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TS 7 – Underground Structures INGEO 2011 – 5th International Conference on Engineering Surveying Brijuni, Croatia, September 22-24, 2011

Borehole Wire Extensometer for Measurement of Small Displacements

Mentes, Gy.

Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, Csatkai E. u. 6-8., Sopron, Hungary, Web site: www.ggki.hu Tel.: +36 99 508348, E-mail: mentes@ggki.hu

Abstract Continuous recording of landslide movements is required for better understanding of the

relationships between triggering factors and dynamics of the movements. On the one hand the vertical resolution of geodetic measurements (GPS measurements and precise levelling) is not sufficient for detecting small vertical movements caused by e.g. rainfall events and evapotranspiration on the landslide area, on the other hand this survey implies tedious and expensive campaigns of monitoring. When observations are not continuous a lot of information relevant to the events is lost. For continuous measurement of very small vertical displacements a highly sensitive borehole wire extensometer was developed. The extensometer is installed in a borehole and consists of an invar wire anchored to the bottom of the borehole. The wire is placed around a pulley which is held by a frame attached to the surface of the ground and is kept in tension by means of an iron counterweight. The position of the counterweight is measured by an inductive position sensor. The measuring range of the extensometer is 5 mm and the scale factor of the instrument is 2.57±0.004 V/mm.

In this paper the construction and calibration of the instrument are described and its practical application on the high loess bank of the River Danube is demonstrated.

Key words: landslides, monitoring, borehole, wire extensometer, small displacements

1 INTRODUCTION In 2002 a geodetic network was established to study the dynamics of the high loess bank

of the River Danube at Dunaföldvár, Hungary (Mentes et al., 2009). Geodetic measurements (GPS, precise levelling) were carried out between 2002 and 2005. Since geodetic measurements are not suitable for the detection of small movements (<1µm), two continuously recording borehole tiltmeters were also installed at the test site, one on the top and the other at the toe of the high bank. The tiltmeters have been recording since 2002. The tilt records showed small periodic daily movements, therefore to better understand the cause of these surface movements a borehole extensometer was developed for continuous measurement of near surface vertical movements of the high bank. The extensometer was installed on the top of the high bank near to the tiltmeter.

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Fiber optic strain sensors are widely used for continuous displacement measurements in landslide monitoring (e.g. Moore et al., 2010) and in engineering geodesy (Brunner, 2009). Until now the electronics of these sensors is complicated and expensive. Corominas et al. (2000) used invar wire extensometers with a cheap electronic sensor for the measurement of landslide displacements. These instruments have a potentiometer as displacement sensor but under rough environmental conditions the potentiometer has contact problems and this is why it is not reliable in continuous field measurements.

Our aim was to develop a cheap extensometer with a very simple electronics and very low energy consumption, which can be left in the field without maintenance for years and which works also in rough environmental conditions. This was the reason for developing an invar wire extensometer with the simplest and most reliable sensor electronics which is not sensitive to the variations of environmental parameters: temperature, humidity, etc. In this paper the construction of the extensometer and results of the measurements are described.

2 CONSTRUCTION OF THE BOREHOLE EXTENSOMETER Extensometers measure the change of distance between their two endpoints. In the case of

wire extensometers one end of the wire is fixed to one end of the distance to be measured, the other end can be moved freely and this movement relative to the other end of distance is measured by means of an electronic transducer which transforms the displacement of the free and of the wire into electric signal. For measurement of real distance variations invar wire is used due to its very low coefficient of thermal expansion. The construction of our instrument can be seen in Figure 1.

