Appendix4-SlopeStabilityAssessment · MCU18/0135

Structures Engineering Design                                                                            18197-001-Rev0
                                                                                                           6 August 2018

Temporary cuts in the very loose to loose sands could be formed at 1V:1.5H for batters up to 2 m in height.
Deeper cuts, if required, should be specifically assessed by Tectonic. Permanent cut batters may be formed
at 1V:3H or flatter. Fill embankments greater than 0.5 m in height should be retained.

4.3       Footings
Footings for the proposed residence, pool and any retaining structures must penetrate through all very loose
and loose sand to found at least 1 m into natural medium dense sand. Based on the Ground Drill boreholes,
founding depths of at least 3.7 m BGL are estimated. Footings founded as per the recommendations above
may be designed using an allowable bearing capacity of 200 kPa. Given the sandy soil profile, steel screw
piles are considered to be a suitable foundation type.
All footings should found such that they are not adversely affected by any adjacent excavations, batter
slopes, trenches, retaining walls, or in ground structures (e.g. swimming pools) that are not designed to
support building loads. Footings should found at least below a plane extending 1 m horizontally from the
base of trenches/excavations/retaining walls/swimming pool, then rising up at 1V:1H, as illustrated in Text
Figure 8.




                     Text Figure 8: Zone of influence diagram – footings to be founded below this line

If any soil conditions encountered during construction are found to differ from those noted above, Ground
Drill and Tectonic must be notified immediately and an inspection carried out to determine if changes to
footing design are required.

4.4       Retaining Walls
Design for retaining structures could be carried out using a triangular pressure distribution applicable to the
wall type and based on the soil parameters given below. The soil parameters given below do not include
allowance for hydrostatic pressures, construction loading, adjacent footing loadings, the effects of sloping
ground and backfill compaction stresses, which should all be considered by the design engineer where
relevant.
          Soil Type: Very loose to loose sand
          Unit Weight: 18 kN/m3
          Friction Angle: 28˚
          Active earth pressure coefficient (Ka): 0.36
          At-rest earth pressure coefficient (Ko): 0.53
          Passive earth pressure coefficient (Kp): 2.77
Retaining walls 1 m in height or greater, should be designed by a professional engineer in accordance with
AS4678-2002 Earth retaining structures.
Retaining structures should be founded as per the recommendations in Section 4.3.
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.




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