sanchez suarez macedo definitivo

Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil.

1

1 INTRODUCTION

While excavating the top heading of the left-side tunnel, during March and May 2012 some soft clayey strata appeared at the top heading as a contact between the bottom limestones and an upper body of very stiff columnar basalts. The tunnel was being excavated under the concept of the so called New Austrian Tunneling Method (NATM); monitoring of convergence indicated that construction was entering a critical geotechnical scenario. Measures taken consisted on improving support and the u se of heavy forepoling systems, including a pause at the working face until stabilization signals were observed. However, after a heavy rain period, the works needed to be suspended again due to reactivation of movements in the interior of the tunnel. Finally the tunnel collapsed. The mobilized mass reached the ground surface causing a landslide of very big proportions.

To carry out the rescue plan of the tunnel it was necessary to implement a new, extensive exploration campaign that included conducting new geological, geophysical and structural

surveys. The information generated from these studies was complemented with that one from the original project and previous studies conducted in the area. Finally, it was possible to build a very complete and accurate geological and geotechnical model of the situation that prevailed at the time.

Figure 1 View of the area of the collapse on the hillside. Note the separation of the two rocky bodies.

The next step was to study the causes of collapse from all points of view: geomechanical

The Xicotepec road tunnel collapse in Mexico; an investigation of the causes and the reconstruction project.

F. A. Sánchez and J. F. Suárez Consultec, Ingenieros Asociados S.C., Mexico DF, Mexico

V. H. Macedo Fomento de Construcciones y Contratas, Xicotepec, Mexico.

ABSTRACT: The road tunnel Xicotepec is under construction since September 2011 as part of a modern Highway project connecting Mexico City with Tuxpan, in the coast of the State of Veracruz. Along the mountain system known as Sierra Madre Oriental, very complex geological formations are passed across, consisting of diverse type of Jurassic sedimentary rocks, coexisting with volcanic deposits from the Holocene. While driving the tunnel through soft clayey strata at the face in combination with and an upper body of stiff columnar basalts at the roof, a very large scale collapse happened. This paper presents a complete, both geological and geotechnical interpretation of the event, supported by two and three-dimensional numerical analyses reproducing the failure with a very good degree of approximation, which served as the basis for the reconstruction designs, same that will also be summarized.


2

geologic, geohydrologic and constructive, with the support of all the instrumentation records generated during construction.

The best accepted hypothesis of the collapse indicated that, due to the very unfavorable position of the columnar basalts with respect to the clayey strata (basalts at the crown and clay at the lower section, see Figure 2), together with the attitude of the joints defining the columns (almost vertical), represented a clear unstable condition. The redistribution of the stress state generated by the excavation process caused plastification and a lowering of strength of the weak unit at the side walls, not being able to carry the weight of the loosened basalts. Moreover, the heavy rains of the preceding days percolated through the rock and saturated even more the clays.

Figure 2 shows a view of the tunnel face before collapse. Note the contact between the stiff columnar basalts and the clayey strata.

Figure 2. Tunnel face in which the contact between columnar basalts and clayey unit can be seen.

Once the mechanism was understood, engineers proceeded to build a large scale numerical model in order to, first, reproduce as closely as possible the phenomenon, to approximate the mechanical parameters of the different geological units via back-analyses and, once counting on a reasonable approximation (using the collapsed model itself), propose and study different construction techniques and ground reinforcement procedures in order to carry out its reconstruction.

Figure 3 shows a geological section before the collapse, while Figure 4 depicts the mechanism and the situation after the event.

Figure 3. Geological interpretation of the situation before tunnel collapse.

Figure 4. Geological interpretation of the situation after tunnel collapse.

2 NUMERICAL MODEL

The numerical model was performed using the finite difference technique with FLAC 3D program. A 3-dimensional mesh with 992,000 hexahedral zones and 1 million 19 thousand nodes was built, covering a large area of the rock mass and the two tunnels.

The mesh included all geological units defined as well as all the geotechnical studies, construction procedures and support types both the original ones (before collapse) and those proposed for the continuation of the works.

2.1 Simulation of collapse

The first part of the study was to conduct a series of back-analyses in order to reproduce the collapse both when and how it occurred; starting mechanical parameters were determined from correlations with the geomechanical classifications made in the new geological surveying campaigns.


3

Figure 5. Mesh view from the Tuxpan side.

Figure 6. Mesh view with sections illustrating the arrangement of layers and tunnel positions.

In the model, construction procedures were programmed as such had been made during excavations including geometry and support measures implemented.

The model was also programmed so that the advances and gaps between the two fronts coincide substantially with the real ones; from the back-analyses it was sought that the collapse occurred in the model for a relative position of the two front faces coinciding with those of the real construction.

