Abstract
Dynamic formulae are a widely used expedient for the control of driven piles to ensure load capacity. These formulae have considerable limitations when used in the prediction of the load capacity on their own, but are very useful in the control of a piling when combined with other tests. This technical note presents an evaluation of the Danish Formula for 54 precast concrete piles, comparing its results with High Strain Dynamic Tests (HSDTs), Static Load Tests (SLTs) and predictions by a semi-empirical static method (Aoki & Velloso, 1975). The data used in the comparison come from three works in the city of Rio de Janeiro, Brazil. All piles were driven with free-fall hammers and in one particular work the piles were relatively short. The predictions of the Danish Formula were evaluated in relation to the pile length/diameter ratio. It was concluded that for short piles - with lengths less than 30 times the diameter - this formula indicates bearing capacities higher than the actual ones. A correction for a safe use of the Danish Formula for short piles is suggested.
Highlights
Dynamic formulae are based on elementary laws of Physics, such as those that govern the conservation of energy or the shock between bodies
Danish Formula results will be compared to results of High Strain Dynamic Tests (HSDTs), Static Load Tests (SLTs) and a static method (Aoki-Velloso method)
As HSDTs are often made during the driving process or shortly after, the static resistance usually increases over time, due to set-up, until reaching the load capacity of a SLT or a prediction by static method, Qult
Summary
Dynamic formulae are based on elementary laws of Physics, such as those that govern the conservation of energy or the shock between bodies. A Wave Equation solution requires, in addition to these data, those related to the driving accessories and the soils (in terms of rigidity, resistance and viscosity), and outputs the set, and the dynamic stresses (stresses along the pile under driving). The first two aspects are inherent to any dynamic method, leading to different load capacities obtained (i) with different driving energies and (ii) with set measurements or HSDTs made at different times after driving. In the evaluation of Danziger & Ferreira (2000), this formula presented a good correlation with results of a Wave Equation solution for steel piles In another evaluation (Silva et al, 2020), the Danish Formula proved to be suitable for the control of a large piling. The predictive capacity of this formula is evaluated in particular in relation to the pile length, resulting in the recommendation of a range for its safe use
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