Going underground

The tunnelling industry has enjoyed a renaissance in recent years, with projects being completed across the globe. Chris Goatman, Kurt Zeidler and Dominic Walkling share their thoughts on innovation, improvements in technology, and what’s ahead for the industry

In the world of tunnelling, Bechtel and GZ continually seek to push the boundaries of what is possible by utilising innovative design, enhanced construction techniques, and emerging technologies for greater efficiencies, improved safety, and better value for money for customers.

Some of the biggest tunnelling advances have come from gaining a better understanding of the versatility and performance of existing materials. The advantages and performance of using Steel Fibre Reinforced (SFR) concrete has been known for years however, its use has typically been limited to small and medium sized tunnels. Bechtel and GZ have been among the early adopters of using SFR in secondary concrete lining for tunnels with a span greater than ten-metres. They have demonstrated not only its viability but also the economic value of steel fibre-only reinforcement in larger tunnels- a step-change for the industry.

The primary advantage of SFR-only concrete, whether cast-in-place or sprayed (shotcrete) linings, is the absence of the costly, resource heavy, and time-consuming installation of rebar reinforcement following the tunnel shape and improved shrinkage control. Even under onerous ground and groundwater load conditions, efficient tunnel linings can be designed and implemented at substantial tunnel sizes. Also, the avoidance of shadowing when using sprayed concrete constitutes a major advantage in terms of the durability of the tunnel linings.

When correctly selected, pre-cast concrete continues to demonstrate benefit in time and costs, particularly for the construction of Emergency Egress Shafts (EES) where constrained space and schedule have required an innovative approach to permanent shaft wall support.

On one recent Bechtel project ESS needed to be more than 30-metres deep, rectangular in design (with internal dimensions of approximately 11-metres by seven-metres), and groundwater heads of 28-metres. Restricted space imposed size restrictions on the lifting equipment and thus weight constraints on the precast panels. The team used hollow-core panels each with a height of 2.1-metres to build the shaft walls. Steel columns were installed inside to facilitate load distribution across strut levels and floors. The hollow cores were filled with high-strength grout and waterproofed. Each panel was equipped along its entire perimeter with sealing strips and re-groutable hoses to seal the joints which are expected to be subject to water pressures of up to 2.8-bar. The use of pre-cast shaft wall panels, structural steel support in combination with pre-cast floor slabs and stairs yielded significant construction time reductions compared to traditional cast-in-place concrete.

Whether bored or mined, the importance of consistent, real-time monitoring and analysis continues to play a vital role in tunnelling projects. The use of robotic theolities to provide continuous real-time measurements of any potential tunnelling induced ground movement through prisms has been common place on major projects for over a decade. Developments around the use of satellite monitoring and Radar Satellite Interferometry (RSI) – a non-invasive surveying technique using Synthetic Aperture Radar images (InSAR) – had many excited however, the frequency of data capture – often weeks – let it down. Now, with more satellites and specialist companies like Sixense, giving high frequency data capture, the industry has another viable option. RSI is becoming increasingly preferable and, in some instances, it is replacing the need for optical systems. The main benefits of satellite monitoring are being able to: monitor a larger area; use older satellite imagery to establish baselines, over years, prior to start of tunnelling to allow for ground movements due to seasonal trends and historic movements to be established. RSI can also quantify the impacts from external works not linked to the project. Unlike prisms installed on buildings around works, RSI is invisible, so third-party consent is not needed for installation and maintenance.

Going forward the use in InSAR to support conventional monitoring during tunnel drives and to fulfil long-term monitoring requirements post-tunnelling is likely to provide further benefits.

Monitoring pictures from recent UK projects are below:

Bechtel is also working with specialist instrumental and monitoring companies including Sixense on passive seismic monitoring – a first of a kind application of this technology to infrastructure. This is an innovative way to detect changes in the properties of the ground over large distances using a network of geophones. By studying variations in the ambient seismic signals, changes in the properties of the ground can be inferred and high-risk areas identified.

The future
Infrastructure investment by governments in the short to near term – as well as pipeline of future projects – remains crucial. Only then can the industry capitalise on the recent growth of expertise, specialist designers and world-class contractors.

In terms of technology, Bechtel and GZ are particularly excited about the potential safety and cost implications of further advances in, and use of, remote technology. Whether that be satellites for monitoring or remote-controlled robotics that will enable monitoring and control of tunnelling from thousands of miles away.

Chris Goatman works at Bechtel, Kurt Zeidler at GZ Consultants and Dominic Walkling at Sixense. Bechtel is a trusted engineering, construction and project management partner to industry and government. Differentiated by the quality of its people and its relentless drive to deliver the most successful outcomes, it aligns its capabilities to its customers’ objectives to create a lasting positive impact. Since 1898, it has helped customers complete more than 25,000 projects in 160 countries on all seven continents that have created jobs, grown economies, improved the resiliency of the world’s infrastructure, increased access to energy, resources, and vital services, and made the world a safer, cleaner place.

For more information, please see www.bechtel.com