8 th Portuguese National Congress on Geotechnical Engineering

Transcription

1 Lissabon April th Portuguese National Congress on Geotechnical Engineering Theme lecture for workshop: Instrumentation of Geotechnical Structures and New Technologies of Information New Developments in Instrumentation and Data Management. Daniel Naterop, Solexperts Ltd. Switzerland ABSTRACT Systematic planning of geotechnical instrumentation is essential for successful application. The checklist presented provides a tool for the systematic planning of instrumentation jobs. Technologies used for other applications have been recently used successfully in geotechnical engineering. Fibre optic systems, Time Domain Reflectometry, Global Positioning System and optical surveying instruments are nowadays also used in many monitoring projects. Loggers at the instrument location, often in combination with radio transmission, have been lately used more frequently in geotechnical applications. An overview of the sensors, instruments and readout elements is presented. More and more computer-based instrumentation is used in geotechnical engineering. The systems provide instantaneous information (data) and warning signals. How can we handle the amount of data and take measures in time? The methods presented include internetbased data visualisation and web servers. The technology is presented together with some selected examples. Both good and bad experiences are mentioned. References: O Connor K.M. and C.H. Dowding 1991 Remote monitoring of rock mass deformation using time domain reflectometry. Field measurements in Geotechnics, Balkema Rotterdam 1

2 1. Introduction In geotechnical engineering our knowledge is based on theoretical concepts, experience, experiments, measurements and observations of the structure s behaviour. In recent years the need for instrumentation and field measurements of geotechnical structures has become more and more important. Some of the reasons are: The observational method in civil engineering combined with good quality data is a valuable tool for small and largescale projects. Safety has always been an important issue. The intensity of construction at difficult locations has increased the necessity of safety. Contracts often allow contractor variants. The risk of new or not yet established construction methods can be reduced with the help of instrumentation. Infrastructure (like roads, railway lines, dams, reservoirs) are often influenced by geologically unstable formations, e.g. landslides, reduction of permafrost. Methods of back-calculation using computer-based modelling are strongly dependent on the input in the model. An important factor therefore is the behaviour measured in situ. Methods of instrumentation, measurement and data handling have been strongly developed. These developments have generally resulted in lower costs for instrumentation and measurements, increased performance of the instruments. The application of new methods in geotechnical instrumentation and Instrumentation and measurements in engineering have to be looked at as a part of planning, construction and maintenance of the structure and have to be planned systematically. 2. A Systematic Approach to Planning Geotechnical Instrumentation References: Lecture Notes of Prof. K. Kovari (Swiss Federal Institute of Technology Zurich) given at FMGM 1992, Oslo Decision making and field measurements in tunnelling (K. Kovari, Ch. Amstad) 25 th OYO anniversary lecture meeting, Tokyo 1979 Geotechnical Instrumentation for Monitoring Field Performance. John Dunnicliff (John Wiley & Sons 1988) In geotechnical engineering, different as it is from many other disciplines of engineering, the forces acting on a structure and the behaviour of the structure itself resisting the applied forces and other influences can only be roughly predicted. The design of soil and rock structures like tunnels, dams, deep excavations and foundations was in the past to a large part a matter of experience. Methods of site investigation, field measurements (including instrumentation) and numerical analysis have in recent decades been introduced and applied increasingly in geotechnical engineering. In the following a short list is presented that gives an overview of some of the points to consider if one is planning instrumentation. For more detailed information we refer the reader to the standard work Geotechnical Instrumentation for Monitoring Field Performance, John Dunnicliff (John Wiley & Sons 1988). 2

3 Geotechnical Problem. Which of the problems that arise are to be solved with the help of instrumentation Forces in structural elements (loads in tiebacks, steel struts) Stresses (in-situ stresses in the rock, earth pressure, stresses in structural elements like piles, diaphragm walls, shotcrete) Pore water pressure and ground water level Wave propagation Other physical or chemical parameters Measuring Principle More or less direct or indirect Pointwise or linewise measurement of displacements Incremental or continuous over time Optical, electrical, or purely mechanical What are the in-situ material properties of Selection of instruments the rock or soil? What accuracy is needed? Determination of the in-situ stresses Safety control and risk management What measuring range is needed? (during construction and the service life of Availability (for purchase, repair and the structure) replacement) Structural behaviour Reliability (Can instrument be replaced with affordable effort and calibrated and checked during monitoring phase) Is reliability near to 100% necessary and at what cost? Back-analysis (calibration of theoretical tools) Check method of construction and advancing state of the art Long-term behaviour monitoring after construction (typically monitoring of dams, tunnels in swelling rock...) Observational method ( design as you go ) Performance tests for instrumentation In many cases different aims are combined in the same instrumentation. What physical parameters are to be measured Deformations (strain, displacement change in curvature) Cost for purchase, instruction, installation, everyday use and maintenance. In what environment and under what conditions will the instruments be operated, installed, stored? Measuring programme Location of points, lines or areas to be measured Frequency of readings in relation to construction phase and loading step 3

