Appendix E - Geotechnical Report

                              PROPOSED RECONFIGURATION OF A LOT
                              216 BLACK MOUNTAIN ROAD, BLACK MOUNTAIN



Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed reconfiguration of a
lot at 216 Black Mountain Road, Black Mountain. The investigation was performed by Tectonic Geotechnical
Pty Ltd for Murray and Michelle Richardson. Murray & Associates are consulting planners for the project.

The investigation comprised digging of test pits at five locations, together with in situ strength testing, and
geotechnical laboratory analyses. This report summarises the results of the investigation and provides
geotechnical recommendations regarding the design of earthworks, footings, site classification, retaining
walls. A quantitative and qualitative slope stability assessment is included. An assessment of positioning for
on-site effluent disposal has also been undertaken.

The proposed reconfiguration will comprise dividing the existing allotment into two separate lots. The
northern lot (Lot 1) will retain the existing house and be of 3.333 ha; whilst the southern lot (Lot 2) will be of
1.366 ha, with a new residence to be constructed on site. A building envelope has been nominated to the
south of Lot 2, and it is understood access will be gained directly off Black Mountain Road via the existing
allotment driveway.

Test pits generally encountered similar subsurface profiles, comprising a surficial covering of topsoil to
around 0.2 m below ground level (BGL), underlain by residual very stiff silty clay to between 1 m and 1.7 m
BGL, then extremely weathered material comprising stiff to very stiff silty clay with cobbles to between 2 m
and 2.7 m BGL, underlain by weathered phyllite rock. It was possible to dig in to the rock for around 0.4 m to
0.7 m before bucket refusal (6 tonne machine) was encountered. Groundwater seepage was not
encountered in any of the test pits at the time of investigation, with soils being dry to moist.

Cut and fill proposals are not known, however, based on the nominated building envelope and the typical
size of a house on a site of this size, depth/height of cut/fill of 1 m has been allowed for, or should otherwise
be supported by an engineer designed retaining wall.

Following clearing and topsoil stripping (estimated at about 0.2 m depth), the exposed subgrade materials
are expected to comprise mostly stiff to very stiff silty clays. The residual and extremely weathered clay
materials found on site may be suitable for re-use as structural fill, subject to appropriate moisture
conditioning.
Footing design may be based upon a site classification equivalent to Class M according to AS2870:2011,
with estimated characteristic surface movements (‘ys’) of 20 mm to 40 mm. Final site classification and
footing design will be dependent on the type of fill material placed and degree of fill placement supervision.

Footings shall be founded in weathered phyllite and may be designed with an allowable bearing capacity of
350 kPa.

Based upon the existing site conditions, and subject to the implementation of recommendations given within
this report, it is our assessment that there would be a Low (L) Risk of global slope instability affecting the
proposed subdivision. This is with a minimum factor of safety against slope instability of 1.5. The extent of
site works, including cut/fill, must be limited to maintain slope stability. Control of stormwater flows via
channels and/or pipes will also be essential to maintain slope stability.

It has been assessed from the results of the investigation that the soil profiles across the likely effluent
disposal area on proposed Lot 2 is equivalent to a Strongly Structured Medium to Heavy Clay (Soil
Category 6). It is recommended that on-site treatment and land application for Lot 2 should comprise a
household sewage treatment plant connected to a surface or shallow subsurface irrigation area of 322 m².
Final irrigation areas and detailed wastewater system design will be dependent upon individual site
assessments at the building stage. There is sufficient room on each lot for these areas, allowing for
setback considerations.




19 June 2018
Report No. 18127-001-Rev0                                ii