ALPACA: Axial-Lateral Pile Analysis for Chalk Applying multi-scale field and laboratory testing

Academic Team: Prof. Richard Jardine (ICL, Project Lead), Dr Stavroula Kontoe (ICL), Prof. Byron Byrne, Prof. Ross McAdam

Research team: Roisin Buckley (ICL) and two further appointments (OU/ICL) starting in spring 2019

Funding: EPSRC (EP/P033091/1) and industrial sponsors

Duration: October 2017 - September 2019

OVERVIEW

The ALPACA project started in October 2017 with funding from EPSRC (£1.38m) and Industry (£390k; contributions from Atkins, Cathie Associates, DNVGL, Fugro, GCG, Iberdrola/SPR, Innogy, LEMS, ØRSTED, Siemens) aiming to develop new driven pile design guidance for chalk sites through a comprehensive programme of high quality field tests, advanced laboratory testing, rigorous analysis and synthesis with other case history data. The Academic Work Group comprises academics and researchers from Imperial College London (project lead) and Oxford University, with the key aim to develop design procedures that overcome, for chalk, the current shortfalls in knowledge regarding pile driving, ageing, static and cyclic response under axial and lateral loading. The research has applications with offshore wind turbines and oil platforms as well as port, bridge and other works.

The project remains open to participation from any further sponsors who are able to contribute financially and take part in the project steering committee. Contact r.jardine@imperial.ac.uk for further details.

BACKGROUND

Large piles are driven to support many safety critical structures, particularly in offshore applications. 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 behaviour is governed by its variable natural structure.

The ALPACA project aims to rectify the poor state of knowledge, building on an earlier Joint Industry Project (JIP) involving Scottish Power Renewables (SPR)/Iberdrola, Imperial College (ICL) & Geotechnical Consulting Group. The JIP, which ended in Sept 2017 and was led by SPR in connection with their Wikinger Baltic Wind Farm project, involved, for the first time, remotely operated underwater static pile load tests. The latter, which were conducted in 40m depth of water with bespoke equipment, were complemented by dynamic stress wave monitoring at three Chalk and Glacial Till dominated locations; see Barbosa et al (2017).

RESEARCH STREAMS

  • FIELD TESTING

The ALPACA campaign, which started in November 2017, involves multiple advanced field tests on 21 tubular steel piles (driven at two scales; 500mm & 136mm OD) with novel fibre-optic instrumentation at an established chalk test site in Kent (Fig. 1) and additional experiments with a 36mm OD mini-pile with pore pressure instrumentation. Focus is given on: 

  • Dynamic driving behaviour and axial responses under static (tension & compression) and cyclic loading 8 months after installation
  • Static and cyclic lateral loading pile tests 9 months after installation

 Figure 1: 500mm OD pile installation by Socotec Nov. 2017, Kent
Figure 1: 500mm OD pile installation by Socotec Nov. 2017, Kent

  • CHALK CHARACTERISATION

Chalk characterisation is being aided by additional seismic and CPTu penetration testing, tests with a new mini-ICP instrumented pile and pile exhumation after testing.

Comprehensive laboratory testing is underway on rotary core samples to examine the static and cyclic behaviours of natural chalk and also puttified samples (Fig. 2), which model the de-structuration that occurs beneath pile tips during driving.

Figure 2: Putty chalk preparation at natural w/c Doughty (2016)
Figure 2: Putty chalk preparation at natural w/c Doughty (2016)

  • ANALYSIS & SYNTHESIS OF DATA

The comprehensive field testing includes novel fibre-optic instrumentation (Fig. 3) which allows the profiles of pile shaft axial and bending stresses to be distinguished under axial, lateral, static and cyclic loading, following development work carried out under the earlier PISA Joint Industry Project described by Byrne et al (2017). Analysis of a combined high frequency dataset from the fibre-optic and Pile Driving Analyzer (PDA) instrumentation will be used to accurately characterise the soil-reactions during the driving process (Fig. 4). Synthesis of the Kent site tests with information from a range of other chalk studies, including the full-scale Wikinger tests and new tests in France will lead to an integrated methodology that will allow far more reliable design for piles driven in chalk.

Figure 3: Careful installation of fibre-optic instrumentation at the 500mm OD piles, Oct. 2017
Figure 3: Careful installation of fibre-optic instrumentation at the 500mm OD piles, Oct. 2017

Figure 4: Indicative time series of fibre optic strain measurements along the length of a 500mm OD pile during driving, Nov. 2017
Figure 4: Indicative time series of fibre optic strain measurements along the length of a 500mm OD pile during driving, Nov. 2017

REFERENCES

Barbosa, P., Geduhn, M., Jardine, R.J. and Schroeder, F.C. (2017) Large Scale Offshore Static Pile Tests – Practicality and Benefits. Proc 8th 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, available online ahead of print: https://doi.org/10.1680/jgeot.17.P.012.

Buckley R. M., Jardine, R. J., Kontoe, S. & Lehane, B. M. (2017b). Effective stress regime around a jacked steel pile during installation ageing and load testing in chalk. Can. Geotech. J. under review.

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 8th Int. Conf. on Offshore Site Investigations and Geotechnics, SUT London.

Doughty L. (2016). Laboratory testing of chalk. MSc Thesis, Imperial College London.