Are we modelling the wrong pile? – Is it time to improve pile testing analysis?

The majority of cast in-situ pile load testing for resistance verification is undertaken using high strain dynamic testing where a hammer strikes the top of a pile, imparting a one-dimensional stress wave down the pile. This ‘stress wave’ is captured using strain and accelerometer gauges to produce force and velocity readings, which are equal when the velocity is multiplied by the pile impedance. The impedance is a function of concrete modulus, concrete wave speed and cross- sectional area. Case Pile Wave Analysis Program (CAPWAP) is signal matching software where the captured force and velocity data are analysed to determine the static soil resistance with respect to the pile head deflection. CAPWAP is a numerical analysis program where the pile is divided into a series of segments, each with individually uniform properties (Impedance). For prefabricated piles, such as reinforced precast, or prestressed concrete, the impedance is uniform for the full length. However, for bored piles, the cross-sectional area can vary with depth, and hence the impedance can vary significantly, especially in soft soils. In many cases, the pile model used in a CAPWAP analysis is not adjusted to reflect the as-constructed pile geometry and only the pile impedance is modified or guessed. Effectively, this is trying to model the static resistance of a pile with a pile model that is not geometrically accurate. This approach to modelling without an accurate pile model can lead to an overestimation of static resistance. There is now technology readily available that can assist with determining the actual cross-sectional area over the full depth of the pile. This paper presents the comparisons of using design vs actual cross-sectional area profiles in soft soil environments. Modelling the actual area increase can lead to more accurate and representative shaft resistance determination.