Tunnel instrumentation 237 0 25 50 75 100 125 150 0 10 20 30 3m dia. Tunnel 2 31 3m 1 2 3 Time, Days Borehole Extension, mm Fig. 14.8 Variation of borehole extension with time. Giri Hydeltunnel through crushed phyllites which squeezed due to high cover pressure of about 300 m. Two extensometers of 5 and 2.5 m depths were installed on the left wall and three extensometers of 7.5, 5.0 and 2.5 m depths were installed on the right wall. No extensometer could be installed on the roof. Tunnel closures were also measured. The data were analyzed and radial displacements u r were plotted against radial distance r for various time intervals as shown in Fig.14.9. The convergence of u r −log r plots at point indicates stabilization of the broken zone between 200 to 300 days after excavation. The broken zone radius (b) at this period was found to be 20.7 and 20.3 m on the left and right wall, respectively. (It can be noted that the radial displacements vs. time curves tend to converge at some radial distance which is believed to be the interface between broken zone and elastic zone within a squeezing ground condition.) The steel ribs buckled after 300 days. This produced a spurt in radial displacements and the broken zone started widen- ing again as indicated by the divergence of u r −log r plots in Fig.14.9. The example clearly shows the usefulness of multi-point borehole extensometers to monitor the development of broken zone around a tunnel under squeezing ground conditions. 3.0 2.0 1.0 0 02351020 b = 20.7 800 400 300 200 100 50 20 Days (a) Left wall Radial Distance, m Radial Displacement, cm 3.0 2.0 1.0 0 0 2 3 5 10 20 b = 20.3 800 400 300 200 100 50 Days (b) Right wall Radial Distance, m Radial Displacement, cm 20 Fig. 14.9 Variation of radial displacement with radial distance within phyllites in Giri Hydeltunnel (a =2.12 m and b =radius of broken zone in squeezing ground). 238 Tunnelling in weak rocks 0 400 800 1200 1600 2000 2400 Time, days −0.50 0.00 1.00 2.00 3.00 Relative displacement between anchors, mm 0.73mm 0.024mm/month 0.00mm/month Anchors 1 and 2 Anchors 2 and 3 Installation of longer rock bolts 7.8.84 1985 1986 1987 1988 1989 1990 25.2.91 23m 57m EL(-)12m EL 45m 1 2 3 Agglomerate Band EL 48.5m 47m EL 93m Fig. 14.10 Monitoring agglomerate band behavior with the multi-point borehole extensometer in the roof of a large underground cavity, India (Goel, 2001). 14.8.6 Observation by borehole extensometer in large underground cavity In one of the large underground opening projects, for example, it has been possible to monitor the roof displacement of 0.024 mm/month (Fig. 14.10). The deformation remains continued for almost 30 months. At this point of time, additional supports of longer rock bolts were installed andsubsequentlyit was observed that the roof movement/displacement had stopped. 14.9 LAYOUT OF A TYPICAL TEST SECTION Layout of an extensively instrumented zone is shown in Fig.14.11. Measurements taken consist of following robust and valuable instruments. (i) Radial support pressure by pressure cells (ii) Load on support by load cells (iii) Depth of loosened rock mass by multi-point borehole extensometers and (iv) Rock closure and support deformation by tape extensometer. Tunnel instrumentation 239 Pressure Cells Load Cells Pin for Support Deformation 0.33m 4-Point B.H. Extensometer 3.0m dia. Fig. 14.11 Layout plan of a typical instrumentation zone. Strain in support can be measured by strain gauges. Instrumentation in the lined and concreted zone should consist of the following: (i) Stress meters Embedded in concrete (ii) Strain meters Besides the above mentioned instrumentation, following data should also be collected: A. Geology – mapping, fracture spacing and orientation, width of fracture zone, alteration and groundwater B. Rock mass quality (Q), rock mass rating (RMR) and geological strength index (GSI) C. Geophysical observations – seismic activity, in situ stresses and their orientation, micro-seismic activity inside opening. Significant researches have been done on the basis of field data from the instrumented tunnels in past. One is missing great opportunity by avoiding the tunnel instrumentation and not collecting new field data, specially in complex geological conditions. REFERENCES Fairhurst, C. (1994). Lecture. Civil Engineering Department, I.I.T., Roorkee Goel, R. K. (2001). Status of tunnelling and underground construction activities and technologies in India. Tunnelling and Underground Space Technology, 16, 63-75. Kastner, H. (1962). Statik does Tunnel - and Stollen Baues. Springer Verlag, Berlin/Gottingen/ Heidelberg. Merrill, R. H. (1967). Three component borehole deformation gauge for determining the stress in rock. Bu. Mines Rep. of Inv., 7015, 38. This