+ STATENS GEOTEKNISKA INSTITUT SGI SWEDISH GEOTECHNICAL INSTITUTE V RAPPORT REPORT No37 LINKOPING 1989 STATENS GEOTEKNISKA INSTITUT SWEDISH GEOTECHNICAL INSTITUTE RAPPORT REPORT No37 Pore Pressure Measurement - Reliability of Different Systems Marius 'fiemblay LINKOPING 1989 ISSN 0348-0755 AB 0STG0TATRYCKU<PG PREFACE This report presents an evaluation of different systems used for pore pressure measurement. The investigation deals with the behaviour of these systems when used for measuring natural variations in the groundwater conditions. The influence on the measurements of changes in the atmospheric pressure is also studied, as well as the various problems occurring with measuring instruments used for automatic reading and recording of the pore pressure in a slope stability monitoring system. The results presented here are therefore of interest for anyone concerned with the choice of a system for long­ term measurement of pore pressure. The evaluation of the different systems is based on measurements performed on· a number of sites located in Sweden. The majority of the results presented here were obtained from a research project carried out in 1984-85 during the author's stay as a guest-researcher at the Swedish Geotechnical Institute (SGI). Some additional measurements found in this report are taken from different projects previously conducted at SGI. The author would like to express his sincere thanks to Dr. Jan Hartlen, Director General of SGI, for making this stay possible and also for his constant interest and encouragement. The author is especially indebted to Mr. Ulf Bergdahl, t he supervisor of this research project, for his constant support and encouragement during this work. His comments during the field work and the discussion of the results were highly appreciated. Special thanks are expressed to Mr. Soren Eskilsson, for his valuable comments regarding the evaluation of the various measuring systems and the interpretation of the results, and also for giving the author the opportunity to use measurements from Orebro and Ska-Edeby. The author also wishes to thank Mr. Jan Wenners trand, formerly of SOI, for his help in the first steps of this research project, and Mr. Olov Lindholm, for his assistance during the perio~ of measurement. Finally, the author wishes to express his gratitude to the staff of SGI for t heir support, especially to the field engineers for their help in installing the instruments, and the information department for their assistance during the preparation of this report. Linkoping, September 1989 Marius Tremblay 3 CONTENTS PREFACE 3 SUMMARY 7 1 . INTRODUCTION . 9 2. LI TERATURE SURVEY . 10 2.1 Instruments and measuring systems ................ ....... 10 2 . 2 Long-term measurements . 16 2 . 3 Pore pressure in natural slopes 16 3. METHODS FOR MEASUREMENT OF PORE PRESSURE ................. 23 3.1 Introduction . 23 3.2 Observation wells vs piezometers ......................... 23 3. 3 Open systerns . 26 • Principle • Perforated pipe • Standpipe piezometer 3. 4 Closed systerns . 28 • Principle • Hydraulic piezometer • Pneumatic piezometer • Electrical piezometer • BAT piezometer 3. 5 Conclusions . 39 4. SOURCES OF ERROR IN PORE PRESSURE MEASUREMENT ......... .. 40 4 .1 Introduction . 40 4.2 General sources of error ......... ... .. ............., .. 40 • Stress adjustment time lag • Hydrostatic time lag • Seepage along conduits • Sealing of joints and connections • Gas or air bubbles in the system • Gas or air bubbles in soil surrounding the tip • Clogging of the filter area • Temper atur e variations • Atmospheric pressure variations • Movement of the tip • Instrument errors - Operator errors 4.3 Special problems with l ong-term measurements 46 • Disaturation • Change in calibration characteristics • Chemical reactions 5 5. EXPERIENCE FROM PREVIOUS PROJECTS .......... ............ 48 5.1 Introduction ........ ...... .............................. 48 5.2 Measurements at Orebro .................................. 48 5.3 Measurements at Ska-Edeby . .... .... ... ........ ...... .. 50 5.4 Conclusions ............................................. 55 6. PORE PRESSURE MEASUREMENTS AT LINKOPING ................. 56 ,6 .1 Purpose of the investigation . .......................... 56 • Comparison of different systems • Influence of the atmospheric pressure 6 . 2 Site conditions . 58 6.3 Description of the instruments 58 • Open standpipe piezometers • BAT piezometers • Vibrating wire piezometers • Resistance strain gauge piezometers 6.4 Installation and measurement ............................. 64 • Description of the installation • Frequency of measurement 6.5 Results . 64 • Measurement of natural variations in pore pressure • Influence of atmospheric pressure on the measurements 7. PORE PRESSURE MEASUREMENTS AT MUNKEDAL ................... 