8.5.12| VERY LOW STRENGTH SOILS

On review of the draft VC logs very soft clay (extremely low strength) are present in VC sample 282-VC-R5-064 at 2.0 m; 282-VC-R5-060 at 1.0 m and low strength CLAY at 1.5 m.

This section is to be updated with geotechnical information on shear strength once final laboratory tests have been received.

8.6 ARCHAEOLOGY CONSIDERATIONS

During the survey there was no obvious archaeological findings observed. Remnant defensive structures from WWII and an historic ship wreck have been described in the relevant sections above (Wrecks and Anthropogenic Debris). However, it is suggested that a full archaeology investigation be conducted on the data collected here by professionally qualified archaeologists in order to assess the possibility of palaeo landscapes that could have been occupied by early man. This type of analysis is out with the scope for this report.

CONCLUSIONS

A topographic, geophysical and geotechnical survey was successfully carried out along the Thor OWF export cable route survey corridors. The following datasets were collected from hull mounted, towed and ROTV platforms: MBES, SSS SBP, and MAG. Geotechnical sampling was completed using vibrocorer (VC), CPT and a Van Veen grab.

The survey was conducted in a safe manner and good quality data was acquired throughout. The survey was conducted in four sections; the onshore topographic survey, the nearshore geophysical survey, the offshore geophysical survey and the geotechnical survey.

ROUTE 2

Route 2 is generally characterised by gentle to moderate seabed slopes. The maximum slope for Route 2 on the landfall section is 37.4° at KP 0.103 (beach dunes) and, sub-sea, 6.1° at KP 21.057.

Maximum and minimum depths of -29.23 m and -1.83 m occur at KP 21.444 and KP 0.340 respectively.

Maximum and minimum elevations of +13.92 m and +0.76 m occur at KP 0.156 and KP 0.280, respectively. Surficial sediments predominately consist of SAND with patches and bands of gravelly SAND to sandy GRAVEL. Discrete areas of DIAMICTON are also present, mainly crossing the corridor from KP 20.854 to the end of the route at KP 21.444.

Occasional boulder fields are common throughout, as are ripples which generally form bands that cross the corridor on a NE-SW orientation.

In the near shore area manmade structures include two WWII concrete bunkers and an area of coastal defence concrete blocks. The former is located in the northern section of the intertidal zone and the latter towards the southern end of the site.

Shallow soils are characterised by an upper SAND unit that varies from silty SAND to SAND and Gravel.

It extends along the entire route and ranges from seafloor to 3.4 m depth BSB. Below this unit is a discontinuous unit formed from channel infill sediments (from homogenous SANDS to laminated silty SAND, GRAVEL and CLAY) which occasionally contain PEAT and or organic material.

There are a number of factors for cable protection within the survey corridor of Route 2 that should be considered during planning of the final cable route.

 Low strength soils were not extensively sampled along the route. From the geotechnical data only one VC (282-VC-R2-004) encountered very soft CLAY within the upper 1.5 m BSB. However, this should be kept in mind during the installation phase as soft CLAY can cause problems during trenching if encountered within the required burial depth.

 Fishing, needs to be considered as it is associated with high risks to the cable if not buried sufficiently. Although trawl scars were not observed, fishing activity is clearly evident as witnessed by the survey teams during the survey campaign.

 A number of manmade structures were observed in the nearshore and should be given as wide a clearance as possible.

 A dedicated UXO survey has not been performed and this should at least be considered prior to the cable installation.

ROUTE 3

Route 3 is characterised by a varied seabed. Several erosional channels and broad sand waves are evident. The maximum sub-sea slope for Route 3 is 9.0° at KP 9.767 where a small bank is encountered.

The maximum slope at the landfall is 37.4° at KP 0.103 where the route crosses the dunes. Maximum

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Surficial sediments predominately consist of SAND for the majority of the route; only in the central portion of the route corridor are areas of layered GRAVEL and gravelly SAND to sandy GRAVEL encountered. DIAMICTON is only really encountered between KP 6.500 and KP 11.000.

Areas of ripples are observed throughout the corridor and generally form thin bands that occasionally cross the corridor in a NNE-SSW orientation. Zones of seabed depressions are only observed from KP 15.000 onwards; the lack of acoustic blanking in the SBP records suggest these are formed from bottom current regime rather than shallow biogenic gas.

Two potential wrecks or wreck debris were located at KP 5.370 (S_R3_0002, DCC 28 m) and KP 5.391 (S_R3_0108, DCC 6 m).

Boulder fields are very common within the corridor and are particularly concentrated between KP 2.980 to KP 4.628, KP 7.923 to KP 11.148 and KP 20.682 to KP 21.861.

Shallow soils are characterised by an upper SAND unit that is typically acoustically transparent. It extends along the entire route and ranges from seafloor to 3.1 m depth BSB. Below this unit is a discontinuous unit formed from channel infill sediments (from homogenous SAND to laminated silty SAND, gravel and CLAY) and reach the limit of SBP penetration at 7.8 m BSB.

