Slope Displacement: Geotechnical Measurement and Monitoring · 2017-11-20 · Slope Displacement: Geotechnical Measurement and Monitoring Solexperts, Switzerland Today, the Chain

Slope Displacement: Geotechnical Measurement and Monitoring Solexperts, Switzerland

Slope Displacement: Geotechnical Measurement and Monitoring Dr. A. Thut, dipl. Eng., Solexperts AG, Schwerzenbach, Switzerland 1 INTRODUCTION In addition to monitoring potential landslides, the geotechnical measurement of slope or bank deformation is essential in the planning phase of buildings, streets, tunnels, and dams. Alongside existing structures, slopes can become unstable due to release of stress on the slope foot following excavation work, or through natural erosion or porewater level changes. Modern automatic geodetic and geomechanical measurements are utilised in advance investigations as well as long-term monitoring. For the permanent monitoring of unstable slopes, geodetic measurements, as well as other various measurements, can be carried out automatically. A PC controls the system, and the measurement values are recorded and can be transferred via GSM or telephone modem to an external station. With the addition of alarm functions, a high level of safety can be achieved. Geomechanical measurements begin where the geodetic measurements leave off, and provide displacement measurements in the slide body itself. The measurement systems, their applications, their predictive strengths, and their limitations are described below. 2 GEOMECHANICAL MEASUREMENTS WITH PORTABLE INSTRUMENTS 2.1 Objectives of Geomechanical Measurement Cracks appearing on the surface of a slope (such as around the edge of a sliding body) are an indication that large displacements have already taken place and that sliding planes have already been formed. In such cases, risk analysis must include supplemental observations with portable instruments. Long-term creep or sliding in an early phase often does not manifest as surface changes. For these situations, only high-precision geomechanical measurements can provide information about movement and potential dangers. Primary displacement and inclination changes within a borehole or on the surface are determined with geomechanical measurements. The goal of this type of measurement is to obtain information about: a) The location of the slip surface b) The movement mechanism c) The volume of the sliding mass d) The rate and rate of acceleration of movement through periodic measurements e) Potential sliding occurring before movement appears on the surface Together with the topography, the geology and the hydrogeologic relationships, geomechanical measurements form a basis for risk analysis for landslides.

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benutzersol

Neuer Stempel


Slope movements can occur slowly over longer periods of time (e.g. Gotschna slope Klosters) without visible signs of sliding. Under such circumstances, geomechanical measurements are employed to study the mechanisms of the slide for the projection of tunnels and the foundation of viaducts (Example: Lehnen viaduct Beckenried, foundation in shafts). 2.2 Measurement Principle: Linewise and Pointwise Measurements There are many instruments available for a variety of needs in geomechanical measurement. Instruments fall in the categories of linewise and pointwise measurement instruments (fig. 1) (K. Kovari 1981). Both measurement principles have their specific applications and complement each other. Linewise measurement is generally carried out with portable instruments. The borehole inclinometer (i.e. Slope Indicator), Sliding Micrometer and Sliding Deformeter are instruments for linewise observation. Measurements with these instruments are labour- and time-intensive, but provide the advantages of comprehensive descriptive information and the opportunity to calibrate the instrument between measurements. Through regular calibration the portable instrument can achieve a high level of accuracy and long-term stability. With linewise observation, continuous displacement profiles can be determined, with the help of inclination measurements over time, meter by meter, along a borehole. With linewise measurement (fig. 1a) the shear plane of a slope can be precisely located, and possible creep can be detected.

Time t

x

Dep

th z

x Anchor

Fig. 1: Linewise and pointwise measurement

Pointwise measurement instruments integrated into an automatic data acquisition system enable the continuous monitoring of deformation. The extensometer, if the anchor is fixed at a point beyond the slip surface, is an instrument for pointwise measurement. With the extensometer, the displacement of the free-moving measurement head on the surface relative to the fixed anchor is measured (limitations of the application see 3.1.1).

