower emissions.
While the benefits of pipe jacking are apparent, they don’t come without some risk. Everyone is familiar with the adage, “a chain is only as strong as its weakest link.” While that principle can apply in many different scenarios from business to sports, it literally applies to pipe jacking.
In pipe jacking, each pipe segment is thrust into the ground, one after the other, building a “pipe chain” underground of increasing length and weight thus incrementally building stress in the pipe. Any failure of an individual pipe segment or joint at any point along the alignment can have potentially devastating consequences to a project, especially in urban areas or on water crossings where accessing the pipeline via a “rescue shaft or tunnel” can be difficult, if not impossible.
Historically, there has been limited knowledge of pipe jacking among owners and engineers, particularly when it comes to what makes an acceptable concrete jacking pipe. This is understandable considering the niche nature of pipe jacking in general and microtunneling in particular. Due to the critical nature of each pipe segment along the pipe string, concrete pipe that has been well suited for open-cut construction for decades simply does not meet the demands of modern pipe jacking.
Today, pipe jacking and microtunneling contractors need jacking pipe built with the end use in mind. Jacking pipes need added strength, joints that allow the pipe string to curve, and gaskets that prevent the ingress of water, soil or bentonite.
This doesn’t mean that jacking pipes need to be complicated in their design or manufacture. There are several pipe manufacturers in North America that make pipe segments that meet the expectations of pipe jacking and microtunneling contractors. We spoke with Robert Ward, an engineer and co-founder/owner of Ward and Burke Microtunnelling, who shared his experience related to concrete jacking pipe for microtunneling projects in the United States and Canada. Ward and Burke Microtunnelling is a member of the North American Microtunneling Association (www.NAMicrotunneling.com), an organization of contractors that promotes education and dialogue on issues related to microtunneling.
“The first thing that an owner should consider is that in many cases the jacking pipe is the final permanent product – nothing else is of any interest in the long run,” Ward said. “Therefore, it is far and away the most important part of the microtunneling process.”
Indeed, upon its commissioning, the jacking pipe might be the final permanent product and carry water, sewage or other utilities, so the longer it lasts, the more cost-effective the initial installation becomes. In addition, that means fewer disruptions to the public for future repair or replacement. It is important to remember that the jacking pipe may also be a casing pipe for a permanent product pipe such as PVC, DIP, etc. Sometimes the annulus between the casing and the final pipe is backfilled rendering the casing pipe temporary in nature, but if the annulus is not filled, the jacking pipe is a permanent structure. In all situations and uses, having a high quality jacking pipe is critical to having a successful project.
So, what are the characteristics that comprise a good concrete jacking pipe? Ward identifies several areas he sees as key. While currently there is not a standard specification for microtunneling pipe, Ward says that parts of several existing ASCE and ASTM standards can be combined to make good reinforced concrete jacking pipe for microtunneling projects. Ideally, these disparate sections will be combined into a single standard that design engineers can cite when writing specifications for microtunneling projects, especially those with difficult geology, long drives, or curves.
The first characteristic needed for a competent jacking pipe is rigidity, Ward said, and that leads to concrete. “During construction, the pipe is going to encounter obstacles that are going to put point loads on it, and that pipe needs to stay in a circular shape,” he said. “The TBM is rigid and it cuts a circle, so it makes sense that whatever you are pushing behind the TBM should also stay a circle. Concrete is rigid and stays in a circle when it gets point loaded.”
“One of the keys to successful microtunneling is reducing the friction so you maintain low jacking forces,” Ward said. “The way to do that is by injecting bentonite around the outside of the pipe. The bentonite often needs to be injected somewhere between 40 and 100 psi. Therefore, the gasket has to be able to withstand 100 psi of external pressure – not 13 psi which is typical for open cut pipes.”
The pressure is key in keeping the annulus open, thus preventing sand and soil from gripping the pipe and increasing friction along the pipe string.
Typical concrete piping systems use gaskets that are designed to keep groundwater out of the pipeline, but do not account for the bentonite lubrication systems. “Some people think that if they are tunneling in dry or shallow ground that they can make do without a gasket capable of resisting higher pressures, but that is not the case. It has nothing to do with groundwater. Failure to resist bentonite pressure will result in sand and soil coming into contact with the pipe, and consequently, pipes getting stuck.”
Curved drives have been common internationally for a number of years, and over the past five years have become commonplace in the United States and Canada. The ability to perform curved drives can eliminate intermediate shafts or avoid obstacles, resulting in more cost-effective projects.
Even if the designed alignment is straight, the ability to steer is an important characteristic for microtunneling pipe. “The pipe needs to be capable of negotiating a 500-m radius curve, regardless of whether the tunnel is curved or straight in theory,” Ward said.
