J001701-006-R-Rev1_Medihub_Geo

Stockwell



4.0     METHOD OF INVESTIGATION
      4.1 Review of Available Information
To assess likely ground conditions, a review of published geological maps and available geotechnical
information was undertaken (refer Section 5.1).

      4.2 Borehole Drilling
Fieldwork was conducted on 19 August 2022 under the supervision of a geotechnical engineer from Core.
Two boreholes (designated BH4 and BH5) were drilled on the proposed development footprint (refer
attached Figure 1; Appendix A).
The boreholes were drilled using a 4WD-mounted drilling rig utilising uncased solid flight augering, to 8 m
below ground level (BGL).
Dynamic cone penetrometer (DCP) testing was undertaken adjacent to/within the boreholes. Undisturbed
(U50 tube) sampling was conducted in suitable cohesive soils, with pocket penetrometer testing undertaken
in the ends of the U50 tubes.
Soil samples were recovered from the boreholes for subsequent laboratory testing (refer Section 4.4).
Borehole positions were recorded in the field using a handheld GPS unit.
Reports of Boreholes are attached in Appendix B, together with explanatory notes. Subsurface conditions
are discussed in Section 5.2.
      4.3 Seismic (Geophysical) Testing
Geophysical testing in the form of multi-channel analysis of surface waves (MASW) was undertaken on
19 August 2022 by geotechnical engineers from Core. This survey was undertaken to supplement the
borehole data.
MASW geophysical surveys use seismic surface (Rayleigh) wave phase and frequency data to estimate
shear-wave velocities of the subsurface materials.
One MASW survey line (denoted Line 1) was carried out across the proposed building footprint. The survey
comprises a single 1-Dimensional vertical sounding recorded at the centre of the geophone array; refer
Image 3. Vertical soundings were carried out at 4 m intervals by pulling the line.
The MASW acquisition comprised the use of a 24-channel array with 4.5 Hz vertical geophones spaced
horizontally at 1 m. The array was connected to a Geometrics Geode, with data recorded using the
Geometrics Seismodule Controller Software (SCS). A sampling rate of 0.5 ms and sample record length of 2
seconds was adopted. The seismic source was a 16 lb sledgehammer striking a steel plate. Vertical stacking
of the seismic source was carried out at multiple shot locations to increase the signal to noise ratio of the
frequency data. Data from multiple source offsets (i.e., shot locations) was gathered during at each sounding
location.
The acquired data was processed using the SurfSeis 6 software. Dispersion images were generated, which
show the frequency versus the percentage intensity of phase velocity, for each acquired field record. A
dispersion curve was then produced by picking the maximum intensity velocities (of the fundamental mode)
across the useful range of frequencies. The dispersion curves for the survey alignment were put through a
10-layer inversion algorithm to produce one-dimensional shear-wave velocity soundings.
The shear-wave soundings were then gridded using Golden Software’s Surfer 22 to produce a two-
dimensional shear-wave velocity section. Sounding positions have been denoted to show the parts of the
section that have been interpolated during the gridding process.
The quality of the produced MASW dispersion curves for each sounding was very good, with high signal-to-
noise (S/N) ratios, for the fundamental-mode dispersion energy, for the frequency range of between 14 Hz
and 55 Hz; which corresponds to a depth range of between about 1 m and 8 m.




J001701-006-R-Rev1                                                                                 Page 3