Geotechnical Investigation Report · OPW26/0149

                                                                                           PTG/04982 – 0001 – Rev0 – Rev0



»   Rock armouring with angular, inert rocks of sufficient mass such that they will not be mobilised or entrained by
    the discharge flow during over-topping events; and,
»   Reinforced concrete armouring with engineer designed baffles to help retard and slow the velocity of any
    discharged water.
As part of the addition of an emergency spillway, it is recommended that the dam height be increased to provide
additional free-board above the designed pond level. The additional free-board will help to reduce the potential for
over-topping, and also help to direct water towards the emergency spillway during extreme weather events.
    9.2.6        Batter Angles
Temporary cut batter angles of 1(v):1(h), or 45°, may be achievable for unsurcharged batters up to 2.0m high where
no groundwater is present. Where surcharges are located within H (height of the batter) of the top of the batter, or
where the excavation extends below the groundwater table, some reduction in this angle will be required.
Care must be taken with unsupported batters to ensure no adjacent structures or services are undermined. Structures
or services are to be set back behind the crests of unsupported batters to a minimum horizontal distance of at least
1.5 times the depth of excavation.
Permanent batter slopes formed in filled ground should be over-filled beyond the intended batter position to ensure
adequate and uniform compaction. Once the over-fill is complete, the final batter should be formed by excavating out
the excess material.
Permanent batter faces formed in both cut and filled ground should be finished at a grade that is no steeper 1(v):3(h),
or 18°.

     9.3           FOUNDATION RECOMMENDATIONS
The investigation undertaken generally indicates competent founding conditions in the location of the proposed
cabins. Shallow piers for the cabins may be designed in general accordance with AS2870:2011 for Class M reactive
soil conditions and the structural loads from the cabins.
In general, borehole BH01 suggests firm to stiff ground conditions are present within 1 m of present ground surface,
however we are also aware that some initial site formation excavation will take place ahead of the cabin foundations
construction. For the preliminary design of the shallow piers for the cabins, the allowable bearing pressures provided
in Table 8 are recommended:
Table 8: Shallow Pier Allowable Bearing Pressures
                         Material Type                                  Allowable Bearing Pressure (kPa)

                     Firm to Stiff Silty CLAY                                         50 – 150
                  Stiff to Very Stiff Silty CLAY                                     150 – 250
                  Very Stiff to Hard Silty CLAY                                      250 – 450
                        Hard Silty CLAY                                                450+


Higher values may be adopted subject to verification of founding conditions at the time of construction.
Given the relatively light loads from the cabins and the competent founding materials present, total settlements are
expected to be small (typically <10-15 mm). Differential settlement across individual cabin footprints is expected to
be negligible where founding levels are consistent.
The carport slabs should be designed as conventional stiffened raft slab in accordance with AS2870:2011 for Class M
reactive soil conditions. Similarly, the raft slabs may be designed assuming an allowable bearing pressure of 100 kPa
where founding upon stiff to very stiff residual clay
Provided slabs are constructed at uniform founding levels and within natural materials only, or a combination of
engineer certified Level 1 fill and natural materials, significant differential settlement is not anticipated.

     9.4           STORMWATER/WASTEWATER EFFLUENT DISPOSAL
Stormwater and wastewater effluent disposal was outside the scope of our brief, and we understand these matters
will be addressed by others. As a general comment, however, we are of the opinion that the near surface soils are
likely to be of low permeability, and therefore would be unsuitable for any form of soakage disposal system for either
stormwater or treated wastewater effluent.


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