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Today, the Chain Inclinometer is also available for installation in boreholes for automatic monitoring of inclination change (see 3.1.2). However, it is more expensive than the Borehole Inclinometer while providing equivalent information. 2.3 Instruments to Determine Displacement Profiles along a Measurement Line 2.3.1 Borehole Inclinometer With the Borehole inclinometer (Hanna 1985) which has a length 0.5 or 1.0 m (fig. 2) an inclination change in relation to the vertical dimension (i.e. by using earth gravity) can be used to observe the lateral excursion in the vertical plain.

Fig. 2 Borehole inclinometer and grooved casing

Instruments for both vertical and horizontal boreholes are available. In vertical boreholes, two planes perpendicular to one another are measured, while with a horizontal probe, only the vertical planes can be measured. These probes have been in use for over 25 years and are employed worldwide. The vertical probes are mostly bi-axial with two inclinometers in two plains, oriented 90° relative to each other. The two axes are referred to as A and B. Predominantly Servoinclinometers are used. A mass is held by force in the vertical plain - this force is proportional to the inclination and is measured by the inclinometer. Technical Data for the Borehole Inclinometer Probe length: 500 mm and 1000 mm Measurement range: ±30° and ±90° Maximum depth: 100 m Accuracy: 1 – 2 mm per 10 m The probe is guided by grooves in the measurement casing (øO 50, 70 or 84 mm). The measurement casing is installed such that one pair of grooves lies in the direction of maximum movement; when installed in a slope, this is the fall of the slope, direction A.

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Positioning of the probe every 0.5 or 1.0 m (depending on probe length) is enabled by markings on the probe cable with respect to the head of the measuring tube. Positioning is not highly reproducible, especially in deep boreholes, and can cause errors while measuring. Settlement and related compressive strain of the measurement casing can also be a source of error and must be taken into consideration. The measurements are always taken in 2 positions; after the first measurement, the probe is rotated 180° and the measurement is repeated. 2.3.2 Sliding Curvometer The Sliding Curvometer (fig. 3) consists of two 1-m long elements, which are connected in a joint. The joint measures the amount of displacement ∆x and ∆y. New possibilities are opened with this instrument, as it is applicable in boreholes of any inclination. Especially at locations that are difficult to access, as landslide areas often are (fig. 4), it is more cost-effective to make horizontal boreholes from the foot of the slope. Technical specifications for the Sliding Curvometer Probe length: 2.0 m Measurement range: + 20 mm per m Accuracy: 0.05 mm per position For a measurement casing 20 m + 3 mm long With the Sliding Curvometer, it is important to take into account that an open polygonal course is measured, and because of this, error propagation is unsatisfactory. However, for determining the location of the slip surface, as well as for establishing the size of the slide mass, the differential displacement is used. Grooved measurement casing is used, and the casing is installed in the same way as for the Borehole Inclinometer.

+x

1m

1m

+y

Groovedcasing

Instrument

Fig. 3 Sliding Curvometer

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Fig. 4: Monitoring an unstable slope with the Sliding Curvometer in a sub-horizontal borehole

2.3.3 TRIVEC All three components of the displacement vector along a vertical borehole axis are measured with the TRIVEC probe (Köppel et al. 1983) (fig. 5). Displacement is measured in the z direction (extension/contraction) along the borehole axis, as well as the x and y excursions in two vertical plains normal to each other. The instrument is a further development of the Sliding Micrometer (Kovari et al. 1979, Kovari and Fritz 1984), with which only the axial extension/contraction is measured along boreholes of any inclination. The TRIVEC is essentially a Sliding Micrometer with two inclinometer sensors for measuring ∆x and ∆y. The two sensors are oriented at 90° to each other in the x- and y-axes. Unlike the conventional borehole inclinometer which is applied in grooved casing, the TRIVEC is applied in measurement casing which consists of a chain of reference points with cone-shaped measuring marks (fig. 6). In comparison to the Borehole Inclinometer, positioning of the TRIVEC probe is precisely defined and exactly reproducible. The measuring marks are located within the telescopic, moveable coupling elements of the PVC casing.

