A combination of three tunnelling methods is being used in north Manchester to provide a long term solution to sewage overflow..
Residential sewage from housing estates around Prestwich will soon no longer pollute a local watercourse in times of heavy rainfall. A 900m long replacement underground pipeline will direct sewage into a hand-shield excavated overflow storage tunnel, designed to safeguard the nearby Singleton Brook, a tributary of the River Irwell.
The sewer consists of five distinct sections of pipeline totalling 665m in length, made up of four microtunnelled drives with a pipejacked tunnel between sections four and five. The pipejacked section followed the line of an existing sewer and effectively served to enlarge this for greater flows.
The project, the Prestwich Golf Course combined sewage overflow scheme, is part of North West Water's pounds2.84M project to replace an ageing Victorian system which discharges sewage to the Brook in times of heavy rainfall. The new system was designed from specifications set down by technical consultant Bechtel.
Design and build contractor Amey Construction reached a milestone on the project earlier this month, when a Herrenknecht micro 8VN 800 tunnelling machine completed the first of the four microtunnels, a 175m long, 975mm diameter section. The machine broke through the concrete outer casing of a second access shaft on 7 October, six weeks after boring began from the first 9m deep access shaft. Site manager Jim Yerkiss says success was due to careful planning and a sophisticated guidance computer.
'The machine was directed with a laser controlled guidance system, which aligned a laser on the front of the cutting head with a fixed laser in the first access shaft. Our engineers had to keep the machine within specified tolerances of 75mm horizontally and 50mm vertically. We managed to keep within 34mm horizontally and were spot on vertically,' says Yerkiss.
Microtunnelling was chosen in preference to pipejacking for safety reasons as Yerkiss explains: 'The internal diameter of the sewers to be microtunnelled is 975mm - too narrow under health and safety regulations for a person to safely operate underground.'
The tunnelling machine has an automated rotating cutting head with 13 teeth. Stiff clay is loosened from the cutting face by pumping water through one of two pipelines. The second pipeline collects the slurry and feeds it back into a vibrating desanding unit, which separates the mix into earth for landscaping and wash water.
Large glacial boulders slowed the machine, but Amey found that by electronically increasing the torque, the cutting action could be applied with a much greater force. After every 2.5m length of microtunnelling, a precast sewer ring was lowered into the first access shaft by mobile crane, and jacked into the cavity behind the machine. A pressure of 200t was required to insert the first pipe section, with each subsequent ring requiring an additional 2.5t of pressure.
The same technique will be used for the remaining three microtunnels.
The more traditional process of pipejacking was used to enlarge the existing sewer. Operatives used handheld pneumatic tools at the face to increase the internal diameter to 1.2m. Precast concrete pipes were inserted using a jacking rig.
Amey has also recently completed the new 250m long overflow storage tunnel, at the end of the pipeline.
Once the contract has been completed, Singleton Brook will meet Environment Agency water cleanliness standards. When other such projects in the area have finished, it is hoped trout will return to the River Irwell. The project is scheduled for completion in January 1999, although Amey is already six weeks ahead of programme.
Mike Walter writes for Barrett, Byrd Associates.