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INTERPRETATION OF TUNNEL INSTRUMENTATION DATA

Manoj Verman Lead Geotechnical Engineer, Golder Associates, Gurgaon–122 002, India. E-mail: mverman@golder.com

ABSTRACT: The paper highlights the importance of field instrumentation of underground structures and focuses on interpretation of instrumentation data which is one of the key elements of the instrumentation process. The role of instrumentation in support modification during tunnel construction and the importance of estimating tunnel convergence and support pressure that occurred before installation of the instruments have been explained with the help of rock mass-tunnel support interaction analysis. The usefulness of field instrumentation has been brought out with the help of case histories. 1. INTRODUCTION

Excavation of underground openings involves inherent difficulties in defining the geologic structures and related geotechnical properties. It is, therefore, necessary to evaluate tunnelling conditions as they are encountered and adjust tunnelling procedures accordingly. It is in this area of timely identification and evaluation of unforeseen rock and soil mass conditions that field instrumentation can help improve both safety and economy of the underground structures during construction and overall performance of the completed structures. One of the key elements of the instrumentation process is interpretation of the data available from the instruments. The paper deals with some aspects of data interpretation with a focus on easy to obtain parameters of convergence and support pressure.

2. TYPICAL INSTRUMENTATION USES FOR UNDERGROUND EXCAVATIONS

Instrumentation may be used for the following applications: (a) Identification of rock and soil mass properties, such as,

strength, deformability, anisotropy etc. (b) Observation of state of stress in rock mass (c) Observation of response of rock mass to disturbances by

construction and operation of the structure (d) Observation of adjacent structures or services either

affected or at risk due to tunnel construction (e) Identification of hazards (f) Working out remedial measures and verification of their

efficacy.

3. TUNNEL INSTRUMENTATION

In tunnelling, undetected or uncontrolled adjustments can cause special problems. Due to space constraint, even minor difficulties can result in major problems and hazards, causing cost and time overruns. An additional factor is the widening technological gap between state-of-art in tunnel excavation/ support and state-of-art in site characterization/prediction

ahead of face. Increased and more sophisticated use of instrumentation is one of the few means available for limiting continued widening of this important technological gap.

Tunnel instrumentation can be used to obtain well distributed and timely information on behaviour of rock mass, per- formance of support system, suitability of specific transition, intersections, portal areas, slopes and adjacent/overlying ground, structures & services. Basic instrumentation, consisting of instruments, such as, tape extensometer, borehole extensometers and load cells, has proven economical and effective in achieving the minimum but important objectives, namely, timely identification and evaluation of instability, performance monitoring of underground structure and providing basis for design modification during construction.

4. SUPPORT MODIFICATION DURING CONSTRUCTION BASED ON INSTRUMENTATION DATA

Monitoring of rock mass behaviour by field instrumentation is an important part of design and construction process of the observational method of tunnel support design which is based on “build-as-you-go” philosophy. The tunneling approach of New Austrian Tunnelling Method (NATM) relies heavily on continuous field monitoring.

The observational method of tunnel support design, based on field instrumentation, laid the foundation of what is called the “convergence-confinement method”. This method is based on the concept of rock mass-tunnel support interaction. It involves study of the interaction between rock mass and tunnel support using the ground reaction (response) and support reaction curves (Fig. 1) which represent the load-deflection behaviour of the rock mass and the support system respectively. The point of intersection of these curves represents the state of equilibrium, i.e., at this point the support pressure required to limit further tunnel deformation is balanced by the support pressure available from the support system. The convergence-confinement method is aimed at estimating the support pressure from the point of

IGC 2009, Guntur, INDIA

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intersection of the ground reaction curve and the support reaction curve.

The application of the convergence-confinement method depends on tunnel instrumentation to a great extent. The predicted ground reaction and support reaction curves (using approaches, such as those suggested by Verman et al. 1995a and 1995b) are revised throughout the tunneling process on the basis of actual measurements during construction, and the amount of support is accordingly increased or decreased, so as to achieve the desirable point of intersection or, in other words, a safe and economical support system. The point of intersection should neither be beyond the minimum value (denoted by point D in Fig. 1) of the required support pressure, nor be much ahead of this value. In the former case, the support will be unsafe and in the latter, uneconomical. In fact the curves may not intersect at all in the former case.

