ALPACA and ALPACA Plus Joint Industry Projects (JIPs): October 2017- April 2022

Academic Team: Prof. Richard Jardine (Principal Investigator, Imperial), Dr Roisin Buckley (Glasgow, formerly Oxford and Imperial), Prof. Byron Byrne (Oxford), Dr Tingfa Liu (Bristol, formerly Imperial), Dr Ross McAdam (Ørsted, formerly Oxford), Associate Prof. Stavroula Kontoe (Patras, formerly Imperial) and Ken Vinck (Imperial).

With additional valuable contributions from Dr Reza Ahmadi-Naghadeh, Angus Campbell, Jamie Crispin, Dr Fabian Schranz, Steve Turner, Dr Kai Wen and Imperial’s technical staff.

Contractor: Socotec UK undertook most of the pile installation and testing.

Funding: £2.5m from EPSRC (EP/P033091/1), ORE Supergen Hub, Royal Society, Atkins, Cathie Group, Equinor, Fugro, GCG, Lankelma, LEMS, Ørsted, Parkwind, RWE, Siemens-Gamesa, Scottish Power Renewables and Vattenfall.

Contact: Prof. Richard Jardine

OVERVIEW

The ALPACA JIP started in October 2017 aiming to develop new driven pile design guidance for chalk sites through a comprehensive programme of high-quality field experiments, advanced laboratory testing, rigorous analysis and synthesis with other case history data. It overlapped for some months with the ALPACA Plus project, which ran from October 2019 to April 2022 and involved driving and testing five far larger (and more expensive) piles to help assess important effects of pile scale. The Academic Work Group started with staff from Imperial College London (project lead) and Oxford University, some of whom moved subsequently to new affiliations. The key aim was to develop more rational, representative and reliable design procedures than were available previously, based on secure knowledge regarding pile driving at chalk sites, the piles’ subsequent ageing behaviour and their responses to monotonic-and-cyclic, axial-and-lateral, loading. The research included intensive characterisation of the chalk at the St Nicholas at Wade in Kent (SNW) test site identified in Figure 1. This work advanced in parallel with advanced field testing on over 40 piles driven at SNW. Close analysis of these experiments led to new design methods which can be applied in developing offshore wind farms and a wide range of other offshore, port, bridge and onshore works.

The ALPACA JIPs advanced in parallel with analytical and database studies, including the ALPHA 3D-FE lateral loading analysis project reported by Pedone et al (2023), the analysis of independent driven pile in chalk testing conducted at other sites, as reported by Jardine (2023) and Vinck et al (2023) and the axial load-displacement analyses described by Wen et al (2023).

 

 BACKGROUND

Large driven piles are often adopted to support safety-critical infrastructure, including almost all large offshore structures. However, the reliability with which their capacity can be predicted is surprisingly low in difficult ground conditions, such as chalk. Chalk is a highly variable soft rock that covers much of England and is widespread under the North and Baltic Seas and poses particular problems because its mechanical behaviour is governed by its variable natural structure. The greatest problems are encountered at low-to-medium density chalk sites.

The ALPACA project rectified the previously poor state of knowledge regarding piles driven in low-to-medium density chalk, building on an earlier JIP which ended in September 2017, involving Scottish Power Renewables (SPR)/Iberdrola, Imperial College (ICL) & Geotechnical Consulting Group. The latter study, which was led by SPR in connection with their Wikinger Baltic Wind Farm project included the first remotely operated seabed static pile load tests. These were conducted with bespoke equipment under 40m of water, and were complemented by dynamic testing at two ages, at three glacial till-over-chalk locations (Barbosa et al 2017). Supplementary tests were also conducted at SNW, as reported by Buckley et al (2017). 

