Mixed Face Conditions and the Risk of Loss of Face in Singapore

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1 Mixed Face Conditions and the Risk of Loss of Face in Singapore J.N. Shirlaw, Golder Associates (Singapore) Pte. Ltd. ABSTRACT: Tunnelling using pressurised face Tunnel Boring Machines (PTBM), such as Earth Pressure Balance (EPB) and slurry shields, generally allows good control of surface settlement. However, there are reports from many projects of occasional, localised, sudden, and very large settlements or sinkholes over PTBM driven tunnels. There are a number of readily identifiable risk factors associated with sudden, very large, localised settlements. The relative influence of these risk factors is identified using detailed data from seven major EPB projects. Ground conditions consisting of a mixed face of soil and rock are particularly problematic. Sample calculations for face pressure in different ground conditions show that the face pressure required to avoid collapse in granular soils, including some high SPT soils derived from weathered rocks, are higher than those required to avoid collapse in a similar tunnel in marine clay. Mixed faces also result in high cutter wear/damage, frequent interventions into the excavation chamber, slow advance rates and dynamic loads from vibration. All of these, in combination, help to explain why mixed face conditions continue to pose particular problems for tunnelling. 1 INTRODUCTION Settlement caused by tunnelling is a major issue in Singapore. One of the advantages of the Earth Pressure Balance (EPB) and Slurry tunnel boring machines that have been introduced in the last few decades is that surface settlements can generally be kept to a very small magnitude. However, local, large settlements or sinkholes have also been recorded on projects using these types of machines. This is illustrated in Figure 1, based on the settlements recorded over the EPB driven tunnels on the North East line. As shown in the figure the results generally showed very good control of settlement, with over 80% of the monitored results giving volume loss values of less than 1% (equivalent to about 14mm settlement over a 6.6m diameter tunnel at a depth of 20m). However, there were a small proportion of measurements that gave a much higher value for volume loss. There were sixteen occasions where there was a settlement of over 150mm, or a sinkhole, over the tunnel. One of these occasions was captured in the monitoring results, the other fifteen were not. An example of a sinkhole from the tunnelling for the North East Line is shown in Figure 2 The large settlements/sinkholes were generally the result of loss of ground at the face of the tunnel. There must have been inadequate face pressure and over-excavation to result in such large settlements. As discussed in Shirlaw and Boone (2004), the number of cases of large, visible settlements (over 150mm) and sinkholes have been recorded for seven major projects involving EPB tunnelling (an updated version is given in Table 1). There were 59 incidents in 86km of tunnelling, an average rate of one incident per 1.46km. 1

2 Cases, percent cases of sinkholes / v. large settlements 0.0 < to to to to to to to 4.99 >5 Volume Loss, percent Figure 1. A summary of the volume losses recorded at 485 monitored settlement points over EPB tunnels, North East Line, Singapore Figure 2. A sinkhole over an EPB tunnel being driven in the weathered rock of the Jurong Formation Table 1. Cases of large (>150mm) settlement or sinkholes over EPB driven tunnels on seven major projects Tunnel Year Length Incidents No./km Singapore MRT Phase St Clair River, Canada Allen Sewer, Canada Sheppard Line, Canada Changi Line, Singapore North East Line, Singapore DTSS Tunnels, Singapore International Conference on Deep Excavations (ICDE 2008)