Page is Intentionally Left Blank 15 Tunnelling machines “Any manager of a project must understand that his success depends on the success of the contractor. The contractors have to be made to succeed. They may have many problems. We cannot always talk within the rigid boundaries of a contract document. No, without hesitation. I go beyond the contract agreement document.” E. Sreedharan, Managing Director, Delhi Metro Rail Corporation 15.1 GENERAL The age-old drill and blast technique is still being used in poor countries due to choice for labor-friendly policies. The time has come for change. We should prepare ourself men- tally for change and for a fast rate of progress also. The applications of modern techniques like NATM and NTM involving automated excavation methods are the need of time. Fig. 15.1 depicts a variety of methods of excavation as a function of strength of rock material (Jethwa, 2001). Table15.1 shows comparative study of the available techniques for tunnelling vis-à-vis some of the important parameters like cost, advance rate of tun- nelling, utilization of money and geometric requirements of a tunnel. A judicious selection of tunnelling technology may be made with the help of Table 15.1 depending upon the culture of a nation. Some nations in Asia prefer to evolve slowly for sustainable growth for a very long time. 15.2 SYSTEM’S MIS-MATCH An effort to increase the rate of tunnelling requires a system’s approach. The system in totality should be improved, specially the weakest link which is the installation of support system in weak rock masses. For example, excavation by a road header will be meaningless if steel-arch supports are not replaced by SFRS (steel fiber reinforced shotcrete) support for weak rock masses. A tunnel boring machine is stuck in a thick fault or shear zone in a complex unknown geological condition, burying the machine. Excessive failure of tunnel face causes jamming of excavating head. So the choice of selection of Tunnelling in Weak Rocks B. Singh and R. K. Goel © 2006. Elsevier Ltd 242 Tunnelling in weak rocks 0 7 70 140 275 Soft ground techniques Road header Tunnel boring machine Drill and blast UCS, MPa Fig. 15.1 Tunnel excavation methods as a function of rock strength (Jethwa, 2001). tunnelling machine depends upon the complexity of geological conditions, poverty of a nation and management conditions. It is wiser to insure TBM always. Unfortunately, active participation of a rock engineer is conspicuously absent from planning to commissioning of the tunnelling projects in many nations. This results in geological surprises which have to be paid for in terms of both time and cost over-runs. There is a great fallacy that automated tunnelling is costlier. It is not true. With advent of modern tunnelling machines, though the initial investment is high, the recurring cost is relatively low in long tunnels (>2 km), except in soft ground tunnelling (Table 15.1). Further, the tunnelling project is completed in shorter time and starts giving economic return much earlier which helps in reducing the cost of interest on the capital investment. It is painful to know that construction of hydroelectric projects is delayed greatly due to the delay in completion of very long and complex tunnel network. Hence, the justification for adopting tunnelling machines but judiciously. 15.3 TUNNEL JUMBO The tunnel jumbo usually consist of light rock drill of high performance which are mounted on a mechanical arms. These arms are moved by hydraulic jacks. The wheeled jumbo is mobile and fast. Initial cost is only a small portion of the overall cost of tunnelling. All booms can be used to drill upwards, downwards, besides horizontally. The number of booms can go up to seven (which was used in Daniel Johnson dam in Canada). The rate of tunnelling goes up with more number of booms and the cost of jumbo also goes up. The main advantages of modern jumbos are: • Faster rate of penetration of drills • Quick realignment of booms (arms) • Versatility of boom movements • Maneuverability of carrier • Low power consumption Tunnelling machines 243 Table 15.1 Comparative study of different techniques for tunnelling projects (Jethwa, 2001). Parameters Conventional drill and blast Automated drill and blast in conjunction with NATM/NTM Roadheaders Soft rock TBMs Hard rock TBMs Cost Initial • ••• •••• ••••• ••••• Running ••••• •• •• ••••• •• Rate of advance Favorable ground 50–60 m/month 200–700 m/month 350–800 m/month 150–300 m/month 500–1500 m/month Unfavorable ground 7–10 m/month 50–60m/month 75–150 m/month 25–50 m/month 100–200 m/month Utilization of money Overall Very inefficient Good Best Best Best Space at face Least Moderate Moderate ••••• ••••• Geometric requirements Shape of