210533 - 108 Black Mountain Range Road, BLACK MOUNTAIN - Slope Stability Report

Total Concept Homes                                                                              20332-003-Rev0
                                                                                                    16 June 2022

The site was classified by Ground Drill as Class P in accordance with AS2870-2011 due to the presence
of fill, poor bearing capacity, and trees. If not for this factor the site would have been classified Class H1
with characteristic surface movements (ys) of up to 55 mm reported by Ground Drill.

On the upslope side of the granny flat building pad, an earthworks cut which ranged to about 1.5 m depth
was present. The upper 1 m BGL (approximately) was observed to featured stiff, silty clay, with varying
gravel content; underlain at the toe by weathered phyllite.
A view of the exposed weathered phyllite in the cut is shown below in Text Figure 4.




Text Figure 4: Looking south along cut (LHS) and detail view of weathered phyllite (RHS)


2.2    Topography
As illustrated in Text Figure 5 on the following page, the wider area around the site consists of a series of
approximately parallel ridgelines and gullies, aligned east-west. The ridgelines within the subject allotment
lead east from Black Mountain Range Road, which traces a crest forming a drainage divide between
eastward and westward draining creek lines.

Just off Black Mountain Range Road, to the south-western frontage of the allotment, a rounded spur
extended through the site of the existing residence and along the northern lot boundary. The granny flat
would be proposed just off the crest of this spur, on ground dipping down to the south-east. Natural slopes
above the granny flat were in the order of about 8°, split by some low-relief terracing works (around 0.5 m
high per bench) inferred to be as a result of previous orcharding. Below the earthworks pad, ground slopes
were around 15°, and developed in to a relatively wide, gully head feature which drained eastward through
an intermittently dammed drainage line.

The cut batter backing the earthworks pad at the granny flat was around 1.5 m depth, and inclined at about
70°. The fill on the downslope side was battered at about 25°.
Ground slopes were measured at the time of the site visit by Tectonic using a hand held clinometer.




                                                         4/13