3 Since the completion of the EPB tunnelling for the NEL and the DTSS there has been the occasional development over a sinkhole over EPB and slurry shield drives for the Circle Line (for examples see Anon (2007a and 2008)). There have also been records of similar problems on other PTBM projects elsewhere (Anon 2007b). 2 COMMON FACTORS IN LARGE SETTLEMENTS/SINKHOLES As discussed in Shirlaw et al (2003), settlements recorded over tunnels in Singapore show that most of the cases of large settlement/sinkhole formation were directly related to the use of an inadequate face pressure. All of the incidents were localised, so the issue is why, suddenly, the machine operator failed to apply adequate face pressure when, for the rest of the tunnelling, sufficient pressure was applied. There appear to be a number of areas where the risk of large settlement/sinkhole is particularly high, as shown in Table 2. Table 2. Large settlement/ sinkhole cases for seven major projects, divided by main risk areas Risk area Number Launching the shield 10 Breaking into recovery shaft 4 Interfaces between stable and unstable soils 6 Mixed faces of rock, or hard cohesive soil, and granular soil 15 Head access for maintenance & cutting tool changes 15 Mechanical failures 5 Others 4 Total 59 It can be seen that the three major factors are: launching, mixed face conditions and head access. The latter two factors can be linked, as the number of maintenance and tool change interventions per unit length of tunnel is much higher in mixed face conditions of rock and soil than in uniform conditions of either rock or soil. So the question is why mixed faces pose a greatly increased risk of major loss of ground during PTBM tunnelling. In this paper, tunnelling through mixed faces of the soil and rock grades of weathered Bukit Timah granite in Singapore will be reviewed. The general conditions giving rise to this condition will be reviewed first. The face pressures needed to keep the face stable, based on theoretical calculations, will be compared with that for some other common ground conditions in Singapore. Finally, the particular issues related to head access will be discussed. 3. MIXED FACE CONDITIONS IN WEATHERED BUKIT TIMAH GRANITE The geological formation termed the Bukit Timah Granite covers a suit of igneous rocks, and some metamorphosed granitic rocks. Apart from granite, the suite of rocks includes (inter alia) granodiorite, micro-granite, adamellite and gneiss. The broad term Bukit Timah granite therefore covers a wide variety of materials in terms of the composition and size of the mineral grains. Deep tropical weathering has resulted in the formation of a thick layer of residual soil (grade VI) at the top of the formation. Below the grade VI granite is, typically, a thin zone of grade V and IV granite, which is generally coarser and more permeable than the overlying residual soil. The transition between the soil grades V/IV and the rock grades (I to III) may be abrupt, or there may be corestone weathering, as shown (idealised) in figure 3. There is considerable variation in the rock/soil grade contact zone, both in terms of the nature and thickness of the grade V/IV, and whether the transition into rock is abrupt or of the corestone type. In some cases the residual soil (grade VI) may transition straight into the rock grades (I to III), with no grade V or IV between. More commonly there is a 0.5m to 2m thick layer of permeable (10-6 to 10-5 m/s) zone of grade V/IV between the rock grades and the residual soil. International Conference on Deep Excavations (ICDE 2008) 3

4 Cohesive soil (grade VI) Sandy soil (Grades IV and V) Particularly close to rock level Figure 3. Idealised profile of rock/soil interface in a corestone weathering profile in Singapore 4 SAMPLE CALCULATIONS FOR SUPPORT PRESSURE AT THE FACE OF THE TUNNEL Simple calculations for the required support pressure at the face of the tunnel can be carried out using the results of centrifuge tests for model tunnels, or by published charts. Here, the results of centrifuge tests on model tunnels, summarised by Kimura and Mair (1984), are used for cohesive soils, and the published charts by Anagnostu and Kovari (1996) for granular soils. For the granular soils it is assumed that there is no seepage towards the tunnel. This requires either that there is a filter cake (for slurry shields) or that the spoil is mixed and conditioned such that it has a significantly lower permeability than the undisturbed ground (for EPB shields). Grade V granite can be easily eroded (Shirlaw et al, 2000), so preventing seepage into the face is a reasonable basis for assessing the required face pressure. Calculations were carried out for the minimum support pressure required to avoid total collapse of the face of a 6m diameter tunnel, with the axis level of the tunnel at a depth of 20m below ground level. Four different soil types were assessed, with the assumed parameters given in Table 3. The calculated minimum support pressures, at tunnel axis level, are also provided in Table 3. No additional load from buildings or surcharge was considered. For the granular soils, a ground water table 1.5m below ground level is assumed. In each case it is assumed that the ground is uniform to ground level. Table 3. Soil parameters used, and the resulting minimum support pressure to avoid total collapse, in the example calculations Soil type C U kpa degrees c kpa kn/m3 Minimum Support Pressure at tunnel axis level, in kpa,to avoid total collapse at the face Marine clay * Residual Soil Fluvial Sand Grade V granite * For the Marine Clay, increasingly large settlements will develop below a face pressure of about 200kPa, in this example. Shirlaw et al (2003) show that the settlement measurements resulting from tunnelling for the North East line were consistent with such calculations. It is notable that the pressure required to avoid total collapse of the face in the grade V granite, with an SPT typically over 30, is higher than that in Marine Clay, and not much different to that in fluvial sand, with an SPT of 5 to 10. This is because al- 4 International Conference on Deep Excavations (ICDE 2008)

