Reference Document - OPW25 0085 (AS Published)

High level footings founded as described above may be designed using allowable bearing pressures of:
    100 kPa in the stiff/medium dense (or stronger) residual soils and future controlled fill.
    200 kPa in the very stiff to hard residual soils.
    400 kPa in the very low strength (or stronger) weathered rock.
Piles founded as described above may be designed using allowable end bearing capacities of:
    150 kPa in the stiff/medium dense (or stronger) residual soils.
    300 kPa in very stiff to hard residual soils.
    600 kPa in very low strength (or stronger) weathered rock.
The above footing recommendations are considered to be minimum requirements from a slope stability
viewpoint. Future residential footing/slab design must also consider site specific ‘soil testing’.
All footings should found such that they are not adversely affected by any adjacent excavations, trenches,
or retaining walls that are not designed to support loads. Footings adjacent to any fill batter crests
should be deepened to found below the toe of the batter. To minimise the potential for any adverse
interaction effects, 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. This requirement is illustrated in
Text Figure 13.




        Text Figure 13: Footing depth required to minimize risk of undermining and surcharging

Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over
the full base area. For bored piles, it is essential that the piling contractor have a ‘mud bucket’ or similar
device to enable clean out of the pile bases. A suitably qualified and experienced structural/geotechnical
RPEQ should inspect the footing excavations to confirm appropriate foundation materials and cleanliness
prior to pouring concrete.
If any soil conditions encountered during construction are found to differ from those noted in the
geotechnical investigation, Tectonic should be notified immediately, and an inspection carried out to
determine if changes to footing design are required.


6.4          Retaining Structures
Design for retaining structures could be carried out using a pressure distribution applicable to the wall
type and based on the parameters listed in Table 5. The values in Table 5 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.




Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                           20