Fig. 1: Selection of Desirable Support from

Convergence-Confinement Method

5. TUNNEL INSTRUMENTATION

There are following four broad phases of instrumentation: (a) Planning of instrumentation scheme (b) Fabrication, calibration, testing and supply of instruments (c) Installation of instruments (d) Collection and Interpretation of data.

All these phases affect the performance of instrumentation and it is important to be careful at all these stages for achieving meaningful results from an instrumentation scheme. The focus in this paper is on interpretation of data.

6. INTERPRETATION OF INSTRUMENTATION DATA

Data interpretation is ideally carried out more or less simultaneously with data collection. But, more often than not, we have a situation where un-analysed data are put away in files with intention of making the analysis later. These un-analysed data can conceal the beginnings of dangerous

behaviour which may eventually become apparent from visual inspection.

6.1 Influence of Data Collection on Data Interpretation

Quality of data available for interpretation is adversely affected by carelessness at data collection stage. Data collection must be made by a trained person with proper Read-out unit in specified way at regular intervals. Importance of raw data must be realized. Observed data must be recorded with utmost care along with details of nearby construction activities, if any.

6.2 Data Interpretation—A Few Tips

Anybody who is engaged in interpretation of instrumentation data would do well to remember the following: • Good or bad, data always speak. One has to “read between

the lines” • Bad data may tell whether:

• Instrument is behaving erratically, or • Data is/are manipulated, or • Data has/have been incorrectly recorded unintentionally • Good data may be used to serve the desired purpose of

instrumentation • All this requires a careful and intelligent analysis of data • Insist on raw data • Cross-check the “unexpected” data with data from nearby

instruments before rejecting it as “incorrectly recorded”. Be doubly sure. Keep sufficient degree of redundancy of instruments for this purpose

• Analyse the data with clear purpose in mind. In any case, first of all look for signs of instability.

6.3 Determination of Unrecorded Instrumentation Data

6.3.1 Unrecorded Data

It is often not possible to commence the closure or load cell observations immediately after the excavation or, in other words, right at the face. This is due to the time consumed for installation of closure bolts and the fact that the protruding part (for attaching the tape-extensometer) of the closure bolt is often found bent or broken when installed close to the face as a result of the fly rock hitting the closure bolts during blasting. Same is the case with load cells which not only take time for installation (the load cells are installed in the support system—like steel ribs or rock bolts—which is normally not installed immediately after the excavation because of practical difficulties), but, like the closure bolts, are also exposed to flying rock pieces during blasting when installed close the face.. Thus, valuable information regarding the initial tunnel closure and support pressure immediately after blasting is almost always lost. To overcome this problem, a graphical method was adopted by Verman (1993) to determine the unrecorded data.

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6.3.2 Estimation of Unrecorded Data

The unrecorded data may be estimated by extrapolating the trend line, as shown in Figure 2. This may be further explained with an example of the Maneri Bhali Stage-II tunnel. Figure 3 shows a plot of the radial tunnel closure with respect to time at Ch. 1568.75 m. The first observation was taken 12 days after the date of excavation. The missing data of the first 12 days were obtained by first plotting the data on a log-log scale and then by extrapolating the initially straight line portion of the curve (Fig. 4). Extrapolation of the straight line portion to 12 days (i.e., 12 days before the date of first observation) on log-log scale, and conversion of the extrapolated value to ordinary scale gives the value of the radial tunnel closure on the date of excavation as –0.16 cm. This implies that an additional value of 0.16 cm has to be added to the radial tunnel closure values to account for the missing data. The revised time versus radial closure curve is shown in Figure 5.

Fig. 2: Extrapolation of Recorded Data to Obtain

Unrecorded Data

Fig. 3: Radial Tunnel Closure Plotted with Time from Date of First Observation onwards at Ch. 1568.75 m in Maneri

Bhali Stage-II Tunnel

6.3.3 Significance of Determining Unrecorded Data

In the example of Maneri-Bhali Stage-II tunnel (Figs. 3 to 5), the unrecorded trunnel closure was found to be 25% of the maximum tunnel closure and occurred within 3.25% time taken for the maximum tunnel closure to take place (Fig. 5). Similar trends have been noticed in the instrumentation data from several tunnels. The high ratio of the unrecorded data as a percentage of the maximum data value occurring within a short time after excavation, underlines the importance of

estimating these missing data, without which the tunnel closures and support pressures are likely to be substantially underestimated.

