Investigation of the load-carrying capacity and efficiency of individual minipiles and minipile groups under vertical loadings

The extension of tree roots into the soil provides a natural analogue for the design of battered minipile systems, whereby principles of biomimicry are applied to develop innovative and efficient geotechnical engineering solutions. This research analyses the capacity of single minipiles as well as minipile groups subjected to compressive (or downward) and tensile (or upward) loadings. Various single minipiles and minipile groups comprising cohesive soils were tested at a site in Dookie, Victoria, Australia. Normalised ultimate load results show that the efficiency of vertical minipiles under compressive loadings is the highest, followed closely by battered minipiles with a 28° angle, while the minipiles with a 45° angle show the lowest capacity. The normalised capacities of single minipiles under tensile loadings show that battered minipiles with an angle of 28° have the highest value, while the vertical minipiles have the second highest value, and minipiles with a 45° angle have the lowest capacity. An analytical model has been proposed to determine the load-carrying mechanism of single minipiles under vertical loading, which decomposes the load into normal and axial loading. For minipile groups, the normalised per-pile capacity in vertical and battered footings reduces as much as 40% and 34%, respectively, when the number of piles is increased under compressive loadings. When tensile loading is applied, the per-pile capacity in battered groups increases while it reduces in vertical groups. Under compressive loadings, battered minipile groups have greater capacity than the vertical minipile groups. System efficiency was estimated from analytical and field test results, with a drop in efficiency when the number of minipiles in a group is increased. This is attributed to the reduction in spacing between the piles with no significant increase in soil block area.