Calculating the Load Capacity and Safety of Helix Piles-Paper

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The study compares two design loads for 50 cm continuous flight auger (CFA) piles at a site in Barra da Tijuca, Rio de Janeiro, to assess the trade-off between construction cost and geotechnical risk. Pile capacity was estimated from SPT data using Cabral’s (1986) semi-empirical method, combining shaft friction and tip resistance. The analysis used three boreholes and a probabilistic framework to estimate resistance variability, reliability, and failure probability.<br /><br />At an allowable load of 800 kN, the average required pile length was about 20 m. The design had an estimated failure probability of 0.30% and required 450 piles; its equivalent foundation cost was approximately R$10.57 million. At 1,200 kN, the average pile length increased to about 25 m, but the design required only 345 piles. Its failure probability rose to 0.65%, while the equivalent cost fell to approximately R$8.9 million, partly due to fewer piles, smaller pile caps, and a shorter construction schedule.<br /><br />Both alternatives were judged to remain within the investor’s stated acceptable risk threshold of 2.5%. The higher-load option therefore offered substantial savings at the expense of a modestly higher failure probability. The authors emphasize that the preferred design should balance cost, structural vulnerability, and acceptable risk, rather than rely on allowable load alone. They also highlight the importance of evaluating shaft-friction variability, since CFA pile tip resistance can vary considerably. Overall, the study supports incorporating geotechnical risk analysis into foundation design to identify cost-effective solutions without exceeding acceptable safety limits.

Keywords

CFA piles

design load

geotechnical risk

probabilistic analysis

SPT data

pile capacity

failure probability

foundation cost

shaft friction

Barra da Tijuca