Appendix D - Geotechnical Report

7.3.2        Footing Design
Footings supporting any structures (including retaining walls) are to penetrate through fill and unsuitable
natural soils (if encountered) to be founded a minimum of 0.5 m into stiff (or stiffer) residual soils.
Due to the depths of foundations recommended, conventional high-level footings (e.g. slab on ground)
should be suitable for some of the lots. Bored piles (or similar) would likely be required where fill
earthworks were undertaken, or where slightly steeper slopes were present (e.g. proposed Lots 2 and 4).
High level footings, so founded, may be designed using a maximum allowable bearing pressure of
100 kPa. A maximum allowable end bearing pressure of 150 kPa could be adopted for steel reinforced,
concrete bored piles. Footings taken to found in the underlying very low strength (or stronger) phyllite
rock could be designed adopting a maximum allowable bearing pressure of 400 kPa.
Footings should be designed for the site classification (at least) nominated above in Section 7.3.1.
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 14.




        Text Figure 14: Footing depth required to minimise 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.


7.4          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 4 overleaf. These values 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.



Geotechnical Investigation
10 June 2026 | 26073-001-Rev0                                                                              19