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by Ariyawansa Jayaweera, Kandy special correspondent

Randenigala reservoir

Victoria reservoir

The renowned geologist, the late professor P. W. Withanage who specialised and versed in the science pertaining to the geological studies and geoscience though not among the living today, but his studies, geological works, writings, sketches and all what he said and wrote have remained and still accepted and thus embraced by the universal literature.

Prof. Withanage in his geological studies and works, many years in the past have showed that many reservoirs, canals, dams, tunnels, water courses and other hydro-related works in Sri Lanka were not done with a real geological study and learning and standard related to geological science and construction technology.

The undermentioned article comprises extracts taken from a forgone public lecture delivered by Prof. Withanage, viz. 2nd Ananda Coomaraswamy Oration - 1988, delivered on August 22nd 1988 at the Lecture Theatre, Department of Geology, University of Peradeniya under the topic and title - "20 years of Dam site and Tunnel Geology Investigations."

The then politicians pooh-poohed and rejected the facts and data included in that oration en masse and disregarded its finger points as well. They even did not consider its value bona fide. They threw it away as worthless as a pice and two pence.

Unlike the Kotmale Project, many inspections and observations in the Victoria Project were done before planning and without implicit guarantee, noted Prof. Withanage. Anyway as mentioned earlier in other projects here too one portion had been forgotten or ignored i.e. the large crevices, fissures and splits passing through the dam on both sides of the roadway. No geological inspections and geologic examinations were held or done.

The most fortunate undertaking in my engineering geology career was the detailed geological mapping of the 5 mile/8 km long Polpitiya tunnel along the Kehelgamuwa ridge on the right bank of Maskeliya Oya.

The detailed subsurface map of Polpitiya tunnel in 1968 on a scale of 1:120 shows clearly, not only the expected vertical and horizontal fractures, but also that most of these fractures which are normally mapped as joints in surface mapping, show definite vertical movements ranging from a few inches to over 5 feet.

It was most interesting to note that the cumulative differential movement along these fractures (over 300) with reference to the Polpitiya tunnel bottom (invert) level indicated a relative vertical 5 ft. uplift/tile from the upstream side of the tunnel from the central highlands of Sri Lanka towards the North.

The discovery of vertical movements along the fractures exposed along the Polpitiya tunnel walls and along the large surface fractures called lineaments in the Kotmale and Victoria Project areas and elsewhere (tectonic movement) provide first hand evidence for the vertical uplift of the Central Highlands (so-called central peneplain) of Sri Lanka to over an anomalous elevation of over 4,000 ft.

The youthful topography with waterfalls around Horton Plains (over 6,000 feet) mountain peaks such as Piduruthalagala (over 8,000 feet) in a very old land surface subjected to weathering and erosion for over 1,000 million years can only be explained by vertical movements along these deep fractures observed in the tunnels and on the surface.

These research programs in the big engineering projects since 1964 by the staff and students of the Department of Geology, University of Peradeniya have opened up the currently active research programme on the past and present movement along the lineament fractures (called lineament tectonica). The lineaments criss-cross the whole island surface and off-shore areas to form a regular mosaic. These lineament studies have a direct bearing on the stability of large dams and powerhouses and leakage from large reservoirs such as Kotmale, Victoria and Randenigala.

Lineaments also provide valuable date for location of potential landslides and subsurface sources of ground water.

"Tunnel is primarily a geologic problem." Therefore the design and construction engineer will expect the geologist to locate the most suitable tunnel trace, also to determine the occurrence and size of the fractures and to identify areas of "bad ground" and seepage zones, that may give rise to tunnel hazards along the tunnel route.

In Sri Lanka, since 1964 the frequent emergency requests for "rescue operation from the contractors, government organisation or their consultants in-charge indicate the lack of detailed studies of the landforms (geomorphology) and geology of the local site areas as well as the regional catchment and surrounding areas of major engineering projects.

This lack of emphasis on detailed investigation of site areas has been the result of the misconceived idea that all regions underlain by old precambrians crystalline rock are stable tectonocally.

Another factor is the lack of appreciation of currently prevalent special geological and tectonic conditions in Sri Lanka-differential tilting and neotectonic (recent) movement along deep lineaments and shear zones that may exist in the project areas. e.g. in Castlereagh tunnel (1964), Polpitiya power house site, Polgolla tunnel, Kotmale and Victoria".