 

RESEARCH STREAMS

FIELD TESTING

The ALPACA and ALPACA Plus campaigns involved over 90 advanced field loading tests on 43 impact driven piles, including open tubular (strain-gauged) steel piles with 139, 508mm, 1220mm and 1800mm outside diameters. The scope included:

• Instrumented monitoring of all piles’ dynamic behaviour under driving
• The piles’ static (tension & compression) axial loading responses at ages up to 420 days after driving, as well as some re-strike testing
• The piles’ behaviour under a wide range of cyclic axial loading conditions
• Equivalent Static and cyclic lateral loading tests on similarly aged piles, including one novel biaxial loading experiment.
• Experiments to assess the specific influences of pile tip conditions, shaft material type (covering mild steels, stainless steels and concrete), length-to-diameter (L/D) ratio, pile wall thickness (D/t) ratio, relative water table depth, loading sense and ageing on pile response.
• Examination of the chalk fabric around driven piles and studies of pile corrosion.

 

 

 

 CHALK CHARACTERISATION

 

Intensive field characterisation was conducted at SNW that encompassed seismic and/or CPTu penetration profiling near each test pile, pressuremeter testing and measurements of groundwater conditions with a piezometer and tensiometer. Over 90 state-of-the art advanced laboratory tests were undertaken on block and Geobore-S rotary core samples covering a wide range of pressures. These experiments established the static and cyclic behaviours of both natural chalk and de-structured puttified samples (Fig. 2). The latter “putty” tests modelled the de-structuration that occurs beneath pile tips and around their shafts during driving, which controls the piles’ response to axial loading.

 

 

 

ANALYSIS & SYNTHESIS OF DATA

Following development work carried out under the earlier PISA Joint Industry Project described by Byrne et al (2017), the JIPs made extensive use of FBG fibre-optic pile strain gauge instrumentation (Fig. 3). Strings of FGB gauges allowed profiles of pile shaft axial and bending stresses to be distinguished under axial, lateral, static and cyclic loading. Analysis of high frequency records from the FBG and Pile Driving Analyzer (PDA) instrumentation allowed far more accurate characterisation of the local soil-reactions developed during driving. The large ALPACA Plus piles, whose driving is shown in Fig. 4, gave vital new information on scale effects. Analysis of the large datasets led to the ALPACA-SNW axial capacity method, which was checked against independent load tests at other sites in the UK, France and Germany (see Jardine 2023 and Vinck et al 2023). New axial cyclic design methods were also developed. Synthesis with understanding from the ALPHA project (Pedone et al 2023) led to better predictive methods for monotonic and cyclic lateral loading cases. A parallel study by Wen et al (2023) showed how the piles’ axial load-displacement behaviour can be predicted accurately.

 

 

 

 

 

 

 

 

 

 

OUTCOMES

The research findings are now being applied in several multi-£bn windfarm projects where chalk strata are encountered at foundation depth. As outlined below, the ALPACA and ALPACA Plus outcomes have been published in open-access papers in major journals, plus other conference and related papers. A further paper (McAdam et al 2024) covering the lateral loading pile tests is currently under final review with Geotechnique.

 

 

 

 

 

 

 

 

 

 

 

PUBLICATIONS FROM ALPACA JIPS

Journals

J1. Liu, T.F., Ferreira, P., Vinck, K., Coop, M.R., Jardine, R.J. and Kontoe, S.K., (2023). The behaviour of a low-to-medium density chalk under a wide range of pressure conditions. Soils & Foundations. 63 1: 16. DOI:10.1016/j.sandf.2022.101268

J2. Buckley, R.M., Jardine, R.J., Kontoe, S., Liu, T., Byrne, B.W., McAdam, R.A., Schranz, F. and Vinck, F. (2023). “Axial cyclic loading of piles in low to medium density chalk.” Géotechnique DOI:10.1680/jgeot.22.00044

J3. Jardine, R.J., Buckley, R.M., Liu, T., Andolfsson, T., Byrne, B.W., Kontoe, S., McAdam, R.A., Schranz, F. and Vinck, F. (2023). “The axial behaviour of piles driven in chalk.” Géotechnique DOI:10.1680/jgeot.22.00041

J4. Ahmadi-Naghadeh, R., Liu, T., Vinck, K., Jardine, R.J., Kontoe, S., Byrne, B.W. and McAdam, R.A. (2022). “A laboratory characterisation of the response of intact chalk to cyclic loading.”  Géotechnique DOI:10.1680/jgeot.21.00198

