Abrasivity and Cutter Life Assessment for TBM Tunneling in Cobbles and Boulders
Steven W. Hunt  Black & Veatch
Glen R. Frank  Lane Construction
ABSTRACT
Assessment of abrasivity and cutter life are necessary to properly manage risks when tunneling in mixed face ground
which includes ground with cobbles and boulders. Maximizing cutter life requires: 1) abrasivity and strength
parameters for soil matrix and rock clasts, 2) methods of combining both, 3) proper selection of cutters and 4)
modified TBM operation including use of boulder detection methods from the TBM to help make operation
decisions. This paper presents approaches for addressing these factors sufficiently to improve tunneling success in
mixed-face and cobbly-bouldery ground. Current laboratory abrasivity testing will be assessed along with equivalent
quartz content correlations to soil matrix and rock clast abrasivity. New approaches to combining abrasivity testing
of both the soil matrix and rock clasts for various cobble and boulder volume ratios will be suggested to estimate
cutter life and intervention intervals. Methods for detecting boulders and using detection data to modify TBM
operations to minimize boulder impacts are provided.
INTRODUCTION
Tunneling in cobbles and boulders or mixed-face ground involves many risks including obstruction, steering,
reduced advance rate and excessive wear to: cutters, cutterhead and the TBM mucking system. The impact on
tunneling varies from negligible to severe depending on the tunnel diameter, tunneling method, ground matrix
conditions and extent and characteristics of the cobbles and boulders or mixed-face ground. Assessment of risks and
selection of mitigation measures require thorough consideration of ground conditions, alignment environment and
tunneling methods.
Assessment of risk level requires adequate ground condition information resulting from a customized subsurface
investigation program that provides adequate data on: geologic units; cobble and boulder volume ratios; sizes,
shapes and distribution of rock clasts (cobbles, boulders or mixed-face rock layers); rock properties including
unconfined compressive strength, quartz content and rock abrasivity; and soil matrix properties including cohesion,
strength, density, grain size distribution, permeability and soil abrasivity.
The total abrasivity of tunnel zone ground results from the combined effects of the soil matrix, gravel, cobbles,
boulders and any mixed-face ground encountered. An assessment of tunnel zone ground abrasivity to estimate wear
of cutters, cutterhead, rock crusher and mucking system is a very important component of risk management for
tunneling in mixed-face conditions. Total ground abrasivity not only includes the primary wear of cutters and
secondary wear of the cutterhead and mucking system, but also includes cutter breakage from impacts with cobbles
and boulders or hard rock layers in a mixed-face condition.
Abrasivity assessment is essential for both the soil matrix and rock clasts. Researchers and practitioners have
developed testing methods and approaches to estimate cutter life and frequency of interventions or intervals between
interventions in all soil conditions and all rock conditions. Abrasivity and cutter wear assessment in mixed-face
ground has not been adequately developed and is a primary focus of this paper. In addition, the paper provides an
overview of cobble, boulder and mixed-face ground advance detection using TBM vibration and other geophysical
data. Such real time data can supplement the subsurface exploration data to allow better adjustments to TBM
operation parameters such as penetration rate and, cutterhead rotation speed. It can also help with decisions on
making interventions to replace worn cutters or to change to cutter types.
GROUND INVESTIGATION
Phased, Multi-Faceted Approach to Subsurface Investigation
Proper subsurface investigation for cobbles and boulders requires a focused, phased, multi-faceted approach (Frank
& Chapman 2001, Hunt 2017). The approach should start with a thorough desk study of local geology, available
geotechnical reports and relevant tunneling case histories. Geologic field reconnaissance should be made to
1
supplement the desk study results, develop a conceptual geologic setting and plan phased subsurface investigations.
Each phase of investigation should focus on both reducing uncertainties and providing sufficient data for baselining
soil matrix and rock clast conditions.
Conventional hollow-stem-auger or rotary wash borings can provide some useful information but are seldom
adequate. Additional exploration is needed using test pits, logged excavations and shafts, large diameter auger bores,
percussion borings, geophysical soundings or sonic coring. Hunt and Del Nero 2010 provide a menu of options with
relative effectiveness. Frank & Chapman 2001 utilized most of these methods to assess ground and cobble and
boulder conditions on a the BWARI project in Columbus Ohio and concluded: “Most successful was the roto-sonic
coring technique, which costs less than double the cost of conventional hollow stem auger borings.”
Hunt 2017 provides a detailed discussion of the nature of ground with cobbles and boulders and explains
development and use of cobble volume ratio (CVR) and boulder volume ratio (BVR) to characterize cobble and
boulder frequencies in geologic units for subsequent use in baselining. Reliable CVR and BVR or percent rock are
not only needed to baseline quantities and distributions but are also needed to evaluate abrasivity, cutter wear and
average intervention frequency.
Laboratory Testing
Laboratory testing to assess abrasivity should be completed in addition to conventional soil matrix and rock testing.
Abrasion testing should include mineralogy, equivalent quartz content assessment and abrasion testing of
representative reconstituted samples. Abrasion testing of rock samples is well established and has standards such as
ASTM D7625, Laboratory Determination of Abrasiveness of Rock Using the Cerchar Method. Abrasion testing of
soil samples has not been standardized and includes over 10 similar but different methods (Hunt 2018). All the soil
abrasion tests involve measurement of steel loss of a ‘tool’ after the tool used has been rolled or rotated through
reconstituted soil samples.
