Slope Stability Report - 22072-001-Rev0

H.M de Groot                                                                                       22072-001-Rev0
                                                                                                    21 March 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.

4.1     Earthworks
Based on the proposed development and the existing site conditions it is anticipated minimal cut and fill
earthworks will be required (i.e. less than 0.5 m depth/height). Should proposed cut/fill exceed 0.5 m
depth/height, the impacts on slope stability must be reviewed by Tectonic before construction.
The shed pad should be proof rolled to identify any soft spots, which should be removed and replaced with
select, compacted fill. The pad surface should be re-graded at a minimum 1V:100H ‘into’ the hillside (north)
to prevent surface water overtopping the fill embankment on the southern side.
Excavations in the natural soils and very low to low strength phyllite should be achievable using conventional
earthmoving equipment (e.g. small to medium sized excavators in the 5 tonne to 12 tonne range). However,
excavations into medium strength (or stronger) phyllite may be difficult and may require the use of ripping
tynes and/or rock hammer attachments. Where excavation into rock is required the contractor must consider
the effects of noise and vibrations.
Temporary batters in stiff (or stiffer) natural soils may be formed at 1V:1H; with permanent batters formed no
steeper than 1V:2H in natural soils or 1V:1H in weathered rock. Flatter batters may be required if
groundwater seepage is encountered. Permanent batters will require erosion protection such as covering
with erosion matting or vegetation. Where spatial constraints prevent adoption of the above recommended
batter slopes, cut or fill batters would need to be supported by retaining walls.
Ground surfaces should be graded to shed surface water runoff away from construction areas. It should be
noted that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion
and sediment control measures must be installed prior to commencement of any further earthworks at this
site.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).

4.2     Retaining Structures
Retaining structures must be founded as described in Section 4.3 below. Retaining structures greater than
1 m high will require engineer design and certification of construction. Retaining structures should be
engineer designed in accordance with AS4678-2002 Earth-retaining structures.
We suggest the following parameters in Table 3 may be adopted for retaining wall design.
 Table 3: Retaining Wall Design Parameters
                                                             Lateral earth pressure coefficients
                          Unit      Friction
 Retained material      weight       angle
                                                         Ka                     Ko
                        (kN/m3)    (Degrees)                                                        Kp
                                                  (Cantilever wall)     (Non-yielding wall)
 Stiff (or stiffer)
                          19           25               0.40                   0.57                2.46
 Clays

 Weathered Rock           21           35               0.27                   0.43                3.69


The values in Table 3 assume that the walls are fully drained and no allowance is included for hydrostatic
pressures, construction loading, up-slope footing loadings, the effects of sloping ground and backfill
compaction stresses.




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