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Inicio Twenty/Twenty It's not the soil, but the water beneath: why a train tunnel...

It’s not the soil, but the water beneath: why a train tunnel in Merida wasn’t a good idea

Tren Maya project included a 4.1 km underground rail inside the city. The idea has been put aside, due to its high cost

By Twenty/Twenty staff

The challenge about planning an underground tunnel for the Mayan Train in Merida was not the kind of the soil and how hard to drill it would be, or the amount of money the government could or not to spend, but how to avoid future problems with the water running like subsurface rivers beneath the city.

“If we speak about engineering or money, it was possible”, said Northwestern University expert in geohidrology, Emiliano Monroy Rios, but modeling or predicting how the subsurface streams will be modified, and the future consequences when the rainy season comes this or next years, is not an easy task.

The experience with floods on different places at Yucatan’s capital city last year which exposed how much the level of the natural aquifer beneath Yucatan can increase with high rainfalls, and caused problems for this city inhabitants, supports his words.

As a matter of fact, there isn’t any study that could determine -with a high level of precisión- the water flowing dinamics under the city and the way it changes if we reach the level or the underground aquifier, or even where we might reach it or not, because it is not all at the same depth.

What do we know about water under Merida?

«It’s too little», Monroy Ríos affirmed whit the knowledge acquired after being one of the researchers who have insisted that Tren Maya´s project has to consider a really huge amount of data about Yucatan Peninsula’s subsoil, because of the caves, hollow and flooded caverns, and cenotes that form interconnected systems beneath this region.

If a fraction of those systems is affected, the effects may reach other areas, either if it is due to pollution or an increase in the groundwater aquifer after natural conditions such as rainfall, he said.

“The problem in the peninsular region is that studying those systems is more complex because they aren’t visible to the naked eye like other areas where there are rivers or lagoon systems.

On those others the modeling can be more precisely and its easy to see where the flow of water will discharge, will overflow or will be displaced as a result of a natural phenomenon or infrastructure work”, added the scientist.

«We have an idea at a regional level, on the Yucatan Peninsula´s platform, as a result of several studies done about how the water flows from south to north,» Ríos Monroy said.

In the ring of cenotes (that goes from west to east connected under the Yucatecan soil) evidence of the flow of water running towards the coast has been found: in the west it flows to Celestún and in the east to Dzilam, according to the available research.

But there are also other currents, and among these, one that passes through Merida and flows towards the north of the city, and from there it goes towards the coastal zone of Progreso and some other nearby areas.

Knowing these aquifer routes has been possible, according to the explanation of Monroy Ríos, through different methods such as modeling from previous data or using tracers.

The tracers are non-polluting chemical elements as tinctures, which aren’t perceptible to the naked eye, which are released at some specific point in the water flows to check if they reach other areas carried by the current previously stimated.

Is it possible to know better the currents under the city?

For the researcher, proposing a complete study of the variations in the direction and water drainage in Merida’s subsoil is complicated.

Especially, if it’s a smaller model of one or more specific areas, as the part of Merida where Tren Maya project was planned to run under, inside a tunnel.

“A reliable modeling is very difficult and we the experts know it, and have already accepted it,» says Monroy Ríos.

Doing the underwater flows modeling in Merida, he warned, to know where it moves, at what speed and what happens if I put this barrier, or if I do an excavation here or somewhere else, is very complicated because we would have to do a lot of studies within specific areas.

Considering the floods that took place this year in Merida, also as a result of rains that exceeded historical records, the researcher stated them as a hard evidence of how does water can find different riverbeds in the subsoil and affect in one area or another, where no similar problem had occurred previously.

It’s known that the first «water body» of the groundwater aquifer in Yucatan is at a depth average of 12 meters, but this year the average reached below the six meters and in some areas it overflowed because of the accumulation caused by torrential and constant rains.

If water surpases its natural level in some area, it will displace and go somewhere else, but it is not easy to know where, according to what the researcher explained.

This means that the tunnel project was something impossible?

«It sounds fanciful, but it is possible,» said Monroy Rios with a little sarcasm about the project that the Fondo Nacional de Fomento al Turismo (Fonatur) presented in december 5th, last year as the alternative to connect the Tren Maya rails with a train station inside Merida.

If all this work was going to be underground they have had to make enough deep so it doesn’t affect the existing urban infrastructure throughout the route, nor the daily activities of people living in that area.

When planning a certain depth, he said, they should take into account the average depth of the aquifer, but it can vary at different points throughout the route.

According to his explanation, this would imply that the construction work should consider a structure not only firm and steady for the train movement, or that could prevent a possible breakdown of the soil layer above the tunnel, but it must also be protected from leakage or even flooding.

Rock type conditions are not an obstacle: «drilling in carbonates (the type of soil in Yucatan) is very easy, it’s a softer rock in comparison to other areas, like the granite that had to be traversed to build the metro subway system, or the deep drainage, in Mexico City. It’s not complicated, it’s easy to cut”.

However, Monroy Ríos believes that if the necessary studies on the currents of the subsoil of Merida aren’t done «the water might continue to flow and generate a current that they will find» throughout the construction work.

And even if the construction doesn’t reach the groundwater aquifer, the margin would be very short with the historical level of the water body (the 12 meters): «if it rises it would reach the tunnel”.

The researcher also acknowledged that this is not a problem, because it can be built with pumping systems and a compact structure that could prevent water from entering.

But the problem, he insisted, is that the water that doesn’t enter the tunnel will keep flowing and it will find another way.

Update:

According to Fonatur, there is no more tunnel project because it was to much expensive. Now this government institution is planning to build an elevated railway from outside of the city to the place where the new train station is planned to be built.

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