At 0.6:28:53 am IST on 26 December 2004 a severe earthquake measuring 9.0 Mw rocked the entire Andaman & Nicobar Islands (A&N Islands). The earthquake’s epicenter at 3.7N and 95E off the island of Sumatra was located very close to the A&N islands. Tsunami waves followed the earthquake – devastating East Coastal regions of the entire island and causing heavy loss of life and property. The severe tremor experienced completely disabled the Island’s infrastructures. The main shock was followed by many aftershocks and as of 15 April 2005 a total of 7512 aftershocks had been recorded, some having epicenters in these islands. An average of 70 aftershocks were being recorded daily as on 15 April.

The earthquake and Tsunami not only devastated the A&N islands but the entire Southern Asia region, even as far as Africa .The earthquake has been rated as the fifth largest since 1900 and the highest ever in the history of A&N islands. The US Geological Survey has rated this earthquake the highest in the world since 1964. About 1000km of the Andaman thrust (Fault Line) has reportedly been broken.

Due to the subduction of the Indo-Australian plate under the Eurasian plate, the A&N islands have experienced uplift and subsidence at different places, as field evidence suggests. At Port Blair, which was stuck by the Tsunami at 7.15am on 26 December, the sea water level has risen by 1.2m, suggesting subsidence of the landmass. In the middle Andamans, emergence of new shallow coral beaches suggests an uplift.

The major damage has been in the southern group of islands like Nicobar, Hutbay, Katchal etc, where the typical flat terrain was most exposed to the monstrous Tsunami waves.

Island geology and topography

The Andaman & Nicobar group of islands consists of a narrow broken chain of 572 islands spread along a curved line 1200km long in the Bay of Bengal. Out of the total, only 36 islands are habited with a total population of 356,152 (2001 figures).

The A&N Island system is believed to have formed in Oligocene Miocene time due to the thrust faulting between the Indo-Australian plate and the micro- Burmese plate .

Structurally the Andaman-Nicobar ridge is dominated by east-dipping nappes having gentler folding in the north part of the arc as compared to tighter folding and more intense deformation within the napes further south off Sumatra. The structures in the Cretaceous-Oligocene sequences are generally more deformed than those developed in younger sequences. Several north-south faults and thrusts are known within the island system and offshore areas. The most extensive is the Jarwa thrust developed in the main Andaman Island. Further, a set of north-south faults slices the sea floor along the eastern edge of the islands in the Nicobar deep and under the western part of the Andaman Sea, the most significant of them being the west Andaman fault. The eastern edge of the islands has a gentle eastward slope and extends to the floor of the Andaman basin marked by troughs. The depth of the trough varies from 2km to 3km below sea level. The north-western margin of the trough is marked by a mosaic of steep and elongated sea valleys and sea mounts such as the Nicobar Deep, Barren-Narcondam volcanic islands, Invisible Bank, Alcock and Sewell seamounts. The Narcondam is now an extinct volcano but the Barren did erupt in the early nineties. The Andaman back-arc spreading ridge bisects the central trough and has produced nearly symmetric spreading for the last 11 m.y. with a half-spreading rate of 1.86cm/year. Average sediment thickness at the Andaman basin is 4km, but it is delimited eastward by the Mergui Terrace at the Malayan continental margin.

The entire island exhibits subdued undulatory topography having a moderate to steep slope with subrounded tops and V-shaped valleys. The southern groups of islands are flatter, whereas the Andaman groups are full of hills and high lands .The highest peak (728m above MSL) is in North Andaman.

Past seismicity

The Andaman & Nicobar Islands are regarded as one of the most active seismic regions in India. They are placed in most severe seismic zone-V of the Indian Seismic Map with expected MMI of IX or greater. The arcuate line of the Islands are said to be located on a small techtonic plate and is referred to as the Andaman plate by Dasgupta (1993) and Burma Plate by Currey et al (1982). This techtonic plate which forms the ridges of the islands is sandwiched between the Indo-Australian plate on the western side and Eurasian plate in the North and East. On the eastern side of the Islands’ arc lies the spreading centers. The Indian lithosphere on the western side subducts below the Andaman (Sunda) plate .The subduction at this interface causes regular seismic shaking of moderate to severe earthquakes in this region.

An earlier event on 31st December 1881 was known to have caused greater damage, which was estimated to have a magnitude of about Mw 7.9 (Ortiz & BIlham 2002).The significant earthquakes of M>6.0 in these Islands are listed in Table 2.

Geological effect

The earthquake on 26 December 2004 has reportedly altered the geographical location of the A&N Islands. This massive thrust of the tectonic plates has heaved the Indian ocean floor towards Indonesia by about 15m ( Source: NASA ). The eastern coast of the Island has gone down by approximately 1.2m, thereby increasing the coast line.The daily tide now moves into the shore, covering roads and flooding paddy fields and houses. The Nicobar Group is the worst hit, rendering 40000 people homeless. The increased sea level has also posed a gigantic task for restoring normalcy and rehabilitation works. The capital city Port Blair located at the southern tip of Andaman District has reportedly shifted by over 1.00m (Source-GSI). As per preliminary studies by GSI, the mean sea level has increased by 1.2m.

