3. Landslide Investigation and Prediction
The flow chart shown in Fig.10 describes the
general investigation procedures in an attempt to understand the mechanism
of origination of disasters associated with slope movement and to predict
the resulting deformation. Investigation items and investigation methods
are shown on Table 1.
3.1 Preliminary Investigation
- (1) Collection of Existing Data, Data Review
- Landslides often occur at specific locations under certain topographic
and geologic conditions. Therefore it is important to utilize existing
data (history of the problem, records of restoration work, and data review)
in order to understand the topography, geology, and properties of similar
landslides. It is also important to understand their relationship with
meteorologic factors, period of activity, existence of any warning sign,
ground water conditions, chronology of topographic change or erosion by
rivers, earthquakes, and other factors which may have a relationship with
the slope deformation surrounding the investigation site area prior to
the detailed investigation.
- (2) Topographic Investigation
- It is necessary to identify any changes in the site topography. That
can be accomplished by recognizing; 1) the overall topographic feature
of the site; 2) understanding the topographic characteristics of the site
slopes; and 3) estimating the regional geologic structure of the site.
Such methods include comparing the aerial photographs of the site and vicinity
taken prior to and after the sliding, and interpreting the topographic
maps and aerial photographs.
In Japan, aerial photographs are taken every few years over the entire
country at a scale between 1/10000 to 1/40000. These photographs are used
to understand the chronologic and topographic changes over the country.
Furthermore, in order to be able to effectively interpret the phenomena
related to microtopography and landslides, large scale aerial photographs
with a scale of 1/8000 to 1/15000 are often taken. By utilizing aerial
photographs, it is possible to interpret landslide phenomena and warning
signs, geology and geologic structure, topography and distribution of vegetation
type. For landslide investigations, it is useful to identify and interpret
the distribution and continuity of knick lines, gentle slopes, gullies
and cracks in the photos to aid in preparing a photo interpretation map.
The map can then be utilized during the field investigation.
The recent popularity of remote sensing using satellite photographs
has been particularly useful for analysis utilizing the thermal infrared
spectrum which is possible to estimate the distribution of slide areas
and ground water, and live vegetation. Remote sensing can be used for analysis
of topographic characteristics and topographic in terrain susceptible to
- (3) Field Investigation
- With an approximate understanding of the overall topographic feature
and knowledge of the distinction(s) of movement and aerial extent of the
sliding block(s) (viewed from the opposite side), a detailed field investigation
plan can be developed to delineate the aerial extent and a general direction
of movement of the landslide zone, assess the geology and geologic structure,
estimate the cause(s) of the sliding, and predict future movement. The
field investigation should not include just the actual landslide area,
but also exists. The field investigation should also include areas where
aerial photographic interpretation is difficult or unclear, and in areas
that could aid in the understanding of particular topographic features
3.2 Drafting a Detailed Investigation Plan
In order to examine the follow item, a detailed investigation which
will satisfy the objectives under the listings in the investigation methods
and observation instruments in the Table 1, should
Survey lines can be established on each moving block on the ground
where the slide mass is expected to be thickest and where the stability
analysis and plan for control works will be emphasized. As a general rule,
the main survey line should be placed where the width of the slide exceeds
100m with subsidiary survey lines established at approximately 50m intervals
Exploratory borings should be drilled on the order of every 30-40m.
At least three borings should be drilled along the main survey line with
one boring drilled at least 5 t0 10 m below the slide plane. During the
early stage of the investigation, it is particularly important to have
an accurate estimate of the configuration and location of the slide plane(s)
an adequate boring depth can be achieved (Fig.12).
Seismic survey lines should be placed at intervals between 50-100m,
and electric survey and radioactivity survey lines should be placed at
20-50m intervals. A survey should be conducted along the main survey line
as well as along the longitudinal survey lines that cross the main survey
line and subsidiary survey lines. For the seismic survey, the survey points
should be established at 5-10m intervals, 20-50m interval for the electric
survey, and at 3-5m intervals for the radioactivity survey. Furthermore,
to verify the results of the geophysical surveys it is important to drill
borings at the survey line intersections.
