Barnabas Ilko Member Name
Senior Structural Engineer
Trevor O’Shannessy Member Name
Principal Geotechnical Engineer
For more than half a century, trenchless construction has proved to be an effective, convenient, reliable and low-impact method to install underground utilities such as pipes and cables from point A to B. There are two main types of trenchless construction methods for installing utilities: Horizontal Directional Drilling (HDD) and the use of Microtunnel Boring Machines (MTBMs). Before you tackle your next project, it is important to understand the benefits and challenges and how these factors can impact your project outcomes.
How can you get the best out of these methods, avoid the common pitfalls and keep your project on track for success?
Understanding the two main methods of trenchless construction
Using HDD methods, small-diameter pipes and conduits can be launched from close to the surface and directionally steered through the ground, which is ideal for curved tunnel alignments such as pipelines that extend from shore to shore under rivers, crossings under major roads or environmentally-sensitive areas. This method is less well suited to gravity pipelines with tight grade tolerances or those requiring regularly spaced maintenance shafts.
Microtunnelling with MTBMs, on the other hand, is typically used to install larger diameter pipes (from approximately 300 mm to 5 m) via a vertical shaft into which the boring machine is placed and pipes are progressively ‘pushed’ or ‘jacked’ through the ground. This method can install pipework at a precise grade at depth and is typically used for sewers and other small-diameter utility tunnels. A variety of boring configurations can be used, as suited to the expected ground conditions.
Why go trenchless?
Microtunnelling or HDD are natural choices where underground utility pipelines for water, sewerage, gas, power or telecommunications need to be installed in dense urban environments, near linear infrastructure or beneath areas of high cultural or environmental value. They may even be the only possible methods when depth or surface conditions and obstacles prevent trenched solutions.
The primary advantage of going trenchless is that life can go on as usual above ground while construction proceeds below the surface. Traffic continues to flow, businesses operate without interruption, and communities are generally much less impacted compared to highly disruptive open excavation alternatives.
Trenchless methods are versatile and productive in a range of weather, suitable for most types of ground conditions, and can be used above or below the groundwater level. They are extremely accurate, reliable and safe and have a relatively low environmental footprint, with less environmental disturbance and community impact.
However, things can go wrong. A trenchless project can become extremely costly if ground conditions aren’t well understood, if the wrong equipment is chosen for the ground condition, or if the launch and reception shafts aren’t well designed and constructed. When an underground tunnelling project goes awry, the recovery costs, program impacts and reputational damage can be high, so it’s essential to invest early in making the process safe, smooth, streamlined and suited to the location.
To reap the rewards and minimise the risk of your project ‘taking a wrong turn’, ask yourself the following questions:
Do you understand the ground and groundwater conditions?
It might seem like stating the obvious, but it is imperative to find out as much as you can about the geology and groundwater along your tunnel alignment. This aspect alone may be the most critical factor in the success or failure of your project.
Key questions to ask include:
- Do you have sufficient geotechnical investigation to understand major ground risks? How much is enough?
- Does the geology vary along the route, what are the variations, and where do the changes occur?
- Might you encounter contaminated ground or naturally occurring ground conditions (such as acid sulphate soil) that would impact management and disposal of shaft and tunnelling spoil?
- Do you have a clear understanding of the groundwater conditions (e.g., pressure and permeability)?
An understanding of the ground is best established in stages, starting from reviewing inexpensive and easily accessible public information early in the project — such as conducting a desktop study — and progressing through to more intrusive investigations such as borehole drilling. This targeted approach enables ground risks to be identified progressively and accommodated during design and construction.
For larger complex jobs, the geological conditions can be modelled in 3D space together with the proposed tunnel alignment and other important project elements such as existing rivers, buildings, underground services (such as deep sewers), building basements, piles and so on. This approach offers greater visualisation of the project conditions and improves project decision-making.
Ultimately, there will always be some degree of geotechnical uncertainty once investigations are complete, so it is important that your choices can accommodate, as far as possible, unexpected conditions that might occur. This is best addressed by compiling a tunnelling risk register (which will be discussed below).
What is the right construction methodology?
In addition to weighing trenchless methods and ground conditions, it is important to weigh important elements related to construction, including the following:
- Tunnelling Plants – The boring plant (MTBMs) can be configured in a variety of ways depending on the dominant ground condition. MTBM types such as auger thrust boring plant are suitable for relatively stable ground conditions such as stiff soils and rock, however they can fail disastrously if used in soft clays, or saturated sandy ground. Slurry and Earth Pressure Balance (EPB) MTBMs are more technologically advanced to deal with potentially unstable soils and can therefore safely deal with a greater range of ground conditions, however they are more expensive to use and require greater amounts of room at the surface for spoil processing. Careful consideration of the right tunnelling plant therefore needs to be given to match the project ground conditions and budget.
- Shafts – Shafts can be a major expense and risk in the project delivery phase. For microtunnelling projects, there’ll be at least one launch shaft at the beginning of the tunnel and a reception shaft at the destination. Shafts need to be sized sufficiently to accommodate the choice of MTBM, thrust block/frame, soft eye, seal frames (if applicable) and the pipe jacking system. Shafts construction methods must be tailored for the anticipated ground conditions. Sealed shaft systems may be required in deep, saturated alluvial soils, whilst shotcrete and rock bolts may offer a suitable solution in rock. Ideally, shafts should be located to minimise disruption to the community and the environment.
- Temporary Works – Close collaboration between the geotechnical engineer, structural engineer and contractors is key to optimal temporary works design and staging. Depending on the application, there may be opportunity to incorporate the temporary and permanent works requirements into a single solution to create a one-pass lining system, avoiding the need for a more expensive two-pass solution.
Have you considered the full range of risks?
Trenchless and underground construction require all project parties to be aware of and manage unknowns and uncertainties. It is important to understand all constraints above and below ground before construction commences. The relocation of shafts and tunnel alignments can be very costly if incorrect assumptions are made during the design development. However, there are tools for the avoidance of such mistakes, such as a risk register.
The risk assessment process and preparation of a project risk register should be carried out in the early phases of the project. The purpose of the register is to identify risks and clarify ownership so they can be adequately allocated and controlled during the delivery phase. The International Tunnelling Insurance Group prepared a guidance document on this subject, titled A code of practice for risk management of tunnel works. This document helps with the understanding and planning purposes of dealing with hazards on tunnelling projects.
Geology is a key risk when going trenchless; however, there are other important factors that can make or break a project and are often overlooked. These include statutory regulations, project program and alignment selection, tunnel spoil and groundwater disposal, noise, vibration, siting of major shafts, third-party impacts and community consultation. Start the conversation early about major risks that can impact the success of the project.
What advice and support do you need?
You are far more likely to achieve success if you draw on a range of reputable specialist expertise. This should include experienced trenchless specialists with good understanding of design and construction issues to help you mitigate the assessed project risks, particularly in relation to ground conditions.
Successful trenchless projects are as much about the preparation and planning as about technology choices and building methods. Don’t be in such a rush that you risk encountering nasty surprises lurking under the surface – at point A, point B, or anywhere between.