CHD pile performance: part I – physical modelling

Abstract

The continuous helical displacement (CHD) pile is an auger displacement pile developed in the UK. It has performance characteristics of both displacement and non-displacement piles due to the way in which it is installed. Based on field experience, it has been shown that the load-settlement performance of CHD piles installed in sand exceeds the current design predictions based upon conservative effective pile diameter and design parameters associated with auger bored or continuous flight auger in situ piles. In an effort to gain a greater understanding of the performance of CHD piles in sand compared with more conventional piling techniques, a programme of physical model pile testing (reported in this paper) and associated finite-element modelling (reported in a companion paper) was undertaken.The model testing programme established that greater shaft resistance may be developed for the piles than had originally been considered. Based upon the results of the model testing, recommendations for more appropriate approaches to the selection of end bearing and shaft resistance factors are made to predict ultimate load capacity in sand.

CHD pile performance: part II – numerical modelling

“…(2013) produced a set of soil-specific correlations for the HST95 sand used in the model tests reported in the companion paper (Jeffrey et al, 2016). The model parameters have subsequently been validated against centrifuge test data (i.e.…”

“…Application of FEM to small-scale model tests Figure 3 shows the geometry of half of a CHD pile that was used in all cases in the FEM to simulate the 1:10 scale model tests reported in the companion paper (Jeffrey et al, 2016). This represents an idealisation (averaging) of measurements taken for all of the physical CHD models following exhumation after the load testing.…”

“…in situ) measurements of relative density (which was uniform with depth). Interface elements were also incorporated between the soil and the ribbed shaft of the pile, with a coefficient of friction R inter = tan δ crit , where δ crit was the critical state (large-strain) concrete-soil interface friction angle reported in the companion paper (Jeffrey et al, 2016). The pile load test was simulated by applying a downwards vertical displacement at the pile head (i.e.…”

“…The authors' companion paper (Jeffrey et al, 2016) described the continuous helical displacement (CHD) pile type and presented a series of small-scale (1g) model tests that were conducted to obtain insight into its installation/formation and performance under vertical compressive loading. It was shown that CHD piles in coarse-grained soils can have a similar or better bearing capacity than a displacement-type pile of the same outer diameter, while saving material (due to its screwtype shape and hence variable cross-sectional diameter).…”

“…relative density) as an input parameter, to enable widespread use in routine design. The use of relative density to parameterise the soil model also permits installation effects to be taken into account in an approximate way, based on apparent changes to soil density around the pile inferred from cone penetration test (CPT) measurements close to the model piles following installation, the data for which can be found in the companion paper (Jeffrey et al, 2016). The constitutive model incorporates stress-dependent stiffness parameters, and so can be validated against the physical model load-displacement test data prior to failure taken from the companion paper and also applied to full-scale field cases.…”