MODELLING THE CAPACITY OF DRIVEN PILES

ABSTRACT

The structural integrity of driven piles is of paramount importance to foundation engineers. Pile load test is frequently carried out on piles selected randomly from vast arrays of driven piles to ascertain their structural soundness. However, pile load tests are expensive and usually carried out on selected piles. This research established a simple empirical mathematical model that is best suited for pile capacity using pile on-site installation parameters in the Niger Delta region of Nigeria. Pile capacity obtained from various mathematical models are compared to bearing capacity obtained from finite element modelling using PileAXL software. PileAXL model was used to predict the pile capacity of the entire thirty-two (32) number driven piles and their settlements at a bridge location in Tungbo-Sagbama area of Bayelsa State, of Nigeria. The safe working load of the driven 406 mm steel cased pile was given as 80tons (800kN). From information gathered, pile load tests were carried out using maintained load test method to 1.5 of the safe working loads (SWL) on two selected piles. The cumulative settlement of these test piles was compared with settlement predictions by PileAXL programme in order to validate the finite element programme. The PileAXL model used an axial load of 1.5 of the safe working loads of pile which is 1200kN. The piles information and data obtained from stratum characterisation of soil were used by the PileAXL model. For simple mathematical on-site modelling, blow count records of driven piles, hammer information, pile information were used by the selected mathematical models. In comparing the prediction of the pile capacity by the PileAXL model, Engineering News Record formula, Modified Engineering News Record formula and Gates method - which are on-site mathematical formulae for pile capacity -were used on the entire driven piles. From the analysis carried out, the average pile capacity using PileAXL model was obtained as 1160.45 kN, while the average pile capacity for Gates method, Engineering News Record formula and Modified Engineering News Record formula were obtained as 1211.4 kN, 3087.08 kN and 2354.61 kN respectively. The Gates method gives the best and most reliable mathematical method compared to the prediction of the PileAXL model. The settlement obtained at axial load (1200 kN) by the PileAXL model at maximum depth of 30m was 27.3 mm, which is satisfactory compared to the cumulative settlement obtained from pile load test of the tested piles which further portrays the reliability of the PileAXL model. This study confirms the economic feasibility and reliability of the Gate method, empowering site engineers to accurately estimate the pile capacity during construction project. This finding highlights the practical utility of the Gate method in enabling efficient and confident decision making by site engineers when accessing pile capacity on site.