By Dr. Eladbor Laloo
‘For if you remain completely silent at this time……deliverance will arise………from another place….. ’- Book of Esther 4:14 (NKV).
I was a young boy when the Umiam Hydel Project was inaugurated in 1966. I vividly remember from newspaper reports that the life span of the Umiam Hydel Project is 50 years. I thought that after 50 years, the dam would be dismantled and a new one constructed. In 1977, I was able to visualize the 50 year life span of the Hydel Project. It was while I was in my 4th Year Applied Geology in the ‘Indian School of Mines’, Dhanbad, where ‘case history’ of dams, with geological problems and designs worldwide, including the Umiam Lake was part of the syllabus in the Engineering Geology Course. I developed a keen interest in it as it was in my ‘home state’, and accessed a lot of data and information.
The Investigation of the Umiam Hydel Project was entrusted to the Engineering Geology Division, Geological Survey of India, (GSI). GSI was entrusted with the work for: (i) Foundation study for design and construction of a concrete dam 71.93 metres high, and 170.67metres long (Main Dam). (ii). Geological study alignment for a tunnel of 2134 metres long to reach the Power House, (from main reservoir to turbine) (iii).To prevent the escape of reservoir water through the low lying areas two earthen dams of 28 metres high and 464 metres long (before reaching the Umroi Airport junction from Shillong), and 15 metres high, and 168 metres long (on the way to Union Christian College) (iv). Geological and structural study of the reservoir area spread over 10 sq km , which includes a total catchment area of 221 sqkm . The problem dealt with here is of the Reservoir and the Main Dam.
Geological investigation: The Geological Investigation work was started in November 1956. Geological mapping and exploratory drilling was carried out to determine the “geological and structural parameters” of the rock formation in the area. In September 1962 the final work was completed and a detailed Project Report (DPR) was finalized.
Geology of the Area: The terms used are purely technical. The rock-types have to be described to understand the write-up. Based on geological mapping and exploratory drilling, the rock types are quartzites, (similar to the ones in the Demthring stone quarry), phyllites, and conglomerate. (loosely cemented boulders) and metadolerites (commonly called metamorphic rocks, which we learnt in secondary school) The rocks are folded and in places faulted (cracks developed). At the main damsite, rocks exposed are hard quartzite and softer phyllites. Tunneling of the Dam was done through quartzites, metamorphosed conglomerate, and meta-dolerite.
Seismicity: Meghalaya then a composite state of Assam experienced a series of earthquakes. The Shillong earthquake of 1897, the Dhubri earthquake 1930, Lakhimpur earthquake 1950. The 1st and the 3rd earthquake was felt up to a distance of 45,32,500 sq km and 29,26,700 sqkm., respectively. The intensity according to the modified Mercalli scale for 1897 was X and in 1950 was VI. Considering these figures, a very high Seismic Factor (SF- needed to be computed for structures to be designed to withstand the forces and deformation caused by earthquakes) is given for the Umiam Dam which is only 16 km from Shillong. The Seismic Factor considered for vertical and horizontal direction is 0.1 and 0.282 respectively. The horizontal factor is twice the figure for the Bhakra Nangal Dam over river Sutlej in Punjab commissioned in 1963. As regards the adequacy of this value, Dr. Okamota of Japan was of the opinion that the ‘Umiam Project Co-efficient’ of 0.2 was sufficient. His observation in the recent earthquake in Japan, that acceleration is less on hard and crystalline rocks, and, as Umiam Dam foundation is based on such rocks (hard and crystalline quartzites) it will not be subjected to heavy acceleration and can resist the vibration.
Faults: ‘Fault’ is defined as a crack with displacement. At one time, a single block of land surface is at the same level. When a crack develops it gives rise to two separate blocks and displaces the rocks. When separated, the rock creates an instability and in due course of time, one of the blocks gives way and is displaced to lower levels. Where are these faults? We are going to deal with the major ones which have created an impact in the present condition of the Umiam Reservoir. (i). A major one is from the relative displacement of the conglomerate bed of the old road, along the 49th and 50th milestone on the Guwahati- Shillong road. (This part is now under water and can be seen when the reservoir water level is very low, in the month of April.) (ii). A series of faults are found in the phyllites of the Guwahati- Shillong road. This area is along the present day road section starting well ahead of the ‘Toyota, Ford, Mahindra Showroom’ and runs up to the main Dam site and beyond. As per definition of ‘fault’ above, the area of the ‘Toyota, Ford, Mahindra Showroom’ and the bed of the Umiam Reservoir are a single mass, at the same height and level ( millions of years ago). Due to faults, it down-thrown the earth mass to the level of the bottom of the present day Umiam Reservoir. (iii). The ‘damsite’ is traversed by another fault. This fault has given rise to several minor faults. (iv). Drilling along the course of the Umiam river gave evidence of a fault along the river. A major fault passes through west of Sumer Hills. Geologically, no dam is to be constructed on a fault-plane. But all rivers are fault-planes, and hence the problem.
