Behaviour of Instrumented Hollow Auger Pile Groups in Sandy Soil

Hollow auger piles are a new solution technique being used more often in recent years as a foundation for buildings of 15 floors or less in the coastal region of Joao Pessoa, Brazil. Designed on "piled raft" type systems, the hollow auger substitutes well the traditional soil improvement solution with compaction piles, in which the ground vibration, inherent to the pile driver process, is no longer admissible in densely populated areas of the city. Hence, this paper addresses the in-situ behavior of this new pile technique in foundation groups of hollow auger piles in a sandy soil deposit, typical of the region. The analyses are based on load tests performed in the actual-size foundations, also scarce for this foundation type. The results indicate that the efficiency factors (eta) of the pile groups is greater than one and close to typical values in the geotechnical literature for sandy soils. For the 2-piled group, eta varied between 1.10 and 1.33. For the 4-piled group, eta varied between 1.05 and 1.25. Settlement ratios (R-s) found experimentally show that the effects of the interaction are more intense at the group with more piles. The results obtained for R-s were 1.08 and 2.01, for 2-piled group and 4-piled group, respectively. The distribution of a load on a pile is analyzed using instrumentation with strain gages, allowing the perception of the division of loads between the shaft and tip. The analyses carried out through the laws of Cambefort show that the average unit lateral friction is fully mobilized and reached a maximum value of 52 kPa, corresponding to a settlement of 18.7 mm. The bearing pressure of the tip was not fully mobilized and reached a value of 5531 kPa, equivalent to a displacement of 83.5 mm.