Axisymmetric centrifuge modelling of deep penetration in sand to model driven piles or the cone penetration test

The advancement of a slender object (a 'probe' or 'penetrometer') into soil (to depths of 10s of metres) is of fundamental interest to Geotechnical Engineers. A 'piled' foundation which is fabricated (as a long cylinder) at the ground surface and 'driven' into the ground is an example of such an object, which may be more than a metre in diameter. It may ultimately be used, for example, in the foundation for an offshore structure such as a wind turbine.At the other extreme the 'Cone Penetration Test' (CPT) is used to by Civil Engineers to characterise soils which are at significant depths in the ground by advancing a cylindrical probe which is only few centimetres in diameter. The resistance which the probe encounters in the ground can be 'extrapolated' to design a piled foundation to carry a given load. However, there is still debate regarding exactly how this should be done, reflecting uncertainty of precisely what happens within the soil when a probe (or pile) penetrates the ground to a significant depth.Many problems in geotechnical engineering are relatively well-suited to 'numerical' modelling. However, the rigorous numerical modelling of deep penetration is unusually difficult because the problem involves large deformations and highly nonlinear behaviour. Considerable advances have however recently been made in beginning to understand some detailed aspects of the fundamental behaviour using 'physical' models. The main drawback of such an approach is that the stresses in the soil in the model are much smaller than they would be at (say) 30m depth in the ground for a piled foundation, and this significantly affects the behaviour of the soil. This can be overcome using a 'geotechnical centrifuge' (where the gravitational acceleration is effectively increased so that higher stresses are generated in a small model).Recent advances have also been made in the use of digital photography to 'track' zones of soil as they move (eg. due to an advancing penetrometer). However, this technique has yet to be used in conjunction with an advancing penetrometer in a geotechnical centrifuge model. More particularly, it has not been used with a cylindrical penetrometer (as a pile or CPT would be). Instead a 'plane strain' idealisation has been used, which effectively assumes that the extent of the penetrometer is infinite in one of the horizontal directions.The main emphasis of the proposal is to develop a (half) cylindrical penetrometer and soil physical model which will allow the associated soil movement to be observed. This technique will then be used in a geotechnical centrifuge, utilising digital photography techniques to track movement of the soil. The results will provide fundamental comparison with the 'plane strain' idealisation which is presently used to observe soil movement, and the (full) cylindrical penetrometer, which does not allow soil movement to be viewed. Indeed, the technique would have potential application in all areas of axisymmetric (circular) physical modelling in geotechnical engineering.Use of centrifuge modelling would mean that the results would be applicable to full-scale situations (such as driven piles). The technique would then be used to observe resistance to penetration simultaneously with soil movement, as applied to topical aspects of behaviour such as 'scale effects' for the probe, the influence of a 'layered' soil profile, and the roughness of the probe shaft.