J001783-001-R-Rev1

Ashley and Caroline Hermes

fully drained and no allowance is included below for construction loading, up-slope footing loadings, and
backfill compaction stresses.
If appropriate, an allowance of 10 kPa should be made for lateral stress induced by compaction plant
operating behind the walls. The effects of surcharge should be included by multiplying the vertical pressure
developed by the surcharge by the appropriate lateral earth pressure coefficient from Table 3. Allowance
should be made for additional surcharge on the wall caused by sloping backfill or natural soil slopes.
Positive permanent drainage (surface and subsoil) must be installed behind retaining walls to prevent the
development of hydrostatic pressures. Drainage material behind the wall should be installed for the full
height of the wall, for a width of at least 0.3 m. The material must be free draining and granular and have a
perforated or slotted drainage pipe at the heel of the wall to rapidly remove the water into the stormwater
system. Drainage outlets must extend beyond the ends of the walls. Installation and maintenance of
drainage will be important to reduce the risk of slope instability on this site.
Retaining wall backfill should be a loose, free draining granular material and should only be lightly
compacted as heavy compaction may raise the earth pressure to above the ‘at rest’ earth pressure. A
geotextile to prevent fine grained soil from migrating into the free draining backfill is also recommended.

Table 3: Retaining structure design parameters.
                                                  Lateral earth pressure coefficient

                                                                                         Non-
                                                        Yielding
                                                                                       yielding
                 Unit
  Retained                   Cohesion,
                weight                           Ka (cantilever)
  material                    cu (kPa)
                (t/m3)
                                           Angle of retained slope (o)      Kp           Ko

                                          0       10      20        30

 Stiff clay       1.8           50       0.41    0.42    0.51       -      2.46         0.58

 Very Stiff
 to Hard          1.9          100       0.36    0.37    0.43       -      2.77         0.53
 Clay
 Very Low
 Strength         2.0          200       0.18    0.18    0.19      0.21     5.6          0.3
 Mudstone


7.3 Site Classification
The site classification derived in accordance with Australian Standard AS 2870-2011 Residential slabs and
footings can provide an indication of the likely magnitude of reactive (shrink and swell) movements
associated with normal seasonal soil moisture variations.
Based on the results of the investigation, the soil reactivity is within the range of Class M, with an estimated
‘ys’ value in the range of up to 20 mm to 40 mm.
The predicted order of movement is based on the existing soil profile (i.e. no cutting or additional filling), and
moisture content changes within soils due to normal seasonal moisture variations only (i.e. does not consider
soil settlement).

7.4 High Level Footings
The proposed residence and all retaining wall footings should be founded at least 0.5 m into the very low
strength (or stronger) mudstone.
For high-level footings, a design allowable bearing pressure of 500 kPa may be adopted.



J001783-001-R-Rev1