Attachment 5 - Geotechnical Assessment · MCU18/0015

RTI 11/055 Folio B391

 

 

 

November 2003 -7-

03632248-001

 

The modelling results are given in Appendix C and are summarised in Table 3 below.

Table 3 —- Summary of Stability Analyses

 

 
   

 

 

 

 

 

 

Subsurface profile for Section A-A (east of creek — clay side) with a 0.8 1
piezometric ground water surface placed at interface between loose to ft
medium dense and dense silty sand. Existing subsurface conditions (east)
interpreted from test pit TP10.

Subsurface profile as for Run#1 (east), but with a piezometric ground water 1.9 2
surface placed at the lower interface of dense silty sand. (cast)
Subsurface profile for Section A-A (west of creek — sand dune side) with a 1.4 1
piezometric ground water surface placed at lower interface of very loose (west)
sand. Existing subsurface conditions interpreted from test pit TP6 and

borehole BH7A.

Subsurface profile as for Run#1 (west), but with a piezometrieground water 0.9 21
surface placed about 0.7 m above lower interface of very loose sand. (west)
Fully specified slip surface with a piezometric ground water surface placed 1.3 3
about 0.7 m above lower interface of very loose sand. (west)

 

 

 

 

The results indicate that the calculated facto of safety for the section east of the creek is
greater than 1.5, provided that the *groundwater/level is maintained at or below the lower
interface of the dense silty sand (inferred colluvium). Experience indicates that a factor of

safety of 1.5 corresponds to,a normally accepted level of risk.

The factor of safety reduces to/léss‘than one if the groundwater level rises within the colluvial
silty sand, resulting in the/poteiitial for slope instability on the creek banks (ie. where the

ground slope exceeds about,15° to 20°).

Stability modelling for the section west of the creek (ie. on the sand dune side) returned a
factor of safety of 1.4, being slightly less than the normally acceptable level for long-term

stability.

As mentioned in Section 2, and as shown in Figure 1, there is a possible landslip located
adjacent to the western side of the creek in the northernmost part of the site. The ground
surface in this area slopes at about 40° towards the east. This possible landslip appears to be
relatively shallow (about 1 m deep) and confined to the loose dune sand. At the toe of the
slip, in the western bank of the creek, colluvium comprising very stiff to hard clay and diorite
cobbles is exposed beneath about 1 m depth of sand. The toe of the slip is located on the
outside of a sharp bend in the creek. When water flows in the creek, it is possible that the toe
of the slip is exposed to high velocity flows, which may scour and undercut the slope, thereby

Golder Associates