Matt Bristow has developed what is known as ‘8-spring model’ for analysing large diameter piles, i.e. piles with length/diameter ratios L/D < 6:
- Pile diameter effects apply to large diameter piles where often additional soil resistances are available. In ‘8-spring model’ all possible soil reactions on the pile are represented by state-of-the-art force-displacement and moment-rotational springs.
- Simple extension of the p-y curve methodology already well-proven for long piles. Suitable for very short piles, i.e. L/D ≤ 2, and any load-level, i.e. both serviceability and ultimate loads.
- The 8№ soil reactions acting on the pile are as follows:
Soil reactions on shaft:
P-y curves – lateral resistance
T-z curves – axial resistance
R-x curves – rotational or tilting resistance
M-x curves – torsional resistance
Soil reactions on base:
Q-w curves – axial resistance
S-v curves – shear resistance
R-u curves – rotational resistance
M-w curves – torsional resistance.
- New formulations have been developed for all new soil reactions and all resistances are non-linear to most accurately represent the real behaviour of the soil and pile resistance. For every point along the pile there are 3№ force-displacement springs and 3№ moment-rotational springs.
- Soil reactions are automatically combined together according to the rate at which each soil resistance is mobilised. A non-linear analysis is used to determine the contribution of each soil reaction. Each soil reaction will vary according to the amount the pile moves or rotates as not all soil reactions are developed at the same rate, e.g. the magnitude of each soil reaction will vary between high or low load-level, etc.
- Methodology based on the recommendations of FHWA RD-86-102, EPRI-EL-2197, and Lam (2013), except model extended to 8№ independent springs and new formulations provided for all soil reactions.
- ‘8-spring model’ essentially parallels the work of the recent PISA design approach, but produced 3 years earlier! However, ‘8-spring model’ has advantage that the moment-rotational resistance on the shaft is defined as a completely independent resistance (rather than being “normalized” with the lateral soil reaction) and there is no reliance on time-consuming black-box technology such as 3-D FEA.
Article
Further information on pile diameter effects and ‘8-spring model’ are contained in the following PDF article (download below)