Tale of the centuries
Recently some Devon residents found that their water supply through a 19th century private main had come to a standstill. Open-cut replacement was out of the question but pipe-bursting posed its own problems.
At the site, located about six miles from Tavistock, an existing 76mm cast-iron water pipe had been in use as the main (private) water supply for three families. Originally installed in 1899, the pipeline ran from a tank surrounding a natural spring in dense woodland, 170m through a plantation, under a private road, 100m through some more accessible woodland, under a river, 100m under a swamp, 60m up a slope through open woodland, under a county road and then 20m under a neighbouring garden.
This range of locations made using a trenchless option to replace the pipe attractive to the owners as access for open-cut works would have been almost impossible without severe environmental damage to land belonging to other people.
The variety of terrain through which the pipeline ran, however, was just part of the problem. The reason the pipeline was in need of replacement was the other.
The existing pipeline, an old water feed from the Duke of Bedford Estate, had exhibited signs of significantly reduced water flows. This was not unexpected, given the age of the pipe, because it was experiencing tuberculation.
But it was the degree of tuberculation that was to make the renovation work more difficult. Over most of its length, the pipeline was almost completely blocked.
The residents, who were responsible for the renovation of the supply pipeline, decided that it would be good to use the existing pipe route but to line the old cast iron pipe with a new 50mm diameter PE pipe. On inspecting the existing pipeline, though, it became obvious that opening up the old pipe bore was not going to be easy. The residents hired a Hammerhead-manufactured Hydroburst 3038 rod-based pipe bursting system for the work, supplied by Gt Gransden-based U Mole.
The Hammerhead HB3038 is a static rod-based pipe bursting system that uses an optional 15.1kW Kubota-driven power pack to provide the hydraulic power for the pipe burster. Once delivered by U Mole the residents formed their own construction team.
As well as having the ability to burst more friable pipe materials, such as cast iron, clay and concrete, when used in conjunction with specially designed splitter heads, the system can also be used for the replacement of existing ductile, steel and plastic pipes.
The HB3038 rig itself is 1,511mm long by 513mm wide by 310mm deep. The unit is primarily aimed at work on smaller pipelines, such as 100mm and 150mm diameters. The unit can generate up to 38 tonnes pulling force.
Given the state of the old pipe, and the desire not to burst but to slipline it, the rig was used more as a cleaning tool. The main problem was how to push the bursting rods through a pipeline that was almost closed off by tuberculation.
In most cases, the rods could be forced through the blockages enabling the necessary bore for the lining pipe to be established. But, in some instances, the tuberculation was so severe that the rod string formed an interference fit over the length of the pipe into which they were passed.
This meant the bursting system power pack was working at its maximum pushing capacity without the bursting rods gaining access to the full pipe length. The lead rod had to be modified to enable this full access to be achieved.
The machine operator welded a ring around the pushing tip of the 44mm-diameter lead rod, making it 48mm diameter. As this was pushed through the existing pipe, it removed sufficient tuberculation so as to make the rods themselves looser in the bore.
This reduced pushing force by a factor of ten, making the rod installation easier.
One section was too encrusted for the burster, giving the risk of rods being pushed out of the main. It was decided that, in this instance, the risk of the rods doing so was acceptable as no other third-party plant such as cables was present.
As expected, the rods did break through and ultimately thrust bored 30m, so an alternative method of laying the pipe was required. This involved pushing a smaller pipe - with tracer wire installed - up the hole left by the rods and then digging it out with the aid of a cable locator. On another part of the project, in the thickest of the woodland, some 160m of replacement was completed using just one start pit.
Out of some 400m of pipeline replacement work, and despite the access difficulties in terms of terrain and existing pipe condition, some 340m of the old pipeline were opened up to allow the slipliner to be installed, with only 60m having to be open cut.
Once each section of pipeline was re-bored, the 50mm diameter PE slipliner pipe was pulled into place using a specially fabricated expanding cone and pulling eye.
Overall it was considered by the makeshift crew to have been a very successful job. It has provided a new water main, which is now supplying the local families with ample water.
With the project undertaken privately, using the hired pipe-bursting rig, as opposed to using the services of a utility company contractor or open-cutting the whole scheme, the residents saved £4,000-plus in costs.
Local project leader Colin Foxhall says: "With our only water supply failing fast, urgent action was needed, and open trenching was out of the question. The results have fully justified the effort put into the project. You have to admire the guys who hand dug it all originally though. They must have been tough."