Page 23 - Demo

4 Geotechnical Investigations
4.1 Published Geology
The published geology for the area1 indicates that the elevated ground is underlain by Matua Subgroup fluviatile sands and gravels, lignites, estuarine sands, lacustrine silts & Pahoia Tephra. These deposits are in excess of 200,000 years old and are overlain by a series of airfall ash deposits. The expected sequence in the elevated area and on the slope is shown on Table 1 below.
The lowlying ground is likely to be underlain by more recent sand, silt and gravel deposits, overlying the Matua Subgroup soils.
Table 1: Expected Geological Sequence
4.2 Investigations Carried Out
An investigation was carried out to determine the ground conditions at the site. The investigation consisted of six Cone Penetration Tests (CPTs) to a maximum depth of 14m and ten handaugers. The ground investigation locations are shown on Figure 1 in Appendix A. The investigation data is included in Appendix B.
4
Layer
Description
Typical Thickness
m
Age
Years
Younger Ash
Typically, stiff sandy SILT or silty SAND, airfall
3
<50,000
Rotoehu Ash
Typically, loose coarse SAND, can also be firm to stiff SILT; airfall
1
50,000 – 65,000
Hamilton Ash
Stiff plastic SILT with very stiff dark brown paelosol at the top known locally as ‘The Chocolate Layer’ airfall
2 to 3
100,000- 350,000
Matua Subgroup
Fluviatile sands and gravels, lignites, estuarine sands, lacustrine silts and airfall tephras older than the Hamilton Ash.
varies
Older than 100,000
1 Geology of the Tauranga Area’ Briggs et al Occasional Report No 22 University of Waikato 1996.
6c Myrtle Drive, Omokoroa O’Brien Geotech Ltd Project 533 February 2017