Canterbury Technical Forum Meeting 53
5:40 pm – 7:00 pm, Wednesday 5 February 2014
Lecture Theatre E6, College of Engineering, University of Canterbury
Presentation: Pore Pressure Response during High Frequency Sonic Drilling and SPT Sampling in Liquefiable Sand
Rick Wentz - Wentz-Pacific
Abstract
Geotechnical investigation using “sonic” drilling methods has become common in Christchurch in the aftermath of the Canterbury earthquakes. The method is particularly effective in coarse alluvial soils, and has become the standard tool for obtaining Standard Penetration Test blow counts (SPT N-values). Routine methods for assessing liquefaction resistance using N-values (Idriss and Boulanger, 2008; Youd, et al, 2001) require in situ testing of undisturbed soils. Current “best practice” requires that SPT be performed in accordance with ASTM D 6066, which does not allow “vibratory” methods. Therefore, questions have arisen regarding the potential for soil disturbance due to sonic drilling. The potential for unreliable SPT values due to soil disturbance during advance of the sonic equipment has warranted evaluation.
This paper summarises an investigation of the possible impact of the sonic method on N-values with direct comparison of N-values from side-by-side sonic and mud rotary boreholes. SPT data were obtained from 4 pairs of boreholes drilled in a variety of potentially liquefiable lithologies. The same drill rigs, rods and SPT hammers were used to reduce variability. A CPT sounding was made adjacent to each borehole paired with supplementary Vs and pore pressure dissipation tests to evaluate soil parameters that influence the susceptibility of the soil to disturbance by high frequency cyclic loading (e.g., stiffness, permeability, relative state parameter).
Sonic drilling is effective for recovering continuous core in alluvial soils. This paper will compare N-values from sonic and conventional mud rotary. A comparison of sonic and mud rotary SPT-based and CPT-based liquefaction analyses will be presented. The effects of soil composition, density, and confining stress will be discussed.
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