Structural Design of Pavements PART VI Structural ... - TU Delft

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27 The way in which the load is distributed depends on the thickness and the stiffness of the layer. In figure 14, the top layer is the stiffest followed by the base and the subgrade. It is obvious that only that part of the pavement that is subjected to stresses, will deform; that is the area enclosed by the cone. This means that the geophone that is farthest away from the load centre (geophone a) only measures deformations in the subgrade while the geophone in the load centre (geophone b) measures the deformations in the subgrade, base and top layer. This implies that if the Boussinesq formula is applied using the deflection value measured by geophone a as input, the modulus of the subgrade is calculated. In case Boussinesq’s equation is used using the reading of geophone b as input, an overall effective stiffness of the pavement is calculated. So the stiffness calculated from the geophone readings going from a to b give information about: the subgrade, the subgrade plus some effect of the base, the subgrade plus the base plus some effect of the top layer, the subgrade plus the base plus the top layer; in short: increasing moduli value will be calculated. All this means that the deflection readings taken at a certain distance from the load centre give in fact information on the stiffness of the pavement at a certain depth. Using this information a so-called surface modulus plot is constructed. On the vertical axis one plots the surface modulus calculated using the Boussinesq formulas and on the horizontal axis one plots the equivalent depth which is equal to the distance of the geophone considered to the load centre. The principle of the plot is schematically shown below. Surface Modulus Equivalent Depth Figure 15 shows the surface modulus plots as calculated using the deflections measured at locations 0.65 and 1 (see table 2). The figure indicates that we are dealing with a weak pavement because the surface modulus values are very low and because the stiffness hardly increases from bottom to top. Only in location 1 some stiffening due to the base and top layer is visible. As shown below, different shapes of the surface modulus plot can be obtained. Surface Modulus Equivalent Depth
28 The drawn line indicates a pavement where the stiffness gradually increases from bottom to top while the dashed line indicates a pavement which has layers with a low stiffness on top of the subgrade. The reason for this might be stress dependent behaviour, lack of compaction, moisture effects etc.. It might very well be that the material with the lower stiffness is in fact the same material as the subgrade material. This is e.g. the case with fill material that cannot be compacted to the density of the existing subgrade. Figure 15: Surface modulus plots for locations 0.65 and 1. The surface modulus plot assists in deciding how many layers should be taken into account in the back calculation analysis. As indicated, the number of layers to be considered is not only the number of physical layers, top, base, sub-base and subgrade; one also has to take into account the fact that within one layer, sublayers may occur with a different stiffness. 6.2 Back calculation of layer moduli: Back calculation of layer moduli from measured deflection bowls is done in an iterative way. The input for the calculations consists of the measured deflection profile, the load geometry used to generate the deflections and the thickness of the layers. Furthermore the cores that are taken from the pavement to determine the thickness of the layers give information on the materials used and the quality of the materials. From the surface modulus plot an estimate is obtained for the modulus of the subgrade and furthermore the surface modulus plot provides information that helps to decide whether or not low stiffness sublayers should be introduced in the analysis. Then moduli values are assigned to the various layers and the deflections are calculated. Next the calculated deflections are compared with the measured ones. If the differences are too large, a new set of moduli is assumed and the deflections are calculated again. This process is repeated until there is a good match between the calculated and measured
Page 1 and 2: CT 4860 Structural Design of Paveme
Page 3 and 4: 2 Table of contents: Preface 3 1. I
Page 5 and 6: 4 1. Introduction: These lecture no
Page 7 and 8: 6 2. Usage and condition dependent
Page 9 and 10: 8 When a swimmer makes a nice dive
Page 11 and 12: 10 Figure 5: Principle of the autom
Page 13 and 14: 12 From the text given above it is
Page 15 and 16: 14 As shown in figure 7, 3 zones ca
Page 17 and 18: 16 Figure 9: Placement of loading p
Page 19 and 20: 18 5. Statistical treatment of raw
Page 21 and 22: 20 Figure 10: Results of a deflecti
Page 23 and 24: 22 Figure 12: Cumulative sum of the
Page 25 and 26: 24 Section Locations Mean SCI SD SC
Page 27: 26 6. Back calculation of layer mod
Page 31 and 32: 30 The clayey subgrade was covered
Page 33 and 34: 32 Section I Temp Force Layer E-mod
Page 35 and 36: 34 [N] [mm] [mm] [MPa] [N] [mm] [mm
Page 37 and 38: 36 Based on these calculations, eva
Page 39 and 40: 38 8. Estimation of the remaining p
Page 41 and 42: 40 Figure 25: Load configuration us
Page 43 and 44: 42 If we assume e.g. that the defle
Page 45 and 46: 44 Figure 27: Relationship between
Page 47 and 48: 46 resistance, some parts of the pa
Page 49 and 50: 48 Log ε = 0.481 + 0.881 log SCI 3
Page 51 and 52: 50 b 3 = 1.0153 b 4 = -7.73 x 10 -6
Page 53 and 54: 52 10. Extension of the simplified
Page 55 and 56: 54 Compressive vertical strain at t
Page 57 and 58: 56 This relationship opens possibil
Page 59 and 60: 58 The remaining pavement life can
Page 61 and 62: 60 estimation of the bitumen and mi
Page 63 and 64: 62 the composition and the degree o
Page 65 and 66: 64 The life of the overlay N can si
Page 67 and 68: 66 Table 7b: Relationship between K
Page 69 and 70: 68 Figure 41: Relationship between
Page 71 and 72: 70 From the nature of the equation
Page 73 and 74: 72 σ σ t Γ ε Figure 43: Strengt
Page 75 and 76: 74 14. Effects of pavement roughnes
Page 77: 76 16. Bekker, P.C.F. Pavement perf

Structural Design of Pavements PART VI Structural ... - TU Delft

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