In addition, to be closer to reality, shotcrete elements, standing at a distance of two meters behind the face, included a hardening law as a function of time, both for stiffness and strength.

The evolution of the Young modulus of shotcrete was taken from the average of the following expressions:

0.6 0.6c 28 exp 28E t E c t

(1)

121 28/

c 28exps t

E t E

(2)

where 0.81c (Mahar, 1975) and 0.25s (Comité Euro-international du Beton, 1993) for rapid concrete and t is time in days (Figure 7).

Figure 7. Stiffness hardening of shotcrete.

Meanwhile, the strength parameters of shotcrete were varied from typical relationships for different ages of plain concrete in accordance with stress-strain curves from European regulations and through the expressions:

*

sin

1 sintan

sin1 2

1 sin

peak

t peak

peak

t peak

f

f

(3)

** *

**

tancos

sin21

1 sin

tan

tan

t cpeak

t peak

peakpeak

f fc

f

c c

(4)

where c* and * are the cohesion and friction angle for a determined age of shotcrete; ; ft a function of time defining the evolución; peak the angle of friction at 28 days (taken as 45°) and cpeak the 28 days cohesion resulting from Mohr-Coulomb failure criterion. With these relations a strength gain law like the one in Figure 8 was obtained.

Steel arches were placed embedded in shotcrete and were simulated via beam elements with stiffness and strength properties of TH-29 structural profiles, such as those used in construction.

In sections reinforced with forepoling systems, pile elements available in the program, where placed.


4

Figure 8. Strength hardening of shotcrete.

Figure 9. Different ages of shotcrete during excavation (Young modulus).

The performed back-analyses had two main objectives: first, to reproduce as close as possible the deformational behavior measured by convergences and, second that the collapse of the left side tunnel occurred at a position and with a shape similar to reality.

For the reproduction of deformations, a representative instrumented section (chainage 754+544) was selected, both for the magnitude of the measured displacement as for the relative position to the collapse zone. This station began to be instrumented when the tunnel face was 16m ahead.

As it is natural while measuring convergence, most of the deformations in a given section occur before and at the right time of the passage of the tunnel face through a determined section Then, what manages to be measured is the subsequent deformational behavior, not less valid, but leaving some uncertainty of which

were the actual displacements during the excavation event.

Figure 10 shows the results of vertical displacement of three points of the convergence section generated during the simulation of the excavation process, contrasted with measured data. The plot represents the deformational behavior (modeled and measured) during the period from March 31th., 2012 (start of monitoring) and June 30th, 2012, (date of collapse), in which 77 meters of the right tunnel were excavated (754 +528 to 574 +467).

Figure 10. History of modeled and measured displacements at station 754 +544, from 03/31/2012 to

06/20/2012, for an advance of 77 meters.

Note in Figure 10 how calculated displacements of the three points are on the same order of magnitude as those measured. Scattering of the instrumented data may be due to rearrangements of the rock mass and the effects of rainfall on clays. The model, as it was created was not capable to simulate temporary rearrangements of the rock mass, but, qualitatively and quantitatively it was considered that their behavior was similar to the one recorded during the measurement campaign, both in terms of time and distance from the tunnel face.

After several calibrations of the model it was achieved that failure occurred during the excavation of the left tunnel at the time in which its front face was in the position 654 +568, i.e. a few meters before it was when the collapse really happened. Given the complexity of accurately reproducing the precise time of failure together with reasonable deformational results, the combination of parameters and boundary conditions reached up to this point where considered to be a valid representation of the geotechnical scenario that produced the failure.


5

During the calibration process, the main units worked were the breccias, paleosoils and loose basalt close to the slope of the hill. The calibration of the breccias and paleosoils was fundamental to approximate the deformational behavior of the whole model and to trigger the mechanism of instability, as it was through the unit of breccias at the left tunnel walls broke, not being able to support the weight of the basaltic mass at the crown. Also, the subsequent landslide was favored by the presence of these paleosoils.

Breccias and paleosoils where modeled using the Mohr-Coulomb failure criterion. The strength parameters resulting from the calibration where: Breccias: c = 50 kN/m2; = 22°, E = 700 MPa y = 0.33 Paleosoils: c = 40 kN/m2; = 20°, E = 150 MPa y = 0.33

Columnar basalts where modeled with the

Ubiquitous Joint Model (UJM). At the collapse zone the joints had an attitude closely perpendicular to the tunnel line with a dip of approximately 85°; close to the surface, they where substantially open and planar (Figure 11).

Figure 11. Columnar basalts at the collapse zone.

