AUTOMATED MONITORING FOR GEOTECHNICAL ENGINEERING PROJECTS

Transcription

1 AUTOMATED MONITORING FOR GEOTECHNICAL ENGINEERING PROJECTS Daniel Naterop, Solexperts AG, 8603 Schwerzenbach, Switzerland Schulstrasse 5, Postfach 230 CH-8603 Schwerzenbach Switzerland Tel.: Fax: INTRODUCTION: Recent developments in the field of geotechnical measurement enable a large number and wide variety of instruments to be integrated into a data acquisition and monitoring system (Solexperts GeoMonitor) for automatic, around-the-clock monitoring. The GeoMonitor is installed and operating at a wide variety of sites throughout Europe for construction projects and for long-term safety monitoring. Included in this paper are three such projects in which the GeoMonitor System is implemented; an arch dam in Switzerland, a tunnelling project beneath a railway embankment in Switzerland, and a lock facility in Slovakia. Two notable developments have been the addition of motion-control motors to optical digital levels to automate measurements and acquire data from vertical displacement measurements, and the integration of Total Stations (theodolites) into the Solexperts GeoMonitor to enable automated 3-dimensional displacement measurements. 2. DATA ACQUISITION AND MONITORING SYSTEM The GeoMonitor system was developed to make the data acquisition and monitoring of complex geotechnical projects like those described in this paper effective and cost efficient. Numeric and graphic displays of both recorded data and real-time measurements can be viewed, reports automated and the sensor and program parameters easily modified. Remote access to the sites via modem allows the system status and program parameters to be viewed and controlled from the office and the site data to be transferred to the office. The DAVIS software enables office personnel, even those not immediately involved with the data acquisition system, to "visualise" the site using computer graphics for evaluation of site conditions and status of the project. GeoMonitor is a modular system for automatic data acquisition and monitoring of up to 240 sensors per site. Fig. 1 is a schematic overview of the system. 1

2 Various Sensors SITE Motorised Optical Digital Level Multiplexer/ Interface Totalstation BUS Cable EXTERNAL STATION MEASUREMENT CONTROL CENTER SGC with Watchdog and Alarms PC with GeoMonitor Software Modem Modem Solexperts DAVIS Software Fig. 1. Solexperts Automatic Monitoring and Data Acquisition System GeoMonitor The PC-based software is designed to work in concert with the Solexperts GeoMonitor Controller (SGC). The ability to monitor a wide range of output signals from previously existing instruments as well as newly developed instruments (analogue current or voltage, digital RS232/485, impulse, etc.) is incorporated into the system. All sensors are connected to the monitoring system with a single bus line. Security against system lock-up and modem access problems is provided by a device referred to as the "Watchdog". The Watchdog continuously monitors the system and phone lines and reinitialises the system if problems occur. The components of the system are protected against electrical over-voltages (i.e., lightning) which is of special importance for projects located in a mountainous region, as is the Valle Di Lei dam. The needs and problems encountered by geotechnical personnel working in the field with data acquisition systems were taken into consideration during the development of the GeoMonitor system. Many of the routine operations performed by geotechnical personnel operating a data acquisition system have been automated and built into the system. Some features used in the projects discussed in this paper are listed below: Real-time numerical and graphical displays of the data with alarm features to alert personnel when a sensor measurement exceeds a given tolerance. (Alarm messages are transmitted to faxes, pagers and telephones.) Automated daily reports keep personnel up-to-date with numerical and graphic output of daily measurements. Data files are automatically separated by week or month to maintain files of manageable size at the Valle Di Lei dam and the Zurich construction site. Binary and ASCII files are written for easy access and security. Calculated sensor measurements are derived from a mathematical expression using actual sensor measurements, and are used to record and display a variety of "realtime" calculated values. At the Valle Di Lei dam, calculated sensors are used to record and display reservoir-level-compensated deformations as well as for the flow through V-notch weirs. Calculated sensors are used at the Lenzburg site to record and display compensated deformations relative to a fixed point. 2

