Slope Stability Report 26-5-2019 OPW19-0193

TRUBUILT
SLOPE RISK ASSESSMENT REPORT



3.4.3      Creep Flow

Based on a review of the surrounding slopes it appears that the soil in the gullies is prone to creep failure.
Generally, this type of failure occurs in thick soil on steep slopes and when the material is saturated. Creep
failures may undergo slow movement with the development of tension cracking and hummocky ground for
several years. Eventually, the failure will reach a critical point where the rate of movement will increase
resulting a large scale rotational or translational failure.

Prevention of creep flow type failures is considerably easier than trying to remediate them once they have
started to move. The following will reduce the risk of the development of creep failures:

      •   Place the wastewater irrigation area and stormwater discharge on a ridge or within the upper half of
          the slope to the gully;
      •   Generally, avoid slopes with a gradient of >45%;
      •   Ensure that the collection and disposal of stormwater flows is designed using the recommendations
          contained in section 4.5;
      •   Ensure that stormwater and tank overflows are discharged via a dissipater structure that will slow and
          spread the flow. Avoid concentrated or point discharges onto the slope;
      •   Ensure that areas down slope of wastewater irrigation and stormwater discharge are vegetated using
          plants with fine fibrous roots and that encourage the growth of ground cover;
      •   Ensure that the lower 25% of the slope into the thalweg of the gully has groundcover vegetation with
          fine roots and trees.

The mitigation method will reduce the likelihood of the event to UNLIKELY and will reduce the consequence
of the event to MINOR, meaning that the assessed risk will be LOW.


3.5        Geotechnical Constraints to Development

Geotechnical conditions on the site mean that the following constraints will apply to the proposed
development to prevent or limit the likelihood of the development resulting in the development of new or re-
activation of existing instability on the site. These constraints comprise:

      •   Placement of wastewater irrigation area – wastewater irrigation area not to be installed on a slope
          with a gradient >45% or less than 50% of the way down the slope between the thalweg and ridge line;
      •   Placement of stormwater discharge – stormwater discharge point not to be installed on a slope with a
          gradient >45% or less than 50% of the way down the slope between the thalweg and ridge line.




LEONTAKIANAKOS-R-001                                Page 11                                            26 May 2019