# Study of a fire in a tunnel during construction phase

Côme Leclerc, Rabah Mehaddi, Zoubir Acem, Bertrand Masselin, Pascal Boulet

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## To cite this version:

Côme Leclerc, Rabah Mehaddi, Zoubir Acem, Bertrand Masselin, Pascal Boulet. Study of a fire in a tunnel during construction phase. Journal of Physics: Conference Series, 2024, 2885 (1), pp.012049. 10.1088/1742-6596/2885/1/012049. [hal-04836290](https://hal.science/hal-04836290v1)

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Submitted on 13 Dec 2024

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# Study of a fire in a tunnel during construction phase

To cite this article: Côme Leclerc et al 2024 J. Phys.: Conf. Ser. 2885 012049

View the [article online](#) for updates and enhancements.

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# Study of a fire in a tunnel during construction phase

Côme Leclerc¹,², Rabah Mehaddi¹, Zoubir Acem¹, Bertrand Masselin², Pascal Boulet¹

¹ Université de Lorraine, CNRS, LEMTA, F-54000 Nancy, France

² Société des Grands Projets, 2, mail de la Petite Espagne – 93200 SAINT-DENIS

Corresponding author: come.leclerc@univ-lorraine.fr

**Abstract.** In this paper the propagation of smoke was studied experimentally and numerically in a tunnel during excavation, considering that the tunnel was closed on the side of the Tunnel Boring Machine, with no specific ventilation system to control the smoke. The analysis of the results focused on the smoke layer in the gallery and the plume at the tunnel exit on the access shaft side, identifying smoke-free zones that could facilitate access by emergency services. The presence of an external wind above the shaft was seen to disturb the flow at the exit, possibly also affecting the smoke flow in the tunnel.

## 1. Introduction

The context of the present study concerns tunneling carried out from a shaft, where a tunnel boring machine (TBM) is placed at the level of the gallery to be dug and progressively moves forward over a length up to several kilometers. In the event of a fire during this phase, there are specific features that make the management of fire and smoke different from that of a tunnel during the operating phase. Ventilation (used for air supply to workers and dust evacuation) is generally stopped by operational choice when the fire is detected. Smoke flows in a natural ventilation regime, with a major effect of buoyancy forces generated by the heat released by the fire. The smoke production and the lack of ventilation can have an impact on the oxygenation of the fire. In addition, smoke propagation is asymmetrical, with only one side open. The heat release rate (HRR) of the fire, the presence or absence of smoke-free zones allowing access for the emergency services, and the characteristics of the smoke plume observed in the access shaft are all crucial issues for fire safety management.

Although tunnel fires are widely studied during the operating phase, they are much less well documented for the construction phase. A general overview of tunnel fires was provided by Ingason [1]. For the construction step, Yao *et al.* [2] and Ingason *et al.* [3] reported specific studies, but the geometry differs from that of interest here. They concern a different type of excavation, carried out simultaneously at both ends, with a connection to the tunnel from the outside via an access gallery. In any case, they highlight the complexity of the smoke flow, possible phenomena of under-ventilation of the fire depending on its position and the geometry of the connection. They also emphasize the influence of the thermal stack effect in the case of a sloping gallery. A great deal of work on tunnels in operation also concerns ventilation and its use to control smoke, sometimes with extraction shafts that bring the case studied closer to our geometry with its access shaft. Among the available studies, Fan *et al.* [4] also considered the effect of an external wind on smoke extraction, a point that we also investigated here. Even though their study concerns a tunnel in the operational phase combining the effects of forced and

<figure><img src="image_5.png" /></figure>
natural convection, they show the contrasting effects of the wind, which, according to the authors, leads to a possible restriction of the smoke plume at the outlet and at the same time creates a zone of depression promoting extraction. Wind speed and the competition between the smoke momentum and wind obviously play a key role in the impact on smoke extraction.

