Attachment F - Geotechnical Report

                                PROPOSED COMMERCIAL & RESIDENTIAL DEVELOPMENT
                                30-32 DOONELLA ST, TEWANTIN




                  Text Figure 6: Footing depth required to minimise risk of undermining




6.3        Retaining Structures
Design for retaining structures could be carried out using a triangular pressure distribution applicable to the
wall type and based on the parameters listed in Table 3. The values in Table 3 assume that the walls are
fully drained and no allowance is included for hydrostatic pressures, construction loading, up-slope footing
loadings, the effects of sloping ground and backfill compaction stresses. Retaining structures should be
engineer designed in accordance with AS4678-2002 Earth-retaining structures.
Table 3: Retaining Structure Design Parameters
                                                                     Lateral earth pressure coefficients
                              Unit
                                         Friction
 Retained material           weight
                                         angle (°)            Ka                        Ko
                             (t/m3)                                                                        Kp
                                                       (Cantilever wall)        (Non-yielding wall)

 Stiff/Medium Dense
                               1.8           27                 0.38                   0.55                2.7
 Materials to 1 m BGL
 Very Stiff Natural Clays
                               1.9           30                 0.33                    0.5                3.0
 > 1 m BGL
 Future Fill                    *            *                   *                       *                  *
Note:    * Depends on fill material type and level of compaction.


Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure, e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill.

6.4        Pavement Design Parameters
Based on laboratory testing results, design of pavements may be based upon a CBR value of 3%, subject to
excavation and replacement of the natural silty clay subgrade with select granular fill (e.g. minimum CBR
15% ‘roadbase’) to a minimum depth of 200 mm.

Proposed pavement subgrades must be ‘proof rolled’ and visually assessed by an experienced RPEQ
engineer to ensure no heaving or unacceptable subgrade deflection occurs. Should heaving or deflection
occur, excavation of ‘soft spots’ may be required, followed by reinstatement with compacted select granular
fill.
The recommended design CBR value is applicable to properly drained subgrades only, and must be
confirmed by subgrade inspection and testing by an experienced engineer following earthworks. ‘Properly
drained subgrades’ require open graded drains, site grades that shed water and prevent ponding, and
subsurface drains adjacent to pavements.



22 June 2018
Report No. 18151-001-Rev0                                   9