25289-001-Rev0 - The Esplanade (1)-compressed

Table 4: Soil Parameters Adopted for Slope/W Analyses
             Material Description            Unit Weight (kN/m3)             Cohesion, c (kPa)       Friction Angle, ɸ (degrees)

         Very Loose/Loose Sands                       17                              0                          27

             Medium Dense Sands                       18                              0                          34

         Dense (or denser) Sand                       19                              0                          36

* Depends on type of fill used and compaction achieved

Commonly adopted industry standards generally require minimum FOS values against slope instability of
1.3 and 1.5 for short-term and long-term conditions, respectively. Administering authorities such as NC
normally require a minimum FOS of 1.5 or greater for long-term conditions. Results of the stability
analyses are presented in Table 5, with the Slope/W outputs shown in Text Figures 9 & 10, overleaf.

Table 5: Slope/W Analysis Results
    Analysis           Description                                                                        Factor of Safety

    Case 1             Unstable, over-steep dune face following potential future erosion cut after
                                                                                                                0.5
                       significant storm event

    Case 2             Slope stability analysis following regression of the slope from Case 1 to a
                       long-term stable profile of 27°, with the eastern property boundary                      1.5
                       protected by a shear pile wall



Results of the analyses indicates the following:
      Case 1 – It was noted that the modelled storm erosion cut would not initially affect the site.
       However, the dune face following the storm erosion cut would have an unstable FOS of 0.5, due to
       the over-steep nature of this profile.
      Case 2 – Following regression of the dune from the future erosion cut to a long-term stable profile
       of 27°, if the eastern property boundary is protected by an engineer designed and certified shear pile
       structure the potential critical slip surface takes the form of a deep-seated failure into the underlying
       dense (or denser) sands. Such a slip surface results in a FOS of 1.5, allowing for retrogression of the
       slope from Cases 1. The pile wall structure would be significant, with deep piles (at least 12 m BGL)
       and a concrete capping beam, possibly in combination with lateral restraint due to the pile ‘upstand’
       of about 5 m.




Geotechnical investigation
19 November 2025 | 25289-001-Rev0                                                                                              14