Geological & Geo-technical Assessment of the subsea

Executive Summary

National Institute of Ocean Technology (NIOT) as a follow up of the earlier survey caried out, detailed Geological & Geo-technical operations in the Sethusamuthram Channel Project.  Earlier 40 locations were identified for Vibro core operations.   However, NIOT carried out 44 Vibro coring operations by deploying 6m long core lines with a very specially designed "A" frame on board vessel Shiva-I.  As agreed with the principles, 4 land bore holes were drilled in the island area forming part of the Adam's bridge and 6 bore holes were drilled in the proposed channel alignment in zone of Adam's bridge.

Samples were collected sectorwise.  In sector AB, 4 vibro cores were collected wherein the measured water depth at the time of collection varied from 6.5m to 9.1m.  The recovery of cores varied from 0.5 to 1.8m.  In general, the top layer is medium to fine sand.  Vibro core AB1 and AB3, intersected hard strata along with sand at depth.  The area being shallower than the required 12m, would require higher dredge, operations including probably in hard formations.  In sector BC, 6 Vibro cores are collected wherein the water level varied from 8m to 12m and the core recovery from Nil to as much as of 2.6m.  Most of the Vibro core had sandy strata. On top we find sandy to clayey sand.  In VC BC-6 to BC-3 at depth calcareous stone was intersected.  In VC BC-1 only, there are no core recovery, inspite of repeated attempts, few pieces of hard shell bed was collected right on the sea bed surface.  Here also, the dredging may have to be carried out to obtain the desired depth of 12m.

In sector CD, 8 Vibro cores were collected where in water level varied from 12.7 to 13.38 m.  The core recovery varied from 1.10 to 2.35m.  All the Vibro cores intersected, softy clay at the top and medium to stiffy clay at depth.  In the present scenario this sector may not require dredging.  The details gathered will be useful when the project authorities go for deepening of the channel.  Regarding sector DE a total of 11 vibro cores were collected with the water level varying from 12.5m to 13.5m.  All the vibro cores intersected, soft clay at top and medium stiff to stiff clay at depth.  Only in DE2 along with clay, small pieces of rocks were obtained at the bottom.  In sector EE4 a total of 13 vibro cores were collected.  The measured water level varied from 7.00 to 12.10 m.  The S-W part vibro cores intersected mostly clay at the top and at the bottom, excepting vibro core EE4/3A and EE4/3B where at the bottom calcareous stone were collected.  The vibro cores in the northern sector had mostly sandy clay on top and sand strata with sand stone at depth excepting EE4/7A had intersected sand stones pieces in depth.  Vibro core EE4/1, EE4/9 and EE4/13 met with calcareous sand stone at the bottom.  All the cores collected were analysed for their geo-technical properties.  Some of the core samples were tested for their calcium carbonate content and for their organic content, the details are given in the table in the report separately. 

Four (4) land Bore Holes were drilled in the various islands on either side of the proposed channel area adjoining the Adam's bridge area.   In Bore Hole 11 from 4.5 to 6.5m calcareous sand stone and rock pieces along with clay and sand in this Bore Hole were encountered.  N value based on SPT test varied, from 9 - 100.  In Bore Hole 4 between 4.5 and 7.5 m peices of calcareous rock occur along with the calcareous sand and in between 7.5 and 12m pieces of sand stones in the clayey silt.  The N values here vary from 9 to as high as 76.  In BH1 between depth 7.5 to 10.5m calcareous  stone pieces were met with.  From 10.5 to 12m it intersected rock pieces and calcareous sand stones.  The N vale varied from 9 to 51.  Again sand  with rock pieces was met with between 15 and 16.5m.  Beyond this the strata is seen to be sandy upto 21m.  In BH2 from 9-12m, it is clay with pieces of calcareous sand  stones and between 12 and 13.5m, hard formation containing calcareous  sand stones / coral, etc.  Again between 17 and 19.5m there are sand stones and coral pieces with sand.  The BH ended with the silty sand at 21m.  The hard formation with clay occurs in the Bore Hole at greater depth than in the seabed where this occurrence is shallow and at less than 2m below the sea bed which has caused either bending of the core barrel or damaging the core catcher.  These aspects should be taken into consideration which planning for dredge operations.

In the offshore domain 6 Bore holes were drilled along the proposed channel alignment in the water depth of 2 to 6m in Zone A, Adam's Bridge.  

Bore Hole No.5:  In the bore hole sand met at depth from the seabed to 9m and the SPT N values varied from 14 to 33.  Between 9m - 12m coral rock with thin partings of sands are seen.  Coral rock within sand partings occur between 12.5 to 14m.  Alternate sequence of sand and coral rocks occur between depths 14 to 18m.  Between 18 to 22m clayey silt with sand stone pieces are met.

Bore Hole No.5:  In this bore hole sands are reported uopto 6m below the sea bed.  Between 6 to 9m coral rock with sand layers occur.  Between 9 to 15m silty sands with gravel occur.  Coral rocks occurs between 15 to 16m.  Beyond 16.5 m mainly sands are encountered.  The SPT values varied in general form 19 to 54.

Bore Hole No.7:  The top 6m below sea bed sands are intersected for which the SPT N values varied between 26 to 44.  Gravel pieces with sands are intersected between 6 to 7.5m depth.  Between 7/5m to 13m mainly sands are intersected for which SPT N values are of the order of 5 to 43.  An alternating sequences of coral rocks and sands are intersected between 13m to 21m for which high order of SPT N values are recorded.

Bore Hole No.7A:  In this Bore hole sands met at depth from the seabed to 7/9m and SPT N values varies from 10 to 37.  Between 7.9 to 9.5m coral rock with thin parting of sand are seen.  Silty sands occur between 9.5m to 12m.  Alternate sequences of sands and coral rocks occur between depths 12m to 23.5m.

Bore Hole No.8:  In Bore hole sands are reported at depth 6m below the seabed.  Between 6 to 10m coral rock with thin sand layers occurs.  Between 10 to 12m sand with gravel pieces.  Coral rock occurs between 12 to 15m.  Beyond 15m upto 23m, mainly sands are encountered.  The SPT N values varies in general from 12 to 57.

Bore Hole No.10:  This bore hole were drilled upto 4.5m from the seabed.  This bore hole were mainly silty sand.  The SPT N valese were 6 and 9.

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