PROVSIONAL DESIGN METHODOLOGY

FOR LOW VOLUME ROADS AND HILL SLOPES MANAGEMENT WITH

JUTE GEOTEXTILES

National Jute Board

(PEA)

Contents

• Provisional Design Approach for Rural Roads

• Evaluation of Reduction in Impact of rain drops on top soil and Run-off Velocity with OW JGT in Hill Slopes.

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DESIGN CONCEPT FOR PAVEMENTS

• Flexible pavements are designed on ‘layer system concept’ to distribute stresses over larger area.

• Design Approach is Semi-theoretical and Semi-empirical.

• Studies carried out have shown that there exists a relationship between pavement thickness, wheel load, tyre pressure and C.B.R.

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Determination of Base Course Thickness

• U. S. Corps of Engineers used the following expression for various loading conditions :

• T = (1)where, P = wheel load (kg), T = Base course thickness (cm), p = Tyre pressure (kg/sq.cm)

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𝑃2

𝑃2

Base Course

• Limitations of the equation are :Properties of material used is not

consideredLoad repetitions also not considered

• Concept of Burmister two layer theory is incorporated into Eqn (1) as different layers of pavement possess different elastic moduli with elastic modulus of top surface is the highest.

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• Base Course thickness is modified by introducing stiffness factor taking into account modulus of elasticity of two layers considering base course and subgrade as a two layer system.

• Now, thickness of pavement is improvised as suggested by Kansas Highway Department Design method:

• T = (2)where, = Elastic Modulus of Sub-grade

= Elastic Modulus of sub-base and base course

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Base Course Thickness with JGT

• Since JGT is laid over sub-grade therefore assuming base course and JGT together as one layer and subgrade as another layer. Eqn (2) is modified as :

• T = (3)• Effect of Vertical stress on subgrade will

depend on mechanical properties of base course, sub-base course and JGT.

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Effect of Number of Passes on Thickness

• Pavement thickness should be sufficient to resist collapse caused by designed traffic.

• Based on performance data, it was established by Yoder & Witczak (1975) and Hveem & Carmany that there is linear relationship between base course thickness and logarithm of load repetitions (N).

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Modified Empirical Equation for Base Course Thickness with JGT

• where, P = wheel load (kg)• T = Base course thickness (cm)• p = Tyre pressure (kg/sq.cm)• = Elastic Modulus of Sub-grade• = Elastic Modulus of JGT (average of warp & weft direction)• = Elastic Modulus of sub-base and base course• N = Cumulative ESAL over 10 years• k = Numerical constant = 0.25

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T = k log N

Corroborative Evaluation

ESAL : 30000 – 60000Thickness (mm)

ESAL : 60000 - 100000Thickness (mm)

ESAL : 100000 - 200000Thickness (mm)

IRC:SP: 72-2007

Without JGT

With JGT

IRC:SP:72-2007

Without JGT

With JGT

IRC:SP:72-2007

Without JGT

With JGT

CBR 2 325 390 300 375 400 310 425 430 330

CBR 3 275 350 270 325 370 280 375 390 300

CBR 4 275 330 250 325 350 265 375 370 280

CBR 5 250 310 240 275 330 250 325 350 265

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NOTE : This relationship needs to be corroborated with the existing design standards from Bangladesh

Inference

• It is apparent that pavement thicknesses with the suggested methodology is close to those recommended in IRC:SP:72 – 2007 for CBR varying between 2 – 5% and ESAL ranging between 30,000 to 2,00,000.

• For ESAL between 1,00,000 – 2,00,000 thickness of pavement determined by the approach is more close to those of thickness in IRC:SP:72 of same ESAL range.

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Check for Design Thickness

Normal stress (σz) at the interface between base course and subgrade should meet the following requirement to prevent subgrade failure i.e. must be less than or equal to bearing capacity of soil as stated by Giroud and Han, 2004.

σz ≤ Nc Cu

Example: Normal Stress At Subgrade (Giroud and Han, 2004) with JGT for CBR 2

σz 116 kN/m^2where, P = 80kN, r =, = 31 (Giroud & Noiray), T = 300mm.

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Bearing Capacity of subgrade soil for CBR 2%

Nc Cu kN/m^2 where, Nc 3.14, Cu 30 CBR = 60kPa

From above it can be inferred that, normal stress in presence of JGT at subgrade interface is less than bearing capacity of subgrade soil.

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Conclusion – Design thickness with JGT is O.K

DESIGN CONCEPT FOR HILL SLOPE MANAGEMENT

• Impact of kinetic energy of rain drops detaches top soil and resultant run-off transports the detached soil particles to drainage outlet (river/stream).

• Soil detachment and transport of detached soil particles by run-off can be controlled if ground has vegetative cover.

• Velocity of run-off will depend on the slope, intensity of rainfall and hydraulic conductivity of soil.

