Deformation behaviour of railway embankment ... - Liikennevirasto

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94 6 FACTORS AFFECTING PERMANENT DEFORMATION 6.1 Introduction A railway embankment is exposed to a large number of load applications during its service life. Although the permanent deformation is normally a fraction of the total deformation produced by each load repetition, the gradual accumulation of a large number of these small plastic deformation increments could lead to eventual failure. Failure and also excessive permanent deformations of the embankment must of course be prevented. To do so the knowledge of the plastic behaviour of unbound granular materials is very important. Most of the research carried out over the years has concentrated on the resilient behaviour of granular materials. In comparison to the resilient behaviour, less research has been devoted to the plastic response and permanent deformation development in unbound granular materials. This is perhaps due to the practical difficulties in studying permanent deformation behaviour. While resilient tests are fairly quick and each laboratory specimen can be studied for a great number of stresses, the monitoring of the build-up of permanent deformations in unbound granular materials and permanent deformation tests are very time-consuming, and usually separate specimens are required for each set of stresses. As a result, greater advances have been made in understanding the resilient response than the long-term performance of granular materials. The permanent strain development in granular materials is affected by several factors: stress level, stress history, number of load applications, principal stress rotation, moisture content, density, grading and aggregate type, and physical properties of aggregate particles. − − − − The importance of the applied stress level is strongly emphasized in the literature. Permanent strain is said to be related directly to the deviator stress and inversely to the confining pressure (Barksdale 1972). However, several studies have suggested that permanent strain is principally governed by some form of stress ratio consisting of both deviatoric and confining stresses (e.g. Brown and Hyde 1975). A number of limited investigations found in the literature (Chan 1990) indicate that reorientation of principal stresses due to moving traffic significantly increases the amount of plastic strains in granular materials. The growth of permanent strain in granular materials is a gradual process during which each load application contributes a small increment to the accumulation of strain. The significance of the number of load repetitions has been recognized many times in the literature (e.g. Sweere 1990, Paute et al. 1993). The amount of permanent strain in granular materials is also influenced greatly by the presence of water. At high levels of saturation, deformation resistance in the material reduces quite rapidly, probably as positive pore water pressure is generated. Proper drainage in granular materials is, therefore, a necessity for improving performances. The reduction in fines content is likely to achieve this but
95 may conflict with the desire to provide a more deformation-resistant mixture by increasing the fines to promote better packing. − − − − − The effect of stress history on permanent strain development has been recognized in the literature but hardly investigated. The study by Brown and Hyde (1975) clearly shows that permanent strain is reduced notably as the result of stress history. However, if previous loading has loosened the material (e.g. by dilating it), the effect of stress history is the opposite. In laboratory permanent strain tests, the effect of stress history is eliminated by using a new specimen for each set of stresses applied. Density, described by the degree of compaction, is believed to have a pronounced impact on the long-term behaviour of granular materials (Thom and Brown 1988, Barksdale 1991). Resistance to permanent strain in these materials is highly improved as the result of increased density. The effect of fines content on the permanent deformation behaviour of unbound granular materials was investigated by Barksdale (1972), Thom and Brown (1988), and Barksdale (1991), who concluded that the permanent deformation resistance of granular materials is reduced as the amount of fines increases. The effect of particle size distribution, or grading, is a disputed subject and different views are found in the literature. As for the effect of aggregate type, it has been suggested that crushed, angular materials undergo smaller permanent deformations compared to materials such as gravel with rounded grains (Barksdale and Itani 1989). 6.2 Stress level The literature available shows that the stress level is one of the most important factors affecting the development of permanent deformations in unbound granular materials. The magnitude of the permanent deformations developed strongly depends on the stress level and increases with rising deviator stress and decreasing confining stress (Werkmeister 2003). Early repeated load triaxial tests reported by Morgan (1966) clearly showed that the accumulation of axial permanent strains is directly related to the deviator stress and inversely related to the confining pressure. Morgan studied the behaviour of sands under repeated loading with an increasing number of load cycles and considered the influence of deviator stress and confining pressure on the accumulation of permanent axial strain. He found a direct dependency between the accumulated permanent axial strain after any given number of load cycles and the deviator stress at a particular level of confining pressure. On the other hand, by maintaining the deviator stress at a constant level, Morgan found that the permanent axial strain was inversely proportional to the confining pressure. Barksdale (1972) comprehensively studied the behaviour of several unbound granular materials using repeated load triaxial tests with constant confining pressure and 10 5 load
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A 5/2006 Deformation behaviour of r
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Finnish Rail Administration Publica
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4 Past investigations have shown cl
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6 Brecciaroli, Fabrizio - Kolisoja,
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8 kuvataan jakamalla jännitykset j
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10 TABLE OF CONTENTS ABSTRACT .....
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12 8.9 Trondheim NTNU/SINTEF’s re
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14 combination of resilient strains
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16 Modelling is an important necess
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18 Figure 2.1:2 Two-dimensional sta
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20 σ xx − σ 3 2 det τ yx σ yy
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22 stresses. As it can be seen from
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24 grains. The resistance to partic
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26 The reason was argued to be that
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28 term matric suction implies the
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30 Figure 3.2.1:1 Idealisation of t
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32 ε ε 2 ν = , (Eq. 3.2.2:4) whe
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34 where sij = σ −σ , (Eq. 3.2.
