Appendix 5. Geotechnical Engineering Report [As Published] · 132007.2399

October 2007                                  - 16 -                         077682076(AB)


                            Table 4 – Presumptive CBR Values

Subgrade Type                                Drainage conditions      Presumptive CBR (%)
Silty SAND (SM)                                        Poor                      5
                                                       Good                      7
Dense SAND (SW, SP)                                    Poor                     10
                                                       Good                     15


For the proposed pavement areas we recommend that a preliminary design CBR of 7% be
adopted for design. The subgrade CBR must be confirmed by inspection and testing
following earthworks to subgrade level.

6.1.4   Drainage

Diversion drainage, in the form of earth bunds or swales, should be placed up slope of
construction areas in order to divert surface water away.

Permanent subsurface drainage should be installed in any areas identified during construction
or earthworks as having wet (as opposed to moist or dry) soil. Natural groundwater ‘springs’
may occur across the site, particularly at breaks-in-slope such as around the edge of the low-
lying south-western area.

Positive permanent drainage must be installed behind retaining walls to prevent the
development of hydrostatic pressures. Diversion drains may be required above the crest of
cut or fill embankments.

Stormwater collected from road surfaces should be directed by swales or pipes to engineer
designed dissipation/detention structures.

Perimeter, subsurface cut-off drains should be installed adjacent to new pavements in order to
intercept and divert subsurface seepage prior to entering the pavement subgrade.

6.1.5   Site Management & Erosion Potential

It is recommended that ‘good’ building practices are adopted so that moisture variations are
minimised in foundation/subgrade materials. In particular we recommend the following:

•   Site drainage should be designed to readily remove surface water and to prevent ponding
    of water on subgrade areas during construction and also adjacent to foundations and
    pavements once construction is completed.

•   Construction methods involving subgrade preparation for earthworks, foundation
    excavation and service trenching should be such that any clay soils are not subjected to




                                     Golder Associates