The use of the UJM model in this case was essential not only to reproduce the collapse, but also to control the extension of the mobilized mass, so that it did not spread beyond where it actually occurred. In the first calibration attempts the Mohr-Coulomb model of was used, however, the collapse generated extended to the

top of the mountain also interesting the right tunnel. It was not until the UJM was implemented that the movements of the slope began to approach the observed reality.

Finally, the collapse was achieved in the model while driving the left tunnel with the set of parameters shown below: Loose columnar basalt: Rock mass: cm = 250 kN/m2; m = 35°; Em = 10 GPa and m = 0.25 Joints: cj = 70 kN/m2; j = 19°.

Figure 12 shows the contours of equal

displacement over a cut of the model, which illustrates the mechanism. Figure 13 shows the contours of maximum shear strain increment.

Figure 12. Contours of equal total displacement over a cut at the chainage 654+590 showing the mechanism of

collapse.

Figure 13. Contours of maximum shear strain increment over a cut at the chainage 654+590 showing the limits of

the collapse mechanism.

2.1.1 Simulation and design for the new excavations

Once reproduced the deormational behavior and the collapse, studies for the proposed solutions aimed at the termination of the right tunnel and the reconstruction of the left tunnel where conducted.


6

Analyses of the proposed new excavations and support systems started from the collapsed state of the model and a reconfiguration of the topography in order to match the actual situation (Figure 14).

Figure 14. Reconfigured model.

Removal of the fallen materials and the excavation of a large injection platform over the left tunnel were simulated. This platform covers the whole collapsed area and the injections were driven from below the actual tunnel floor to some meters above the crown (Figure 15).

Figure 15. Model showing the injection platform.

Figure 16 shows a scheme of the injections proposed. First, all the fallen materials like earth and large blocks where removed and the platform constructed. From the platform a mesh of 9 boreholes per section where drilled and injection executed covering an important area around the collapsed tunnel.

For the right tunnel a heavier reinforcement and a special construction procedure for the critical zone, which included a curved, closed and reinforced concrete invert, structurally jointed with the tunnel walls, was proposed.

Structural analyses for the support systems of the right tunnel where carried out using 2D finite elements. In order to do so, the bi-dimensional model was calibrated according to

the 3D results and measurements as well as for the real advances of the front face registered during construction.

Figure 16. Scheme of the injection procedure.

Also, a relaxation law based on monitored convergence and time was determined and related to the hardening process of shotcrete.

In the finite element simulations different stages of ground relaxation and aging of shotcrete where implemented together with the adjustment of some mechanical parameters of rocks and clays. The Jointed Rock Model available in PLAXIS was utilized to represent the anisotropic nature of the basalts.

Figure 17 shows the shape of the relaxation law (as percent of the total deformations instrumented) vs. time and age of shotcrete.

Figure 17. Relaxation of the ground as a function of time (and ages of shotcrete ) before, during and after passing

through the control station in the model..


7

Figure 18 shows the result of these calibrations. Note that 2D results are qualitative and quantitatively in good agreement with the 3D simulations. Also note that the 3D model was previously calibrated with field instrumentation (Figure 10).

Finally, the collapse of the left tunnel and side hill was also reproduced with the 2D finite element model (Figure 19).

Once reproduced the failure scenario, the next step was to reconfigure the model to the actual situation and simulate the subsequent excavations, according to the new procedures and reinforcement proposed, in order to carry out structural designs.

For the left tunnel, a special construction procedure was proposed, which comprises, in the first phase, the excavation of two small lateral galleries in the lower section along the entire collapsed length, supported by steel frames and shotcrete and two large reinforced cast concrete footings at the base. In a second phase, opening of the top heading sustained by a heavy forepoling system, steel frames and reinforced shotcrete. This section will also include a structurally attached, curved concrete invert. Figure 22 shows the construction scheme in the finite difference mesh.

Figure 18. Results from the calibration of the 2D finite element model with respect to the 3D finite difference

simulations.

In the analyses of the new excavations, both for the termination of the right tunnel and the rescue and conclusion of the left one, multiple convergence control sections, same to be implemented during construction, were placed. It will be fundamental to lead an intensive behavioral control where, in case of larger movements than expected, adapt the excavation processes and/or further strengthen the supports systems.

Figure 19. Collapse in 2D finite element model.

Figure 20. Bearing capacity diagram for the top heading support system of the right tunnel.

Figure 21. Construction and support scheme for the collapsed zone of the left tunnel.

Figure 23 shows the expected deformational response at the intrados of the control section at chainage 654+570, located at the middle of the collapsed zone. Note that the deformations expected, according with the model, are to be tolerable.


