Tunnel Face Stability & New CPT Applications - Geo-Engineering

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144 4. High-Speed Piezocone Tests in Sandscharacterisation methods primarily yield a soil classification, but may also give a first indicationof other properties like fines content, sensitivity, overconsolidation ratio (OCR) or relative density.Other correlations have been published which yield a more precise determination of soilproperties like relative density based on CPT or CPTU measurements, but most are valid onlyfor a limited range of soils [108].Of those soil properties the relative density, fines content and stress state are of major importancein determining the liquefaction potential of a soil layer. In the case of flow liquefaction,the main parameter is the density of the soil in relation to the critical density, which is used asan indication whether or not the soil is contractant. In the case of cyclic liquefaction, the finescontent and stress state are also needed to make a correct assessment of the cyclic resistanceratio (CRR), either indirectly by correlating with the individual parameters or directly by usingq c -CRR correlations. The cyclic resistance ratio is defined as the cyclic stress ratio (CSR)= τ/Gs v ′ at which liquefaction occurs and the determination of the CRR is currently mademainly through the q c -CSR correlation charts based on the database of field cases compiled byStark & Olson [154], complemented with data from Suzuki et al. [142].This method does not directly use the friction and excess pore pressure measurements,although corrections on the effective q c used as index to the charts can be made. The CRRfurther depends strongly on the number of stress cycles N applied to the soil. Correlationsbetween N and CRR exist, but these do not generally extend to a single load cycle and assuch the extension to flow liquefaction of these methods is not possible, besides the fact thatthis extension would be somewhat speculative. Furthermore these methods remain indirect andempirical and as a result cannot easily incorporate the effects of a combined static and dynamicload, as in combined flow and cyclic stress liquefaction. Considerable effort is applied nowadaysto new techniques to measure in-situ, in a wide range of soils, the CRR in a more direct way, usingprobes containing vibrating elements and/or using shear wave response of the soil [143, 133].When attention is limited to contractant soils however, which under saturated conditionsshould generate excess pore pressures during shear deformation and contraction, and whichare therefore susceptible to flow liquefaction and are generally highly susceptible to cyclicliquefaction, it should be theoretically possible to measure these excess pore pressures when inan in-situ test a shear deformation is applied to the soil. Of those contractant soils, loose packedclean sands containing few to no fines are most susceptible to liquefaction.During cone penetration testing the soil around the cone is subjected to a combined compressionand shear deformation and excess pore pressures are generally generated. Whether theseexcess pore pressures are detected by a piezometer at the shoulder of the cone (see figure 4.1)depends mainly on the permeability of the soil. In soils with a permeability less than 10 −7 m/s,fully undrained conditions may be expected and the full excess pore pressures generated shouldbe measured in CPTU. In the interval 10 −7 < k < 10 −4 m/s partially drained conditions maybe expected, whereas above 10 −4 m/s fully drained conditions are most likely. Now the loosesands with low fines content that are of particular interest exhibit such large permeabilities thatat regular penetration speed no excess pore pressures are measured at the cone shoulder duringregular CPTU, even if they are generated by the shear deformation.If the penetration speed is increased, however, the dissipation time between the generationof the excess pore pressures and the observation by the piezometer is shortened. This mightcreate the possibility to observe excess pore pressure in contractant soils before they dissipatecompletely. If so, this would create a more direct observation of the contractancy of the soil andtherefore a means to identify liquefiable soils. Using a specially designed miniature cone in alarge triaxial cell this behaviour has been observed by Canou et al. at very high speeds [57].To test whether this behaviour can also be measured using a regular CPTU cone, a number of
4.2. Determination of Liquefaction Potential 1453Frictionsleeve21ConeFigure 4.1: Location of filter elements and excess pore pressure measurement in different conedesigns and their numberingtests has been executed in the calibration chamber at the Geotechnical Laboratory, at the regular2cm/s speed as well as higher speeds. During these tests pore pressures have been measured.The possibility to determine the liquefaction potential using such a method has been comparedwith methods based on correlation charts using q c from regular CPT.First the determination of liquefaction potential based on correlation charts will be described.Then the possible influences of the penetration speed on CPT and CPTU results will be investigated,followed by an overview of the various tests that have been executed in the calibrationchamber and their results.4.2 Determination of Liquefaction PotentialThe method of determining the liquefaction potential of the soil as described by Robertson &Fear [142] can be summarized as follows. First the normalized cone resistance q c1 is determinedasq c1 = q ( ) 0.5c pap a σ v′ (4.1)where p a , the atmospheric pressure, is used a a reference pressure. q c1 is subsequently usedas an index to a database of field cases. The dividing line between liquefied and non-liquefiedsites, for a magnitude M=7.5 reference earthquake, and for clean sands with less than 5% fines,defines the CRR. For q c1 between 30 and 160 it is defined as( qc1) 3CRR = 93 + 0.08 (4.2)1000This relation is valid for clean sands only. For sands with a fines content higher than 5% acorrection can be made to the q c1 value [108, 142].
