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Discussion: Challenges of earth pressure balance tunnelling in weathered

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 Cite this article Discussion
Zhang N, Shen SL, Zhou AN, Lyu HM and Shirlaw JN (2021) Paper 2000148
Discussion: Challenges of earth pressure balance tunnelling in weathered granite Received 25/07/2020;
with boulders. Accepted 24/09/2020;
Proceedings of the Institution of Civil Engineers – Geotechnical Engineering 174(1): 91–95, Published online 04/11/2020
https://doi.org/10.1680/jgeen.20.00148
ICE Publishing: All rights reserved

Geotechnical Engineering

Discussion: Challenges of earth pressure

balance tunnelling in weathered granite
with boulders
Nan Zhang MSc An-Nan Zhou PhD
PhD candidate, Department of Civil Engineering, School of Naval Associate Professor, Discipline of Civil and Infrastructure, School of
Architecture, Ocean, and Civil Engineering, Shanghai Jiao Tong University, Engineering, Royal Melbourne Institute of Technology, Victoria, Australia
Shanghai, China (Orcid:0000-0001-5209-5169)
Shui-Long Shen PhD Hai-Min Lyu PhD
Professor, Key Laboratory of Intelligence Manufacturing Technology Post-Doctoral Fellow, State Key Laboratory of Internet of Things for Smart
(Shantou University), Ministry of Education, and Department of Civil and City, University of Macau, Macau, China (Orcid:0000-0002-2641-8099)
Environmental Engineering, College of Engineering, Shantou University, J. N. Shirlaw MSc, CEng, MICE
Shantou, Guangdong, China; Discipline of Civil and Infrastructure, School Senior Consultant, Golder Associates (Hong Kong) Ltd, Wan Chai,
of Engineering, Royal Melbourne Institute of Technology, Victoria, Australia Hong Kong, China (corresponding author: nshirlaw@golder.com)
(Orcid:0000-0002-5610-7988)

Contribution by J. N. Shirlaw per two working days for an EPB drive in weathered tuff in
The authors are to be commended for presenting a detailed, mixed-face conditions of 85–99% rock. On the same drive, the
warts and all, case history of earth pressure balance (EPB) tun- disc cutters needed to be changed more than seven times as fre-
nelling in mixed ground formed by the weathering of granite quently in 85–99% rock, where EPB pressure had to be applied
(Zhang et al., 2020). Among the other issues brought out in to stabilise the saprolite portion of the face, compared with
the paper are the following. open-mode tunnelling in a full face of rock grades of tuff. So,
most of the problems described by the authors are well known
& An overall average advance of 1.05 rings per day and longstanding issues related to EPB tunnelling in mixed
(1.89 m/day) for the 550 ring section, with an average of ground resulting from the weathering of strong crystalline rock.
just 0.67 rings per day (1.2 m/day) achieved in zone 8,
which was mostly in mixed ground with a high proportion One problem identified in the paper that was not recorded in
of granite rock. these previous cases was the locally severe water leakage at the
& Two sinkholes. joints and bolt pockets in the tunnel lining, as shown in
& Excluding the sinkholes, a maximum measured settlement Figures 9(a) and 9(b) of the paper. The authors state that this
at the ground surface of 290 mm. was due to wash-out of the grout. Wash-out of the tail void
& Extensive use of pre- and post-grouting to reduce the risk grout would result in uneven loading on and distortion of the
of further sinkholes or excessive settlement and to help fix rings, with steps and lips at the joints. This would be exacer-
boulders in position. bated by the shove loads applied to the lining. The leakage
& As shown in Figure 17 of the paper, on average 66.7% of would be consistent with there being a consequent mismatch
the cycle time per ring was devoted to cutter changes between the meeting faces of the gaskets. It is likely that there
(28.2%), grouting from the surface (26.2%) and cleaning was damage to some of the segments due to applying shove
out the tail shield (12.3%). pressures to deformed rings. A segmental lining that leaked so
& Localised problems with severe water inflows at the joints badly during construction (Figure 9(c))) and suffered local
in the segmental lining, ascribed to wash-out of the tail damage would be a major concern for long-term durability.
void grout.
The tunnelling clearly experienced major problems with ground
Many of these problems are familiar from previous documented control when operating in a mixed face of rock and soil grades
cases of EPB tunnelling in mixed ground resulting from the of weathered granite, and this is consistent with previous experi-
weathering of rock, particularly when the fresh rock is strong or ence worldwide of EPB tunnelling in comparable conditions.
stronger, and crystalline. Babendererde et al. (2005) discuss an
EPB-driven tunnel in Portugal in mixed soil and rock grades of In their conclusions the authors recommend:
weathered granite: there were ‘several’ collapses and extremely
slow progress. Shirlaw et al. (2003) reported four sinkholes and & drilling of probe holes from the tunnel boring machine
three further major ground losses over EPB tunnel drives in (TBM) to fine-tune the understanding of the ground
Singapore at soil/rock interfaces within weathered granite. conditions
Shirlaw (2016a) recorded average progress rates of less than 1 m & real-time monitoring of cutter wear.

