Instrumentation used for the surface deformation investigation includes extensometers, ground tiltmeters, movement determination by survey methods including transverse survey, grid survey, laser survey from the opposite bank, movement determination by aerial photographs, and G.P.S.(Fig.13) provides an example of instrumentation.
- 1) Extensometer The extensometer is used to measure relative movement by comparing the extension of two points. The extensometers are generally installed across the main scarp, at transverse crack and transverse ridges near the toe or front portion of the slide and parallel to the suspected slide movement (Fig.14). By arranging a series of interconnecting extensometers from the main scarp to the toe of a complex landslide that has many moving slide blocks, the resulting data could aid in clearly delineating the individual slide blocks. Measurements should be accurate to within 0.2mm, and the magnitude of the movement and daily rainfall data should be included to establish the relationship between the measurable movement and the precipitation rate (Fig.15).
- 2)Tiltmeter The ground tiltmeter is useful for determining the deformation at the head and toe portions and sometimes along the flanks of the landslide, or to assess the possibility of future deformation. A level type tiltmeter is most conventional. The tiltmeter is capable of measuring the N-S and E-W components (Fig.16) The magnitude of tilting and tilt directions can be determined directly from the instrument panel. Furthermore, in order to determine the characteristics of the deformation, the results are shown chronologically along with the daily rainfall totals. The relationship between the magnitude of tilting and the cumulative effect of tilting, rainfall totals and groundwater levels are shown on Fig.17.
- 3) Simple Method to Measure Movements One of the simplest methods to determine landslide movement is to drive wooden stakes across a tension crack along the direction of slide movement (Fig.18). Then attach horizontal boards to the stakes, and saw through the boards. Any movement across the tension crack can be determined by measuring the space between the sawed portions of the boards.
- 4) Determination of Movement by Surveying (Transverse Survey, Grid Survey, Laser Survey From the Opposite Bank, Movement Determination by Aerial Photographs, and G.P.S.).
- Transverse Survey:
- This survey method establishes transverse survey lines across the landslide blocks with closely-spaced survey stakes. The survey stations should be established both within the slide and outside the slide on stable ground.
- Grid Survey:
- This survey method involves constructing grid lines across the entire landslide as well as stable ground outside of the landslide. The survey stakes are driven at the intersection of the grid lines.
- Laser Survey from the Opposite Bank:
- A control point is established along the opposite bank on stable ground, and survey stakes are positioned within the slide. It is most effective where the movement in large.
- Movement determination by Aerial Photographs:
- For landslides with a large component of movement, aerial photographic determination is the most useful. An accurate movement can be measured by annual or bi-annual flying.
- G.P.S.:
- Global Positioning System is the state of the art technology that uses signals from satellites to determine the three-dimensional positioning of the slide. G.P.S. has been used in recent landslide investigations where a high degree of success has been reported.
- 5)Automating Survey System In the past, measurements of slope deformation have been performed manually. Recently, automatic survey systems using data loggers and computers have adopted (Fig.19). The instrument set-up in the field has been designed for easy installation, and is weatherproof, durable, maintenance-friendly and economical. (Fig.20) (Fig.21) (Fig.22) (Fig.23). Through remote control in real time and rapid geographical data processing, it is possible to store long term data accurately and effectively and would provide an early warning of slide activity, thereby reducing landslide hazard. Furthermore, the recent development in the informationalized constructions systems and adopting the safety control at the construction site using the real time facilitates the planning through construction stage (Fig.24).
There are three main advantages in using the automating survey system.
- 1. Surveillance of the conditions of landslide failure: Issuance and cancellation of landslide watch and warning announcement based on the velocity of movement, piezometric pressure and variation in the rainfall amounts. Prediction and forecasting of the landslide failure.
- 2. Understanding of the conditions of the landslide deformation: Chronological measurements of movement velocity. Determination of the slide plane depth. Determination of the relationship between the slope deformation and factors of slide occurrence (pore water pressure against the slide plane, critical pore water pressure related to the time of sliding, rainfall and snowmelt).
- 3. Effectiveness in determining landslide control works: Measurement of the amount of earth movement and pore water pressure. Measurement of the earth pressure affected by piles and collection wells. Determination of the effectiveness of construction.
- 1)Borings
- The majority of the borings drilled are larger than 66mm. Core samples are recovered from the borings and are stored in core boxes. Boring logs should be prepared along with photographs of the core samples. The boring logs shall include such information as: geologic and soil description; color; hardness; lithologic description; degree of weathering; alterations and fractures; strike and dip of bedding joints; boring conditions; initial and stabilized ground water levels; and rate of core recovery.
- 2) Geophysical Surveys
Geologic assessment based on the boring data obtained from the drilling site should include a discussion regarding the differentiation of moving earth blocks, semi-moving earth, and stable ground. Clays within the slide plane generally have a high moisture content, are highly sticky and plastic and are often associated with abrasion scars and slickensides. During drilling, squeezed earth could occur near slide plane. Slopes where advanced relaxation of the bedrock formation has occurred will often exhibit gentler slopes than that of the unaffected bedrock zone. Formations can bend or form a kink bend near the lower limit of this zone, and could develop into a slide plane. In translational dip slope slides, the slide plane in many instances will develop along a thin, weak bed of mudstone, tuff bed or coal seam sandwiched between hard and competent beds. Borings can sometimes easily miss these thin beds. Therefore, the possible existence of slide planes along there weak beds typically consist of about 10cm, and must be considered even though the boring may not indicate they are actually present.
