Slope inclinometers are devices used to monitor slope movement and stability. They are installed in cased boreholes in the ground and use probes to measure the tilt of the casing over time. This provides data on the magnitude, rate, direction, and type of any landslide movement occurring. Proper installation and regular monitoring are needed to get accurate readings, as inclinometer measurements can be affected by random errors, systematic errors, and limitations in field accuracy. Expert interpretation of the data is important to understand slope behavior and design remediation solutions if needed.

1. INCLINOMETERS
Prepared by
Saurav Poudel
Roll no: 11

2. Introduction
Slope inclinometer is a device for monitoring the onset and continuation of
deformation normal to the axis of the borehole casing by passing a probe along the
casing (Dunnicliff 1988).
Slope inclinometer commonly used today is derived from a prototype built in
1952 by Stanley D. Wilson.
 The inclinometer first became commercially available in the late 1950s from the
Slope Indicator Company which Stan Wilson founded (Green and Mikkelsen 1986,
1988).
Inclinometer monitors deformation normal to the axis of the casing which
provides a profile of subsurface horizontal deformation.

3. Major purposes of Slope Inclinometers
 To measure the angle of slope or inclination of a surface, such as a hill, mountain, or
incline.
 To determine the stability of a slope or incline, such as in the case of a potential
landslide or rockfall hazard.
 To monitor the movement or settlement of a slope or incline over time, such as in the
case of a construction project or mining operation.
 To assist in the design and construction of structures on sloping or inclined surfaces,
such as roads, bridges, or buildings.
 To aid in the exploration and mapping of natural landscapes and terrains, such as in the
case of geology or surveying studies.
 To determine the degree of deviation from vertical or horizontal in drilling and boring
operations, such as for oil and gas wells or underground tunnels.

4. Instrumentation
The inclinometer probe measures the inclination of
the casing with respect to the vertical using one or
two force-balanced servo-accelerometers.
A uniaxial probe requires four passes to measure
the tilt of the casing in four different directions,
while a biaxial probe only requires two.
The biaxial probe contains two accelerometers, one
measuring tilt in the plane of the inclinometer
wheels and the other measuring tilt in a plane
perpendicular to the wheels.
The A-sensor in a biaxial probe is oriented parallel
to the wheels, and the B-sensor is oriented
transverse to the wheels.

5. Installation and monitoring of inclinometers
Slope inclinometers are installed within boreholes which are shafts relatively small in
diameter bored in the ground.
Segments of the casing are placed in the borehole until the desired depth. The casing
must be installed 3-6 meters deeper from the point where displacements are expected to be
eliminated.
Two grooves are then installed carefully in order not to apply too much torque on the
casing.
The guide grooves must be kept clean via water hosing or brushing during the installation
so that tracking problems will not emerge.
Grout, a mixture of water, Portland cement and bentonite, is pumped through the pipe
and into the borehole. The purpose of using grout is to bond the slope inclinometer with the
surrounding soil.
Once the system is installed, the internal part of the casing must be kept clean so that the
probe travels via the grooves without obstructions.

6. Importance of slope inclinometer data
Slope inclinometers/indicators are used to determine the magnitude, rate, direction,
depth, and type of landslide movement.
This information is usually vitally important for understanding the cause, behavior, and
remediation of a landslide.
Magnitude and location of movement
The probe measures the tilt of the casing which can be converted to a
horizontal movement.
The angle θ is the angle of tilt measured by the inclinometer probe, and L is
the measurement interval.
The deviation from vertical, i.e., horizontal displacement, is determined by
the sine function and expressed as follows:
Deviation from vertical = L * sin θ

7. Rate of movement
The rate of movement can be assessed
from a plot of cumulative displacement
versus time data.
. The rate of movement is also of
importance to investigate the effect of
rainfall, slope loading, toe excavation, and
remedial measures on slope stability.
. The rate of movement is frequently more
important than the magnitude of movement
because it determines whether the slide is
accelerating, decelerating, or continuing at
the same rate.
Inclinometer data plotted in terms of displacement
versus time

8.  Direction of movement
 Knowing the direction of movement
can reduce the number of cross-
sections that need to be considered in
the stability analyses and remedial
design because the various cross-
sections should be parallel to the
direction of movement.
 The direction of movement can also
be used to determine if the slide is
moving as a single unit or not, which
can facilitate determining causation
and remediation.
Magnitude and direction of movement
using the A-and B-axes

9. Inclinometer Accuracy
 The precision of inclinometer measurements is influenced by various factors such
as the design of the sensor, the quality of the casing, probe, cable, and readout
system.
 It is essential to ensure that all grooves are thoroughly cleaned and that the sensor
unit is calibrated on a regular basis.
 According to product literature from Slope Indicator, the system field accuracy is
empirically ±7.8 mm per 30 m of casing, subject to certain qualifications.
 If the movement measured in a slope inclinometer falls within the system field
accuracy, the consultant or expert should interpret the movement with caution
and within the context of the expected error of the probe.

