Slope Stability Report 26-5-2019 OPW19-0193

TRUBUILT
SLOPE RISK ASSESSMENT REPORT



3.4.1      Wedg e Failure

Wedge failure occurs when a block of rock falls from a batter slope due to two or more intersecting planes of
weakness daylighting on the slope face. The Kin Kin Beds have distinct foliation that results in the excavated
material parting easily along the foliation planes and there are known tectonically formed joint planes. Note
that this style of failure may not be able to be stopped, the risk will increase with time as this material will
degrade in the cut with time, but the likelihood and impact can be reduced through the following means:

     •    The material is to be cut no steeper than 1(V):1(H), if the foliation is parallel or sub parallel to the face
          and at a shallower angle than 45°, then the slope should be cut back to the foliation angle;
     •    Care should be taken to ensure that water in not allowed to flow over the batter slope and that water
          is not allowed to pond on the slope above the cut;
     •    Periodic inspection and removal of wedges as they start to occur is prudent;
     •    It may also be prudent to cover the slope with a non-woven geotextile (MRTS27 Class B or greater) or
          a semi-rigid finish like Concrete Canvas. It is not recommended to cover the slope with finishes that
          would prevent water draining from the slope (ie grout or sprayed concrete).

The mitigation method will reduce the consequence of the event to INSIGNIFICANT, meaning that the
assessed risk will be VERY LOW.


3.4.2      Shallow Sl umpi ng

The shallow slumping is more likely with fill materials from crushed phyllite and containing sericite due to the
particle shapes causing issues with compaction resulting in lower strength soil. The following mitigation
methods will overcome the likelihood of shallow slumping from occurring:

     •    The water tanks are to be constructed on cut platforms with rock material across the entire base. The
          tanks should be bedded onto a free draining sand layer and the overflows connected to the
          stormwater management system;
     •    Where fill will be placed, to soil must be stripped and the slope cut into a series of 0.3m high steps to
          key the fill into the slope. Requirements contained in sections 4.4 and 4.4for compaction and over
          construction are mandatory for fill materials on this site. It is recommended that fill materials be
          compacted using a vibrating plate compactor with a minimum mass of 250kg in loose layers not
          exceeding 200mm thick;
     •    The top of fill platforms should be graded so that they are free draining and water should not be
          allowed to pond on the fill platforms;
     •    Fill batter slopes shall be protected from turbulent or concentrated water flow.

Note that due to the natural grade of the slope, the fill batter slopes will need to be finished about 1(V):1.5(H)
to avoid fills with high apparent heights.

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



LEONTAKIANAKOS-R-001                                  Page 10                                               26 May 2019