Design of Unpaved Roads A Geotechnical Perspective

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1 - CGTR NERIST Design of Unpaved Roads A Geotechnical Perspective Arindam Dey Assistant Professor Department of Civil Engineering Geotechnical Engineering Division IIT Guwahati 2 Introduction Road Network in India Over 42 lakh kms (CIA, 212) 48% Unpaved Roads (MoRTH, 28) Unpaved roads (Includes haul roads and access roads) Sand or stone aggregate placed directly over the local soil subgrade No permanent surfacing before immediate application Constant passage of traffic over time Settlement Rutting 1

2 3 Layout of the Presentation Unpaved Roads Quasi-Static Analysis and Formulations : Design Charts Influence of Traffic Numerical FE Modeling: PLAXIS 2D v212 : Safe Designing Methodology 4 Axle Load on an Unpaved Road: Load Distribution (Giroud and Noiray, 1981) Total load Replaced by Equivalent single axle load (P) Evenly distributed (4 wheels) Axle load expressed in terms of Contact areas of tires (A c ) Tire inflation pressures (P c ) Contact area Equivalent rectangular contact area (LxB). Assumption: Equivalent uniformly distributed contact pressure (P ec ) Geometry of unpaved road, vehicle axle loads and contact areas [Adopted from Giroud and Noiray(1981)] Dual tire prints Equivalent Contact Dimensions: Present Study On-Highway trucks Off-Highway trucks L = B 2 B = P Pc L = B 2 B = P P c 2 2

3 5 Load Distribution by Aggregate Layer on Subgrade Load-Distribution Shape Pyramidal Load-dispersion angle (α ) α = α = π 4 φ agg 2 Stresses generated on the aggregate-subgrade interface With geotextile p, Without geotextile p Load distribution by aggregate layer on the subgrade soil (a) Without geotextile (b) With geotextile [Giroud and Noiray(1981)] Aggregate thickness P p = + γ h ( B + h α )( L + h α ) P p = + γ h 2 ( B + 2 h tan α )( L + 2 h tan α ) 2 2 tan 2 tan p With geotextile h, Without geotextile h = π c u ( π 2 ) p = + c u 6 Idea from Past Research Proposed design charts Uses only undrained cohesion c-φ soil: Strength Parameters Cohesion Angle of internal friction Conventional Design Charts Over estimation of aggregate layer thickness Only a degenerated condition Present Study Improvisation over the Giroud & Noiray s Model (1981) Accounts internal friction angle of soil subgrade Reveals substantial reduction in aggregate thickness (Compared after inclusion of φ) Accounts moving load as static Maximum axle load Quasi-Static Analysis Detailed parametric study Influence & Sensitivity of various contributory parameters. Design Charts (with & without Geotextiles) for several combination of various parameters 3

4 7 Quasi-static Analysis of Unpaved Roads Unpaved roads: Aggregate cover Acts as load-dispersing mechanism Reduce subgrade stresses Minimizes hindrance for passage of vehicles Shear strength of the soil (Mohr-Coulomb expression) Allowable bearing capacity (q all ) (Terzaghi, 1943) Bearing capacity factors q all τ = c + σ tan φ cnc + γ h Nq +.5 γ B ' N = FOS N N q γ c 3π φ 2( ) π φ 4 2 = e 2cos( + ) 4 2 = 2( N + 1) tanφ N = ( N 1) cotφ q q γ 8 Design of Unpaved Road without Geotextile Net pressure on the subgrade soil P p = + γ h ( B + h α )( L + h α ) 2 2 tan 2 tan Net Pressure Allowable Bearing Capacity (FOS=1) P γ h cnc γ h Nq.5 γ ( B 2h tan α ) N 2( B + 2h tan α )( L + 2h tan α ) + = γ Load distribution by aggregate layer on the subgrade soil without geotextile [Giroudand Noiray(1981)] Solution Required thickness of aggregate layer on c-φ subgrade without geotextile 4

5 9 Design of Unpaved Road with Geotextile Subgrade soil Undrained and Incompressible Geotextile Stretched wavy shape due to settlement under tires and heave in between them Generation of tension membrane effect Reduction of pressure by geotextile : p g Kinematics of unpaved roads with geotextile [Giroud and Noiray(1981)] Pressure transferred to subgrade soil, p* (portion AB) * p = p p g 1 Design of Unpaved Road with Geotextile Pressure (p*) Allowable bearing capacity of the subgrade soil P p* = γ h pg cnc γ hnq.5 γ ( B 2h tan α ) N 2( B + 2h tan α )( L + 2h tan α ) + = a pg = Kε a 1+ a = ( B + 2h tanα ) 2 2s K Tension-elongation modulus ε Elongation of Geotextile s Function of rut depth(r) b + b ' ε = 1 a + a ' γ FOS =1 Solution Required thickness of aggregate layer on c-φ subgrade with single layer of geotextile Shape of deformed geotextile [Giroud and Noiray(1981)] 5

