Piezometers for Measuring the Water Table to Monitor Landslides - Case Study Example

PIEZOMETERS FOR MEASURING THE WATER TABLE TO MONITOR LANDSLIDES Introduction The presence of ground water in a rock slope can have a detrimentaleffect on the soil stability of the slope by decreasing the shear strength of potential failure surfaces. Water pressure in tension cracks, small animal borings or similar almost vertical fissures and discontinuities, reduces stability by increasing the forces that induce landslides. It is important that “measurement or calculation of this water pressure forms part of site investigations for stability studies” (Wyllie et al, 2004: 109). Drainage is one of the most effective and economical means available for improving the stability of rock slopes. Appropriate design of drainage system is possible only if the water flow pattern within the rock mass is understood; and for this purpose, the measurement of hydraulic conductivity and water pressure are carried out. Piezometers are used for measuring water pressure within a slope in order to control the stability of the slope by drainage. Piezometers are devices that are sealed within the ground, generally in boreholes. They respond only to ground water pressure in the immediate vicinity and not to ground water pressure at other locations. Piezometers can also be used to measure the in situ hydraulic conductivity of rock masses, using variable head tests (Wyllie et al, 2004: 120). The purpose of this paper is to discuss the role of piezometers in preventing landslides, identify the different types of piezometers, examine the advantages and disadvantages of each type of piezometer, compare the type of soil that they can be used in, and their applications. Discussion Piezometers measure groundwater pressures, which is useful for “effective stress stability analyses of landslides and to observe the variation of pore pressure vertically in the landslide” (Cornforth, 2005: 82). Certain factors have to be taken into consideration when planning a piezometer installation to measure water pressures in a rock slope. It is important that the drill hole should intersect the discontinuities in which the ground water is likely to be flowing. For example, the hole should intersect the persistent beds in sedimentary rock which has low persistence joints. Zones of fractured or sheared rock should be located for positioning the completion zone of the piezometer, since ground water flow would be more concentrated in these areas. The length of the completion zone in rock is usually longer than in soil, because of the requirement to intersect discontinuities. Fault zones are geological features; if they contain crushed rock they act as conduits for ground water, or if they contain clay gouge, they may act as barriers to ground water flow (Wyllie et al, 2004: 120). Further, the geology of the slope determines the number of piezometers and the number of completion zones in a single piezometer. For example, low hydraulic conductivity shale and comparatively high hydraulic conductivity sandstone constituting a sedimentary deposit, may call for the installation of completion zones in each rock unit. “The hydrodynamic time lag is the volume of water required to register a head fluctuation in a piezometer standpipe” (Wyllie et al, 2004: 120). Time lag is significant in rock with low hydraulic conductivity, and also depends on the type and dimensions of the piezometer. Standpipe piezometers have a greater hydrodynamic time lag than diaphragm piezometers, because they require a greater movement of pore or joint water, to register. A long hydrodynamic time lag is referred to by the term “slow response time”. In rock slopes where pressure fluctuations in joint water pressure is not significant, a standpipe piezometer is considered to be most suitable. On the other hand, a diaphragm piezometer with a much shorter time lag would be more appropriate to measure the response of the ground water pressures to a drainage system such as a series of horizontal drains; or to detect water pressures of a brief duration in response to precipitation (Wyllie et al, 2004: 120). Another important factor is the rock type, for which the filter material in the completion zone of the piezometer should be appropriate. Clay shales or weathered micaceous rocks require fine grained filter material that will not be clogged by rock weathering products washed in from the walls of the hole. When selecting piezometer types, cost and reliability have to be taken into account. Pneumatic and vibrating wire piezometers are expensive and require costly readout units; whereas standpipe piezometers are simple to install, and can be read with inexpensive well sounders. In situations where the slope is moving and the piezometer may be lost, the more economical standpipe piezometers would be suitable for installation (Wyllie et al, 2004: 121). The Use of Piezometers, Different Types of Piezometers, their Advantages and Disadvantages, the Type of Soil Optimal for Each Type, and their Most Suitable Applications Measurement of ground water pressure is required not only for stability analysis but also to monitor changes in groundwater conditions resulting from seasonal impacts, mining activity and drainage of subsoil water. Piezometers are transducers that convert pressure to a more easily measurable form of output such as elevation head, electrical voltage or electrical current. Piezometers are generally installed in boreholes, and should be sealed into the holes so that pressures are measured at the sampling points only. Poor sealing would lead to improper ground water pressure readings and hence would cause an error in the determination of groundwater flow characteristics. For valuable output information, it is essential to maintain a completely effective seal of the piezometer. The simplest