rainfall induced landslide on sh-77 : a case study

Rainfall induced landslide on SH-77 : A Case Study

Devendrakumar Tiwari (Student, M.Tech, Dept. of Transportation and Planning Engineering, Sandip

University)

Prof. Mahesh Endait (Faculty and Project guide, Dept. of Civil Engg., Sandip University)

Abstract : This study basically focusses on the remedial measure that can be provided to a landslide that occurred on a State

Highway Road number 77. The landslide occurred during the monsoon season of the year 2019 in the month of July. The main

reason suspected for the cut was heavy rainfall in the region. For the purpose of analysis of the failure Plaxis 2D software based on

the Finite Element Method was used in this study. The study was conducted by preparing two models one with the suspected cause

that is excessive moisture due to heavy rainfall without the retaining wall and the second with the suggested remedy in the form of

retaining wall. After analysis of both the models it was observed that the total displacement in the model with retaining wall was

less as compared to the model without retaining wall.

Keywords : Plaxis2D, Finite Element Method, Retaining Wall, Excessive Moisture.

1. INTRODUCTION

Landslides in hilly areas are one of the major concerns in infrastructure development. Engineers since very

long have tried to overcome this natural phenomenon by various methods. Landslides are generally

movement of earth mass under gravity due to various reasons, like erosion, earthquake, volcanic activities,

increased moisture content and/or human intervention. There are various known techniques to overcome or

reduce the effects of landslides, some of which may be, construction of retaining structures like retaining

walls, gabions, geosynthetic materials etc., by providing anchorages to the suspicious section or the section

which is under the risk of landslide or soil stabilization using chemicals or soil replacement and proper

compaction.

Arbanas (2015) referring Cruden (1991) describes landslides as ‘the movement of a mass or rock, debris or

earth down a slope’. Landslides can occur due to various reason such as increased moisture content of the

land mass, movement of the land mass due vibration caused due to construction activities, can be included

under human interference, weak or fractured rock mass below the ground or volcanic activities.

Landslide was observed in a road section in Jawhar – Parali region on chainage 48/800 km (Technogem

Consultants Pvt. Ltd.) . The area comes under hilly terrain characterization. Jawhar – Parali is situated near

the Western Ghats region. The region is said to observe significant rainfall in the months July – October

(Climate-Data.org). The average rainfall in the region is about 3287mm (Climate-Data.org). The problem

of landslide occurred in one of the section of this road whose chainage is mentioned as 48/800 km. It

happened in month of July (2019), monsoon season, when after heavy rainfall (in the mentioned region) a

part of the side slope of the road subsided.

The preliminary conclusion drawn at the site of occurrence was the heavy rainfall that occurred in the region

which apparently increased the moisture content of the soil mass leading to the subsidence of the slope

under the action of gravity.

1.2 OBJECTIVE OF THE STUDY

1. Numerical Modeling of the landslide using Plaxis 2D software.

2. Suggesting the solution of retaining wall and analyzing its behavior in Plaxis 2D.

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Volume 8, Issue 9, 2020

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http://aegaeum.com/ Page No: 1194

1.3 LOCATION OF STUDY

The study area is located in the Jawhar region which is in the hilly regions of Maharashtra in Thane district.

In the study area the road which is categorized as SH-77 is considered. Since, the road is situated in the

hilly regions the area is prone to receive heavy to very heavy rainfall during the rainy seasons. The rainfall

recorded by Indian Meteorological Department in this area in the year 2019, the year in which the landslide

occurred, showed a higher rainfall when compared to the rainfall recorded in the previous year. The data

recorded for the region in the year 2018 was 1299.0 mm while that in year 2019 was 1948.4 mm which is

higher than the previous year. It can be considered as one of the reason for the landslide.

Figure 1. Google Earth imagery showing the landslide affected area of Jawhar – Parali SH-77 (Lat. –N 19.95˚,

Lon. – E 73.23˚)

Table 1. Pavement composition of Jawhar – Parali road SH-77

Sr. No. Chainage Side

Pavement Composition in (mm)

Remarks

BT BBM WBM/Granular

Layer

Total

Thickness

(mm)

75 37/000 L.H.S 60 0 170 230

76 37/500 R.H.S 50 0 160 210

77 38/000 L.H.S 70 0 150 220

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78 38/500 R.H.S 50 0 160 210

79 39/000 L.H.S 40 0 150 190

80 39/500 R.H.S 50 0 140 190

81 40/000 L.H.S 60 0 160 220

82 40/500 R.H.S 50 0 150 200

83 41/000 L.H.S 50 0 140 190

84 41/500 R.H.S 60 0 160 220

85 42/000 L.H.S 50 0 170 220

86 42/500 R.H.S 60 120 180 360

87 43/000 L.H.S 50 110 170 330

88 43/500 R.H.S 60 120 180 360

89 44/000 L.H.S 70 110 150 330

90 44/500 R.H.S 70 0 130 200

91 45/000 L.H.S 60 0 120 180

92 45/500 R.H.S 50 0 130 180

93 46/000 L.H.S 50 0 140 190

94 46/500 R.H.S 40 0 150 190

95 47/000 L.H.S 60 0 110 170

96 47/500 R.H.S 70 0 130 200

97 48/000 L.H.S 60 0 120 180

98 48/500 R.H.S 50 0 130 180

99 49/000 L.H.S 50 0 140 190

100 49/500 R.H.S 40 0 150 190

101 50/000 L.H.S 50 0 150 200

102 50/500 R.H.S 40 0 160 200

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Figure 2. Typical Cross – Section of the Project road Jawhar – Parali SH-77

1.4 SCOPE OF STUDY

Landslides are one of the common problems faced by engineers. This study will be helpful in finding a

remedy for avoiding or lowering the damage caused due to the landslide by using PLAXIS 2D Software.

