OPW052025 Report - Part 2

Table 4: Retaining Wall Design Parameters

                             Unit         Friction           Lateral earth pressure coefficients
 Retained material           weight       angle              Ka                   Ko                    Kp
                             (kN/m3)      (Degrees)          (Cantilever wall)    (Non-yielding wall)
 Stiff (or stiffer)          18           25                 0.41                 0.58                  2.46
 Residual Soils
 Very Low Strength           21           33                 0.29                 0.46                  3.39
 (or Stronger)
 Weathered Phyllite
 Very Low Strength           21           35                 0.27                 0.43                  3.69
 (or Stronger)
 Weathered Basalt
 Future Fill                 *            *                  *                    *                     *
Note:   * Depends on fill material type and level of compaction.

At a minimum, retaining structures must be founded as per the recommendations in Section 6.3.2 above
with the footings also designed to account for lateral loads imposed by the retaining wall. Detailed
design must include an assessment of internal and global stability, together with required bearing
capacity and founding depth, by the retaining wall design engineer.


6.5       Pavement Design
Pavement design will be dependent upon the actual subgrade materials present after earthworks.
Following earthworks subgrades are expected to comprise mostly silty clays with varying gravel content
and gravelly clays. Laboratory testing for the high plasticity silty clay soils with no visual gravel content
returned a CBR values of 1.5 % with a relatively high swell value of 5 %.
Where the reactive silty clays with no visual gravel content are present in the subgrade, and a hard
surfacing (e.g. concrete) is to be used, a nominal over-excavation and replacement to 300 mm depth with
‘roadbase’ (or similar granular materials) would be required to achieve a design CBR value of 3 %.
Alternatively, gravel surfacing could be used with an expectation of some future maintenance. Where the
clays contain a significant proportion of gravel or are gravelly clays (i.e. >30 % gravel) a design CBR of
3 % could be adopted, subject to the subgrade passing a ‘proof roll.’
Pavements may be designed using the CBR value of the imported fill material where such filling is at least
500 mm deep.
The performance of pavements should also be considered against the anticipated ground surface
movements, taking normal seasonal moisture variation into account, as discussed in Section 5.2.1 above.
Consideration could be given to subgrade stabilisation using lime or cement treatment for hard surfaced
pavements traversing the basaltic soils towards the upper parts of the site. Input and advice from a
pavement design specialist should be sought in this regard to confirm volumes of mixing etc.
Proposed pavement subgrades must be ‘proof rolled’ and visually assessed by an experienced RPEQ 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.




Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                                  23