LEMTA has carried out an initial study of the dynamics of fire and smoke in a tunnel during excavation, in partnership with the Société des Grands Projets, as part of a work on the Grand Paris Express project. To do this, we designed an experimental model (1:22 scale) and analyzed the results of numerical simulations carried out using the code Fire Dynamics Simulator (FDS). We considered a section of gallery with its access shaft. A heptane pan was set to simulate a fire, and the mass loss of fuel was used to determine the Heat Release Rate (HRR) of the fire. The temperature field in the gallery was determined using thermocouple trees at several longitudinal positions, in order to characterize the smoke flow and its evacuation towards the access shaft. The method was already used successfully in Mehaddi et al. [5] to investigate the use of a water curtain for fire and smoke containment. This study showed that water activation induced the loss of smoke stratification and that smoke could not be contained (at least in the studied conditions). It is therefore necessary to let the smoke flow, preserve its stratification downstream and analyze its evacuation in the access shaft. This is the main focus of the work presented here, with a particular attention paid to the effect of an external wind on the smoke flow inside the shaft and the tunnel.

In the following sections, we describe the model and its metrology, we summarize the numerical parameters set in FDS, we validate them by comparison with experimental data, and present a set of results for the fire dynamics and the smoke flow in the shaft. Finally, we draw our conclusions.

## 2. Experimental study

A model has been adapted from [5], with a tunnel with diameter 40 cm and length 2 m (shortened as compared to [5] to be placed inside a wind gallery in order to mimic the wind effect with a wind velocity in the range between 0 and 5 m/s (Figure 1 and 2). The tunnel emerges into a cubic shaft measuring 1.20 m on each side. The tunnel is made of 1 cm thick steel for the first meter, which is dedicated to the fire area, and 1 cm thick polycarbonate for the rest of the model to allow the visualization of the smoke flow. K-type thermocouples are set on 4 trees, with a 2 cm vertical gap between thermocouples (see positions A, B and C in the shaft at 30, 60 and 90 cm from the tunnel exit, plus position D in the tunnel 10 cm before the exit). A heptane pool fire was used, with a high precision balance to evaluate the mass loss rate and deduce the HRR using the combustion heat 44.6 kJ/kg as suggested in [6]. The case of a stationary prescribed HRR equal to 5.5 kW will be presented here (equivalent to 12.5 MW at the real scale considering the standard scaling rules in L$^{5/2}$ for a 1:22 scale). In the presented cases, the wind will be activated after 150 s. Uncertainty and repeatability were checked as in [5] and are not discussed here.

<figure><img src="image_6.png" /><figcaption>Figure 1: Sketch of the model, with the tunnel and its access shaft.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 2: Picture of the model set inside the wind gallery.</figcaption></figure>
### 3. Numerical simulation

FDS was used for the simulations (see [7-8] for a detailed code description and submodel evaluation), reproducing the geometry, with an open space above the shaft to allow for inlet conditions simulating the wind effect, introducing thermal properties and wall thickness to reproduce the actual boundary conditions. Simulations were carried out with a prescribed HRR based on the measured mass loss rate, involving three successive linear steps: from 0 to 2.75 kW in 5s, up to 3.3 kW between 5s and 80s, up to 5.5 kW between 80s and 160s, and finally constant at this value. A regular mesh was used, involving cubic cells with a side of 1 cm. This provides cells 10 times smaller than the so-called D* characteristic length suggested in FDS users' guide for a correct mesh design and 40 cells along a diameter in order to represent accurately velocity or temperature profiles in the gallery. This seems reasonable for a grid-independent analysis (which was also demonstrated in [5] in similar conditions).

