Geotechnical Stability Assessment · MCU19/0008

Project No. PG-2096                                                                                                             Page | 9
April 2019
Ref: PG-2096, 2019-02-21, GR VER 2
Andrew and Michelle Douglas – Geotechnical Stability Assessment - Proposed Residence, 22 Arakoon Crescent, Sunshine Beach




TABLE 3                EARTH PRESSURE COEFFICIENTS (NON-SLOPING CREST BACKFILL)

                                                           Unit               Friction
                                                                                                 Active             At Rest   Passive
Material                                                   Weight             Angle
                                                                                                 Ka                 Ko        Kp
                                                           (kN/m3)            (degrees)
                  Loose                                    18                 26                 0.39               0.56      2.56
Sands             Medium Dense                             18.5               30                 0.33               0.50      3.0
                  Dense                                    19                 32                 0.31               0.47      3.25

Preference should be given to adopting thin soil layers and using small hand-
controlled compaction equipment during backfilling against retaining walls. This is in
order to limit the stress applied to the walls during construction. Should heavy
compaction be required, then wall stresses will be well in excess of Ko and
temporary propping should be used.

It is recommended that all retaining walls be drained for full height in order to
minimise hydrostatic pressure build-up behind the wall. Additional guidelines on wall
drainage are provided in Appendix G of AS 4689-2002.

9.0 SITE MANAGEMENT

To maintain the long term performance of the structure, good management of the
soil conditions and the development is vital throughout the life of the development.

The following are some specific comments with respect to site management.

          It is important that the site be well drained. The ground around the building
           should slope away at 1 in 20 for 2 metres and then fall to the stormwater
           system to prevent ponding of water adjacent to the building.
          Founding soils should not be allowed to become saturated.
          Service trenches under the buildings should be kept to a minimum. Saturation
           of the on-site material will result in an increase in potential ground surface
           movements.
          Footings should be poured immediately after excavation. If footings cannot
           be poured on the same day as excavation, a blinding layer of 50mm
           thickness is recommended.
          Trees, garden beds and other vegetation should be planted at a distance at
           least equivalent to three quarters of their mature height away from the
           structures. This will assist in minimising shrinkage movements in the expansive
           on-soils.