J5. Vinck, K., Liu, T., Jardine, R.J., Kontoe, S., Ahmadi-Naghadeh, R., Buckley, R.M., Byrne, B.W., Lawrence, J.A., McAdam, R.A. and Schranz, F. (2022). “Advanced in-situ and laboratory characterisation of the ALPACA chalk research site.” Géotechnique DOI:10.1680/jgeot.21.00197

J6. Liu, T., Ahmadi-Naghadeh, R., Vinck, K., Jardine, R.J., Kontoe, S., Buckley, R.M. and Byrne, B.W. (2022). “An experimental investigation into the behaviour of de-structured chalk under cyclic loading.” Géotechnique DOI:10.1680/jgeot.21.00199

J7. Buckley, R.M., McAdam, R.A., Byrne, B.W., Doherty, J.P., Jardine, R.J., Kontoe, S. and Randolph, M.F. (2020). "Optical fibre Bragg grating measurements during impact pile driving in chalk." ASCE Journal of Geotechnical and Geo-environmental Engineering 146 9:04020082. doi.org/10.1061/(ASCE)GT.1943-5606.0002293

J8. Jardine, R.J., Buckley, R.M., Byrne, B.W., et al. (2019). "Improving the design of piles driven in chalk through the ALPACA research project." Revue Francaise de Géotechnique, 190008s. DOI: 10.1051/geotech/2019008

Conferences

C1. Liu, T., Ahmadi-Naghadeh, R., Vinck, K., Jardine, R.J., Kontoe, S., Buckley, R.M., Byrne, B.W. and McAdam, R.A. (2023). “Laboratory investigation of the cyclic loading behaviour of intact and de-structured chalk”. Proc. 8th Int. Symp on Deformation Characteristics of Geomaterials. IS-Porto. Porto.

C2. Vinck, K., Liu, T., Jardine, R.J., Kontoe, S., Buckley, R.M. Byrne, B.W., McAdam, R., Ferreira, P. and Coop, M.R. (2023). “The monotonic behaviour of a low- to medium-density chalk through in-situ and laboratory characterisation”. Proc. 8th Int. Symp on Deformation Characteristics of Geomaterials. IS-Porto. Porto.

C3. Liu, T., Vinck, K., Ushev, E. and Jardine, R.J. (2023). “In-situ and laboratory characterisation of stiff and dense geomaterials for driven pile analysis and design”. Keynote Lecture, 8th Int. Symp on Deformation Characteristics of Geomaterials. IS-Porto. Porto. To appear in Soils and Rocks.

C4. Liu, T., Ahmadi-Naghadeh, R., Buckley, R.M., McAdam, R.A., Jardine, R.J., Vinck, K., Kontoe, S. and Byrne, B.W. (2023). “Cyclic characterisation of low-to-medium density chalk for offshore driven pile design.” SUT OSIG 9^th International Conference on “Innovative Geotechnologies for the Energy Transition”. London.

C5. Jardine, R.J., Liu, T., Kontoe, S., Schranz, F., Buckley, R.M., Byrne, B.W., McAdam, R.A. and Vinck, K. (2023). “Driven pile behaviour in low-to-medium density chalk: The ALPACA JIP outcomes.” SUT OSIG 9^th International Conference on “Innovative Geotechnologies for the Energy Transition”. London.

C6. Vinck, K., Jardine, R.J., Willow, A., Liu, T. and Byrne, B.W. (2023). “The effects of steel corrosion on the interface shearing behaviour of chalk.” SUT OSIG 9^th International Conference on “Innovative Geotechnologies for the Energy Transition”. London.

C7. Vinck, K., Jardine, R.J., Buckley, R.M., McAdam, R.A., Liu, T., Kontoe, S., Byrne, B.W., Ferreira, P. and Coop, M. (2023). “Monotonic characterisation of low-to-medium density chalk for driven offshore pile design.” SUT OSIG 9^th International Conference on “Innovative Geotechnologies for the Energy Transition”. London

C8. Buckley R.M., Byrne, B.W., Doherty, J.P., Jardine, R.J., Kontoe, S., McAdam, R.A. and Randolph, M.F. (2021). "Measurements of distributed strain during impact pile driving". BGA Conference Piling 2020, Durham, UK (March 2021).

