Preliminary Geotechnical Investigation · OPW21/0315

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not likely to be suitable where there is a need to build close to the site boundaries and adjacent to in-
ground services or where excavations will extend below the groundwater table.

It is therefore anticipated that bulk excavation will require positive support comprising temporary
measures such as sheet piles. Sheet piles could either be installed to act as a cantilever or be
anchored or propped. Anchoring would allow the use of a lighter section sheet and would provide
better control of soil movements. These movements will be both lateral and vertical occurring at the
crest of and behind the excavated faces as a result of relaxation of these faces.

The design of flexible retaining walls should be specifically engineer designed in accordance with
AS4678 (AS 4678, 2002) using a triangular pressure distribution and the earth pressure parameters
given in Table 2. Flexible walls are those which are free to rotate or tilt (such as cantilevered walls)
and should be designed using active (Ka) earth pressure coefficient. Rigid walls are those which are
restrained against rotation or tilt (ie. single anchored/propped walls) and should be designed using the
at-rest earth pressure (Ko).

Table 2: Earth Pressure Coefficients (non-sloping crest backfill)
              Material                 Unit       Friction       Active        At Rest         Passive
                                     Weight        Angle          Ka             Ko              Kp
                                     (kN/m3)     (degrees)
           Loose sands                  17           30           0.33           0.50             3.0
      Medium dense sands                18           33           0.30           0.45             3.3

Passive resistance (Kp) at the toe of the wall should be ignored in the zone where future disturbance
(eg. services trenches) could occur.

The effects of surcharge in the retained zone should be included by multiplying the vertical pressure
developed by the surcharge by the appropriate lateral earth pressure coefficient given in Table 2.
Allowance should also be made for the surcharge due to sloping crests if applicable.

The basement structure will need to be ’tanked’ with the basement walls and floor slabs designed to
resist full hydrostatic pressures.


7.3    Site Preparation and Earthworks

All existing sand fill encountered on site should be considered as ‘uncontrolled’. However, it is
anticipated that the sand fill will be excavated and removed as part of the basement excavation.

At bulk excavation level the exposed subgrade is expected to predominately comprise medium dense
natural sands. Trafficability across the sand subgrade will be impossible except for tracked equipment
or 4WD vehicles. Surface loosening of the sand is anticipated due to excavation works. In order to
improve trafficability across the loosened surface sands, the exposed subgrade should be compacted
with a small vibrating roller to achieve a minimum density index of 70%. If the sand is dry, it should be
saturated and left for a short period prior to vibration. To improve subgrade performance and site




Proposed Residence                                                                      95120.00.R.001.docx
33 Wyuna Drive, Noosaville                                                                     October 2020