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 Geotechnical Engineering Discussion: Challenges of earth pressure
Volume 174 Issue 1 balance tunnelling in weathered granite
with boulders
Zhang, Shen, Zhou, Lyu and Shirlaw

There is also mention of the ground control problems being a Special Administrative Region). Reference can also be made to
result of encountering boulders and using ‘unreasonable’ oper- the guidelines issued by the German Tunnelling Committee
ations parameters, but there is no discussion of how this (DAUB, 2016). The Hong Kong and German guidelines can be
should be rectified. contrasted with the process described by Zhang et al. (2020),
where it is reported that the face pressure was ‘dynamically
There are fundamental issues with using a pressurised EPB adjusted during the tunnelling process by the tunnelling engin-
TBM in mixed ground derived from the weathering of strong eers and TBM operators’. This dynamic adjustment appears to
crystalline rock, as documented in previous projects. Shirlaw have been based on the excavated volume, as determined from
(2016a) showed how the penetration index, the specific energy counting the number of skips of material removed.
and the cutter wear during EPB tunnelling in mixed-face con-
ditions increased dramatically as the proportion of rock In Figure 7 of the paper it is indicated that the typical volume of
increased, particularly once the proportion of strong rock material excavated was 200 m3/ring. For a 1.8 m long ring and
increased to over 50% of the total face area. As recorded by an excavated diameter of 9.15 m, the theoretical volume of
Shirlaw et al. (2017) and Kwong et al. (2019), the use of slurry ground excavated is 118 m3/ring. However, the volume would
TBMs in similar mixed-ground conditions in weathered increase due to a number of factors, including bulking, the
granite in Hong Kong did not experience similar problems addition of conditioning agent and the volume of any water
with ground control. With the slurry TBMs, reasonable rates flowing into the face and around the TBM skin. In Figure 7, the
of progress were achieved despite the need to spend significant volume excavated appears to be typically 170–180 m3/ring up to
amounts of time changing cutters due to the abrasive nature of ring 250, and thereafter around 220 m3/ring, but fluctuating dra-
the rock and the saprolite; nor was there need to use grouting matically with a maximum value of about 360 m3/ring. This
to aid the tunnelling, except at break-in and break-out. A com- degree of variation is indicative of major loss of ground and
parison of the performance of EPB and slurry TBMs in poor face control. However, counting skips is an extremely crude
mixed-face conditions resulting from the weathering of granitic measure of the volume of excavated material. Even the more
gneiss is provided in the construction of phase I of the sophisticated methods of measuring excavated material such as
Bangalore metro (Kenyon, 2016). The slurry TBMs on the belt weighting or belt scanning are still relatively inaccurate. As
east–west line required approximately one year of construction discussed in GEO Report 298 (HKIE Geotechnical Division,
time less than the EPBs on the north–south line to complete 2014), such systems are important as they can provide an indi-
comparable lengths of tunnel; in addition, one of the final cation that major over-excavation is occurring. However, the