- (1) aerial extent of the slide, differentiation of moving blocks and
identification of the direction of movement
- (2) location and shape of slide plane(s)
- (3) nature of landslide block(s)
- (4) possibility of further or future movement on slopes above the existing
- (5) possibility of further, future or accelerated sliding
- (6) distribution of ground water
3.3 Detailed Investigation
- (1) Investigation of Surface Deformation
- The investigation of surface deformation is conducted to define the
boundaries of the landslide, size, level of activity and direction(s) of
the movement, and to determine individual moving blocks of the main slide.
The presence of scarps and transverse cracks are useful for determining
whether the potential for future activity exists.
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.
- 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.
- (2) Investigation of Geologic Structure
- In most cases, the investigation of geologic structure relies on exploratory
borings; however in cases where the bedrock distribution is ambiguous or
a better understanding of the regional geologic structure is needed, then
a geophysical exploration (seismic survey, electrical survey and radioactivity
survey) is combined with the boring data.
- 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.
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
- 2) Geophysical Surveys
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.
- (3) Evaluation of Slide Plane
- Determining the slide plane for actively moving landslides utilize
the fact that the rates of movement differ significantly along the slide
plane. Depending on the requirements for surveying accuracy and magnitude
of movement, the appropriate instrumentation shall be selected from the
following representative instruments;
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
- 2) Inclinometer
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).
- 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
- 4) Other Methods
- Other methods to evaluate the slide plane include: slide plane detection
probe; creep wells; and sounding penetration test.
- (4) Ground Water Investigation
- Investigation of ground water, which is a driving force of sliding,
includes determining ground water level, pore water pressure, ground water
logging, ground water tracing test, pumping test, water quality analysis,
electricity survey, geothermal survey, and geophysical logging (electric
logging and radioactive logging). Based on the results of the above measurements
and tests, ground water control works can be planned and designed.
- 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
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.
- 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
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.
- (5) Geotechnical Investigation (Rock Mechanic Tests)
- In order to conduct slope stability analyses and to design appropriate
control measures for landslides, physical properties such as strength of
slide plane, location and depth of slide plane and stable ground areas
must be determined. The following tests are generally performed; physical
tests, Standard Penetration Tests, soil mechanic tests (unconfined compression,
tri-axial compression, box shear, ring shear, and in-situ shear (along
the slide plane)). In order to obtain the earth reaction coefficient for
the design of the restraint works, there is a current tendency to conduct
more horizontal loading tests and plate loading tests to determine the
modules of deformation. Furthermore, the intensity and degree of alteration
of the slide plane clays are evaluated by X-ray diffraction methods. The
results have also been applied to analyze the origin of the slide plane.
3-4 Prediction of Landslide
- (1) Landslide Distribution Map
- Most of the new landslide are reactivated old failures in landslide
terrain, and unless there are special causes, it is extremely rare that
non-landslide terrain fails. Those topographic characteristics can be interpreted
from aerial photographs and topographic maps, and be verified through field
Furthermore, bedrock landslides and weathered bedrock landslides with
past movement at the time of sliding is small, and sheared bedrock and
topographic features related to the early stages of sliding that were subjected
to creep deformation in the deeper portions often do not exhibit clear
landslide topographic characteristics. Because of these reasons, double
ridge topography associated with mountain deformation, parting ridges,
breaks-in-slope, knick lines, distribution of old and scarps, bulging at
the tip of ridge lines, discrepancy in the geologic distribution "following
the investigation, geologic structure degree of shearing degree of creep
and other factors must be considered when evaluating landslide topography.
Landslide distribution maps with the above descriptions are generally
limited to small areas, however, recent regional maps covering the entire
country of Japan have been published (Fig.29).
- (2) Landslide Prediction
- Now it is possible to predict the timing of a slope failure by interpreting
the rate of deflection measured by extensometers placed across tension
cracks of a slope. Failure predictions rely on extensometers placed across
scarps, and areas will be considered "off-limits" when the rate
of movement exceeds 2 to 4 mm/hour. Based on the change in the rate of
movement, the following three methods are commonly used to predict the
timing of landslide movement.
- 1) Graphic solution using the tertiary creep curve (Fig.30)
- 2) Graphic solution of surplus time using semi-log paper (Fig.31)
- 3) Method using a inverse number of rate of movement (Fig.32)