3 File Solexperts Data Visualizer "DAVIS" Options SOL EXPERTS AG Clinometer 2 Ni20 (X) 3 Ni20 (Y) 4 Ni20 (X) 5 Ni20 (Y) 6 Ni20 (X) 7 Ni20 (Y) Digital Level 8 Na Na Temp Pore-water Sensor Window Pressure Nivel 20 Level Sensor Number 8 Cha Data Window Ala # 1 21-Apr-94 17: Ala # 2 NA Graph Window # 3 # 4 Level Pillar Sensor 8 # # # 7 # Apr 28-Apr 5-Mai 12-Mai 19-Mai 26-Mai Fig. 2. Solexperts Data Visualisation Software DAVIS Monitoring complex geotechnical projects often results in great quantities of data that quickly become unmanageable. Without a tool to easily and efficiently obtain a comprehensive overview of the data, little is gained from the monitoring efforts. The DAVIS software in combination with the GeoMonitor, provides an efficient way to handle and review data from multiple sensors and manually taken measurements. The software maintains a data base of the project measurements. A pictorial display of the project site shows each sensor's location, alarm type and status, together with sensor, data and graphics windows for a selected sensor (Fig. 2). 3. VALLE DI LEI ARCH DAM The Valle Di Lei double arch dam is situated at the border of Switzerland and Italy. The dam itself is in Switzerland, while the majority of the reservoir is in Italy. The dam was constructed in the 1960s; it has a height of 142 meters, a crest length of 635 meters, and a reservoir capacity of about 2 million m3 of water. 3.1 Monitoring System In 1994 the Solexperts GeoMonitor automatic monitoring system was installed to automate measurements which had previously been taken manually. The system included sensors for measuring precipitation, air and concrete temperatures, seepage, porewater pressure, pendulums and extensometers for measuring deformations, and most importantly, reservoir level. Figure 3 is a schematic overview of some of the instrumentation. All sensors are integrated into the automatic monitoring system which provides the client with a tool for safety control and analysis of dam behaviour. The newly developed sensors used in the system are described in the following sections. Reservoir level meters Fixed re-installable Micrometer (FIM) Seepage measurement Extensometer (3-point) Piezometric pressure measurement Concrete temperature measurement Plumb line Rain gauge Air temperature Inverse plumb line with telecoordinometers Retrievable piezometric pressure cell Fig. 3. Instrumentation of Valle di Lei Arch dam. -14 m -27 m 3

4 3.2 Retrievable Electric Pore-water Pressure Sensors MUX/ Interface Standpipe To Measurement Control Center Cable in protective tube Sensor housing with electric sensor Sensor seating Standpipe tip Fig. 4. Porewater Pressure Sensor Pore-water pressures are an important factor in many geotechnical engineering projects. Until recently, the use of high-accuracy electric sensors was often avoided for long-term monitoring of pore-water pressures because the sensor was permanently installed, and the failure of the sensor meant loss of the entire piezometer. To enable monitoring of pore-water pressure for long periods of time (over years) and to increase usefulness and flexibility, the retrievable electric porewater pressure sensor was developed. At the Valle Di Lei dam, 8 boreholes situated downstream from the dam are equipped with 1 to 3 retrievable electric porewater pressure sensors each. These systems provide high accuracy, protection from overvoltages (i.e. lightning) and the possibility to easily replace a sensor should the need arise. The piezometer system includes a retrievable electronic sensor which is placed within the stand-pipe tubing (Fig. 4). A seat that seals the sensor housing against the stand pipe is located near the lower end of the piezometer to decrease borehole storage effects. This system has proven reliable and offers several advantages suited to this project: All stand pipes were installed and grouted in place prior to the sensors being inserted into the stand-pipe tubing This allowed verification of the piezometer's operation. Water levels were initially manually monitored to evaluate which piezometers were of interest prior to installation of the sensors. Only the piezometers of interest were equipped with the electric sensors for automatic monitoring. By placing the sensor within the stand pipe, but not seating the sensor, the fluid within the piezometer was in direct communication with the screened interval and could be monitored. Hydraulic tests (e.g., slug tests) can then be conducted to determine hydraulic parameters. If a sensor fails, replacement costs are low and replacement is quick in comparison with permanently installed sensors that require the drilling and instrumentation of an entirely new borehole. After completion of the project, the sensors, cables, etc. may be retrieved and reused for other applications. 4

