Attachment 5 - Slope Stability Assessment · RAL20/0005

Mr Peter Loxton                                                                                 16162-001-Rev0
                                                                                                   22 July 2016

2.0        DESCRIPTION OF EXISTING CONDITIONS
2.1       Geology
Available geological information1 indicates that the site is underlain by the late Triassic to Jurassic age Tiaro
Coal Measures, which comprise predominantly sandstone and siltstone.

The CSI investigation comprised two boreholes (BH1 and BH2) drilled to depths of up to 1.9 m (BGL) by
hand auger at the southern and northern ends of the proposed addition, respectively. In situ soil strength
was assessed by conducting dynamic cone penetrometer (DCP) tests adjacent to each of the boreholes,
extending to depths of up to 3.5 m BGL.

The boreholes and DCPs indicate very loose natural clayey sand to depths of between 0.4 m and 1.1 m
BGL, then loose sand, becoming dense from about 1.5 m to 2 m BGL, and possibly tending dense from
approximately 2.5 m to 3.5 m BGL (then extending to the depth of investigation). These soils are inferred to
comprise alluvium deposited within the watercourse, and are likely to overlie stiff residual clays and
weathered rock of the Tiaro Coal Measures at depth.

Groundwater was encountered at 0.4 m BGL in the CSI boreholes, with the sands described as “wet”.

CSI has classified the site as Class P (per AS2870) due to the presence of low bearing capacity of the
sands, however indicate that the site has the characteristics of Class S (low reactivity to normal soil moisture
content changes).

2.2       Topography
As shown in Text Figure 3, the proposed building site (at the rear of the property) is located at the western
edge of a natural watercourse, where the ground surface slopes very gently (<1°) towards the north. In the
western part of the block, the natural ground surface rises up to a well-rounded ridge which peaks further
west near Alderley Terrace (likely underlain by residual soils and weathered rocks of the Tiaro Coal
Measures). Ground slopes in this part of the site form a relatively planar surface and were recorded using a
hand held clinometer ranging up to about 10° (18%) (Ref. Text Figure 4 over page). Cut/fill batters around
the existing house have formed a level carparking area at the front (Ref Text Figure 5 over page).




                                                          Site




Text Figure 3: Local ground surface contours (courtesy NSC MapInfo)




1
    1:100,000 Gympie Special Geology Map, Sheet 9445 & Part 9545, DME, 1999




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