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Graphic element Research > Growth > Research projects > Previous projects > New Products and Materials > Concrete improvements for complex tunnel designs
Graphic element Concrete improvements for complex tunnel designs
     
 
Theoretical and practical research has improved the mechanical performance of sprayed concrete and created new confidence in the material as a combined primary and secondary tunnel lining. As a result, tunnel designers now have more flexibility when designing complex junctions and access points. This could lead to improved underground transportation systems, and has already been used in one project to provide greater access for those passengers with limited mobility.
 

Sprayed concrete has been widely used by the European construction industry in stabilising rock tunnels. It has also been used as a primary liner for soft ground tunnels, but has never been seriously considered as part of the permanent works. This is because of a lack of confidence in the long-term structural and durability performance of the material and the absence of a suitably detailed modelling and design process that takes into account the complex interactions between ground and lining. The process does, however, have the potential to provide tunnel designers much greater flexibility in terms of complex tunnel junctions and passenger access points in underground transportation networks. This potential created an impetus to examine the possible problems with a view to finding solutions and, as a result, a consortium of European construction companies and researchers came together under a Brite-Euram project. After four years of work, new equipment and formulations have now been developed that allow sprayed concrete to be used to its full potential.

Finding the right recipe

One of the first aims of the project was to characterise concrete mixes with respect to strength, durability and application properties. The project co-ordinator, Mott MacDonald, a UK construction and civil engineering company, worked closely with British SME Sprayed Concrete on this aspect. Sprayed Concrete specialises in wet application techniques. This involves mixing the concrete in a conventional manner and then adding air and an accelerator at the spraying nozzle. The technique produces far less dust than the alternative 'dry' application whereby water is added at the nozzle. Consequently, it is better for both the health of workers and in terms of material wastage.
A large number of concrete mixes were characterised and several important guidelines for mix design were established. The mixes were further refined with help from two other project partners, Taywood Engineering Ltd, another UK civil engineering and construction company, and Imperial College of Science and Technology in London. As a result, the project identified a shortlist of mixes with good durability properties, extremely good flexural toughness, and tensile strengths that were improved with steel fibre reinforcement.
Project partner Dragados, a Spanish construction company, then put the mixes to the test in Spain. Favourable results there proved their potential for the pan-European market, an important factor given the regional variations in aggregate types and quality, and in cements. Dragados has subsequently benefited from its involvement in the project by being able to market improved pre-bagged cement products to its national sprayed concrete market.

The age question

One of the key aspects of the research was the need to develop a time dependent behavioural model which can be used for design purposes. Consequently, the research partners, Imperial College and the Institute of Mechanics, Materials and Geostructures (IMMG) in Athens developed mathematical models to describe and predict the mature behaviour of sprayed concrete based on extensive testing of conventional cast concrete. A series of large-scale tests which involved spraying circular fibre reinforced concrete linings on to a simulated clay surround, were implemented by Taywood to generate data to calibrate and update the models. Further work on ageing is now ongoing at Mott MacDonald.
IMMG also looked at the characteristics of the concrete in the first few hours after application and discovered that the concrete has considerable strength and stiffness at only two hours after spraying. As a by-product of this work an instrument capable of providing reliable in situ strength predictions based on the bearing strength of the concrete surface was developed.

Giving confidence

The research has provided good grounds for users to be confident in the use of sprayed concrete, either individually or in combination with other techniques, as an important element in tunnel construction.
Since the completion of the project, Mott MacDonald has been involved in a number of construction projects that have specified sprayed concrete, including an access shaft and non-circular tunnels designed particularly for passengers with limited mobility at Paddington Underground Station in London. Outside of the UK, the company has designed both primary and secondary fibre-reinforced spray concrete linings in the eastward extension of the San Diego LRT, and a primary liner in the Kastanousa Rail Tunnel in Greece.

Cordis RCN: 6716
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