5 3.3 V-Notch Weir Seepage Measurements Seepage along the drainage gallery in the Valle Di Lei dam is automatically measured using two V-notch weirs (Fig. 5) Existing V-notch Transducer Suspended mass Transparent hydraulic interconnection Calculated weir flow (ml/sec) Reservoir elevation Seepage Reservoir elevation (m above MSL) Nov 94 Dec 94 Jan 95 Feb Fig. 5. V-notch Weir Fig. 6. Seepage and reservoir level Measurement devices are hydraulically connected with the water behind each weir and measure changing buoyancy forces on a suspended mass with changing water level. Because no moving parts are involved in the measurement, high sensitivity is achieved. Water-level measurements are recorded and flow through the weirs is calculated and monitored. The plot in Fig. 6 shows the calculated seepage flow from the left side of the dam. As can be seen in the plot, both the seepage and reservoir elevation decrease until the reservoir reaches approximately 1904 meters. When the reservoir drops below this elevation, the decreased stress on the concrete blocks of the dam allows increased seepage through the block joints. 3.4 Telecoordinometer to Monitor Pendulums Optoelectronic Telecoordinometers (from ISMES and Huggenberger Switzerland) are used to measure four pendulums in the Valle Di Lei dam that show the effect of changing reservoir levels and structural deformations from temperature changes. A collimated light beam is used to make a contactless measurement of the pendulum's wire. The light beam produces a detectable shadow on two CCD sensors - one each for the x and y directions. (See Fig. 7.) Each telecoordinometer has a measuring range of 50 mm with a resolution and reproducibility of 0.01 mm. The plot in Fig. 8 shows measured deformations and reservoir elevation. 5

6 Cover CCD Array sensors Optical Units Base Plate Measurement and test references Supporting shelf Deformation (mm) 47.0 Laser Pendulum Reservoir Elevation Reservoir Elevation (m above MSL) December 1994 (Days) Fig. 7. Optoelectronic Telecoordinometer for Fig. 8. Telecoordinometer (x-direction) and reservoir Pendulums level 3.5 Fixed re-installable Micrometer (FIM) for Deformation Measurements within the Foundation In recent years a total of 8 boreholes up to 35 m in length have been installed with Sliding Micrometer casing within the dam and its foundation. Linewise differential deformations have been periodically monitored using a portable Sliding Micrometer. A Fixed re-installable Micrometer (FIM) was then installed in one of these boreholes and provides real-time deformation measurements to the GeoMonitor system. The FIM is positioned at an area of interest which was determined by the linewise Sliding Micrometer measurements, between 1 and 7 meters depth (Fig. 9). Measurements display a high accuracy and sensitivity to very small deformations and show a strong relationship between the reservoir elevation and deformation along the borehole axis (Fig. 10) Borehole with Sliding Micrometer casing Deformation (mm) FIM Measurement Reservoir Elevation Reservoir Elevation (m above MSL) 6 Fixed re-installable Micrometer temporarily installed for continuous deformation measurement December 1994 (Days) Fig. 9 Position of Fixed re-installable Micrometer (FIM) Fig. 10. Deformation and reservoir level 6

7 The FIM provides monitoring flexibility. It can be temporarily removed from the borehole to allow linewise measurements over the entire casing length with the portable Sliding Micrometer, and re-installed afterwards. If desired, a series of FIMs are fixed within the same borehole to provide differential deformations over several sections of the borehole. The FIM can be installed in Sliding Micrometer, Sliding Deformeter of TRIVEC casing. 4. TUNNELLING UNDER A RAILWAY EMBANKMENT, LENZBURG, SWITZERLAND The SBB Bern-Zurich line is one of the main connections in the Swiss railway system. Along this line, in the region of Lenzburg, an overflow canal is being constructed to reroute overflow from a small river during periods of high water. A section of this canal was to pass through an embankment, across which this line of the SBB passes. Measurement Control Center Measurement Profile 1 Reference Bar-coded Staffs N Dam Digital Level High water overflow canal Aabach Aabach Tunnel Railway lines Fig.11.Bar-code measuring staffs mounted directly on the train rails on the Lenzburg embankment dam Fig. 12. Layout of the measuring points and the motion-controlled optical digital level Under the protection of an injected shield, a tunnel with a diameter of approximately 3m was driven through the embankment by pipejacking. Solexperts was contracted to install a monitoring system which would continually monitor the stability of the embankment and the train rails throughout the tunnelling phase, and which would automatically trigger alarms if critical settlements occurred. The combination of the motion-controlled optical digital level with the GeoMonitor data acquisition and monitoring system was ideal for this project. From April to November 1997, 38 bar-coded measurement staffs were monitored hourly by a single motion-controlled Zeiss DiNi10 level. Most of the 50 cm-long staffs were mounted directly to the train rails. The other staffs were mounted on light poles and railings which were fixed to the body of the embankment. As a reference point, one measuring staff was mounted approximately 30 m from the axis of the tunnel, where it would not be affected by the tunnelling activity. The level was mounted on the crown of the embankment directly above the tunnel axis. Although this is where the largest settlements were expected, any movement of the level itself was compensated for in the absolute settlement calculations. The position of the level enabled measurements at the greatest distances on both sides of the tunnel axis. 7

