By Dr Eladbor Laloo.
What is the Umiam Reservoir? I was a young boy when the Umiam Hydel Project was inaugurated in 1966. I accompanied my father to the inaugural function and recall one speaker remarking that the project life span is 50 years. I thought then that the dam would be dismantled after 50 years and a new one would be constructed. Eleven years later when in the 4th Year of my Applied Geology Course in the ‘Indian School of Mines’, Dhanbad, I learnt of the problem of the Hydro Project as a student of Engineering Geology Course. It was a ‘case history’ among other dams, worldwide, which generated a lot of geological challenges during construction. I developed a keen interest in this dam, it being in my ‘home state.’ I was able to access a lot of data and information on the subject.
The Investigation of the Umiam Hydel Project was entrusted to the Engineering Geology Division of the Geological Survey of India,(GSI). The GSI was entrusted with the work for: (i) Foundation study for construction of a concrete dam 71.93 metres (236 ft) high, and 170.67metres (560ft), long(Main Dam). (ii). Geological study alignment for a tunnel of 2134 metres (7000ft) long. (From main reservoir to Sumer) (iii). To prevent the escape of reservoir water through the low lying areas, two earthern dams of (a) 28 metres (92ft) high and 464 mtres (1520ft) long (before reaching the Umroi Airport junction from Shillong) and (b) 15 metres (50ft) high, and 168metres (550ft) long.(on the way to U.C.C.) (iv). Geological and structural study of the reservoir area spread over 10 sq km which includes a total catchment area of 221 sq km. The problem dealt here is of the Reservior and not the Main Dam.
Geological investigation: The investigation work was started in 1956. Geological mapping and exploratory drilling was carried out to determine the geological and structural parameters of the rock formation. In September 1962 the final work was completed with regards to foundation mapping, grouting (sealing of cracks and fissures, which will be dealt in future course of the write-up), and treatment of faults etc.
Geology of the Area: This item is purely technical; ‘Greek’ to a lay person. The rock-types must be mentioned to understand the continuity of the write-up. Based on geological mapping and exploratory drilling, the rock types are quartzites, (the one you can see in the Demthring stone quarry), phyllites, and conglomerate. (loosely cemented boulders)., and metadolerites. The rocks are folded and in places faulted. At the dam-site, hard quartzite and softer phyllites are exposed. Tunneling was done through quartzites, metamorphosed conglomerate, and, meta-dolerite.
Seismicity : Assam experienced a series of earthquakes. Shillong was hit in 1897, Dhubri in 1930, Lakhimpur in 1950. The 1st and 3rd earthquake was felt at a distance of 45,32,500 sq km and 29,26,700 sq km., respectively. The intensity according to the modified Mercalli scale for 1897 was X and 1950 VI. Considering these figures, a very high seismic factor 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 Dam. As regards the adequacy of this value, Dr. Okamota of Japan was of the opinion that the ‘Barapani Project Co-efficient’ of 0.2 be sufficient. He pointed out by his observation in recent earthquake in Japan that acceleration is less on rocks and as such dams founded on rocks will not be subjected to heavy acceleration.
Faults: ‘Fault’ is defined as a crack with displacement. At one time, a single block of land surface were at the same level. A crack developed, giving rise to two separate blocks. Once separated, the rock creates instability and in due course of time, one of the blocks gives way and is down-thrown to a lower level, displacing the two blocks. Where are these faults? (i) A major one is recorded from the relative displacement of the conglomerate bed of the old road, along the 49th and 50th mile stone on the Gauhati- Shillong road. Now under water it can be seen occasionally when the reservoir water level is very low. (ii). A series of faults are found in the phyllites of the Gauhati- Shillong road. This area starts at a place well ahead of the ‘Toyota Showroom’ and runs up to the main dam-site and beyond. As per definition of ‘fault’ above, the area of the ‘Toyota Showroom’ and the level bed of the Umiam Reservoir are a single mass, at the same height and level millions of years ago. Then a fault took place, collapsing the earth mass to the level of the bottom of the present day reservoir. (iii). The ‘dam-site’ is traversed by another fault. This fault has given rise to several minor faults. (iv). Exploratory drilling along the course of the Umiam river gives evidence of a fault passing 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 rock-displacement of the formation. When that happens one block slip 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-plane to be crushed and myolonised (reduced to powder). When this happens it causes fragmented boulders and a huge void 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. It requires a proper technical treatment.
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 jointed. (iii). Open joints. (iv). Fractures and seepage. (v). Soft phyllites.
How the treatment was done? The width of the fault plane varies from place to place and the depth of excavation of the rock surfaces vary. 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 was excavated and broken rock pieces removed. Later, the excavated part was sealed with ‘cement concrete’ of suitable 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 devastating earthquake of 1897, Ward’s Lake, Shillong, was razed to the ground. The dam was reconstructed. Five to six years back the level of the Lake was below the optimum level. The cementing material on the masonry wall of the dam had decayed and water is leaking heavily. The leakage was stopped using ‘epoxy’ with compressed air to fill in the cementing layer between the sandstones. This proves that cementing material in any case has its life span.
The cementing material of the Umiam Lake compounded by high pressure generated by the volume of water, through the fault-plane, the joints, fissures, fractures, seepages and soft phyllites also has its their life span. The inaugural speech of 50 years is the life span of the cementing material. But within 45 years the reservoir started leaking. In its 45 years lifespan, everyone through the media has admitted that enough ‘silt’ (geologically, defined, as the finest disintegration of quartz) has accumulated along with other sewage. But, it is the ‘silt’ that has been the mainstay of the lake. It is the ‘silt’ deposited in the decaying cementing material that has partly sealed the leakage. During my 35 years as a field geologist, any leak in the bottom of the steel tank for drilling operation, was sealed by filling it with six inches of soil cover.
One agency has recommended ‘de-siltation’ of the Umiam Lake to give it a lifespan of 100 years; another a lifespan of 400 years. But this is not the solution. In February 2008, the Union Ministry for Environment & Forest, put Pushkar Lake, which existed since the 4th Century in Ajmer, Rajasthan, on the list of five lakes under the National Lake Conservation Project (NLCP). Tetra Tech India Limited was asked to prepare a DPR. De-siltation was recommended and the Union Government sanctioned Rs. 48.3 crore. It was entrusted to Water and Power Consultancy Services of India (WAPCOS). But after de-siltation, the lake dried up. The proposal was opposed by Senior Geologists (hydrologist) of the State Department, who foretold that the lake would ran dry. De-silting increases seepage lost. Let us remember that GSI spent 6 years to make a complete report on the project and there is no overnight solution to the problem.
Others attributed the dropping of the level of Umiam Lake to environmental problems. In May 2012, I visited Myntdu-Leshka Project. The level of the Leshka Lake was almost optimum; same is the case with the Kopili Project. Whereas, Umiam lake was pathetic. In November 2012, the Umiam level has gone down by seven feet from the optimum level. Is this due to environmental factors?
Finally, there is a story of two crows. One is a wise thirsty crow, who raised the level of the water in a beaker by dropping pebbles till the level of the water reached her beak. Then there is the silly thirsty crow who is so lazy, break the bottom of the beaker with its beak; drank the water to its heart’s content. When it become thirsty, it returned to find that the beaker had dried up. Are we the wise crow or the silly crow?
N.B. The author is a senior geologist by profession. The views expressed are personal and not of the organization he works for. Email; [email protected]