The Brahmaputra River, the Life-Line of Assam, Has Steep Gorges And

Geotechnical Problem of Tilting & Subsidence of Sunken Well for High Level Jetty at Pandu Port, Guwahati, Assam Dr. Barin Chatterjee, Consultant Engineering Geologist Abstract Introduction: 1. On the southern bank of the west-ward flowing Brahmaputra River, within 200 m from the left abutment of the road-cum-rail bridge between Pandu and Saraighat a High level jetty is under construction. Chronology of events which necessitated this investigation to be taken up was tilt and shift of well no.BW-2 on 22.1.2012 night. Background information: 2. The well No. BWl had been sunken down to its designed founding level i.e. 19.5m with bottom plugging completed. There was a sudden increase of tilt in well No.BW2 and it was found as 1 in 71 in place of 1 in 126. It was reported that there was a likely shift of the well which resulted stoppage of work and survey had to be conducted with Total Station equipment. On thorough inspection it was observed that the pulling station had failed as the strands for pulling the well were found snapped. The site workers reportedly felt sudden disturbance at around 10.30 PM nearby the area of BWI. The report of the total Station survey indicated that there was a shift of well No.BW2 to the tune of about 3.98 m whereas the same was earlier measured on 15.O1 .2O12 as only 0.521 m which indicates that the activity of creep type of movement was in progress at least during 15-22 January, 2012. 3. It was reported that the ground between the BW2 and AW had sunk and the-then existing pond between AW and BW2 had increased in size and depth. At various places in the close vicinity of the well, wide cracks and settlement of ground had been observed; the river-bank line near the BW2 had shifted beyond 7.5 m. towards the river along with temporary Bamboo sheet piles for supporting the working platform used in sinking of the well. All these evidences have been photographically recorded by the client, as reported. 4. River morphology: The Brahmaputra River, the life-line of Assam, has steep gorges and a rapid in Arunachal Pradesh which on entering in Assam becomes a braided river channel (at times 16 km wide) and then with its tributaries, creates a flood plain of 80–100 km wide and 1000 km long. With its origin in the Angsi Glacier, located on the northern side of the Himalayas in Burang County of China's Tibet Autonomous Region as theYarlung Tsangpo, it flows across southern Tibet to break through the Himalayas in great gorges (including the Yarlung Tsangpo Grand Canyon) and into Arunachal Pradesh (India) where it is known as Dihang or Siang. It flows southwesterly through the Assam Valley as Brahmaputra and then southerly through Bangladesh. 5. About 2,900 km long, the Brahmaputra is an important river for irrigation and inland waterway transportation. The average depth of the river is 38 m and maximum depth is 120 m. The river is prone to catastrophic flooding in spring when the Himalayan snows melt. The average discharge of the river is about 19,300 cubic meters per second and floods can reach over 100,000 cubic meters per second. It is a classic example of a braided river and is highly susceptible to channel migration and avulsion. It is also one of the few rivers in the world that exhibit a tidal bore in the hill, to be discussed in further details in para-12 of the latter part of this report. The River Brahmaputra is navigable for most of its length; Guwahati being on the same bank of the river, is connected to National Waterways No 2, with a terminal at Pandu. It is used for movement of bulk & general cargo, passenger vessels and tourist vessels. 6. Morpho-tectonic aspects: Geomorphic signatures have added a discrete domain of the foredeep zone in front of the rising mountain. With its thick pile of valley fills this mega unit is balancing the Isostatic rise of the tectonic mountain mass. Sandwiched between the continental craton to the south and lofty mountains to the north, the foredeep has record of contemporary deformation abundant in the frontal Himalaya, Mishmi and Belt of Schuppen. Three such sites are short-listed for a ready reference. Presence of distinct young fault scarp in the Ultapani- Lalbhita sector in the Assam- Bhutan border attests to occurrence of late Pleistocene- early Holocene large earthquake; while for the Bhareli Dun type valley and Manabhum anticline segments, morpho-tectonic and other geologic-geodetic studies required to tag the deformation history. The region is one of the most seismically active belt of the world with Himalaya and the Indo-Burmese fold-thrust packet from the N/NW and E/SE respectively squeezing the Brahmaputra valley with its crystalline basement between the Main Frontal Thrust (MFT) and the Naga Thrust (NT); from the exposed Mikir hills the basement plunges to the northeast and lies below 6.5-7.0 km of Tertiary sediments near the Mishmi front. The 250 km x 100 km stretch of Assam shelf between the Himalayan frontal belt and Belt of Schuppen, up to the Mishmi frontal thrust is relatively less seismic compared to the area located further southwest encompassing the Meghalaya Plateau- Mikir Hills segment. 