Appendix B - Geotechnical Investigation Report

Stockpiles of construction materials, excavated materials, existing footings, etc. should be kept well clear of
the batter crests to avoid surcharging the slopes.

We expect that retaining walls will be constructed within the temporary batters, temporarily propped,
backfilled and permanently supported by the structure.



6.5     Retention Design Parameters
The following characteristic earth pressure coefficients and subsoil parameters may be adopted for the
design of retaining walls that will be propped by the structure and any landscape retaining walls:
     For retaining walls that will be temporarily propped, backfilled and permanently restrained by the
      structure, we recommend the use of a triangular lateral earth pressure distribution with an ‘at rest’ earth
      pressure coefficient (ko) of 0.6 for the soil profile, assuming a horizontal backfill surface.
     For landscape retaining walls, where we assume minor movements will be able to be tolerated, we
      recommend the use of a triangular lateral earth pressure distribution with an ‘active’ earth pressure
      coefficient (ka) of 0.4 for the soil profile, assuming a horizontal backfill surface.
     A bulk unit weight of 20kN/m3 should be adopted for the retained profile.
     Any surcharge affecting the walls (e.g. nearby footings, compaction stresses, sloping retained surfaces,
      construction loads etc) should be allowed for in the design using the appropriate above earth pressure
      coefficient.
     Conventional retaining walls should be designed as drained and provision made for permanent and
      effective drainage of the ground behind the walls. Subsurface drains should incorporate a non-woven
      geotextile fabric, such as Bidim A34, to act as a filter against subsoil erosion. The subsoil drains should
      discharge into the stormwater system.
     Lateral restraint of retaining walls founded in the soil profile below bulk excavation level or adjacent
      surface levels may be provided by the passive pressure of the soil below these levels. A ‘passive’ earth
      pressure coefficient, Kp, of 3 may be adopted, provided a Factor of Safety of at least 2 is used in order to
      reduce the high deflections that are associated with achieving a full passive case. Localised excavations
      in front of the walls e.g. for buried services etc must also be taken into account when checking the
      embedment depth design. We note that where the ground surface on the low side of the wall slopes
      away from the wall, the lowest ground level within 5m of the wall should be adopted for assessing the
      level at which passive resistance will be effective.
     Friction between the base of a retaining walls and the underlying bedrock can be calculated using a
      friction angle of 35°, provided the base is clean, rough and dry when the retaining wall footing is poured.



6.6     Footing Design
The ASI report has assessed the residual clays to have a high potential for shrink/swell reactivity with changes
in moisture content. However, considering the limited thickness of reactive residual clays and clay fill,
characteristic surface movements in the range for a Class S to Class M site are expected and are likely to be
differential across the proposed house. However, we expect trees to be removed as part of the work and so

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