Design of Pavements on Expansive Clay Subgrades

Transcription

1 Design of Pavements on Expansive Clay Subgrades Robert L. Lytton Professor, Fred J. Benson Endowed Chair Zachry Department of Civil Engineering Texas A&M University Foundation Performance Association Houston, Texas December 12, 2012

2 Outline Performance of pavements on expansive clays Roughness Cracking Pavement monitoring program Suction envelopes for design Prediction of movement Edge of pavement Wheel path 2

3 Outline, cont. Prediction of roughness Longitudinal cracking over expansive soils Design countermeasures Crack spacing Features of design program WinPRES WinPRES demonstration 3

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22 Exponential Suction Profile for Extreme Wetting and Drying Condition nπ nπ U(Z,t) = U e +Uoexp - πntz - cos Z2 α α U(Z) = U e +Uoexp - 0 nπ Z α Suction (pf) Mitchell (1979) Volumetric Water Content Fort Worth Interstate Vertical Movement (in) Moisture Active zone Depth (ft) Wet Dry 6 8 Wet Dry 10 Wet Equil Dry

23 Soil Suction, pf Depth, ft Copyright John T. Bryant (2008) 23

24 Figure 1 - Total Soil Suction Histogram for Frequency More Total Soil Suction, pf 24

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26 Performance Criteria for Engineering Structures Engineering Structure Performance Criteria Foundations Differential movement: vertical and lateral and allowable stresses Differential movement and allowable stresses Total vertical and lateral movement; lateral pressure; allowable stresses Pavements Roughness spectrum, International Roughness Index, Longitudinal cracking Roughness spectrum, Pilot and Passenger acceleration Retaining Walls Lateral pressure and movement, allowable stresses 26

27 Performance Criteria for Engineering Structures, cont. Engineering Structure Performance Criteria Pipelines Roughness spectrum, allowable stress, fatigue criteria, corrosion Slopes Downhill movement, shallow slope failure, slope stability Canals Combination of the performance criteria of retaining walls, pipelines, and slopes; thermal and shrinkage cracking; permeability of the cracks and joints Moisture Barriers Reduction of the movement of water in the soil and of total vertical movement Land Fill Covers and Liners Moisture and leachate transmission (including the effects of cracks) 27

28 The Design Problem How do you design a foundation to perform successfully when you have poor site conditions? Vegetation Drainage Slopes 28

29 Answer: Design for the worst that they can do Site condition Problem Limiting Condition Vegetation Poor drainage Slopes Drying shrinkage Swelling Downhill creep, shallow slides Wilting point pf=4.5 Clay wet limit pf>2.5 Uphill offsets, drainage control 29

30 Answer: use suction envelopes to determine the worst that they can do 30

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32 (-) Suction Ground Surface Wet Season Equilibrium Dry Season Depth 32

33 (-) Suction Ground Surface Dry Season Wet Season Equilibrium Depth Water Table Suction of the Water 33

34 Field Conditions U e = exp( TMI) Field capacity Equilibrium pf Wilting point 4.5 pf Root zone 4 Depth (ft Moisture active zone nπ UZ ( ) = Ue ± U0 exp - Z α 34

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36 Volume Change % fc = % 2µ m % No.200sieve LL / %fc γ h γσ % 2µ m = γ0 % No.200sieve = γ h 1 h 1+ h θ θ PI / %fc Zone III (Covar and Lytton, 2001) (Lytton, 1994) 36

37 Volume Change V hf σ f = γh log10 γσ log10 V hi σ i (Lytton, 1977) H H V = f V f = pf f = 0.5 when drying; f = 0.8 when wetting n V = fi Z i= 1 V i i Volume Mean Principle Stress-Suction surface 37