Another important aspect of the unrecorded data is their influence on the observed support reaction curve and, therefore, on the point of intersection of the ground reaction and the support reaction curves.

Fig. 4: Radial Tunnel Closure Plotted with Time on log-log

Scale at Ch. 1568.75 m in Maneri Bhali Stage-II Tunnel

Fig. 5: Radial Tunnel Closure Extrapolated to Date of Excavation at Ch. 1568.75 m in Maneri Bhali Stage-II

Tunnel

This is shown in Figure 6. It is often seen that the dates of excavation, support installation, and first observation are different. The correct coordinates of the point of intersection “C”, as is clear from Figure 6, are XDOE and XDOSI, where XDOE is the final closure extrapolated to the date of excavation, and XDOSI is the final support pressure extrapolated to the date of support installation. This is because while the ground reaction curve starts at point “A” immediately after excavation (or even before excavation; according to Daemen 1975, some tunnel closure takes place ahead of the face), the support reaction curves comes into picture only after the supports are installed (denoted by point “B”). It would, therefore, be incorrect to extrapolate both the support pressure and the tunnel closure to the date of excavation to obtain the observed support reaction curve. Similarly, extrapolation of both the support pressure and the tunnel closure to the date of

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support installation would be incorrect. The support reaction curve obtained by using the “unextrapolated” or, only the recorded data is also incorrect in the same way.

Fig. 6: Influence of Unrecoreded Data on Support Reaction Curve (SRC) and its Point of Intersection

with Ground Reaction Curve

7. CASE HISTORIES

The author and his former colleagues at Central Mining Research Institute (CMRI, now known as CIMFR) and at Advanced Technology & Engineering Services (ATES) have been involved in instrumentation of several tunnels and caverns and have reported significant findings, especially in tunnels in Himalayas (Goel 2001; Jethwa et al. 1977; Jethwa et al. 1980; Verman 1993; Verman et al. 1997a; Verman et al. 1997b; Dube 1979; Dasgupta et al. 1999; Viladkar et al. 2008) where the recorded tunnel closures and support pressures are generally larger than those in more competent rock masses in the Indian peninsula. To highlight the usefulness of instrumentation even with low values of these parameters, some results are presented here briefly from the instrumentation of the caverns of Koyna Hydropower Project, Stage-IV (Maharashtra) and Sardar Sarovar Project (Gujarat).

The power house and transformer hall caverns of Koyna Hydropower Project, Stage-IV were instrumented with several borehole extensometers and rock bolt load cells (Verman & Jethwa 1996). Detailed analysis of the data obtained from each instrument was carried out. This involved the analysis of a large data base generated over a period of more than 5 years. This led to not only the routine findings, so essential for the design-as-you-go philosophy, but also threw up some significant results of special interest.

7.1 Warning about Impending Rock Fall

One of these results of special interest was the warning given by the instrumentation data about impending rock fall in transformer hall area. Data obtained from a multi-point borehole extensometer installed in the roof at Ch. 62.5 m are plotted with

time in Figure 7. Although the observation started 303 days after excavation of full cavern width at this section, the roof convergence was still showing a continuously rising trend, pointing towards instability. The rock bolts installed 77 days after the date of first observation were clearly inadequate and the rising trend continued till a chunk of “bolted” rock mass collapsed and the roof convergence stabilised. Thus, instrumentation provided enough warning about inadequate support and a timely action could have prevented the collapse.

Fig. 7: Trend of Roof Convergence in Power House Cavern

of Koyna Project, Stage-IV

7.2 Collapse Averted due to Instrumentation

In a similar situation at Sardar Sarovar Project, the warning sounded by instrumentation was heeded in time and collapse prevented. Here, the presence of an agglomerate band above the roof of the power house cavern had raised doubts about the roof stability due to the apprehension about opening up of the joints between the agglomerate band and surrounding basalt as a result of the cavern excavation (Verman et al. 1991). To keep a watch on the behaviour of agglomerate band, a borehole extensometer was installed from the top and going through the band and terminating close to the cavern roof (Fig. 8). Even after a period of more than 3 years, the

Fig. 8: Stabilising Effect of Longer Rock Bolts in Power

House Cavern of Sardar Sarovar Project

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roof convergence was showing a rising trend (Fig. 8), although at a low rate of only 0.024 mm per month. While the rate of increase of convergence was low, the trend was ominous and one (or both) of the joints of agglomerate band was slowly but surely opening up. This led to the decision of installing longer rock bolts in order to stitch the joints and it resulted in stabilising the rising trend (Fig. 8).

