The people of Chiapas, in southwestern Mexico, near the border with Guatemala, suffer more than their share of natural disasters. Hurricanes, earthquakes, monsoons, and flash floods batter the region year after year. During the rainy season, from May to October, severe flooding often overflows area rivers, inundating nearby cities and countryside. People and valuable livestock drown every year, and the general property damage is devastating.
Every year, Mexico’s National Water Commission (Comisión Nacional del Agua) and the Ministry of Infrastructure for Chiapas State (Secretaría de Infrasestructura del Gobierno de Chiapas) spend millions of dollars to protect the population by repairing flood-damaged dikes. Typically, they use riprap, gabions, cable blocks, and tetrapods.
Cutting costs and improving protection
However, in 2009, they tried a new idea that is far less expensive, but offers equal or better performance — Tensar Triton polymeric marine mattresses filled with small rocks. The project, carried out through Tensar’s Chiapas distributor, Ecomex, addressed dike failures along the Xelajú, Coatán, and Zanatenco rivers which, in turn, frequently flood the small cities of Motozintla, Tapachula, and Tonalá.
“Sudden downpours can create flooding conditions with these rivers,” said Rodrigo Valencia, Tensar International manager for Mexico and Central America. “High flow volume erodes river embankments and sets the stage for flooding and property damage.”
Jorge A. Nava G, an engineer with Ecomex, met with staff from the commission and its general contractor, Pakal Dredging of Chiapas (Dragados Pakal de Chiapas, S.A. de C.V.). He proposed a value-engineered solution for using Tensar’s Triton Systems to prevent future dike failure. Ecomex analyzed each of the three rivers, developing site-specific designs for the Triton marine mattresses.
“We were sure the work could be completed more quickly and cost effectively using the Triton Systems,” Valencia said. “Our analysis showed that Triton could save the Chiapan government 30 to 40 percent, while delivering equivalent or better performance.” Subsequent contractor comments have confirmed these savings.
Ecomex’s job was to stabilize and reinforce the dikes to prevent undermining and failure. The design had to accommodate a high flow volume, as well as the limited availability of equipment in the area.
“The system is easy to install because the compartments can be filled with small stones,” said Victor Mendez, project manager with Pakal Dredging. “There are no large boulders in the Chiapas region, but we have an abundance of small stones.”
This use of readily available, natural fill material means Triton Systems are much less expensive than conventional solutions such as riprap. The highly resilient, flexible cells also conform to land contours and site configurations while resisting scour far better than rigid systems.
The Triton mattresses were constructed onsite by Pakal Dredging using Tensar BX1500 Geogrid, which resists corrosion as well as biological and chemical degradation. Each mattress measured between 12 to 20 inches thick by 13.1 feet wide by 40 feet long.
Ecomex specified mattresses with internal baffles to create transverse compartments at 3-foot intervals. Each compartment was filled with 4- to 6-inch stones. With the exception of the Zanatenco River project, which required importing stone from a quarry 15 miles away, the crews were able to use local stone for the construction.
Pakal Dredging placed a total of 4,000 units, using a stack-bond pattern. The project required 5 million square feet of geogrid material.
“Using the Triton system enabled the owners to restore the dikes with significant cost savings,” Valencia said. “It’s also important to note that the system was easy to install. That’s an important benefit in a region where technically skilled labor is at a premium and very expensive to hire.”
The mattresses have performed well in the subsequent rainy seasons. “There were a couple of big storms, and the contractor found no damage to the mattresses,” Valencia said.
|Marine mattress benefits
According to Tensar, its Triton marine mattresses offer the Chiapans the following benefits compared with traditional construction:
• strength through their combination with Tensar Uniaxial Geogrids;
• ultraviolet stability, with a minimum of 2 percent carbon-black additive;
• ease of installation without costly dewatering;
• corrosion resistance; and
• slope support as steep as 1.5:1.
Steve Knapp, a writer for Knapp Communications, specializes in the engineering and geosynthetic industry.