Significant population growth and its concentration in major cities, combined with a better appreciation of environmental issues and protection, demand a better way to provide water and wastewater services. As our infrastructure ages and access to water resources grows increasingly uncertain, we need innovative, cost-effective and minimally-disruptive methods to protect our water supplies.
Water is one of our most important resources, and its transport and access is a major concern in both the developed and developing world. Water is the lifeblood of our cities and communities — but it relies entirely on a well-functioning circulatory system. Water conveyance systems move water and wastewater from points of collection to facilities for treatment, and finally for distribution or disposal. Water supply tunnels, aqueducts, intake and outfall structures, interceptors, trunk and drainage pipelines are major components of water distribution. Pumping stations and facilities for flow control, surge abatement, collection, storage, and disposal are also a necessary part of the system.
However, much of this critical infrastructure is not easily accessible for maintenance or repair. Aging infrastructure has become an increasingly crucial issue, because when this infrastructure is underground and needs to be replaced or rehabilitated, access can be a problem. Often times, in the intervening years since its initial construction, roads, utilities, and other infrastructure have been built above, restricting our options for underground access.
When this occurs, traditional open-cut construction may not be suitable for pipe replacement or rehabilitation for any number of factors:
- The underground space in the public right-of-way is congested with pipelines and utilities
- Traffic management in urban areas
- Street pavement damage and the cost of surface restoration may be prohibitive
- Located under runways or taxiways which prohibits road closure
- Direct and indirect business losses
- Large amounts of soil that could be contaminated
In these cases, trenchless technology is the solution. Trenchless technology minimizes the construction impacts associated with a project, and causes less surface disruption. There are significant benefits to using trenchless technology — and many different methods that can be used.
Tunnel Boring Machines (TBM)
One method of trenchless access is by using Tunnel Boring Machines (TBM). TBMs are used to excavate tunnels with minimal disturbance to the surrounding ground. Compared to the cut and cover approach, TBMs significantly reduce the disturbance of traffic and the associated environmental impacts in urban areas. They can be used to tunnel through anything from sand to hard and competent rock.
For deeper, longer tunnels in urban areas, or for a tunnel crossing major bodies of water, a pressurized-face tunnel boring machine is the best fit, because it is capable of handling the full range of expected ground conditions. A single-pass, precast concrete segmental lining forms the tunnel behind the TBM. The selection and design of the precast segmental liner is critical for successful application. The segments are equipped with waterproofing gaskets and act as the structural support system and water barrier.
There are two major TBMs used in soft-ground tunnelling: Earth Pressure Balanced (EPB) and Slurry Type Shield Machines. An EPB TBM will perform better where the ground is silty. A slurry TBM is ideal in loose water-bearing granular materials. However, with the application of appropriate ground conditioning agents, the range of ground conditions for each machine can be extended. TBM technology has advanced significantly in the last 15 years, allowing for the construction of larger, deeper, and longer tunnels in more difficult ground conditions.
This technique has been used in several WSP projects including the Twinning of West Trunk Sewer (Region of Peel) 3.0m & 2.7m diameter and Bathurst Langstaff Trunk Sanitary Sewer (York Region) 2.7m diameter.