One significant limitation of these methods is the maximum grain size of the reconstituted soil sample. The first soil
matrix abrasion testing methods were limited to testing with sample grains smaller than 20-mm (0.8-inches) – fine
gravel and smaller. Larger gravel is crushed to 20-mm or smaller which allows the quartz content of the larger
gravel to influence the sample abrasivity. More recently developed methods allow testing of soil mixtures with large
gravel and finer, and some even allow very small cobbles up to ~100-mm (3.9-inch) in the reconstituted soil sample
used for testing. The various soil abrasion test results generally provide an index value of relative abrasivity that in
some cases are correlated to cutter life estimate methods (Hunt 2018).
None of the soil matrix abrasion tests consider the effects of cobbles and boulders. Cobbles and boulders are
generally not crushed and do not increase the abrasivity of reconstituted samples consequently. While crushing of
cobbles and boulders and balanced incorporation of fragments into the reconstituted samples tested would influence
of the abrasive quartz content, it does not allow the cutter breakage factor of the cobbles and boulders to be
evaluated. If portions of the tunnel zone ground are anticipated to have cobble plus boulder volume ratios over ~ 1-2
percent, soil abrasion testing alone is not adequate to assess abrasivity and cutter life – the rock clast abrasivity and
breakage impacts should also be considered.
In addition to soil abrasion tests, studies and case histories indicate that the equivalent quartz content (EQC) of the
ground has been correlated to abrasivity and wear. EQC is determined by microscopic examination of a thin section
to determine the mineral contents by percentage. Results are multiplied by the Rosiwal abrasivity index for each
mineral to obtain the EQC (Thuro & Plinninger 2003, Moridzadeh et.al 2016). EQC has been correlated to
abrasivity and cutter wear of both rock and soil matrix ground. Logically, the EQC of the cobble and boulder content
can be added to the EQC of the soil matrix to better assess cutter life – see additional discussion in a later section.
Baselining Cobbles, Boulders and Mixed-Face Ground Conditions
Cobble and boulder risks cannot be assessed and mitigated unless an adequate subsurface investigation program is
completed and cobble and boulder conditions are carefully baselined. Cobble and boulder conditions can and should
be baselined for tunnel projects where the subsurface investigation indicates that portions of the tunnel zone will
have a combined cobble volume ratio (CVR) and boulder volume ratio (BVR) over approximately 1 percent or
2
more. Previous studies have shown that BVR values over as small as 0.1 percent can have significant impacts on
microtunneling (Hunt & Mazhar 2014) and that many projects with boulder impacts had BVR values ranging from
0.1 to 2 percent (Hunt 2017). Where BVR values exceed approximately 2 percent or where CVR + BVR values
exceed approximately 5 percent, the potential impacts can be very significant and costly.
The primary cobble and boulder conditions (rock clast in soil matrix) to consider baselining include:
• Quantities and size ranges for anticipated cobbles, including cobble volume ratios for geologic units
baselined.
• Quantities and size ranges for anticipated boulders, including boulder volume ratios for geologic units
baselined.
• Cobble and boulder volume ratio or quantity distributions along tunnel (isolated and clusters).
• Soil matrix mineralogy (equivalent quartz content) and matrix strength (cohesion or degree of
cementation), density, abrasivity and permeability.
• Rock clast mineralogy with equivalent quartz content and rock type descriptions including both native and
erratic clasts.
• Rock clast unconfined compressive strength and Cerchar abrasivity index (CAI) ranges (statistical data and
histogram).
• In addition to the above, baseline the extent and rock quality designation (RQD), Geologic Strength Index
(GSI) or other rock quality parameters for mixed-face ground with rock layers.
Of these items, the most important are the soil matrix type, density and strength; cobble and boulder volume ratios;
maximum boulder size and size ranges; and rock clast strength and abrasivity ranges. Additional conditions to
consider baselining include: estimated cobble and boulder quantities for size ranges per length of tunnel, cobble and
boulder angularity and shapes, and gravel volume ratios (as a percentage of total excavation volume and not just in
grain size distribution curves for the gravel and finer soil matrix).
ESTIMATING CUTTER LIFE
Abrasion, Wear and Cutter Life Concepts
The factors influencing rates of abrasion and cutter wear have been studied extensively for both soil matrix and rock
conditions. In general, these studies have shown that the rate of abrasion and cutter wear increases as the quartz
content or equivalent quartz content increases and as the unconfined compressive strength or density of the ground
increases. Abrasivity and cutter life in rock are discussed to a limited extent but are not covered in detail.
Clay with a low quartz content is much less abrasive than granular soil with a high quartz content. Other conditions
such as density, strength, water content, cutter lubrication and anti-abrasion additives are also factors. Cobbles and
boulders often have a high equivalent quartz content and tend to significantly increase the total equivalent quartz
content of a soil with cobbles and boulders. In addition to increasing wear in proportion to increasing equivalent
quartz content, cobbles and boulders tend to cause cutter breakage which compounds the wear impacts.
The travel distance of a cutter depends on its radial position, the cutterhead rotation speed, TBM advance rate and
tunnel distance advanced. Gauge and perimeter cutters have the largest travel distance per cutter rotation and
therefore generally wear faster than other cutters per length of tunnel bored.
The maximum travel distance of a specific cutter before it is worn excessively can be estimated from ground
condition and anticipated abrasivity rates, but it depends on many factors: TBM operation parameters including:
variability of ground conditions and abrasivity; type of cutter (scrapper-ripper vs disc cutter); cutter steel
composition and inserts for abrasion resistance; cutter shape and size; cutter locations on cutterhead; mucking
system type and use of abrasion reduction additives; advance rate and rotation speed; impacts with boulders or rock
clasts causing cutter or cutter housing breakage; and plugging of disc cutters causing loss of rotation. Aspects of
steel selection and hardening for TBM cutters, cutter housings and cutterhead in addition to cutter size and other
metallurgical factors on wear are discussed in Del Nero 2020.
