1. Geotechnical Validation for Sydney Tunnels Tim Nash, Bernard Shen, Rob Bertuzzi
16th Australasian Tunnelling Conference
30 October - 1 November 2017, Sydney
Pells Sullivan Meynink

2. Objective
Application of Sydney Classification System (Pells et. al., 98) to rock masses in mined tunnels
Practical challenges and suggested improvements/alternatives
Endoscopes - pros and cons

4. Routinely Determining UCS
OPTIONS
CHALLENGES
1. Coring and testing •
Timeframe
Equipment
in tunnel
Coring
correct interval/orientation
Expensive
2. Point load testing •
Sample size and number In
situ
position
Relies on good UCS correlation
Lacks
repeatability
3. Field guide to strength
(normally adopted) •
No access to tunnel face/crown/sidewall
Small ranges of UCS in classification
Sample size/insitu position

5. UCS – Improvements Field estimation
Specify field est. strength intervals rather than specific UCS values
adequate SI to substantiate field est. strengths
Only at critical locations - specify min. UCS values that require testing, not for routine verification

7. Defect Spacing
Challenge 1: Distinguishing between a true defect and rock fabric
“a discontinuity, fracture, break or void in the material across which there is little or no tensile strength.”

8. Groundwater seepage/Iron staining
Infill
Overbreak or delamination back to a plane
Shearing observed in an endoscope

9. Defect Spacing Challenge 2: Scale
Consider 6 m high tunnel face with 3 bedding planes, 2 being 0.5 m apart. The defect spacing could be reported as 0.5 m (min) or as 2 m (average over defined interval).
Addressed by Bertuzzi and Pells (2002)
“the classification system be applied to portions or units of rock mass having similar UCS, defect spacing and seam characteristics”

10. Defect Spacing Suggested solutions
Specify intervals over which class should be assessed (assumes portions or units of rock mass have similar UCS, defect spacing and seam characteristics)
Specify min defect spacing if important to design
Establishing common approach for all mappers at beginning of construction (training)

11. Allowable Seams Definition
“…clay, fragmented, highly weathered or similar zones”
Challenge: Access - permissible only after ground is fully supported, almost always includes shotcrete
At best, the character and infill thickness is based on remotely observed behaviour

13. Allowable Seams Suggested solutions
Seams specifically defined in design to be practically verifiable and confidently assessed at (near) the face.
Example for endo: “allow any subhorizontal defect infilled with clay or any fragmented/highly weathered rock that is between 25 mm and 200 mm thick”.

14. Endoscopes Holes drilled in crown
Commonly with rock bolting rig, top hammer percussion, mm diameter
Challenge: Percussion drilling can inadvertently damage the borehole - rifling, overbreak and fracturing of the rock mass

15. Endoscope inspections

17. Endoscope Inspections
Endoscope inspections becoming more common for alleged validation of
Defect type and characteristics
Rock mass class
Geological structural models
Lithological changes
Hole closure caused by shearing along defects
Validation of many of these aspects by endoscope is virtually an impossible task.

20. Endoscopes Suggested improvements
Non percussive/rotary drilling techniques, hole diam. ≥ 65 mm and well flushed
Avoid incorporating the identification of infill thicknesses <25 mm
Consider endos as informative method to assess lithology and shearing/hole closure

21. Concluding Remarks Incumbent on tunnel designers to
consider the inherent difficulties and accuracy in field observations
align with common, pragmatic solutions for validating geotechnical / geological attributes
Outline how each design component will be validated at start of construction (collaboration).