Risks involved in vertical shafts construction for the eastern drainage tunnel in Mexico Riesgos involucrados en la construcción de pozos verticales para el túnel de drenaje oriental en México

This article describes the main stages involved in the construction of the vertical shafts (large-diameter vertical wells), which are necessary for the subsequent construction of the tunnel´s sections. The different risk situations existing during the construction of the Eastern Drainage Tunnel in the valley of Mexico City (in Spanish, "Tunel Emisor Oriente") are analyzed. In order for this 52 km-long and 7.5 m-wide tunnel to carry part of the city’s sewage, 25 shafts must first be built, ranging from 55 to 150 meters deep. The magnitude of such a project implies working in different geographical areas and varied geological strata involving the presence of groundwater, which increases the risks due to possible landslides or flooding during excavation. As digging will occur in different types of soil, varying procedures must be used depending on soil type. Likewise, due to the magnitude of this kind of project, detailed scheduling and planning are required as simultaneous works on different fronts are necessary to meet deadlines. The study mentions that, while projects like these involve high risks for workers, analysis of activities and situations are conducted precisely to demonstrate that such risks can be considerably reduced.


History of the Project of the Eastern Drainage Tunnel
Mexico City's metropolitan area is built over a closed watershed, it was composed by a system of lakes comprising five large lakes: Texcoco, Xaltocan, Zumpango, Xochimilco and Chalco. During the highest raining seasons, the five lakes merged into one of an area of more than two thousand square kilometers. This explains the periodical flooding the inhabitants of the region suffered since the foundation of Tenochtitlan (Aztec capital before the Spaniards conquered it and changed its name to Mexico), as well as the great importance to make draining works in order to control and extract rainwater and sewage from the valley.
The construction of Mexico City over the location of the five lakes brought two permanent problems: the need to extract to rainwater in order to prevent flooding and the sinking due to the over extraction of the water from the aquifer. As a result, in 1975, the Central Drainage Tunnel 50 km long was opened, which is the main structure of the actual deep draining system of the valley (Aguilar, 2011).
Nowadays, the capacity of Mexico City's draining system has been surpassed, causing serious problems. It is more than enough to compare its capacity in 1975 with the actual one which is 30% lower, and the population of the area has doubled since then. The decrease is mainly due to the sinking of Mexico City caused by the overextraction of water from the groundwater reserves of Mexico City's valley (Alberto, 2010) In order to fully solve the draining system problem, it was decided to build a new tunnel below the city: The Eastern Drainage Tunnel "EDT" (Tunel Emisor Oriente in Spanish), with a capacity of 150 m 3 per second of polluted water. Originally, the total estimated cost of the Project was 12 billion pesos (National Water Commission, 2011).

Geotechnical conditions of Mexico City's Valley
Due to the great length of the EDT, the project had to be completed in soils with different properties, for example: soft soil (muds) under the lake area, which main feature is its high compressibility, and whose consolidation process is in the first stage with a sinking yield of 30 cm per year (Contreras, 2010).
Other important aspects to consider is that the tunnel runs on the edge of a draining channel with polluted water (with no concrete wall or coating), and in some other area, it runs just 10 m from a lagoon (Holguín et al., 2010).
In general, the vertical shafts described in this paper are built in soft soils which main features are its low shear resistance, high compressibility, low permeability, and the underground water in hydrodynamic conditions (Juárez et al., 2010).

Risk Management
We have to say and accept that, in general, the activities required in the construction sector involve its own risks and can appear at any time. This is particularly important in the underground projects that have a high risk in all its phases. For such reasons, it is compulsory to consider that those situations can appear at any time, even with the most detailed and strict prevention planning, and thus the planning and programming will have to include the activities necessary to face all the unforeseen situations, that is, even consider how to get the risk control stage involved. According to the principles of risk management, it is necessary, and it is justified that the preliminary studies required to analyze and understand the subsoil where to project will be constructed.
On the other hand, before starting the execution of the project, the construction company must plan and schedule the high-risk activities, so that by the time these activities are done, they can be executed in the predicted time, with the necessary resources and on site. This means that at the construction site the scheduled activities will be followed to continue with the stages of organization and management, and only this way can we prevent improvising and consequently minimize the labor risks.