Castlereagh - Norton tunnel

During the excavation of the 20,000 ft. long Castlereagh-Norton tunnel along the right bank of Kehelgamu Oya, a tributary of Kelani Ganga a highly decomposed and highly sheared kaolinized shear zone (160m) 500 ft. wide was encountered unexpectedly in 1964 near stations (270m) 810 ft. to 1200 ft. (400m) very close to the centreline of the Castlereagh dam along the hillside. "Huge amount of water started flowing into the tunnel and as the excavation was carried on, new springs were frequently discovered".

The contractors "feared that tunnel might collapse or even worse suddenly would drain the Castlereagh lake through a fault zone into the tunnel."

At the request of the contractor in 1964, a detailed geological and geotechnial survey was carried out. Weathered zones and places of potential heavy seepage from the reservoir were located for further tunnelling. But as expected, the tunnelling operations "came to a stand still" at station 80 m (250 ft) from the intake on account of bad rock "running ground" and heavy seepage from the reservoir.

The completion of the tunnel was delayed by one year resulting heavy losses to the contractor and the government, which had to provide thermal power for one additional year. Detailed field mapping of the tunnel route and a few more bore-holes across the tunnel trace near the intake earlier during the feasibility stage could have located these series of narrow kaolinised shear zones with seepage and possibly avoided the delay.

Laxapana-Polpitiya project
(MOP stage I)

In the case of the Laxapana-Polpitiya tunnel, which runs through the Kehelgamuwa ridge for 25,400 ft. no serious adverse conditions were encountered during tunnelling between 1966-1969.

However, the constructional history of the power-house pit or excavation at the end of the penstocks from the outlet of the tunnel was a different story.

The siting of the Polpitiya power house excavation is a glaring example of the failure on the part of the Consultant Geologist (from a foreign country) to observe and interpret the local geology.

Although the geology report indicated the presence of laterite overburden ranging in depth from 75 to 100 feet, the site selected for the power house and the tailrace (outlet for the water from the turbines) was a marsh with peat deposit and boulders along an earlier river channel 60 feet above the present Maskeliya Oya.

The heavy seepage downslope movement of the landslides and earthslips along the weathered shear zones damaged the construction equipment and delayed the excavation for months. In addition the government of Sri Lanka had to pay 3,000,000 as damages to the contractor as result of 'changing conditions'.

Canyon-Laxapana tunnel
(MOP stage II)

The 18,500 feet long unlined pressure tunnel is along the right bank of the Maskeliya Oya, upstream of the Laxapana power house. In 1970, a detailed geological survey along and across the proposed tunnel route was carried out to determine the feasibility of shifting the tunnel trace towards the Maskeliya Oya to reduce the length of the proposed tunnel and also of the adits as recommended by the foreign geologists.

The new tunnel route No. 3 with only two adits as recommended on the basis of the results of the detailed geological survey was accepted. On account of the exhaustive study of the geology of the project area, no unexpected tunnelling problems were encountered and the project was completed according to schedule.

Moussakelle-Canyon Tunnel (MOP stage III) Kotmale and Victoria dams and tunnels

During the excavation of the Moussakelle-Canyon tunnel extremely adverse tunnelling conditions occurred at stations 6,600 ft and 12,960 ft and nearly 390 ft. of tunnel had to be abandoned as a result of wrong selection of the original tunnel trace.

In the case of Kotmale project, a series of investigations by several foreign and local organisations had been carried out. However, according to the reviewers and panel of experts "the available knowledge on the geology of the project area is yet incomplete and is ambiguous and does not permit to enunciate a final judgement." To rectify these deficiencies the Department of Geology, University of Peradeniya, carried out a detailed regional study of the whole site area.

If these detailed survey was carried out earlier, with a careful follow up of subsurface drilling a good deal of money could have been saved during the designing stages.

Victoria project

Unlike in the Kothmale project, for the Victoria project considerable preliminary information through deficient in some respects were available during the feasibility and designing stage.

However, as in the other projects referred to earlier, no regional study of the geology and tectonics especially along the lineaments around the project site area had been made.

In November 1981, during the excavation of 3 mile long tunnel (22 ft. diameter) extremely adverse tunnelling condition were encountered at station 15,300 ft. and 300 ft old excavated tunnel and partly completed surge shaft were abandoned and the tunnel route relocated on a new trace.