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Grout

Measurement casing

Measuring mark(cone-shaped)

Probe head(ball-shaped)

Inclinometers

Inductivedisplacementtransducer(LVDT)

Guide rods

Measuring mark(cone-shaped)

Probe head(ball-shaped)

Fig. 5 TRIVEC

Fig. 6 Stepwise placing of the TRIVEC in the measurement casing

Measurement casing

Instrumentmeasuringposition

Coupling/measuring mark

The measuring marks hold the two sphere-shaped heads of the probe in place for a measurement. If the two measuring marks are displaced in relation to each other as a result of displacement in the rock formation or soil, the change in distance (extension/compression) and the change in inclination is determined from the difference between measurements at two different times. The measuring marks are cone-shaped whereas the probe heads are sphere-shaped; with the ball-cone seating principle, the location of the centre of the sphere is precisely defined. This simple seating principle explains the high measurement accuracy of the system. Technical specifications for the TRIVEC Probe length: 1000mm Measuring range: +7.5°, adjustable to +15° Accuracy per position: ∆z ±0.003 mm/m

∆x, ∆y 0.005 mm/m Accuracy per 10 m casing: ∆z 0.01 mm ∆x, ∆y 0.05 mm

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The measuring marks and probe are notched such that when the probe is rotated 45°, it can glide within the casing to the next measuring mark, and measurements are taken each meter.

x+

Measuring positionSliding position

Fig. 7 Probe in sliding and measuring positions

For the measurement, the probe is again rotated 45° and pulled into place in the measuring marks. The probe heads are at each end of a telescoping tube, braced with a spring. An Invar calibration tool is used to check the functioning of the probe and to determine a zero point, as well as to amplify the signal from the inductive displacement transducer and the clinometers. It consists of two measuring marks with a precisely defined distance (Amstad et al., 1987) for the z axis, and with the possibility to generate a precisely defined inclination with the help of end blocks. TRIVEC Measurement Examples Gotschna Slope, Klosters, Switzerland The Gotschna slope in Klosters has been continuously surveyed for 50 years by geodetic means. Movement can be up to 50 mm per year. TRIVEC measurements were conducted at several locations in preparation for a new tunnel for the railway and for a Klosters bypass. One borehole was drilled from the existing railway tunnel to a depth of 40 m and TRIVEC measurement casing installed (fig. 8). The measurement line intercepts the slide zone. The measurement results are shown in fig. 9. Included are the differential displacements in the x and z directions, as well as the profile of the displacement vectors. This is a typical example of a slide with two definite slip surfaces. It is interesting to compare the differential displacements in the z-axis with the horizontal displacements in the x-axis. At a depth of 10 m, differential settlements show 2 peaks, and horizontal displacement shows only one peak. The shearing appears to be accompanied by a consolidation or possibly erosion below the shear zone.

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Alluvials

Train tunnel

x

z

0 40m

Moraine?

?

?

Fig. 8 Position of the instrumented borehole in the RhB railway tunnel at Klosters

Strain z [mm]

10

20

30

Dep

th [

m]

3 9 14 22 27 35

3 9 14 22 27 35

Inclination change z [mm/m]

2 4 6

10

20

30

40

8 10 12-2

Months afterzero measurement

Dep

th [

m]

10

20

Dep

th [m

]

20

10 20

10

x [mm]

z [m

m]

0.5 1.0 1.5

Displacement vectors

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Fig. 9 Displacements in the sliding slope with the distribution of expansion and the deformation vectors In two other zones of the Gotschna slope, 4 additional boreholes were instrumented with TRIVEC measurement casing (fig. 10). The boreholes B17 and B18 display a typical example of displacement vectors in an upward shear.

