Geotech Report · MCU19/0008.02

Project No. PG-2096                                                                                                                Page | 8
December 2022
Ref: PG-2096, 2022-11-03, GR VER 1
Keith & Jeannette Ince – Geotechnical Stability Assessment - Proposed Residence, 22 Arakoon Crescent, Sunshine Beach




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.

The design of flexible and rigid retaining walls could be undertaken using a triangular
pressure distribution and the earth pressure parameters given in Table 3. 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 (i.e. single anchored/propped walls) and should
be designed using the at-rest earth pressure (Ko) and a trapezoidal pressure
distribution as per Figure 1.


                                                                                                  FIGURE 1
                                                         0.25H




                                                                             H
                                     0.4  H




                                                          0.25H

                                                                                                KH
                                 Trapezoidal                                                 Triangular




Passive resistance (Kp) at the toe of the wall should be ignored in the zone where
future disturbance (e.g. 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. Allowance should also be made for the surcharge due to
sloping crests if applicable.

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