tunnel Any Any Horse-shoe Circular Circular, horse-shoe, rectangular Cross section of tunnel Any Any 2.5–10 m 1–10 m 2–13 m Maximum gradient Any Upto 30 degree 15 degree <10 degree <10 degree Applicability Geology Universally applicable Universally applicable Sensitive to change Very sensitive to change Very sensitive to change Rock strength All strength All strength Medium hard to hard Soft and clayey Hard to very hard Operational parameters Ground disturbance and overbreak •••••• Moderate with good controlled blasting measures Least Moderate to least Least Operator’s skill • Moderate ••••• •••••• ••••• Support requirement •••••• Very high to high •• High to low •• Speed of work • Good to very good •••••• Fair Extremely good Public safety • •• ••••• Moderate ••••• Quality of work • Poor to good •••••• ••••• •••••• Hard Rocks – Automated D&B with NATM/NTM; Soft Rocks Roadheaders with NATM/NTM. • – Very Low, •• – Low, •••– Low to medium, ••••– Medium to high, •••••– High, ••••••– Very high. 244 Tunnelling in weak rocks • Longer bit and steel life • Considerably less noise • Improvement in environmental conditions The vertical drilling mechanism is used for drilling boltholes and horizontal booms are used for drilling blast holes. 15.4 MUCK HAULING EQUIPMENT Efficient removal of excavated rock blocks (muck) is an important operation. Use of belt conveyers is very economical and efficient. Belt conveyers load into the muck cars hauled by diesel, electricity or battery. As the area available is limited in a tunnel driving the mucking equipment should occupy minimum working space. Rail track should be well laid on rock mass and should be maintained well for efficient operation. The rail lines move upwards in squeezing rock conditions or swelling rocks. In former case, rock anchors should be installed in the floor and shotcreted using SFRS. In the latter case, swelling of rocks should be prevented by spraying shotcrete immediately all round the tunnel including the floor to prevent ingress of moisture inside the rock mass. However, the inverts delay mucking. Fig. 15.2 shows Haggloader 10 HR which is mounted on a rubber tired chassis. It is more mobile than other Haggloaders. It uses digging and gathering arms in the front of the machine. The muck is brought into the transport equipment by a conveyer (shown by inclined line). This model is highly efficient and safe for the operator. The classic books of Singh (1993) and Bickel and Kuesel (1982) describe various other machines used for tunnelling operations. Fig. 15.2 Haggloader 10 HR, principal data. Tunnelling machines 245 15.5 TUNNEL BORING MACHINE (TBM) After nearly 150 years of development, the TBM has been perfected to excavate in fair to hard rock masses. The TBM has the following technical advantages. • Reduction in overbreaks • Minimum surface and ground disturbance • Reduced ground vibrations cause no damage to nearby structures, an important consideration for construction of underground metro • The rate of tunnelling is several times of that of drill and blast method • Better environmental conditions – low noise, low gas emissions, etc. • Better safety of workers Engineers should not use TBM where engineering geological investigations have not been done in detail and the rock mass conditions are very heterogeneous. Contractors can design TBM accordingto the given rockmass conditions which arenormally homogeneous non-squeezing ground conditions. TBM is unsuitable for the squeezing or flowing grounds (Bhasin, 2004). The principle of TBM is to push cutters against the tunnel face and then rotate the cutters for breaking the rocks in chips (Fig. 15.3). The performance of a TBM depends upon its capacity to create largest size of chips of rocks with least thrust. Thus, rock chipping causes high rate of tunnelling rather than grinding (Kaiser & McCreath, 1994). The rate of boring through hard weathered rock mass is found to be below expectation (see Chapter 16). Disc cutters are used for tunnelling through soft and medium hard rocks. Roller cutters are used in hard rocks, although their cost is high. A typical TBM is shown in Fig. 15.4 together with the ancillary equipment. The machine is gripped in place by legs with pads on rocks. The excavation is performed by a cutting head of welded steel and convex shape, with cutters arranged on it optimally. The long body of TBM contains the four hydraulic P Fig. 15.3 Mechanism of failure of rock by cutter (Bickel & Kuesel, 1982). 