form of piezometer is similar to an observation well, other than the fact that the screened or slotted section at the bottom of the pipe has a length of a meter or less, and rests in a pocket of sand. An impervious material is used to backfill the drill hole around the pipe if the casing is removed, otherwise if the casing is left in place above the screened section, the space between the casing and the riser pipe is backfilled. If the phreatic surfaces are to be determined for different layers in the deposit, with screen at an appropriate depth, separate installations are made (Ralph et al, 1996: 54). The soil types that are best suited to piezometers are pervious such as gravel and sand. In finer soils, the time for equilibrium or the time lag becomes excessive, and in case the ground water level itself is subject to fluctuation the results may be erroneous. The time lag can be reduced by decreasing the diameter of the riser pipe and consequently reducing the flow into the piezometer required to make the measurement. This can also be done by the use of electrically or pneumatically operated sensors that allow measurement of the water pressure, with negligible flow. The choice from among the various alternatives depends on factors such as the particular characteristics of the site and the requirements of design or construction (Ralph et al, 1996: 54). Piezometers should also be installed with subsoil water drainage systems “to monitor drawdown levels in the excavated area” (ASCE, 1999: 18). Continuous evaluation should be carried out by reading piezometers daily and plotting the readings. Everyday, pumping records should be maintained to determine the quantity of water removed by the drainage systems to help in determining inadequate seepage control and other requirements. The relative volume demand is the volume of water required to operate the device. “Those piezometers which have a high volume demand are not well suited for measuring rapid changes in groundwater conditions in low permeability materials” (Kliche, 1999: 77), on the other hand, low volume demand types can accurately reflect even small changes in ground water pressures. Improved knowledge of soil mechanics helps to relate soil behaviour to slope stability and consequent possibility of landslides. The science of slope stability evaluation still remains a challenge, though it is increasingly progressing to higher levels of accuracy and successful implementation of remedial measures and prevention of landslides (Duncan & Wright, 2005: 1). Standpipe Piezometers The open standpipe piezometer is composed of an open tube which connects a ground zone with the ground surface. The lower end is open or perforated, and acts as the sensing element which communicates directly with the ground surface through the bore of the pipe. The level of fluid is measured by sounding or by lowering a chalked tape or electrical probe into the tube to measure the water level. Usually the probe contains electrical contacts, and when it touches the water, a circuit is completed and a signal is given out in the form of a buzzer or light. The system is widely used in highly permeable soils (Sarsby, 2000: 263) such as coarse-grained granular soils, and free-draining rock masses (Hunt, 2007: 298). The read-out equipment is a water level finder. The advantages of this type of piezometer are that it is simple to install, easy to read and reliable, not prone to freezing, has a long history of effective operation, is self-deairing and any airbubbles that may collect in the sand filter escape on their own, up the tube. The “purge bubble” principle can be used to take remote readings with the standpipe piezometer (Sarsby, 2000: 263). However, the disadvantage is that there has to be access to the top of the hole, and there can be substantial time lag in low conductivity rock. The tube should be straight for the entire length of the system, and there is a high relative volume demand. The device cannot measure negative pore pressure, cannot be used in areas that are subject to inundation unless offset standpipe is used, it has to be guarded during construction, no central observation station is possible, and requires a sounding probe. Further, there is a likelihood of difficulties in small diameter tubes if water levels are significantly below 100 ft., or dip less than 45 degrees (Kliche, 1999: 77). Porous Tube Piezometers (PTPs) PTPs are used to measure piezometric water levels in embankments, foundations and abutments in selected zones of materials. The PTPs may be installed in drill holes or placed in an embankment during construction. The porous tube assembly is encircled by pervious material and closed by bentonite or grout into a specific influence zone within the dam or foundation. Water enters the porous tube and rises to a level in the standpipe that represents the piezometric elevation (Jansen, 1988: 763). The device is applicable for the following puposes: “to determine the uplift and pore pressure gradients in foundations; measuring the elevation of ground water in stand pipes, bore holes and wells; to determine the flow pattern through earth or rock fill dams, to delineate the phreactic line; and stability investigation against landslides” Slotted Pipe Piezometers (SPPs) The function of SPPs is the same as that of PTPs, that is measuring of piezometric water levels. The difference is that instead of a porous tube for the water to enter, a machined slotted pipe is used. Selecting a porous tube piezometer over a slotted pipe piezometer is normally based on the grain size of the materials at the measuring elevation or the influence zone. Fine grain materials are potentially capable of causing clogging of the slotted pipe (Jansen, 1988: 763). Observation Wells Coarse-grained sandstones and highly fractured rock have a permeability greater than 10-4cm/s and