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Título

THE IMPORTANCE OF A RELIABLE SPT TEST ON COSTS OF DEEP FOUNDATIONS

Resumo

Wave-equation analysis on piles may be done to select a hammer mass to install the deep foundations at the design length. GRLWEAP is a software commonly used for this purpose. In this project site, located in São Bernardo do Campo, SP, Brazil, 17cm-square precast concrete piles were driven by a 2.8-ton free fall hammer with a drop height of 30 cm for a workload of 25 tons. Moreover, the SPT tests showed clayey soil with low Nspt values of 1 blow/30 cm in the first 3m depth, followed by a layer of the same material with Nspt values between 4 blows/30 cm and 26 blows/30 cm up to 5m depth. Then, the SPT tests indicated higher than 50 blows/30cm between 6m and 7m depth for the clayey soil. The GRLWEAP analysis indicated that 17cm-square precast concrete piles would be embedded at 6m depth with a blow count higher than 860 blows/m for a drop height of 30 cm. However, during the pile driving, the soil demonstrated low strength at 6m depth for most of the piles, and the piles were driven between 5.9 m and 12.1 m depth, and the average pile penetration was 9.3 m. Once the average pile penetration was 3 m longer (70% longer) than the predicted lengths by the GRLWEAP, it was requested a new SPT test to confirm the soil profile. However, it was not possible due to the timeline restrictions. Then, five Dynamic Load Tests were performed on the piles with pile lengths between 8.8m and 12.1m, and the results confirmed a low-soil resistance at 6m depth. The mobilized loads were between 67.3 and 87.0 tons. In addition, the PDA indicated good results for the piles that reached longer depths than the ones predicted by the GRLWEAP, although the SPT suggested a lower pile length. This case study shows the importance of a reliable SPT test. Due to errors in the SPT test and timeline restrictions, the contractor’s budget was severely affected. The PDA was fundamental for the quality assurance of the deep foundations, and the conclusion is that if the piles were embedded at 6 m depth according to the SPT tests, then a low pile capacity would be reached once the predicted blow count of 860 blows/m was not observed at this depth. Moreover, all the piles would be restriked to reach the workload with the minimal factor of safety.

Palavras-chave

SPT Test, Costs, GRLWEAP, Dynamic Load Test, Precast Concrete Piles

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