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mONITORING
THE INFLUENCE OF SLOPE RECTIFICATION  
MEASURES AT BADULUSIRIGAMA LANDSLIDE

 

T.W.A.S Navodya

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University of Moratuwa, [email protected]

Prof. S.A.S Kulathilake

University of Moratuwa

 

Abstract:

Keywords:  Landslides;
Landslide rectification; Landslides monitoring instrumentation; Extensometer;
Inclinometer

 

1.   
Introduction

Badulusirigama
landslide happened initially in 2011 year causing life damages and property
damages. (Amada, 2016)After that area was
recognized as a slides affecting region. Reasons for sliding are found to be unplanned
developments in hilly areas. Anyway afterwards government has taken number of
rectification measures in the form of surface and subsurface drainage systems,
reinforced techniques, soil nailing and toe retaining structures.  Though these measures are taken
place it’s a must to check for the efficiency and will they are enough to
withstand sliding. In order number of monitoring devices such as extensometer,
tilt sensors, inclinometer, etc. had been installed These
instruments help to obtain data about the slide and by analysing these data can
determine whether the slide is still active or not. If the rectification
measures are enough then the slide should not be still in progress. Therefore by
analysing data obtained from about instruments, we can conclude whether these
rectification measures are enough. This research, will ascertain the
effectiveness of the rectification measures adopted in Badulusirigama, by using
the monitored data.
This project based on the process of collecting data then analysing those data
and coming to conclusions about the efficiency using those analysed data. In
the process we have to collect data from already installed instruments in the
project of NBRO. Those instruments are extensometer, inclinometer, piezometer
and strain gauge which have installed in the site of rectification, measure for
the progress of slide by measuring movement of surface, horizontal movement of
subsurface, fluctuation of ground water content and characteristics of slip
surface with rainfall. And by analysing those data only, we should approach to
a conclusion about rectification measures. (Risk Awareness & Future Challenges, 2016)

2.     
Landslides

Movements of soil and rocks can be
happened in different ways under the gravity. These movements cause
inconvenience to the life of human and their properties. Landslides occur along
a well-defined failure surface and moving material largely remains in contact
with underlying material. Factor of safety depends on shear strength parameters
of the slope material and pore water pressure distributions. Factor of safety
can be defined as,

                                                                                                                                             (1)

?f
= Shear strength, ?m = Shear stress mobilized for equilibrium.
Accuracy of the value of FOS depends on accuracy of
used data. Shear strength of saturated soil can be expressed as,

                                                                                   (2)

Shear strength of unsaturated soil can
be expressed as,

                                                                                (3)

                                                    (4)

Here Ca = apparent cohesion, C’=
effective cohesion, ?’= angle of friction, ? = total stress and u = pore water
pressure. (kulathilake) In dry weather
periods, ground water table is low and high negative pore water pressures or
matric suctions extend at the upper levels. So, relatively large negative pore
water pressures in upper 1-2 m of a slope can be obtained using instruments
using tensiometers. With the infiltration of rainwater, matric suction will be
reduced or completely destroyed. The loss of matric suction causes the
reduction of factor of safety. Landslide is an unstable slope. If Factor of
Safety is larger than 1, slope is stable or no landslide. If Factor of Safety
is less than 1, slope is unstable or landslide occurs. (Kulathilake)

