KEEPING BLACKPOOL SAND IN ITS PLACE
- Blackpool, England's most popular coastal resort, is famous for its Tower, white-knuckle rides, illuminations and sandy beaches. The sand that attracts the tourists may, however, be less popular with the town's engineers.
- Much of Blackpool is underlain by sand and peat, and has a high water table. A consequence of this is that long-therm infiltration into defective sewers below the water table encourages a loss of fine material from the ground aruond the pipe, leading to settlement at the surface. Migration of fines can occur through leaking joints, fractures and defective laterals. In some cases there is an infiltration/exfiltration cycle resulting from periodic surcharging of the system, and this can result in the loss of fines into sewers and laterals both above and below the water table. These effects are compounded by larger-scale ground loss into the more serious pipe defects such as breaks and collapses.
- As part of its Management Contract under the Sewer Services Agreement with North West Water Ltd, Blackpool Borough Council has recently been tackling infiltration and ground erosion problems in parts of the Central and South Shore districts of the town. Known as the South Shore Sewer Renovation Scheme, the project was initiated because of long-standing highway settlement problems in various streets, requiring frequent regrading and repairs to the road surface. Serious deformation of the carriageway had been experienced along the line of several sewers, and the migration af sand into the sewers was suspected to be the cause.
- This chemical is then pumped out rapidly, and the sewer is refilled with a second non-toxic chemical (S2) which reacts with the residue of the first to form an impermeable gel. The effect is to create a waterproof mass around points of leakage eliminating infiltration and exfiltration and also enhancing structural stability. Once S2 has had time to react, the surplus is pumped out and the sewer is cleaned by water jetting to remove any residue of gel.
- Depending on the condition of the sewer and the nature of the ground, two or three applications of the system may be required to achieve a full seal. In severe cases, the first application may be carried out using a much reduced penetration time for chemical S1, in order to achieve a preliminary reduction in permeability and chemical loss. Most sewers treated in the Blackpool contract required either two and three applications for complete sealing.
- A thorough pre-survey is essential to locate all lateral connections and establish the level to which it is safe to fill the system, and wherever possible stoppers are inserted into the upstream end of laterals. An inflatable stopper is also inserted in the downstream end of the section of sewers to be treated and the required head is achieved by filling the upstream manhole to a predetermined level.
- O. C. Summers has made every effort to advise residents about the work, writing to everyone affected before the contract started and then contacting them individually 24 to 48 hours before their section of sewer was to be treated. Pumps are available on site if house connections do show signs of surcharge despite the warnings, but there have been few problems of this nature. Local Councillors have also been informed about the work, so that they can respond to any queries from their constituens.
- The chemicals S1 and S2 are brought to site in a road tanker divided into two compartments each holding approximately 9,000 l. If this is insufficient to treat a complete lenght of sewer, as may be the case with larger diameter pipes or long lenghts, the sewer can be divided into two sections with an inflatable stopper, and each section treated separately. Surplus chemicals are returned to the tanker for reuse, and various tests are carried out to ensure that they remain within specified limits. In practice, only about 30% of S1 is usually reclaimed, the remainder having penetrated the ground.
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- TARGET
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- Over two-thirds of the contract had been finished by the middle of February 1998, the time of No-Dig International's visit, and the project was on target for completition by the last week of March. Cris Doyle, O. C. Summers' Sanipor Agent, remarked, "The project has gone well to date. We were initially concerned that the consumption of materials in the larger diameter pipes would be rather high, but in practice this has not been the case and we have found them no more difficult to treat than the smaller sewers."
- Giving his impressions of the project, Will Britain said, "There has been excellent progress. The contract is on time and within budget, and we are also pleased with the safety aspects of the work."
- The project is a valuable demonstration of the contribution that trenchless technology can make to solving old problems costeffectively and with minimal disruption.