3
The frequency of cutter change intervention intervals depends on extent of critical cutter wear. The extent of cutter
wear leading to an intervention depends on many factors including: those listed above for a specific cutter; number
and location of cutter changes during the previous intervention; excessive loss of overcut from gauge cutter wear;
reduced advance rate and increased thrust and torque from worn cutter inefficiency; locations of low permeability,
stable ground locations that would allow atmospheric interventions; risk of rapid cascading cutter loss from adjacent
cutter overstressing after a cutter failure; risk of wear damage to the cutterhead and cutter housings and other factors.
The number of cutters changed during an intervention may range from 10 to 100 percent (Farrokh & Kim 2017)
making prediction of cutter change intervals very difficult.
Cutter Life in a Soil Matrix
The ten plus soil abrasion tests previously referenced directly or indirectly provide correlations to cutter wear
parameters such as cutter life index (CLI), Soil Abrasivity Index (SAI), or cutter tool life parameters:
• Sc – spiral cutter travel distance in km/cutter,
• Hm - average cutter consumption per length of tunnel in m/cutter (Bruland 1998), and
• Vc - average cutter life per excavated tunnel volume in m3/cutter (also called Hf – Bruland 1998).
These parameters allow estimates of distance bored, Li, before an intervention is required to change cutters.
Koppl & Thuro 2013 evaluated data from 18 slurry TBM drives for 10 reference projects and developed empirical
equations to estimate cutter life Sc for disc or scraper cutters and cutter change interval, Li, for anticipated ground
conditions. The method uses SAI values to characterize soil conditions. An equation is provided that allows SAI to
be estimated from estimated average EQC, D60 and shear strength values for anticipated soil matrix conditions.
After a base Sc is estimated, equations are provided to allow Sc adjustments for cutter tip width, penetration rates
and number of scrapers versus disc cutters. The maximum achievable tunnel advance distance, Lc, of an individual
cutter can be computed from a semi-empirical equation using the adjusted Sc, cutter track radius and estimated
penetration rate. An estimated intervention interval, Li is based on the lowest estimated Lc values, criticality of
specific cutters and extent of worn or damaged cutters considered unacceptable. The affects of cobbles and boulders
can be partly considered in the method by modifying the SAI for the soil matrix to consider the higher EQC
resulting from cobble and boulder content. Modifications of the method to consider CVR+BVR would be useful.
Another approach to estimating maximum intervention interval, Li, values is to estimate average cutter tool life, Vc,
using soil abrasion test correlations or case history data. Vc is the average cutter life from the tunnel volume bored
divided by the number cutters that are replaced over a tunnel interval. After estimating the number of cutters to be
changed (Ci), then Li = Vc x Ci / A where Li is in m, Vc is m3/c, and A is the bored tunnel area in m2.
Jakobsen et.al 2013 present charts that relate Vc to EQC, SAI, and SAT (Soil Abrasion Test from NTNU). Figure 1
shows a relationship between Vc and EQC from over 20 cases. It shows that Vc values for soil matrices generally
ranges from about 50 to 1400 m3/cutter with most of the data between 200 and 1200 m3/cutter.
Figure 1. Tool life, Vc, versus equivalent quartz content, EQC (from Jakobsen et.al 2013)
4
Abrasivity and Cutter Life of Ground with Cobbles and Boulders or Mixed-Face Conditions
The abrasivity of the cobble and boulder component should be considered in addition with soil matrix abrasivity
when the BVR exceeds 0.5 percent and combined CVR+BVR exceeds approximately 2 percent. The objective is an
assessment of total ground abrasivity for each reach with similar geology along the tunnel zone. It must be estimated
by combining the soil matrix abrasivity with the abrasivity of cobbles and boulders. The latter involves an
assessment of the intact rock clast abrasivity as indicted by Cerchar abrasivity index tests and then modification for
cobble and boulder conditions by consideration of concentration (combined cobble and boulder volume ratio), size,
angularity and distribution within the soil matrix. Cutter breakage should also be considered as discussed below.
The abrasivity of rock clasts (cobbles and boulders) or rock layers can be assessed by completing Cerchar abrasivity
Tests. The Cerchar Abrasivity Index (CAI) is commonly related to rock properties such as unconfined compressive
strength, Mohs Hardness, Vickers Hardness number of the rock (VHNR), EQC and cutter life.
Farrokh & Kim 2018 provide correlations of CAI and other TBM operation parameters with cutter life, Vc =
(1577×d×PR×D) / (CAI×RPM×N), where Vc is cutter life in m3/cutter, CAI is Cerchar Abrasivity Index, d is cutter
diameter in inch, PR is TBM penetration rate in m/hr, D is tunnel diameter in m, RPM is revolution per minute, and
N is number of cutters on the cutterhead. Farrokh 2013 provides charts for Vc versus UCS and quartz content for
five rock types. Hunt 2018 summarizes data on cutter tool life for various rock and soil types and shows that tool life
roughly correlates to equivalent quartz content. Other correlations of Vc with CAI, EQC, rock abrasivity index
(RAI) and similar parameters exist.
Two approaches were tentatively proposed in Hunt 2018 for estimating cutter life and intervention interval for soils
with cobbles and boulders. The first approach was a semi-empirical correlation of reported intervention intervals and
BVR data. The second approach was based on proration of tool life by bore volume estimates for the soil matrix and
rock clasts for various levels of BVR. Average tool life by bore volume, Vc, in m3/cutter, can be estimated using
correlation charts by researchers for tunneling in soil and rock. Hunt 2018 proposed equations for computing an
equivalent Vc value for five ranges of CVR+BVR. The resulting combined Vc values can be used to estimate
intervention intervals. Modifications to the Vc combination method are given below.