3.     
Landslide Rectification
Measures

Landslide rectification measures can be
divided into 4 categories as, modification of slope geometry, drainage,
retaining structures and internal slope reinforcement. Usually slope geometry
is adjusted by cutting slope according to a safe angle. Also adding and removal
of materials which is inside the slope can be done. Drainage systems can be
mainly divided into two categories as surface drainage and sub surface
drainage. Dewatering and vegetation planting methods are also drainage systems.
Berms and cascades are surface drainage systems which allows to flowing of
water rapidly down the slope. Also vegetation will cover the surface and hence
infiltration of water can be reduced. Subsurface drainage systems allow
movement of water which is inside the soil and accelerate the pore water
pressure dissipation. In some situations, External stabilization such as earth
retaining systems and internal stabilization systems such as soil nailing is
needed. Gravity Retaining Walls, Passive piles, piers and caissons, cast in
situ reinforced concrete walls, Reinforced earth retaining structures, Buttress
counter forts of coarse grained material, and Retention nets for rock slope
faces, Rock fall attenuation or stopping systems are the earth retaining
systems. Rock bolts, Micro piles, Soil Nailing, Anchors (pre-stressed or not), Grouting,
Stone or lime/cement columns, Heat, Treatment, Freezing, Electro-osmotic
anchors, Vegetation planting are internal stabilization systems. (kulathilake) It is essential to
monitor all these stabilization measures periodically and attend to whatever
the necessary maintenance and repair work promptly. Otherwise, they will not be
able to perform the designed functions.

4. Landslide Monitoring
Instrumentation

Landslide monitoring is important to get
idea of the failure surface, type of the landslide and identifying the
deviations of safety factors. Landslide monitoring can be determined by
displacements and deformations of slope surface and subsurface. There are
various methods to monitoring landslides. They are satellite and remote sensing
techniques, Photogrammetric techniques, geodetic techniques and physical
techniques or instrumentation. The methods or instruments are selected
according to the purpose of the monitoring and movement type. (Savvaidis) Geotechnical
instrumentation is used to analyse slope stability and behaviour of slope failure.
We can identify inclinometer, extensometer, piezometer, tilt meters, water
level meters and geophones as popular geotechnical instruments.

4.1Extensometer.

Small displacements of soil surface can
be obtained by along the borehole axis by rod extensometer. Also lateral
deformation of structures can be found. Cable percussion drilling is used for
installation of device. Anchors, protective pipe, reference head are the main
parts of the extensometer. Anchors are arranged inside the bore hole with rods.
Reference head is placed at borehole collar. After the installation bore hole
is grouted. Below part of the rod is anchor and upper part is reference head. A
change of a rod distance indicates a happening of a movement. Extensometer has
ability to give high resolution measurements. Multi-point extensometer has one
reference head with six rods and anchors. (Guide to Geotechnical
Instrumentation, 2004) Temperature corrections, quality of
installing instrument affect the accuracy of extensometer readings. As the
limitations, there is limited range to measure and the displacements are
measured against an assumed point. Establishment of the instrument is very
important.

4.2Inclinometer

Inclinometer is used to monitor lateral
deformation of the subsurface. Inclinometer is placed inside in a borehole by
drilling. In each installation, there are micro-electro-mechanical system
accelerometer sensors.(Wan & Standing, 2014) When installing the instrument, it has to be installed properly below
the potential area of movement such that not to translate. Otherwise it may not
identify the total amount of movement. (Amarathunga & Bandara) We can obtain slip
surface of the movement and depth to the failure plane by using this
instrument. Sensors can measure inclination of the inclinometer casing.
Changing of an inclination reading indicates that there is movement. Value of
the deformation can be obtained by difference between inclinometer readings.(Allan Widger, Jorge Antunes, J. Kelly, Q. Vu, & Wayne Clifton,
2006) Also we can get the rate
of movement. When taking reading an electronic probe is inserted through the
casing by wheels. Usually we take two set values as perpendicular directions. (I, Perera, K.M.T, W.G.B.T, & H.M.T, 2012) The rate of movement
has to be considered because we say whether sliding is accelerating. (Stark & Choi, 2008)

4.3 Pipe strain gauge

The gauge is attached to PVC pipe such
that towards the direction of landslide surface. The slip surface can be
identified by reading depth of the accumulation of strain value. PVC pipe is
good for active landslides because of the possibility and appropriateness for
the site installation. Pipe strain gauge gives more accurate values when
measuring after about 7 days of installing. Surface of the strain gauge can
easily scrawl so life time may short to 1-2 years. Pipe strain gauge records
data continuously. So, data can be collected once per day. (The manual for
landslide monitoring, analysis and countermeasure, 2013)

