Mexico City, and its metropolitan area, was built over what was once an area of five lakes - Chalco, Texcoco, Xaltocan, Xochimilco and Zumpango. As it has grown, the city has gained ground from the lakes, and consequently faces a major threat of floods. The East Outlet Tunnel (Túnel Emisor Oriente), one of the biggest sewage projects in the world, will help to avert a catastrophe.
Sinking 10 centimeters a year
Over the last 100 years, parts of Mexico City have sunk by nearly 12 meters – the height of a 4-storey house. Huge volumes of groundwater are pumped above ground to serve the needs of nearly 20 million inhabitants, which has led to the city sinking at the rate of 10 centimeters annually.
The bulk of the city is constructed on swampy subsoil, and since more groundwater is extracted from this terrain can be replaced by precipitation, it becomes even more compact and the city above continues to subside. As a result, various infrastructures in the city, including buildings, roads and sewage systems, have been extensively damaged. To compound matters, the city faces problems of flooding during the rainy season.
Mexico City thus had two artificial water outlets and a drainage system in operation until 1925, when subsidence caused by the extraction of underground water unleashed a huge flood.
In the 1930s, the metropolis witnessed the start of a massive population explosion, growing from one million inhabitants to two million by 1940, three million by 1950, more than five million in 1960, and more than ten million in the 1970s. During this time, numerous drainage facilities were built, including regulating dams, kilometers of drain lines, pumping plants, west intercepting sewers, and piping for the La Piedad, Churubusco and Consulado rivers.
“Many thought it would save us definitely…”
1967 saw the start of work on a new construction project nicknamed “The Deep Drainage System”. It embodied a novel approach, consisting of two intercepting sewers with a diameter of 5 meters and a combined length of 18 kilometers, at a depth that varied from 30 to 50 meters. The intercepting sewers discharged into the 50 km deep emitter outlet with a diameter of 6.5 meters. Inaugurated in 1975, the system was considered by many to be definitive.
Loosing 30% of its capacity
But that proved to be a grave error of judgment. The central emitter outlet continues to sink, presenting serious problems. Its gradient is becoming increasingly more precarious and has in the meantime lost around 30% of its original capacity of 170 m³/s. In some places pumps have to be used to carry the sewage because the gradient has reversed. Year for year, serious floods occur because the wastewater system is no longer able to cope with massive downpours or almost double the population.
The central emitter outlet remains closed for repair and maintenance during low water level months. This means an alternative emitter outlet is needed to maintain operational capacity throughout the year since the current deep drainage system is insufficient for the present requirements of the Valley of Mexico.
New “Túnel Emisor Oriente”
The new planned “Túnel Emisor Oriente” should prevent flooding in the Valley of Mexico, reduce the risk of drainage system failure and implement a procedure that allows inspection of the drainage system without interrupting operations. The concrete will be pumped to a depth of around 95 meters over a maximum distance of 1000 meters. Approximately 230,000 m³ of concrete are required for the segment that ICA is responsible for.
230,000 m³ of concrete are required
The solution chosen to meet the above specs consists of the two additives Plastiment Liquid and SikaViscoflow. Application of Sika ViscoFlow® technology in the form of a combination of Sika ViscoFlow®-10, Sika ViscoFlow®-20 E, Sika Visco-Flow®-50 and Sika® Plastiment® Liquid produced high-quality concrete which fulfilled workability retention time requirements. What is more, the compressive strength/time specifications were more than satisfied by the combination of mix design and Sika admixtures.
Sika Mexico delivers solutions for this project as well as on-site technical support, thus contributing cutting-edge technology to Mexico’s development effort. The Mexican Water Board CONAGUA has embarked on a major project in the form of the Túnel Emisor Oriente scheme, which is unparalleled anywhere in the world and absolutely essential for Mexico City itself.
Apart from being able to coordinate the groundwater table more effectively in future to prevent the city from subsiding further, the more than 60 km long collector – designed to provide a more effective sewage system and enhance the quality of life in the mega-city – is of decisive importance.
The authorities and the population are convinced that the world’s biggest sewage project will reintroduce an element of quality to life in the city.