Pore pressure response during high frequency sonic drilling and SPT sampling in liquefiable sand

The potential for systematic variations in SPT N –values between mud rotary and sonic drilling methods has been the focus of a pilot investigation at a site that exhibited extensive liquefaction during the 22 February 2011 M 6.3 Christchurch Earthquake. In an attempt to quantify the impact of drilling methods on N-values, 2 pairs of closely-spaced sonic and mud rotary boreholes were advanced in deposits of predominantly silty sand and sand silt with SPT tests at 1.5 m intervals. Pore pressure transducers (PPTs) were employed at depths ranging from 4.6 to 17.5 m to monitor the effects of soil disturbance imparted by the sonic drilling procedure. The excess pore pressure response provides an indirect correlation to the level of soil disturbance due to drilling and sampling. The PPT data demonstrated clear trends during the sonic drilling process. Key preliminary findings indicate that: 1) transient pore pressures are relatively small as close as 300 mm from the sonic casing (ru < 0.15); 2) excess pore pressures generated during advance of the casing are largely dissipated by the time of the SPT test; and, 3) changes in pore pressure during normal, non-vibratory and common aspects of the drilling process can be as large as that due to the core barrel advance with vibration.

The direct comparison of the N-values demonstrates weak trends in the ratio Nrotary/Nsonic; however, the close proximity of the boreholes to each other may have affected the N values. Additionally, much of this difference is considered to be within the range of variability of SPT data in uniform soil deposits and no systematic trend is evident on the basis of the small database obtained in this first phase of the investigation. Additional SPT data from similar investigations are being collected in order to develop a statistically significant basis for comparison.