Article Source: http://www.theage.com.au/victoria/city-loop-safety-fears-20110921-1klbd.html#ixzz1YrFvhlAJ
India Vs Global Experience:
The Metro Rail tunnels in Kolkata, which lack the presence of surface protection systems in most of its areas inside the tunnels, have already undergone repairs.
Article Source: NBMCW December 2008 issue.
Continuous surface treatment on the interior surfaces can make underground structures waterproof and dry, which may not be achieved by injection or grouting. The surface treatment can be in the form of waterproofing plasters or in other forms. The surface treatment is provided after arresting water leakages at point leaks, construction joints, honeycomb areas and at structural discontinuities, as at locations of inserts.
The provision of a waterproofing treatment on the interior surfaces of a concrete tunnel structure will thus prevent not only water leakages and the wash-out of the products of cement hydration and other components of concrete but also early corrosion in reinforcing bars. The treatment will consequently enhance the durability of tunnel structures of concrete.
The waterproofing treatments, provided on selected areas of the Metro Rail tunnels in 1983 and later in other areas in 1995, remain effective till date as a testimony to the long term effectiveness and durability of the particular cement-based surface protection system for underground structures.
The Indian standard IS 456:200015 recognizes the problem of early distress in concrete structures as well as the causes for such early distress when it states in its Clause 8, Durability of Concrete: "One of the main characteristics influencing the durability of concrete is its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious substances." The interior surfaces of tunnels are generally exposed to air and thus these surfaces, if not protected, will permit the ingress of oxygen and carbon dioxide. While the diffusion of carbon dioxide may lead to depassivation of reinforcing bars, making corrosion in such bars possible, the diffusion of oxygen into the structures will lead to corrosion in the rebars in the presence of the moist environment inside the structure.
Author: Dr Anil K Kar, Chairman, Engineering Services International, Salt Lake City, Kolkata