6 11 Parameters MATLAB codes Compute required aggregate thickness (with and without geotextile layer) Sensitivity plotted For various important parameters Range of various parameters chosen (Indian traffic conditions) Axle Load (P) Tire inflation pressure (P c ) Angle of internal friction of aggregate (φ agg ) Angle of internal friction of soil (φ) Soil cohesion (c): Unit weight of soil and aggregate (γ): 3 kn 36 kn (MoRTH, GoI, 25; IRC-37-21) 15 kpa 75 kpa (AFJM, 1994; Khanna and Justo, 21) [Covers the broad domain of soil- purely cohesive soil to rocky subgrade] 5 kpa [Covers purely cohesionless soil to hard clay in the subgrade] 19 kn/m 3 [Kept same- No significant variation in γ ] Track widths of Indian Cargo vehicles (e) Tension-elongation modulus of geotextiles (K) Factor of safety (FOS): m 1-5 kn/m (Giroud and Noiray, 1981) 1 2 [FOS =1 (ultimate bearing capacity) other FoS (allowable bearing strength)] 12 Without Geotextile Design Charts: Effect of Cohesion of subgrade soil Soft soil: Very low c and φ values Immensely thick aggregate layer Optimum cohesion of 3 kpa to substantially reduce the aggregate layer thickness With Geotextile Adopt some subgrade modification techniques for soils with natural cohesion less than 3 kpa Application of geotextiles can be a solution Stiffer clays Theoretically no necessity of aggregate layer 6

7 13 Design Charts: Effect of Angle of Internal Friction of subgrade soil Subgrade containing coarser soil particles Without Geotextile Enhanced angle of internal friction Increase in bearing strength Substantial reduction in the required aggregate thickness Analysis by Giroud and Noiray (1981) With Geotextile Based on purely cohesive soils Results in overestimated results for natural subgrade soils having cohesionless particles as well 14 Design Charts: Effect of Axle Load and Tire Pressure Increment in Axle Load Required aggregate thickness is higher Obvious observation With Geotextile With Geotextile Increment in tire inflation pressure Does not significantly affect the required aggregate thickness for lower axle loads Lower tire inflation pressure Higher equivalent contact area c=5 kpa, ϕ agg = 35, soil ϕ = 5, K=1 kn/m, r=.3 m, e=1.7 m c=5 kpa, ϕ agg = 35, ϕ soil = 5, K=1 kn/m, r=.3 m, e=1.7 m 7

8 15 Benefit from Geotextile: Tensile Strength Depicts benefit of geotextiles : Enhanced tensile strength of geotextile Reduction in required aggregate thickness With Geotextile Zero tensile strength Absence of geotextile Efficacy of geotextiles Degree of improvement (If) 1% improvement theoretically signifies that aggregate cover is not necessary With Geotextile K i = Thickness at final K value I K K i f = K K 1 = Thickness at initial K value 16 Effect of Rut Depth Lower rut depths Negligible or Nil efficacy of geotextiles Reconfirms the finding of Holtz and Sivakugan (25) With Geotextile Larger rut depth Large deformation Enhanced mobilization of membrane tension Increased efficiency of the geotextile Substantial reduction in h With Geotextile 8

9 17 Benefit from Geotextile: Comparison Comparison shows a reduction of ~2 mm Aggregate layer thickness With Geotextile: (K= 4 kn/m) With Geotextile: (K= 1 kn/m) The reduction in thickness increases with increase in tensile strength- Economy 18 Typical Quick Design Charts: Unreinforced and Reinforced Case Without Geotextile With Geotextile 9

10 19 Influence of Traffic N=1 N=1 N=1 2 Influence of traffic Webster and Alford s Expression h m.19 log N s = ( CBR ).63 For rut depth=.75 m Standard Axle load= 8 kn Bearing capacity of subgrade Black s Expression (Field test data) qu = 1 CBR Equation: Multiple passage Need to extend the applicability Rut depth ( ) log Ns log Ns 2.34 r.75 h m.81log Ni log Pi 1.89r 5.95 = ( q ).63 u Other Axle Loads P s P i N N s i P i = Ps 3.95 Solution Technique : MATLAB Yields cumulative aggregate thickness (hm) 1

11 1/19/ Design Charts without Geotextile: Multiple Passage Multiple Passage: Without Geotextile Multiple Passage: Without Geotextile P = 8 kn P = 19 kn 22 Design Charts with Geotextile: Multiple Passage Multiple Passage: With Geotextile Multiple Passage: With Geotextile K = 5 kn/m, e = 1.7 K = 1 kn/m, e =