Shafts
Shafts are vertical or inclined accesses that serve to carry out through them all auxiliary operations in a tunnel construction ( Figure 1): digging, ventilation, pumping, salvage material extraction, vertical transport, electric and compressed air installations, personnel access, etc. Shafts also serve to capture water from surface collectors to lead it to the deep drainage tunnel (Luna, 2010).
The underground works of the drainage system and the subway in Mexico City, by their magnitude and subsoil characteristics, left great knowledge in the construction of shafts in soft soils; this has resulted in the possibility of having an advanced planning of the construction of the shafts and to obtain the labor risks analysis to minimize the possibility of having accidents (Méndez, 2009).

Constructive Procedures of the Shafts
Shafts built in the first stage of the drainage system, in soft clay soils of the transition zone and under the lake area of Mexico City, were made with the Mexican technique (Solum), with the French technique (Soletanche), and two others with the Italian technique (Icos). The procedures are quite similar, excavations are done by sectors and stabilized with bentonitic mud; then with a tremie pipe the walls are poured with concrete; then the core is excavated, and at the end the bottom is filled also with concrete.
During the execution of these tasks countless technical problems appeared, but the attention paid in the procedures derived in the successful construction of the shafts in clay soils. This in turn served as a great experience of the behavior of subsoils, mainly found in Mexico City's Valley (Monroy et al., 2010).

Solum Technique a) This technique is the most used in construction of shafts in clay soils and these are the procedures:
After marking the center of the shaft in the ground and the boundaries of the lining, the area is divided into six equal parts, each with an angle of 60 o . Then, a sequence of vertical tunnels of 0.60m in diameter and forming a ring are drilled, with a separation of about 0.50m from each other, so leaving a part of the ring soil undrilled.
Everything is stabilized with bentonite mud. Once the drilling of the ring is finished. The remaining material is extracted with a grab crane, always replacing the extracted material with equal amounts of bentonite mud. After excavating this first annular sector, the walls must be poured, which consists in lowering the frame by pouring concrete from the bottom through a tremie pipe, displacing the bentonite mud by difference of densities. The same drilling and framing procedures are performed for the next annular sector alternately until the drilling and framing of the wall of the shaft is finished (Mooser, 2009). b) The core excavation is done with a grab crane up to the depth where no expansions are yet present due to discharges of the ground, accordingly to soil mechanics calculations and with observations form instrumentation and measurements. Upon reaching this level, all work is suspended, and the weight of the excavated material is replaced by an equivalent volume of water to prevent swelling. The core excavation can then proceed, extracting the necessary material under water until it reaches the designed depth ( Figure  2).  (Paniagua, 2002).

Labor risks in the construction of the shafts
Once the different construction procedures for the shafts have been described, it can be deducted that, in general, the technique to be chosen depends to a large extend on the characteristics of the ground, and therefore, as a first point, it is essential to have the subsoil preliminary results and, in particular, of the area where it is intended to build each shaft.

Labor risk analysis in the construction of the shafts
The Mexican Official Regulation (NOM 031-STPS-2011; section 5.3) requires that the employers of the construction sector have the analysis of potential risks for all and each one of the activities they perform. The risk analyses are defined as the documents containing the characteristics of the construction work and the associated risks to each of the activities carried out, as well as the preventive measures for each identified risk.
For this purpose, there is a scheme in which, for each activities and processes, the potential dangers and risks are presented, thus allowing to obtain at the end the degree of risk for each activity.

It should be noted that this analysis is poorly valid if the company does not have: a) Experience in the activities to be carried out (project type) b) Work safety experience c) Commitment and sincerity to labor security
The recommended risk analysis schemes (Table 1) it has to be mainly considered: -The number of people at risk -The existence of constructive procedures -The existence of risk prevention training -The number of times the worker is exposed to risk -The degree of injury that could suffer the worker In the same risk analysis scheme, at the end, a score is obtained and can be translated into degree of potential risk exposure for the workers. The range of risk are classified from trivial to intolerable (Table 2). Therefore, if the analyses are done with professionalism and sincerity (without the aim of concealing the degree of risk), they should be indispensable tools in the prevention of labor risks.
It must be mentioned that it is also very important that those responsible for the area of occupational safety and hygiene are empowered (authority and support) in the field of action, starting by providing the personnel with individual protective equipment (IPE), in accordance with the activities to be carried out, with the quality specified in the regulations and for each of the workers involved Below is the analysis of potential risks for the main activities carried out during the construction of the shafts. Preliminary activities (Table 3). These activities mainly involve the layout and leveling; work is carried out at ground level, thus the predominant risk is the direct exposure to the inclement weather, but the existence of animals in the area can also be considered as risks, so all that contains or is being involved in the work area (e.g: crime or other social problems) must be considered. Construction of the concrete crown of the shaft: the concrete crown of the shaft is a concrete structure that serve as vertical guide so that the excavation equipment can be aligned. Therefore, one exterior and one interior are required. These activities start at ground level and up to a depth of 2m, so the most common hazards are direct exposure to sun light, the use of tools and machinery and loads lifting of that can result in risks for the workers (Table  4).  (Figure 3)

