The excavation was conducted at shallow depth in a strong basaltic rock mass. ... C. L. 1997. Manual of tunnels and underground workings (in Spanish). Madrid.
EUROCK’2004 – Salzburg, Austria
A method for continuous monitoring of tunnel deformations during construction and service phases C. Dinis da Gama Professor and Head, Geotechnical Center, IST Technical University of Lisbon, Portugal
ABSTRACT Current safety requirements for both construction and operation of tunnels demand reliable monitoring and data processing methods. The conventional techniques of convergence measurements are characterized by intermittent recordings, frequently interrupting construction procedures and are not applicable to survey tunnel stability upon its completion. The new extensometric method is aimed to override those difficulties by means of installing sets of electric resistance strain gages (or fiber optics sensors) on the steel arches of any tunnel support system, thus providing continuous recordings of deformations, even beyond the construction phase. A description of laboratory and field tests there were performed to validate the process, as well as the mathematical models formulated to implement it, are presented in association with examples of application. Comparison of field results with those provided by conventional techniques indicated good agreement in terms of obtained convergence values.
1
INTRODUCTION
Tunnel construction nowadays requires adequate monitoring for detecting both any signs of instability as well as preventing environmental impacts on tunnel’s vicinity. The most common method of evaluating stability conditions during excavation is by convergence measurements conducted by means of diametrical or perimetral changes registered by mechanical or optical systems (Jimeno, C.L., 1997). With such techniques it is possible to detect anomalous displacements along tunnel periphery and then act towards stabilization, either by strengthening the existing supports or reinforcing the surrounding ground with appropriate treatment methods. In general, convergence monitoring is performed along prescribed sections of the tunnel, separated by distances that decrease as instabilities occur, and vice-versa. A common feature of this process is that at each measurement section most other construction operations must be interrupted, causing delays in the progression of the work. To overcome those disadvantages, an extensometric technique was devised in terms of measuring those displacements through the installation of strain gages, or fiber optics sensors, on the internal face of the steel arches forming the usual support system. The new method has a certain number of advantages with respect to the conventional practice, namely in terms of: (i) continuity of displacement data acquisition, (ii) availability of convergence data at real time, (iii) non-interference with ongoing excavation operations, (iv) lower implementation costs and (v) the possibility to monitor tunnel behaviour upon its construction, i.e. along its service life.
2
LABORATORY TESTING
Research started at laboratory with small scale models of steel arches, actually formed by metallic bars in which electrical resistance strain gages were installed (see Figure 1).
Figure 1 – Several lab models of monitored steel arches
By submitting the model arches to external loads (either symmetrical or non-symmetrical) the measurement of strains at several points (5 to 7) were performed, as well as the monitoring of convergences between those points by the use of LVDT transducers. A clear linear correlation between strains and convergences was recorded, as it is depicted in Figure 2. 900 800 y = 77,797x R 2 = 0,9923
-6
STRAIN, ε (10 ) DEFORMAÇÕES,
700 600
y = 61,59x R 2 = 0,9829
500 y = 56,509x R 2 = 0,9833
400
y = 43,419x R 2 = 0,9807
300
y = 13,751x R 2 = 0,993
200 100 0 0
2
4
6
8
10
12
14
16
D EDISPLACEMENT SLOCAMENTOS, δ (mm)
Figure 2 – Variation of deformations measured at 5 strain gages and relative displacements between those points
As the linear elastic behaviour was observed in laboratory scale, a mathematical model was developed to sustain this interpretation.
3
MATHEMATICAL MODELLING
Using the theory of elasticity a set of expressions relating the displacements u, v with normal
strains ε r , ε θ for a bi-dimensional state of strain in polar coordinates with radius r are (Benham & Warnock, 1973): εr =
∂u ∂r
εθ =
u r
(1)
As the strain gages measure the hoop strain ε θ its relation with the radial displacement u may be obtained by the expression: εθ =
u r −u
so the radial displacement is given by:
(2)
u=
rε 1+ ε
(3)
By introducing in these expressions the experimental values obtained in the lab, a good agreement was found between the displacements measured with LVDTs and by calculation with this algorithm. A further result was obtained by means of representing graphically the deformed arch upon its loading (see Figure 3).
Figure 3 – The initial and final geometry of an arch submitted to external loading
4
FIELD TESTS
A further set of investigations were performed in a tunnel under construction in the Lisbon subway network. The excavation was conducted at shallow depth in a strong basaltic rock mass. Tunnel diameter was 10.24 m and excavation was achieved by heavy hydraulic impact hammers, with support formed by one meter spaced TH-29 steel arches, in addition to shotcrete. The installation of strain gages was accomplished in seven points of the arches internal face and their measurement done periodically by conventional strain indicators. As the conventional method of convergence readings was still under effect, a comparison of results from both methods was possible, indicating good agreement (see Figure 4). An interesting output was obtained through the representation of convergences at different time intervals (see Figure 5). Due to the fact that same orders of magnitude were obtained through both the conventional and the extensometer method, an increased confidence on the new method performance was reached.
4 5
6 7
3
2 1
DATES Measured Calculated 05-Jun 17-Jun 26-Jun 11-Jul 11-Sep
C2-6 M C
C4-6 M C
C2-4 M C
0 3.0 2.5 2.3 3.4
0 2.0 2.2 2.8 3.7
0 3.2 2.3 2.7 4.3
0 0.6 1.17 1.87 3.54
0 0.73 2.20 2.83 3.57
0 0.81 2.90 3.73 4.98
Figure 4 – Comparison of convergences showing measured values (M) by the conventional method and calculated (C ) by the extensometric method
Figure 5 – The evolution of convergences (in mm) in the tunnel section.
5
CONCLUSIONS
The so called extensometric method was applied on steel arches both at laboratory and field scales, providing excellent agreement with conventional techniques of convergence monitoring. Besides the validation obtained through the application of a mathematical elastic model, the following advantages of the new technique are: - Greater versatility in monitoring tunnel convergences continuously; - Increased availability during both construction and service phases; - Less interference with other tunnelling operations; - Possibility of supplying alarm signals (either sonic of red lights) when excessive values are recorded; - Use of “on-line” data transmission to display results in a network of computers; - Possibility of using fiber optics sensors in stead of electrical strain gages to upgrade method reliability; - Low implementation costs.
ACKNOWLEDGEMENTS The author thanks all parties involved in this research that led to the registration of the extensometric method as Portuguese Patent No. 103058, in January 10, 2004.
REFERENCES Benham, P. & Warnock, F. 1973, Mechanics of Solids and Structures, Pitman, London Gama, C. D. 1996. Correlation between rock mass classes, convergence rates and support densities for underground coal mine excavations.Eurock’96,Torino,pp.825-830. Jimeno, C. L. 1997. Manual of tunnels and underground workings (in Spanish). Madrid.
