Musa-nagoya (1)

Third International Conference on Geotechnique, Construction Materials andEnvironment, Nagoya, Japan, Nov. 13-15, 2013, ISBN: 978-4-9905958-2-1 C3051

LANDSLIDES IN BANGLADESH: CAUSES AND RECOMMENDATION

Musahaq Ali, Zakaria Hossain and Shigeko HaruyamaGraduate School of Bioresources, Mie University, Japan

ABSTRACT

This paper gives an account of the investigation carried out to reviewand analyses of floods and landslides in some selected areas in Bangladesh.The study aimed to determine the main causes of failure of slope orlandslides and to suggest remedial measures for minimizing naturaldisasters in the country. Information on landslide and slope failure werecollected from daily newspapers, available internet sources, statisticalyearbook, and technical research reports. On the basis of such information,five major devastations were taken into consideration in this paper fordetailed study. The results of analyses and discussion revealed that themajor causes of landslides were due to illegal cutting slope by public.Other causes were identified as poor planning and faulty construction ofstructures neighboring to landslide site. Analyses of slope stability withand without pore water demonstrated that the factor of safety of slopedecreased with an increase in pore water pressure during rainfall causingthe landslide devastation in Bangladesh during rainy season.

Keywords: Landslide, Flood, Heavy Rain, Bangladesh, Stability Analysis

INTRODUCTION

Heavy rains and landslides inBangladesh occurred during the lastcouple of years had killed at least329 people and left stranded over1950000 people in the south-eastBangladesh. The landslides are alsoa great concern in Bangladesh asthey threaten the life and propertyof the people in the hilly areas ofBangladesh such as Chittagong,Rangamati, Cox’s Bazar, Hobigonjand like other natural disastersthose frequently occurred inBangladesh such as tropicalcyclone, flood, draught etc. Therecent Cox's Bazaar, Bandarban andChittagong landslides in 2012 thatoccurred on June 27 is anotherdevastating natural disasters since

the independent of Bangladesh interms of human casualties damageand personal. The indiscriminatecutting of hill that made it verysteep slope, faulty construction ofhomes, embankments, roads andmiscellaneous structures in themountainous areas, worsen by theheavy rains; had increased theoccurrences of frequent landslidesespecially during the last twodecades in the southern part ofBangladesh [1]. A catastrophicsuper landslide like that tookplace in 2007 was the most damagingone, claimed 130 lives, renderednearly 150 families homeless andleft over 300000 people withoutsecure accommodation.. The tragiclandslide of 1986 and 1989 claimed51 and 300 lives respectively [2].

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GEOMATE-Nagoya, Nov.13-15, 2013

In recent years, the loss of lifeand property which occurred due tothese landslides still causingstaggering situation such aslandslides in 2009, 2010 and 2011had killed at least 81 peoples andaffected more or less all the sevenlandslide prone districts of thehilly region. Landslides andrelated slope instability phenomenawas not only the problem inBangladesh but also outbreak manyparts of the world [3]. In Japan, alot of devastating landslidesoccurred almost every year. It canbe noted that a wealth ofexperience in understanding,recognition and treatment oflandslide hazards is needed [4].Particular area requiring attentionconcerning the selection and designof appropriate, cost-effectiveremedial measures which in turnrequires a clear understanding ofthe mechanisms and processes thatcaused the landslide [5]. Especialattention had been required for thehills of Chittagong in Bangladeshas it was particularly badly hitcaused devastation due to heavymonsoon rainfall. Sudden landslidesnot only destroying lives, crops,agricultures, poultry, fisheriesand many other things but also itcreates simultaneous accumulationof silts in the river beds andfloodplains. Unfortunately, verylittle or no study is availablewhich specifies the main reasonsfor such disasters. It is thereforefelt that a research reviewing thecauses of landslide in Bangladeshshould be undertaken. In order tofulfill the above objectives,information on landslides orfailure of slopes was collectedfrom different sources such asdaily newspaper, web page,statistical year book and technicalreports. Discussion has been made

to reveal the causes of landslidesand recommendations are made forminimizing the landslide hazard inBangladesh

LOCATION OF LANDSLIDES

The geographical position oflandslide occurred in Bangladesh isshown in Fig. 1. The place ofoccurrence is mainly Chittagong andCox’s Bazar district as shown inthe south-East location on the mapof Bangladesh.

