Appendix B - Geotechnical Assessment Report

Mr Tony Rossiter                                                                               21436-001-Rev0
                                                                                             18 November 2021

4.1.4 Batter Slopes
Given the space available for construction, it is expected that a 1V:3H downstream face; 1V:2.5H upstream
face; and 3 m wide crest can be adopted for the design of the dam. Batters for the sides of the cut-off trench
beneath the dam should be formed no steeper than 1V:1H provided that the depth does not exceed 1.5 m.
Assuming typical soil parameters for reworked natural sandy clay compacted as detailed in Section 4.1.3,
the FOS against slope failure should be at least 1.5, with a resultant Low Risk of slope instability affecting
the dam. Stability analysis should be carried out if a fill embankment higher than 2 m is proposed.

4.1.5 Spillway Construction
The width of the spillway inlet and outlet should be determined to ensure that the maximum likely flood flows
can be carried at safe velocities of less than 2.5 m/s.
It may be necessary for the surface of the spillway be protected with rock embedded in concrete or
shotcrete. Rock (e.g. rip-rap) embedded in concrete may be a preferred option to assist in dispersing strong
flows. A key-in trench should be installed a minimum of 300 mm deep at the upstream edge of the inlet, and
the downstream edge of the outlet, in order to prevent undermining or lifting of the concrete lining of the
spillway.

4.2   Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction area, to prevent saturation of working subgrades.



5.0     REFERENCES
Design and Construction of Small Earth Dams, Nelson, K.D., EA Books, 2010 (3rd Ed.)

Guidelines for the Construction of Earth-Fill Dams, Water Resources Policy #2008/1, Water Resources
Division, Dept of Primary Industries and Water, Tasmania, 2008
Queensland Dam Safety Management Guidelines, The State of Queensland (Dept of Natural Resources and
Mines), 2002
Guidelines for constructing and maintaining aquaculture containment structures, The State of Queensland
(Dept of Primary Industries and Fisheries), 2007




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