OPW052025 Report - Part 2

▪    Prevent drying of the clay soils during construction by minimizing exposure of cut batters and
     excavation subgrades. This may be achieved by measures such as covering batters with
     geosynthetic filter fabric (‘Bidim’) or HDPE (‘black builders plastic’), staging earthworks to minimize
     soil exposure before covering with structures or surface layers (e.g. topsoil), and placement of
     ‘blinding’ layers across excavation subgrades and footing bases (e.g. crushed rock or concrete). If
     clayey subgrade materials are allowed to ‘dry out’, significant softening could occur on ‘wetting up’,
     resulting in subgrade strengths less than expected for stiff clays.
▪    Install and maintain temporary drainage to prevent saturation of the site during construction.
▪    Seal the ground surface around building perimeters (e.g. concrete or paved pathway).
▪    Plant shrubs/trees at least a distance from buildings equal to the mature height of the shrub/tree.
▪    Install landscaping along proposed roads that will minimise the potential for adverse impacts on
     pavements and kerbs (e.g. preferably shrubs, grasses, etc.).
▪    Maintain stormwater drains to prevent overflow into soils.

6.3.2         Footings
Footing requirements would be subject to geotechnical review once building design plans and
earthworks details are available for the proposed residence; however, the following may be used in
developing structural design details.
All footings for the proposed buildings and any ancillary structures (e.g. retaining walls etc.) must
penetrate through any future fill and found at least
0.5 m below ground level (existing) into stiff to very stiff residual soils.
It is anticipated that normally dimensioned, high-level footings or mass poured shallow concrete pads
(‘bucket piers’) could be considered for most of the building. Footings founded as recommended above
in stiff (or stiffer) clays/silts could be designed using a maximum allowable bearing pressure of 100 kPa
(high level/pads), or up to 150 kPa maximum allowable end bearing pressure for steel reinforced,
concrete bored piles (e.g. if this was preferred for post footings). For deeper piles taken to socket into the
underlying very low strength (or stronger) weathered basalt, a maximum allowable bearing pressure of
400 kPa could be adopted for design.
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 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 14.




Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                              21