Intervention Intervals from BVR Correlation
For the first approach, a database of soft ground tunnel projects was used. Projects that had reported or interpreted
TBM and MTBM intervention intervals, and reported BVR and/or CVR data or data to allow interpretations were
evaluated. Figure 2 shows the result with a plot of average cutter change intervention interval, Li, versus BVR. It
shows that impacts on Li are small for BVR values less than approximately 0.5 percent and are slightly more than Li
values typically experienced for soil without cobbles and boulders. The impact of boulders on Li rapidly increases at
BVR values over 0.5 percent (which correlates to CVR+BVR = 1 to 2.5 percent) and approaches the Li values for
rock at BVR values over 1-2 percent which correlates to CVR+BVR values in the range of ~3 to 10 percent.
If the subsurface exploration program provides CVR and BVR data, then the chart in Figure 2 could be used to help
estimate intervention intervals and extent of cutter changes. The chart does not distinguish between cutter types, but
the data generally shows similar intervention intervals for disc cutters and heavy block rippers and smaller
intervention intervals for scraper and pick cutters. Additional reliable data would help increase the value of the chart,
but unfortunately most case histories for tunneling and microtunneling in ground with cobbles and boulders don’t
report boulders encountered, interpretations of boulders encountered or baselined CVR or BVR values. Many case
histories also do not report intervention data. Hopefully, future projects will begin reporting both more often.
Intervention Intervals from Soil and Rock Cutter Life Charts
The second approach involves estimation of the total ground abrasivity is outlined in Table 1. The table provides
suggested trial equations for estimating tool life, VcC+B, for ground with cobbles and boulders by combining Vc
estimates for soil matrix and rock clast samples for CVR+BVR ranges. After estimating the number of cutters to be
changed during an intervention = Ci, then the interventional interval, Li = Vc x Ci / A where Li is in m, Vc is m3/c,
and A is the bored tunnel area in m2.
5
For an example case assuming Vc soil = 500 m3/c, Vc rock = 100 m3/c and CVR+BVR =5% percent and assuming a
4 m OD TBM with 25 cutters and A= 6.38m2, then VcC+B may be estimated as 70 percent Vc soil plus 30 percent Vc
rock resulting in VcC+B = 0.7 x 500 + 0.3 x 100 = 380 m3/cutter. Adding a 20% reduction for cutter breakage results
in VcC+B = 304 m3/c. Assuming Ci = 6 cutters (24%) changed at an intervention, for the case with no cobbles and
boulders, the intervention interval would be Li = 500 x 6 / 6.28 = 478 m. Assuming Ci = 6 cutters changed at an
intervention, for the case with 5% cobbles and boulders, the intervention interval would be Li = 304 x 6 /6.28 = 290
m or 188 m less. The reduced intervention interval with cobbles and boulders would be 290 / 478 = 61% or 39% less
than the case with cobbles and boulders.
The percentages shown in Table 1 are preliminary and need to be researched further, but this approach should
provide a better estimate of total ground abrasivity due to cobbles and boulders than present methods which ignore
the impact of cobbles and boulders. The method could be used in conjunction with the relationship between BVR
and Li shown in Figure 1 to better estimate the impact of cobbles and boulders on cutter wear and life.
Table 1 – Preliminary total ground abrasivity and cutter life Vc for ground with cobbles and boulders
%CVR+BVR, Total Ground
Total Ground Abrasivity Approach
% Rock
Abrasivity
≤1
Soil matrix abrasivity Vc soil, m3/cutter from soil matrix abrasivity testing or other data
2-5
Very Low
VcC+B = (70% Vc soil + 30% Vc rock clasts)
6-20
Low
VcC+B = (50% Vc soil + 50% Vc rock clasts) x 0.80*
21-50
Moderate
VcC+B = (30% Vc soil + 70% Vc rock clasts) x 0.80*
51-90
High
VcC+B = (20% Vc soil + 80% Vc rock clasts) x 0.80*
91-100
Rock abrasivity
Vc rock, m3/cutter from correlations to CAI, EQC, other data for rock
*For CVR+BVR from 6 to 90 percent, decrease VcC+B by 20% to account for mixed-face impact cutter breakage
Figure 2 – BVR versus Li or average cutter change intervention interval (disc, ripper and scraper cutters),
from Hunt 2018
Cutter Life and Intervention Interval Case History Data
To help assess the cutter life Vc approach, Table 2 lists Vc, Hm, and average Li values along with TBM diameter,
number of cutters changed and intervention intervals for 30 bored tunnel reaches in soil, mixed-face and rock. The
table also lists the provided or estimated average percent rock for mixed-face ground or CVR+BVR. Using the data
6
in Table 2, cutter life Vc versus percent rock or CVR+BVR is plotted in Figure 3. It shows that cutter life rapidly
decreases as the percentage of rock increases from 0 for soil only conditions to 10 percent rock or 10 percent
CVR+BVR. From 10 to near 100 percent rock, the cutter life slowly decreases. Due to less cutter impact breakage,
cutter life is often higher for 100 percent rock than for mixed-face ground, particularly for rock with low to
moderate UCS and lower CAI.