8 4.1 Motion-Controlled Optical Digital Level Solexperts has developed programmable motion-control and focusing motors for several optical digital levels, which enable the levels to be integrated into Solexperts monitoring system GeoMonitor. The motion-control motor aims the level at a bar-code staff which is mounted on the structure to be monitored. Then, with a second motor, the optics are focused on the staff. The GeoMonitor System takes a measurement, saves the measurement value, and directs the level to the next bar-code staff. The GeoMonitor software takes the measurement values from each staff and compares them to a reference staff, calculating real-time absolute settlement and heave This Solexperts development provides the following advantages for vertical displacement measurements: Fully automated, around-the-clock monitoring (with an illuminating spotlight for night-time measurements) of vertical displacement of any number of measurement points No cable required to connect the instrument to the measurement points - only one cable is required for connecting the level to the GeoMonitor. Real-time, automatic calculation of compensation for temperature influences 4.2 Procedure At the Lenzburg site, the motion-controlled digital level and 38 bar-coded staffs were installed before the onset of tunnelling in order to determine the normal behaviour of the embankment. During two weeks of pre-tunnelling observation, practically no movement was detected. A second monitoring phase, during which the soil directly over the projected tunnel was stabilised with injections of a cement/bentonite mixture, slight settlement of up to 12 mm was observed due to drilling of the first series of boreholes for grouting. Based on these measurements, the drilling method of the subsequent boreholes was adjusted. Instead of drilling all the boreholes from one side of the embankment, the boreholes were then drilled from both sides of the embankment. 4.3 Outcome Unexpectedly large settlements of the embankment were recorded as soon as pipe jacking for the tunnel started (marked on the graph as phase 3). Jacking had to be halted several times because settlements and difference in settlements had reached or passed acceptable limits. During some of these interruptions, e.g. on the weekend of July 12th and 13th 1997, the train rails had to be elevated to compensate for the large amount of settlement. 8

9 Level [mm] Measurement Profile 1 Calculated settlement relative to reference point Reference measurement: May 23, 1997 A-1 B-1 03-Jun-97 17:00 10-Jun-97 17:00 20-Jun-97 17:00 10-Jul-97 17:00 11-Jul-97 17:00 11-Jul-97 22:00 C-1 D-1 E-1 F-1 Heave G-1 Settlement Position [m] Fig.13 Calculated settlement relative to a reference point; settlement increased on July 10 and 11 during the first phase of driving the tunnel. Level [mm] Settlement relative to fixed point 18 drilling inj. boreholes injection Axis 1 Axis 2 Axis May 30-May Fig. 14 Settlement corresponding to the individual stages of construction (DAVIS graphic). driving tunnel 9-Jun 19-Jun 29-Jun 9-Jul 19-Jul 29-Jul 8-Aug 18-Aug 1997 Heave Settlement Within about 10 days after pipejacking was completed, virtually no further settlement was being detected, and within 4 weeks reached a maximum of 68mm above the tunnel. On-line measurements allowed the differential settlement of the embankment to be monitored hourly around the clock. This system provided the construction supervisors with an automatic, efficient, and reliable monitoring method. In combination with the Solexperts DAVIS Data Visualisation Software, the system provided those responsible for the safety of the project with a continual overview of the site measurement data, accessible by modem from the office. 5. LOCK FACILITY, GABCIKOVO, SLOVAKIA 5.1 Geotechnical Monitoring during Operation The Gabcikovo lock facility was built on the Danube in Slovakia between 1978 and The locks and power station lie about 60 km downriver from Bratislava near the Hungarian border. These locks are an important element of the Gabcikovo-Nagymaros System, and perform the following functions: Flood control for the area bordering the Danube Year-round assurance of minimum 3.5 meters water depth in the Danube (required of European waterways) Electricity generation via 8 Kapplan turbines with a combined capacity of 720 MW The double-lock system of Gabcikovo is one of the largest in Europe. Each lock chamber has a length of 275 m, a width of 34 m and a maximum difference between upstream and downstream water levels of 23 m. 9