this permits the monitoring of ground water pressures in open holes. These observation wells cannot be used in monitoring ground water pressures in rock. Hence, the major limitation of observation wells is that “they create a vertical connection between strata, so their only application is in consistently permeable rock, in which the ground water pressure increases continuously with depth” (Willey et al, 2004: 121). In contrast to observation wells, which may be open to the entire column of soil through which they pass, piezometers measure the water pressure at a selected level in the system. Closed Tube or Hydraulic Piezometers The relative volume demand is low to medium. The read-out equipment is normally Bourbon gauge or mercury manometer. It is a relatively cheap system, can measure negative pore pressure, can be used in areas subject to inundation, relatively little interference with construction, and can be read at central observation stations. On the other hand, the drawbacks are that the observation stations need to be protected against freezing, the device is fairly difficult to install, more expensive compared to open systems, it is sometimes difficult to maintain on air free system, most types are fragile, some types have limited service behaviour records, and is not suitable for general borehole use since it requires read-out location not significantly above lowest water level (Kliche, 1999: 77). The device is optimally applied in earth dams with soils of low to medium permeability (Hunt, 2007: 298). The twin-tube hydraulic piezometer is applicable in: “uplift and pore pressure gradients in foundations, embankments and abutments, in the monitoring and control of dewatering and drainage, hydrological investigations and water supply undertakings, and in shallow underground works and surface excavations”. Pneumatic Piezometers Pneumatic piezometers are very similar in installation and design to hydraulic piezometers, except for the fact that there is gas in the lines of the pneumatic devices. These devices are composed of a valve assembly and a pair of air lines that connect the valve to the surface. The valve is placed in the sealed section of the piezometer to measure the water pressure at that point. Air is pumped down the supply line until the air pressure equals the water pressure acting on a diaphragm in the sealed section. The water pressure pushes against the flexible diaphragm, forcing the diaphragm to warp and close a tube. Pneumatic pressure is then applied against the diaphragm through a tube connecting the transducer to a readout unit. The pneumatic pressure that is required to force the diaphragm back to its original position is the water pressure impacted on the transducer, which is the read-out equipment. All excess pressure is released through a vent line (Jansen, 1988: 763). A rapid response time is achieved while using pneumatic piezometers, and they are simple to operate. The relative volume demand is low to very low. They are particularly useful in situations where there is no access to the collar of the hole, since even in remote locations readings can be made and measurements taken. The elevation of observation station is not dependent on the elevation of piezometer tip. Electrical source is not required in the pneumatic system, as also protection against freezing and de-airing are not required. They are suitable for low conductivity rock installations. The main disadvantages of this type of piezometer are the risk of damage to the lines during construction or during operations, the requirement to maintain a calibrated readout unit, and the requirement for periodic “de-airing” of monitoring system. Also, the equipment is fragile and requires careful handling during installation. Further, it is often difficult to detect when escape of gas starts, negative pressures cannot be measured, condensation of moisture occurs in cell unless dry gas is used, and requires careful application of gas pressure during observation to avoid damage to cell (Kliche, 1999: 77). This piezometer is best applied in fine-grained soils, and slow-draining rock masses (Hunt, 2007: 298). Electronic Transducers The advantages of using electrical transducers for measuring water pressure, is the highly rapid response time, and the capability to record and process the results at a considerable distance from the slope. Also, the relative volume demand is negligible, negative pressures can be measured. The device is ideal for remote monitoring, since the installation is simple. The relative water demand is negligible. However, the system is comparatively expensive, especially in the case of extensive cable lengths. Some zero drift is possible. (Kliche, 1999: 77). Common electrical transducers include strain gauges and vibrating wire gauges that measure pressure with a high degree of accuracy. All transducers should be thoroughly tested and calibrated before installation. Certain types may be vulnerable to blast damage. Hence, it is important to take into consideration that the long term durability and reliability of these sensitive electrical instruments may not equal the design life of the slope; so their maintenance and possible replacement should be planned earlier (Willey et al, 2004: 122). The application of this device is recommended in fine-grained soils and slow-draining rock masses (Hunt, 2007: 298). Vibrating Wire Strain Gauge Piezometer “Like other closed system piezometers, the vibrating wire piezometers are used in embankments where they are less likely than standpipe piezometers to be damaged by consrtuction equipment” (Jansen, 1988: 763). They are also used to check the accuracy of adjacent instruments, and for monitoring negative pore pressures. The piezometer tip contains a porous disc that allows water pressure to enter and press against a stainless steel diaphragm. A high-strength