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36 addition, when both the complexi
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38 supplement continuum mechanics m
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40 Figure 4.2:1 A typical variation
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42 Figure 4.2:3 Modulus of resilien
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Page 48 and 49: 46 material (Uzan 1985). Dilation i
Page 50 and 51: 48 Figure 4.5:1 Resilient modulus o
Page 52 and 53: 50 The grain size distribution of g
Page 54 and 55: 52 The grain sizes that determine t
Page 56 and 57: 54 4.6.3 Fines content A simple cha
Page 58 and 59: 56 grained than for fine-grained ag
Page 60 and 61: 58 and the shape of the particles a
Page 62 and 63: 60 Figure 4.7:1 Effect of the wetti
Page 64 and 65: 62 Figure 4.7:3 Modulus of resilien
Page 66 and 67: 64 Figure 4.7:6 Dry -density as a f
Page 68 and 69: 66 Figure 4.7:7 Resilient response
Page 70 and 71: 68 The form index can be measured u
Page 72 and 73: 70 Hovinmoen are gravel materials.
Page 74 and 75: 72 5 MODELLING OF RESILIENT DEFORMA
Page 76 and 77: 74 5.2.2 Resilient modulus as a fun
Page 78 and 79: 76 M r k2 k3 ⎛ θ ⎞ ⎛ q ⎞ =
Page 80 and 81: 78 the model showed good representa
Page 82 and 83: 80 discussed. However it can be exp
Page 84 and 85: 82 et al. (2003) calculated the res
Page 86 and 87: 84 p u = unit pressure (1 kPa), δp
Page 88 and 89: 86 Figure 5.2.5:2 Vertical resilien
Page 90 and 91: 88 2 1 ⎛ ⎞ = A q ε ν p ⎜1
Page 92 and 93: 90 Figure 5.3:2 An example of the c
Page 94 and 95: 92 by incorporating a coefficient o
Page 98 and 99: 96 cycles. He drew the general conc
Page 100 and 101: 98 accumulated strain, ε p , is co
Page 102 and 103: 100 Figure 6.3:1 Effect of loading
Page 104 and 105: 102 load” is not equal to the fai
Page 106 and 107: 104 Chan (1990) noticed, in his stu
Page 108 and 109: 106 Youd (1972) studied the behavio
Page 110 and 111: 108 Figure 6.5:2 Comparison of plas
Page 112 and 113: 110 ample fine-grained fractions th
Page 114 and 115: 112 of a dolomitic limestone with a
Page 116 and 117: 114 Fuller curve (Equation 4.6.2:3)
Page 118 and 119: 116 Figure 6.6:7 Elastic and plasti
Page 120 and 121: 118 Figure 6.7:1 Dependence of the
Page 122 and 123: 120 0,7 0,6 Elastic limit Failure s
Page 124 and 125: 122 Figure 6.8.1:1 Influence of fin
Page 126 and 127: 124 explain why the differences in
Page 128 and 129: 126 was then argued that the variat
Page 130 and 131: 128 7 MODELLING OF PERMANENT DEFORM
Page 132 and 133: 130 Figure 7.2:1 Mohr-Coulomb repre
Page 134 and 135: 132 where K p (N) = bulk modulus wi
Page 136 and 137: 134 1993)) have reported that at le
Page 138 and 139: 136 ε 1,p = accumulated permanent
Page 140 and 141: 138 Barksdale (1972) conducted repe
Page 142 and 143: 140 Figure 7.6:2 Relationship betwe
Page 144 and 145: 142 also be employed for pavement d
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144 repeated loads, although adapta
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146 series of specimens (or in a mu
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148 Figure 7.7:5 S-N design models
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150 In conclusion, both Huurman (19
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152 aim to simulate as closely as p
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154 by the American Association of
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156 Confining pressure is applied t
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158 particles. During the test, the
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160 method takes the average value
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162 Figure 8.7.2:2 Schematic illust
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164 through each platen is provided
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166 measured for the 600 mm central
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168 displayed on the monitor in gra
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170 unbound granular materials. The
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172 Figure 8.8.2:2 Permanent axial
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174 performance-related (functional
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176 the six supports of the vertica
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178 under the same isotropic stress
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180 9 CONCLUSIONS 9.1 Resilient def
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182 − Permanent deformation resis
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184 Balay J., Gomes Correia A., Jou
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186 Cheung, L.W. (1994). Laboratory
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188 El abd, A., Hornych, P., Breyss
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190 Hoff, I., Nordal, S., and Norda
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192 Kolisoja, P. (1996b) Large Scal
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194 Mitry, F.G. (1964). Determinati
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196 Plaistow, N.C. (1994). Non-Line
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198 Tam, W.A. and Brown, S.F. (1988
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200 Wellner, F. (1996). Influence o
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