In response to an Underground Asset Management Study in 1997, the Hong Kong SAR started the Replacement and Rehabilitation Programme of Water Mains with a view to improve about 3,000 kilometres (40 per cent of the total pipeline network) of aged water mains. Since the start of the works, close-fit polyethylene lining techniques have increasingly been used to rehabilitate water pipelines. This family of techniques involves insertion of a high-density polyethylene (HDPE) liner after temporarily reducing the cross section, either radially or by folding, to facilitate installation. The liner is then reverted/re-rounded, typically by pressurised water, air, steam or a combination of these methods.

Insituform Asia Ltd. (Insituform) introduced InsituGuard, its close-fit HDPE lining system in 2007. Using two installation techniques, InsituFlex, a concentrically reduced pipe (CRP) system and InsituFold, a longitudinally folded pipe system, the system is fully structural by design, with an SDR of 17 providing a 10 bar pressure rating. The use of InsituGuard can significantly reduce the amount of trenching required and maintain or even enhance flows. Insituform looked closely at the key material properties of the lining systems in advance and confirmed that they met the key requirements within the European standard.

The Insituflex system employs a radial reduction technology that temporarily reduces the liner’s diameter concentrically by as much as 20 per cent. A series of drive rollers pushes the pipe into place while also squeezing it to a smaller diameter. Rather than assert high pulling loads on the liner, a winch is used to pull the nose of the liner around bends and past dislocated joints. This process places less axial strain on the liner and leaves less residual tension in the final liner pipe system which consists of the liner, host pipe and fittings. This approach also helps minimise axial retraction of the pipe at the end pits during its return towards its original diameter. If the liner has to be cut into at a later time this reduced retraction makes the insertion of new fittings easier.

The InsituFold system folds HDPE pipe into a “heart” shape, which reduces the cross-section of the liner by up to 40 per cent. As it exits the machine, the folded liner is then banded to maintain the reduced shape prior to its insertion into the host pipe.

These bands are broken by internally pressurising the pipe with water after insertion. As with the InsituFlex installation process, this helps ensure that there is less retraction if the pipe has to be cut into at a later time.

Testing times

In order to have the InsituGuard system approved a range of tests and trials were initiated. Some of these were to satisfy Insituform that the InsituGuard system did not have a deleterious effect on the performance of the pipes. Other tests were requested by the client to confirm that the system would meet contractual obligations.

The InsituGuard liner has a pressure rating of PN10, which is classified as fully structural and independent according to the American Water Works Association (AWWA) and the European Committee for Standardization (CEN). The structural requirement of the liner is set to cope with the typical internal operating pressures found in Hong Kong water pipelines which range from 3 bar to 10 bar.

The specifications for close fit lining systems require meeting a mixture of prescriptive and performance criteria. The reduction of flow capacity should not exceed 3 per cent. Any reduction in flow capacity due to loss in bore cross-sectional area due to the thickness of the liner is typically offset with the smoother inside surface of the HDPE liner. Pipe material quality is specified to meet international standards.

Because customers have indicated they want a system that is repairable within the client’s water resumption performance pledge, an operation and maintenance manual and video demonstrating the repair method is required. In addition, technical matters such as ensuring that the ovality of reverted pipe does not exceed 2 per cent must be proved.

Concentrically reducing the diameter of the HDPE pipe to fit into a host pipe means that the diameter almost inevitably becomes non-standard. However, it is necessary after lining the host pipe to bring the liner back to a size which can be successfully electrofusion welded. It is essential to choose an electrofusion coupler that can cope with some variation in the external diameter dimensions of the re-rounded host pipe. The results obtained in testing pipes and couplers are shown in the following section.

In the series of tests, a number of pipe samples were processed by the proprietary systems, followed by reversion with pressurised water. Some of the samples as required by BS EN14409-3 contained a butt-fusion in the middle.

In addition to the hydrostatic pressure tests, 14 samples were also tested to determine the tensile strength of butt fusion joints – seven from the original pipe and seven from the processed pipe. All results showed satisfactory compliance with the standard.

Onsite in Hong Kong

Hong Kong’s existing water supply network is mostly located below busy city streets. Therefore, it is required that any system must be capable of being installed within this environment. Productivity, system compatibility, safety and the ability to negotiate some bends are all factors in determining whether a system is suitable for carrying out rehabilitation works in Hong Kong.

A site demonstration of the preparation, installation, reversion and fittings installation of both the InsituFold and InsituFlex installation methods was conducted in October 2007. The demonstration consisted of the simulated underground water pipelines. Two DN 300 mm pipes were lined using the InsituFold installation method and two DN150 pipelines with the InsituFlex installation system. This setup modelled the type of pipelines following cleaning that were anticipated to be encountered during the site work. An intermediate pit was formed at the bends so that the upper sections of the bends could be removed along the lines after insertion and reversion to illustrate what occurred at these points.

Because of the folding process the client was concerned that in operation there may be some restriction in flow for longitudinally folded pipe due to residual ovality of the previously folded pipe. A site trial was requested to demonstrate the compliance with the ovality requirement in a working condition. The requirement in the contract was that the pipe must be circular within 2 per cent. It was determined that there was no distortion in the circular shape for the concentrically reduced pipe, thus meeting the ovality requirement. Its circularity was not a matter of concern.

In the trial, an area at Location B was open between two host pipe sections to enable measurement of the in-pipe condition. The pipe was then pressurised and maintained at 10 bar during measurement.

Measurements taken at several locations confirmed the results met the contract criteria.

The client rightly took a cautious approach in accepting new techniques. Various tests, demonstrations and trials were conducted to verify the characteristics of the various methods. These included elements that related to:

• The material properties of the processed pipe
• Installation
• In-service operation
• Maintenance.

Major steps in order to gain the acceptance of the rehabilitation system have been successfully undertaken. The InsituGuard system met all requirements and is now being used on a number of contracts within Hong Kong.