Assessment of the Results of the Geotechnical Study by NIOT for Sethusamudram Shipping Canal Project
D.N.Seshagiri Former Director, Geological Survey of India
Blasting under water:
Blasting under water requires greater care and experience than similar operations above water. The pressure is greater on all sides.
Underwater blasting entails so many problems that this branch of blasting technique is very special. The cost of drilling and loading have been very high when divers have had to do the work, and at the same time it has been considered necessary to have a larger number of holes and from three to six times as much explosive as in ordinary bench blasting. A fundamental fact is that conventional underwater blasting has been carried out in such a way that there has been a great risk of flash- over between different holes. From the point of view of safety alone this is unsatisfactory. In short, it is very complicated.
Vibration hazards are more severe, for, water bearing deposits are very effective transmitters of shock waves. (Ref. Du Pont Blasters’ Handbook-15th Edn.).The characterization of underwater rounds as opposed to ordinary blasting is, first the risk of unintentional flash- over and, second, the larger quantity of charge. This makes the estimates not only more complicated but also more uncertain since there is no assurance how a flash over will not occur. Apart from that a flash-over causes a larger total charge to be ignited within a certain time, there is the fact that faulty ignition sequence causes greater fixation and also for this reason stronger ground vibration. (Ref. Modern Techniques of Rock Blasting. (U.Longefors & B.Khilstrom. John Willey & Sons 1963).
The difficulties of underwater blasting and vibration effects have to be kept in mind. Controlling vibration in underwater blasting is a very difficult task. Charges have to be heavy to obtain optimum fragmentation. Controlled blasting techniques such as smooth or cushion blasting or pre-splitting cannot be adopted.
The blasting effects on the marine life and ecology do not appear to have been studied. The possibility of tidal surges due to underwater blasting has to be thought of since this will be akin to a very shallow earthquake. Will all these lead to an environmental disaster?
The contractual obligations and responsibilities will also need consideration. Does the dredging contractor(s) is aware that hard rocky strata is likely to be encountered and even in clays dredging may not be successful in some cases and other methods of removal has to be resorted to. Unless these aspects are made explicitly clear it may lead to a situation wherein negotiated settlement (which has to be avoided to ensure transparency) or arbitration will become necessary.
Other Methods of removal:
Hard material from seabed has been removed with out the use of explosives. Not only rock, even icebergs have been removed.
Use of Expansive Cement is one of them. This requires boreholes to be drilled in which the Expansive cement is placed. A “Free Face “has to be made available.
Hydraulic hammer fitted to a hydraulic excavator is commonly used. As recently as early 2004, the Belgian Company “Dredging, Environmental and marine Engineering” (DEME) had built the world’s most powerful self-propelled cutter suction dredger.
To design the cutter head, the rock mechanical properties of the (sea bed) rock will be necessary as in the case of Tunnel Boring Machines. A Rock Mass Rating has been developed incorporating a) Strength of the intact rock material (UCS), b) frequency of discontinuities (FI) and c) the thickness of the various layers. Such details are not available for the SSCP.
Very recently, rock dredging has commenced at Viaskapatnam port to deepen the inner circle to facilitate the entry of bigger vessels. Grab dredgers, hoper barges and drilling and blasting equipment will be used to execute this project. (Source: The Hindu Business Line. Sep.03, 2005).
It is fervently hoped that in SSCP drilling and blasting will not be permitted.
For the preparation of this note, the following documents have been consulted:
a) Geological and Geo-technical assessment by NIOT. Executive Summary
b) NIOT Vibrocore Data for Sethusamudram Shipping Canal. L&TRamboll
c) Bed Profile-Channel Section A-B,B-C(Adam’s Bridge) andE-E4(Plak Strait).L&T Ramboll
d) Logs of B.H.Nos. 1, 2, 4 & 11 (Adam’s Bridge). L&T Ramboll