Table 2 – Cutter life and intervention intervals data tunnels in soil, mixed face ground and rock
Project
Name,
Location
TBM
ED,
m
Number
cutters
changed
Vc,
m3/c
Hm
(m/c)
Number
of Interventions
Avg
Li, m
Avg %
rock,
C&B
Ground
References
Bangalore Metro,
India, TBM 1
6.44
381
61.1
1.9
100
7.2
30.0%
Silty sand - clayey silt and
residual soil
W2012.54,
2016 Gudge,
Bangalore Metro,
India, TBM 2
6.44
349
67.0
2.1
103
7.0
30.0%
Silty sand - clayey silt and
residual soil
W2012.54,
2016 Gudge,
Chennai Metro,
India, R134-259
6.63
144
44.8
1.3
5
37.4
40.0%
Mixed Face Silty Sands/
Clays, Weathered Granite
R2019.83
Chennai
India
Metro,
6.63
253
88.7
2.6
19
34.2
20.0%
Mixed Face Silty Sands/
Clays, Weathered Granite
R2019.83
Tehran Metro, L7
First 6500m
9.16
1169
366.6
5.6
73
89.0
7.0%
gravely sand w clay/silt +
sandy gravel w clay; w C+B
TU2017.21
Tehran Metro, L7
12+500-15+690
9.16
449
459.0
7.1
36
89.0
5.0%
gravely sand w clay/silt +
sandy gravel w clay; w C+B
TU2017.21
Tehran Metro, L7
15+690–18+998
9.16
364
605.0
9.1
37
89.0
3.0%
gravely sand w clay/silt +
sandy gravel w clay; w C+B
TU2017.21
Busan Line IIa, S230, Busan Korea
7.28
30
294.6
7.1
1
212.4
3.0%
alluvial soil: silt, sand,
boulder clay
Bae & Kim
2001
Busan Line IIb, S230, Busan Korea
7.28
41
39.0
0.9
2
19.2
33.0%
Mixed: 67% silt, sand,
boulder clay and 33% rock
Bae & Kim
2001
Busan Line IIc, S230, Busan Korea
7.28
45
15.5
0.4
1
16.8
50.0%
Mixed: 50% silt, sand,
boulder clay and 50% rock
Bae & Kim
2001
Busan Line IId, S230, Busan Korea
7.28
9
77.7
1.9
1
16.8
100.0%
100% hard rock, 39-235
MPa, RQD 0-70%
Bae & Kim
2001
Piedmont Tunnel,
Italy
3.60
59
151.1
14.8
11
79.6
100.0%
Gneiss, mica schist, Avg+
110 MPa, GSI 50-60
Oggeri
&
Oreste 2012
Rossaga
Hydroelectric
7.23
315
115.2
2.8
100.0%
Granodiorite, mica gneiss,
schist
W2017.238
Yamanli II Tun.
Adana Turkey
4.31
58
1660.0
114.1
6.7
100.0%
limestone, 30-80 MPa,
average RQD is 75-80%.
W2015.153
Yinhanjiwei Tun
N Project, China
8.02
322
1057.8
21.0
6.7
100.0%
Phyllite/schist, + Phyllite/
Metasandstone, 31-92 MPa
W2016.77
Yinhanjiwei Tun
S Project, China
8.02
649
114.2
2.3
6.7
100.0%
10% Quartzite + 90%
Granite 85-169 MPa
W2016.77
Singapore Tunnel
A, Ring 150-600
7.46
67
130.2
3.0
26
7.7
100.0%
granite, 71 to 185.9 MPa
TU2016.133,
2016 Shirlaw
Singapore Tunnel
A, Ring 150-600
7.46
11
191.7
4.4
7
7.1
50.0%
mixed granite and soil
TU2016.133,
2016 Shirlaw
Singapore Tunnel
A, Ring 150-600
7.46
86
146.2
3.3
34
8.4
6.0%
clayey completely
decomposed. granite
TU2016.133,
2016 Shirlaw
Singapore Tunnel
A, Ring 150-600
7.46
37
161.5
3.7
12
11.8
4.0%
Completely decomposed
granite
TU2016.133,
2016 Shirlaw
Singapore Tunnel
B
9.23
112
130.6
2.0
34
6.4
100.0%
Weak Tuff
TU2016.133,
2016 Shirlaw
Singapore Tunnel
B
9.23
191
18.3
0.3
16
3.2
82.0%
Mixed 85-99% Tuff
TU2016.133,
2016 Shirlaw
Singapore Tunnel
B
9.23
76
118.7
1.8
12
11.5
60.0%
Mixed 50-85% Tuff
TU2016.133,
2016 Shirlaw
3
7
Project
Name,
Location
TBM
ED,
m
Number
cutters
changed
Vc,
m3/c
Hm
(m/c)
Number
of Interventions
Avg
Li, m
Avg %
rock,
C&B
Ground
References
Singapore Tunnel
B
9.23
84
356.6
5.3
15
30.8
15.0%
Decomposed 15-50% Tuff
TU2016.133,
2016 Shirlaw
Singapore Tunnel
B
9.23
12
803.9
12.0
2
74.1
3.0%
Decomposed <15% Tuff
TU2016.133,
2016 Shirlaw
Singapore
Thomson E Coast
6.66
281
78.1
2.2
791
0.8
35.0%
Mixed-face, granite +
residual clay w C+B
2017 Connors
Guangzhou metro
line 9, China
6.25
281
123.3
4.0
8
125.6
10.3%
Limestone, mixed-face,
silty clay
2018
et.al
Brightwater West,
BT-4 West Reach
4.67
32
1716.3
100.3
1
1604.0
0.1%
Non-glacial clays silts,
sands + Pre-Fraser Glacial
R2011.80,
N2010.90,
Brightwater West,
BT-4 East Reach
4.67
168
359.6
21.0
9
352.9
0.2%
Glacial silts, clays and
sands with little G, trace CB
R2011.80,
N2010.90,
Eglinton-SC,
Toronto
6.52
300
655.8
19.7
9
589.6
0.2%
Glacial silts, clays and
sands with little G, trace CB
T2016.27
R2015.79
Elbaz
Figure 3. Cutter life m3 per cutter versus percent rock or CVR+BVR
Cutter life, Hm in m/cutter versus percent rock or CVR+BVR is plotted in Figure 4. It shows that cutter life rapidly
decreases as the percent of rock increases from 0 for soil only conditions to 5 percent rock or CVR+BVR. From 5 to
near 100% rock, the cutter life slowly decreases. The cutter life is often higher for 100 percent rock than for mixedface ground, particularly for rock with low to moderate UCS and lower CAI.
