Breaking the mould, not the surface
Cory Higgins, founding director of Terebro, recounts some recent triumphs for trenchless technology
Asingle factor - government policy on new housing development - is enough to dictate that the majority of sewer pipe installation will take place in areas of redevelopment, or at least within existing conurbations. That spells problems for open-trench installation, as main sewers typically follow road networks that are already liable to excavation by other service contractors.
Water companies acknowledge the advantages of trenchless techniques, such as auger boring and microtunneling, in minimising impact on the travelling public, infrastructure and the environment. Framework agreements and supply chain management appear also to have helped trenchless pipe installation become established. Even so there is little doubt that, in the UK, excavation is still the first-choice method for constructing new sewers.
It has been estimated that open-trench work incurs losses, in terms of disruption, environmental impact, spoil-tipping and transport costs, of £2B/pa. In Terebro's experience, centred on auger boring, the prevailing opinion is still that trench-digging is the least-cost option, despite the evidence against such a contention.
A city built on sand
The opposite is true in Germany, for instance, where Terebro's Bohrtec- guided auger machines are produced. There the open-trench approach is becoming a last resort for the utilities, and in Berlin excavation as a first choice is outlawed, with the result that auger boring is often specified at the contract tendering stage. The process now accounts for 90% of the gravity sewer installed in Berlin.
As the city lies on sandy clay soils, the procedure is simplified, due to the soft, consistent ground conditions. This allows contractors to operate without subcontracting the auger boring to specialists. If they prefer not to tie up capital and maintenance costs in auger boring machinery, the rigs can be hired individually, along with an operator. This approach is also increasingly favoured in the UK.
But what about other trenchless techniques? Directional drilling is an alternative to auger boring, but it requires a fall of at least 1 in 100 to ensure that a gravity sewer will function properly. This is because the drill tends to waver off-line if it hits a stone or rock, introducing sections with a shallower gradient that may need periodic swabbing out in service. The problem is that a fault like this can only be detected by CCTV, after the sewer has been installed.
Using laser guidance technology, auger boring is far straighter than directional drilling, and it is capable of a high degree of accuracy. Special measures, such as double pilot rodding and bentonite injection, can also be used to assist in hard ground conditions.
The advantages of the method were illustrated recently when an auger boring rig and operator were hired by Barhale Construction from Terebro Trenchless Solutions. Two auger bore drives were completed for new sewers as part of an £8M flood alleviation scheme in the south-west of Derby.
The overall scheme, partly under Sinfin Golf Club's course, provides a new relief sewer to reduce flows within the existing network. The route takes in approximately 3.9km of new sewers, ranging in size from 600mm to 1,500mm, through the golf club, school grounds, recreation ground, fields and highways. The auger bore works, for an additional 150mm-diameter sewer, were carried out at the clubhouse.
The game must go on
The original plan was to install the sewer by open cut, at depths of up to 3m, across the main entrance to the clubhouse and adjacent putting green. The alignment then passed through an area of trees and crossed an access road. As this would have been highly disruptive to the golf club's members and staff, contractors Barhale Construction, in partnership with North Midland Construction, proposed auger boring for these sections. Approval was given by the client, Severn Trent Water.
Terebro made two drives with a combined distance of 80m, using one of their optically-guided BM400 augerboring rigs, and installed 150mm pipe sections. Using a trenchless method meant that there was no disruption, the clubhouse was available at all times, and the putting green was undisturbed. No trees had to be removed and root damage was avoided.
There was a time saving of approximately 1.5 weeks compared to open-trench methods. As a result, the additional cost of choosing trenchless installation was paid back in time saved, low reinstatement costs and reduced risks of service damage and compensation claims from the club.
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