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

Total Concept Homes                                                                              20332-003-Rev0
                                                                                                    16 June 2022

4.0        RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached. This document should be referred to by those
preparing the concept and designs for future residential development at the site.

4.1       Site Layout and Geotechnical Constraints
The general building location denoted in Text Figure 2, and as visited by Tectonic during our walk-over is
considered reasonable from a slope stability viewpoint, although this should not extend further downslope to
the east on to the slightly steeper slopes (measured at up to 15°).

Tectonic must be consulted to assess the impacts on slope stability if the granny flat is moved elsewhere, or
if the extent of earthworks is altered (other than as recommended in this report).
To achieve a Low Risk of instability, the following is recommended:

          All retaining walls exceeding 1 m high and/or subject to surcharge loads must be engineer designed
           and certified, which must include an assessment of global and internal stability.
          Unretained fill heights shall be limited to no more than 1 m, such as for landscaping. Any proposed
           fill exceeding 1 m must be fully supported by an engineer designed and certified retaining structure,
           unless assessed and approved by a suitably qualified and experienced geotechnical RPEQ.
          Design and construction of footings shall be undertaken in accordance with Section 4.3.
          Formalised drainage should be constructed to ensure all stormwater is directed away from the
           proposed and existing structures.

4.2       Earthworks
In construction areas, all vegetation, tree stumps and large roots must be ‘grubbed out’, with any organic rich
topsoil removed. Any slumped materials on the batter face and crest of the existing fill batter should be
selectively removed, and recompacted as per the recommendations below. It is recommended that finished
subgrades be proof rolled using a minimum 4 tonne rubber tyred vehicle or drum roller to identify any ‘soft
spots’. Any such soft spots should be over excavated and replaced with compacted select fill.

Although trafficability should generally be good in ‘dry’ conditions, the soils could be subject to strength loss
from repetitive vehicle loading (particularly from rubber tyred plant) or if they become wet. Due to the
potential for poor trafficability during and after wet periods, placement of a minimum 150 mm thick granular
working platform may be required, particularly along access ways.

Fill materials should be compacted at moisture contents within the range of -2 % to +2 % of optimum
moisture content for Standard Compaction. Confirmatory compaction testing must be carried out at regular
intervals and further details for control and testing of fill are given in Australian Standard AS 3798-2007
Guidelines on Earthworks for Commercial and Residential Developments.

Fill (site won and imported) should have a maximum particle size of 100 mm for an uncompacted layer
thickness of 200 mm; and shall be compacted by repeated rolling with a roller to achieve a dry density ratio
of at least 95 % of the Standard Maximum Dry Density for cohesive soils, or 70 % Dry Density Index for any
imported cohesionless soils. Topsoil must not be re-used as structural fill, but may be used for landscaping.

Any imported fill should comprise a good quality material meeting the following specifications:

          Soaked CBR ≥ 10 %
          Shrink-swell index (Iss) ≤1 %
          Maximum particle size ≤ 100 mm

Following stripping of topsoil and any organic laden surface soils, fill must be ‘benched’ into the natural soil,
with batters over filled by 0.5 m (horizontally) and then trimmed back to the well compacted material.

It is anticipated that excavations (e.g. footings, underground services) could generally be completed by
medium size excavators (8 tonne to 10 tonne) with toothed bucket attachments. However, the ground profile
indicated by the Ground Drill boreholes suggests that weathered rock could be expected from the current cut




                                                       9/13