4.4 Tilt meter

Tilt meter is used to measure small inclination
changes in surface of the land. Tilt meter has dual axis sensor. Tilt meter has
accuracy with 0.10. We can only get measurement of a small area
which can represent the landslide area. (Uhlemann et al., 2016)

4.5 Water level meters

Existence of water will reduce the
factor of safety of the slope. If a crack is filled with water, then it makes a
tension on the slope. Tension force is depended on height of water column in
the crack. So, ground water level gives an idea about that height. During rainy
seasons, ground water level may rise up. So factor of safety may reduce due to
higher hydraulic pressure. (I, Perera, K.M.T, W.G.B.T, & H.M.T, 2012)

4.6 Piezometers

Measurement of pore water pressure is
very important because many landslides are induced by heavy rain. We can
identify 4 types of piezometers as, standpipe piezometer, pneumatic piezometer,
vibrating wire piezometer and electrical resistance piezometer. Standpipe
piezometer is an open hydraulic piezometer. It has a porous water intake
connected with the standpipe. This is sealed when inside the borehole. Water
can go in to the pipe. When pores water pressure deviates, the water level also
deviates. This device gives a direct measurement of water level also it hasn’t
underground sensing components. But this gives slow responses in low
permeability soils. Pneumatic piezometer performs by a gas pressure. This has
two pneumatic tubes. When taking a reading, input tube is connected to the
pneumatic indicator. Then gas is flowed through the tube. Then gas is offed
when the flow is in other tube. Take the reading when pressure is stable.
Quickly readings can be taken by this instrument. Operator must be very careful
and get in touch with this instrument. (Guide to Geotechnical
Instrumentation, 2004)Vibrating wire piezometer has an
automatic data recording system. Also this gives quick responses to changes in
pore water pressure. This has a tensioned steel wire and an electromagnetic
coil. A diaphragm is arranged to identify pressure changes. If there is any
change wire will be tensioned and vibrates. Hence it makes a signal and we can
get readings through a readout device.(Allan Widger et al., 2006)

4.7 Geophones

A Micro seismic monitoring technique is used
in this device. This measures vibrations which are induced by the movement of
landslide. Also this can catch the characteristics of the signal such as
frequency, amplitude.(“Micro seismic investigation of
an unstable mountain slope in the Swiss Alps – Spillmann – 2007 – Journal of
Geophysical Research: Solid Earth – Wiley Online Library,” n.d.)

5. Analysis of the monitoring
data

5.1 Pipe strain gauge data analysis

Data can be collected as per a day. If we plot
a graph as X axis is the Time and Y axis is the depth of boring. Graph
represents the changes in strain over time. Slip surface can be identified as
the significant deviation in the graph. (The manual for landslide
monitoring, analysis and countermeasure, 2013)

5.2 Inclinometer data analysis

We can’t get horizontal movement of the
inclinometer casing directly. But the inclinometer probe can measure the tilt
angle of the casing. So we convert it into the horizontal movement as follows.

                                     (5)

L is the measurement interval (usually 0.5m). ? is the tilt angle. Value of tilt will be a measurement interval’s
function. Bottom of the casing is a fixed end. So it can’t move to a lateral
direction. A lateral movement of casing with respect to the fixed bottom can be
taken as the vertical variation. So value can be plotted as Slope change vs
Depth profile. This represents the lateral movement at each particular depth.
So this profile shows the failure plane. We should find the direction of
movement because it is parallel to with our critical cross section. Finding the
movement direction can reduce the effort of stability analysis. Also the
location of critical cross section and movement direction may important to
obtain the shear strength parameters and the design rectification measures. (Amarathunga & Bandara) X and Y are directions
of inclinometer casing which are perpendicular to each other. Using X and Y
axis details of the inclinometer, we can find the horizontal displacements. Normally
X-axis of the casing is installed towards the movement direction. Probe can be
inserted along the casing grooves. Casing grooves are also set as in X and Y
directions. So probe can also be adopted at two perpendicular directions. Hence
horizontal segment of sliding of assumed direction can be obtained by inclinometer
as for transverse direction as well as parallel direction. (Mikkelsen, 2003) If the movement is
not a single sliding unit, it is difficult to align the X-axis towards the
direction of movement. In such cases, vector calculations are done for X and Y components
and exact magnitude and direction can be obtained. Rainfall pattern data is
also important to compare the results of inclinometer data monthly.