Kalpong hydro project (North Andaman)

The 5.25MW Kalpong hydroelectric power project, commissioned in September 2001 on the Kalpong river, is the first hydro plant commissioned in North Andaman. The project harnesses the water resource of the river Kalpong, utilising a 149.36m net head. The project has all components of a hydro scheme except a surge shaft. It includes: a 34m high, 138m long concrete dam; 27m high, 146m long rockfill dam; 300m long open cut channel and 256m long approach channel; 110m high, 108m long rockfill dyke and three more earthfill dykes of average height 8m; 133m long tunnel; 650m long penstock pipe; and a 31.50m x13.2m surface power house for three 1.75MW Francis turbines designed to generate 14.83MU energy per annum. The generation alone saves the island 100M INR per annum, as it was originally only supplied by diesel power plants.

Concrete dam

In a severe earthquake of magnitude 9.0, the concrete dam is the most vulnerable structure. As against the full reservoir level (FRL) of el 206.66m, the water level of the reservoir on 26 December was el 204.90m – only 1.76m less than the FRL.

The Kalpong concrete dam is 34m high above the deepest foundation level of el 176.5 and 138m long at its crest elevation of 210.50m. The dam consists of 11 blocks in which three blocks are overflow blocks and the rest are non overflow blocks, four blocks in each bank. The spillway consists of three blocks with a total width of 24m. Non-overflow blocks are each 15m long with the last two abutment blocks in the left and right flank measuring 12.5m each.

An inspection cum drainage cum foundation gallery is provided along the dam profile measuring 1.8m x 2.25m. Pressure relief holes are provided downstream of the gallery at 3m c/c spacing with depth in rock varying between 6.0m to 15.2m in the various dam blocks. All the seepage water inside the gallery is collected in the sump well through drains. This is further disposed of downstream of the dam by two 15hp dewatering pumps installed in the sump chamber. A 1200mm diameter sluice pipe with knife edge valve has been provided in the body of the spillway section with its centre line at el 190.0m for silt flushing and to regulate the flow downstream as and when required.

The dam foundation has been provided with both upstream and downstream shear keys in spillway blocks .

An ungated spillway at full reservoir level has been provided to discharge a standard project flood (SPF) of 405m3/sec. The ungated overflow section has been designed for passing an outflow discharge of 150m3/sec at M.W.L. 209.40. A flip bucket-type energy dissipation structure has been provided to regulate the flood discharge downstream.

Post earthquake inspection of concrete dam

The post earthquake inspection revealed no physical distress or failure of abutment slopes, cracking etc. There was also no sign of any structural damage. However prominence of block joints reveals the dam did shake during the earthquake. Insignificant seepage was observed from the block joint of L2-L3, but this will subside over time. Seepage increased from the pressure relief holes and dam body but within safe limits. There was a slight rise in electrical piezometer readings – they only indicated a rise of about 0.2kg in the uplift pressure of the central spillway block. This might be due to opening of the foundation rock fissures as a result of the shaking, but these will subside and revert back to normal levels once the openings stabilise. The structure was found to be in a healthy condition.

The details of Electrical Piezometer readings as logged before and after the major shock shows that the behaviour of the structure was quite satisfactory (see table 3). The details of block joint meters are shown in Table 4.

Joint meter reading deviation by 1mm and less indicates shaking of the dam due to the severe earthquake and prominence of the block joints. The left bank downstream earthen slopes were in stable condition, as was the downstream slopes of the right bank. The downstream protection works, like PCC walls, RR stone masonry walls etc were in a stable condition with no visible distress/disturbance. The dam top road, parapet wall, upstream slopes and protection measures were also found to be in stable condition with no visible disturbance.

Overall, the concrete dam was found to be structurally safe and appeared to have withstood the earthquake.

Rockfill dam

The rockfill dam of the Kalpong project has been constructed 15km upstream of Diglipur on the right fork of the Kalpong river. The dam, with a total length of 146m and its crest at el 212.00m has a maximum base width of 116m. The maximum height of the dam is 27m from the average foundation level.

The rockfill dam with non overflow section is provided with a central clay core with 3m wide transition filter zones of sand and gravel on the upstream and downstream, both for controlling internal seepage in steady and drawdown condition. The clay core has a slope of 1H:2V in the upstream and 1H:3V in the downstream.

Due to the severe shock, a longitudinal crack (varying from 1mm to 70mm) has developed on the Rockfill dam top road. There is no rise/change in the toe drain discharge, and no sign of piping. However damage has been observed to some concrete ribs on the upstream and downstream faces supporting the riprap. There is no apparent damage/crack in the riprap. The cracks in the ribs and on the top road indicates shaking of the entire structure. The Concrete ribs and riprap have supposedly guarded the rockfill materials from disturbance. The dam parapet walls are also not disturbed/disaligned, indicating good health of the structure. The toe drain water was also clean, indicating no piping action.

Minute inspection was carried out to locate any kind of slope distress or upheaving, on both the upstream and downstream area. The entire area was found to be structurally stable and in good condition.