steel bear frame pulley

steel weight

surface

concrete block

invarwire

concrete bedconcrete weight

PVCcasing

data loggerproximitysensor

Figure 1 Construction of the borehole wire extensometer

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One end of the invar wire is anchored to the bottom of the borehole by a concrete mass which is fixed to the borehole by concrete during installation. This part of the borehole is not caused by PVC tube for the reason of a stable connection of the lower end of the wire to the ground at the bottom of the borehole. At the upper end of the borehole which is about half a meter below the surface there is a concrete block around the borehole ensuring a good connection to the ground. This concrete block holds a steel bear frame with a pulley which can revolve round its axle fixed to the frame. The invar wire is placed around the pulley and it is spanned by a steel weight. The vertical motions of the ground between the ends of the borehole cause the displacement of the steel weight position of which is measured by an electromagnetic proximity sensor type BALLUFF. The sensor is in a closed casing, so it is not sensitive to humidity. The measuring range of the sensor is 4.5 mm. The sensor is installed in a way that at rest it is about 2 mm far from the steel weight, so the movement of the weight can be measured in the range of ± 2 mm. This solution provides a contactless measurement. The sensor needs 12 V power supply and its output voltage is in the range of 0-11 V. The data is collected by a datalogger with a rate of 1 sample/hour. The batteries and the datalogger are placed in a thermally insulated steel box dug in the ground.

The proximity sensor was calibrated by a laser interferometer in laboratory. According to our investigations the sensitivity of the sensor depends on the dimensions and shape of the steel piece the displacement of which is to be measured. For this reason the calibration was made by the steel weight of the extensometer used as counterpart of the proximity sensor. The steel weight was immovable and the proximity sensor was fixed to the stage of a microscope and moved against the steel weight by the micrometer screw of the microscope. The displacement of the proximity sensor was measured by means of a HP 5508 laser interferometer in the range of 0-5 mm far from the steel weight. The characteristic of the electromagnetic proximity sensor was measured several times. Figure 2 demonstrates a series of the measurements and the regression line fitted to it. The steepness of the regression line gives the sensitivity or the scale factor of the sensor. The average scale factor of the sensor obtained from repeated measurements is 2.57±0.004 V/mm. The linearity error of the sensor is less than 0.03 mm.

Figure 2 Characteristic of the BALLUFF proximity sensor

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3 OBSERVATION OF THE VERTICAL DISPLACEMENTS OF THE HIGH BANK OF THE RIVER DANUBE AT DUNAFÖLDVÁR, HUNGARY

The vertical extensometer was installed close to a high sensitive borehole tiltmeter on the

top of the high (30-50 m) loess bank of the River Danube at Dunaföldvár (Figure 3) in June, 2005. The length of the instrument is 3 m for observation of the small vertical movements of the upper layer of the high bank due to the pore pressure variations of the soil which are caused by the precipitation and vital processes of the vegetation.

The analogue output signal of the extensometer is digitalized and stored by a datalogger Campbell Scientific XR 10. A battery of 12 V with a capacity of 55 Ah provides for the supply voltage of the extensometer and the datalogger. The instrument can work more than 50 days without changing the battery. The extensometric data and the temperature of the borehole bottom were measured and stored every hour.

Figure 3 Digital Terrain Model of the Dunaföldvár test site with the location of the instruments (made by A. Gyimóthy).

Red points are GPS monuments, blue points are the locations of the tiltmeters. Figure 4 shows the daily averages of the extensometric data (VD), the water level

variations of the River Danube (DWL) and the precipitation (P) from September 2, 2005 till December 31, 2010 to demonstrate the relationship between vertical displacements and the water level and precipitation.This connection is not always obvious because the vertical displacement depends also on the groundwater level which was not recorded continuously in this period. Higher groundwater level causes higher pore pressure in the underground of the high bank and this causes a contraction in the upper layer of the high loess bank in contrast with the precipitation which soaks the upper layer of the high bank, increasing the pore pressure and so causing a vertical dilatation of the soil. This effect can be seen very well in Fig. 4. During the high water of the River Danube the groundwater level is higher than it is usually causing small contraction in the above described manner. This effect, especially during the high speaks of the water level of the river can also be seen in Figure 4.