5.3 Extensometer data analysis

Extensometer gives relative displacements among
two sliding surfaces. So readings can be plotted against time axis. Hence we
can identify the movement patterns for a particular time period. From analysing
those graphs, we can identify the section which is with sudden variations.
After the removing of errors, exact critical movement pattern can be obtained. (I, Perera, K.M.T, W.G.B.T, & H.M.T, 2012)

5.4 Water level meter data analysis

 

References:

Amada, K. (2016, October-December).
Landslide Mitigation – The Badulusirigama. (K. Amada, Ed.) JICA News From
Sri Lanka, 10, 4.

 

Amarathunga, M., & Bandara, R. (n.d.). Use
of Inclinometer for Landslide Identification with Relevance to Mahawewa
Landslide. National Building Research Organization.

 

Guide to Geotechnical Instrumentation. (2004). Washington, USA: Durham Geo Slope
Indicator.

 

I, I., Perera, K.M.T, B., W.G.B.T, K.,
& H.M.T, I. (2012). Instrumentation and Monitoring of Mahawewa Landslide
off Walapane in Central hills of Srilanka. ACEPS. National Building
Research Organization.

 

Kulathilake, P. S. (n.d.). Slope
stability.

 

kulathilake, P. S. (n.d.). Stabiliztion of
slopes. NBRO Workshop.

 

Mikkelsen, P. E. (2003). Advances in
Inclinometer Data Analysis. Symposium on Field Measurements in Geomechanics,
(p. 13). Oslo.

 

Risk Awareness & Future Challenges.
(2016). NBRO International Symposium (p. 351). Colombo: National
Building Research Organisation.

 

Savvaidis, P. (n.d.). Exisiting
Landslide monitoring systems and techniques. The Aristotle university of
Thessalonaki, Geotechnical Engineering.

 

Stark, T. D., & Choi, H. (2008). Slope
inclinometers for landslides. springer-Verlag.

 

(2013). The manual for landslide
monitoring, analysis and countermeasure. In R. Bandara, M. Somaratne, L.
Indrathilaka, N. Amarathunga, S. Tsukamoto, S. Fujisawa, . . . Y. Uchikura.
NBRO & JICA DiMCEP.

 

Allan Widger, R.,
Jorge Antunes, P., J. Kelly, A., Q. Vu, H., & Wayne Clifton, A. (2006).
Instrumentation and Real Time Monitoring of a Landslide on Highway No. 302 Near
Prince Albert, Saskatchewan.

 

Microseismic
investigation of an unstable mountain slope in the Swiss Alps – Spillmann –
2007 – Journal of Geophysical Research: Solid Earth – Wiley Online Library.
(n.d.). Retrieved January 18, 2018, from
http://onlinelibrary.wiley.com/doi/10.1029/2006JB004723/full

 

Uhlemann,
S., Smith, A., Chambers, J., Dixon, N., Dijkstra, T., Haslam, E., … Mackay, J.
(2016). Assessment of ground-based monitoring techniques applied to landslide
investigations. Geomorphology, 253, 438–451.
https://doi.org/10.1016/j.geomorph.2015.10.027

 

Wan,
M. S. P., & Standing, J. R. (2014). Lessons learnt from installation of
field instrumentation. Proceedings of the Institution of Civil Engineers –
Geotechnical Engineering, 167(5), 491–506. https://doi.org/10.1680/geng.13.00054