71 7 .1 Purpose of the investigation . 71 7 .2 Site conditions . 71 7.3 Choice of the installation depths .... ...... .. .. .......... 73 7 .4 Presentation of the system . 73 7.5 Presentation of the results .. ..................... .. .... 74 7.6 Particular problems during the period of measurement 76 • Improper connection o f the transducers • Malfunctioning of the transducers • Difficulty of re-installing t he transducers 8. DISCUSSION - CONCLUSIONS - RECOMMENDATIONS .............. 81 9. REFERENCES . 83 6 SUMMARY This report presents the results of investigations performed at the Swedish Geotechnical Institute (SGI} regarding different systems used for measuring pore pressure. The purposes of the investigation were: • to study the behaviour of different systems when measuring natural variations in the groundwater conditions; • to study the influence of changes in atmospheric pressure on the measurements; • to determine the essential characteristics of a measuring system (piezometer and recording instrument} required for reliable long­ term measurement of pore pressure. The first part of the report includes a presentation and a short evaluation of the measuring systems most commonly used in Sweden. The advantages and disadvantages of these systems are presented, and their applications and shortcomings are briefly discussed. Measurements performed at Orebro and Linkoping show that measuring systems based on different principles may be used for following natural variations in the groundwater conditions in clayey soils . The investigation shows that even open systems can be used for this purpose. The study of the behaviour of relative pressure transducers performed at Ska-Edeby indicates clearly that severe problems may occur if the device used for the transmission of the atmospheric pressure to the t ransducer becomes defective. In such a case, variations in the air pressure may affect the pore pressure measurement and make the interpretation difficult. In order to understand and clarify the phenomena observed at Ska­ Edeby regarding the influence of the atmospheric pressure, an analysis of this problem was also made at Linkoping. Pore pressure measurements performed with absolute pressure transducers were compared with the values obtained from relative pressure transducers. This investigation shows that changes in atmospheric pressure affect directly the pore pressure measurements performed with absolute pressure transducers. Therefore, these variations must be taken into account in the interpretation. 7 After the observations made at Linkoping and Ska-Edeby, it was concluded that it is preferable to use absolute pressure transducers and to correct the pore pressure measurements for the variations in atmospheric pressure observed on the site. Finally, a pore pressure measuring system was chosen and installed in a slope in the municipality of Munkedal. The system comprises BAT filter tips and electrical transducers fabricated at SGI. It was integrated in a slope stability monitoring system tested on this site. Pore pressure measurements were performed on a continuous basis for more than 18 months using an automatic reading and recording system. The results pointed out the various problems related to long-term measurement of pore pressure. The most valuable operational characteristic of the system is the possibility to remove the electrical transducer from the filter tip, which permits the following operations to be performed: • control of the measurements with a manual reading device; • calibration of the transducers whenever necessary; • replacement of defective transducers. The experience acquired during this period confirmed the suitability of the system chosen for these measurements, its flexibility being one of the most important characteristics. 8 1. INTRODUCTION This investigation is the first step of a program undertaken at thi Swedish Geotechnical Institute to develop a slope stability monitorin~ system in which the pore pressure is one of the essential parameters. The purpose of the investigation is to study different systems usec for measuring pore pressures in order to find the type of instrumentation most appropriate for such a monitoring system. The analysis of slope stability is one of the geotechnical problemt where groundwater conditions are of great importance. In suet analysis, the water flow distribution in the slope and its vicinity, as well as the pore pressure variations in the various layers, have tc be analysed. Despite the fact that the soil behaviour is greatly affected by th1 groundwater conditions, very often the only information available t1 the geotechnical engineer is the groundwater level under which he ha: to assume a hydrostatic