Fig.1 Geographical location oflandslide

Death of People During Past FiveYears

The number of death of peopleoccurred during the past five yearsare given in Table 1. It is notedthat there were many otherlandslides took place after theindependence of Bangladesh in 42year. In this paper, the recentcasualties are emphasized. As canbe seen in Table 1, the devastation

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that took place in 2012 claimedover 118 lives along with over250000 peoples left homeless orstranded without properaccommodation.

Table 1 Recent death of people dueto landslide Date of occurrence

Deathof

people

Left stranded

Place

2007 June11 130 30000

0Chittagonj

2009 July4 6 50000

0Hobi-gonj

2010 June15 58 20000

0Cox's Bazar

2011 July1 17 70000

0Chittagong

2012 June27 118 25000

0

Chittagong,Cox'sBazar

Fact findings

The Chittagong district is secondlargest city and Cox’s Bazar toptourist city of Bangladesh close toBay of Bengal facing heavy rainsduring monsoon season in the monthof June and July. Large part of theport city of Chittagong are badlyaffected by floods (Fig.2) causingmultiple landslides (Fig.3).

Fig.2 Heavy rain causing flood inChittagong city (source: BBC)[6]

Fig.3a Destruction of houses due tolandslide (2012) (source::reliefweb)[7]

Fig.3b Destruction of houses due tolandslide (2007) (source:reliefweb)[8]

It is known that there are lot ofilligal cutting of hills in theChittagongcity took place everyyear by the influencial people.Owing to cutting of natural slope,the slope became heay stiff andsometimes the inclination were over70 to 80 degree or close tovertical as can be seen in Fig.4.

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Fig.4 Natural slope was illegallycut to stiffer inclinationsusceptible to failure (source:Daily star)

CHECKING STABILITY OF SLOPE BEFOREAND AFTER CUTTING

In order to check the effect ofrainfall on the stability of slope,it is necessary to perform theanalyses considering the existenceand non-existence of pore waterinside the slope for both thenatural and cut slopes. Consideringthe facts above, the analyses wereperformed using the method ofdetermination. The main feature ofthis method is that it calculatesthe factor of safety of a slopebased on moment equilibrium ofsimultaneous blocks dividing thewhole slope into two major blocksof left and right sides as shown inFig.5.

Fig.5 Forces acting on a slopefor stability check

Where, Hi is the horizontalcomponent of internal force and Vivertical component of internalforce both are acted at thevicinity of i-section owing to theself-weight of the right-side soil-block when considering the free-body of left side soil block. Inthe same way, considering the freebody of the right-side soil-block,the Hi and Vi are acted in theopposite direction due to thereaction of the left-side soil-block. The self-weight of right-side soil-block is denoted by Wir.The center angle formed by thelines connecting to the center ofthe arc and the point which is thecross-section of slip line andacting line of Wir is denoted byir., The horizontal distancebetween the i-section and theacting line of Wir is denoted byXir and the vertical distancebetween the point of action ofinternal forces and the slip lineis given by Li. Alike the right-side soil-block,

the parameters Wil, il, Xil are

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also defined in the case of left-side soil-block.Another feature ofthis method is that the point ofaction of internal forces to theslip circle (Li) is common for bothleft and right group. The radius ofthe circular arc is defined by r.The horizontal distance between thebottom edge of the slip circle andi-section is defined as Xi and thecenter angle formed by the linepassing through the cross-sectionof i-section and slip-line with thevertical direction is defined asi. The details of this method canbe found elsewhere (Hossain andInoue, 2007). By using thedeterminate method, the factors ofsafety of the landslide section arecalculated by the followingequation.

