Appendix B - Geotechnical Assessment Report

Mr Tony Rossiter                                                                              21436-001-Rev0
                                                                                            18 November 2021

4.0     RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.

4.1        Dam Embankment Construction
No specific design details were available at the time of reporting however it is anticipated that embankments
for the dam construction will be in the order of 1 m to 2 m deep/high. If earthworks in excess of these
heights are proposed, Tectonic must review the design details and reassess slope stability prior to
construction. It is recommended that a suitably qualified engineer be consulted regarding the design of the
dam so that hydraulic and civil design matters are appropriately resolved.

4.1.1 Suitability of Existing Soils for Re-use as Dam Fill
Appendices 2 and 3 from the Guidelines for constructing and maintaining aquaculture containment structures
(2007) provide recommendations for the suitability of various soil types for re-use as structural fill for dam
embankment construction (in relation to small dams for aquaculture ponds).
The soil types encountered by the GD investigation in this area comprised mostly sandy clays with some
gravel content that were noted to have low dispersion potential. Such materials are expected be generally
suitable for use as structural fill for dam construction.

4.1.2 Site Preparation
After stripping and excavation to the design levels, the subgrade is expected to comprise mostly stiff sandy
clays, that may become firm (or softer) if high moisture content develops.
Following stripping, for the dam embankment footprint, ‘proof rolling’ using a fully laden water cart or
minimum 10 tonne drum roller should be carried out in order to identify any ‘soft spots’. Treatment of ‘soft
spots’ may involve excavation and replacement with select fill or tyning, appropriate moisture adjustment and
compaction. Proof rolling should be carried out in the presence of an experienced engineer.
Reactivity of the clay at subgrade level which will form the lining of the dam should be considered, as the
clay may develop cracking when dried out, which could lead to leakage. Should such leakage not be
considered to be an issue for the dam being constructed on this site, no further action would be required (in
dealing with the clay). However, if leakage is an issue, the clay should be covered with a 200 mm to
400 mm thick layer of inert/non-reactive material (e.g. gravel, sand or clayey sand), or be mixed with suitable
additives, such as gypsum at a rate of about 2.5 t/ha (this should be confirmed by agronomy assessment), to
reduce the potential shrinkage.
Use of a geosynthetic liner (e.g. HDPE) could also be considered to aid in reducing infiltration losses.
A temporary surface diversion drain should be installed upslope of the proposed dam site to shed surface
water runoff away from the construction area.

4.1.3 Filling
Fill for the dam embankment should be compacted at moisture contents within the range of +1 % to +2 % of
optimum moisture content for Standard Compaction. Fill should be placed in maximum 150 mm to 200 mm
thick (loose thickness) layers and be compacted by at least four to six passes with an pad foot compactor (or
similar) to achieve a dry density ratio of at least 95 % of the maximum Dry Density Ratio (Standard
Compaction method).
Details for inspection and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on
Earthworks for Commercial and Residential Developments. Confirmatory compaction testing must be
carried out at regular intervals. Filling should be carried out under Level 2 geotechnical testing. It is
recommended that the frequency of fill density testing carried out should be in accordance with Type 3 per
Table 8.1 of AS3798.
The above requirements should also be applied when constructing the key-in trench beneath the dam.




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