5 most all of the support pressure is required to resist the water pressure. The difference in behaviour, and required support pressure, between the residual soil (grade VI weathered granite) and the grade V/IV is also very significant. The calculations above are for the bare minimum pressure for face stability, and so consideration needs to be given to; The variability of the face pressure The effects of vibration, which can be an additional destabilising effect in the granular soils Any loads from buildings Neither of these effects is considered in the example calculations. In weathered granite, failure to maintain an adequate pressure can be due to, inter alia: Inability to create a suitable spoil (for EPB operation) out of a mixture of cut rock and granular soil Unexpectedly breaking into a mixed face of soil and rock, from a full face of rock, without the required pressure to stabilise the soil Mechanical problems with the machine, including wear of the screw conveyor (for EPB shields), blockages (for slurry shields) or failure of the main pressure bulkhead (for both EPB and slurry shields) Human error, either by the operator not applying the required pressure, or by a failure to communicate the correct pressure to the operator Calculation error. As the water pressure is dominant in the calculations for granular soils, lack of information on the actual water pressure can be a cause of such error 5 THE RISK OF LOSS OF GROUND DURING HEAD ACCESS IN MIXED FACE CONDITIONS Head access is required very frequently in mixed rock and soil conditions in weathered granite. This is because: Granite is highly abrasive and causes wear to the cutting tools and other moving parts Small boulders or pieces of rock that enter the excavation chamber can cause damage or blockage The transition from soil to rock results in impact damage to the disk cutters, as shown in figure 4. In mixed zones of very strong rock and soil, interventions can be required very frequently, possibly every 1m to 10m. In contrast, if the tunnel is in soft soil, such as the soils of the Kallang Formation, little or no maintenance or cutting tool changes may be required for a full tunnel drive. As discussed above, there is commonly a zone of grade V/IV weathered granite just at the soil/rock interface. This zone is generally significantly more permeable than the overlying residual soil, and behaves as a granular soil. This zone, if encountered, is unstable under the effects of seepage. As a result, either compressed air or ground treatment (grouting or freezing) need to be applied to stabilize the face and allow safe interventions into the head for maintenance when the rock/soil interface is at or close to the level of the tunnel. Ground treatment is typically carried out from the surface, and it takes several weeks to complete a section to allow access. It is therefore uncommon to use ground treatment for the type of ad hoc interventions required for cutting tool changes and routine inspections and maintenance. However, where the tunnel is in highly abrasive and damaging mixed zones of soil and rock over a significant length, it is worth considering creating a series of pre-installed blocks of treated ground that can act as safe havens and allow extended head access for major maintenance. International Conference on Deep Excavations (ICDE 2008) 5