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Figure 4 Vertical displacement (VD), water level variation of the River Danube (DWL) and

precipitation (P) data between 02.09.2005 and 31.12.2010. The gaps in VD data series are due to datalogger problems.

Inspecting the curve of vertical displacement the question arises whether there is a

seasonal effect in the extensometric data record or not. To get an insight into the feature of the extensometric signal the effect of the temperature was also investigated. Figure 5 shows the daily averages of the temperature and vertical displacements. Apparently there is a close correlation between the two data series but there is not a constant phase shift between them. The correlation coefficient is very small: 0.14. Shifting the curves to each other to reach the best fitting, the correlation coefficient will be not higher than 0.2. That means that the temperature has very small direct effect onto the vertical extensometer, so it can be left out of consideration.

One reason for the phase shift between vertical displacement and temperature is that the temperature is measured at a 3 m depth in the borehole which causes a constant phase shift. The other reason is that the precipitation and the evaporation cool the soil which hinders the penetration of the heat into the deeper layer of the soil. The random rainfalls cause a random phase shift between the vertical displacements and temperature. The phase shift varies between 30 and 90 days.

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Figure 5 Relationship between vertical displacement (Ext.) and borehole temperature

To show the short periodic vertical movements the hourly recorded extensometric data

were filtered by a high pass filter with a cut-off frequency of 0.04 cycle/hour to eliminate the long- periodic displacements. Figure 6. shows the short periodic displacement variations and the borehole temperature. The vertical displacement amplitudes are somewhat higher between May and September when the surface temperature (about 30-60 days phase lead relative to borehole temperature) is high and the plants need more water for their vital processes than in other period of the year (Bódis and Mentes, 2010).

Figure 6 Seasonal variations of short periodic vertical displacements (Ext.) and the borehole

temperature Figure 7 shows a short record of the water level variation of the River Danube,

precipitation, vertical displacement and tilt data. The vertical displacement varies mainly due to the precipitation and the displacement amplitudes slightly depend on the water level of the river. While the vertical displacement amplitudes are high during and after rainfalls, the tilt amplitudes are high during dry periods (Bódis and Mentes, 2010) and depend much more on the water level of the River Danube then vertical displacement amplitudes.

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Figure 7 Short periodic vertical displacements (VD), tilt, precipitation (P) and water level

variations of the River Danube (DWL) between 12.09.2005-19.02.2006

4 CONCLUSIONS The developed vertical extensometer is a cheap instrument for the measurement of vertical

displacements of local geodynamic processes, e.g. observation of landslides, ground and object motions in engineering geology and geodesy. Its length can be some 10 metres depending on the order of the magnitude of the motions to be measured since its measuring range is determined by the measuring range of the electromagnetic proximity sensor. The demonstrated example of application for landslide movements on the high loess bank of the River Danube proves that the instrument has very good temperature stability and high resolution to study the movements and deformation of the ground due to rainfall, groundwater variations and vital processes of the vegetation. Acknowledgements

This research was supported by the Hungarian National Scientific Research Fund (OTKA) in the research project K81295. The author is very grateful to Frigyes Bánfi and Tibor Molnár for their assistance in the development of the instrument.

REFERENCES

BÓDIS, V.B. - MENTES, Gy. 2010: Investigation of connection between surface mass movements and vegetation. (in Hugarian with English abstract), Geomatika, 2010, (in print).

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BRUNNER, F.K. 2009: Faseroptische Sensorik: Ein Thema für die Ingenieurgeodäsie? Vermessung und Geoinformation 97, 335-342.

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MENTES, G. - THEILEN-WILLIGE, B. - PAPP, G. - SÍKHEGYI, F. - ÚJVÁRI, G. 2009: Investigation of the relationship between subsurface structures and mass movements of the high loess bank along the River Danube in Hungary. Journal of Geodynamics, 47, 130-141, doi:10.1016/j.jog.2008.07.0005.

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