8. CONCLUSIONS

Instrumentation of und`rground openings during construction is vital to evaluate tunnelling conditions to adjust tunnelling procedures and support requirements accordingly. Correct interpretation of instrumentation data is crucial for the success of any instrumentation project. Properly interpreted data can yield valuable information about tunneling hazards and inadequacy or otherwise of the support system, thus leading to a safe and economical construction.

REFERENCES

Daemen, J.J.K. (1975). “Tunnel Support Loading Caused by Rock Failure”, PhD Thesis, University of Minnesota, U.S.A.

Dasgupta, B., Sharma, M.K.V., Verman, M. and Sharma, V.M. (1999). “Design of Underground Caverns for Tehri Hydropower Project”, India by Numerical Modelling, 9th International Congress on Rock Mechanics, Paris.

Dube, A.K. (1979). “Geomechanical Evaluation of Tunnel Stability under Failing Rock Conditions in a Himalayan Tunnel”, PhD thesis, Department of Civil Engineering, University of Roorkee, India.

Goel, R.K. (2001). “Status of Tunnelling and Underground Construction Activities and Technologies in India”, Tunnelling and Underground Space Technology, 16, pp. 63–75.

Jethwa, J.L., Singh, Bhawani, Singh, B. and Mithal, R.S. (1977). “Rock Pressure on Tunnel Lining in Swelling and Viscous Rocks”, Intern. Symp. Soil-Structure Interaction, Univ. of Roorkee, India, Jan. 1977, pp. 45–50, (Pub) M/s. Sarita Prakashan, Meerut, India.

Jethwa, J.L., Singh, B., Singh, Bhawani and Mithal, R.S. (1980). “Influence of Geology on Tunneling Conditions and Deformational Behaviour of Supports in Faulted Zones—A Case History of Chibro-Khodri Tunnel in India”, Engineering Geology, 16 (3 & 4), pp. 291–319.

Verman, M., Jethwa, J. and Singh, B. (1991). “Monitoring of a Large Underground Power House Cavity”, 7th International Congress on Rock Mechanics, Aachen, Germany.

Verman, M.K. (1993). “Rock Mass-Tunnel Support Interaction Analysis”, PhD Thesis, Department of Civil Engineering, University of Roorkee India, p. 267.

Verman M., Singh, B., Jethwa, J. and Viladkar, M. (1995a). “Determination of Support Reaction Curve for Steel-Supported Tunnes”, International Journal for Tunnelling and Underground Space Technology, Vol.10, No. 2, pp. 217–224.

Verman, M., Viladkar, M., Singh, B. and Jethwa, J. (1995b). “A Semi-Empirical Method for Design of Support Systems in Underground Openings”, International Journal for Tunneling and Underground Space Technology, Vol. 10, No. 3, pp. 375–383.

Verman, M. and Jethwa, J. (1996). “Instrumentation of Caverns of Koyna Hydro-electric Project”, Stage-IV, Indian Geotechnical Conference. Madras, India.

Verman, M., Singh, B., Viladkar, M. and Jethwa, J. (1997). “Estimation of Mobilised Cohesion around Underground Openings”, International Journal of Rock Mechanics & Mining Sciences, Vol. 34, No. 5, pp. 851–858.

Verman M., B. Singh, M. Viladkar and J. Jethwa. (1997b) Effect of Tunnel Depth on Modulus of Deformation of Rock Mass, International Journal of Rock Mechanics and Rock Engineering, 3, Vol. 30, pp.121–127.

Viladkar, M.N., Verman, M., Singh, B. and Jethwa, J.L.(2008) Rock Mass Tunnel Support Interaction Analysis—Part–II: Support Reaction Curves, Journal of Rock Mechanics & Tunneling Technology.