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Tunnel Face Stability&New CPT Appli
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Dit proefschrift is goedgekeurd doo
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viAcknowledgementsKolla här, min
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viiiAbstractThe tunnel wound on and
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xContents3.2.4 Finite Element Model
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xiiList of Symbols× ij i = j norma
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xivList of Symbolsλfirst Lamé con
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1.2. Outline of this Thesis 3suppor
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Chapter 2Stability Analysis of the
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2.1. Introduction 7fluidsoilzs Fα
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2.1. Introduction 9Figure 2.3: Unsu
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2.1. Introduction 11q sq sq sααα
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φ = 40 ◦φ = 35 ◦2.1. Introduc
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2.1. Introduction 15T 1 T 1E1 E 1G
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2.1. Introduction 17N Deformation<
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2.1. Introduction 19GC45 ◦ + φ/2
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2.1. Introduction 21Figure 2.19: Ce
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2.1. Introduction 23Figure 2.21: In
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2.1. Introduction 25D (m) Soil type
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2.2. Wedge Stability Model 27the su
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2.2. Wedge Stability Model 29xyhz =
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2.2. Wedge Stability Model 31s ′
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2.2. Wedge Stability Model 33q 02ad
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2.2. Wedge Stability Model 35200400
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2.2. Wedge Stability Model 37with a
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2.2. Wedge Stability Model 39s(z t
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2.2. Wedge Stability Model 41∆p f
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2.2. Wedge Stability Model 43has be
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2.2. Wedge Stability Model 45from t
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2.2. Wedge Stability Model 47γF(t/
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2.2. Wedge Stability Model 49s min
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2.2. Wedge Stability Model 5140∆s
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2.3. Model Sensitivity Analysis 53P
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2.3. Model Sensitivity Analysis 55i
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2.3. Model Sensitivity Analysis 57
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2.3. Model Sensitivity Analysis 63t
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2.3. Model Sensitivity Analysis 65o
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2.4. Comparison with Field Cases 67
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Figure 2.57: Overview of soil strat
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Page 107 and 108: 2.4. Comparison with Field Cases 93
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Page 111 and 112: 2.6. Conclusions 97the effective st
Page 113 and 114: 2.6. Conclusions 99has been made th
Page 115 and 116: Chapter 3Horizontal Cone Penetratio
Page 117 and 118: 3.2. Interpretation of Cone Penetra
Page 123 and 124: 3.3. Calibration Chamber Tests on S
Page 143 and 144: 3.4. Scale Model Tests 129Figure 3.
Page 145 and 146: 3.4. Scale Model Tests 1315210Figur
Page 147 and 148: 3.5. Field Tests 133Figure 3.35: Pl
Page 149 and 150: 3.6. Modelling Horizontal Cone Pene
Page 155 and 156: 3.7. Conclusions 141W H/V 043Ticino
Page 157: Chapter 4High-Speed Piezocone Tests
Page 161 and 162: 4.2. Determination of Liquefaction
Page 163 and 164: 4.4. Calibration Chamber Tests 149
Page 165 and 166: 4.4. Calibration Chamber Tests 1518
Page 167 and 168: 4.4. Calibration Chamber Tests 153v
Page 169 and 170: 4.4. Calibration Chamber Tests 155-
Page 171 and 172: 4.5. Conclusions and Recommendation
Page 173 and 174: 4.5. Conclusions and Recommendation
Page 175 and 176: Chapter 5Conclusions and Recommenda
Page 177 and 178: 5. Conclusions and Recommendations
Page 179 and 180: Bibliography[1] M.Y. Abu-Farsakh, G
Page 181 and 182: Bibliography 167[30] B. Belter, W.
Page 183 and 184: Bibliography 169[65] J.K. van Deen.
Page 185 and 186: Bibliography 171[99] E. Kreyszig. A
Page 187 and 188: Bibliography 173[133] Committee on
Page 189 and 190: Bibliography 175[171] F.W.J. van Vl
Page 191 and 192: Curriculum VitaeWout Broere was bor
Page 193 and 194: SamenvattingGraaffrontstabiliteit &
Page 195 and 196: Samenvatting 181spanningen ook in d
Page 197 and 198: NawoordIn de afgelopen jaren heb ik
Page 199 and 200: Appendix ABasic Equations of Ground
Page 201 and 202: A.1. Semi-confined System of Aquife
Page 203 and 204: A.2. Transient Flow 189withS s the
Page 205 and 206: ....Appendix BBasic Equations of El
Page 207: B. Basic Equations of Elasticity 19
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