Fig. 10 Displacement vectors in a cross section of the sliding slope at Klosters; boreholes 17 and 18 show upward shear

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Bärentritt at Klöntalersee For the slope stability study of the rock slope at Bärentritt, 4 boreholes up to depths of 85 m were equipped with TRIVEC measurement casing. Fig. 11 shows the displacement vectors for two boreholes in a cross section of the slope. In borehole SB2, the first slip plane is found at a depth of 72 m and a second, less distinct plane is found at 25 m. The displacement vectors run parallel. The borehole SB3 is located in the sedimentary filling in the Nacken Valley. The measured deformations are a result of a following block slide triggered by the sliding of the downhill massive.

33 mm

Distometer

Messlinie58 mm

35 m

m59 mm

Fig. 11 Displacement vectors in the rock slope at Bärentritt

2.3.4 Sliding Micrometer and Sliding Deformeter combined with the Borehole Inclinometer The Sliding Micrometer and the Sliding Deformeter belong to the family of portable measurement instruments that determine the strain distribution along a borehole axis. The measurement casing is conceived the same as for the above-described instruments. The choice to use a Sliding Micrometer or a Sliding Deformeter is determined by the required precision. The Sliding Micrometer, with the higher accuracy of +0.003mm/m is more appropriate for applications in rock and concrete with high deformation moduli, and the Sliding Deformeter, with an accuracy of +0.03 mm/m is more appropriate in more deformable formations. When measurement casing with both guide grooves and measurement marks is installed, the Sliding Micrometer or the Sliding Deformeter can be applied in combination with the Borehole inclinometer in order to determine all 3 components of the displacement vector. The horizontal components are not measured at the exact same point as the vertical, because the two instruments have a different construction and thus a different measuring position (fig. 12). The inclinometer is seated approximately 10 centimetres below the seating of the Sliding Micrometer/Deformeter.

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The advantage of combining measurements from both instruments is the high accuracy of the strain measurements along the borehole axis. The displacement vector, which with few exceptions has a vertical component, enables the early detection of deformation.

Fig. 12 Measurement position for the Sliding Micrometer/Deformeter (left)

and the Borehole Inclinometer (right)

X

Z

xi

t

Zi

Pi

slip zone

ZiPi xi

l

Pi-1Pi-1

Fig. 13 Sliding zone between two neighbouring measurement markings a) Sliding downwards: shortening b) sliding upwards: lengthening

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3 AUTOMATIC AND PERMANENT MONITORING For safety reasons in situations where the public is endangered, e.g. roads or towns located below unstable slopes, linewise measurement is supplemented by or replaced with permanent monitoring systems consisting of various sensors and automatic data acquisition. These measurements, introduced as pointwise measurements, primarily involve deformation measurements performed for example with extensometers, optical displacement measurement with automatic levelling instruments, theodolites, or inclination measurement with permanently installed Clinometers. Also, the measurement of piezometric elevations is classed as pointwise. In existing TRIVEC measurement casing, permanent strain gages and inclinometers can be installed to continuously monitor the critical slip zones. An automatic system such as the Solexperts GeoMonitor (which will be described here as a representative of all such systems) is employed for the acquisition of data. A PC with the data acquisition software and the SGC (Solexperts GeoMonitor Controller), which controls the sensor measurements, is at the heart of the system. Each sensor has its own address. The SGC, directed by the PC, takes addresses each sensor, one after the other over a single cable, a so-called bus cable. With this system, hundreds of sensors can be serially connected in a modular system according to the users‘ needs.

motion-controlledoptical digital level

Modem

Total tation

SolexpertsDAVIS

Software

PC withGeoMonitor

SoftwareSGC withWatchdog

and Alarm switches

BUS cable

Messzentralevor Ort

Monitored slide area

External Station

Modem

Multiplexer/Interface

Various sensors

Fig. 14 GeoMonitor, an automatic and permanent data acquisition system with a single bus cable The measurement values are automatically saved, calculated, and displayed in numeric and graphic formats. The data can be transferred at any time to anywhere in the world via modem from the on-site PC to a remote office. Alarms can be set to go off when measurements exceed defined boundaries - triggering such things as flashing lights, automatic fax notices, or pagers.

