Tunneling expert Dr. Anthony Harding describes a recently conducted research project to investigate a more environmentally friendly alternative for designing non-ferrous, low carbon segmental lining for tunnel construction.
By: Dr. Anthony Harding, CH2M HILL principal tunnel engineer
Around the world, from the United States and Canada to the United Kingdom, Singapore, Australia, the United Arab Emirates, and beyond, some of the world’s largest tunnels connect people and places, and keep rivers clean from sewer overflows. When designing these tunnels, engineers must take into account the loadings, pressure and long-term durability requirements that will be placed on the tunnel over its lifecycle.
Tunnel segmental linings, commonly associated with Tunnel Boring Machines (TBMs), form the wall of the tunnel and are used to support the ground outside the tunnel. Segmental linings come in a variety of shapes and sizes. Usually linings consist of precast concrete segments or are made from cast iron or steel.
A team of Australian tunnel experts recently conducted a research project to investigate a more environmentally friendly alternative for designing non-ferrous, low carbon segmental lining for tunnel construction. Rather than using conventional concrete made with ordinary Portland cement, the team used geopolymer concrete, an innovative material which relies on minimally processed natural materials or industrial byproducts to drastically reduce its carbon footprint by reducing the carbon dioxide required to produce the material by 70%. They also used macro-synthetic fibres (plastic fibres) in the concrete to eliminate the need for a steel cage.
Traditionally, steel bolts have been used to facilitate the build of segmental linings, and while bolts on the circumferential joints have been replaced with plastic dowels in recent years, bolts are still used in the longitudinal joints because a practical alternative has not been identified. The research project aimed to address this issue and provide the missing link by developing a practical non-ferrous alternative so all steel bolts could be removed.
First, the industry was canvassed widely in order for the project team to understand the requirements the bolts fulfil and determine how a non-ferrous solution might improve the design. Surprisingly, the team discovered little consensus among experts on what the bolts actually do. The original function of bolts was to hold smaller segments, with no other support, together until the ring was complete and grouted into the ground. However, building techniques and segment sizes have changed over the years, while bolts are relatively unchanged.
In addition, technology has evolved over the last 20 years, and today mechanical segment erectors and rams on the TBM hold the segments in place during the construction phase. Therefore, bolts are no longer required to hold the rings together like they once were.
Despite the absence of a consistent view on the contemporary function of bolts, sufficient requirements were identified and the research team came up with numerous ideas to solve the problem, including two promising solutions—one of which is installed on the segment just prior to erection and push fits into position as part of the normal erection process. The other is post-installed onto the face of the segments. Both solutions save erection time, as well as eliminate steel from the linings. We’re currently working to develop a prototype for testing on a tunnel construction project.
Dr. Anthony Harding is a principal tunnel engineer at CH2M HILL. He has broad experience in the delivery of tunnel projects, spanning road, rail and water sectors, with particular expertise in Tunnel Boring Machine tunnels. He led the design of the world’s largest solely steel fibre reinforced concrete segmental lining for the Brisbane Airport Link project, saving the client money and improving durability of the finished tunnel. He also led the design of the Blue Plains Tunnel project in Washington, D.C. and a novel shotcrete opening support in a TBM tunnel that allowed the TBM to remain operational during cross passage break-out and construction. He is located in the firm’s Brisbane, Australia office.