A qualitative validation was sought by comparing the numerical and the experimental smoke flows visually. As will be shown later in a series of figures presenting instantaneous maps of the smoke flow, the general pattern observed experimentally was well reproduced, namely : (i) the space is filled by smoke upstream the fire location (on the TBM side where the tunnel is closed), (ii) the smoke flows in the upper part of the tunnel downstream (on the exit side), but a smoke free layer is preserved in the bottom part with a height about 40% of the total height, (iii) the plume exits in the shaft, along the shaft side above the tunnel exit, leaving a smoke free space on the opposite side of the shaft.

A quantitative validation was also obtained by comparing the simulated and measured temperatures in the shaft (Figure 3). The difference with ambient temperature is plotted as a function of time and at various vertical positions for trees B and C in the shaft. Prior to wind activation, smoke does not reach tree B continuously (Figures 3(a) and 3(b)), but some smoke puffs reach the upper thermocouples. This looks like a noisy value numerically, but it is an observation of the limit reached by the plume, close to the middle of the shaft. On the contrary, the smoke does not reach tree C (Figures 3(c) and 3(d)) before 150 s. After 150 s, the fan is activated and the wind set at a constant value of 3.5 m/s induces a mixing of air and smoke in the shaft, as evidenced by an increase in temperature both at position B and C for all vertical positions revealing smoke everywhere in the shaft, with a quite satisfying agreement between experiment and simulation regarding the average increase in temperature.

<figure><img src="image_8.png" /><figcaption>Figure 3: Temperature difference with ambient value (measured data in figures (a) and (c) and simulation results in figures (b) and (d)) on tree B at 60 cm from gallery exit (figures (a) and (b)) and tree C at 90 cm (figures (c) and (d)).</figcaption></figure>
4. Wind effect

Results for different wind velocities above the access shaft as shown in Figure 4 (1.5 m/s, 3.5 m/s and 4.5 m/s from the left to the right respectively), for different times (from the top to the bottom). The blower is activated after 150 s, explaining why subfigures (a) to (c) are identical. Then, the plume is dragged by the wind. The higher the wind speed, the greater the disturbance to the plume, with smoke density increasing throughout the shaft and eliminating the smoke-free zones. The recirculation of the smoke in the shaft induces a mixing with air in the whole space, even disturbing the smoke-free layer initially observed downstream the fire in the gallery, in the bottom part. This mixing still provides oxygen to the fire that continues to burn.

<figure><img src="image_9.png" /><figcaption>Figure 4: Smoke flow visualisation as a function of time (from top to bottom), for HRR equal to 5.5 kW and three wind velocities (1.5 m/s on the left, 3.5 m/s at the middle and 4.5 m/s on the right)</figcaption></figure>

It is interesting to note that for a velocity of 1.5 m/s the wind activation starts to disturb the plume flow (Figures d and g), but the buoyancy of the smoke is sufficient to counterbalance the momentum induced by the outside wind and the shaft is finally cleared after 220 s (Figures j and m). The plume remains entrained horizontally above the shaft, but a smoke-free area is clearly observed in the shaft on the side opposite to the tunnel. In addition, the smoke-free zone in the lower part of the gallery downstream of the fire is preserved. It seems that there is a critical velocity near 3 m/s (based on complementary computations not shown here) above which the smoke-free areas are lost.
Beside this observation of the smoke flow, a quantitative analysis has been done, computing the streamlines in a vertical plane centred in the median plane of the gallery (Figure 5), for different wind velocity, between 0 and 4.5 m/s. The soot concentration is superimposed in order to highlight the smoke area. Plotted data were obtained in stationary regime, averaging the numerical results over 50 s, between 400 and 450 s after the fire ignition.

