slope stability · MCU22/0172

Integra Partnerships                                                                                      22339-001-Rev0
                                                                                                       13 September 2022

4.3     Site Classification
Due to the insufficient bearing capacity of the very loose to loose sand and probability of ‘uncontrolled’ fill
associated with the demolished residence, 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.

4.4     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. 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 1.5 m and 2.2 m BGL in the boreholes. A minimum founding depth of 3 m BGL (existing ground
levels) is recommended.
In areas of cut exposing medium dense (or denser) sands at the subgrade level high-level footings may be
adopted. Elsewhere, piles would be required. It should be noted that excavations for concrete strip/pad
footings (or similar) may loosen the base. Therefore, the use of a vibrating plate would be required to
densify any high-level footing bases prior to the placement of reinforcing steel. High level footings, where
achievable in deeper cut areas, may be designed using an allowable bearing pressure of 100 kPa provided
that they are founded in the natural medium dense (or denser) sands. Settlement under such loads is
expected to be in the order of up to 20 mm or 1% of footing width.
Given the sandy soil profile and depth of footings required below existing ground levels, steel screw piles
could be adopted outside of deeper cut areas. 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.

Steel reinforced concrete bored piles founded as noted above may be designed using an allowable end
bearing pressure of 300 kPa. Parameters for screw pile design are given in Table 3 below. Settlements
under such pressures should be no more than about 25 mm.

Table 3: Screw Pile Design Parameters


                                                                                            Ultimate           Allowable
                                                                         Bearing
                                 Unit weight       Friction angle,                          Bearing             Bearing
      Founding Material                                                  Capacity
                                  (kN/m3)                Ø (°)                            Capacity fbult      Capacity fball
                                                                        Factor, Nq
                                                                                             (kPa)               (kPa)

 Medium dense natural
                                      18                 32                  25                1200             400 (Note 1)
 sand
 Dense to very dense (or
                                      19                 36                  35                1800                600
 denser) natural sand
 Notes:
     1)    Allowable pressure based on a working stress factor of safety value of 3 on the ultimate pressure. If a limit state
           design methodology is adopted the ultimate bearing capacity may be used and must be multiplied by a
           geotechnical reduction factor Øg determined in accordance with AS2159-2009 to obtain the Design
           Geotechnical Strength (Rd,g).




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