drives on the north–south line was carried out by a slurry critical control for ensuring face stability is the face pressure,
TBM that had completed work on the east–west line and was which can be measured accurately, rather than the relatively
brought over to replace the originally planned EPB TBM inaccurate volume measurement. The continuous maintenance
(Kenyon, 2016). It is reported that, for the next phase of the of adequate face pressure is a proactive measure to provide face
Bangalore metro, the client specified the use of slurry TBMs, stability, whereas volume measurement is reactive. With volume
based on the experience of phase I (Wallis, 2018). measurement, control measures can only be taken after signifi-
cant ground loss has already occurred and been identified. The
The use of slurry shields has become common in Singapore target face pressure should be based on calculation from the
and Hong Kong when it is anticipated that weathered granite known ground and groundwater conditions, as outlined in GEO
including a significant proportion of mixed-face tunnelling will Report 298 (HKIE Geotechnical Division, 2014) or DAUB
be encountered (Shirlaw, 2016b). If EPB TBMs are used, it is (2016), with procedures to adjust the target pressure if warranted
typically recommended that an additional active face support by observations during tunnelling (Shirlaw and Boone, 2009).
system using pressurised slurry is incorporated into the TBM,
as developed for the Metro Do Porto tunnelling (Babendererde From the information in Figures 3 and 6 of the paper, it is
et al., 2005). There is no indication in the original paper that evident that the face pressure applied was in places less than the
such a system was incorporated into the TBM in Shenzhen. water pressure. It is noted in the paper that many of the ground
control problems occurred in the mixed ground in zones 6 and 8,
Shirlaw et al. (2017) discuss the basis for the calculations used in where the greatest face pressure was applied. However, this was
Hong Kong to establish target face pressures for slurry TBM also the area where the water pressure was greatest, as the tunnel
operation and during interventions; these calculations are was on a down grade. In these zones the applied face pressure
treated as part of the temporary works design and are prepared appears to have been generally less than the water pressure shown
and independently reviewed before tunnelling. In Hong Kong, in Figure 3. The significant proportion of time devoted to clean-
such calculations are typically carried out based on the guide- ing the tail skin shows that the spoil was very wet and sloppy,
lines provided in GEO Report 249 (Golder Associates (HK) and sliding off the conveyor belt. This is consistent with the face
Ltd, 2009) for slurry TBMs and GEO Report 298 (HKIE pressure being lower than the water pressure – the result would
Geotechnical Division, 2014) for EPB TBMs (these two docu- have been seepage of groundwater towards the TBM, destabilis-
ments were published by the government of the Hong Kong ing the saprolite and increasing the water content of the spoil.

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 Geotechnical Engineering Discussion: Challenges of earth pressure
Volume 174 Issue 1 balance tunnelling in weathered granite
with boulders
Zhang, Shen, Zhou, Lyu and Shirlaw

