PROVSIONAL DESIGN
METHODOLOGY
FOR LOW VOLUME ROADS AND HILL
SLOPES MANAGEMENT WITH JUTE
GEOTEXTILES
National Jute Board
(PEA)
Jute Geotextiles
Contents
• Provisional Design Approach for
Rural Roads
• Evaluation of Reduction in Impact
of rain drops on top soil and Runoff Velocity with OW JGT in Hill
Slopes.
Jute Geotextiles
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.
Jute Geotextiles
Determination of Base Course Thickness
𝑃
𝑃
2
2
Base Course
• U. S. Corps of Engineers used the following
expression for various loading conditions :
• T= 𝑷
𝟏.𝟕𝟓
𝑪𝑩𝑹
−
𝟏 𝟏/𝟐
𝒑𝝅
(1)
where, P = wheel load (kg), T = Base course thickness (cm), p
= Tyre pressure (kg/sq.cm)
Jute Geotextiles
• Limitations of the equation are :
Properties of material used is not
considered
Load 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.
Jute Geotextiles
• 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= 𝑷
𝟏.𝟕𝟓
𝑪𝑩𝑹
−
𝟏 𝟏/𝟐
𝒑𝝅
𝟑
𝑬𝒔𝒈
𝑬𝒃𝒄
(2)
where, 𝑬𝒔𝒈 = Elastic Modulus of Sub-grade
𝑬𝒃𝒄 = Elastic Modulus of sub-base and base course
Jute Geotextiles
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= 𝑷
𝟏.𝟕𝟓
𝑪𝑩𝑹
−
𝟏 𝟏/𝟐
𝒑𝝅
𝟑
𝑬𝒔𝒈
𝑬𝑱𝑮𝑻 +𝑬𝒃𝒄
(3)
• Effect of Vertical stress on subgrade will
depend on mechanical properties of base
course, sub-base course and JGT.
Jute Geotextiles
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).
Jute Geotextiles
Modified Empirical Equation for Base
Course Thickness with JGT
T= 𝑷
𝟏.𝟕𝟓
𝑪𝑩𝑹
−
𝟏 𝟏/𝟐
𝒑𝝅
𝟑
𝑬𝒔𝒈
𝑬𝑱𝑮𝑻 +𝑬𝒃𝒄
x k log N
• where, P = wheel load (kg)
• T = Base course thickness (cm)
• p = Tyre pressure (kg/sq.cm)
• 𝑬𝒔𝒈 = Elastic Modulus of Sub-grade
• 𝑬𝑱𝑮𝑻 = Elastic Modulus of JGT (average of warp & weft direction)
• 𝑬𝒃𝒄 = Elastic Modulus of sub-base and base course
• N = Cumulative ESAL over 10 years
• k = Numerical constant = 0.25
Jute Geotextiles
Corroborative Evaluation
ESAL : 30000 – 60000
Thickness (mm)
ESAL : 60000 - 100000
Thickness (mm)
ESAL : 100000 - 200000
Thickness (mm)
IRC:SP:
72-2007
Withou
t 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
NOTE : This relationship needs to be corroborated with the
existing design standards from Bangladesh
Jute Geotextiles
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.
Jute Geotextiles
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 =
𝑷
𝟐
𝝅(𝒓+𝑻 𝒕𝒂𝒏𝜶)𝟐
𝐏
where, P = 80kN, r =
𝟐
𝛑𝐩
= 116 kN/m^2
, 𝜶 = 31 (Giroud & Noiray), T = 300mm.
Jute Geotextiles
Bearing Capacity of subgrade soil for CBR 2%
Nc Cu = 𝟏𝟖𝟖 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.
Conclusion – Design thickness with JGT is O.K
Jute Geotextiles
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.
Jute Geotextiles
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.
Jute Geotextiles
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.
Jute Geotextiles
DIRECTION OF RUN-OFF
Weft yarns of jute
geotextiles acting as
micro-barriers
X-SECTION
Incipient
storage of
water
Direction of run-off
Warp
Weft yarns of
jute fabric
ISOMETRIC VIEW OF SLOPE
Jute Geotextiles
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:
𝐦𝐚𝐬𝐬
=
𝐚𝐫𝐞𝐚
ρ*i*t*cosθ
(1)
• Substituting (1) in familiar eqn of Kinetic
energy per unit area expressed as :
Ek =
𝛒𝐢𝐭𝐯 𝟐 𝐜𝐨𝐬𝛉
𝟐
(2)
Jute Geotextiles
• 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 :
E’k =
𝛒𝐢𝐭𝐯 𝟐 𝟏−𝐂𝐯 𝐜𝐨𝐬𝛉
𝟐
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
Jute Geotextiles
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.
Jute Geotextiles
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.
Storage
d
Weft of open
mesh JGT
Storage
--- --- -- L
1:n
-- -- -- --
Jute Geotextiles
• The aspect of overland storage has been
analyzed by Sanyal (2006) which establishes
the following relation for slope 1: n.
S=
𝐍 𝐱 𝐝𝟐 𝟒𝐧− 𝝅 𝟏𝟎𝟑
mm3/m2
𝟖
where, S = storage by weft yarns (mm3/m2)
d = Diameter of yarn (mm)
N = Number of weft yarns per meter
Jute Geotextiles
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.
Jute Geotextiles
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’
m
a
mg sinθ
θ
µk mg cosθ
mgcosθ
mg
As derived from free body diagram :𝑭x = ma = mg sin𝛉 - µk mg cos𝛉
a = g sin𝜽 - µk g cos𝜽 where µk = N x h
Jute Geotextiles
• 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 :
v2 = 2{g sin𝛉 – Nhgcos𝛉}s
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)
𝛉 = Angle of inclination of hill slope
Jute Geotextiles
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
V1
V2
Weft yarns of JGT
 Before meeting breaker, N = 0 then 𝑽𝟐𝟏 = 2sg sin𝜽
 After meeting the 45 barriers in 1 metre, N=45 then
𝑽𝟐𝟐 = 2sg sin𝜽 – 90sg cos𝜽
Jute Geotextiles
• Reduction in Run-off velocity in 1 metre along the
slope (%) :
=
𝑽𝟐𝟏 − 𝑽𝟐𝟐
𝑽𝟐𝟏
x 100 = (45 h cot𝜽) x100 %
• With 4mm dia of weft yarns and 𝜽 = 30
32% run-off reduction in 1 metre
The approach is evidently theoretical
based on certain assumptions and requires
corroboration with field data.