Slope Stability Assessment

Paul & Janelle DeSouza                                                                         22198-001-Rev1
C/- Sarah Waller Architecture                                                                     25 July 2022

4.3       Footing Design
All footings supporting the proposed residence (and ancillary structures, e.g. retaining walls) must extend
through any existing or future fill and natural soils, to found a minimum 300 mm into the underlying competent
weathered sandstone (i.e. equivalent to 300 mm deeper than test pit bucket ‘refusal’ depths, Ref. Section 2.2
above), or at shallower drilling/excavation refusal in consistent low strength (or stronger) material (i.e. not
corestones or ‘floaters’).
Footings so founded may be designed using an allowable bearing capacity of 400 kPa.

With the moderately steeply sloping nature of the site and depth to rockhead, depending on the final
configuration of the house, it is expected that a combination of the following could be adopted:

          Concrete reinforced bored piles, in areas of proposed future retained filling.
          Normally dimensioned, conventional slab on ground beams, in deepest areas of earthworks cut,
           where low strength (or stronger) sandstone is exposed at surface.
          Mass concrete pad footings, socketed into the underlying weathered sandstone may be a preferred
           where residual soils and XW material are present at surface, but the underlying sandstone is present
           within 500 mm of the ground level.
Footing design should also account for potential soil reactivity, which has been determined by Ground Drill to
be Class H1, with an expected characteristic surface movement (ys) of 41 mm to 50 mm. Filling in building
areas would have an adverse effect on the site classification (particularly with site-won clays) with more
onerous requirements likely (i.e. Class H2 or Class E); and should be considered as part of the earthworks
optimisation at this site.

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/batter slopes/excavations/retaining walls,
then rising up at 1V:1H, as illustrated in Text Figure 10 on the following page. Interaction with the existing
retaining wall to the west of the site area should also be considered.




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