38 Suction vs. Pressure vs. Volume Surface 38

39 Formation of Suction vs. Pressure vs. Volume Surface 39

40 Calculated Vertical Movement 40

41 Transverse Distribution of Vertical Movements (in) Section A Section B Section C Swelling d (ft) Shrinkage

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43 Predicted Roughness vs. Time; Fort Worth I- 820 B 4.5 Loss of Serviceability 140 Increase in Roughness 4.0 Soil 120 SI 3.5 IRI 100 Soil Time (yrs) Time (yrs) 43

44 Predicting Changes in IRI (R) dr = β ( H ) + dt Pavement categories: 1 2 Moisture barriers with paved medians β 1 = 0.619, β 2 = Moisture barriers with sodded medians β 1 = 1.583, β 2 = Control section with and without medians β 1 = 2.701, β 2 = β 44

45 IRI vs. PSI 45

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47 Climatic Conditions Thornthwaite Moisture Index (TMI, 1948) Roadside Drainage Conditions Longitudinal Drainage Cut Hill 2.3 pf Slope Valley 2.0 pf 2.0 pf Flat Fill 2.5 pf 2.2 pf 2.2 pf 2.6 pf 2.3 pf 2.3 pf Lateral Slope TMI 100R 60DEF = E p R = runoff moisture depth DEF =deficit moisture depth E p = evapotranspiration 47

48 Acceptable Predicted Performance Flexible Pavement Fort Worth I-820 A 48

49 Acceptable Predicted Performance Rigid Pavement Austin SR-1 49

50 Longitudinal Cracking over Expansive Soil Expansive soil Experiences volumetric change when subjected to moisture variation Longitudinal crack Initiates in shrinking expansive subgrade Propagates to pavement surface 50

51 Practice of Lime Treatment 51

52 Without Geogrid Reinforcement Asphalt Crack Base Subgrade (Expansive soil) C L * Rong Luo, Texas A&M University 52

53 With Geogrid Reinforcement Asphalt Base Geogrid Subgrade (Expansive soil) C L 53

54 Transverse Stress Distribution in Pavement (Crack at Edge of Shoulder) 54

55 0.200 Number of Crack= Number of Crack=2 80 Stress Intensity Factor (MPa*m^.5) Number of Crack=3 Number of Crack=4 Number of Crack=5 Upper Tensile Strength Lower Tensile Strength Geogrid Tensile Strength (kn/m) Geogrid Stiffness (kn/m) 55

56 Transverse Distribution of Vertical Movements 56

57 Edge Moisture Variation Distance, e m pf Suction envelopes P = maximum permissible vertical strain e m 2u o Design suction range e m = αt π 2u pf 0 ln( ) pf = p γ h 57

58 Longitudinal Crack Spacing Pavement Initial Suction pf 2u 0 Tensile Strength of Soil Wilting Point 58

59 Shrinkage Strain ε s 1 = (1 + pf ) h ( ) 6 [ γ pf ] 59

60 Distance to First Shrinkage Crack x 1 = αt π 0 ln( ) 2u 0 2u pf 60

61 Diffusivity α 2 m = [ S γh ] sec ( ) ( )

62 7 pf int 1 pf(w) S 0 0 Sw = pf( w) pf w, water content w sat intercept pf intercept = (%fine clay) 62

63 Alternative Use built-in empirical expression: α = S γ h where: S = (LL) (PI) (% No. 200) γ = γ h 0 %-2μm % -No.200sieve 63

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65 Field to Laboratory Diffusion Coefficient Ratio Field α/laboratory α 0 65

66 Program WinPRES 66

67 Soil Properties 67

68 Lane/Barrier configuration 68

69 Initial Serviceability 69

70 Diffusivity Slope of SWCC Suction Compression Index 70

71 Traffic/Reliability 71

72 WinPRES Demo 72

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76 Soil Survey of Harris County, Texas Lake Charles series 76

77 Lake Charles series, cont. 77

78 Soil Survey of Harris County, Texas Engineering properties and classifications 78

79 Soil Survey of Harris County, Texas Engineering test data 79

80 Soil Survey of Harris County, Texas Profile of Lake Charles clay 80

81 WinPRES Demo 81