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3.1 Sensors for Automatic Monitoring 3.1.1 Rod Extensometers By installing fixed anchors at great enough depths (beyond the possible slip zones, to act as a reference point) the surface movement as well as the relative movement between anchor and measuring head can be detected by an integrated displacement transducer. Multiple-point extensometers enable the measurement of displacement between up to 12 anchor points in a borehole.

Mechanical orelectricalmeasurement

fixedmeasuringhead

Grout

Fixed anchor

Formable shist

Rock

uo

Fig. 15 Simple extensometer mono rod Potentiometric or inductive displacement transducers carry out the measurement of displacement. The measuring range can vary widely; normally a range of 50 mm is built in, with an accuracy of 0.1 mm. The measuring range for mechanical measurement at the end of the rod is around 300 mm. 3.1.2 Fixed Clinometer and Chain Inclinometer The danger of falling formation blocks normally engenders, besides translation, the danger of rotation (fig. 16). The change in inclination of such blocks is measured automatically and continuously with mounted inclinometers.

Fig. 16 Displacement of a rock on a plastic plane, translation and rotation

Fig. 17 BL 200 A

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To permanently measure the inclination change in boreholes and measurement casing, the chain inclinometer can be employed. The elements, which are equipped with uni- or bi-axial Clinometer probes, come in lengths of 1.0, 2.0 and 5.0 meters. For reasons of cost, electrolytic levels are used. The measurements are temperature-dependant and are therefore either predominantly lab-compensated or corrected by software in the data acquisition system. The accuracy is about 0.1 mm/m; temperature effects and drift are discussed in separate literature (Dunnicliff John 1988). Inclination can also be measured with portable instruments such as the Clinometer BL 200 A (fig. 17). In an example situation, at the potential rockslide in Oelberg, the displacement measurements taken with extensometers were supplemented with measurements with this portable Clinometer. A high-precision (0.005 mm) calibration device is used to determine the zero point and to calibrate the instrument. Temperature effects can be cancelled out by conducting the measurements in two positions; i.e. the instrument is turned 180° for the second measurement. 3.1.3 Fixed Strain and Inclination Measurement Instruments for Sliding Micrometer and

TRIVEC Measurement Casing Special installable instruments have been developed for application in existing measurement casing from linewise measurement instruments (TRIVEC, Sliding Micrometer, etc.) for the continuous, automatic observation of displacement. The instruments, the Fixed re-Installable Micrometer FIM (fig. 18) and the Fixed re-installable Inclinometer FIN, are installed in measurement casing in highly deformable zones and are automatically and continuously monitored with the automatic data acquisition system GeoMonitor. The high-precision instruments can be removed for calibration or for reuse in other boreholes (Naterop et al. 1991).

Measurementcasing

Probe braced betweentwo measuring marks

Fig. 18 FIM, Fixed re-Installable Micrometer in TRIVEC or Sliding Micrometer measurement casing

3.1.4 Pressure Sensors Piezometric elevations can be measured at up to 5 levels in a borehole. For this purpose, piezoresistive pressure transducers are most often used (fig. 19). If possible, these sensors should be installed so that they can also be removed or replaced. Technical Specifications for Piezoresistive Pressure Transducers: Measurement range: numerous ranges starting at 1 bar Accuracy: 0.2% of the measurement range at a constant temperature 0.5% of the measurement range at temperatures between 0 and 50°

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The installation is carried out in steps, with the most important factor being the sealing of the borehole between the various levels. Each sensor is installed in a 1-m filter section filled with quartz sand. The seal between these filter sections is made with a clay-cement suspension.

