Microtunnelling projects for the installation of new underground infrastructure are often motivated by costs. The chosen method has to either maximise the present value of the net benefits, or minimise the costs of providing the service. Traditionally, open-cut excavation methods were used for such projects, however, these methods are expensive. With asset owners focusing on costs, there are a number of areas to be considered where trenchless technologies such as microtunnelling can provide a more cost-effective solution.
Unlike open-cut methods, which require trenches to be dug to match the length of the pipe, microtunnelling has a smaller site footprint, as it only requires an entry and exit pit. In terms of costs, especially in high-density areas, the reinstatement of surfaces such as sidewalks, pavement and landscaping is expensive – it is estimated this accounts for around 70 per cent of an open-cut project’s direct costs. As microtunnelling results in less surface disruption, the costs of site rehabilitation are greatly reduced with less human labour, excavation and backfill costs.
Time is money and the longer a pipe installation takes the more it will cost. The need to dig up and refill large sections of land for open-cut methods increases the duration of a project. Microtunnelling can reduce construction costs by decreasing the amount of time personnel and equipment are required on site. While timing for each project differs depending on specifications and site conditions, it is possible to complete projects such as the installation of an enveloper pipe and/or gravity sewer in as little as 48 hours.
In high-density areas, open-cut methods can cause major disruption to businesses and the general public, leading to increased social costs. These social costs may include disruption to traffic and to business activities, damage to existing paved surfaces, adverse environmental impacts, and disruption to normal life patterns of the people living, working and shopping around the construction zone. While these costs are generally paid for by the community and not realised as a direct project cost, it is important to take this into account when choosing a method. Other social costs can be directly related to the project, such as traffic management.
The cost to excavate in rock is often very expensive on a typical open-cut project, with time, excessive noise, and the total vibration of the process key issues on such jobs.
Microtunnelling is increasingly becoming a viable alternative due to reduced costs and production in such projects. By only coring a hole through the rock rather than excavating trenches through layers of rock there are significant cost and productivity advantages.
Similarly, in wet flowing ground, like below the water table and in sand/silt, the cost of dewatering and shoring, especially where sheet piling is required, is expensive. Microtunnelling is seen as a cost-effective solution as only the launch and exit pits require dewatering.
Contaminated ground is another area where microtunnelling has distinct cost advantages. Excavating the absolute minimum amount of ground necessary to install a pipe reduces the amount of contaminated ground that needs to be treated.
Whenever excavation starts getting deeper we see more reasons to look at microtunnelling. The cost of microtunnelling changes minimally with depth as opposed to open-cut. As a result there is a point of depth where microtunnelling is more cost-effective. The depth this occurs varies depending on ground conditions, job location and other obstructions. In an urban environment in a road reserve area it can be at 2m deep, and in a greenfield setting it might be 5m plus.
Obstructions (surface or subsurface)
The ability to create a keyhole pipeline is an advantage of microtunnelling for areas where there are surface obstructions such as roadways, power poles, trees, lakes, waterways, hills, and other structures, and subsurface obstructions, including other utilities, pylons, and foundations. The keyhole pipeline involves excavating a small shaft in accessible areas where obstructions can be navigated. This allows the pipe to be installed around any congestion, as minimal ground needs to be excavated to install the pipe.
With open-cut methods becoming increasingly expensive due to rising indirect costs of fuel, spoil waste disposal, and environmental and social impacts, microtunnelling provides a cost-effective option for pipe installation.
The points that have been discussed are only some of ways that microtunnelling can reduce the costs of a project, with the Louisiana Technical Institute estimating – based on bid data – total overall savings of up to 75 per cent can be realised.
There are of course other non-cost related advantages such as environmental advantages and minimal invasion on cultural heritage issues
About Stuart Harrison
Global microtunnelling pioneer Stuart Harrison is the Managing Director of Edge Underground, where he specialises in on-grade microtunnelling installations with millimetre accuracy. Stuart is also the inventor of the Vermeer AXIS Guided Boring system, and he is constantly working to improve the effectiveness of this and other trenchless systems used in the installation of gravity sewers. To discuss your next microtunnelling installation, contact Stuart on 1300 JACKED or at [email protected].