slope stability

Integra Partnerships                                                                            22340-001-Rev0
                                                                                             14 September 2022

3.2.3       Excavations
Excavations through the fill and natural soils encountered in the boreholes should be achievable using
conventional earthmoving plant (e.g. small excavators). Non-surcharged temporary cuts in the very loose to
loose sands could be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up to
2 m in height. Although unlikely, if groundwater seepage is encountered or if sloughing of dry sand occurs at
the batter surface, flatter batters or temporary retention may be required. Deeper cuts, if required, should be
specifically assessed by Tectonic.
Permanent batters up to 1 m high (vertical height) may be battered no steeper than 1V:2.5H. All remaining
permanent cut/fill batters must be supported by engineer designed retaining walls. Any permanent soil and fill
batters will require erosion protection (e.g. revegetation or surface protection).

If the above temporary batter slopes cannot be achieved, due to spatial restraints, temporary shoring would
be required (e.g. sheet piles, contiguous pile wall etc.). Excavations in close proximity to existing structures
and infrastructure, including on neighbouring properties, will need to be specifically designed to prevent
undermining.

If undermining is a potential issue the neighbouring structures may need to be underpinned or excavations
may require temporary shoring. The advice of a suitably qualified and experienced structural RPEQ must be
sought regarding any potential undermining and design and construction of any underpins/temporary shoring
(if required).
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).

Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).

3.3     Site Classification
Due to the insufficient bearing capacity of the very loose to loose sand and probability of ‘uncontrolled’ fill
associated with the demolished residences, the site is classified as Class P in accordance with AS2870-
2011 Residential Slabs and Footings. However, it is assessed that the existing soil profile would exhibit
the characteristics of Class S, with the estimated characteristic surface movements (‘ys’) ranging up to
about 20 mm. Due to the potential variability in subgrade density, and allowing for some settlement of the
loose sands, a site classification of Class M is recommended in relation to assessment of flexibility
requirements for ground slabs, footing and plumbing design).

The use of reactive fill materials (clays) can adversely affect the characteristic ground surface movement,
and must be considered during design if fill material other than sand is proposed to be used on the site.

3.4     Footings
No design loadings were available at the time of reporting. High level footings should not be utilised due to
insufficient bearing capacity and the potential for excessive settlements in the loose upper sands
encountered in the boreholes. Footings for the proposed residence and ancillary structures (e.g. water
tanks, retaining walls etc.) must penetrate through any fill materials and unsuitable natural soils (i.e. very
loose to loose sands) to found at least 0.5 m into medium dense (or denser) sands which were encountered
at depths between 1.8 m and 2.5 m BGL in the boreholes. A minimum founding depth of 3 m BGL (existing
ground levels) is recommended.

Given the sandy soil profile and depth of footings required below existing ground levels, steel screw piles
could be adopted. Due to the presence of loose sands, together with the close proximity to surrounding
infrastructure, open bored piers or driven piles are not considered to be suitable for this site. Steel reinforced
concrete bored piles could be feasible; however, the use of sacrificial liners (e.g. ‘formatube’) would be
required to prevent the caving of upper loose sands.




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