Average intervention interval versus percent rock or CVR+BVR is plotted in Figure 5. It shows that intervention
intervals rapidly decrease as the percent of rock increases from 0 for soil only conditions to about 3 percent rock or
CVR+BVR. From 3 to near 100 percent rock, the cutter life slowly decreases. Intervention intervals in 100% rock
are often higher due to elimination or reduction of impact breakage.
Figures 2-5 all support the Vc soil and rock proportions suggested in Table 1 which favors Vc soil for cobbles and
boulder or mixed-face percent rock less than about 3 percent and favors Vc rock values with a reduction for
breakage for ground with higher concentrations of cobbles or boulders or percent mixed-face rock.
8
Figure 4. Cutter life, m per cutter, versus percent rock or CVR+BVR
Figure 5. Average intervention interval versus percent rock or CVR+BVR
Cutter Breakage
Cutter breakage may occur anytime cutters are impacting (bashing) cobbles and boulders instead of slicing, scraping
or progressively chipping through the rock clasts. Breakage tends to occur from dynamic stress concentrations
during impacts. Higher stress impacts will cause more breakage. Higher stress impacts may result from:
• Higher TBM advance rates and cutterhead rotation speeds,
• Harder, higher strength cobbles and boulders,
9
•
•
•
Larger boulders,
Boulders embedded in a dense, strong unyielding soil matrix that minimizes boulder movement, and
Eccentric impacts.
In one study, Lo Faro et.al 2019 found that cutter breakage as a percent of total cutter replacement ranged from 0 to
90 percent with the highest percentages of breakage in mixed-face ground. On the Big Walnut Augmentation
/Rickenbacker Interceptor (BWARI) Project for the City of Columbus Ohio, where BVR values varied from 0-2.5
percent, DiPonio et al 2007 found that “one-third of all the ripper cutters were being broken by boulders,”
Cutter breakage as a percent of total cutter replacement will increase as CVR+BVR or percent rock increase from 0
to about 10 percent then level off. The cutter breakage as a percent of total cutter replacement may range from 20 to
30 percent when percent rock or CVR+BVR values range from 10 to 90 percent. Lo Faro et.al 2019 reported cutter
breakage percentages ranging from 0 to 90 percent with 10 to 30 percent breakage common in mixed-face rock.
Additional and more thorough studies of case history data with BVR, cutter wear and breakage data would be useful
to better determine breakage impacts on cutter life and the resulting decreases in tunnel advance before a cutter
change intervention is required. Based on the data evaluated, Table 1 suggests that cutter life, VcC+B be decreased by
20 percent for mixed-face ground with percent rock from 6 to 90 percent to account for cutter impact breakage in
mixed-face conditions.
BOULDER DETECTION
Pressure Balance Tunneling in Bouldery Ground
Utilization of a pressure balance tunnel method in boulder-laden soils poses a significant challenge to the cutterhead
and attached cutting tools including disc cutters. These tools are designed and capable of excavating through rock
(including boulders) but the conditions presented by bouldery ground are significantly different than those presented
by a full face of rock.
Rock excavation with discs is based on fragmentation of the rock due to shear and tensile forces developed with the
penetration of the disc into the rock. In general, the force from the initial penetration results in crushing of the rock
into a powder under the tip of the disc. This crushing of the rock is very energy intense and the thrust to penetration
depth is very high during the initial phase. Once the penetration of the disc reaches a critical depth, shear and tensile
forces start to overcome the strength of the rock and the amount of force necessary to increase the depth of
penetration is significantly reduced. This continues until the depth of penetration reaches an optimum point where
the average size of the rock chips being created are maximized. Once this optimum depth of penetration is
surpassed, the energy being generated by the disc penetration is not enough to overcome the resistance to fracture
due to the increased length of the fractures required to create rock chips. This results in a rapid increase in the forces
applied to the disc with additional penetrations beyond the optimal penetration.
In order to properly excavate through boulders, the advance rate must be compatible with the bearing capacity of the
cutter disc and provide penetration rates suitable for rock. Typical pressure balance tunnel boring machines are
limited to a cutterhead rotational speed of less than 4 RPM due to the limitations imposed by the main bearing
protection sealing system. Typical optimal penetrations for disc cutters into hard rock is less than 0.2 inches (5 mm)
per revolution, which results in advance rates of well under 1 inch (25 mm) per minute. This is less than 25% of
what would typically be expected in ground not containing boulders.
If the assumption is made that there is always a boulder in the face then the advance rate would need to be limited to
25% of what the system is capable of, and since the majority of the tunnel would not have boulders, most of the
tunneling would be very inefficient.
If the assumption is made that there is no boulder in the face, then encountering a boulder at penetrations more than
4 or 5 times greater than what the disc is designed for, will result in shock loading and the failure of disc cutters,
unless the operating parameters are quickly modified. This will result in delays in the tunneling due to the need for
interventions into the cutterhead chamber for cutting disc replacement.
10
What is needed is a system that provides the TBM operator with real-time data indicating that the cutterhead is about
to or is encountering a boulder. This will give the operator an opportunity to adjust the operating parameters for the
TBM compatible with the disc cutters cutting the boulder as they are designed to do - with a penetration per
revolution of less than 0.25 inches (6 mm).
Different boulder detection type systems have been developed and several are available especially for larger
diameter machines. These include systems for sensing ahead of the face for boulders or obstructions (mainly sonic
or seismic based) and for sensing boulders or obstructions at the face itself, such as instrumented cutters and systems
that monitor the vibration of the TBM cutterhead and main drive system. We will focus on the vibration-based
system in this section.
