mONITORINGTHE INFLUENCE OF SLOPE RECTIFICATION MEASURES AT BADULUSIRIGAMA LANDSLIDE T.
W.A.S NavodyaUniversity of Moratuwa, [email protected] S.A.
S KulathilakeUniversity of Moratuwa Abstract: Keywords: Landslides;Landslide rectification; Landslides monitoring instrumentation; Extensometer;Inclinometer 1. IntroductionBadulusirigamalandslide happened initially in 2011 year causing life damages and propertydamages. (Amada, 2016)After that area wasrecognized as a slides affecting region. Reasons for sliding are found to be unplanneddevelopments in hilly areas. Anyway afterwards government has taken number ofrectification measures in the form of surface and subsurface drainage systems,reinforced techniques, soil nailing and toe retaining structures. Though these measures are takenplace it’s a must to check for the efficiency and will they are enough towithstand sliding. In order number of monitoring devices such as extensometer,tilt sensors, inclinometer, etc. had been installed Theseinstruments help to obtain data about the slide and by analysing these data candetermine whether the slide is still active or not.
If the rectificationmeasures are enough then the slide should not be still in progress. Therefore byanalysing data obtained from about instruments, we can conclude whether theserectification measures are enough. This research, will ascertain theeffectiveness of the rectification measures adopted in Badulusirigama, by usingthe monitored data.This project based on the process of collecting data then analysing those dataand coming to conclusions about the efficiency using those analysed data. Inthe process we have to collect data from already installed instruments in theproject of NBRO. Those instruments are extensometer, inclinometer, piezometerand strain gauge which have installed in the site of rectification, measure forthe progress of slide by measuring movement of surface, horizontal movement ofsubsurface, fluctuation of ground water content and characteristics of slipsurface with rainfall. And by analysing those data only, we should approach toa conclusion about rectification measures.
(Risk Awareness & Future Challenges, 2016)2. LandslidesMovements of soil and rocks can behappened in different ways under the gravity. These movements causeinconvenience to the life of human and their properties.
Landslides occur alonga well-defined failure surface and moving material largely remains in contactwith underlying material. Factor of safety depends on shear strength parametersof the slope material and pore water pressure distributions. Factor of safetycan be defined as, (1)?f= Shear strength, ?m = Shear stress mobilized for equilibrium.Accuracy of the value of FOS depends on accuracy ofused data. Shear strength of saturated soil can be expressed as, (2)Shear strength of unsaturated soil canbe expressed as, (3) (4)Here Ca = apparent cohesion, C’=effective cohesion, ?’= angle of friction, ? = total stress and u = pore waterpressure.
(kulathilake) In dry weatherperiods, ground water table is low and high negative pore water pressures ormatric suctions extend at the upper levels. So, relatively large negative porewater pressures in upper 1-2 m of a slope can be obtained using instrumentsusing tensiometers. With the infiltration of rainwater, matric suction will bereduced or completely destroyed.
The loss of matric suction causes thereduction of factor of safety. Landslide is an unstable slope. If Factor ofSafety is larger than 1, slope is stable or no landslide. If Factor of Safetyis less than 1, slope is unstable or landslide occurs.
(Kulathilake)3. Landslide RectificationMeasuresLandslide rectification measures can bedivided into 4 categories as, modification of slope geometry, drainage,retaining structures and internal slope reinforcement. Usually slope geometryis adjusted by cutting slope according to a safe angle. Also adding and removalof materials which is inside the slope can be done. Drainage systems can bemainly divided into two categories as surface drainage and sub surfacedrainage. Dewatering and vegetation planting methods are also drainage systems.
Berms and cascades are surface drainage systems which allows to flowing ofwater rapidly down the slope. Also vegetation will cover the surface and henceinfiltration of water can be reduced. Subsurface drainage systems allowmovement of water which is inside the soil and accelerate the pore waterpressure dissipation.
In some situations, External stabilization such as earthretaining systems and internal stabilization systems such as soil nailing isneeded. Gravity Retaining Walls, Passive piles, piers and caissons, cast insitu reinforced concrete walls, Reinforced earth retaining structures, Buttresscounter forts of coarse grained material, and Retention nets for rock slopefaces, Rock fall attenuation or stopping systems are the earth retainingsystems. Rock bolts, Micro piles, Soil Nailing, Anchors (pre-stressed or not), Grouting,Stone or lime/cement columns, Heat, Treatment, Freezing, Electro-osmoticanchors, Vegetation planting are internal stabilization systems. (kulathilake) It is essential tomonitor all these stabilization measures periodically and attend to whateverthe necessary maintenance and repair work promptly. Otherwise, they will not beable to perform the designed functions.
