DISPLACEMENT MONITORING IN CABRIL DAM USING GNSS


able to measure small-amplitude movements, either low-frequency or high-frequency[1]. Although these characteristics make GNSS use possible in a wide range of applications, GNSS observation in dams aims to measure, at strategic points, the three displacement components, usually of small amplitude and exhibiting slow variations (low frequency variations). The permanent displacement observation using GNSS at notable points is an important contribution to the dam’s structural response monitoring over time.


This type of monitoring requires installation of a GNSS station at the dam points to be monitored, and at least one GNSS station located outside of the dam, but within its proximity, at a suitable location (fixed station), which is designated as a reference station, to allow the use of GNSS in relative mode – its most accurate mode. This method measures the coordinate difference between a station positioned in the dam structure and the reference station located outside of the dam (Figure 2). This distance is called the “base” or “vector”. If one of the stations, being the reference station, is fixed, the permanent base measurement allows the horizontal component of the second GNSS station to be directly measured (located at the object point).


Figure 2. GNSS system installed at Cabril dam (to measure displacements at the top of the central section)


3. Cabril Dam and GNSS System


Cabril dam is located on the Zêzere River and has been in operation since 1954. It is the highest arch dam in Portugal, with a maximum height above the foundation of 132m. Its crest stands at 297m of elevation, and has a length of about 290m. Cabril dam is a double curvature arch dam located over a granite mass rock foundation.


A GNSS system, financed by FCT, was installed at Cabril dam by LNEC and EDP in 2016 to enable continuous automatic measurement of the three displacement components at the top of the central section[2], with a sampling frequency of 20Hz. This system includes a GNSS receiver located at a point outside the dam (about 200m away), which is being used as reference, and another GNSS receiver that was installed at the crest centre, the object point


Vol XXXI Issue 3 DAM ENGINEERING 151


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