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Ground investigation: Changing tides

Tidal power could offer potential for the UK to achieve its renewable energy targets and ground investigation for the first scheme was critical to gaining planning consent.

The UK government has committed to increasing its production of energy from renewable sources from 5% to 15% but achieving this calls for diversification in the types of energy exploited. Tidal power is viewed as one of the key sources of energy that could meet government targets and deliver long-term, low-cost, reliable and predictable power to communities.

3513 lagoon wall visual revc jan2014 lr

Ground investigation contractor ESG has said that ground investigation work for the project played a pivotal role in the scheme gaining planning consent.

One of the first tidal schemes looks set to be one planned for Swansea Bay Tidal Lagoon which would capture the energy on the 6.5m tidal range from the mouth of the Severn Estuary. Ground investigation contractor ESG has said that ground investigation work for the project played a pivotal role in the scheme gaining planning consent.

The completed structure would be located southeast of Swansea and include a 9.5km seawall from between the mouth of the River Tawe and Crmlyn Burrows to create an 11.5km2 lagoon.

The project is classed as a “nationally significant infrastructure project” under the Planning Act 2008 so planning permission falls under the remit of the Planning Inspectorate. ESG has said that ground investigation work was vital for the front end engineering design which was used for the application, which has now been granted.

On-site challenges

The initial ground investigation, designed by Atkins, was carried out by ESG as principal contractor. The variable nature of the environments between the beach and nearshore demanded the use of a range of investigation techniques for the different site zones: Zone A (furthest offshore), Zone B (intertidal) and Zone C (upper intertidal/beach).

1254 arrival and rock pools

The UK government has committed to increasing its production of energy from renewable sources from 5% to 15% but achieving this calls for diversification in the types of energy exploited

Given the high costs of marine ground investigation, a key challenge was making sure the scope of the investigation would properly support the planning application.

ESG drilled exploratory holes spaced over hundreds of metres focusing on the proposed turbine area, the bund wall footprint, potential dredging zones and the central ridge identified in the geophysical survey.

The ground investigation was phased so that preliminary information could be gained from cone penetration tests (CPT) and underwater sediment sampling (vibrocoring), prior to finalising the overwater borehole requirements.

Zone C works comprised four dynamically sampled boreholes using a restricted access rig with adjacent CPTs from ESG’s medium crawler unit.

Seven overwater boreholes were drilled by ESG from a jack-up platform. These were carried out by cable percussion boring through the superficial deposits using a Dando 3000 rig, and continued with Geobor S triple tube rotary coring of the underlying bedrock using a Beretta T51.

Nine CPTs were carried out to a maximum depth of 17m from a survey vessel using a Datum Neptune 5000 seabed CPT unit. These were used to provide additional information on the soil conditions between boreholes. Dissipation tests were also performed in conjunction with CPTs to assess consolidation properties of the softer superficial deposits.

Ten vibrocores were carried out using a 6m long, 100mm diameter vibrocorer operated from the survey vessel, to recover samples for laboratory testing. These were then used to determine the suitability of near surface materials for dredging and reuse in the bund wall construction.

Testing programme

Soil and rock samples obtained during the investigation tested to determine index properties, strength and stiffness, permeability and chemical composition relating to concrete durability. The test information was used, together with the field test data, to provide parameters for preliminary stability and seepage analysis for the proposed embankment and cut off wall. Assessment of the material was conducted to also determine its suitability for reuse in the construction of the seawall, as well as the suitability of the material for reuse and disposal from a geoenvironmental perspective.

According to ESG, the phased approach taken by the company meant that there was much more control and flexibility available than is typical in similar projects.

“The preliminary ground investigation of the bay has allowed us to obtain complex data regarding the geological makeup of the bay,” said Atkins principal engineering geologist Simon Holt. “This information informed the early stages of the planning process and demonstrated the bay is safe to undergo development and formed.”

Western landfall building

The completed structure would be located southeast of Swansea and include a 9.5km seawall from between the mouth of the River Tawe and Crmlyn Burrows to create an 11.5km2 lagoon

 

 

 

 

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