There are a number of factors for cable protection within the survey corridor of Route 3 that should be considered during planning of the final cable route.

 Low strength soils were not extensively sampled along the route. From the geotechnical data only one VC (282-VC-R3-019) encountered soft to firm CLAY within the upper 1.0 m BSB. However, this should be kept in mind during the installation phase as soft CLAY can cause problems during trenching if encountered within the required burial depth.

 Fishing, needs to be considered as it is associated with high risks to the cable if not buried sufficiently. Although trawl scars were not observed, fishing activity is clearly evident as witnessed by the survey teams during the survey campaign.

 A dedicated UXO survey has not been performed and this should at least be considered prior to the cable installation.

ROUTE 4

Route 4 is characterised by an undulating seabed with gentle gradients. The maximum sub-sea slope for Route 4 is 6.0° at KP 17.155. At landfall the maximum slope is 43.0° at KP 0.167. Maximum and minimum depths of -29.67 m DTU15 MSL and -3.75 m DTU15 MSL occur at KP 24.207 and KP 0.477, respectively. Maximum and minimum elevations of +19.16 m and +1.26 m occur at KP 0.134 and KP 0.245, respectively. Initial surficial sediments from the extent of the nearshore data to KP 2.000 consists of SAND with frequent, discrete outcrops of DIAMICTON.

Boulder fields are sparse along the entire route, being located as discrete bands at approximately KP 2.000, KP 4.000 and KP 17.000.

SAND with occasional areas of gravelly SAND to sandy GRAVEL and GRAVEL make up the sediments within the route corridor. Ripples, present where GRAVEL sediments are observed are not considered to be a major concern for cable lay.

Three main areas of depressions are present within the survey corridor. They cross the survey corridor between approximately KP 10.500 to KP 11.400, KP 13.000 to KP 16.500 and KP 22.200 to KP23.000.

Two contacts (S_R4_0075, DCC 105 m and S_R4_0076, DCC 93 m) are observed as potential fishing debris, comprising a possible clump weight and rope.

Shallow soils are characterised by an upper SAND / silty SAND. It extends along the entire route and ranges from seafloor to 3.0 m depth BSB. Below this unit is a discontinuous unit formed from channel infill sediments (from homogenous SANDS to laminated silty SAND, GRAVEL and CLAY) and reach the limit of SBP penetration at 7.7 m BSB.

There are a number of factors for cable protection within the survey corridor of Route 4 that should be considered during planning of the final cable route.

 Low strength soils were not extensively sampled along the route. From the geotechnical data only one VC (282-VC-R4-037) encountered very soft CLAY within the upper 2.5 m BSB. However, this should be kept in mind during the installation phase as soft CLAYS can cause problems during trenching if encountered within the required burial depth.

 Fishing, needs to be considered as it is associated with high risks to the cable if not buried sufficiently. Although trawl scars were not observed, fishing activity is clearly evident as witnessed by the survey teams during the survey campaign.

 A dedicated UXO survey has not been performed and this should at least be considered prior to the cable installation.

ROUTE 5

Route 5 is characterised by an undulating seabed with gentle gradients. The maximum sub-sea slope for Route 5 is 5.0° at KP 15.655. Maximum and minimum depths of -27.22 m DTU15 MSL at KP 19.041 and -3.75 m DTU15 MSL at KP 0.477, respectively. Maximum and minimum elevations of +19.16 m and +1.26 m occur at KP 0.134 and KP 0.245, respectively.

Surficial sediments are dominated by SAND throughout the Route 5 survey corridor. Discrete outcrops of DIAMICTON are present from KP 1.000 to KP 2.000.

Occasional boulder fields are present essentially in the nearshore secretin only, forming two bands that extend across the corridor at approximately KP 2.000 and KP 4.000.

Two large area of depressions was observed crossing the survey corridor between KP 10.757 and KP 14.697 and KP 15.634 and KP 18.145. No areas of acoustic blanking were noted within the SBP data.

A channel, dividing two sand banks is observed crossing the route at KP 20.670. The eastern sand bank also contains sand waves, indicative of active mobile sediments.

One contact, a possible wreck, was located at KP 4.226, (S_R5_0532, DCC -328 m).

There are a number of factors for cable protection within the survey corridor of Route 5 that should be considered during planning of the final cable route.

 Low strength soils were not extensively sampled along the route. From the geotechnical data only VC, 282-VC-R5-064 and 282-VC-R5-060 encountered very soft CLAY within the upper 2.0 m BSB.

However, this should be kept in mind during the installation phase as soft CLAYS can cause problems during trenching if encountered within the required burial depth.

 Fishing, needs to be considered as it is associated with high risks to the cable if not buried sufficiently. Although trawl scars were not observed, fishing activity is clearly evident as witnessed by the survey teams during the survey campaign.

 A dedicated UXO survey has not been performed and this should at least be considered prior to the cable installation.

The survey corridors present challenges in a number of areas and careful routing as well as engineering

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