8

Figure 22. Construction procedure in the FD mesh.

Additionally, the model includes virtual inclinometers matching the real position to use during construction. A series of results for different construction events where plotted, which will be useful for comparison and control of the deformational response of the rock mass (Figure 24).

Figure 23. Modeled displacement history of the inner line of the tunnel at the control station 654+570 for the

excavation of top heading, bench and invert.

Analyses showed that the critical construction phase will be the advance of the lateral galleries. According to the calculations, large displacements are to be expected before the installation of the support system, which consist on closed steel frames (TH-29) and shotcrete. The indication given in design was to drive the excavation very slowly and with short advance lengths wile rapidly installing the reinforcement.

Once the section is closed, it is expected that deformation should stabilize quickly.

Figure 24. Modeled response of some virtual inclinometers placed at the model for different

construction events.

Figure 25 shows horizontal displacement contours at the steel frames and the shotcrete wall of the galleries (intrados). Figure 26 shows the displacement history of the left gallery before, during and after the pass of the excavation trough the control station

Figure 25. Expected displacements at the steel frames and shotcrete for the excavation of the lateral galleries.

Note in Figure 26 the great magnitude of displacements at the left wall and the rapid stabilization after the installation of the support system. However, since these deformations will occur during excavation there will be no possible way to measure them and they will be masked by the same excavation procedure.

The real effects of this construction phase will have to be monitored via the data obtained from the inclinometers.


9

Figure 26. Displacement history of the left gallery before, during and after the pass of the excavation trough the

control station.

In Figure 27 one can see the large magnitude of displacements at the side wall corresponding with the gallery. However, once constructed the stiff cast concrete footings at the base all ground pressures should be absorbed by the structure as it can be deduced from Figure 28.

Figure 27. Horizontal displacements at the end of the construction process.

In these calculations the effect of the injections was not taken into account, first because it resulted extremely difficult to establish a priori values on how these treatments improve the rock mass resistance and second because it was assumed that if the proposed procedures are able to provide an adequate level security without considering the treatment, then the calculations will be more on the safety side.

Finally, stability analyzes for different times of the reconstruction of the left tunnel where conducted; it was verified that with the removal of the collapsed part of the hillside, in order to build the injection platform, as well as with the proposed construction procedures and support

systems, the works should be carried out with a good margin of safety.

Figure 28. Displacement history at the top heading beginning with the construction of the galleries, measured

from: a) extrados and b) intrados.

Figure 29 shows a view of the injection platform and the injection works during the autumn 2013.

In Figure 30 the potential collapse mechanism at the end of the excavations and for a safety factor of 1.67 is shown In Figure 31 the behavior of one of the nearby to tunnel inclinometers for different levels of the safety factor is depicted.

Figure 29. View of the injection platform.

3 CONCLUSIONS

Using advanced continuum analysis techniques it was possible to simulate with a good degree of approximation the behavior of the


10

excavations and the collapse occurred in the left body of Xicotepec tunnels.

Figure 30. Contours of equal shear strain increment at the end of the calculation process. (SF=1.67).

Figure 31. Response of one inclinometer for different values of the safety factor at the end of the reconstruction

of the left tunnel.

Although the problem deals with a heavily fractured rock mass, through continuous equivalent modeling techniques it was possible to determine adequate mechanical properties that resulted in a response of the model, which is in accordance with the field measurements.

Then, using a model of anisotropic strength, it was possible to reproduce the collapse at a time of the excavations and shape of the mobilized mass, sufficiently accurate to what really occurred.

From the calibration of the model via back-analyses, new excavations procedures where designed, which will start at the end of 2013 in order to complete construction. Forecasts of the behavioral response of the site will be possible to verify by instrumentation and will serve to keep a tight control of the safety levels established by this investigation.

FINAL REMARKS

At the date of submission of the present paper, reconstruction works for the left tunnel have not jet started; only the injection program has been concluded.

Authors hope that for the time of the Congress, information regarding the behavior of the rock mass will be available for presentation.

REFERENCES

Comite Euro-International du Beton: Bulletin D’information No213/214 CEB-FIP Model Code 1990 (Concrete Structures). Lausanne.

Weber, J.W. 1979: Empirische formeln zur beschreibung der festigkeitsentwicklung und der entwicklung des e-modulus von beton betonwerk und fertigtechbik. Betonwerk U Fertigtel-Tech. Volume: 45. Issue Number: 12

Mahar, J. W. 1975: Shotcrete practice in underground construction: Final Report. Dept. of Civil engineering. University of Illinois at Urban Champaign, Springfield VA.

sanchez suarez macedo definitivo

Documents