Rescuing Mexico City's drainage

This article describes a case of real application in which different procedures were carried out, in order to rescue the deep drainage of one of the most populated cities in Latin America and prevent its flooding with wastewater.

by: Pedro Sánchez

In October 2007, a catastrophic flood took place in the city of Villahermosa in Tabasco, State of Mexico, triggering a state of emergency that endangered thousands of lives. The flood had many repercussions in the capital, Mexico City, where 20 million people lived with the potential danger of a similar flood. The aging of the city's sewer system had not received proper maintenance for 35 years due to time and cost considerations.

This situation was not only a municipal concern in Mexico City, but also a geographical one. For many years, this city has had to fight continuous flooding due to the nature of its location.

The fact of having as a precedent being a city founded on land reclaimed from an ancient lake plus the need to evaluate the waters of the city, forced the authorities to build a massive system of canalization to drain the waters outside the limits of the city.

After the devastating experience in Villahermosa, the government recognized that it could no longer ignore the deterioration of Mexico City's drainage system and decided to inspect the main line of this system to assess its condition.

Inspection
The public body "Sistema de Aguas de la Ciudad de México," in charge of the city's sewer system, ordered a team of engineers to inspect the pipe system that has been in service for more than 35 years. Due to its large size, 65,616.7 meters long and 6.5 m in diameter, the personnel in charge of carrying out the inspection had to use small boats to carry out the reconnaissance route. The inspection was carried out to verify the conditions of the tunnel and the damage suffered by the concrete walls of the pipe.

Once the tunnel was emptied, engineers were able to use an all-terrain vehicle to make the route and assess the damage in detail. Based on the findings of the engineers, the authorities decided to protect the pipe with a resistant coating, in order to extend the life of this important engineering work.

Problem
According to the engineers' report, the area at the top of the pipe had been attacked by acid vapors generated by the decomposition of organic matter in the water. The decomposition of the drainage water produced sulfuric acid, high amounts of methane, and hydrochloric acid, all of which accelerated the aggressiveness of the chemical attack on the pipe.

Along with a decrease in pH, the consequent risk of passivation in the concrete was greater, thus affecting the steel of the pipe structure. Passivation is known as the protective layer that covers the steel in concrete and protects it against accelerated deterioration due to corrosion. In addition, the transport of all types of debris had generated considerable wear by abrasion on the concrete walls, resulting in cracks and exposed steel to the point of compromising structural integrity, of not receiving timely treatment.

The applied product is a two-component, high-performance epoxy coating designed to protect metal and non-metallic surfaces against attack by aqueous solutions. An attractive feature of this product is its ability to be applied by spray in "wet" conditions of the substrate and the environment, it is also able to resist up to 50°C in immersion conditions and is recommended to be applied in two layers of 10 thousandths of an inch each, for a total dry film thickness of 20 thousands.

The product consists of a base and a solidifier which are mixed in proportions of 3 parts of base by 1 part of solidifier. The presentation of this product is available in three colors, which helps to easily determine where the two required coating layers have not been applied. For this specific application the coating was applied in two different colors beige and gray.

Works
The equipment used for this application was a plural pump which works automatically, dosing the base and solidifier components of the product properly. The base and solidifier of the product were placed separately in two hoppers located on either side of the pump. Each hopper was equipped with a silicone rubber heating band to increase the temperature of the base of both and solidifies up to 65C. in order to lower the viscosity of the components and thus facilitate the application of the coating.

A special vehicle was custom designed to carry the spraying equipment, the product, the crew, and a scaffolding, the latter allowing personnel to reach the top of the pipe without inconvenience. Because the surface on which the carriage was to be moved was not flat, the carriage was constructed with diagonal wheels, which facilitated movement along the pipe walls.

Personnel
All personnel in charge of the application wore safety harnesses and personal protective equipment (PPE), including respirators, goggles, suits and gloves. Applicators were trained to carry out work in confined spaces. Gas detectors were used to constantly monitor the air quality inside the pipe. Industrial fans located on the surface were also used to ensure that fresh air constantly recirculated in the tube.

Each member of the work team had to inform a safety supervisor each time they accessed or left the tunnel so that in this way the personnel in charge of the safety of the work had control of the people who had accessed the tunnel in the event of an emergency.

As usual in this type of work, a safety meeting was held every morning before accessing the tunnel located 70 to 80 meters underground to talk about the potential risks present during the working day and to review the steps to follow in the event of an emergency situation.

Surface preparation
The preparation of the surface was carried out by a qualified company, hired for this purpose. The surface was pressure washed using 5,000 psi of jet water to clean the substrate and achieve a minimum of 3 thousand surface profile as stipulated by NACE Regulation No. 5/SSPC-SP 1210. Environmental parameters such as surface temperature, dew point and relative humidity were considered, keeping track of all these parameters.

Application
The application was made by Ivresse International, a local company in Mexico City." The office of the Water System of Mexico City" established a period of 28 days to carry out the application of the product. The approximate total area of deteriorated concrete to be covered was 12,166.9 m2. The work was executed in the dry season, during the dry season when the water level in the system was the lowest.

For this application, the tunnel was strategically divided into several sections of 87.5 m each. The application during the first day consisted of spraying a layer of 10 thousands of the product in a section of 87.5 m of tunnel. The next day, a second layer of 10 mils was applied in the same coated section the day before, and a layer of 10 mils was covered in the next 87.5 m section of the tunnel. The same procedure was repeated until the completion of the work. In all cases, the first layer was gray and the second was beige. This was done intentionally to facilitate the application of the second layer and avoid holes and failures. All the work lasted 30 days, the post-application inspection was carried out by an independent team qualified to ensure the correct application of the standards.

Conclusions
The epoxy coating resistant to severe dry conditions or under immersion has proven to be an effective solution to extend the life of the drainage system in Mexico City. Not only will this product help prolong the operational capacity of this system, but it will also provide resistance against the attack of the generated acids.

It is recommended that this unique drainage system be inspected periodically to carry out the relevant maintenance work and thus avoid high costs of major maintenance.

* Technical Service Engineer at Belzona Inc. He can be reached at 2000 NW 88 Court, Miami, FL, (305) 594-4994, www.belzona.com, [email protected].