4 Method of data transmission (cable, radio link, fibre optic cables, manual) Redundancy of data Who is responsible for instrumentation (organisational chart includes all relevant organisations involved) Data processing How are readings taken Checking accuracy and performance of instruments Checking consistency Assumptions for data evaluation Transforming data to information Data handling that can be easily operated by the persons responsible (data handling using data visualisation, Internet) Contractual organisation of the instrumentation In view of selection of suitably experienced person Method of request for quotation (e.g. specific types of instruments as descriptive specifications or evaluate performance) Instrumentation is provided by subcontractor of the main contractor (as assigned suppliers or not) Instrumentation is provided by subcontractor of the project engineer or client In view of maintaining instrumentation during construction work (damage) In view long term monitoring after construction is completed In view of fact that instrumentation is often considered unnecessary and a disturbance, involving a money loss to the project by the main contractors to carry out the instrumentation work (continuity and motivation). Quality assurance Interpretation of results (not a part of the instrumentation) Problem solution (not a part of the instrumentation) 3. New Instrumentation technologies 3.2 Time Domain Reflectometry Time Domain Reflectometry (TDR) is a measuring system originally developed to detect and to locate breaks in power transmission cables and communication lines. Using a TDR-cable tester an electrical pulse with an ultra fast rise is applied to coaxial cable and the reflection is detected. Pulse reflection from the changes in geometry, caused by a crimp, a kink a short circuits, cable break and the presence of water along the tested cable, is superimposed on the input pulse and forms a reflected TDR-signature. This TDRsignature consists of many individual reflections of the cable. The characteristics of a TDR signature are determined not only by the magnitude of cable deformation but also by the type of cable defect. Assuming a constant pulse propagation velocity, the distance to the cable defect is proportional to the elapsed time between initiation of the input pulse and the arrival of the reflected pulse. To locate the defect of the cable, the tester converts this time difference into a distance and by inspection of the TDR signature and magnitude of the defect determined. 4

5 TDR cable tester readout unit This technique has been adapted in the last years to monitor: Movements in rock and soil for landslideunderground mining etc. Therefore a special coaxial cable is grouted inside a borehole installed inside a landfill Schematic drawing of TDR-instrumentation for landslide monitoring TDR-signal and inclinometer reading at landslide monitoring Time-domain reflectometry is nowadays also a well-known technique for determining water content in unconsolidated soil. This TDR measuring probe is basically wave-guide system, embedded in the soil or rock material. The propagation velocity of the pulse depends critically on the water content of the soil or rock and can be measured with high accuracy. For soil moisture measurements usually dual rod TDR probes are used. This classical Dual Rod TDR consists of two metallic electrodes, which are electrically insulated by a special foil or coating to avoid an electrical short-circuit by water with high conductivity. 5

6 TDR-probe type dual rod probe The Global Positioning System (GPS) is a worldwide radio-navigation system consisting of a constellation of 24 satellites and their ground stations. GPS measures the corresponding distance and calculates its position using these satellites as reference points accurate to a matter of metres. Theoretically, three satellites are enough to define two exact points. To measure enough for geotechnical applications up to 8 or more satellites are used. TDR-probe for monitoring water contents on a surface The relatively low cost of the borehole instrumentation (a coaxial cable) and the ease of taking the readings (manually but also automatic readings can be taken) makes TDR a valuable alternative or addition to the instrumentation using inclinometers and extensometers. TDR cannot detect the amount of movement and its direction but the location (depth) of a shear zone. 3.3 Global Positioning System GPS General description Trying to figure out where you are and where you're going is probably one of man's oldest pastimes, as navigation and positioning are crucial to so many activities. Not so long ago the U.S. Department of Defence decided that the Military Forces had to have a very precise form of worldwide positioning. And fortunately they had the kind of money ($12 billion!) it took to develop something really sophisticated. Two measurements define a circle. Three measurements define a point. GPS receivers have been miniaturised to just a few integrated circuits and so are becoming very economical. This makes the technology accessible to virtually everyone, which is why these days GPS is being installed in cars, boats, aeroplanes, movie making equipment, farm machinery, and even laptop computers. GPS is an excellent instrument for land surveying, monitoring of deformations and 6