246 Tunnelling in weak rocks Cutters Cutting head bearing Drive motor Gripper pads Conveyor Cutting head Sprocket wheel meshes with ring gear on cutting head Thrust jacks Support leg Transformers Fig. 15.4 Tunnel boring machine and ancillary equipment (Bickel & Kuesel, 1982). Main legs Hydraulic thrust cylinders Cutting head Rear support legs Step 1: Start of boring cycle. Machine clamped, rear support legs retracted Step 2: Start of boring cycle. Machine clamped, head extended, rear support legs retracted Step 3: Start of boring cycle. Machine unclamped, rear support legs extended Step 4: End of reset cycle. Machine unclamped, head retracted. Machine now ready for clamping and beginning boring cycle Fig. 15.5 Method of advance of a rock tunnelling machine (Bickel & Kuesel, 1982). jacks to push forward the cutting head and also drive motors which rotate the cutting head for chipping rocks. Fig. 15.5 shows schematically a method of advance of the cutter head. This figure shows how TBM is steered and pushed ahead in self-explaining four steps. Typically even when a TBM operates well, only 30 to 50 percent of the operating time is spent on boring. [...]... of Germany 6. 490 m 3.8 m (7 m including tail skin) 252 MT 57 m 1 to 7 rpm 4000 kNm Precast segmental RCC 5. 7 m 6 (5+ 1 key) 280 mm 1.2 m M- 45 16 tons EPBM gasket and hydrophilic seal 3 MW for each machine 10 m per day 28.8 m per day Herrenknecht of Germany 6. 490 m 3.9 m (6. 9 m including tail skin) 3 25 MT 70 m 1 to 6 rpm 4377 kNm Precast segmental RCC 5. 7 m 6 (5+ 1 key) 280 mm 1.2 m M- 45 16 tons EPBM... Location Infrastructure (a) Total (b) Ratio (a)/(b) At grade (surface) Elevated (super structure) Long span bridge Cut and cover Tunnelled 25 100 250 100 to 200 150 to 50 0 30 30 30 40 50 55 130 280 140 to 240 200 to 55 0 0. 45 0. 75 0.90 0.70 to 0.80 0. 75 to 0.90 Metro tunnels 259 Fig 17.1 Precast lining in a metro tunnel (Ref: http://www.railwayage.com/sept01/ washmetro.html) The work culture of Delhi Metro... involving partial or complete reconstruction Beams lose bearing, walls lean badly and require shoring Windows broken by distortion Danger of instability 0.1 to 1 1 to 5 Max tensile strain (%) due to subsidence Less than 0. 05 0. 05 to 0.0 75 0.0 75 to 0. 15 0. 15 to 0.3 5 to 15 or a number of cracks greater than three 15 to 25 but also Greater than 0.3 depends on number of cracks Usually greater than 25 but... suggested to use σcm when the angle is more than 45 degree and σtm in case the angle is less than 45 degrees It may be noted here that penetration rate is more in case the angle is zero degree σcm = 5 · γ Q1/3 c ( 16. 2) σtm = 5 · γ Q1/3 t ( 16. 3) where Qc = Q · qc /100, ( 16. 3a) Qc = Q · qt /100, ( 16. 3b) = Q · (I50 /4) and γ = Density in gm/cm3 Equations ( 16. 2) and ( 16. 3) for the estimation of σcm and σtm are... 255 Equation ( 16. 10) also demonstrates instability in fault zones, until (−)m is reduced by pre- or post-treatment Example Slate QTBM ≈ 39 (from previous calculations with 15 tnf cutter force) From equation ( 16. 7), PR ≈ 2.4 m/h Since Q = 2, m1 = −0.21 from Table 16. 1 If the TBM diameter is 8 m and if CLI = 45, q = 5% and n = 1%, then m ≈ −0.21 × 1.1 × 0.89 × 0.87 × 0.97 = −0.17 from equation ( 16. 6)... estimate of QTBM is as follows: QTBM = 8.8 15 1 0 .66 20 20 15 × × = 39 × × × 10 × 9 6 1 1 20 20 5 15 20 According to Fig 16. 2, QTBM ≈ 39 should give fair penetration rates (about 2.4 m/h) If average cutter force were doubled to 30 tnf, QTBM would reduce to a much more favorable value of 0.04 and the PR would increase (by a factor 22 = 4) to a potential 9 .6 m/h However, the real advance rate would depend... follows: AR ≈ 5 (QTBM )−0.2 · T m ( 16. 8) One can also check the operative QTBM value by back-calculation from penetration rate: QTBM ≈ 5 PR 5 ( 16. 9) 16. 6 ESTIMATING TIME FOR COMPLETION The time (T ) taken to penetrate a length of tunnel (L) with an average advance rate of AR is obviously L/AR From equation ( 16 .5) , one can therefore derive the following: T = L PR 1/(1 + m) ( 16. 10) Rock mass quality for tunnel... force (Barton, 20 05) Example Slate Q ≈ 2 (poor stability); qc ≈ 50 MPa; I50 ≈ 0 .5 MPa; γ = 2.8 gm/cm3 ; Qc = 1; and Qt = 0. 25 Therefore, σcm ≈ 14 MPa and σtm ≈ 8.8 MPa Rock mass quality for tunnel boring machines (QTBM ) 253 The slate is bored in a favorable direction, hence consider σtm and RQD0 = 15 (i.e., . advance Favorable ground 50 60 m/month 200–700 m/month 350 –800 m/month 150 –300 m/month 50 0– 150 0 m/month Unfavorable ground 7–10 m/month 50 60 m/month 75 150 m/month 25 50 m/month 100–200 m/month Utilization of. planar, unaltered). The estimate of Q TBM is as follows: Q TBM = 15 6 × 1 1 × 0 .66 1 × 8.8 15 10 20 9 × 20 20 × 20 20 × 15 5 = 39 According to Fig. 16. 2, Q TBM ≈39 should give fair penetration rates (about. rate: Q TBM ≈ 5 PR 5 ( 16. 9) 16. 6 ESTIMATING TIME FOR COMPLETION The time (T ) taken to penetrate a length of tunnel (L) with an average advance rate of AR is obviously L/AR. From equation ( 16 .5) , one