6 Figure 4. Disk cutters damaged by tunnelling through mixed zones of soil and rock in weathered granite Compressed air is commonly used to allow ad hoc interventions into the excavation chamber. Prior to the introduction of PTBMs, compressed air was used, with open face shields, to tunnel in weathered granite in Singapore and Hong Kong (Shirlaw et al 2000). The compressed air pressure applied in weathered granite was typically set to balance the water pressure at a level between tunnel axis and invert. Compressed air, in this application, did not support the effective stresses in the soil. The use of a balancing compressed air pressure prevents the pore water from seeping towards the face. If a lower compressed air pressure is used and there is seepage, the seepage will cause increased effective stresses in the soil, reducing stability. Grade V granite is also easily eroded by seepage (Shirlaw et al 2000), and loss of ground through erosion will further reduce the stability of the face. Compressed air pressure is constant over the height of the face, so a perfect balance cannot be achieved with water pressure which increases with depth. In practice, there is excess air pressure in the crown and a small under pressure at invert level. In sand, the excess pressure in the crown dries out the sand, which then starts to run. Grade V granite usually has some residual cementation, and is not pure sand, and so it is more stable under compressed air than a sand. However, lengthy interventions will still result is a gradual loss of stability, due to drying in the upper part of the face and slow erosion in the lower part. With open face shields, for long interventions, the ground was supported by either sprayed concrete or timber. For slurry TBMs, the filter cake formed by the bentonite on the face can help stability by acting as a separating layer between the compressed air and the soil. However, over long interventions the filter cake, unless renewed, can dry out and become ineffective. The risk of loss of ground during interventions is due to a number of factors, including; The temptation for the contractor to use a lower compressed air pressure than the static water level, due to the increased working time this gives in the chamber The change from slurry or EPB pressure support to compressed air; during this change, the pressure in the head may vary more than under normal TBM driving Gradual loss of face stability over time, as discussed above The destabilising effects of vibration on face stability; the vibration can be due to turning the head to access alternate cutting tools or at the restart of tunnelling. Cutting into a mixed face of soil and rock leads to greater vibrations than excavation in soil 6 International Conference on Deep Excavations (ICDE 2008)

7 In mixed face of strong or very strong rock, the slow progress and frequent interventions means that an local loss of ground may migrate upwards before it can be grouted at the tail of the TBM The local conditions in a mixed face of weathered Bukit Timah granite vary considerably: the soil grades can exhibit significant variation in terms of degree of weathering and residual cementation, and the Bukit Timah granite includes a variety of igneous rocks, resulting in variation in the nature of the resulting saprolites. It is therefore possible to get away with particular procedures and compressed air pressures for some interventions, but not for others. 6 CONCLUSIONS A review of a number of PTBM tunnelling projects in Singapore shows that there have been a significant number of cases of large, but generally localised, settlements or sinkholes. There is a particular risk for excessive loss of ground when tunnelling through a mixed face of soil and rock, both during normal excavation and during the, often frequent, interventions into the excavation chamber. Overall, the rate at which cases of excessive ground loss have been occurring has tended to reduce as procedures have improved. However, there remains a higher than normal risk of excessive ground loss when tunnelling in a mixed face of rock and soil. PTBM tunnelling through the interface between rock and soil grades of weathered granite is difficult because: The stability of the face is highly sensitive to the face pressure used The adverse effect, on stability, of vibration during excavation in a mixed face The need for frequent interventions into the excavation chamber of the PTBM for maintenance and cutting tool changes Further improvements in working practices can further reduce the risk of loss of ground when tunnelling using a PTBM in mixed face conditions of rock and soil. However, the challenges posed by the geological conditions mean that there will continue to be an increased risk in these conditions compared with tunnelling in uniform conditions of soil or rock. Detailed site investigation, to identify where the interfaces are present, combined with judicious alignment design to minimise the length of the tunnel in mixed face conditions could help to further reduce the overall risk. REFERENCES Anon. (2007a). Sinking feeling along Telok Blangah Road. Straits Times, Singapore. August 20, 2007 Anon (2007b). KCR Tunnelling suspected in TST cave-in. South China Morning Post, 4 June Anon (2008) Circle Line work causes cave-in off Holland Road. Straits Times, Singapore. 25 May Shirlaw, J.N., Hencher, S.R and Zhao, J. (2000). Design and construction issues for excavation and tunnelling in some tropically weathered rocks and soils. Proceedings GeoEng2000 Technomic Publishing, PA, USA. Volume 1. November 2000, pp Shirlaw, J.N., Ong, J.C.W. Rosser, H.B., Tan, C.G, Osborne, N.H. and Heslop P.J.E. (2003) Local settlements and sinkholes due to EPB tunnelling. Proc. ICE, Geotechnical Engineering I 56, Issue GE4, October 2003 pp , pp Shirlaw, J.N., Boone, S. The risk of very large settlements due to EPB tunnelling. 12 th Australian Tunnelling Conference, Brisbane, April 2005 International Conference on Deep Excavations (ICDE 2008) 7