JCS
τ = σ′ tan(ϕr + JRC × log( ′ )
σ
Where: JRC = joint roughness coefficient
JCS = compressive strength of rock (2.5MPa)
σ' = effective normal stress
φr = residual friction angle
Typical residual friction angles published in the literature of 30 degrees for siltstone and 32 degrees for
sandstone may be adopted.
These parameters assume that jointing is clean and tight. Where the joint thickness is found to be
larger, or where infill materials exist, lower defect properties will be appropriate.
7.9
Rock Mass Rating (RMR) Classification
The rock mass rating (RMR89) system (Bieniawski, 1989) is used to specify stand-up times and rock
support requirements. The RMR classification scheme has been traditionally developed for relatively
hard rock mass penetrated by discontinuities with known spacing, persistence and surface condition
characteristics that tend to decrease the cohesion and frictional properties of the intact rock (Carvalho,
et al., 2007). Refer to Appendix E, Section E11.
The application of RMR (and other rock classification schemes such as GSI) have been called into
question for low strength rock materials near the soil interface such as weakly cemented (typical
strength, questionable application) and uncemented ECBF (highly questionable application).
RMR values have been assigned for EU2 and EU3 materials. The following observations were
considered when assigning RMR values:
a) The fracture spacing pattern (refer to Section 7.5.3) is ‘clustered’. Fracture spacing is typically
widely spaced with local zones (commonly associated with faulting) where fracture spacing is
closely spaced. Several drillholes were noted to have higher than typical fracture density. For this
reason, a separate RMR value assignment is required for a) typical rock mass conditions; and b)
local closely spaced rock mass. It is recommended that the observational method is adopted
during construction to map the tunnel excavation ground conditions and additional support
installed if geological features are identified. In addition to jointing, EU2 is characterised by a subhorizontal, decimetre-spaced bedding fabric which is inherently persistent and pervasive
throughout the EU2 rock mass. Discrete bedding planes (in contrast to gradational contacts) are
commonly low tensional strength. Rare bedding partings lacking any tensional strength are also
present which increase the risk of roof slabbing failure. Low tensional strength sub-horizontal
bedding and bedding plane partings may have higher support requirements, to prevent release of
wedges on bedding surfaces in the tunnel crown. In contrast, bedding is ‘semi-confined’ in the
tunnel walls and does not form a releasing surface for over-break. For this reason, a separate
RMR value assignment is required for a) EU2wall; and b) EU2roof.
b) EU3 is typically massive and does not have a well-developed bedding fabric. However, like EU2,
the jointing pattern is typically clustered with local zones (normally in association with faulting)
where fractures are closely to very closely spaced.
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c) Although considered unlikely, landslides do occur in ECBF along low angle clay-rick claystone
beds within the ECBF and along the interface of sandstone-siltstone layers within weathered
ECBF. Where bedding is gently inclined out of the tunnel wall (unfavourable) and claystone layers
are present the potential for ‘mega-wedge’ release on cross-joints perpendicular to bedding is a
possibility. Both moist fissured and wet thin claystone beds were encountered in some Stage 2
drillholes although these beds were not common Observations on the ground conditions during
construction should consider the potential for clay layers and wedges.
Based on the above observations the several RMR classes have been defined to accommodate the
variety of expected rock mass conditions – refer to Table 7-7 below:
Table 7-7: RMR classes
Class
Application
EUs1
Do not use RMR for EUs1 – refer to Section 7.9.1 below
EU2typ
General: typical widely spaced fracture spacing
EU2local
EU2roof
Local: closely to very close fracture spacing (often associated with faulting)
EU2roof: Bedding is typically decimetre spaced which is similar in density to the EU2local
fracture spacing. RMR for EU2 roof and EU2local will be approximately the same although
bedding is pervasive and persistent.
Therefore use EU2local as proxy for EU2roof
EU3typ
General: typical widely spaced fracture spacing
EU3local
Local: closely to very close fracture spacing (often associated with faulting)
Rock mass ratings assigned for each RMR class are presented in Table 7-8 and Table 7-9 below
using the 1989 version of the RMR classification scheme (RMR89). The methodology for calculating
RMR89 is summarised in Section E11 in Appendix E.
Table 7-8: RMR89 Typical case for EU2 and EU3
RMR input parameter
EU2typ
EU3typ
Comments
Intact rock strength
1
2
EU2 1-5 MPa
EU3 5-25 MPa
Spacing*
32
30
EU2: use ‘EU2* composite’ fracture spacing of 2.5 fractures/m –
refer to Table 7-2 above
EU2: use ‘EU2* composite’ fracture spacing of 2.5 fractures/m –
refer to Table 7-2 above. Drop 2 points to account for undersampling of common sub-vertical fracture set
JCond89
16
20
JCond89 parameters taken from Table 7-4 above
Groundwater
5
5
Dripping
Joint orientation adjustment
-5
-5
‘Dip 0-20 – Irrespective of strike’ = Fair
RMR89
49
52
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Table 7-9: RMR89 for local cases: EU2local, EU3local and EU2roof
Parameter
EU2local
EU2roof
EU3local
Comments
Intact rock strength
1
2
EU2 1-5 MPa
EU3 5-25 MPa
Spacing*
27
27
EU2 and EU3: ‘EU2/EU3 local closer spacing’ fracture spacing
of 5 fractures/m – refer to Table 7-2 above
JCond89
16
Water adjustment
5
Joint orientation adjustment
-5
RMR89
44
20
5
JCond89 parameters taken from above Table 7-2 above.
