Tunneling Technology Bores Ahead

Engineering and Mining Journal, Dec 2005 by Carter, Russell A

The principle on which liner plate assemblies work is to simply distribute and transmit the load to the surrounding earth. As a steel liner plate ring takes load vertically, it tends to deflect inward at the top and outward at the sides. The ground resists deflection of the lining by developing a passive force equal in magnitude and opposite in direction to the force exerted by that of the lining.

The company notes in its literature that there are various methods for determining the required strength of tunnel linings-but the type of ground, location and depth of cover, size and length of the tunnel, level of ground water, superimposed loading, and history always guide these calculations. One of the most important factors in any calculation for the effectiveness of a tunnel support is the ability of the undisturbed ground to resist movement. Soft ground conditions may be stable or unstable in nature, and each presents its own problems.

American Commercial liner plates can be purchased in 16- and 24-in. (41 to 61 cm) width as corrugated or smooth plates. Liner plates are formed from one piece of metal with longitudinal and circumferential flanges. The plates can be galvanized and/or bituminously coated and can include grout holes with plugs. For special conditions, gasketed liner plates and tapered liner plates are also available.

At the other end of the spectrum of tunnel lings are cast-in-place or sprayed-on cement products and the equipment required to apply them. Degussa, through its underground construction arm, UGC International, offers customers a service package that provides both a wide range of sprayable cements, injection waterproofing, fire protection and grouts, as well as the application equipment from Degussa's Meyco division for wet- or dry-method placement.

Meyco's Oruga model is one of its most recent introductions, a crawler-mounted mobile spraying manipulator unit for applying shotcrete. It is suitable for both wet and dry methods. The 1,670-kg unit has a spray height of 7.94 m and width of 11.1 m, and is powered by a 13-kW diesel engine. The Oruga features a Meyco Rama 4 spraying manipulator, powered by a 7.5-kW electro-hydraulic drive unit. The operator can sit at the rear of the machine, or can have access to all unit functions via a cabled or radio remote control.

In June 2005, another Degussa unit, Degussa Admixtures, announced the availability of Masterseal 345, a dry-process sprayable membrane for waterproofing concrete. Masterseal 345 is applied in a sandwich between layers of sprayed or cast-in-place concrete and exhibits excellent bond strength and good elasticity. The product's advantage, according to the company, lies in the bonding properties between the membrane and concrete lining on both sides. The strong bond eliminates potential groundwater paths.

Masterseal 345 can be applied even in tunnels with complex geometries. Applications rates can be up to 100 m^sup 2^ per hour, using standard spray equipment. According to Chris Gause, underground construction manager for Degussa Admixtures, "Many of our recent worldwide projects have adopted this approach and have shown total project savings up to 25% over the double shell method." Masterseal 345 is a continuous waterproofing system without discrete joints and without the need for waterstops and compartmentalization.