7 displacements in geotechnical and other civil engineering projects. Some of the possible applications are: displacement and deformation measurement of large dams base data recording for maps landslide monitoring machine guidance With an advanced set-up of GPS it is possible to measure with an accuracy better than a centimetre! This system named differential GPS (DGPS) involves the use of two receivers, one that is stationary and another that is roving around making position measurements. The stationary receiver has to be erected at a point, which has been very accurately surveyed. This reference station receives the same GPS signals from the satellites as the roving receiver. Instead of calculating its actual position, it attacks the equations backwards. The difference between what the travel time of the GPS signal should be and the actual value results in a error correction factor. The receiver then transmits this error information to the roving GPS, which uses it to correct its measurements. The accuracy of the measurements can be improved with: point to be observed has minimal signal shade (no trees, steep mountain slopes, houses, etc.) prolonged measurement period selection of a measurement time with good satellite constellation post-processing with special software including additional data from other instrumentation Application for machine guidance The "Unique" Airport Zurich justifies its name in the building sector, in that it employs a uniquely accurate slip-form pager with automatic direction and height control for the construction of taxiways. At a width of 6m and a thickness of 36cm, the taxiways are being built with an accuracy as low as a millimetre. All taxiways of the 5th expansion of the airport are being built in this way. A new type of machine guidance system with Total Stations and GPS constantly monitors the position of the machine and feeds it with steering information. Metal stakes and control strings are no longer required and thus do not slow down the work Taxiway pavement at Unique Zurich Airport 7

8 Bulldozer equipped with GPS The system included Leica 300 sensors (receivers) one receiver, as a object point, was installed on the roof of a large building and the other receiver, as a reverence point on the roof of a shaft of the mine. Data transmission between the two sensors was done by radio link and the data from the reference point to the PC, where data processing took place, was transmitted by data cable. This so-called GOCA-system provided, based on spline-adjustment and trend estimation, the following data for horizontally directed and vertical displacements Leica GPS 500 System receiver on pole Applications for geotechnical engineering projects Example of online subsidence monitoring for the Lohwies-Hall in Reisbach/Saarland (Subsidence Area) Ref GOCA project example Reisbach and Trimble web site. In this area in Germany in recent decades underground mining and the excavation of large areas were carried out. This resulted in large settlements at the surface that endangered the stability of many buildings. A permanent monitoring of this subsidence area first using a classical deformation network was established by the Karlsruhe University of Technology in cooperation with the Deutsche Steinkohle Co. and from spring 1999 on using GPS measurements deformations were measured during a 7 months period. During this time horizontally directed deformations of up to 25cm and vertical directed deformations of up 90cm were measured. In recent years GPS has been used more and more in geotechnical engineering projects. This has included applications like: Landslide monitoring: In New Zealand a national network to monitor land movement has been created. Therefore 20 Trimble Continuously Operating Reference Stations (CORS) GPS receivers have been commissioned for use in a network and for field work to monitor land movement and the movements of the earth's crust to millimetre accuracy. This network will also provide accurate control points for New Zealand surveyors, who will be able to access surveying data through an online internet-based bulletin board service. In Japan and some other 8

