Analysis of a rapid load test on an instrumented bored pile in clay

Abstract

Rapid load testing methods for piled foundations are generally easier and quicker to mobilise than classic static tests, and are less complex to analyse than dynamic load tests. A recently developed rapid load pile testing method known as the Statnamic test is seeing greater use in the UK for the assessment of piles. For foundation design, it is necessary to derive the equivalent static load-settlement curve from the rapid load test data by eliminating inertial and damping effects. Existing methods of test analysis generally provide good correlation with static tests for sands and gravels, but overpredict pile capacities by up to 50% for clays. In order to gain an insight into the behaviour of rapid load pile testing in clays, a full-scale pile instrumented with accelerometers, strain-gauged sister bars and a tip load cell was tested in a glacial lodgement till near Grimsby, UK. The soil around the pile was also instrumented with radially arrayed buried accelerometers. The test pile was subjected to rapid loading tests, the results of which were compared with constant rate of penetration and maintained load static tests on the same pile. Results from the field testing have been analysed using non-linear viscous parameters obtained from laboratory model and element tests to represent rate-dependent clay shear resistance in the post-yield phase of loading. Shaft frictions derived from the strain-gauged reinforcement in the pile have been compared with shear strains and stresses derived from accelerations in the surrounding soil to give an insight into the load transfer mechanisms for a rapidly loaded pile in clay.

CHD pile performance: part I – physical modelling

“…Although the stiffness of the cement paste was not directly measured, these pastes typically have a maximum Young's modulus of 29 MPa, which reduces with porosity (Mindess et al, 2003) or water/cement ratio (typical range 20-29 MPa). This compares favourably with the range of stiffness for cast in situ pile concrete of 24-28 MPa reported by Brown et al (2006), although it is acknowledged that concrete stiffness is dependent on the mix design and the aggregate type. Piles were consistently tested 24 h after casting.…”

“…The lowest strain gauge (gauge 1) was approximately 30 mm above the tip of the CHD pile. The gauges measured axial strain within the pile when load testing was carried out and were used to determine the axial load distribution as outlined by Brown et al (2006) and Jeffrey (2012). Typical axial load distribution curves for the model CHD pile at increasing pile settlements (normalised by pile diameter D, or D f in the case of the CHD pile) under axial compressive load are presented in Figure 5.…”

CHD pile performance: part II – numerical modelling

“…In performing these calculations, the shaft and base diameters were taken as D f and D core , respectively Table 2 and the shaft and base capacities were computed using the analytical model from the companion paper (these capacities are included in Table 5). Fleming's method also requires three stiffness parameters -namely, Young's modulus of the concrete E c (based on the range of 24-28 GPa reported by Brown et al (2006)), the shaft-soil flexibility factor M s (0·001-0·004, after Fleming (1992)) and Young's modulus of the soil at the pile base E b (30-100 MPa for medium dense sand, after Azizi (1999)). In Tables 3 and 4, a range is given for these predictions -the smallest settlement represents the stiffest combination (E c = 28 GPa, M s = 0·004, E b = 100 MPa) and the largest settlement represents the most flexible combination (E c = 24 GPa, M s = 0·001, E b = 30 MPa).…”

Bored pile design in stiff clay II: mechanisms and uncertainty

“…Extrapolation techniques based on hyperbolic load-settlement relations have proved reasonably accurate and offer the opportunity to make settlement predictions after only 1 day of testing (Fleming, 1992). Faster constant rate of penetration (CRP) tests and quasi-dynamic tests rely on rate effect corrections, and generally require prior correlations with ML tests on the same soil (Brown et al, 2006). The increasing use of monitoring systems during construction, and in service, will provide better justification of settlement predictions in future, although obviously not of failure conditions.…”