What happens when a fault occurs? It causes a rock-displacement of the formation. When a rock-displacement occurs one block slips downwards and the rubbing of the two rock surfaces during displacement generates a very high temperature causing the rocks on both sides of the fault-planes to be crushed and myolonised (reduced to powder). When this happens, it causes fragmented boulders and a huge void ( open space) between the two rock surfaces. With such conditions at the bottom of the Reservoir and the Main Dam, can it hold water? The answer is ‘no.’ Hence, it requires a geological and technical treatment, known as ‘grouting’
Defects of the foundation rocks and their treatment: The following areas were outlined for treatment. (i). The main fault zone at the central part of the Dam site and other minor faults and crushed zones. (ii). Rocks which are extremely joined. (iii). Open joints. (iv). Fractures and seepage. (v). Soft phyllites.
How the treatment was done?: Removal of crushed, soft rocks for a depth of two to three times the width was recommended to expose the fresh rock surface. Both sides of the fault plane were excavated; all broken rock pieces were removed. Later, the excavated part was sealed with ‘cement concrete’ of specific proportion to prevent leakage. Grouting with angle holes from both upstream and downstream parts penetrating the fault zone was carried out.
Can a cementing material last forever? During the earthquake of 1897, Ward’s Lake, Shillong, was erased to the ground. It was reconstructed with stone masonry, lime and sand as cementing materials. Around fifteen years back the level of the lake was below the optimum level as the cementing material of the dam had decayed and was leaking heavily. The old cementing material was removed by compressed air and filled by ‘epoxy’ as a cementing material using compressed air between the sandstones. The leakage stopped. This proved that cementing material in any case has its life span.
The cementing material of the Umiam Lake, compounded by high pressure with the volume of water, has its life span as well. At the inaugural speech it was stated that 50 years is the life span of the ‘cementing material,’ but within 45 years the reservoir is leaking. During its lifespan, it was presumed that enough ‘silt’ (geologically, defined as the finest disintegration of quartz) had accumulated. Geologically, it is the ‘silt’ that has been the mainstay of the lake. ‘Silt’ deposited in the cracks of decaying cementing material has partly sealed the leakage. As a field geologist for 36 years, I can say that leaks in the bottom of the steel tanks for drilling operations are sealed by filling them with six inches of soil cover.
Due to low volume of water in Umiam Lake, some environmentalists had recommended de-siltation to increase the life span of the Umiam Lake to 100 years., another one suggested its increase to a lifespan of 400 years. In February 2008, the Union Ministry for Environment & Forest, put Pushkar Lake, existing since the 4th Century in Ajmer, Rajasthan, on the list of the National Lake Conservation Project (NLCP). Tetra Tech India Limited prepared a DPR. for de-siltation. Rs. 48.3 crore was sanctioned. Water and Power Consultancy Services of India completed the de-siltation, and the lake dried up. The proposal was opposed by geologists of the State Department, who foretold that the lake would run dry as de-silting leads to leakage. Others attributed it to environmental problems. In May 2012, due to low rainfall, Umiam Lake the level of the Lake was pathetically low. I visited the Myntdu-Leshka Project; the lake was at its optimum. The same is the case of the Kopili Project. Is it an environmental problem?
The Dy. C.M, Prestone Tynsong, in his press briefing, announced that a second bridge would be constructed spanning the army boating facility to Nehru Park. Based on the above facts, let us not destabilize the equilibrium of the Umiam Lake and the Main Dam. Let us not shake the floor of the Lake, it may cause a catastrophe, if the floor leaks. It is not an ideal location for the second bridge. With due humility, it is suggested that the proposed bridge be located downstream, somewhere near the Industrial Area. Let a ‘cantilever’ bridge be designed with no pillars in the Umiam River to maintain overall equilibrium in the area since the ‘fault plane’ continues all along the Umiam. The Jadukata Bridge – a cantilever bridge created history for the State. Here are the excerpts.
“When in Meghalaya visit the Jadukata bridge. The longest span cantilever bridge in India. When it comes to beauty, it’s not just one of the most gorgeous bridges in the country but in the world.” Can we redo it?
Finally, let us base our confidence and hope in the Umiam Hydel Project. It was constructed after computing all geological, structural, seismic data, consultancy with experts from the world’s highest seismic zones all put together.
(The author is former Jt. Director (Geology) DMR, Shillong)