9 countries GPS networks have been used for a number of years to continuously monitor land movements and to provide accurate control points for surveyors and various organisations. Deformation monitoring of large structures (e.g. bridges, dams, high rise buildings). Dam deformations at the Diamond Valley Lake in Southern California are being monitored with 7 Leica type CRS1000 receivers located on the dam to be monitored and 2 receivers as reference points located about 8km away located on stable rock. Specially developed algorithms are being used for high-precision real time monitoring and alert. The accuracy for the horizontal position of a single point is about 10mm and for the vertical position about 50mm. If each end of the reservoir is analysed independently, precision can be improved to about 4mm horizontally and 10mm vertically. Ref: Epoch by Epoch positioning applied to dam deformation monitoring at Diamond Valley Lake, Southern California (Y. Bock, P. J. de Jonge, D. Honcik, M. Bevis, L. Bock, S. Wilson) GPS is nowadays a highly advanced technology allowing positioning of a point up to just a few mm accuracy. Some points to focus on are: The cost of a single point to be monitored is relatively high. GPS is therefore interesting in combination with other methods (e.g. optical surveys, pendulums, extensometers). Depending on the position of the satellites used for calculation, the method applied and the requirements of accuracy of a single measurement, data processing and an alert can take from a few minutes to several hours. Data has to be transmitted to and from the receiver using cables or radio link and the receivers require power supply (batteries, solar panels or cables). Receiver on a point with known coordinates serves as reference for Differential GPS. GPS cannot be used inside buildings, in tunnels and caverns. Also near buildings (e.g. at the downstream side of a concrete dam) and in a steep valley the performance of a GPS receiver can be reduced to a large extent. GPS receivers have to be visible from a series of GPS-satellites. 9

10 during grouting. This set-up allowed monitoring one point only with each single digital level. The instrument itself could be located on a pier that was assumed to be stable. To read other points the instrument had to be redirected to this new point and re-focussed to its distance. Leica GPS System 500 hand-held receiver 3.4 Motion-controlled digital levels Digital levels, used for surveying and in civil engineering, combine the function of a traditional level with image processing. Therefore, instead of staffs with a metric scale, barcode staffs are used. The digital level reads the bar-coded staff with a CCD-camera and transforms the image with image processing software into a mm-reading. High quality instruments on the market are accurate to 1/10 mm. Schematic layout of an optical level This gave us to the idea of developing a unit to redirect (rotate) and focus the instruments with a motion control unit. This control unit has been adapted to be mounted on the digital levels of Zeiss and Leica. The level is controlled with two stepper motors, one for the rotation of the level and one motor for focussing the optics. A separate interface basically includes surge protection, power supply, control of the spotlight mounted on the level to illuminate the staffs also at night and connectors for the data bus line. A temperature sensor also connected to the interface records ambient temperatures and enables, if necessary, temperature compensation of the readings. The motioncontrolled level is often placed within the area of vertical movements. 2 to 3 bar-coded staffs are placed in the area where no settlements or heave are expected. The level takes readings on these reference points to detect its own vertical position. This optical measuring principle allows readings of staffs at distances ranging from 2 to 100m from the level. The illumination on the level reaches to a distance of up to about 40m. For staffs that are further away, external staff illumination has to be installed. Normally, 0.5m long pieces of a staff acting as reference and measuring points are mounted on poles to a wall or under a ceiling. In the course of a project to refurbish a weir in the Rhine in Switzerland Solexperts used digital levels to monitor different weir piers 10

11 Light for staff illumination Solexperts Motion control unit Motion controlled Optical Digital Level In some projects several levels have been used in a chain to monitor large areas or structures, using on-line calculation to determine and display real-time settlement data for the entire system. The accuracy achieved with digital levels depends on the set-up of the instrument, reference and measuring staff, the repetition of individual readings on one staff (mean values or median) are then used for further calculation) and the stability of the reference points. Also atmospheric conditions will have an influence (mainly refraction). Based on experience of very different projects measurements are accurate to within +/- 0.3mm to 0.5mm. After some small scale projects in Switzerland, this type of instrument has been used very successfully in different projects (a selection of some): Berlin, Potsdamerplatz. A existing underground railway station was monitored over a period of 6 years. (40 points were measured in 1 to 3 hour intervals) Bratislava. An adjacent old building had to be monitored during nearby excavation for an underground car park Berlin. Bridge monitoring during the construction of a lock situated next to the bridge foundation Underpinning of an old building in Zurich Chemnitz. Monitoring of a railway bridge during underpinning of two bridge piers. The following figure shows settlements and heave measured on a tall building in Berlin during grouting under the foundation and piling next to the foundation. After completion of grouting and piling the building was vertically aligned by means of compensation grouting. GSW Berlin, cross section, geology, constructional measures, instrumentation Settlement and heaving due to the construction measures 11