That requires the pipe’s tailskin extending 7 in. to allow the flexibility for the joint to open and close without losing pressure on the gasket. According to Ward, where the tailskin is embedded in the wall of the concrete pipe, it needs an angle that acts as both an anchor and a waterstop. Additionally, the tailskin needs a studded anchor every 12 in. welded to it around the perimeter or rebar anchors welded to the tailskin for the purpose of preventing the tailskin from coming loose and allowing water to leak around it. Finally, a hydrophilic strip glued onto the inside of the tailskin is needed to act as an additional waterstop. (See Figure 1: Typical Joint Detail for Concrete Jacking Pipe)
In pipe jacking, the ability of the pipe to accept axial loads is key to success. The pipe has to be able to be pushed through the ground without any failures. Regardless of the length of project, a stout pipe is needed because, in an imperfect world, difficulties arise and a microtunneling contractor may need to apply extra force to keep the pipe string moving.
Furthermore, concrete jacking pipes should be wet cast vs. dry cast, Ward said. Although dry cast pipes, typically used for open cut projects, have the same ultimate strength as wet cast, wet cast pipes have a higher resistance to strain and can carry an axial load of approximately 2,000 tons on an 84-in. pipe on a radius of 1,500 ft.
“From our experience, when strain exceeds 0.002, dry cast concrete pipes begin to lose their ability to resist load,” Ward said. “Wet cast concrete pipes have very little drop off in their ability to resist load and can continue to carry load even at strains exceeding 0.002 and up to 0.005 or more. ACI/ASCE designs for an ultimate strain capacity of 0.003, but it is still good to know there is sufficient load capacity above this strain in a ‘doomsday’ scenario.”
“High strains on the pipe are not an everyday occurrence, but you have to prepare for the day when things go wrong,” Ward said. “You may need to push really hard on the pipe to get you out of a tight spot, and wet cast pipe will be able to take those loads.”
Another benefit of the wet cast process is the smooth exterior of the jacking pipe. This smoother pipe surface reduces the skin friction on the pipe string, thereby lowering the required jacking force being applied to the pipe to complete the drive.
Pipe Wall Thickness
In addition to its rigidity, concrete pipe is well suited for pipe jacking projects because of its wall thickness. The thick walls of concrete pipe give an ability to accommodate intermediate jacking stations (IJSs or interjacks), which are typically installed on long microtunneling drives as a means of applying thrust in addition to the jacks at the entry pit. Interjacks are placed periodically along the pipe string between pipe segments and this additional thrust capability serves almost as an insurance policy in case an extra push on the pipe string is needed.
The diameter of the interjack cylinders is approximately 6 in., and a wall thickness of at least the diameter of the interjack cylinders is needed.
“Concrete pipe, with its thick wall has an inherent economic advantage over all other products because it can be used with interjacks at very high loads,” he said. “Again, you might not need to use interjacks often, but when you need them you don’t want a pipe that gives up.” Compared to other pipe materials, interjack stations in concrete pipe tend to be more economical to fabricate, install during jacking and then to remove when the jacking is complete.
Sourcing the Pipe
Ward says that many concrete pipe manufacturers are more than capable of delivering high-quality jacking pipe. The crucial aspect is creating a high-quality mold. The tolerance of jacking pipe needs to be higher than typical open-cut concrete – on the order of 1/16th of an inch. Ward and Burke has worked with concrete pipe manufacturers in the United States and Canada that are producing world-class concrete pipes for world-class projects.
“Once you have the mold set up, the pipe is simple to make,” Ward said. “It doesn’t cost a lot more to make the right pipe, and it will save you a lot of money vs. getting the pipe stuck.”
There has been some discussion within the industry about the need to write a new specification related to concrete jacking pipe, but Ward says that existing documents can be used as guidance for making concrete pipe that meets the needs of microtunneling contractors, beginning with ASCE 27-00, and its follow-on ASCE 27-17: “Standard Practice for Direct Design of Precast Concrete Pipe for Jacking in Trenchless Construction.”
“ASCE 27-17 is the bible for microtunnelers,” Ward said. “You need to know it and abide by it.”
For strength of the pipe, ASTM C76-19: “Standard Specification for Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe” can be used as a reference document. The standard identifies five different classes of pipe, with Class V being the most robust. Ward suggests using Class V pipe for all pipe jacking projects (except when tunneling in beach sand or rock, where Class IV may be acceptable).
Standards regarding the gaskets for pipes up to 50 psi pressure rating can be found under ASTM C361-16: “Standard Specification for Reinforced Concrete Low-Head Pressure Pipe.” AWWA’s C-300 pipe specification should be used for cylinder encased microtunnel concrete pipes for pressure ratings between 50 and 150 psi.
ASCE has also published the book “Standard Construction Guidelines for Microtunneling,” which covers the planning, design, pipe materials and construction of microtunneling (CI/ASCE 36-15).
By choosing properly designed pipe for the job, owners can achieve successful projects that meet the demands of the public for generations to come.
Robert Ward, co-founder of Ward and Burke, was the source of this article on behalf of the North American Microtunneling Association (NAMA). NAMA and its members endorse the contents of this article.