A problem discussed in the paper is the effect of the corestone Authors’ reply
boulders within the weathered rock on the tunnelling. From The authors would like to thank the contributor for providing
the description in the paper it appears that the boulders were detailed and valuable discussions regarding the article (Zhang
coming free from the saprolite matrix and damaging the et al., 2020). The authors’ responses are as follows.
cutting tools by rolling around the head. Occasionally,
boulders also clogged the screw conveyor. The TBM was Ground control
equipped with disc cutters capable of cutting rock. Ideally, the Boulders lowered the advance rate and aggravated the ground
boulders should be cut in situ, which would avoid the problems disturbance, resulting in difficulties in terms of tunnel face
described in the paper. Boulders will only be cut if the saprolite support (Ren et al., 2018a, 2018b). For ground control, the
matrix surrounding them is stable and strong enough to rotation speed of the cutter head should be decreased to
prevent the boulders being plucked from the face by the reduce the impact in boulder-laden strata. The TBM is rec-
forces imposed during cutting. As already discussed, the face ommended to advance with a low penetration rate to minimise
pressure applied was often inadequate to balance the water the vibration when encountering boulders (Elbaz et al., 2018a,
pressure and stabilise the saprolite. In particular, the resulting 2018b). Interventions should be conducted with shorter inter-
seepage would have destabilised and eroded the saprolite. vals for the timely replacement of disqualified cutting tools,
So, the boulders would have readily come free, consistent with since damaged cutters may accelerate the failure of cutting
the description in the paper. The often inadequate face tools nearby.
pressure used during tunnelling would thus have been a major
factor in causing the problems related to the corestone In addition to water leakage, machine control is another criti-
boulders. cal factor influencing lining quality. In full-face rock strata,
vibration leads to difficulties in tunnelling along the designed
Another significant problem discussed in the paper was the tunnel axis (DTA). Moreover, the trajectory control system of
wash-out of the tail void grout. Wash-out cannot occur unless TBMs shows a time lag effect, which hinders the timely adjust-
there is groundwater flow that transports the grout away from ment of the pose and trajectory of the machine (Wang et al.,
the tail void around the rings. The obvious transport system in 2018). Deviation from the DTA is subsequently beyond the
this example is from the tail void, along the overcut around the required range. Consequently, this issue brings about difficul-
shield skin and into the excavation chamber. This will occur if ties in lining installation and poor-lining quality, which can
the face pressure is less than the groundwater pressure and the aggravate water leakage problems. Figure 9(c) of the paper
ground is sufficiently permeable for the pressure difference to shows a submerged work zone. This was mainly attributed to
result in significant flow. Once this has occurred, there is the drainage through grouting holes in the segment (see Figure 18)
potential for before construction of the next ring.

& the partially grouted rings to distort, creating extra flow Selection of TBM type
paths by leakage through the joints in the lining, as shown Geological investigations conducted before the excavation
in Figure 9 of the paper showed the presence of both soft layers and hard rock.
& subsequent grouting into a water-filled tail void, potentially Therefore, a combined TBM was used for the studied section
resulting in segregation of the grout and continuing to adapt to the frequently varying strata. The conventional
problems with ineffective grouting. shield machine, which was designed for tunnelling in soft
layers initially, was improved to increase the working perform-
The use of a tail void grout with a 4 h setting time is ance when encountering hard strata. For example, the stiffness
likely to have been another factor in the reported wash-out and strength of the cutter wheel were increased to minimise
problems. the deformation in the case of direct contact with hard strata.
The disc cutters were improved to be capable of high-strength
Based on the evidence presented in the paper, the root causes cutting rock. The main bearing and main drive were also
of many of the problems experienced on this drive were a improved to provide enough power when encountering rock.
result of the choice of TBM, the failure to establish the correct
operating pressures and inadequate procedures to maintain A slurry TBM may have been helpful to increase the tunnelling
face stability. These are fundamental issues for tunnelling in performance in zones 6 and 8, where heavy over-excavation
the very onerous and abrasive mixed-ground conditions that occurred. The highly weathered granite in both zones remained
were identified in the ground investigation prior to tunnelling. stable as undisturbed soil, but was extremely easy to disturb by
Given the demonstrated limitations of the TBM and the oper- water alternation or ground disturbance. Therefore, a slurry
ating procedures, the contributor considers it unlikely that TBM or a pressurised slurry support system would be rec-
refining the knowledge of the ground conditions and cutter ommended to maintain the stability of the tunnel face and
wear, as recommended by the authors, would have prevented mitigate ground settlement. However, a slurry TBM requires
the problems that occurred during tunnelling. both water supply and slurry treatment. Since the case reported