Figure 3. Activities required for the construction of the Milan walls
The most common hazards in these construction processes are the dirt moving done with heavy machinery (grab cranes, cranes, tractor shovels and trucks), the use of welding and cutting equipment, open air excavations, the handling of bentonitic muds and the reinforced steel grid hoisting (Table 5).  (Table 6). Primary coating. For the cases where the terrain is firm or rocky the procedure for the construction of the shaft changes. In these situations, the nucleus is excavated to an average depth of 1.5m, a circular or polygonal metal ring (which will be used as shore) and concrete (pressure-poured) with an average thickness of 30cm.

Figure 4. Workers' descent in an improvised bucket
The most representative hazards in this process are related to the workers' descent to the bottom of the shaft, even though it is forbidden the use baskets or buckets for the transport of personnel, the rush of activity and the tolerance of the construction managers cause the staff to take risks (Figure 4). on the other hand, load lifting and concrete pouring are also important in terms of risk degree, although they don't reach a fatality risk (Table 7).  (Table 8), but the risk degree highly increases when these activities are performed at great depths and topographers require measurements or fixing tools in hard-to-reach places and without protection ( Figure 5).   For such of activities, which represent a high risk (Table 9), specially for processes 1 and 3, the start of the construction should not be authorized until it is demonstrated with evidence on site that constructive procedures, a detailed risk analysis, an evacuation plan, machinery check-up (cranes) and the certification its operator are available ( Figure 6).  (Table 10). It consists of a structure formed by concrete and a reinforced steel grid, which reinforces the primary wall (primary coating). The associated risks in these activities are very high, as staff must work is the heights to form the steel grids that form the final wall (Figure 7).

Figure 7. Assembly of the grids in the heights
Another risk also with of high-risk degree, is the maneuvers necessary for the assembly of the platform that will serve as a sliding formwork (Figure 8).

Conclusions
Even when it has been noted that risk analyses are an indispensable tool in the security and hygiene areas, many corporations still consider that labor risk prevention represents only a great expense, and they do not see it as an investment, thus originating that risk analyses are a simple requirement to be met and not an opportunity to avoid risks of accidents on site.
In addition, companies have hired staff that do not meet the required profile to be responsible for the safety of workers.
Some companies still have the wrong idea that worker's safety depends solely on them, and they act after an accident occurs instead of preventing it. For example: during the construction of the Eastern Drainage Tunnel, it was noted that personnel responsible for worker's safety on site had experience as firefighter and/or paramedic. Certainly, the prevailing working culture in construction sites shows a tendency towards unsafe behavior often it seams that workers enjoy going against the rules; personnel are reluctant to use IPE with the argument that they interfere with their activities.
Another factor that negatively impacts job security is the time available and almost always tight for the execution of the construction projects. This situation results in that even the project managers themselves justify risky acts during the any process.
Worker's security in construction sites, while still showing deficiencies, has improved greatly compared to previous years. Much remains to be done and significant progress and results can only be expected only if prevention of construction labor risk is supported by high levels, starting with government authorities, workers Union, clients and entrepreneurs of the sector.
Internally, in the construction companies risk prevention must be considered from a systemic point of view, according to which, everyone must engage in a, active way. Thus, constructive procedures can be planned jointly with the security and hygiene area, and the most important, that the resulting strategies are followed on site.

Recommendations
The personnel responsible for the worker's safety must have knowledge of construction procedures to have the possibility to issue recommendations and supervise that workers are not exposed to risk of accidents. On occupational safety issues, construction companies should think more about Prevention and not so much in Reacting after.
Construction entrepreneurs must see the area of safety and hygiene as an area of opportunities and not as an expense. Safety and hygiene personnel must show high commitment and dedication to their activities and to protect their coworkers.
Job safety in constriction sites will only improve substantially when workers will be trained in prevention, construction chiefs will lead by example and the culture of risk prevention will be promoted.
Finally, all the above recommendations reveal the urgency of having construction managers who, in addition to construction experience, must have a solid knowledge in project management, in order to plan, program, organize, direct and control each and every stage and area involved in the project.