4. Landslide MonitoringInstrumentationLandslide monitoring is important to getidea of the failure surface, type of the landslide and identifying thedeviations of safety factors. Landslide monitoring can be determined bydisplacements and deformations of slope surface and subsurface. There arevarious methods to monitoring landslides. They are satellite and remote sensingtechniques, Photogrammetric techniques, geodetic techniques and physicaltechniques or instrumentation. The methods or instruments are selectedaccording to the purpose of the monitoring and movement type. (Savvaidis) Geotechnicalinstrumentation is used to analyse slope stability and behaviour of slope failure.We can identify inclinometer, extensometer, piezometer, tilt meters, waterlevel meters and geophones as popular geotechnical instruments.
4.1Extensometer.Small displacements of soil surface canbe obtained by along the borehole axis by rod extensometer.
Also lateraldeformation of structures can be found. Cable percussion drilling is used forinstallation of device. Anchors, protective pipe, reference head are the mainparts of the extensometer.
Anchors are arranged inside the bore hole with rods.Reference head is placed at borehole collar. After the installation bore holeis grouted. Below part of the rod is anchor and upper part is reference head. Achange of a rod distance indicates a happening of a movement. Extensometer hasability to give high resolution measurements.
Multi-point extensometer has onereference head with six rods and anchors. (Guide to Geotechnical Instrumentation, 2004) Temperature corrections, quality ofinstalling instrument affect the accuracy of extensometer readings. As thelimitations, there is limited range to measure and the displacements aremeasured against an assumed point. Establishment of the instrument is veryimportant.4.2InclinometerInclinometer is used to monitor lateraldeformation of the subsurface. Inclinometer is placed inside in a borehole bydrilling. In each installation, there are micro-electro-mechanical systemaccelerometer sensors.
(Wan & Standing, 2014) When installing the instrument, it has to be installed properly belowthe potential area of movement such that not to translate. Otherwise it may notidentify the total amount of movement. (Amarathunga & Bandara) We can obtain slipsurface of the movement and depth to the failure plane by using thisinstrument. Sensors can measure inclination of the inclinometer casing.
Changing of an inclination reading indicates that there is movement. Value ofthe 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 rateof movement. When taking reading an electronic probe is inserted through thecasing 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 movementhas to be considered because we say whether sliding is accelerating. (Stark & Choi, 2008)4.
3 Pipe strain gaugeThe gauge is attached to PVC pipe suchthat towards the direction of landslide surface. The slip surface can beidentified by reading depth of the accumulation of strain value. PVC pipe isgood for active landslides because of the possibility and appropriateness forthe site installation.
Pipe strain gauge gives more accurate values whenmeasuring after about 7 days of installing. Surface of the strain gauge caneasily scrawl so life time may short to 1-2 years. Pipe strain gauge recordsdata continuously. So, data can be collected once per day.
(The manual for landslide monitoring, analysis and countermeasure, 2013)4.4 Tilt meterTilt meter is used to measure small inclinationchanges in surface of the land. Tilt meter has dual axis sensor. Tilt meter hasaccuracy with 0.10.
We can only get measurement of a small areawhich can represent the landslide area. (Uhlemann et al., 2016)4.5 Water level metersExistence of water will reduce thefactor of safety of the slope. If a crack is filled with water, then it makes atension on the slope.
Tension force is depended on height of water column inthe crack. So, ground water level gives an idea about that height. During rainyseasons, ground water level may rise up. So factor of safety may reduce due tohigher hydraulic pressure. (I, Perera, K.M.T, W.G.
B.T, & H.M.
T, 2012)4.6 PiezometersMeasurement of pore water pressure isvery important because many landslides are induced by heavy rain. We canidentify 4 types of piezometers as, standpipe piezometer, pneumatic piezometer,vibrating wire piezometer and electrical resistance piezometer. Standpipepiezometer is an open hydraulic piezometer. It has a porous water intakeconnected with the standpipe. This is sealed when inside the borehole. Watercan go in to the pipe. When pores water pressure deviates, the water level alsodeviates.
This device gives a direct measurement of water level also it hasn’tunderground sensing components. But this gives slow responses in lowpermeability soils. Pneumatic piezometer performs by a gas pressure. This hastwo pneumatic tubes.
