Using Geological Data to Understand Distribution of Rock Mass Properties

SRK News | Issue 54: Rock Engineering and Slope Stability

   
    
A4   |   Letter

This case study details how geotechnical properties were linked to geology alteration types. This allowed SRK to use the geology database to determine spatial distribution of zones of very weak and extremely weak rock.

Channel iron deposits (CID) are Tertiary aged iron-rich fluvial sedimentary deposits occupying meandering palaeochannels in Western Australia. The deposits consist of weakly cemented goethite-hematite pisolite and are an important source of iron ore. The channels are incised into basement lithologies and are typically 450- 750m wide and up to 100m deep. The palaeochannel deposits are many tens of kilometres in length. The banks of the palaeochannel typically dip between 15°- 25° but locally can dip up to 35°.

The deposit is divided into upper and lower units. The Upper CID is unaltered and shows a prominent pisolite texture. This unit shows uniform geotechnical conditions: medium strong and widely jointed. In the lower region of the channel the rock has undergone variable alteration as a result of post depositional groundwater movement. The Lower CID unit transitions from moderately strong rock showing pisolitic texture to very weak rock with zones of friable, soilstrength ochre with increasing alteration. Clay pods are also common within the Lower CID unit. Logged strength is supported by laboratory testing values, showing strength varies from noncohesive soil through extremely weak to weak rock. Between 20-30% of the rockmass was classified as very weak or less. Thus, whereas the Upper CID unit is a uniform material showing similar rock strength and jointing throughout the deposit, the Lower CID unit is weak and highly variable.

Geotechnical logging and laboratory testing were used to geotechnically characterise the Lower CID material, but the geotechnical drillholes were too widely spaced to meaningfully interpret the distribution of the weak zones.

Geotechnical properties were compared with logged geological properties, including alteration type, alteration intensity, geological hardness, colour, weathering and major mineral components. SRK then established a strong relationship between alteration type and intensity and the geotechnical characteristics of the Lower CID unit.

SRK used the greater geological database to interpret trends across the deposit, emphasising locality and extent of both clay pods and the highly altered Lower CID. By linking geotechnical characteristics to logged geological properties, SRK was able to use the ~30,000m geological database in preference to the ~600m geotechnical database to understand spatial distribution of weak and extremely weak rock, and adapt the pit design accordingly.

Diane Walker: dwalker@srk.com.au

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