12 23 Summary Accounting both subgrade strength parameters (c & φ) in present study Realistic estimation of h- Economical design Influential contributory parameters Axle load (P) Subgrade strength parameters (c and φ) Angle of internal friction of aggregate (φ agg ) Parameters having minimal effects on required aggregate thickness Tire inflation pressure (for lower axle loads) Location of vehicles Tensile strength of geotextile Significantly affects degree of improvement (in terms of reduction in h) Beneficial effect of geotextile is highlighted for higher rut depths (elevated tensioned membrane effect) Cumulative Aggregate Thickness Based on Empirical formulas not for N > 1, 24 Need for Continuum Modeling Lack of practical applicability Very low value of subgrade strength High aggregate thickness (Vice-Versa) Construction Failure Unbound Aggregate Mechanically unstable Punching Failure Fine content required 4-8% (IRC:SP:77-28) To bring Precision Limit Equilibrium - Finite Element Approach LE: Simplification for complex numerical problem Resort to FE approach Propagation of failure 12

13 25 Modeling tool PLAXIS 2D v212 Performs deformation & stability analysis Geotechnical applications Convenient GUI : allows automatic generation of 2D FE mesh (Global and Local refinement) Realistic construction simulation : allows activating, de-activating element clusters, loads 26 Present Study: Model Description Properties Subgrade Aggregate Constitutive Model Mohr-Coulomb Mohr-Coulomb Unit Weight (γ) 19 kn/m² 19 kn/m² Elastic Modulus (E) 6 MPa 2 MPa Poisson s Ratio (ν).4.3 Initial Void Ratio (eint).5.1 Model Type Plain Strain Geometry Uniform cross-section Same stress state perpendicular to cross-section Model Geometry 2 Layered System Subgrade & Aggregate Side Slopes 3H: 1V (stable side slope) Model Boundaries 13

14 27 Model Geometry: Unreinforced Model Unreinforced Model Geometry Finite Element Mesh: Triangular- 15 nodded 28 Failure under Aggregate Load Checking Stability csoil, φsoil, φagg: Analytical results cagg =.1 (to avoid numerical instability) Result: Failure in Subgrade During lay of aggregate 14

15 29 Critical/Limiting Failure Conditions Subgrade Failure Aggregate Load Allowable Bearing Capacity of Subgrade γ h s = c N +.5γ BN s,min c s FOS γ Failure Zone Cmin chart 3 Failure under Vehicular Loading Checking Stability cs,min, φsoil, φagg: Analytical results cagg =.1 (to avoid numerical instability) Result: Failure in Aggregate Due to punching Mechanical Instability Absence of Fine soil 15

16 31 Critical/Limiting Failure Conditions Aggregate Failure Stress intensity under tire Allowable Bearing Capacity of Aggregate Result P 2tL = c N γ tn a,min c +.5 a FOS γ With each φagg : Different FOS Variation in axle load: Varying stress distribution angle different stress intensities Stability Check : Excessively strong Subgrade 32 Combined Configuration Subgrade cs,min, φsoil : Determined from failure only due to aggregate load Aggregate ca,min, φagg : Determined only from punching failure of aggregate under vehicle load Loading Subgrade fails: enhancement of parameters required (cs,min csa,min ) 16

17 33 Estimation of enhanced Subgrade cohesion (csa,min) Parametric Trial-and-Error determination Determination of the property enhancement zone Total Deviatoric Strain plot Total Displacement plot 34 Design Methodology: Unreinforced Case Geometry and Cluster Parameters: Design Charts If subgrade fails : Aggregate load Find cs,min and test model with this value If subgrade passes : OK Fails : Fine Tune Passes: Test Aggregate under load Fails: Find ca,min and test model Passes: Find csa,min and test model Passes : OK Fails: Fine Tune Passes: OK Fails: Fine Tune 17

18 53 Thickness Reduction using Geotextile Model Number Model Load P = 3 kn P = 8 kn P =19 kn P = 24 kn P = 36 kn Initial model height 1.14 m.72 m 1.1 m 1.24 m 1.46 m Reduced height without geotextile Reduced height with geotextile Reduction due to geotextile.5 m.7 m 1.5 m 1.15 m 1.46 m.4 m.55 m.65 m.75 m 1.2 m.1 m.15 m.4 m.4 m.2 m Percentage Reduction 2 % % 38.9 % % % 36 Conclusions To impart mechanical stability to Stacked Unbound Aggregate Mixture of aggregate, sand, fine-sized particles Stress distribution angle Presence of fine material in cluster voids Varies with varying axle load even for same φ value Modification of Strength Parameters: Subgrade Based on strain concentration pattern under operational conditions Advantage of Numerical MOdelin Coupled stress-deformation based analysis (c-φ soil) Different from conventional stress based stability of only cohesive soil 18

19 Thank You 19