<figure><img src="image_10.png" /><figcaption>Figure 5: Streamlines and soot mass fraction for different wind velocity. All data were obtained from numerical results with a time integration between 400 and 450 s.</figcaption></figure>

As already observed, the smoke plume leaving the shaft is inclined by the horizontal wind flow, with a disturbance that increases with wind speed. This creates a vortex inside the shaft. It already exists in Figure (5a) in the absence of wind, at the interface between the outgoing plume and the incoming airflow, but it descends to the level of the shaft and merges with the recirculation loop in the shaft in windy conditions. Above a speed of 3.5 m/s, this vortex disappears and the concentration of soot increases sharply inside the shaft and tunnel due to the recirculation of smoke. The smoke is partly diluted and some outside air remains entrained and can feed the fire via the bottom layer in the lower part of the tunnel. This was not so obvious, since it was questionable whether the effect of the outside wind could act as a cover for the shaft and lead to overpressure in the tunnel. Figure 6 shows the relative pressures in the tunnel and shaft with and without wind (for the case of 3.5m/s only, but the trend is similar for all wind velocities which were simulated). In the case without wind (Fig.6a), the pressure in the tunnel and the shaft is fairly homogeneous, slightly negative. With wind (Fig.6b), this negative pressure in the shaft is significantly higher in magnitude, with on the contrary a rise in positive pressure
around the fire zone. These data confirm the contrasted effect of wind, the smoke exit is disturbed but the conclusion on its evacuation is not straightforward. This should be further investigated to evaluate how the smoke flowrate is affected and with what consequence for the condition inside the gallery.

<figure><img src="image_11.png" /><figcaption>Figure 6: *Relative pressure without wind (a) and for a wind velocity of 3.5 m/s (b). All data were obtained from numerical results with a time integration between 400 and 450 s.*</figcaption></figure>

**5. Conclusion**

We carried out an experimental and a numerical study of a tunnel fire during the construction phase at the model scale. The numerical results obtained with the FDS code are in good agreement with the experimental data collected. They lead to the following conclusions:

- The fire fills the tunnel upstream of the fire point (on the closed side), but a free layer of smoke remains in the lower part downstream, covering 40% of the total height of the tunnel.

- The smoke escapes into the access shaft, producing a plume attached to the wall above the tunnel exit, leaving a smoke-free zone on the opposite side of the shaft (which would allow access for emergency services or fire-fighters if necessary).

- However, the presence of wind above the shaft can disrupt this flow regime and cause the plume to be dragged along, recirculating in the shaft and filling in the areas initially free of smoke.

- Below a critical speed (of the order of 3 m/s at the scale of the 1:22 scale model studied), the buoyancy of the smoke may be sufficient to clear the shaft and restore a free zone similar to the case without wind. However, the presence of puffs, wind change, or other instabilities has not been studied at this stage and would require further analysis.

The work underway will continue with a detailed analysis of the critical speed that fills the access shaft and the consideration of variable conditions. A study of the effects of slope and induced stack effect, as well as sensitivity to the HRR and position of the fire, is also planned.

**References**

[1] H. Ingason, Li Ying Zhen, A. Lönnermark, Tunnel Fire Dynamics, Springer (2015).

[2] Y. Yao et al., Numerical study of the characteristics of smoke spread in tunnel fires during construction and method for improvement of smoke control, *Case St. in Th. Eng.* 34 (2022) 102043

[3] H. Ingason et al., Fire incidents during construction work of tunnels, *SP Report* (2010), p.88

[4] C.G. Fan et al., Effects of ambient wind on thermal smoke exhaust from a shaft in tunnels with natural ventilation, *Applied Thermal Engineering* 117 (2017) 254–262

[5] R. Mehaddi et al., Use of a water mist for smoke confinement and radiation shielding in case of fire during tunnel construction. *Int. J. Therm. Sci.* 148 (2020), 106156.

[6] V. Babrauskas, Estimating large pool fire burning rates, *Fire Technol. 19 (4) (1983) 251–261*.

[7] K. McGrattan et al., Fire Dynamics Simulator, User’s Guide, NIST Spec. Pub. 1019 6th Ed. (2023)

[8] K. McGrattan et al., Fire Dynamics Simulator, Technical Reference Guide Volume 3: Validation NIST Spec. Pub. 1018-3 6th Edition (2023)