Canterbury earthquakes 2010/11 Port Hills slope stability : Laboratory measurement of P and S wave velocities of drillhole rock samples : RS Lab Report 003

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Bruce, Z.R.; Lyndsell, B.M. 2017 Canterbury earthquakes 2010/11 Port Hills slope stability : Laboratory measurement of P and S wave velocities of drillhole rock samples : RS Lab Report 003. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/20 59 p.; doi: 10.21420/G2Z88K

 

Abstract: P-wave and S-wave velocity measurements were carried out in the GNS Science Rock and Soil Mechanics Laboratory at Avalon on cored drillhole samples of rocks recovered during the ground investigations performed in the northern margin of the Port Hills following the 2010/11 Canterbury Earthquake Sequence. The velocities were measured by ultrasonic pulse transmission through the core lengths using an Olympus pulse generator and SBEL 200 kilohertz P- and S-wave piezoelectric crystals mounted in transmission and receiving platens. Waveforms used to manually pick arrival times were recorded using a Pico digital oscilloscope coupled with a Pico datalogger. The twenty-two samples tested, comprising basaltic lava, lava breccias and tuffs and epiclastic sediments, were selected from cores remaining from the laboratory testing programme in 2013 to determine strength and deformation properties. Being selected from remaining core material specimen lengths were typically short, normally between 35 and 85 mm in length. The highest velocities were measured in the lava samples, with P-wave velocities between 5,500 and 5,800 m/s and S-wave velocities between 2,700 and 3,100 m/s. Lower velocities were measured in the lava breccias, tuffs and epiclastic sediments. Lava breccia velocities ranged between 1,300 and 3,950 m/s for P-waves and 650 and 2,000 m/s for S-waves respectively, while tuffs and epiclastics ranged between 1,450 and 3,050 m/s for P-waves and 700 and 1,450 m/s for S-waves. These materials also showed a greater range in P:S wave velocity ratios (1.4 – 2.6 compared with 1.9 – 2.1 for lavas). Wave transmission through the breccias and tuffs, which were also typically in a moderately to highly weathered state, was also attenuated often requiring the stacking of waveforms reducing confidence in picking arrival times. Dry density values measured during the P- and S- wave measurements were similar to those measured during strength and deformation testing in 2013. Comparison between deformation modulii values determined statically in 2013 and dynamic modulii calculated from the P- and S- wave velocities gave expected trends. Dynamic to static modulus ratios (Ed:Et) were the highest for the lower strength lava breccias, tuffs and epiclastic sediments (between 3 and 5) and lowest for the higher strength lavas (1.0 – 1.5). Comparison of downhole S-wave velocities measured in drillholes in 2013 drilling programme with those measured in the laboratory in 2015 showed a wide range of ratios (Vslab:Vsfield of 0.1 to 2.2) indicating that the two data sets are not directly comparable. (auth)