Geotechnical Investigation Report · OPW26/0149

                                                                                               PTG/04982 – 0001 – Rev0 – Rev0



Whilst groundwater was not encountered in our investigation, we have modelled an assumed phreatic surface at
approximately 7 m depth. Additional analyses have also been undertaken to model a hypothetical, elevated
groundwater condition to simulate a potential rise in groundwater following a period of prolonged, intense rainfall. For
the elevated groundwater models, the phreatic surface is modelled at a depth of approximately 3 m.
Further to the above, we have undertaken two further scenarios of the proposed profile: soon after construction, before
the pond is filled, and shortly after construction as soon as the pond has been filled.
The geotechnical parameters for the modelled units have been interpolated from the borehole logs and laboratory test
results. Geotechnical parameters for the proposed fill material have been assumed. All geotechnical parameters used
in our analyses are summarised in Table 6 below.
Table 6: Geotechnical parameters utilised in analysis.

                                                                                                        Undrained
                                                                        Drained Parameters
                                                   Unit Weight                                          Parameters
                 Material
                                                    γ (kN/m³)
                                                                    Cohesion          Friction        Shear Strength
                                                                     c’ (kPa)         Ø’ (deg)           su (kPa)

   1 – St – Vst Silty CLAY (Natural)                     18             5                23                 20

      2 – H Silty CLAY (Natural)                         18             6                25                 60

      3 – Vst – H Silty CLAY (Fill)                      18             5                20                 50


Drained parameters were adopted for long-term stability assessments, while undrained parameters were utilised for
the short-term stability cases, i.e., shortly after the dam has been constructed. The undrained parameters were
selected as conservative lower-bound values to help bound the short-term risk in the loading scenarios analysed.
The analyses were undertaken using the Slide2 software by RocScience and were conducted using the Spencer
method for interslice forces. Analyses were undertaken for both circular and non-circular failure surfaces, using the
Auto Refine and Cuckoo search methods respectively. Additionally, the non-circular analyses were optimised to refine
the returned Factor of Safety (FoS) value within the search areas defined.
For the assumed groundwater profiles, a minimum Factor of Safety (FoS) of 1.5 was targeted for long term stability.
For the hypothetical, elevated groundwater scenarios, a minimum FoS of 1.3 was targeted. The results of the global
stability assessment are summarised in Table 7, with outputs of the analyses presented in Appendix 3.


Table 7: Summary of Global Stability Analysis.

                                                                                      Circular         Non-Circular
        Scenario                Pond Level           Groundwater   Parameter Set
                                                                                      Surface            Surface

     Existing Profile                n/a                 Assumed      Drained            2.9                2.9

     Existing Profile                n/a                  Raised      Drained            2.3                2.3

    Proposed Profile                 Full                Assumed      Drained            1.8                1.8

    Proposed Profile                 Full                 Raised      Drained            1.7                1.7

    Proposed Profile               Empty                 Assumed    Undrained            3.3                2.9

    Proposed Profile               Empty                  Raised    Undrained            3.3                2.9

    Proposed Profile                 Full                Assumed    Undrained            3.1                2.6

    Proposed Profile                 Full                 Raised    Undrained            3.1                2.6


Based on the geometry and geotechnical parameters adopted, the analyses indicate that the proposed dam and
associated pond is expected to remain stable through all loading scenarios analysed.



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