7. Geological set-up: The regional geological setting indicates the Bramhaputra valley in Assam is in a state of perpetual flux due to rapid Geomorphological changes with annual devastation by floods; rapid migration of channels, erosion of land and severe earthquakes occurring in the valley. The zone as such is geologically most unstable and a preliminary study indicates that the valley is floored by Quaternary sediments with isolated inselbergs of hills and ridges of Precambrian gneiss-schist complex. The origin of the Assam valley which was evolved in the last two million years is a story of alluvium of the foreland depression in between the young Himalayas to the north and the ancient Shillong plateau to the south. The imagery study suggests that the alluvium rests in fractured pavements, the different blocks of which have undergone differential vertical movements giving rise to uplifted and sunken blocks. 8. Seismotectonic consideration: The Jetty site on the left bank of the Bramhaputra river in the northern fringe of the Shillong plateau and its northern extension forming the basement of the alluvium and the unfolded Tertiary rocks of Assam basin form a triangular crustal block bounded by regional thrusts/ faults. The frequency of earthquake occurrences in the area indicates the presence of very high shear stress and the area thus predictably shows very high seismic activity. A reference to the epicentral map in the Earthquake Atlas of India published by the Geological Survey of India indicates the high Seismicity in the area. 9. Taking into consideration the seismotectonic framework of the area, the three major seismic lineaments close to the site, are: (1) The Main Boundary Fault/Thrust zone in Arunachal Pradesh towards north-east, (2) The Dauki Tear Fault along the southern margins of the of the Shillong plateau along which earthquake of magnitude 7 and 8.7 respectively have occurred in the past, and (3) Dhubri tear fault towards west. A NNE-SSW trending fault is also noted in the imagery containing the present site, which is known for its micro-earthquake events studied by the Geological Survey of India. 10. Site Condition: From the (SPOT) imagery study it is clear that the river Brahmaputra in this area has a major location of bank erosion on the right bank (northern bank) within 500 m to the upstream and opposite to the Pandu Port site. This happens to be the more vulnerable area as evident from the steep bank under active toe-erosion. The narrowness of the gorge near the Pandu-Saraighat Bridge is mainly controlled by the inselbergs. Further towards upstream and downstream of the bridge the river bulges out with development of diara, which can be advantageously used by stabilization process to narrow the width of the river in the upstream. However it should be borne in mind that where the river becomes narrow from an otherwise wide branching pattern, the flow conditions can give rise to increase in velocity resulting bed scour. The river pattern observed from older period indicates that deep scouring of bed rock took place immediately upstream of the Bridge. Bathymetric and land survey map of the Pandu Jetty area, prepared on 24.01.2013 Geotechnical discussion: 11. The detailed contour map recently prepared by Total Station survey over the land and by echo-sounding survey under the sub-water region has immensely helped in the present geological interpretation. Depositional and structural features are inter- related. Differential compaction has produced drape over reefs near the strand line edge. In the map, prominent reef lines are marked as f-f’, which trend NNW-SSE (across the strand line) and suspected to be neotectonic deformation. One such conspicuous reef line trends NNW-SSE between N1070: E 5.5 to N1030: E 6.7. Another such reef line exists at the location N1035: E 3 to N1025:E3.5. These reef lines are easily picked up in the Bathymetric map, while their ground ward continuation is practically untraceable due to the effects of constructional activities. These cannot be correlated with the recorded evidence of neotectonic movement in the area towards SE direction on the northerly slope of the Shillong plateau. (Published Neotectonic map of Geological Survey of India). Furthermore, most reflection recording and processing discriminate against such neotectonic deformation because of the use of arrays and stacking velocities which do not optimize such events.

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