11. Is there three dimensional limit equilibrium modelling? [LV-P]

Refer to report: 25^th February 2020 – Godre’r Graig Primary School – Preliminary Investigation and Additional Assessment 7234e.02.3302

Based on conditions at the time of the assessment, intrusive investigation could only be undertaken in areas of cleared vegetation and where it was safe to do so. Geophysical survey was implemented to supplement the Ground Model.

2D limit equilibrium slope stability modelling was undertaken utilising GeoStudio SlopeW to assess the Adequacy Factor (AF)/Factor of Safety (FoS) within the slope. The FoS approach was adopted as it considers the ratio of disturbing forces against restoring forces and gives a simple indication to stability, i.e., stable, or marginally stable.

The slope profile adopted was that of the tip above the school; the modelled section has been drawn perpendicular to contours where nearby borehole information has been obtained, supplemented by the geophysical data. The slope profile has been surveyed and visual observations and slope measurements made during the geomorphological mapping were checked against what was considered the steepest section.

In order to understand the tip stability in modern day design situations/sensitivity, the model was run adopting partial factors for Design Approach (DA) 1, Combination 1 (C1) and Combination 2 (C2); this suggested inadequate FoS.

Refer to report: 22nd September 2021 – Godre’r Graig Tip Remediation Options Report – 7234e.04.3564

Monitoring shows downward movement of the Quarry Spoil Tip (towards the school). Our previous assessment suggested that the Quarry Spoil Tip was Marginally Stable, i.e., that it was likely to fail at some time in response to destabilising forced reaching a certain level of activity. The information from the inclinometers suggest that the Quarry Spoil Tip is moving and is Actively Unstable, i.e., destabilising forces are producing continuous or intermittent movements.