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

Total Concept Homes                                                                                  20332-003-Rev0
                                                                                                        16 June 2022

2.4    Surface Drainage
Broadly, the natural surface drainage follows the ground surface contours, which over the proposed
development area tended to the gully to the south-east/east. An upslope catchment does exist at the granny
flat site, extending to the crest of the spur running through the lot (the highest topographical point locally
being around the location of the existing residence).
A gravel trackway running from the residence to the site of the proposed granny flat was noted to be locally
eroded, appearing to direct water to the earthworks pad. Some minor slumping at the crest of the fill
embankment was noted, with some rill erosion evident.

Surface water is expected to drain well from the site considering the moderate to steep natural gradients
and relatively low permeability of the soils in the immediate subsurface, and relatively shallow depth to
apparent ‘rockhead.’

2.5    Vegetation
In the vicinity of the proposed granny flat, the vegetation mostly consisted of maintained grass. Sparse
shrubs and large size mature trees were noted on the sloping ground to the west of the proposed granny
flat, which were observed to be relatively ‘plumb’. Tilting or distorted growth can sometimes be an indicator
of slope instability.

2.6    Buildings and Other Structures
The existing residence comprised a one storey, low set, masonry walled structure, constructed over a cut
and fill earthworks pad. The residence was mostly on cut to the south, which ranged up to around 1.5 m
depth. Fill on the northern side of the residence was supported by a grouted rock wall and ranged up to 2 m
height. The grouted rock facing supporting the fill did not show any signs of global instability having affected
the structure; with the residence generally being in good structural condition, other than some minor cracking
and separation of the concrete apron to the slab. It was noted that some of the cracking coincided with the
daylighting point for underslab pipework (assumed to be for wastewater and water).




Text Figure 8: Looking west along grouted rock wall (LHS); & cracking in apron surrounding existing residence




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