Fiber optic geotechnical instrumentation has been designed for use at El Mauro tailings dam in Chile. IWP&DC takes a look at the world’s first project for this type of application
Located in the mountains north of Santiago, Chile, El Mauro tailings dam is being built as part of the Los Pelambres mine project. One of the largest private copper producers, Los Pelambres is owned 60% by Chilean Anaconda-Antofagasta (Luksic group), 15% by Nippon Mining & Metals Co, 10% by Mitsubishi Materials Corp, 8.75% by Marubeni, 5% by Mitsubishi Corporation and 1.25% by Mitsui and Co.
Approximately 1.4km wide, El Mauro will have a final height of 240m, at an altitude of 938m asl. Work began on the infrastructure of the dam following environmental approval received in 2004. This approval allows Los Pelambres to increase its mineable reserves to 2.1B tones of ore taking into account the increased storage capacity of the dam. Expected to cost around US$450M, the dam is scheduled to be completed in 2007.
In September 2005, Los Pelambres awarded a C$287,000 contract to roctest Ltd for instrumentation at El Mauro, including the installation of fiber optic sensors and on site training. Chilean Roctest representative Geomediciones was responsible for the design of the instrumentation project – a first for this type of application with fiber optic instruments.
Immune to electromagnetic interferences and static electricity found in high altitudes, fiber optic instruments can offer an advantage over the traditional vibrating wire technology sensitive to harsh environments. This is important as Los Pelambres mine is located at an altitude of 3200m where dry air produces static electricity. The area is also affected by earthquakes, which will be monitored by the installation of seismographs connected to the fiber optic instruments so that high-speed dynamic measurements can be taken during a seismic event.
The El Mauro project involves 30 Fiber Optic Piezometers model FOP (Dynamic and Static automated data sampling), four temperature sensors FOT-N and one barometric piezometer for correction of recorded data, with cable lengths from 600 to 1300m up to the instrumentation house. Two data acquisition systems consisting of one DMI-32 for static readings and one BUS System for dynamic readings have also been supplied. At the instrumentation house a complete server specially developed by GeoMediciones operates software that downloads and analyses the dam 24 hours a day with online transferred data.
Fiber optic piezometers
The FOP series of fiber optic piezometers is designed to measure pore-water or other fluid pressures. The design is based on non-contact deflection measurement of a stainless steel diaphragm, as opposed to more conventional measurement techniques.
The pressure transducer is based on Fabry-Perot interferometry. Pressure creates a variation in the length of a Fabry-Perot cavity consisting of the inner surface of the stainless steel diaphragm on one side and an optical fiber on the other.
The FOP is designed to be embedded in earthfills, at concrete/earth interfaces or inserted into boreholes and small-diameter pipes. The end of the FOP is fitted with a high or low air entry filter, which protects the sensing element from solid particles, allowing this model to sense only the fluid pressure to be measured. The filter is designed to be easily removable for saturation.
The FOP series has been designed to provide the following features:
• Intrinsically safe
• Immune to EMI/RFI/Lightning
• Long term reliability
• Accuracy of ± 0.25% F.S.
• High resolution
• Rugged stainless steel construction for harsh environments.
FOT-N Temperature Sensor
The compact size of the FOT-N temperature transducers, together with their resistance to corrosive environments, makes them suitable for temperature measurements in harsh environments.
Like the FOP series, FOT-N is designed to be intrinsically safe and is immune to EMI/RFI/lightning. The sensors can be used at a temperature of up to 350°C and provide high accuracy. The temperature gauges are based on the thermal expeansion of highly stable glass, which, according to Roctest, allow precise, stable and repeatable measurement.
The model is fitted with the extrinsic Fabry-Perot fiber optic transducers enclosed inside a protective stainless steel tube.
Fiber optic multi-channel field datalogger (Model DMI)
The DMI is a multi-channel universal fiber optic field datalogger ideally suited for performing multi-point temperature, pressure, strain and displacement measurements in applications hostile to non-fiber optic transducers.
The DMI datalogger is designed for applications that require continuous monitoring of a large number of measuring points. The DMI datalogger is capable of measuring the absolute cavity length of Fabry-Perot transducers with great accuracy. It is compatible with fiber optic transducers, including temperature, strain, pressure, displacement and force and load. Roctest’s transducers feature complete immunity to microwave and RF radiation, high temperature operating capabilities, intrinsic safety and non-invasive use.
The DMI has a 0.01% F.S. resolution (without averaging) and 0.025% F.S. precision. Use of a Flash ROM allows the user to easily upgrade the signal conditioner firmware. The DMI has a non-volatile memory buffer that can store up to 50,000 data samples and datalogging sequences. Duration and other operational parameters are easily programmable using RS-232 remote control and user-friendly software.
Fiber optic multi-channel system (Model BUS)
The Model BUS is a multi-channel, simultaneous-reading fiber optic signal conditioner with fast sampling rates and compatibility with most of Roctest Telemac’s fiber optic transducers.
The BUS system is designed to offer the speed and versatility required for the most demanding experiments. The BUS system allows simultaneous multi-channel measurements at sampling rates of up to 1000Hz, making it useful when multi-channel, simultaneous fast data recording is required.
Each channel has its own 10V analog output and is independently controlled through the RS-232 communication port. The sampling rate for each channel can be set individually to 100, 500 or 1000Hz.
The BUS system comes in a 19inch industrial rack chassis that can incorporate from 1 to 8 channels. BUS Systems can be cascaded for an even larger number of channels.
Fabry-Perot sensor technology
The Fabry-Perot sensors in use at El Mauro are used with broadband white light source instead of laser light. This highly sensitive technique can make precise, absolute and perfectly linear measurements without stabilisation means, according to Roctest. Fabry-Perot sensor is based on interferometry principles.
The gage construction is illustrated in Figure 1. The extrinsic Fabry-Perot gage consists of two semi-reflective mirrors facing each other. These mirrors are deposited on the tips of multimode optical fibers and these fibers are spot fused into a capillary. The air gap between the mirrors is called the Fabry-Perot cavity length (lcavity) and the distance separating the welded spots is called the gage length (Lg) and dictates the gage operating range and sensitivity.
A portion of the white light launched by the readout unit is reflected by the first semi-reflective mirror. The rest of the light travels through the Fabry-Perot cavity and is partially reflected a second time by the next semi-reflective mirror. The light from the two reflections will interfere and travel back in the readout unit toward a detector.
When the gage is bonded to a substrate, the strain transferred to the gage is converted into cavity length variation and the strain is given by the following equation:
The design of the pressure sensor is based on a non-contact measurement of the deflection of a stainless steel diaphragm, as opposed to the more conventional measuring of the diaphragm’s deformation. When the gage is under pressure, there is a variation of the Fabry-Perot cavity length made by the inner surfaces of the stainless steel diaphragm on one side and the tip of an optical fiber on the other side. Readings are via simultaneous-reading fiber optic signal conditioners and designed to offer the speed and versatility required, allowing simultaneous multi-channels measurement at sampling rates of 1000 Hz, ideal when complex fast data reading is required giving a perform of multi-point temperature, pressure, strain and displacement measurements.
The authors of the article are Claus Fahrenkrog, CEO of Geomediciones and Jacques Boily P.Eng from Roctest.