This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractThe load distribution among piles in a group varies such that the inner piles often carry a smaller share of the total load compared to the exterior piles, which is a result of increased soil-pile interaction. The main objective of this paper is to establish the relative effectiveness of pile groups with no inner piles (perimeter group), when compared to the more common grid configuration. The numerical investigation utilised the finite element programme ABAQUS and considered a range of variables that affect pile group behaviour including: number of piles, pile spacing, length/diameter ratio and soil strength.It was demonstrated that a complete grid group was less efficient than a perimeter group, where efficiency is defined as the load capacity of the whole group expressed as a ratio of the number of piles in the group multiplied by the load capacity of a single isolated pile.Efficiencies close to unity were observed for some perimeter groups. Perimeter groups also showed a 'block' type group failure could occur, where piles were placed at a spacing of less than 2.0 d centre-to-centre. This often, but not always, led to a reduction in the efficiency of the pile group.Key words: Pile groups, mini piles, numerical analysis, efficiency 3 IntroductionIt is generally accepted that pile groups in clay have an efficiency of less than one, where efficiency is defined as the load capacity of the whole group expressed as a ratio of the number of piles in the group multiplied by the load capacity of a single isolated pile. In this context, the term 'pile groups' is generally considered to refer to groups in a grid arrangement, e.g. a group of 5x5, consisting of 25 piles. It was unknown if perimeter groups, where all inner piles are removed (Figure 1), would also have an efficiency of less than one.A research project investigated the problem using centrifuge modelling of a range of pile groups, using the facility at City University London (Rose, 2012). The model piles simulated bored piles in firm kaolin clay; they were loaded under monotonic axial loading conditions and tested to failure. Centrifuge testing was complex and extensive and details of the experimental work are beyond the scope of this paper.A numerical investigation of the problem has been conducted using the finite element (FE) method. The initial analyses agreed with and confirmed the main findings of the centrifuge experiments. Further analyses were undertaken to extend the scope of the experimental research. The variables considered in the analysis include the geometry of the group (shape and length/diameter ratio), the number of piles, the pile spacing and the soil strength. These properties will affect the soil-pile interaction and consequently the behaviour of the group, which will influence the efficiency. Such pile groups may be installed when it is necessary to carry large loads in confined spaces ...

Initial applications of micropiles have involved retrofitting foundations of existing buildings. In these applications, the overall performance of the micropiles–raft (MPR) foundation system is similar to a piled raft foundation where the load is transmitted through both the raft and micropiles. However, there is no guidance available regarding the performance of MPR foundations. In this study, geotechnical centrifuge testing was conducted to investigate the behavior of MPR foundations in sand and evaluate their performance characteristics. The study investigated the effect of raft flexibility on a number of important design parameters, including raft total and differential settlements, raft contact pressure, raft bending moment, and load sharing between the raft and micropiles. In addition, the use of micropiles as settlement reducers was investigated. The results showed that the micropiles carried 42%–59% of the applied load for the MPR configuration considered, which resulted in redistribution of the raft contact pressure. It was found that the Poulos–Davis–Randolph (PDR) method can be used to evaluate the performance of MPR systems with relatively stiff rafts; however, it is not applicable for MPR with flexible raft. A correction factor was proposed to account for the raft flexibility in the PDR method.

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