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National Jute Board (PEA)

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

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National Jute Board (PEA)

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  1. PROVSIONAL DESIGN METHODOLOGY FOR LOW VOLUME ROADS AND HILL SLOPES MANAGEMENT WITH JUTE GEOTEXTILES National Jute Board (PEA)

  2. Jute Geotextiles 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.

  3. 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.

  4. Jute Geotextiles 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) Base Course

  5. 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 surfaceis the highest.

  6. Jute Geotextiles • Base Course thickness is modified by introducing stiffness factor taking into account modulus of elasticity of two layers consideringbase 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

  7. 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 = (3) • Effect of Vertical stress on subgrade will depend on mechanical properties of base course, sub-base course and JGT.

  8. 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).

  9. Jute Geotextiles Modified Empirical Equation for Base Course Thickness with JGT T = k log N • 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

  10. Jute Geotextiles Corroborative Evaluation NOTE : This relationship needs to be corroborated with the existing design standards from Bangladesh

  11. 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.

  12. 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 ≤ NcCu Example: NormalStress At Subgrade (Giroud and Han, 2004) with JGT for CBR 2 σz116 kN/m^2 where, P = 80kN, r =, = 31 (Giroud& Noiray), T = 300mm.

  13. Jute Geotextiles Bearing Capacity of subgradesoil for CBR 2% NcCu 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

  14. 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.

  15. 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 surfacerunoff 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.

  16. Jute Geotextiles ESTIMATION OF VELOCITY AND KINETIC ENERGY OF RUN-OFF ALONG THE SLOPE • Assumptions : • Run-off component of precipitation is considered only. • Neglecting absorption & storage of water by JGT. • Hydraulic conductivity of soil and percolation is neglected. • Taking into account laws of dynamics, kinematics and gravitation. • Considering weft yarns of JGT as frictional barrier.

  17. 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

  18. 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: = ρ*i*t*cosθ(1) • Substituting (1) in familiar eqn of Kinetic energy per unit area expressed as : Ek = (2)

  19. 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 : 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 E’k =

  20. 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.

  21. Jute Geotextiles Storage - - - - d Weft of open mesh JGT - - - - - Storage - - - - - - - - - L 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. 1:n

  22. Jute Geotextiles • 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 S= mm3/m2

  23. 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.

  24. 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’ µk mg cos m a mg sin mgcos mg As derived from free body diagram :- x = ma = mg sin - µk mg cos a = g sin - µk g coswhere µk= N x h

  25. 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 : 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 v2 = 2{g sin – Nhgcos}s

  26. 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 = 2sg sin • After meeting the 45 barriers in 1 metre, N=45 then= 2sg sin – 90sg cos

  27. Jute Geotextiles • 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.

  28. THANK YOU

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