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Reinforcement and deformation monitoring of a railway embankment in France
Compiled and edited by Ron Bygness
Introduction
This project was the first practical realization of the RAFAEL research project for the use of high-strength geosynthetics for base courses over cavities, including the first “intelligent” geosynthetic. This product combines the benefits of a high-strength geocomposite with those of optical fibers. It enables designers to both reinforce soil structures and detect and evaluate possible deformations and settlements to provide timely maintenance. Usage can increase safety factors and enable cost-effective construction, especially in areas prone to sudden settlements, such as karstic and mining zones, or with embankments on piled foundations.
The railway line between Mouchard and Bourg in the east of France crosses areas where underlying strata are suspected to contain large cavities. Since damage had occurred in the past, the client (SNCF–French Railways) decided to reinforce one of the most sensitive sections with geosynthetics.
To avoid any additional risks, a permanent monitoring of the section was required. Here, the detection geosynthetic product was an optimum solution, fulfilling both technical and economical requirements. In addition, it is insensitive to electromagnetic interference, which is unavoidable in the vicinity of electrified rail tracks.
Mouchard-Bourg rail line
The superstructure of the single-track section consisted of 500mm of ballast underlain by a 500mm filter blanket. The section reinforced and monitored with the detection geosynthetic had a length of 50m. The following surface-settlement criteria “s” were chosen for design and monitoring:
- warning at s = 6mm
- reduction of speed at s = 9mm
- closure and maintenance at s = 21mm
The design of the geosynthetic was carried out using an analysis method derived from the French Railways RAFAEL project from the late 1990s (see sidebar). In this case, an ultimate tensile strength of 300 kN/m was required. The configuration of the sensors assumed a cavity diameter of 1.2m. The design led to a sensor spacing of 850mm between both the longitudinal optical lines and the sensors along each line. This resulted in a total of 300 sensors within the entire section that was monitored.
To minimize impediment to traffic, all work was completed in just one night between 23:00 (11 p.m.) and 6:00 (6 a.m.). In this 7-hour timespan, the top layers were removed, the geosynthetic installed, and both base course and ballast reinstalled again. The line was reopened for rail traffic as scheduled at 7:00 (7 a.m.). Despite the difficult conditions (darkness, lack of space, bad weather) the tight time schedule was met. This was only possible because Geodetect had been equipped with all necessary cables and connections at the factory.
Initial measurements showed that the system fulfilled all requirements.
The product and its applications
What is it and how does it work?
The Geodetect system was developed for the measurement of strain, allowing the survey of earthworks structure with or without reinforcing geosynthetics, such as in roads, railways, retaining walls, tunnels and underground structures, and pipes for gas, water, and oil.
This system may be used in all applications where a survey is required, either for critical structures or construction following the observational construction method.
This system also measures and follows the strain in soil and enables transmission of a warning signal as soon as its elongation reaches a pre-set limit. This permits intervention at an early stage before further development of soil subsidence results in damage to the structure. Thus, it can significantly reduce the cost of rehabilitation works, and railways or roads may still be used to a limited extent until maintenance is comlpeted.
Description
Geodetect is a geotextile-based monitoring system, consisting of a high-strength geotextile, equipped with optic fibers connected to a monitoring device.
Both permanent and temporary monitoring are possible, depending on the needs of the project:
- Using a portable and weather resistant registration device, individual and punctual data can be covered from different structures. This is useful when data have to be covered over a long period of time, but permanent monitoring is not necessary.
- For permanent monitoring and recording of the data, the use of a datalogger is recommended, which may be installed on site. The data can be transferred by standard telecommunication cables or via GPRS to a central unit.
Performance
This product offers an economic solution to ensure the safety of the structure (embankment, building, pipeline, railway) and prevent the damage and its consequences in the sensible areas (soil subsidence, compressive soils, etc.). Primary advantages are:
- high reliability (insensible to the environment, such as electro-magnetic interference)
- very high sensitivity (deformations of < 0.01%)
- optimized solution for the reinforcement, the elongation measurement and the warning
- high durability in soils
- interrogation system adjusted to the needs of the user
- survey and monitoring of very large areas is possible
- applicable to a wide range of geosynthetics for reinforcement in combination with the strain device.