10 Fig. 15: Overview of the Gabcikovo lock system. Solexperts and the Slovakian daughter company Geoexperts were contracted by the operators of the lock system to develop a measurement program for geotechnical monitoring, and instrumentation for the lock chambers and the surrounding foundation soil. The objective of instrumentation is the determination of possible deformation and displacement in the concrete structure and concrete blocks in the region of the lock chambers as well as the lock gates during operation of the locks. In addition, during monitoring activities, the safety of the entire dam installation should be ensured. At the beginning of 1997, a large portion of the sensors and the measurement control centre for data acquisition were installed. For the continuous acquisition of measurement data and on-line analysis, the Solexperts GeoMonitor was put into operation. 3D-displacement TCA 1800 with meteorological sensors Danube Reference Water level 2D/3D-jointmeters Water level Sliding Micrometer with 4-point Extensometer Fig. 16: Schematic cross-section of the locks indicating the positions of installed instruments. 5.2 Description of Sensors To monitor the lock system, two motorised Total Stations (Tachymeters with integrated distance measurement) were employed, which were controlled by the GeoMonitor System. In addition, further instruments were also installed, as illustrated in figure 6. Approximately 80 measuring points for the two motorised Leica TCA 1800 Total Stations were placed on the lock and monitored continuously and automatically for position (X and Y) and level (Z). These measuring points consist of passive reflectors (miniprisms) which are installed on measuring pillars located on the upper edge of the side walls of the locks and on the upstream and downstream lock gates. 10

11 Using readings from 4 reference points, position and level changes of the two Total Stations are detected and compensated. At the same time, atmospheric temperature and pressure are measured to correct the distance measurements on-line. With this system of measurement with the Total Stations, point-specific displacement of less than 1 mm can be detected even though difficult conditions exist (large temperature fluctuations, strong winds, quickly changing positions of the Total Stations due to filling and emptying of the locks). With the automatic measuring system Solexperts GeoMonitor, the measurement results, (coordinates and displacement vectors) are available for evaluation and interpretation immediately after each measurement is taken. The advantages of an automatically controlled Total Station are: No cable connection between the instrument and the measurement point; the only cable required is to connect the Total Station to the measurement control centre. Continuous displacement measurements of any number of measurement points. Detection of three-dimensional movement. Accuracy of the Total Station is 1 mm in position and level. Automatic, real-time compensation for measured influences (temperature, etc.) Large measurement range (optimal targeting of reflectors up to distances of 1000 m) Rapid measurements Illumination of the measurement points is not necessary for 24-hour monitoring Alarm functions for warning of measurements which exceed preset boundaries. Alarms can trigger a variety of devices, such as flashing lights, sirens, faxes, pagers and telephones. Additional instruments were installed at the site for geotechnical and hydrogeological monitoring and ensuring the safety of the lock system. These instruments were also integrated into the GeoMonitor System for data acquisition. Additional instruments installed at the site were: Piezometers with electronic pressure transducers to monitor the groundwater level changes at the sides and between the lock chambers. Electric Pressure Transducers to monitor the water level in both lock chambers as well as upstream and downstream of the lock facility. 2-D and 3-D jointmeters with electric displacement transducers to monitor the expansion joints between the 8 blocks of the lock chambers at floor level. Sliding deformeter for linewise measurement of vertical displacement of the soil surrounding the facility, to a depth of 40 m. For automatic measurements, these measurement lines are supplemented with 4-point extensometers. 11

12 5.3 Site-specific Monitoring Setup In the measurement control centre, a PC is installed with GeoMonitor software and project-specific DAVIS Software to record measurements and to oversee the site. The DAVIS software is installed with a graphic overview of the site and the sensor types, locations and status. With a mouse click, data from one or several sensors are presented in list format, as line graphs, or as displacement profiles which show the displacement vectors of position changes of the measurement points. Solexperts Data Visualizer "DAVIS" File Options Help Gabcikovo Raw data TCA 2 Coord. Difference X Y Z Fig. 16: Site-specific DAVIS window showing sensor types and locations. 5.4 Outcome of Automatic Monitoring Automatic instrument monitoring in combination with linewise deformation measurements proved to be a very efficient system for geotechnical observation, obtaining data and ensuring safety at the Gabcikovo Lock facility. Combining geotechnical sensors with geodetic instruments within a data acquisition system provides and efficient method for monitoring the condition of a structure and the underlying soil. The large number of measurements within a short period of time enables detection of previously unavailable correlations. For example, the movement of the lock wall during filling (approx. 15 minutes) correlated strongly with the change in water level within the lock and in the surrounding soil. 12