Some geologists are of the opinion that a rockfill dam at Victoria would have been more suitable in view of the presence of a lineament through the axis of the tunnel and the reservoir.

Need for seismic and geodetic monitoring

The presence of large fractures (lineaments) showing definite vertical and horizontal shear movements and the occurrence of recent movements in the main project areas strongly indicate the necessity of long-term seismic monitoring and revelling preferably installation of three-dimensional central seismic network.

Since recent years, it has become apparent that the impounding of reservoirs large and moderate in size (Victoria and Kotmale are large reservoirs even according to world standards) may induce seismicity even in areas hitherto considered aseismic (Brundsden, 1980). Also "absence of recent earthquakes strong enough (m. 3.5) to be felt does not ensure the absence of currently active faults on the island, which could be triggered by the creation of a large reservoir (Deere et al. Report on Kotmale project 1980).

As pointed out by Simpson (1986) it is in areas of relatively low natural seismicity that the reservoir induced earthquakes can have their greater impact on engineering design and the largest of induced earthquake has occurred in these areas. e.g. Koyna (6.5), Kariba (5.8), Hsinfengkiang (6.1) and Aswan (5.5). Up to now in Sri Lanka, no increase in microseimicity, important precursor for reservoir induced seismicity earthquake (RIS), has been observed during impounding of the new reservoirs - Victoria (1984), Kotmale (1985) and Randenigala (1986). However, the time-lag of reservoir-induced seismicity has varied elsewhere from 5 years (Koyna, India), 7 years (Oroville, USA) to 17 years (Aswan) since the first filling of the reservoirs.

In addition to seismic monitoring, checking of the elevations (relevelling) around the reservoirs and along tunnels should be carried out to check the impact of large volume of water in the reservoir and seasonal drawdown on the surrounding terrain at least at 5-year intervals. For this purpose permanent bench marks should be installed preferably before impounding of the reservoir.

Environmental factors

In addition to the geological factors in selecting the tunnel routes and dam sites, environmental factors should be considered by both engineer and the engineering geologists in-charge of the project. "Not everything technically possible is also ecologically acceptable" (Langer 1982).

For this reason during the selection a tunnel alignment, construction of a dam and impounding of a reservoir, engineering geologists and engineers must be aware of the necessity of taking into account not only of "factors affecting reliability and efficiency of constructions, but also to a no less extent, the problems of environmental protection and the rational use of the environment."

State has taken steps during the Mahaweli accelerated programme to mitigate and minimize the environmental hazards. But the drying up of all wells along the 5 mile long unlined Polgolla tunnel and in the case of Kotmale in spite of a detailed geomorphological survey by a team of experts, the social and economic impact, as a result of the reservoir-induced landslides and creep, on the building and villagers in the 2000 year old villages, such as Nawangama, Pusulpitiya, Niyangandora, Kalpitiya, Kadadora and Gangewala (covering an area of 4 sq. miles) were not recognised earlier.

Recently over 500 families (over 2,000 persons) have been vacated from these villages and settled on lands allocated in the neighbouring tea estates.

Maintenance of large engineering structures

The work of the geologist and engineer has not ended when engineering structures - dams, reservoirs, tunnels and power-houses are completed. The unlined tunnels such as Polgolla, Bowatenne and Polpitiya require ground inspection periodically after commissioning to check rockfalls and the condition of lined sections.

The current repairs undertaken along the Polgolla unlined tunnel show the systematic approach of the newly established Department of the Mahaweli Authority - headworks, administration, operation and maintenance (HAOM).

However apart from the routine repairs, the period of dewatering of the tunnel should be made use for installing permanent bench marks along the tunnel (if bench marks have not been fixed earlier) and to relevel the invert (bottom) and also the two sides of the lined sections to determine any recent displacements along these weak zones.

"The new technology of large dam is only imperfectly understood." In Sri Lanka there are 3 large dams and reservoirs Kotmale, Victoria and Randenigala.

Therefore additional monitoring of these dams as well as the surrounding areas especially on either side of lineament fractures in the site areas should be continued on a long-term basis during maintenance. Possible leakage from reservoirs and the effects of rapid drawdown and filling are other important factors that should be monitored.

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