Furthermore, using the data from the borings, the following information must be assessed or determined.
1. Evaluation of slide plane 2. Ground water level measurements 3. Ground water logging 4. Ground water tracer tests 5. standard penetration tests, Horizontal loading tests, In-situ tests such as in site permeability tests 6. Sampling for soil tests 7. Various geophysical logging
Geophysical surveys (seismic survey, electric survey and radioactivity survey) are conducted to understand the approximate geophysical conditions of the slide itself and the surrounding area. P-wave refraction surveys are the most common seismic survey. Other methods, such as S-wave and P-wave shallow refraction, are seldom used. Electric survey is the specific resistance method and is applied to determine the distribution of aquifer(s) and to understand the geologic structure. These surveys include the development of the geotomography method. A natural radioactivity survey is used to determine the locations of small scale fracture zones and cracks.
1. Pipe strain gauge 2. Inclinometer 3. Multi-layer movement meter
- 1) Pipe Strain Gauge
- P.V.C. pipes with strain gauges are inserted into the boreholes, and the movement is estimated by the change in the strain as the P.V.C. pipe bends. The accuracy of the strain gauge increases as the intervals of the gauge narrows, however, it is acceptable to widen the space as much as 1m for investigations involving very thick slide materials and when it is difficult to handle the survey extension wires. Two of the lowest strain gauges must be anchored into the bedrock below the slide plane so that data from within the intact formation can be obtained. Furthermore, annular space between the borehole and the pipe must be filled with concrete following the gauge installation. The instruments should last for one to two years (Fig.25).
- 2) Inclinometer
- 3) Multi-Layer Movement Meter
- several wires are anchored at different depths within a borehole with the attached wires extended to the ground surface. The magnitude of the displacement of each wire segment can be measured directly using a ruler. It is possible to install 20 to 30 wires per borehole. This method is not suitable for landslides with small displacement. This instrument is most effective where the slide movement is so large that some of the other instruments cannot be used. Applying the same principle, a vertical extensometer can be constructed by fixing a wire on the bedrock at the bottom of the borehole (Fig.27).
- 4) Other Methods
- Other methods to evaluate the slide plane include: slide plane detection probe; creep wells; and sounding penetration test.
A grooved casing is inserted into the borehole extending into the bedrock formation, and have an adequate quality of grout placed into the borehole to assure a positive contact with the borehole. By lowering a probe equipped with a tilt sensor, deformation in the casing can be detected and movement of a landslide can be determined. An accurate measurement is possible where the deformation of a landslide is relatively small. As a landslide movement increases, the borehole and casing will bend making insertion of the probe difficult or will exceed the tilt detection limit of the instrument. (Fig.26).
- 1) Ground Water Level Observation
- As a general rule, ground water levels should be measured in all the boreholes. In some of the more important boreholes, continuous rainfall data will be kept by an automatic recorder to determine the correlation between the slide movement and rainfall and ground water level, and will collect data on the ground water distribution and movement regime.
- 2) Pore Water Pressure
- 3) Ground Water Logging
- Locations of ground water flow and flow directions can be determined by measuring the increasing specific resistance of ground water in flow over time. The measurements will be continued often lowering specific resistance of ground water by injecting a salt solution into the borehole. There should be at least two borings for ground water logging at the head portion of the landslide where abundant ground water is expected. The measurement results should be recorded along with the boring logs, and the relationship between the location of ground water flow and bed, and magnitude and variation of specific resistance of ground water should be discussed. Furthermore, the results of the analysis should be recorded along with the cross sections in order to understand the overall ground water flow (Fig.28).
- 4) Ground water Tracer Tests
- Tracers such as a soluble dye, or inorganic chemicals (NaCl) are injected into a borehole. Water samples are then collected chronologically from springs, other boreholes, wells and ponds within or outside the landslide, and are analyzed for the tracer to estimate the ground water flow direction(s) and permeability. This data is used for basic information for the design of dewatering works.
- 5) Drawdown Test
- In order to estimate the yield and to calculate the coefficient of permeability, water within a borehole is pumped to certain levels after raising the boring casing every 2 to 3m. A time-recovery curve can then be plotted using Jacob's and other formulas, and the coefficient of permeability can be determined.
- 6) Water Quality Tests
- Water quality tests are an effective method to examine the distribution of the ground water regime and flow directions where the subject landslide is very large and the ground water system is expected to be complicated. Specific tests include determination of water temperature, Cl-, SO2-4, HCO-3, Na+, K+, Ca++, and Mg++ content, pH, alkalinity, electric conductivity, SiO2, and others. The test results are classified according to the analytical data and composition.
- 7) Geothermal Investigation
Ground water levels in boreholes will often reflect seepage from highly fractured formations or indicate the water level of a predominant aquifer. Therefore, for stability analysis, it is best to measure pore water pressure along the slide plane. Sometimes it is difficult to accurately estimate the depth of the slide plane. In such cases it is desirable to install piezometers in the beds with low seepage or low shear strength. The standard piezometers that are used in landslide investigations must be durable, and open piezometer water level type.
This procedure utilizes ground temperature measurements throughout the study area, including ground temperatures near the ground water veins. By measuring the temperature differences at non-ground water areas and near ground water veins, it is possible to isolate the ground water veins where the temperature difference between the two is large. By conducting the geothermal investigation in summer months or winter months where near surface ground temperature is influenced by air temperature, good results have been obtained for the isolation of relatively shallow ground water.