Inner casing (removable)

Piezometer casing

Pressure sensor(removable)

Filtertip with geotextile

• Readout unit• GeoMonitor

Sensor seating

Sensor cable

Fig. 19 Extendable pressure transducer for porewater pressure measurement

3.1.6 Motion-controlled, Automatic Digital Level For automatic, continuous level measurements, digital levels such as the Leica NA 3003 or Zeiss DiNi 10 (fig. 20) can be integrated into the GeoMonitor System. Solexperts has developed motion control and focusing motors so that these instruments can automatically monitor numerous points for vertical displacement.

Fig. 20 Motorised optical digital level, Zeiss DiNi 10

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3.1.6 Automatic, Optical Geodetic Measurements with Total Stations Motorised total stations such as the Leica TCA 1800 (fig. 21) can carry out the automatic monitoring of displacement in the x, y and z directions. Modern software enables multiple total stations to be installed in a net, with an online, robust least-squares compensation of net being executed in real time. A relatively high accuracy can be achieved with a motorised total station: ∆z < 0.5 mm ∆y, ∆y < 1.0 mm at distances from 150 m

Fig. 21 Motorised Total Station, Leica TCA 1800

3.2 Overvoltage (Lightning) Protection Electrical surges due to lightning can damage automatic measurement systems. Therefore, it is of great importance and high priority that the system components include protection against overvoltage. 4 CONCLUSION Geomechanical measurements are elemental for the study of unstable slopes. The most commonly measured displacement components are strain and inclination change. Either linewise or pointwise measurements can be made - linewise measurements measure all three orthogonal components of displacement along a measurement line, enabling a detailed interpretation of the slide mechanism, such as shearing with or without consolidation, creep, etc. Pointwise measurements are, together with automatic measurement systems, of significance for continuous and automatic acquisition of displacement data. The data observed over time can provide predictive information about the development of the potential slide. Alarm functions provide an additional element of safety. All measurements can be interpreted on the basis of geologic and hydrogeologic studies; on the other hand, measurement results can supplement geologic and hydrogeologic studies.

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REFERENCES Hanna, T.H. (1985): Field Instrumentation in Geotechnical Engineering, Trans Tech

Publications

Köppel, J., Amstad, Ch., Kovari, K. (1983):

The Measurements of Displacement Vectors with the "TRIVEC" Borehole Probe, Proc. Int. Symp. on Field Measurements in Geomechanics, Vol. 1, Zurich, Balkema, Rotterdam

Kovari, K., Amstad, Ch., Köppel, J. (1979):

New Developments in the Instrumentation of Underground Openings, Proc. 4th Rapid Excavation and Tunneling Conference, Atlanta, U.S.A.

Kovari, K., Amstad, Ch. (1983) Fundamentals of Deformation Measurements, Proc. Int. Symp. on Field Measurements in geomechanics, Vol. 1, Zurich, Balkema, Rotterdam

Kovari, K.,Fritz, P. (1984): Recent Developments in the Analysis and Monitoring of Rock Slopes, Prac. IV Int. Symp. on Landslides, Vol. 1, Toronto, Univ. of Toronto Press

Kovari, K. (1988): Methods of monitoring Landslides Vth International Symposium of Landslides, Lausanne

Naterop, D., Köppel, J. (1991) FIM: A new Precision Fix Installable and again Removable Strain Meter Field Measurments in Geomechanics 3rd International Symposium 9-11 September 1991, Oslo, Balkema, Rotterdam

Otto, B., Hauenstein W. (1997) Stability of Rock Slopes at the Banks of Reservoirs Selected case study from a Swiss Hydropower Scheme, ICOLD 19, Florence 1997, Vol. Q74, R

Dunnicliff, J. (1988) Geotechnical Instrumentation for Monitoring Field Performance, John Wiley and Sons 1988

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Slope Displacement: Geotechnical Measurement and Monitoring · 2017-11-20 · Slope Displacement: Geotechnical Measurement and Monitoring Solexperts, Switzerland Today, the Chain