Boulder Detection with Vibration Monitoring
In the earlier days of pressure balance tunneling, the TBM operator was positioned within the shield, and many
operators were able to hear and feel the vibrations that were created when the cutterhead encountered a boulder. This
allowed the operator to modify the operational parameters of the TBM in a timely manner, which resulted in the
TBM being able to excavate through the boulder at a slower speed without the need for an intervention to repair the
cutterhead.
In November 2004, tunneling commenced on the Big Walnut Augmentation Rickenbacker Interceptor (BWARI)
Project in Columbus OH, with a 16 ft (4.9 m) diameter EPB TBM that was specifically designed for the bouldery
conditions expected to be encountered on that project. The TBM successfully completed the project without the need
of compressed air interventions. This was due in part to the fact that the project was in a very rural area, but also in
part due to the TBM operator’s station being inside the TBM shield within 7 meters of the face. The operator was
able to sense when there was a boulder or boulders in the face and quickly modify the operating parameters of the
TBM.
In September 2008, tunneling commenced on the Brightwater West project just north of Seattle WA, with a 15 ft
(4.6 m) diameter EPB TBM that was specifically designed for glacial and interglacially deposited material. Despite
the fact that significantly fewer boulders were expected when compared to the previous project in Ohio, the TBM
was significantly impacted by boulders including one hyperbaric intervention at 3.5 bar. This was more than
partially due the TBM design having the operator in a separate cabin outside of the shield and over 20 meters from
the face.
Given the knowledge that the operator would not be able to sense when a boulder was in the face, the contractor had
a vibration monitoring system mounted on the TBM and some data was gathered. This was the contractor’s first
attempt to use the TBM data acquisition system to detect boulders at the face using vibration sensors. Although
efforts to correlate the data with the boulder encounters were not successful, much was learned concerning what
additional steps needed to be taken in order to gather the kind of information needed to detect boulders in the face.
In June 2011, tunneling commenced on the University Link 230 (U230) project in Seattle WA, with a 22 ft (6.7 m)
diameter EPB TBM that was specifically designed for the glacial and interglacially deposited material expected on
the project. The tunnel drives on this project were only 3,750 feet (1,143 m) long and the majority of the alignment
was in interglacially deposited sands, silts and clay. There was a risk of encountering boulders and the contractor,
working with the Colorado School of Mines installed a second generation vibration based boulder detection system.
This system was able to detect known changes in the geology, but few boulders were encountered (Walter 2013).
In June 2014, tunneling commenced on the North Link 125 (N125) project in Seattle WA, with a refurbished 22 ft
(6.7 m) diameter EPB TBM, which was originally designed for the glacial and interglacially deposited material
expected on the project. The tunnel drives on this project were up to 8,000 feet (2,438 m) including significant
lengths expected to contain a high density of boulder concentration. Once again working with the Colorado School
of Mines, the contractor installed a vibration-based boulder detection system on the TBM. This system was able to
identify boulders in the face and eventually allowed the tunneling operator to modify the TBM operational
parameters in a way that facilitated continuous excavation through known boulders without the need for hyperbaric
interventions into the cutterhead chamber (Buckley 2015, Buckley, et al 2017)
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The last 15 or so years of working on the development of a vibration based boulder detection system has resulted in
a system that is capable of detecting the presence of a boulder (and conceptually other similar obstacles) as it is
being encountered by the cutterhead of the TBM. This system can be installed on the project site as the TBM is
being readied for launch provided that the back of the cutterhead chamber is available for the placement of
accelerometers. The system requires calibration prior to the launch of the TBM after the TBM assembly has
progressed to the point where the TBM cutterhead can be turned at maximum rotation speed. The system
components are small in size and there is no requirement for sending or receiving information through the cutterhead
chamber, as the vibration being sensed is transmitted through the structure of the cutterhead and cutterhead drive.
Therefore, this system can be mounted on nearly any pressure balance TBM including most microtunneling
machines.
The accuracy of the system is dependent on the effectiveness of the calibration process that takes place prior to the
launch and during the startup of the drive. The natural vibration of the TBM cutterhead rotation can be characterized
when spinning at different rotations in air prior to launching and blows can be manually applied to the face of the
cutterhead as it is spinning, and the signature of these blows characterized. Based on the force of the blows a certain
signature range is classified as a potential boulder impact and when these are encountered during tunneling. The
operator will be notified and will go through a prescribed protocol to change from optimal parameters for standard
face conditions to optimal parameters for excavating through the boulder(s).
Plan for Boulder/Obstruction Detection on the Seattle Ship Canal Project
The Seattle Ship Canal Project is a CSO storage tunnel project consisting of approximately 14,000 feet (4,267 m) of
18’10” (5.7 m) tunnel on the north side of the Ship Canal in Seattle WA. The tunnel will be entirely in soil deposited
in a glacial and inter-glacial environment, with well over 50% of the material expected in the tunnel envelope
consisting of glacial till. The amount of till in the heading is expected to result in the capability of the TBM to
excavate through boulders having a large impact on the success of the project. The number of boulders anticipated
is approximately 3 times more than what was anticipated on the N125 project, which experienced significant delays
associated with repairs to the cutterhead from damage due to encountering boulders in the face.
LANE Construction is currently planning on installing a vibration-based boulder detection system on a
Herrenknecht TBM prior to a TBM launch planned for June 2021. The system will be installed and initial calibration
conducted once the TBM is lowered into the shaft and prior to the launch bell being closed around the shielded
portion of the TBM in the shaft bottom. Additional calibration will need to be performed after the TBM is
excavating, as the ground will alter the vibrational signature of the operating TBM.