steel wire that is fixed to the center of the diaphragm at one end and to an ‘end-block’ at the other end, is set to a predetermined tension during manufacture. Pressure applied to the diaphragm causes it to deflect, subsequently changing the tension and the resonance frequency of the wire. A coil magnet assembly with a read-out device vibrates the wire and measures the wire’s vibration frequency. From the readings using calibration charts or an equation using simple gauge factor, pore pressure is calculated Resistance Strain Gauge Piezometer In the resistance strain gauge piezometer, water pressure is allowed to enter an internal diaphragm through a porous disc that filters out soil particles, preventing them from entering. The deflection of the internal diaphragm is measured using an elastic wire electrical resistance device as the sensing instrument. This consists of two coils of fine steel wire wound on ceramic spools. “One of the coils increases in length and resistance with strain, while the other decreases” (Jansen, 1988: 763). Change in resistance is caused mainly by stress, and not by the change in the dimensions. The ratio of the electrical resistance of the two coils is directly proportional to the change in gauge length, and the total resistance of the two coils is directly related to temperature. To calculate the proper resistance ratio, within the calibration range of the device, the temperature must be determined. Both the ratio as well as the total resistance should be accurately measured. The sensing elements are immersed in corrosion resistant oil for protection, with a small quantity of nitrogen, to allow the expansion. The oil also acts as a heat absorber, reducing the effect of heating when readings are taken. Multi-completion Piezometers “For slopes excavated in rock types with differing hydraulic conductivities, it is possible that zones of high ground water pressure exist within a generally depressurized area” (Willey et al, 2004: 122). In such circumstances it would be necessary to measure the ground water pressure at a number of points in a drill hole. For this purpose, multiple standpipe piezometers are installed in a single drill hole with bentonite or cement seals between each section of perforated pipe. In an NX borehole, the maximum number of three standpipes can be installed, because the placement of filter and effective seals becomes very difficult if more pipes are installed. An alternative method of measuring water pressure at a number of different points in a drill hole, is to use a Multi-port (MP) system, that enables the measurement of hydraulic conductivity and retrieval of water samples. The MP is a “modular, multiple-level monitoring device employing a single closed access tube with valved ports” (Willey et al, 2004: 122). In a single well casing, several different levels of a drill hole can be accessed by the valved ports. The modular design permits as many monitoring zones as required, to be established in a drill hole. The MP system has been used in drill holes that are as deep as 1200 metres. Conclusion This paper has highlighted the significance of Piezometers for measuring groundwater pressure, in order to determine soil stability and consequent likelihood of landslides occurring in slopes of various soil and rock compositions. The different types of piezometers, both open and closed varieties have been identified; their advantages and disadvantages, the type of soil that each is most suitable for, and their applications have been discussed. It is observed that all the types of piezometers are sifnificantly useful for specific soil qualities, and serve their purpose usefully in different terrains and locations. The evaluation of soil stability continues to pose a challenge, in spite of advanced techniques and devices. In certain conditions, the piezometer may not be useful for measuring groundwater pressure. Carter & Gregorich (2007: 1072) state that the piezometer method of measuring the water table is not practical for rocky and gravelly soils, as “advancing a piezometer pipe or establishing a seal between the pipe and the soil are usually problematic”. It is recommended that the initial change in the piezometer water level is by rapid water removal or by the use of a slug, rather than by rapid water addition. This is because, accurate readings may not be possible if there is partial plugging of the piezometer cavity due to introduction of silt and clay into suspension by the addition of water. Hence, it is recommended that research should focus on developing practical solutions to the problems of sealing in unsuitable soils, to obtain evidence for best practice in increased efficiency and optimal use of peizometers. References ASCE (American Society of Civil Engineers). 1999. Construction control for earth and rockfill dams. The United States of America: ASCE Publications. Carter, M.R. & Gregorich, E.G. 2007. Soil sampling and methods of analysis. The United States of America: CRC Press. Cornforth, D. 2005. Landslides in practice: investigation, analysis and remedial/ preventative options in soils. New Jersey: John Wiley and Sons. Duncan, J.M. & Wright, S.G. 2005. Soil strength and slope stability. New Jersey: John Wiley & Sons. Hunt, R.E. 2007. Geotechnical investigation methods: a field guide. The United States of America: CRC Press. Jansen, R.B. 1988. Advanced dam engineering for design, construction and rehabilitation. London: Springer. Kliche, C.A. 1999. Rock slope stability. The United KingdomL SME’’s Publications. Ralph, K.T., Peck, B. & Mesri, G. 1996. Soil mechanics in engineering practice. New York: Wiley Publications. Sarsby, R.W. 2000. Environmental geotechnics. London: Thomas Telford Publications. Wyllie, D.C., Mah, C.W. & Hoek, E. 2004. Rock slope engineering: civil and mining. 4th Edn. London: Taylor & Francis. Read More