SEABED DYNAMICS - France Energies Marines

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SEABED DYNAMICS

Challenges of rocky substratum

Elisabeth PALIX, EDF Renewables

FEM S&T Tribune 2020 | Seabed Dynamics1

French geological context


Offshore windfarm development in France A01 & AO2


• A01o Fécamp: chalk, GBS

o Courseulles: Marl/limestone, MP

o St Brieux: Granite, Jacket

o St Nazaire: limestone/sand, MP

• A02o Le Treport: Chalk, jacket

o Noirmoutier: limestone, jacket

Work done by the French Comity of Soil Mechanic


Off the French coasts, the most commonly encountered sedimentary rocks include chalks, marls and limestone rocks. Calcarenites, volcanic rocks (basalt) or metamorphic rocks (granite, gneisses) can also be encountered.

9.7.3. DrillingRecommendations for bored piles construction are provided in the NF EN 1536 (2015) standard. A few essential points are reminded below: …

9.4.3.1. ULS conditionsThe French experience of land foundations on bored piles (NF P 94-262, 2012), backed by reliability analyses achieved using the database mentioned in paragraph 9.4.2.1,

What are the challenges for foundations in/on rocky substratum?

• How to adapt site investigation and laboratory tests to rocky conditions?

• How can we install foundations in rock?

• What will be the behaviour of the rock / pile interface?

• What will be the behaviour of the rock mass under pile lateral loading?

• What will be the impact of cyclic loading?


Fécamp: GBS example

• Can we drive through a flint band?

• Is the upper chalk strong enough?

• What is the evolution of stiffness properties with time?

6 FEM S&T Tribune 2020 | Seabed Dynamics

Jacket cases in chalk


Very low friction during driving (10 – 20 kPa)

CIRIA (2002)• 20 kPa for low and medium density chalks• 120 kPa high density chalk

NF P 94-262 (2012): ultimate friction• soft: 5 - 20 kPa• weathered: 20 – 40 kPa• intact: 40 – 50kPa

CFMS recommendations (safety factor in chalk):In chalks, given the uncertainty associated to the models presently available to assess the capacity in these materials, it is recommended to adopt: γR1= 1.30. This factor may be lowered if representative and well-documented piles tests (as per paragraph 9.4.2.3) allow an in depth understanding of the phenomena involved and improve significantly the degree of confidence in the assessment of capacity.

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Jacket cases in chalk


• ALPACA (Axial Pile Capacity in Chalk) JIP

• Wigkinger offshore pile tests

• Le Treport onshore pile tests

Driven or drilled and grouted monopiles


simplification

Onshore pile tests in calcarenite

FEM S&T Tribune 202010

• 11 piles of 0.76 and 1.2m diameter

• Different installation methods tested: driving only, drive/drilled/drive and drilled and grouted

• Lateral load applied at 5m above ground level

• 4 static + 4 cyclic lateral load tests

Implementation of observations and results in FEM


SOLCYP+ project


Axial cyclic response:

• Limestone / grout + chalk / steel interface tests (3SR Grenoble)

• Pile in chalk: synthesis of French knowledge and modelling (LGCgE Lille)

Lateral cyclic response:

• Lateral loading in sand through centrifuge tests (IFSTTAR Nantes)

• Behavior and modelling of monopiles in limestones (onshore tests)

Conclusions

• Rock composition (e.g. carbonate content), structure (e.g. porosity) and history greatly affect its behaviour

• Rock is a very heterogenous material by nature (=> heterogeneity of results)

• Foundation design and installation really need to be looked at together

• Still some work in progess in the industry (i.e. ALPACA and SOLCYP+ JIPs)

• Can we rely on the ageing effect in chalk?

• Many other topics have not been discussed today: hole stability, cable protection, jacking on hard ground…


SEABED DYNAMICS - France Energies Marines

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