Slope Stability Assessment

Paul & Janelle DeSouza                                                                               22198-001-Rev1
C/- Sarah Waller Architecture                                                                           25 July 2022

During our walk-over assessment at the site, a number of exposed, relatively flat topped corestones were
noted to be present at the topographical high point forming the site of the residence. These were of
boulder size, with the material described as coarse grained, pale grey stained yellow brown, moderately
weathered sandstone. There were extremely weathered sections around the corestones, which enabled
the excavation of test pit TP2 (Ref. below).

A view of the exposed sandstone corestones is given below in Text Figure 4.




                                                                                           Exposed corestones




Text Figure 4: General view of exposed corestones (looking south), and detail view inset

2.2       Subsurface Conditions
The Ground Drill soil test included subsurface investigation within the proposed building envelopes via.
test pits using a 5.5 tonne excavator with toothed bucket and ripping attachments. Test pits TP1 to TP3
were dug progressively from the south-eastern corner of the residence, the central/highest point and the
north-western corner, respectively.

Detailed summaries of each test pit are given below:

          TP1 encountered a surficial layer of loose, silty sand to 0.2 m below ground level (BGL), then stiff to
           very stiff, sandy and silty clays (residual soils) to 1.5 m BGL, where extremely weathered (XW)
           materials were encountered as very stiff to hard sandy clays. Distinctly weathered, low strength (or
           stronger) sandstone was logged at 2 m BGL, where bucket refusal occurred.

          Test pit TP2 was excavated between corestones (Ref. Section 2.1 above), with a relatively deep silty
           sand and clayey sand profile, being loose to 1.5 m BGL, where distinctly weathered sandstone
           resulted in bucket refusal.

          Test pit TP3 on the downslope, northern side of the residence, encountered loose silty sand to 0.5 m
           BGL, then stiff to very stiff sandy clays to 1.6 m BGL, then becoming dry and hard to 2.3 m BGL,
           where distinctly weathered sandstone was encountered. It was possible to penetrate the sandstone to
           2.5 m BGL under firm ripping conditions in the confined test pit base.

          Test pit TP4 was excavated approximately within the expected footprint of the shed and encountered
           loose silty sand to 0.5 m BGL, then stiff, sandy clay to 1.5 m BGL, where distinctly weathered
           sandstone was logged, resulting in bucket digging refusal.
Groundwater seepage was not encountered during the digging of the test pits, however “very moist” soils
were logged by Ground Drill in test pit TP2 just above the inferred sandstone layer at a depth of
1.2 m BGL and below the upper sand at 0.5 m BGL in test pit TP3.




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