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 Geotechnical Engineering Discussion: Challenges of earth pressure
Volume 174 Issue 1 balance tunnelling in weathered granite
with boulders
Zhang, Shen, Zhou, Lyu and Shirlaw

Guang-He
expressway

North line South line North line South line

(a) (b)

Figure 20. Localised tunnel path through (a) expressway and (b) residential area

in the paper was in the centre of Guangzhou city, it was excavated material in the soil chamber, which further clogged
difficult to arrange a slurry treatment device in this location. the cutter head opening and resulted in heavy flat wear of the
Figure 20 shows the localised tunnel path in the urban centre. disc cutters on the centre part of the cutter wheel. Accordingly,
As mentioned in the paper, some tunnel projects in Shenzhen the authors recommended increases in the dosage of the con-
have employed a slurry-type TBM; for example, a slurry-type ditioning agent and the concentration of the foaming agent
TBM is being used on Chunfeng tunnel, under construction. solution in order to decrease cutter head clogging in the highly
weathered granite stratum.
Tunnel face pressure
During the tunnelling process of each ring, the thrust cylinder Completely weathered granite is sensitive to ground disturb-
strokes, which indicate the advancing distance of the machine, ance and water alternation and shows poor stability for
were carefully monitored by the TBM operators. In general, boulders to be cut in position. A high face pressure will stabil-
these strokes should correspond to the volume of the excavated ise the tunnel face, but will also lead to the formation of mud
earth estimated by counting skips. Inadequate strokes indicate cakes.
that over-excavation is probably occurring. In this case, the
TBM is ceased and surface grouting is expected to be imple- Wash-out problem
mented. Therefore, control measures can be adopted before The authors agree with the discusser that the tail void caused
significant geohazards are observed. by the wash-out of grout resulted in the segment uplifting in
the full-face rock strata, which might have led to the elliptic-
The face pressure setting is very important for maintaining shape deformation of the segments. This aggravated the
tunnel face stability. As mentioned in the paper, spewing groundwater leakage through the joints. A modified grout with
mainly occurred in zone 7 of the full-face rock strata. The increased proportions of sand and fly ash could reduce the
advance rate was very low here (5–10 mm/min) due to the low initial setting time to ease the wash-out problem (Zhang et al.,
penetration rate when cutting rock. The excavated material was 2018).
mainly a mixture of rock powder and groundwater, which
could not be transported by the conveyor belt. As an exper- Concluding remarks
iment, the applied face pressure was adjusted up to 3.8 bar A slurry TBM is preferable in mixed-face strata of completely
(380 kPa), which was higher than the water pressure, by and highly weathered granite owing to its capability of fine-
adding air pressure. However, the pressure in the excavation tuning face pressure, maintaining face stability and reducing
chamber was hard to maintain for the entire tunnelling ground settlement. However, site conditions and environmental
process, and spewing occurred during pressure relief. factors become the major concerns for the selection of TBM
Therefore, drainage through grouting holes in the segment was type. In the project considered in the paper under discussion
adopted on site before tunnelling the next ring. (Zhang et al., 2020), there was not enough space for slurry treat-
ment equipment (e.g. a mud pond) as the construction site was
The face stability in zones 6 and 8 was very poor. The increase in an urban centre. In addition, the transportation of slurry is
in the applied face pressure subsequently aggravated the for- subjected to regulatory limitations during the daytime in the city
mation of mud cakes. This can be attributed to the increase of centre. Large slurry treatment equipment may have also had an

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 Geotechnical Engineering Discussion: Challenges of earth pressure
Volume 174 Issue 1 balance tunnelling in weathered granite
with boulders
Zhang, Shen, Zhou, Lyu and Shirlaw

impact on the surrounding environment, for example ground construction-troubles-and-Phase-II-preparations.php (accessed

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Ren DJ, Shen SL, Arulrajah A and Cheng WC (2018b) Prediction model
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