J001701-006-R-Rev1_Medihub_Geo

Stockwell



experienced engineer (RPEQ), and advice from specialist contractors should be sought regarding the design
and construction. The temporary support system adopted would need to prevent the loss of support to
adjacent buildings and other structures (if any), whilst also acting to limit groundwater seepage (if any) into
the excavation.
Shoring will also be required to enable safe personnel entry into trenches (with vertical sides) deeper than
1 m. The above recommendations do not supersede any existing safety regulations or legislation applicable
to excavations (e.g. limits on personnel entry into trenches).

     6.2 Basement Design
An engineer-designed soldier pile wall with shotcrete infill could be used for temporary excavation stability
and could also form the permanent wall, as well as potentially carry vertical loads from the structure above.
Anchored supports or bracing will be required until the ground floor is constructed.
For preliminary design, walls supported by multiple rows of anchors or props could be designed using a
rectangular earth pressure distribution of 0.25γH kPa over the full height of the wall [where H is the total
vertical height of the wall in metres and γ is the bulk unit weight of the soil retained (suggested value
20 kN/m3)]. It is probable that the earth pressure will be below this value at the top and toward the base of
the wall. Core should be contacted if further details are required. Surcharge loading should be multiplied by a
lateral coefficient of 0.25 to calculate the resulting additional lateral pressure on the wall.
Groundwater was not encountered during drilling (refer Section 5.3); however, given the proposed basement
depth (in the order of 6 m BGL), the presence of groundwater against the basement walls cannot be
discounted. A ‘wet’ basement design may be viable (subject to regulator approval) – possibly designed to
accommodate hydrostatic pressure to a nominated groundwater level (say 3 m BGL), with inflow/relief valves
into the basement for higher groundwater levels. In our experience, ‘wet’ basement designs are not favoured
by regulators (e.g. Council). A ‘dry’ (or ‘tanked’) basement design may therefore be required, designed to
withstand hydrostatic pressure from groundwater at a preliminary design level of 2 m BGL (e.g. about
40 kPa) provided relief valves are installed at this level.
Installation and monitoring of a groundwater standpipes to 6 m BGL are recommended to confirm the above
recommendations regarding groundwater/hydrostatic pressure. The standpipe can be installed during
proposed fieldwork on Lot 2 (proposed Retail outlets development) and monitoring will need to include a
significant wet period to provide meaningful results.

     6.3 Foundation Design
    6.3.1 Site classification
The site classification derived in accordance with Australian Standard AS 2870-2011 Residential slabs and
footings is not directly applicable to developments of this type but can provide an indication of the likely
magnitude of reactive (shrink and swell) movements associated with normal seasonal soil moisture
variations.
Due to the presence of fill the site is classified Class P. The existing soil profile on the site could be classified
as equivalent ‘Class ‘M’ in accordance with AS 2870-2011 The estimated characteristic ground surface
movement due to seasonal moisture variation (‘ys’ value) for this site is estimated to be in the order of 20 mm
to 40 mm.
It should be noted that the estimated order of movement is based on the existing soil profile (i.e. no cutting or
filling), and moisture content changes within soils due to normal seasonal moisture variations only.

    6.3.2 High-level Footings
Ground conditions at basement level are likely to comprise very stiff/hard residual clays (and possibly
weathered rock). A high-level pad, strip or raft footing system in this material is considered appropriate and
could be designed using an allowable bearing pressure (ABP) of 300 kPa. Settlement is estimated to be less
than 30 mm. Preliminary analysis of the raft suitability could be carried out using the geotechnical design
parameters provided in Table 1 below.



J001701-006-R-Rev1                                                                                       Page 6