C9. Buckley, R.M., Jardine, R.J., Byrne, B.W., Kontoe, S., McAdam, R.A., Ahmadi-Naghadeh, R., Liu, T., Schranz, F. and Vinck, K. (2020). “Pile behaviour in low-medium density chalk: Preliminary results from the ALPACA project.” ISFOG 2020, Austin, Texas (August).

C10. Jardine, R.J., Buckley, R.M., Byrne, B.W., McAdam, R.A. and Kontoe, S. (2019). “Research to improve the design of driven piles in chalk: the ALPACA project.” Coastal Structures Conference 2019, Hannover, Germany (Oct).

C11. Jardine, R.J., Kontoe, S., Liu, T.F., Vinck, K., Byrne, B.W., McAdam, R.A., Schranz, F., Andolfsson, T. and Buckley, R.M. (2019). “The ALPACA research project to improve design of piles driven in chalk.” Proceedings of the XVII European Conference on Soil Mechanics and Geotechnical Engineering (ECSMGE), Reykjavik, Iceland (Sep).

C12. Jardine, R.J., Kontoe, S., Liu, T.F., Vinck, K., Byrne, B.W., McAdam, R.A., Schranz, F. and Buckley, R.M. (2019). “Rationalising the design of piles driven in chalk through the ALPACA project.” Extended abstract, 2nd International Conference on Natural Hazards and Infrastructure, ICONHIC, Chania, Greece (June).

 

Papers from linked projects

Jardine, R.J. (2023). “Time-dependent vertical bearing behaviour of shallow foundations and driven piles”. 6^th ISSMGE McClelland Lecture. Proc. 9th Int. Conf. on Offshore Site Investigations and Geotechnics, SUT London. pp 27-81.

Pedone, G., Kontoe, S., Zdravković, L., Jardine, R. J., Vinck, K., & Liu, T. (2023). “Numerical modelling of laterally loaded piles driven in low-to-medium density fractured chalk”. Computers and Geotechnics, 156, https://doi.org/10.1016/j.compgeo.2023.105252

Wen K., Kontoe, K., Jardine, R.J. and Liu, T.F. (2023). “An axial load transfer model for piles driven in chalk”. ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol 149, No 11. https://doi.org/10.1061/JGGEFK.GTENG-11368.

Vinck, K., Liu, T.F., Mawcet, J., Kontoe, S. and Jardine, R.J. (2023). “Field tests on large scale instrumented piles driven in chalk: results and interpretation”. Canadian Geotechnical Journal. Vol. 60, No 10. https://doi.org/10.1139/cgj-2022-0441.

 

Background papers

Barbosa, P., Geduhn, M., Jardine, R.J. and Schroeder, F.C. (2017) “Large Scale Offshore Static Pile Tests – Practicality and Benefits”. Proc 8^th Int. Conf. on Offshore Site Investigations and Geotechnics, SUT, London.

Buckley R. M., Jardine, R. J., Kontoe, S., Parker, D. & Schroeder, F. C. (2017a). “Ageing and cyclic loading behaviour of piles driven in a low to medium density chalk”. Géotechnique, https://doi.org/10.1680/jgeot.17.P.012.

Byrne, B.W., McAdam, R.A., Burd, H.J. Houlsby, G.T., Martin, C.M. Beuckelaers, W.J.A.P, Zdravkovic, L., Taborda, D.M.G, Potts, D.M., Jardine, R.J, Ushev, E., Liu, T.F., Abadias, D., Gavin, K., Igoe, D., Doherty, P., Skov Gretlund, J., Pacheco Andrade, M., Muir Wood, A., Schroeder, F.C, Turner, S and Plummer, M. (2017) “PISA: New Design Methods for Offshore Wind Turbine Monopiles”. Keynote. Proc 8^th Int. Conf. on Offshore Site Investigations and Geotechnics, SUT, London.