Figure3. Actual site picture of SH-77 Jawar-Parali Road, India, July 2019.

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2. Methodology

Plaxis 2d is used for analysis of the behavior of the soil. The Plaxis 2D software is used for the simplistic

study which was required for this preliminary study. Before the analysis in Plaxis 2D some field operations

and data collections along with laboratory test was required to be executed so that the input data required

for analysis with the software is obtained.

Procedure followed in plaxis 2d

A) Field data collection and Lab tests.

The project road in consideration was an overlaying project under PWD Jawhar. Various site operations

such as Axle Load test, Origin Destination and geotechnical investigation of the site was done.

After the landslide occurred various soil tests in the laboratory was conducted such as liquid limit and

plastic limit determination, Grain size analysis, modified proctor test and CBR test. The soil sample

from the field were collected and taken back to the lab to perform the above mentioned test. The soil

was collected in a plastic bag so that the moisture in the soil is not lost and most representative and

correct result is achieved. The tests and data was provided by Technogem Consultants Pvt. Ltd.

The results obtained are as follows-

Table Number 2. Grain Size Analysis

DETERMINATION OF GRAIN SIZE ANALYSIS

(As per IS 2720 Part - 4)

IS SIEVE

NO.(MM)

MASS

RETAINED

(gm)

%

RETAINED

%

CUMULATIVE

RETAINED

%

PASSING Remark

1 2 3 4 5 6

100 0.00 0.00 0.00 100.00

Gravel

75.00 0.00 0.00 0.00 100.00

53.00 2473.00 27.24 27.24 72.76

26.50 1936.00 21.33 48.57 51.43

9.50 1730.00 19.06 67.63 32.37

4.75 1046.00 11.52 79.15 20.85

Sand

2.36 477.00 5.25 84.40 15.60

0.850 226.00 2.49 86.89 13.11

0.425 235.00 2.59 89.48 10.52

0.075 579.00 6.38 93.27 6.73

Silt &

Clay

Pan 376.00 4.14 93.62 6.38

REMARKS: Gravel 67.63 %

Sand 19.27 %

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Silt & Clay 6.38 %

Table 3. Liquid Limit and Plastic Limit Test

DETERMINATION OF LIQUID AND PLASTIC LIMIT VALUES OF THE FRACTION

PASSING IS 425 Micron sieve

( As per IS 2720 Part V - 1985 )

Sr.No Liquid Limit Plastic Limit

1 2 3 4 1 2 3

1 No.of Blows 39 32 20 11 - - -

2 Container No 87 86 19 123 - - -

3 Wt.Cont. + Wet soil (

gms ) 67.47 65.88 59.03 45.13 30.39 32.23 32.03

4 Wt.Cont. +Dry soil ( gms

) 55.51 55.32 50.58 39.01 28.55 30.57 30.28

5 Wt.of Container ( gms ) 15.1 16.32 20.97 18.46 20.10 22.00 21.50

6 Wt. Of water ( 3-4 ) (

gms ) 11.96 10.56 8.45 6.12 1.84 1.66 1.75

7 Wt. Of Dry Soil (4-5) (

gms ) 40.41 39.00 29.61 20.55 8.45 8.57 8.78

8 % Moisture ( 6/7 x 100 ) 29.60 27.08 28.54 29.78 21.78 19.37 19.93

Results:

1) Liqiud Limit ( L.L ) 28.75 %

2) Plastic Limit (P.L ) 20.36 %

3) Plasticity index ( P.I )

= 8.39 %

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Table 4. Modified Proctor Test

MODIFIED PROCTOR TEST

( As per IS 2720 Part -8 )

Consultant -: Technogem Consultants Pvt. Ltd.