0
Dip 0-20 – Irrespective of strike’ = Fair
-5
49
EU2 and EU3: Dripping
EU3 special case: Water inflow
44*
*Special case water inflow
7.9.1
Uncemented Sand (EUs1)
Applying a rock mass classification to uncemented sand (EUs1) is controversial. The primary question
is: should EUs1 be treated as a soil mass or rock mass owing to its extremely low intact ‘rock’ strength
(IRS)?
The RMR system takes into account low intact rock strength and accommodates IRS < 1 MPa
therefore it can be argued an RMR can be assigned to EUs1.
On this basis an argument exists for ‘un-jointed’ EUs1 mass to be assigned an RMR of 85 (completely
dry) to RMR 70 (flowing groundwater) depending in groundwater conditions and primary permeability
of the EUs1 material. Prior construction experience indicates ‘dripping’ is the likely groundwater
scenario. However, running sands are sometimes encountered which suggests locally, flowing water
could also be possible.
This RMR range would result in classification as ‘good rock’ (RMR 61-80) with an indicative (RMR
assigned) stand-up time of 1 year for a 10m span which is a very risky and probably unrealistic
proposition – depending upon ground water conditions.
Although the RMR system supports assignment of an RMR for EUs1, it is Aurecon’s view that
corresponding estimates of stand-up time, and excavation and support requirements determined using
RMR as an input parameter assume non-soil conditions (i.e. assume jointed rock mass and possibly
hard-rock conditions). Moreover, the RMR recommended support requirements does not take into
account additional support requirements to counteract squeezing/deformation inherent in low modulus
materials at the soil/rock interface.
Case studies of tunnel failures constructed in uncemented sand (for example, Kaohsiung Mass Rapid
Transit system in Taiwan) suggest design using additional rock strengthening techniques proven to
support excavations in uncemented sand should be investigated.
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7.10
Geological Strength Index (GSI)
7.10.1
Design values
Despite limitations discussed below in Section 7.10.2, GSI values have been determined using several
methodologies as summarised in Table 7-10 below.
As a sanity check Hoek et al. (2005) suggest tectonic undisturbed decimetre bedded sedimentary
rocks (alternating sandstone and siltstone) would generally be located in the area indicated ‘M1’ on
Figure 7-8 which is consistent with the GSI values determined in Table 7-10 for EU2 ECBF. The area
‘M2’ on Figure 7-8 indicates typical GSI range for rock material that is brecciated in fault zones and
could be considered a lower-bound case for ECBF.
Hoek et al. (2004) also note that reaches of ECBF rock lacking or only having few discontinuities can
be treated as intact with engineering parameters given by direct laboratory testing – this case is
considered directly applicable for EUs1 uncemented sandstone.
Table 7-10: Geological Strength Index determinations
Method
EU2
EU3
Comments
Using RMR parameters:
Hoek et al. (2013)
1.5 × 𝐽𝐽𝐽𝐽𝐽𝐽𝐽𝐽𝐽𝐽89 + 𝑅𝑅𝑅𝑅𝑅𝑅 ⁄2
JCond89 = 16
RQD81CORE = 90
JCond89 = 20
RQD81CORE = 90
Refer to graphical representation
on Figure 7-8
Typical: GSI = 69
Typical: GSI = 75
RQD81CORE = 90
Local: GSI = 60
RQD81CORE = 80
Local: GSI = 70
Jr = 2
Ja = 3
RQD81CORE = 90
Jr = 3
Ja = 2
RQD81CORE = 90
GSI = 66
GSI = 76
IRS = 1
Spacing = 32
JCond89 = 16
Groundwater = 5
Orientation Adj = -5
IRS = 2
Spacing = 30
JCond89 = 20
Groundwater = 5
Orientation Adj = -5
1+32+16+(15-5)
= 59
2+30+20+(15-5)
= 62
Range: 56-87
Average: 71
Range: 56-87
Average: 71
Using Q parameters:
Hoek et al. (2013)
52 × 𝐽𝐽𝑟𝑟 ⁄𝐽𝐽𝑎𝑎
𝑅𝑅𝑅𝑅𝑅𝑅
+
(1 + 𝐽𝐽𝑟𝑟 ⁄𝐽𝐽𝑎𝑎 )
2
Estimate from RMR:
Hoek et al. (2013)
𝑅𝑅𝑅𝑅𝑅𝑅′89 − 5
Indicative GSI ranges for tectonically
undisturbed sedimentary rock
masses
(Hoek et al., 2005)
a) Substitute RMR89 water
adjustment for +15 points
b) Ignore adjustment for fracture
orientation
Used as a sanity check
Suggest use GSI 75 since
EU3 lacks bedding fabric
*For EUs1, use GSI=100
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Figure 7-9: GSI for tectonically undisturbed but lithologically varied sedimentary rock masses (Hoek, et al., 2005)
7.10.2
Application
GSI was developed as a hard rock system based upon a qualitative assessment of lithology, structure
and condition of discontinuity surfaces. It is roughly equivalent to RMR but unlike RMR or Q it is not
dependent on RQD and has no rock mass reinforcement or support design capability – its only
function is the estimation of rock mass properties especially for weak, tectonically disturbed and
heterogeneous rock masses (Marinos et al., 2005).
The original GSI classification system was based on the assumption that the rock mass contains a
sufficient number of ‘randomly’ oriented discontinuities such that it behaves as an isotropic mass and
therefore could not be applied to a rock mass such as the ECBF which clearly defined dominant
bedding fabric. In 2005, this assumption was relaxed when the GSI system was extended to
encompass tectonically undisturbed but lithologically varied sedimentary rock masses such as
decimetre interbedded ECBF (Hoek et al., 2005).
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