SDI-Unistride at Cerne Abbas: Resin the bar (Utilities special report)

The chalky ground and valley location of the 1,000-year-old settlement of Cerne Abbas makes the village prone to flash floods and burst pipes. CJ reports on how drainage specialist SDI-Unistride used acrylate injection to seal the joints.

The ancient Dorset village of Cerne Abbas is famed for its beautiful valley setting and its giant carved into the chalk hill. The stunning location may attract tourists, but it also makes the area prone to flooding, damaging the area's drainage system.

Groundwater levels throughout the area are only just below the surface, and sometimes above, with parts of the village boasting natural ponds and with river courses having to be culverted along some village streets.

In the more recent years of the village's existence, when mains drainage systems were installed to serve the residents, the majority of the pipework was laid well below the existing water table. Over the intervening years, as the pipes have aged and joints deteriorated, this has led to a significant amount of infiltration through defective joints, with up to 30% of joints being affected on any particular pipe length.

Infiltration can vary from a mere trickle, to gushing flows, depending on the prevailing ground water situation and the degree of deterioration in the pipeline. This inflow of fresh water not only reduces the capacity of the drain to handle foul flows, but also ultimately adds to the burden on the treatment plant into which these flows are fed.

Wessex Engineering and Construction Services (WECS), the construction division of Wessex Water, undertook a study to establish the best way to complete the repairs to minimise, if not eliminate, the infiltration.

Investigations showed that the majority of the infiltration on the Cerne Abbas network was through defective joints, so WECS undertook to find a system of sealing that would counter it without the need to excavate the whole pipeline system. There was also a need to ensure that, whatever system was chosen, the work would not interfere with the tourist activity that, during the summer months in particular, is so vital to this small rural community.

It decided to use the trenchless system known as acrylate injection sealing (or gel sealing). The contract was awarded to Hampshire-based drainage contracting specialist SDI-Unistride, one of just five companies in the UK able to carry out this type of work, and was scheduled for seven weeks duration at a cost of some £70,000.

"This is a fast, efficient and sustainable means of sealing our sewers using cutting-edge technology without the need for excavation in the highway," says Julian Britton, senior engineer at WECS. "It therefore minimises the disruption to our customers. Preventing groundwater from entering our sewers is an important issue for us, as it means that we use less electricity in pumping sewage to treatment at the works, again saving money."

SDI-Unistride used its CUES-manufactured joint sealing rig to carry out the work. Managing director James Fulker says: "Gel sealing is a quick and relatively inexpensive way of sealing pipe joints and manholes. As many as 70 joints per day, or up to four manholes, can be tested and sealed. Our rig is unique as it is completely self-contained and requires no other support vehicles or equipment to operate. For the customer this means greater efficiency and even less disruption."

Sealing cycle
Once a section of pipeline has been CCTV surveyed and cleaned to remove silt and obstructions, a typical sealing cycle involves the establishment of the acrylate injection system set up between two existing manholes. With the sealing truck at one manhole and a control winch at the other, a winch cable is set up between the two access points.

On this winch line, a CCTV camera is positioned so that all sealing work can be observed from within the pipeline by the operator, who sits in the control cabin on the truck, ensuring accurate positioning of the sealing unit and enabling the operator to check the progress of the operation. Next to the CCTV camera is positioned the sealing unit or packer itself. This in turn is connected to compressed air, water and sealing resin feeder pipes. On the CUES rig, feed lines are specially designed as a single 'cable' to avoid any chance of entanglement in the pipe.

The sealing packer comprises a framework carrying a packer that can be inflated at the ends, with a mid section that allows the sealing resin to be injected at a set point between the end seals. Each packer is designed to fit into a specific pipe diameter - in this instance 150mm.

To complete a single seal, the rig operator winches the camera/sealing packer arrangement into position over a pipe joint with the packer straddling the joint. In this way, when the end seals are inflated, the joint is, in effect, isolated from the rest of the pipe. With the end seals inflated, air pressure is applied at the joint to test its state of repair. If after a pre-set time the air pressure has not dropped, the joint is taken as sound. If air pressure is lost, the joint has to be sealed as it is obviously leaking. Where major infiltration occurs, a preliminary pressure test is not always used as the leak is obvious.

On the Cerne Abbas project, SDI-Unistride and WECS agreed a preset time for the pressure test of two minutes, although in some cases tests of up to five minutes may be used.

Once a joint has failed the pressure test, and with the end seals still inflated and sealed, the two-part resin is pumped into the annulus around the sealing packer between the sealed ends. Just before the resin is expelled into the annulus, the two parts pass through a static mixer, which produces the final resin mix. The pressure pumping the resin into the annulus forces the mixture into the joint, filling it. As the resin mix cures and hardens, this seals the defective joint against further infiltration.

To test this, once the resin pass has been completed and the resin has cured, a further air pressure test is carried out. If the joint again fails to hold the air pressure a further resin sealing pass is made. This cycle continues until the joint is completely sealed, although it is rare that more than two passes are required.

Seepage control
The acrylate polymer sealing gel used on the Cerne Abbas project is known as AC-400 grout. It is a monomer system grout designed specially for ground water control soil grouting, seepage control in concrete and infiltration control in sewer networks. Acrylate grout was introduced to the industry in 1980 after the removal of acrylamide grout as an accepted product in 1978 due to toxicity problems.

The gel time for the resin can be pre-determined by the operator and is established by varying the concentration of chemicals in one part of the resin mixture. The gel time can be varied from just a few seconds, to a matter of minutes depending on the situation at hand. Where large infiltration is experienced, the quicker the gel time the better, otherwise there is the risk the resin will be washed out of the joint before setting. Where less infiltration occurs, but where the defect may need a significant amount of resin to fill it, gel time needs to be longer to allow for sufficient flow of resin to occur before curing takes place.

The speed and efficiency that is offered by the system means that several joints can be sealed during the course of a single shift and with the work being carried out through existing manholes, no excavation is required. This limits the duration of any road closures and other interference to everyday life for residents. There is also no need to block the pipeline during the sealing process as the sealing packer is hollow, allowing flows to continue to run even while the work proceeds, so keeping customers on line at all times.

The system also utilises a small crew of just three people on site, with just one needed to operate the system during a sealing run because the control cabin is set up to give the operator full control over both the truck-end operations and the remote winch-end systems. This means other sections of pipeline can be prepared and readied for sealing by the other crew members while the first is being completed.

The truck has a full workshop facility on board for cleaning and on-site repair of the equipment, again limiting the potential for downtime during a project.

The schedule of works for the Cerne Abbas Project, consisting of joint sealing over 960m of pipelines, was completed on time to finish before the main start of the tourist season at Easter 2006.

[Contract Journal, 1 June 2006, p32-33]