1) Sample -: 1 Sample No. :

1

2) Location -:

3) Source -: Subgrade Sampling Date

-:

4) Ward-: P Testing Date -:

5) Road -:

No. of Layers

-: 5

5) Degree of Compaction Heavy

6) Blows per layer 55 BLOWS

7) Height of drop 450 mm

8) Volume of cylinder 2250 Checked By:

9) Tested By

DENSITY DETERMINATIONS

Test No 1 2 3 4 5 6

1) Water Added

2) Wt.of Cyl. + Wet soil ( kg ) 10917 11320 11510 11419

3) Wt. Of Cylinder ( kg ) 6484 6484 6484 6484

4) Wt.of Wet soil 4433 4836 5026 4935

5) Wet Density y, kg/mm3 1.970 2.149 2.234 2.193

MOISTURE DETERMINATIONS

1) Container No 1 2 3 4

2) Wt.of Cont. + Wet soil ( gm ) 154.00 123.00 142.00 125.00

3) Wt. Of Cont+ Dry soil ( gm ) 135.00 102.00 112.00 98.00

4) Wt.of Container ( gm ) 23.00 11.00 17.00 20.00

5) Wt. Of Water ( gm ) 19.00 25.00 30.00 27.00

6) Wt. Of Dry Soil ( gm ) 112.00 91.00 95.00 78.00

7) Moisture Content w% 16.96 27.47 31.58 34.62

8) Dry Density Yd = Y/(1+(w/100)) 1.684 1.686 1.698 1.629

Maximum dry density 1.690 gm/cc ( From Graph )

Optimum moisture content 31.50 % ( From Graph )

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Figure 4. Bore Log Data

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B) Modeling using Plaxis 2D

The next part after field data collection and lab test is analysis by using Plaxis 2D. For this purpose, 2

models were created namely one which resembles the site condition when there was excessive moisture

in the soil due to heavy rain and the second model was made with a retaining wall as a possible remedy

to reduce the overall settlement. The values of properties from Table 5 and 6 were used for preparation

of the models.

Figure 5. Modelled geometry without Retaining wall

Figure 6. Modelled geometry with Retaining wall

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Table 5. Material Properties of the Retaining wall (Source [29])

Material Properties of Plate

Properties Unit Value

Axial stiffness (EA) kN/m 4.25 x 106

Flexural rigidity (EI) kNm2/m 2.56 x 105

Poisson’s ratio(ʋ) --- 0.15

Weight per area kN/m/m 20.4

Table 6. Material Properties of the Soil

Layer Young's Modulus

(E (KN/m2)

Poisson's Ratio

(ν)

Cohesion

(c)

KN/m2)

Basalt Rock 2.29 *10^5 0.2 304

Gravelly Soil 96*10^3 1 1

Murum 1.3*10^4 1 1

Sand 1.3*10^4 0.2 1

Clay 50*10^3 0.4 25

C) Calculation phases

In the initial phase (Phase 0), the bitumen layers are deactivated in the second phase the complete road

is activated and in the third stage the load on the system is activated. For the load that is to be applied

that data was obtained from the Axle Load Test .

The load taken into consideration for the modelling of the test model is based on the axle load data

obtained from Technogem Consultants Pvt. Ltd. The material used for construction of the pavement

was Bitumen, Water bound macadam and murum.

The axle load obtained were in the form of Equivalent Standard Axle Loads (ESAL’s). The value was

4.621 ESAL’s. According to IRC 37:2001 the load of 80 KN (1 ESAL) is considered to cause more

damage to the pavement hence it is considered for the design of flexible pavement. Thus, in this study

the load acting upon the pavement is considered as a Uniformly Distributed Load (UDL) acting along

the transverse length of the pavement. So the load acting on the test section was thus taken and

converted from ESAL to UDL as follows:

Total Load in ESAL = 4.621

1 ESAL = 80 KN

Total Load in KN = 80*4.621 = 369.68 KN

Now, Load Acting on the pavement = 369.68/ (5.5*1) = 67.21 KN/m2

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D) Mesh and Output-

Standard 15-node elements will be used. The software divides the geometry into smaller elements to

perform the Finite Element Method of calculation.

3. RESULTS AND DISCUSSIONS

A basic 2d model with properties as mentioned in methodology was used for the numerical modelling

purpose.

1. Model 1: In this model the effect of excessive water is considered.

2. Model 2: In this model a retaining wall with backfill is suggested as a remedy.

After the application of the load on the geometry of the road under consideration following output was

obtained from Plaxis 2D:

1. Model 1: This model or geometry of the soil was prepared considering the higher level of the water

and to analyze the behavior of the soil mass in this condition. The rise in pore water pressure or

moisture content can be one of the important factor in the slope failure. [1][5][9] [10] [14] [20].

The result obtained in the final output for the Total Displacement or Settlement of the soil was 513.33*10-

3 m.

Figure 7. Deformed Mesh Showing Total Displacement for Model 1

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2. Model 2: In this model the geometry preparation was done considering a remedial measure in the form

of a retaining wall to prevent the landslide. After the appliocation of the load and final analysis the output

obtained was for Total Displacement or Settlement which was 111.44*10-3 m.

Figure 8. Deformed Mesh Showing Total Displacement for Model 2

4. CONCLUSIONS

From the Figure 7 and 8, showing the final output for model 1 and 2 it can be seen that the total

displacement for model 1 that is the geometry without the retaining wall is 513*10-3 m and that of model

2 that is the geometry with retaining wall is 111.44*10-3 m. On comparing both the values it can be

observed that the value obtained for model 1 is higher than the value obtained for model 2 which suggest

that the displacement in model 1 without retaining wall is more than that in model 2 with retaining wall.

It can hence be concluded that retaining wall as a remedial measure can be used on the study area since,

the displacement of the soil mass is reduced to some extent.

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rainfall induced landslide on sh-77 : a case study

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