No-dig solution for Colombo’s colonial sewers

The need for the rehabilitation of sewerage pipelines in Sri Lanka’s capital, Colombo, became clear in 1997 during a study performed by UK consulting company WS Atkins International. In addition, subsidence of roads due to collapsed sewerage pipelines further proved that rehabilitation was necessary.

The main sewerage pipelines were built over 80 years ago and their structural condition had deteriorated considerably. Atkins’ study also proved massive infiltration of groundwater into the sewerage pipelines, estimated by the client, the National Water Supply & Drainage Board (NWS&DB), to be 20-40% of the total flow.

The Colombo Sewerage Rehabilitation Project (CSRP) was awarded to the Danish contracting company, Per Aarsleff, in November 2004. NWS&DB invited bids for the project in February 2004 after securing a soft loan from the Danish government through the Danish International Development Agency (DANIDA).

Scope of work

The project value is approximately e22 million and work began on 9 March 2005. The project period was 32 months and the scope of work consisted of rehabilitation of the most critical sewerage pipelines in the northern catchment of Colombo and the largest pumping station in the city, as well as rehabilitation of one pumping station in the southern catchment.

The scope of work included in the project was selected on the basis of the earlier classification of the pipelines as critical, and according to the severe impact that failure or collapses would have on the city’s entire sewerage system.

Pumping stations

The construction of the new pumping station at Madampitiya was made difficult by the hard, metamorphic gneiss rock in the area and the depth of the existing main sewerage pipeline. The construction of the pumping station was therefore done inside a cofferdam constructed by secant pile walls and heavy steel shoring.

The new pumping station will be equipped with coarse screen, automatic fine screens and vortex for removal of sand. The new pumps will transport the sewerage to an existing collection chamber, from where it is discharged into the sea.

In this existing pumping station, electrical and mechanical equipment will be replaced to accommodate the projected incoming flow for year 2020. Both pumping stations will be equipped with frequency converters and a SCADA system to optimise pump efficiency and thereby reduce the running costs of the pumping stations.

Pipelines

The most critical sewer pipeline in Colombo was established in 1906. It is located under the main roads of the city. The topography of Colombo results in shallow slopes for the pipelines and is also the main reason for the critical condition of the sewers at a depth of 5-10m.

Combined with a high groundwater table, the depths of the sewers would make replacement by conventional excavation difficult and costly. Furthermore, excavation of the sewers would disturb traffic to an unacceptable level, considering the location of the pipelines.

It was therefore decided to use trenchless (or no-dig) technology, which, it was estimated, would be 7-15 times faster than replacement by excavation. Based on the 1997 surveys, the requirements for no-dig methods were decided:

• Self-sustaining rehabilitation; new products should be able to sustain all external and internal loads independently
• Product life expectancy of a minimum of 50 years
• The flow capacity of the existing pipelines must not be reduced.

After cleaning and CCTV inspection of the existing sewer pipelines, two methods were selected for the rehabilitation of the sewer pipelines: cured-in-place pipe (CIPP) and glass-fibre reinforced plastic (GRP).

CIPP

For CIPP, a new pipe is installed inside the existing pipeline. The flexible liner is first impregnated with resin, then taken to the installation site, installed and cured on site. On this project, CIPP for small pipelines was used, ranging from circular 450mm to egg-shaped 1050 x 710mm pipes.

The dry liner is made of polyester felt and impregnated with polyester resin while under vacuum. A CIPP installation takes 3-4 days, depending on the dimension and length of the rehabilitated section. This timescale includes setting up necessary installation equipment, inversion of the liner, curing, cooling and reopening of the rehabilitated section.

The technical improvements to the sewer system consist of:

• New self-sustaining pipeline with an expected life of more than 100 years
• Elimination of groundwater infiltration
• Increase of flow capacity by approximately 10%
• Reduced siltation due to improved self-cleaning effect.

GRP

The second no-dig method used on the project was GRP. Aarsleff GRP involves filament-winding technology. The pipes up to 6m long are produced by a computerised filament-winding machine at a factory close to the installation site.

After initial curing, the 6m pipe is cut into smaller pipes and the socket and spigot ends are ground. The specially produced socket is installed in the socket end of the pipe and a special gasket is mounted in the spigot end.

The pipes are lifted into the existing sewer pipeline, transported to the correct location, placed and jointed with the other GRP pipes.

In principle, a new pipeline is constructed inside the existing one; the GRP pipes are transported inside the existing pipeline using a specially constructed trolley. The annular space between the existing and new pipelines is filled with grout after installation of GRP in the entire section.

The trolley is also used to connect the GRP socket and spigot ends. This method reduces the cross-sectional area of the existing pipeline by about 10-15%. Due to a much lower friction in the new pipeline, the flow capacity is actually increased by up to 5%.

The technical improvements to the sewer system are:

• New self-sustaining pipeline, with an expected life of more than 50 years
• Elimination of infiltrating groundwater
• Reduced siltation due to improved self-cleaning effect.

Benefits

The benefits of the CSRP is that future road collapses are unlikely and that the sewerage system is undergoing continual repair. Furthermore, there are now lower operational costs at the pumping stations due to reduction of infiltrating groundwater. Better management of sewerage means that there is a lower rate of contamination of surface water by sewage.