This is a technology that was developed using prior research funding from both the National Science Foundation and
from industry. The system involves hardware (up to 4 triaxial vibration sensors and a data acquisition system) and
software (algorithm to process vibration data in real-time and to report vibration signals that suggest boulder
impacts). The adoption of this system into a new TBM and in different ground conditions requires some research
effort.
The project will involve the installation of vibration monitoring sensors on the atmospheric side of the excavation
chamber bulkhead and the installation of a data acquisition (DAQ) system connected via ethernet to capture and
record the data. Installed algorithms will process the vibration data in real time and provide this information to the
researchers and project personnel. The system will be calibrated before and during early tunneling.
SUMMARY
Hundreds of papers with relevant information on abrasion and wear have been written. Aspects of some were
presented in this paper. Below is a summary of concepts and observations on tunneling in cobbles and boulders or
mixed-face ground to consider:
1. Cobbles and boulders or percentage of rock are important factors influencing wear, breakage and cutter life.
2. Subsurface investigation methods are available to reasonably characterize cobble and boulder conditions
including use of CVR and BVR (Hunt 2017).
3. As the gravel, CVR and BVR increase, total ground abrasivity increases and cutter life decreases.
4. Ground with a CVR + BVR over 50 percent tends to behave like a mixed-face soil-rock interface condition.
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5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
Boulder size matters – boulders over approximately 30 percent of the excavated diameter are less
ingestible, require fracturing and increase the risk of severe cutter wear and obstruction.
Cobble and boulder clasts are survivors of geologic transportation and wear processes and as a result
generally have high unconfined compressive strengths and high Cerchar Abrasivity Index values which
increase total ground abrasivity and cutter wear.
Boulders create a mixed-face condition a similar to a soil-rock interface. Mixed-face conditions with large
differences in strength and compressibility result in cutter impacts and cutter breakage that significantly
decreases cutter life from that attributable to abrasive wear alone. A cutter life reduction by 20% is
suggested to account for impact breakage.
Energy consumption matters – the energy required to commutate (fracture) rock clasts to gravel size for
slurry muck transport is directly related to the rock clast strength and the degree of commutation required
for passage from the crushing chamber into the slurry transport system (Hunt 2017). The higher the cobble
and boulder volume ratios, the higher the energy demand. The higher the cobble and boulder strengths, the
higher the energy demand. Energy demand correlates with total ground abrasivity and wear. The higher the
energy demand to commutate cobbles and boulders, the greater the abrasion and wear impacts.
Different cutter types (disc cutter, ripper, scraper, conical) affect TBM performance, cobble and boulder
excavatability, abrasive wear and breakage and thus cutter life and intervention intervals. The most optimal
cutter type tends to change with changes in soil matrix condition and CVR, BVR and rock content
percentages.
Soil matrix strength is an important factor in cutter fracturing of rock clasts versus plucking. Goss 2002
found that the ratio of UCS rock to UCS soil must be less than 600 for disc cutters to cut rock clasts instead
of plucking them. Kiefer et.al 2008 provide more detailed information on extent of cutting versus plucking
for single and multi-kerf cutters.
TBM penetration rate reduction is generally required to facilitate disc cutter cutting of rock clasts and to
minimize impact breakage of cutters. A penetration of 10 mm or less per rotation may be required to
maximize cutting vs. plucking (Babendererde 2003).
Disc cutters tend to be more effective than ripper or scraper cutters when the CVR+BVR or rock content is
over about 20 percent.
Ripper cutters tend to bash and pluck rock clasts up to ~ 30 percent of the excavated diameter and are more
effective in lower strength soil matrix.
Lower strength soil matrix results in a higher incidence of boulder plucking and pushing aside the TBM.
Cutter life is directly related to helical cutter travel distance, Sc. Gauge and perimeter cutters travel more
(higher Sc) per cutterhead rotation and therefore tend to wear faster than interior cutters.
Interventions are generally needed when a sufficient number of gauge and perimeter cutters are severely
worn and are affecting advance rate and overcut or excavated diameter. Cutters generally have a wide
range in wear and need for replacement of partially worn cutters often depends on intervention costs and
risks. The percentage of cutters changed in an intervention may range from 10 to 100%.
Severe wear or breakage of a cutter in rocky ground (CVR+BVR or rock percentage over ~50 percent) can
result in progressive overstressing of adjacent cutters and rapid cascading failure of cutters and need for a
major cutter change intervention.
Impact vibrations can be measured by sensors in a TBM and used to predict or verify upcoming cobble and
boulder conditions and allow more focused mitigation measure decisions.
TBM penetration rate and rotation speed affect mixed-face impacts and vibrations. Reduced penetration
rates and rotation speed generally help minimize cutter impact damage and increase cutter life.
When CVR+BVR exceeds ~ 20 percent, a reduced MTBM penetration rate is also generally needed to
allow rock crushers to crush clasts and the mucking system to remove clasts from the excavation chamber
and prevent choking or MTBM cutterhead stalling.
The quartz content of the ground is directly related to abrasivity and wear and the concept of equivalent
quartz content (EQC) of both the soil matrix and rock clasts can be used to estimate abrasivity of ground
with cobbles and boulders.
Rock abrasivity index (RAI) is a function of rock clast unconfined compressive strength and abrasivity and
therefore an important indicator of rock clast abrasivity.
Soil abrasivity index (SAI) is a function of soil matrix EQC, D60 grain size and shear strength (Koppl &
Thuro 2013) and can be used with cutter and TBM parameters to estimate cutter life and intervention
intervals in soil. Modifications of the approach that consider CVR+BVR values can be used to estimate
cutter life and intervention intervals in ground with cobbles and boulders.
13
24. Rock and soil testing that provides correlations to cutter life, Vc, in m3/cutter can be combined with
percentages of rock or CVR+BVR to estimate cutter life and intervention intervals in mixed-face ground.
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