When taking a reading, input tube is connected to thepneumatic indicator. Then gas is flowed through the tube. Then gas is offedwhen 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 carefuland get in touch with this instrument. (Guide to Geotechnical Instrumentation, 2004)Vibrating wire piezometer has anautomatic data recording system. Also this gives quick responses to changes inpore water pressure.
This has a tensioned steel wire and an electromagneticcoil. A diaphragm is arranged to identify pressure changes. If there is anychange wire will be tensioned and vibrates. Hence it makes a signal and we canget readings through a readout device.
(Allan Widger et al., 2006)4.7 GeophonesA Micro seismic monitoring technique is usedin this device. This measures vibrations which are induced by the movement oflandslide. Also this can catch the characteristics of the signal such asfrequency, amplitude.(“Micro seismic investigation ofan unstable mountain slope in the Swiss Alps – Spillmann – 2007 – Journal ofGeophysical Research: Solid Earth – Wiley Online Library,” n.d.) 5.
Analysis of the monitoringdata5.1 Pipe strain gauge data analysisData can be collected as per a day. If we plota graph as X axis is the Time and Y axis is the depth of boring. Graphrepresents the changes in strain over time. Slip surface can be identified asthe significant deviation in the graph. (The manual for landslide monitoring, analysis and countermeasure, 2013)5.2 Inclinometer data analysisWe can’t get horizontal movement of theinclinometer casing directly. But the inclinometer probe can measure the tiltangle 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’sfunction.
Bottom of the casing is a fixed end. So it can’t move to a lateraldirection. A lateral movement of casing with respect to the fixed bottom can betaken as the vertical variation. So value can be plotted as Slope change vsDepth profile. This represents the lateral movement at each particular depth.
So this profile shows the failure plane. We should find the direction ofmovement because it is parallel to with our critical cross section. Finding themovement direction can reduce the effort of stability analysis. Also thelocation of critical cross section and movement direction may important toobtain the shear strength parameters and the design rectification measures. (Amarathunga & Bandara) X and Y are directionsof inclinometer casing which are perpendicular to each other.
Using X and Yaxis details of the inclinometer, we can find the horizontal displacements. NormallyX-axis of the casing is installed towards the movement direction. Probe can beinserted along the casing grooves.
Casing grooves are also set as in X and Ydirections. So probe can also be adopted at two perpendicular directions. Hencehorizontal segment of sliding of assumed direction can be obtained by inclinometeras for transverse direction as well as parallel direction. (Mikkelsen, 2003) If the movement isnot a single sliding unit, it is difficult to align the X-axis towards thedirection of movement. In such cases, vector calculations are done for X and Y componentsand exact magnitude and direction can be obtained. Rainfall pattern data isalso important to compare the results of inclinometer data monthly.
5.3 Extensometer data analysisExtensometer gives relative displacements amongtwo sliding surfaces. So readings can be plotted against time axis.
Hence wecan identify the movement patterns for a particular time period. From analysingthose 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 FromSri Lanka, 10, 4.
Amarathunga, M., & Bandara, R. (n.d.).
Useof Inclinometer for Landslide Identification with Relevance to MahawewaLandslide. National Building Research Organization. Guide to Geotechnical Instrumentation. (2004). Washington, USA: Durham Geo SlopeIndicator.
I, I., Perera, K.M.
T, B., W.G.B.T, K.,& H.M.
T, I. (2012). Instrumentation and Monitoring of Mahawewa Landslideoff Walapane in Central hills of Srilanka. ACEPS. National BuildingResearch Organization. Kulathilake, P. S.
(n.d.). Slopestability. kulathilake, P. S. (n.d.
). Stabiliztion ofslopes. NBRO Workshop. Mikkelsen, P. E. (2003). Advances inInclinometer Data Analysis. Symposium on Field Measurements in Geomechanics,(p.
13). Oslo. Risk Awareness & Future Challenges.
(2016). NBRO International Symposium (p. 351). Colombo: NationalBuilding Research Organisation. Savvaidis, P. (n.
d.). ExisitingLandslide monitoring systems and techniques. The Aristotle university ofThessalonaki, Geotechnical Engineering. Stark, T. D., & Choi, H.
(2008). Slopeinclinometers for landslides. springer-Verlag.
(2013). The manual for landslidemonitoring, 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 NearPrince Albert, Saskatchewan. Microseismicinvestigation 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, fromhttp://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 landslideinvestigations.
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 offield instrumentation. Proceedings of the Institution of Civil Engineers -Geotechnical Engineering, 167(5), 491–506.