12 3.5 Motorized Total Station General description of the Motorized Total Station A Total Station in this context is an electronic theodolite to measure the vertical and horizontal angles equipped with an electronic distance measuring device (EDM). It is used for three-dimensional coordinate measurement. The values (distance and angles) are displayed digitally and are normally transmitted to a memory card. Advanced Total Stations have been equipped with servomotors to position the telescope on to the targets combined with automatic target recognition, to aim the telescope very accurately to the centre of the reflective target (prism). Since the locations where the Total Station can be placed are unstable, a network of points which are called reference points is installed at places where no movements are expected. The local coordinates of the station are determined for each scan record using the recent measurements. The redundancy of measurements requires an adjustment that is done by the Helmert-transformation. The number of measured reference points, the standard deviation and the residuals are recorded allowing statistical evaluation and filtering out of erroneous measurements by setting a limit e.g. for accuracy. Due to the influence of temperature and air pressure on the distance measurement (measuring medium is light) sensors for atmospheric conditions are part of the monitoring system. In recent years Total Stations have been used a lot for the automatic monitoring of deformations and displacements in geotechnical and other civil engineering projects. Some of the applications are: Monitoring of structures adjacent to and above tunnels in construction Deformation measurement of a large lock Monitoring of the deformation of the walls of a large and deep excavation Landslide monitoring Dam monitoring Therefore Total Stations are combined with a monitoring system (e.g. the Solexperts GeoMonitor) that enables immediate calculation of the results, checking the displacements calculated for alarm limits and automatic transmission of the results and alarms to the engineers in charge. The accuracy achieved with Total Stations depends on different factors. The main points to look at are Accuracy of the selected Total Stations Type of target and number of repetitive readings on reference points and measuring points Distances and angles to the reference points and the measuring points on the structure to be monitored Atmospheric conditions, refraction 12

13 With this system displacements of a single point in a range of about a 100m (distance between the Total Station and the measuring points) can be detected within 1-3 mm accuracy. To optimise the performance of Total Stations, one should beforehand plan carefully the layout of the system and carry out a pre-analysis to detect the accuracy and from this optimise the layout of the set-up. Total Stations are optical precision instruments that include servomotors and technically highly advanced elements for positioning the telescope and taking readings. In normal automatic and continuous use Total Stations need servicing and re-calibration. This should not be neglected. Following pictures how a building with the measuring points installed at a tunnelling project in Bremen and a graph with settlement over time. 3.6 Extensometers with Logger and Radio Data Transmission Different types of extensometer are used to measure displacements along predefined sections. Borehole extensometers, single or multipoint, are fixed in position in the borehole using a cement-clay-water grout or using different types of expandable anchor. The rods within a protective tube are joined together in the extensometer measuring head. Reading of the extensometer is carried out using a dial gauge or a portable displacement transducer. If an extensometer has to be read from a remote location (e.g. for a potential landslide area with difficult access to the extensometer location) displacement transducers are installed and by means of a cable the individual transducers are connected to a readout box or logger. On a potential landslide with the possibility of rock fall cables may get damaged. Also in tunnelling cables are often affected and damaged by blasting and anchoring. A new type of extensometer developed by Solexperts in Switzerland with a measuring head that includes displacement transducers and the logger in combination with radio link data transmission is briefly described below. The main features of this type of extensometer are: Extensometer head, made of stainless steel, can be completely installed within a borehole of only 60 to 70mm diameter and is fully sealed against 10 bar water pressure To install extensometers in vertically upwards directed boreholes and in formations with water overpressure a mechanical packer to seal the borehole head can be pushed over the extensometer head. Grouting pipes and vent lines are fed through the packer to allow grouting under also difficult conditions. 4 to 8 displacement transducers can be integrated within the extensometer measuring head. The measuring range of the transducers varies from 50mm to 250mm or more. The logger, also within the sealed extensometer head, includes signal conditioning, and storage for values. With a 13

14 software, operated on a palmtop or laptop PC, the logger is set up and data is downloaded. Downloading is normally done via cable link or using a small size radio transmission module allowing data to be transmitted over distances of 100m or more. All electronic parts, transducers, the logger and the radio link module can be reused for other extensometers and removed for changing batteries, recalibration etc. Extensometer head with radio transmission module Displacements recorded over time in Swiss tunnel with different excavation stages 14