(1)

where, n indicates the number ofvertical slices, indicateseffective angle of internalfriction and indicates effectivecohesion of soil. Here, H is thehorizontal force component, V isthe vertical force component, W isthe self weight of slice, is theangle to the center of the circle,u is the pore water pressure and lis the base length of the slicealong the circle. The ηgiven in theabove equation is defined asfollows.

(2)

Figures or Tables should be sizedthe whole width of a column, as

shown in Table 1 or Fig.1 (Figs. 1and/to n) in the present example,or the whole width over twocolumns. Do not place any textbesides the figures or tables. Donot place them altogether at theend of manuscripts.

RESULTS AND DISCUSSION

The properties of soil obtainedby laboratory tests are given inTable 2.The results of analyses ofnatural slope (Fig.6) are comparedto that of the cut slope (Fig.7) inTable 3. Table 1 Materials propertiesused in stability checking

Parameters ValuesDry density, t/m3 01.64Optimum water content, % 21.4

Specific gravity 02.723Cohesion, kN/m2 95.71Angle of internal friction, degree 24.21

Sand, >75m (%) 16.00Silt, 5-75m (%) 44.00Clay, <5m (%) 40.00Liquid limit 52.0%Plastic limit 29.7%Plasticity index 22.3Unified Classification System

CH

Fig.6 Before cut: facing of natural

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slope

Fig.6 After cut: facing of naturalslope

The factors of safety of thenatural slope before cutting are1.98 and 2.35 with and without porewater pressure respectively. Thisindicates that the stability of theslope decreases due to the increasein the pore water pressure duringrainfall. After cutting the hill,the safety values are obtained as1.48 and 1.10 respectivelyindicating that the factor ofsafety decreased significantly withthe increase in pore water pressurewhich leads to the failure of theslope during rainfall because therecommended factor of safely valuesare over 1.8 or more.

Table 3 Results of stabilitychecking

Porewaterpressur

econditi

on

Factor ofsafety

before hillcutting

Factor ofsafety

after hillcutting

Withoutporewater

2.35 1.48

Withporewater

1.98 1.10

CONCLUSION

From the results and discussiongiven above, the followingconclusions can be drawn:1. The natural slope in the failurelocation in Bangladesh was stableboth in the dry season and rainyseason as the factor of safety arefound to be equal or more than therequired factor of safety of astable slope.2. The major cause of landslide wasfound to be man-made throughillegal hill cutting by influentialpeople of Bangladesh.3. The other causes of landslideswere identified as the increase inpore water pressure duringrainfall.4. It is recommended that unlessserious steps are taken to correctthis way of hill, the landslides inBangladesh are plausible to turninto even worse with time andachieve eventually devastatingmagnitude.

REFERENCES

[1] Roy, P. (2007) Mindless hill-cutting caused mudslide, TheDaily Star, 2007-06-12.

[2] Schuster, R.L. (1991)Landslides analysis and control,National Research Council, p.1-10.

[3] Sikdar, Z.H. (2007) Over 100people were killed and hundredsmore injured due to devastatinglandslides in these areas on June11, The Daily Star, 2007-06-12.

[4] Iai, S., Mimura, M., Chigira,M., Kamai, T., Sidle, R.C., Suwa,H., Saito, T and Tobita, T.(2006) Geo-disaster predictionand geo-hazard mapping in urbanand surrounding areas, progressreport in FY 2005, Annuals of

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Disas. Prev. Res. Inst., KyotoUniv., (49) C, 23-38.

[5] Chigira, M. and Yokoyama, M.(2005) Weathering profile of non-welded ignimbrite and the waterinfiltration behavior within itin relation to the generation ofshallow landslides, EngineeringGeology, 78, 187–207.

[6] BBC“Heavy rains and landslidesin Bangladesh kill 90”

[7] reliefweb“Bangladesh: SituationReport No.233 (as of 26 June2012)”

[8] reliefweb“Bangladesh Landslideand Flood Situation ReportNo.234”

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Musa-nagoya (1)