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CONTROL OF SOIL EROSION WITH JGT

• Appropriateness of JGT in soil conservation can be explained as :JGT acts as a cover over the soil which lessens the

direct impact of K.E of rain drops.Pose speed breakers by weft yarns of jute fabric across

the direction of flow to reduce velocity of surface runoff successively.

Ensures overland storage as jute is excellent water absorbent (nearly 5 times its dry weight).

Facilitate growth of vegetation on bio-degradation of fabric so that its roots could hold the soil against detachment.

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ESTIMATION OF VELOCITY AND KINETIC ENERGY OF RUN-OFF ALONG THE

SLOPE• Assumptions :

o Run-off component of precipitation is considered only.

o Neglecting absorption & storage of water by JGT.o Hydraulic conductivity of soil and percolation is

neglected.o Taking into account laws of dynamics, kinematics

and gravitation.o Considering weft yarns of JGT as frictional

barrier.

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Weft yarns of jute fabric

Warp

Direction of run-off

Incipient storage of water

Weft yarns of jute geotextiles acting as micro-barriers

ISOMETRIC VIEW OF SLOPEISOMETRIC VIEW OF SLOPE

DIRECTION OF RUN-OFF

X-SECTION X-SECTION

PART1 : DETERMINATION OF EXTENT OF REDUCTION OF IMPACT OF KINETIC ENERGY OF RAIN DROPS

• Mass of water per unit area impacting a bare soil surface is given by Gabet & Thomas,2003:

= ρ*i*t*cosθ (1)

• Substituting (1) in familiar eqn of Kinetic energy per unit area expressed as :

Ek = (2)

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• Modifying (2) by introducing ‘Cv’ coverage by JGT, considering the fact JGT will prevent impact of raindrops after touching ground in Eq. (2) to yield an effective kinetic energy E’k :

where, ρ = density of water (1000 kg/ m3),

i = rainfall intensity (m/s),

t = storm duration (s),

Cv = percentage of area covered by JGT.

v = terminal velocity of raindrops

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E’k =

INFERENCE

• Reduction in impact of K.E. (%)

= (1 – Cv) x 100%

With 500gsm OW JGT of 40% coverage there will be 60% reduction in impact of rain drops on topsoil.

Understandably, larger is the extent of percentage of cover over soil, less will be extent of od detachment and migration of soil particles.

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PART2 : DETERMINATION OF EXTENT OF OVERLAND STORAGE

• Overland storage is interception of run-off.• If a portion of the overland flow can be

intercepted as storage, erosive force will get somewhat reduced.

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- - - - - - -- -

- - - - - - - - -

L

Storage

Weft of open mesh JGT

Storaged

1:n

• The aspect of overland storage has been analyzed by Sanyal (2006) which establishes the following relation for slope 1: n.

where, S = storage by weft yarns (mm3/m2)

d = Diameter of yarn (mm)

N = Number of weft yarns per meter

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S = mm3/m2

INFERENCE

• Neglecting absorption by JGT, there will be Effective storage of 0.44litres of water by 4mm diameter of 45 consecutive number of weft yarns of 500gsm OW JGT.

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PART3 : DETERMINATION OF EXTENT OF SUCCESSIVE REDUCTION OF VELOCITY OF RUN-OFF • Considering an object of mass ‘m’ moving down the plain

surface with acceleration ‘a’ meeting a barrier on way posed by weft yarns of JGT with no ground friction, the barrier effect (posed by jute yarns) denoted by ‘µk’

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a

mg sin

µk mg cos

mgcos

 mg

θ

m

As derived from free body diagram :- x = ma = mg sin - µk mg cos

a = g sin - µk g cos where µk = N x h

• For assessing run-off velocity running across direction of flow, we revert to Newton’s basic equation :

v2 = 2as (with initial velocity of run-off is zero)

• Substituting above equations will give :

where, v = Runoff velocity (m/s)

s = Distance between consecutive weft yarns (m)

µk = Barrier coefficient = N x h

N = Number of weft yarns of JGT across the slope

h = Thickness of weft yarns of JGT (m)

= Angle of inclination of hill slope

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v2 = 2{g sin – Nhgcos}s

INFERENCE• Assuming velocity at any point along the slope be V1 and after

meeting 45 barriers of weft yarns in 1 metre of 500 gsm OW JGT run-off velocity be V2

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V1

V2

Weft yarns of JGT

Before meeting breaker, N = 0 then = 2sg sin After meeting the 45 barriers in 1 metre, N=45 then =

2sg sin – 90sg cos

• Reduction in Run-off velocity in 1 metre along the slope (%) :

= x 100 = (45 h cot) x100 % • With 4mm dia of weft yarns and = 30

32% run-off reduction in 1 metre

The approach is evidently theoretical based on certain assumptions and requires corroboration with field data.

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THANK

YOU