use of macro synthetic fibre reinforcement in underground civil construction, The

Concrete, Apr 2007 by Ridout, Andrew

The Scandinavian tunnelling market has traditionally been a leader in the use of new materials and tunnelling techniques. Macro synthetics for sprayed concrete reinforcement have been embraced by Scandinavia and Norway in particular.

It has only been within the last ten years that macro synthetics have been able to compete with other types of sprayed concrete reinforcement such as steel fibres and welded wire fabric. This has been achieved by using specialised fibre manufacturing techniques and polymeric materials resulting in macro synthetics achieving similar performance at a lower cost. Macro synthetics also have the advantages of being non-rusting, light and easy to handle, and reduce the wear on pumping equipment.

The specification for sprayed concrete in Norway is Publication No 7(1) that calls for two classes of energy absorption in an EFNARC(2) panel - E700 and E1000, corresponding to 700 and 1000 Joules respectively at 25mm deflection of the panel.

The work towards identifying macro synthetics as a replacement for steel fibre commenced in Norway in 2002 when Veidekke Entreprenør AS, Heavy Construction Division, undertook a full research programme to evaluate the performance of these fibres in sprayed concrete reinforcement. Since then several other subsea tunnels have either used or have been specified with macro synthetic fibre including the North Atlantic tunnel at Kristiansund in north-west Norway, the Finnfast tunnel just outside Stavanger in southern Norway and the Halsn0y tunnel in western Norway.

Vadheim test programme

The tunnel chosen for the test programme was the E39 Bogstunnelen in Western Norway, a 3.5km tunnel between Teigen and Bogen at Vadheim. Testing was carried out using both macro synthetic and steel fibres by spraying test panels as well as a full-scale in-situ trial in a working tunnel.

Christine Hauck, of the Heavy Construction Division of Veidekke, says panel tests, as part of the project, show that the energy absorption classes of E700 and E1000 were fulfilled by dose rates of 5kg/m^sup 3^ and 7kg/m^sup 3^ of Barchip Kyodo macro synthetic fibre respectively. Both these results were comparable to steel fibre dosages of 25kg/m^sup 3^ and 35kg/m^sup 3^ for the same energy absorption classes. Once the panel tests had been completed successfully, a full trial could be undertaken in the running tunnel.

A 100 linear metre length of tunnel was fully sprayed with alternately macro synthetic fibre reinforced shotcrete and then steel-fibre-reinforced shotcrete. This ensured as far as possible that the sprayed concrete of both types was subjected to the same ground conditions and therefore loadings. The in-situ sprayed concrete was monitored for six to eight months and it was concluded that there was no difference in the sprayed concrete crack pattern although the steel fibres on the surface and in some localised cracking showed significant rusting. This comprehensive test programme proved the ability of macro synthetic fibre for sprayed concrete reinforcement as an alternative to steel fibre.

Halsney tunnel

In 2005, a contract was awarded for the construction of the Halsn0y tunnel in western Norway. The tunnel crosses the Hardanger fjord from Sunde to Halsnøy Island 135m below sea level. The single-pass two-lane tunnel has a cross section of approximately 90m^sup 2^ and excavation was undertaken by the drill-and-blast method.

Dependent on the ground conditions, which were monitored after each blast, sprayed concrete was designed to meet the specification of either 500 or 700 Joules (EFNARC panel(2). Macro synthetic fibre had proved it could meet the specification but as the Halsnøy tunnel passed beneath a fjord it had the added advantage of not being prone to the corrosive effects of sea water leaching through the rock. Initially, just the portals were to be sprayed with macro synthetic fibre but as the initial works progressed well they were used for the full tunnel. In excess of 35 tonnes of fibre was used.

Sweden

One project to use macro synthetic fibres for sprayed concrete reinforcement in Sweden is the Årstaäng Bergrumsgarage (Underground Car Park) project in Årstadal, south of Stockholm city centre. This project by Frijo Entreprenad AB involves the conversion of three large caverns, originally used as cold storage for ice cream, to an underground car park to serve the growing redevelopment of the area.

The caverns vary in size from 7-8m high, 16-2Om in width with lengths of 85,125 and 195m with a total area of approximately 7400m^sup 2^. The project involves the heightening of the roofs in order to accommodate one level of parking and the construction of a number of cross-passages for the movement of the cars in and out of the car park, prior to the installation of the precast concrete decks.

Once the cavern enlargement and other excavation had taken place, each was sprayed with a lining of between 40-60mm using macro synthetic fibres as the reinforcement. The specification for the project called for a flexural strength greater than 4.0MPa and a residual energy (toughness) of 2.5MPa at 3mm deflection. This was achieved with 10kg/m^sup 3^ of Barchip Shogun 48mm structural synthetic fibre. Testing was carried out to ASTM C1018-97(3).