10. Random errors versus systematic errors
 Random errors are typically no more than ±0.16 mm for a single reading interval and
accumulate at a rate equal to the square root of the number of reading intervals.
 On the other hand, systematic errors are about 0.13 mm per reading under controlled
laboratory conditions and accumulate arithmetically.
 Systematic errors are more important and significant than random errors and should be
avoided as they may mask shear movements at slip surfaces.
 Mikkelsen (2003) provides methods to detect and correct systematic errors in data
processing.
The main types of systematic errors are;
• bias-shift error,
• sensitivity drift,
• rotation error, and
• depth positioning error.

11. 1) Bias-shift error
The sensor bias is the reading of the probe when it is vertical.
Bias-shift error is related to a small change in the bias of the inclinometer probe over time
Usually eliminated during data reduction, but sometimes introduces errors if there is a
change in the bias between opposite traverse readings, i.e., 180° apart, or if the opposite
traverse readings are missed
Calculation:
BSE = (0:01 mm)bN
b= bias in units and N= number of reading intervlas.
2) Sensitivity drift
The causes of sensitivity drift are a drift in the operation amplifier in the pre-amplifier of the
probe
The sensitivity drift is directly proportional to the magnitude of the readings, and it varies
between data sets but is relatively constant for each data set (Mikkelsen 2003)
This error is difficult to identify and if the error is recognized, it is easy to correct by having
the probe factory calibrated and then applying a suitable correction factor (Mikkelsen 2003).

12. 3) Rotation Error
The rotation error occurs when the inclinometer
casing deviates significantly from vertical.
If the accelerometer sensing axis in the A-axis is
rotated slightly towards the B-axis, the A-axis
accelerometer will be sensitive to inclination in
the B-axis.
The rotation error can be detected by identifying
that the casing is severely out of vertical
alignment by the shape of the casing
deformation and by observing that the lateral
displacement graphs in both directions (A- and
B-axes) resemble each other (Cornforth 2005)
Calculation:
Rotation error angle ( Δ) = sin -1 (r/s)
4) Depth Positioning Error
The depth positioning error results from the
probe being positioned at different depths
than the “zero” readings in the casing.
Depth positioning errors are not common in
most landslide cases, but it is time
consuming to quantify and correct the depth
positioning errors in practice.

13. Pitfalls and precautions during using
inclinometers
Inaccuracies due to sensor design and quality: The design of the sensor and quality of the casing,
probe, cable, and readout system can affect the accuracy of inclinometer measurements.
Need for regular cleaning and calibration: Regular cleaning and calibration of the sensor unit is
necessary for accurate readings.
Presence of random and systematic errors: Even with regular cleaning and calibration, there can still
be errors in the readings due to both random and systematic errors.
Field accuracy limitations: The system field accuracy of inclinometers is limited, and any movement
within this range should be viewed with caution and not immediately concluded as a landslide.
Need for proper installation: Proper installation of the inclinometer sensor and proper measurement
techniques are essential for accurate results.
Need for expert interpretation: Inclinometer readings should be interpreted by a trained and
experienced expert to ensure accurate and reliable results.
Need for regular monitoring: Regular monitoring is necessary to detect and track changes over time,
particularly in areas prone to landslides.

14. CONCLUSION
 Slope inclinometers are used to determine the magnitude, rate, direction, depth, and type
of landslide movement.
 The inclinometer must be located well below the potential zone of movement to capture
the total amount of movement.
 Many inclinometer measurements fail to achieve these intended aims due to ignorance of
many factors which must be addressed during installation, monitoring, and data reduction.
 The precision of inclinometer measurements depends on various installation factors, such
as proper placement in stable ground, using the right backfill to fix the inclinometer, and
using a flexible casing to detect small movements.
 However, even with these precautions, there can still be systematic errors in the data
reduction phase, such as bias-shift error, sensitivity drift, rotation error, and depth
positioning error.

15. Thank you!