Appendix H - Slope Stability Assessment

Brett Hage                                                                                            22328-001-Rev0
                                                                                                   14 September 2022

4.4    Footing Design
Footings for the proposed building and any ancillary structures (e.g. retaining walls) must penetrate through
any future fill to ‘socket’ at least 1 m into natural residual soil comprising stiff (or stiffer) clay or a minimum of
0.3 m in to low strength (or stronger) weathered rock. Based on the boreholes and our site observations,
depending on the construction methodology adopted, normally dimensioned, conventional high-level footings
would be appropriate in upslope areas where bulk earthworks cuts are undertaken, and extended on the
downslope side via the use of steel reinforced concrete bored piles (or ‘bucket piles’).
Footings could be designed with a maximum allowable bearing pressure of 100 kPa when founded in stiff
residual clays, 200 kPa in very stiff to hard residual soils; or 400 kPa in very low strength (or stronger)
weathered rock.
Footing design should also account for the characteristic surface movement (ys), indicated by the site
classification given above. The re-use of the site-won clay materials for filling works could result in greater
estimates of characteristic ground surface movement and should be considered by the design engineer once
earthworks details have been confirmed.
All footings should found such that they are not adversely affected by any adjacent excavations, batter
slopes, trenches, or retaining walls that are not designed to support building loads. Footings should found at
least below a plane extending 1 m horizontally from the base of trenches/excavations/retaining walls, then
rising up at 1V:1H, as illustrated in Text Figure 11.




             Text Figure 11: Footing depth required to minimise the risk of undermining and surcharge loading

Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. Should bored piles be adopted, it is recommended that the piling contractor have a ‘mud
bucket’ or similar device to enable clean out of the pile bases. ‘Spinning’ of the augers is not acceptable
and will not sufficiently remove drill spoil from pile bases. A suitably qualified and experienced RPEQ
should inspect the footing excavations to confirm appropriate foundation materials and cleanliness prior to
pouring concrete.




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