Addendum Report - Slope stability assessment - 400 Cooroy Mountain Rd, Cooroy

Project DMS                                                                                    17385-002-Rev0
                                                                                               19 January 2022

An experienced engineering geologist completed a site-walkover assessment on 13 January 2022, in order
to make an appraisal of the current site conditions in the location of the pool deck. A two tier masonry
block retaining wall (~2.5 m total retained height) was present along the western and south-western edges
of the proposed deck area; with an irregularly shaped concrete pool shell to the south-east.

From the base of the wall and pool shell exposures some more recently tipped fill was noted, left inclined at
around 25° (Ref. Text Figure 2 below). Below this, exposures of residual soils were noted to the east of the
pool shell. The tipped fill was estimated to be in the order of about 0.5 m to 1 m depth over the slope. The
overall natural ground slope in this area was estimated at around 14°. Measurements of slope angles were
taken on site using a hand-held inclinometer.




Text Figure 2: View looking west over area of proposed deck showing tipped fill
In our previous investigation report (Ref. 17385-001-Rev0, dated 27 November 2017), test pits TP2 and
TPS were located to the south-eastern part of the new pool deck. These encountered stiff to very stiff,
residual sandy clay soils to depths of between 0.4 m and 0.7 m BGL, with highly weathered rhyolite below
to excavation refusal depths of between 1.2 m and 1.3 m BGL (depths at the time of the original
investigation fieldwork, 9 November 2017).

We understand that the pool deck is a structurally separate element from the adjacent retaining walls and
pool shell, and that being the case, we would recommend that all footings for the proposed pool deck are
taken through any recently placed fill, and residual soils, to be founded a minimum of 0.3 m into very low
strength (or stronger) weathered rock. It is understood that steel reinforced concrete bored piles are
proposed for the footing construction, which is considered suitable from a slope stability viewpoint. Footings
so founded could be designed using a maximum allowable end bearing pressure of 1 MPa. If the tipped fill
described above is to be left in place it should be trimmed no steeper than 1 V:2.5H (or flatter) and
protected from erosion with vegetation.

The risk assessment for the new pool deck has been carried out according to the AGS 2007 Practice Note
Guidelines for Landslide Risk Management. Based on the extents of the pool deck shown in Text Figure 1 ,
and that the footings are constructed as above, it is our assessment that a Low Risk of landslide could be
achieved, and that the assessment given in Table 3 of our previous report remains valid to the proposed new
pool deck construction. Considerations in our previous investigation report regarding other geotechnical
matters (such as siting of the deck, earthworks and drainage); would also be critical in maintaining a Low
Risk of slope instability.




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