SUPERSEDED Reference Document

                                                                                                Page 7 of 13




pressure coefficient. Rigid walls are those which are laterally restrained against rotation or tilt (ie.
single anchored/propped walls) and should be designed using the at-rest earth pressure
coefficient (Ko).

Table 3: Earth Pressure Coefficients (non-sloping crest backfill)
 Material                           Unit        Friction     Active         At Rest       Passive
                                    Weight      Angle        Ka             Ko            Kp
                                    (kN/m3)     (degrees)
 Loose sand                         17          30           0.33           0.50          3.0
 Medium dense sand                  18          33           0.30           0.45          3.3
 Dense to very dense sand           19          36           0.25           0.40          3.8

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 3. 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.

It is recommended that factors of safety of 2 against overturning and sliding stability and 1.5 for
global stability, be adopted in the design of all retaining walls.

For limit state design methods, the ultimate parameters provided in Table 3 will need to be
factored in accordance with AS 4678 (2002). Guidance on the selection of material strength partial
factors is provided in Section 5.2 of AS 4678 and is dependent upon the nature and state of the
in-situ soils.

6.3    Site Preparation and Earthworks

All existing sand fill encountered on site should be considered as ‘uncontrolled’. However, it is
anticipated that the majority if not all of the clayey sand/sand fill will be excavated and removed
as part of the basement excavation.

At basement excavation level the exposed subgrade is expected to predominately comprise
medium dense natural sand. Trafficability across the sand subgrade will be impossible except for
tracked equipment or 4WD vehicles. Surface loosening of the sand is anticipated due to
demolition and excavation works. In order to improve trafficability across the loosened surface
sand, 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 trafficability, it is recommended
that, in areas where loose to medium dense sand is exposed at subgrade level, they be overlaid
with 150 mm of well compacted select granular fill with a minimum CBR value of 15%.




Proposed Residence                                                                    229073.00.R.001.Rev0
13 Mermaid Quay, Noosaville                                                                     June 2024