J001701-006-R-Rev1_Medihub_Geo

Stockwell



      5.2 Subsurface Conditions
The boreholes generally encountered very stiff to hard residual clay. Some very stiff clay fill (or reworked
natural soil) was encountered in BH5 to 0.9 m BGL. In BH4 the residual clay tends to extremely weathered
rock from about 6 m BGL.
The seismic testing confirms the results of the borehole drilling and indicates inferred extremely weathered
rock from about 7 m to 8 m BGL.
      5.3 Groundwater
Groundwater was not encountered during drilling. This is in line with our previous experience in this area
where groundwater was also not encountered during drilling. However, groundwater seepage may have
been slow to develop through the low permeability clays within the boreholes. Our previous experience on
the commercial developments immediately to the south-west, as well as the residential development to the
north-east, is that groundwater was encountered at about RL 3 m to RL 4.3 m AHD (i.e. equivalent to about
5.7 m to 7 m BGL at Lot 22).
Groundwater levels on the site may vary depending on weather/climate and human influences (e.g. surface
drainage).

6.0     GEOTECHNICAL COMMENTS & RECOMMENDATIONS
      6.1 Excavation & Stability
Excavation for the proposed double-level basement is expected to mainly encounter very stiff or hard clays.
Excavation of such materials should be achievable using conventional small to medium size earthmoving
equipment (i.e. 12 to 20 tonne excavators). Larger equipment will improve excavation productivity. Ripper
attachments may be required where weathered rock is encountered or within confined excavations.
The subgrade of the approximate 6 m deep basement excavation is expected to comprise very stiff/hard
residual clay or extremely weathered rock. Trafficability of this material should generally be good, except
when exposed to water. It would be prudent to allow for the placement of a 150 mm thick layer of aggregate
across the base of the excavation and elsewhere on the site where vehicle/machinery access is required, to
assist with access and trafficability in the event of rain and/or groundwater seepage.
Working platform assessments must be carried out by a suitably qualified person (RPEQ) for heavy
machinery or equipment (e.g. crane or piling rig, if required) prior to trafficking and operations.
Groundwater inflow/seepage may be encountered during the basement excavation, depending on the
groundwater level at the time of the excavation. Based on the relatively low permeability of the residual
clays, such inflows could likely be managed through a series of sumps in the excavation base and pumping
out from these sumps. The excavation base should be graded to direct water to the sumps, and/or a system
of shallow drains could be considered. Discharge of captured groundwater will be subject to relevant
Council/State guidelines.
Subject to space limitations, the basement excavation could be temporarily battered, followed by wall
construction. Temporary batters should be no steeper than 1V:1H in the residual clays but must should be
assessed (particularly for possible fissured clays) by a geotechnical RPEQ. Steeper batters may be possible
in the lower portions of the excavation where weathered rock may be found, subject to inspection by a
geotechnical RPEQ. Batters in uncontrolled fill (if encountered) may need to be less steep (about 1V:2H).
If battering is not suitable, then a soldier pile wall with shotcrete infill could be considered (and could form the
permanent wall; refer Section 6.2).
Piles will likely need to be propped, braced or anchored to limit bending moments and deflections and to
provide control of soil movements behind the wall. These movements may be both lateral and vertical. Such
movement has the potential to damage adjacent adjoining in-ground services and structures (if any). A soil-
structure interaction analysis would need to be undertaken to assess such movement as part of the
temporary stability design. Temporary basement support must be designed by a suitably qualified and



J001701-006-R-Rev1                                                                                      Page 5