TECHNICAL | MECHANISED TUNNELLING


Table 1: Grades of weathering Grade


Term VI V IV III II I Residual and colluvial soils Completely weathered rock Highly weathered rock


description


All rock converted to soil. Original rock structure completely destroyed. Point of geological pick indents easily and deeply, and without sound when struck by hammer.


All rock discoloured and converted to soil but original rock mass structure still visible. Geological pick indents easily, and dull sound with hammer.


All rock discoloured. Original rock mass structure present, largely intact. Geological pick does not indent easily, and dull sound with hammer.


Moderately weathered rock Rock is mostly discoloured with local original colour. Original rock mass structure is well preserved. Geological pick scratches surface, and intermediate sound with hammer.


Slightly weathered rock Fresh rock Source: Ref: Dearman, 1976; ISRM 1977


Discolouring only near joints. Original rock mass structure is fully preserved. Geological pick scratches surface with difficulty, and ringing sound on hammer strike.


Rock has its original aspects and no discolouration. Geological pick scratches surface with high difficulty, and ringing sound with hammer.


Table 2: Rock Classification


ISRM grade


R0 R1 R2 R3 R4 R5 R6


description


Extremely weak rock Very weak rock Weak rock


Medium strong rock Strong rock


Very strong rock Extremely strong rock


Table 3: Cerchar Abrasivity Index (CAI) CAI


0.3 – 0.5 0.5 – 1.0 1.0 – 2.0 2.0 – 4.0 4.0 – 6.0 6.0 – 7.0


compressive strength (MPa) 0.25 – 1.0


1.0 – 5.0 5.0 – 25 25 – 50


50 – 100 100 – 250 >250


Approx range of axial


(MPa), by point load testing Ambiguous results


Ambiguous results Ambiguous results 1 – 2 2 – 4


4 – 10 >10


Cerchar (1986) (pin hardness 54) Not very abrasive Slightly abrasive Abrasive


Very abrasive


Extremely abrasive -


and measure the disintegration behaviour of rocks, which helps in the layout design of slurry circuit and sizing the slurry treatment plant (STP). Some rock types tend to swell in combination with


strongly influence the design and application


of MTBMs. They are key factors for machine type recommendation, cutterhead design and cutter selection, and so are also necessary to help predict performance of the tunnelling operation. The deformation modulus or the modulus of elasticity


(E-Modulus, or Young’s modulus) is of less interest for the appropriate selection and design of a MTBM. Rock abrasivity can be characterised by: the Cerchar


abrasivity index (CAI); the drilling rate index (DRI) / cutter life index (CLI) / bit wear index (BWI); and, equivalent quartz content (Equ). The former is industry standard and considered easy to obtain. But for the second group – DRI / CLI / BWI – only a few labs can measure those, making them less common for use in design and application of small-diameter tunnels. When rock at a project site is mostly sedimentary,


another interesting geotechnical property is slaking. A few tests (L.A. Abrasion apparatus value (LAV), slake durability index test, and LCPC abrasivity test) describe


24 | January 2024


water. This geological feature is important to measure and understand, for example for the design of the overcut and selection of the right bentonite mixture, respectively. Quantification tests can include: pressure swelling and swelling displacement tests (HUDER & AMBERG, HENKE & KAISER, PAUL (DGEG), IRSM, THURO & SPAUN). With such reliance in construction on these important


geotechnical parameters, the site investigation campaign with boreholes must be both sufficient and well planned. The locations of rock/ground sampling must be carefully planned against the proposed tunnel alignment, and there be enough performed, and at a suitable frequency, to support generation of quality data for the overall planning. Investigations should also include photos of the boreholes and note of the detailed geotechnical profile, thereby allowing the geological and geotechnical information to be inter-connected between the sampling points. Additional to evaluating the geotechnical values of


samples, the whole rock mass can be characterised – or classified – from a geotechnical perspective by adding information about the fracturing of the body, and information on groundwater presence. Rock mass classification is the assessment of


geological conditions for construction engineering – how the geomechanical behaviour of the whole could


Point Load Index


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