No-dig carves niche for itself
When trenchless technology was first developed, it was just a rumble in the underground - many people were resistant to it. Nick Sheehan reveals it has come a long way - in variety of techniques, and bringing people round to its way of thinking.
Experience of trenchless, or no-dig, techniques has grown over the past 20 years - some have been widely adopted by water companies, others have faded to oblivion. What remains is a clutch of proven trenchless methods.
While some low-impact, trenchless techniques are used to install new water and sewer pipes, the majority of work is in rehabilitating existing ones. Foremost among rehabilitation techniques is lining.
For potable water supplies, one possible approach is to slipline polyethylene (PE) pipe into the host main. But, for gravity sewers, unaided by internal pressure, pipes have to contend with ground loads. In these circumstances, the stiffness of PE makes it unsuitable for sewers at practical wall thicknesses.
The solution is to use a rigid lining material, less prone to deformation under external loading. As liners still need to be flexible during insertion, the cured-in-place-pipe (CIPP) system was devised.
Typically, a CIPP is a composite product with a flexible fibre tube that is pre-impregnated with a polymer resin. When the liner is positioned in the host pipe and expanded to full diameter, the resin is cured, or set, to form a permanently rigid pipe lining.
Lateral connections to small lined pipes, which have been problematical in the past, are now achievable with the aid of robotic cutting and installation machines.
Systems differ in many ways, but perhaps the key variables are environmental impact and structural strength.
Pollution can occur with conventional CIPP liners when they are heated, to activate curing, by flushing the liner through with hot water. This causes styrene - a volatile, potential carcinogen - to wash out of the liner.
Hence the development of EcoCIPP, which is made by Perco Engineering services. This system uses ultraviolet light to activate curing.
It also uses less energy than conventional CIPP, as water heating is not required.
Provided the sewer is not badly distorted, an EcoCIPP liner can be deployed straight into the old sewer pipe. This is following cleaning, thorough inspection and removal of any obstructions. The liner is winched through the pipe between manholes, and then closed at both ends using a sealing system.
It is inflated with compressed air, which presses the liner against the bore of the existing pipe.
The final step is to draw the system's UV curing lamp/camera unit along the length of the liner, hardening the resin and forming a high-strength pipe-in-pipe.
If the condition of the old pipe is too bad for lining, or the sewer needs to be upsized, pipebursting is an option. Perhaps the simplest technique is rod-pulling, in which a steel-bursting head is pulled through the pipe.
The Perco Expandit system bursts progressively, using hydraulics, to cut out vibrations that can damage other buried services and surface structures. It can also pull pipe strings behind it, or short snap-together pipe sections from a manhole. The hydraulic bursting action breaks up the pipe and forces the fragments into the surrounding earth.
The Expandit mole is kept on course by a chain winch, while a hydraulic ram pushes the device forward. Short pipe lengths - whether in clay, concrete, or PE - are lowered into the manhole and jointed as they follow the bursting head along the course of the pipe.
Trenchless technology has even more difficulties to overcome when installing new pipes in virgin ground. Sophisticated laser guidance has made accurate, point-to-point installation a matter of routine but variations in ground conditions make it important to select the right technology for the job.
Microtunnelling was developed 50 years ago in North America.
Capable of working in very soft, very hard, or mixed ground, it is unbeatable for accuracy, and installs pipe diameters ranging from 250mm to 3,000mm. One of its particular advantages is that microtunnelling completes the work in a single pass.
Another advantage is that the closed-face aspect of the machine - preventing material washing back past the circumference of the cutting face - allows it to operate deep below the water table.
Spoil is continuously fluidised and removed down the new pipe. While the water is recycled, the dry solids can either be reused on site, or removed to landfill.
The closed-face, controlled-spoil removal and low vibration has the net effect that ground settlement can be virtually zero. This is an important benefit when operating near buildings, or creating under-track rail crossings. Even pressure pipes for clean-water supply can be installed, by microtunnelling to insert lost steel casings that can then be sliplined with PE pipe.
From the same family of pipe-jacking techniques as microtunnelling, auger boring is traditionally used to install smaller pipe diameters and requires more consistent ground conditions.
The auger system and the insertion of new pipe operates section by section, as parts are lowered into position at one end of the bore, and removed as they emerge from the other.
A simpler process in some ways than microtunnelling, auger boring can quickly install pipes of 150mm diameter to 1,200mm.
As the auger screw advances through the ground, it is shielded inside a steel pipe through which the spoil is screwed back to the launch pit or manhole by the auger flights. Ground settlement is still low and work can proceed below the water table.
Guided auger boring is effected by using laser light from a theodolite at the launch point.
The light travels through hollow pilot rods as they are pushed, in series, through the ground to establish the direction of the bore. Any deviation is corrected by rotating the pilot rods.
Following the pilot rods, the steel casings and augers are headed by a reamer, which opens out the bore to the product pipe diameter. Pipe sections are jacked into place, or steel casings inserted to allow slip lining.
When maximum accuracy is not needed, unguided auger boring - in the right ground conditions - can achieve drives of 300m in a single shot.
In this procedure, the auger rig is set up and aligned in a small, excavated pit with a prepared base. The finish point can be another pit, shaft, or manhole.
Trenchless technology has a reputation for minimising disruption.
In the past there was an understandable resistance to it, as individual techniques were relatively new. But trenchless work is now undertaken by numerous specialist contractors with a much wider range of methods and experience.
Consequently, water companies, as other utilities, are now able to limit the true costs of streetworks by using trenchless technology with confidence.
Nick Sheehan is a founding director of Perco Engineering Services.T: 01604 590200