slope stability report

HAZARD MITAGATION – CONSTRUCTION RECOMMENDATIONS

We consider the primary risks for landslip occurring within this allotment is over excavation or, stormwater scour
and vegetation removal.

The construction of any cut/fill platforms within the site can lead to an increased risk of landslip.

Any over excavated (steep) cuts may lead to landslip. Similarly, steep fill batters may be susceptible to slip or
erosion.

Any exposed fill batters are susceptible to weakening from saturating rains and stormwater scour. This risk is
compounded, where poor compaction is achieved when placing the fill. Any fill batter profiles should be limited
to a maximum grade of 1 to 3 (vertical to horizontal). Where possible the fill platform should be benched into the
natural profile after the removal of all organic material including topsoils. The fill material should be placed and
compacted to achieve a minimum relative field density of 95%. Surface stormwater runoff should be diverted
around fill batters. Vegetating the fill batter can improve the batters ability to resist slip and or scour. Where
these recommendations are unachievable, benching, shoring or retaining measures should be implemented.

Any permanent exposed cut batters should be limited to a maximum slope of 1 to 2 (vertical to horizontal). The
face should be protected from stormwater scour with cut off drains above the cut. Catch drains should also be
provided along the base of the cut. The drains must dissipate all stormwater from above and below the cut.
Vegetating the cut batter can improve the batters ability to resist slip and scour. Steeper batters should be
retained with engineer designed retaining walls.

All excavations must be adequately battered or retained to prevent collapse. Temporary excavations for retaining
walls can have exposed cut batters of 1 to 1.7 (vertical to horizontal) taken from the base of the excavation.
These earthworks must incorporate shallow surface spoon drains at the base and along the high side of the cut.
Drains must have an efficient outflow down-slope and clear of the construction area. Any exposed soil face
should also be protected against the potential for stormwater erosion. Saturation of the exposed cut batter from
prolonged rainfall will potentially de-stabilise the cut batter. Programming of the works must consider this
possibility. Temporary back propping of the slope may be required. Should these recommendations be un-
achievable the builder should seek guidance from this office on temporary shoring, benching or other suitable
techniques.

The allotment will support a typical form of residential construction. Structural foundations should be designed
by a competent person in accordance with AS 2870, accepted engineering theory and the geotechnical
conditions.

All earthworks should be constructed in accordance with AS3798-1996 ‘Guidelines on earthworks for commercial
and residential developments’ and sound engineering theory.

The builder must ensure the stability of neighbouring structures is maintained through not undermining
their foundations. This is achieved via restricting any earthworks to above a line radiating downwards
at an angle of 1 in 1.2 (vertical to horizontal) from the base of the foundation/structure to be protected.

Retaining walls for landscaping purposes would need to be constructed with consideration of maintaining stability
of the surrounding ground during construction. Such measures would include limiting the slope of any
excavations to a slope of 1 vertical to 1.7 horizontal and protecting the works against stormwater scour. The
retaining wall designs must allow for the lateral soil pressures expected including additional surcharging from
sloping backfills.

Although we consider the allotment stable in its current form there will always be a risk of landslip under severe
stormwater conditions.

Measures that will reduce this risk include the following but are not limited to.

The finished ground surface should be leveled and graded to ensure no concentration of stormwater runoff
leading to ground scour can occur.

Any collected stormwater from areas other than the roof should be disposed of downslope from the proposed
dwelling and should also ensure no ground scour can occur.




 SCG Consulting Engineers                                   4                                     SCG Ref: 30378(SL)