The non disturbance of the dam profile indicates that the structure withstood the shaking. The reason for the longitudinal crack on the dam top surface road may be attributed to the low tensile strength of the bitumen surface.

The damage was rectified by the user agency by opening the crack up to the depth of crack (which was above the impervious clay core top). This was refilled with filter materials and the top was sealed with bitumen pre-mix.

Rockfill saddle dyke and earthen dykes

The intake saddle dyke is located on the saddle at the left bank ridge of the left fork. The lowest ground level in this saddle is at el 202.80m. At this location, a dyke 108m in length has been constructed to prevent the overflow of the reservoir. The rockfill section with central clay core is 10m high above the deepest foundation level. The upstream and downstream slopes are protected with stone riprap bounded by concrete ribs.

Both the upstream and downstream slopes were found to be stable and no physical distress, upheaving, subsidence, piping or slope disorder was observed. There was no seepage found in the toe of the structures. The stone pitching works and parapet were found to be in order. However a longitudinal crack varying from 1mm to 30mm wide has developed on the dyke top road on the bitumen premix surface, in the middle segment. There was also some damage to a few concrete ribs on the upstream and downstream faces supporting the riprap, thus indicating shaking of the structure.

Penstock and saddle supports

There was no distress in penstock alignment, anchor blocks and saddle supports. The power house was running during the earthquake, thus the penstock was in full operational condition. The perfect alignment of the penstock indicates that the structure successfully absorbed the shock. The saddle supports in the exposed region of the U-shaped nallah were also found to be structurally sound.

Power house civil works

The complete power house complex was found to be structurally sound except for a minor, negligible hairline crack developed in the joint of beam and hollow block near the transformer wall, indicating the shaking of the building

Turbines and generators

The Kalpong hydroelectric power station has three generating units of 1750kW each, operating under a rated net head of 149.36m with design discharge of 1.35m3/sec. The generating voltage of 3.3kV is stepped up to 33kV through 2500KVA, three phase, 3.3/33kV generator transformers. The turbines are horizontal shaft, Francis type, with an output of 1842kW at rated head and directly coupled to the generator with a speed of 1500rpm. The turbine consists of spiral casing, runner, draft tube, guide vane assembly, DT cone, DT bend, governing equipment etc.

Effect of earthquake on machines

At the time of the occurrence (6.29am) of the earthquake on 26 December, unit 1 of the power plant was under operation. Soon after the shock, the lines tripped on over current and the unit was stopped through emergency stop by the field staff. The intake valve was immediately closed. At about 9.30am, an attempt was made to restart the unit, however smoke was observed coming from the side of the speed sensors mounted next to the shaft seal. The unit tripped on loss of primary signal and a rubbing sound was observed at lower speed. The unit was closed. At the time of the earthquake unit 2 and 3 were on standstill. When staff attempted to start the other two units, both would not operate, even though the MIV was fully open. This indicated jamming of the shaft. As a result, the entire power plant was shut down. The reason for such shaft jamming may be attributed to the fact that the mass of the turbine and the generator connected with the shaft are different. As such, in the event of tremendous ground acceleration, the force acting on these two units are significantly different. As the whole unit is fixed on a foundation frame, the shaft and the gaps become the weakest portion for movement – hence the jamming.

As a result, the power supply from the project was disrupted totally, plunging the entire north Andaman into darkness.

Restoration measures

Primary inspection indicated jamming of the shaft, MIV of unit-1 and also possible relative settlement of foundation .

Detailed inspections suggested no settlement of foundation but indicated relative movement of the turbine and generator due to the severe shock, which jammed the shaft, requiring fresh alignment and adjustment of bearing pads. The labyrinth gap of unit 2 and 3 was 0.35mm as against 0.5mm at the location. No major damage to the machines/control room was found. Similar action was taken on unit 2 and 3 to realign the machines, and unit 2 and unit 3 was fully restored on 12 and 18 January 2005, with the first unit restored on 4 January.

Impact of earthquake in other areas

The earthquake & tsunami of 26 December 2004 had quite a devastating impact on life and property of these islands. As per official reports:

• 3100 dead and about 6000 missing as on 15th April 2005.

• The eastern coast of the Andaman Island has sunk by 1.2m

• Trinket Island has been divided by two parts.

• The rise in water level has partially submerged some of the southern islands – namely Katchal, Kamorta, Kundul, Teressa.

• In Port Blair, the main airport was flooded and runway cracked. The runway has since been restored.

• Port Blair ports and jetties were damaged and two vessels sunk

• The newly constructed 268m long Austin bridge connecting north and middle Andaman developed settlement of deck slab and is now closed for traffic.

• The main powerhouse of Port Blair (20MW diesel generator) is closed due to tsunami devastation

• Electricity and water supply of all southern group islands completely hampered.

• Water supply to Port Blair City restored on 11 January, through temporary diversion of pipe lines.

• Sea water ingress has rendered about 7000Ha of paddy fields as waste land

• Permanent rise in mean sea level has forced many settlements including market areas to evacuate.

Author Info:

A K Dash is Senior Manager (Civil) at NHPC Ltd in Port Blair


Table 1
Table 2
Table 3
Table 4