Engineering Geology

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E n g i n e e r i n g G e o l o g y strength, then as displacement increases, the resistance decreases, again to a limiting value that is termed the residual strength. Skempton noted that in moving from peak to residual strength, cohesion falls to almost, or actually, zero and the angle of shearing resistance is reduced to a few degrees (it may be as much as 10∞ in some clays). He explained the drop in strength that occurred in overconsolidated clay as due to its expansion on passing peak strength and associated increasing water content on the one hand. On the other, he maintained that platey clay minerals became orientated in the direction of shear and thereby offered less resistance. Failure occurs once the stress on clay exceeds its peak strength and, as failure progresses, the strength of the clay along the shear surface is reduced to the residual value. It was suggested that under a given effective pressure, the residual strength of clay is the same whether it is normal, or overconsolidated (Fig. 5.4). In other words, the residual shear strength of clay is independent of its post-depositional history, unlike the peak undrained shear strength that is controlled by the history of consolidation as well as diagenesis. Furthermore, the value of residual shear strength, ¢ , decreases as the amount of clay fraction increases in a deposit. In this context, not only is the proportion of detrital minerals important but so is that of the diagenetic minerals. The latter influence the degree of induration of a deposit of clay, and the value of f r ¢ can fall significantly as the ratio of clay minerals to detrital and diagenetic minerals increases. f r The shear strength of undisturbed clay is frequently found to be greater than that obtained when it is remoulded and tested under the same conditions and at the same water content levels. The ratio of the undisturbed to the remoulded strength at the same moisture content is termed the sensitivity of clay. Skempton and Northey (1952) proposed six grades of Figure 5.4 Peak strength and residual strength of normally consolidated (N–C) and overconsolidated (O–C) clay soils (after Skempton, 1964). With kind permission of the Institution of Civil Engineers. 224
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Engineering Geology
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Engineering Geology Second Edition
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Preface As noted in the Preface to
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Contents 1. Rock Types and Stratigr
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Contents Field Instrumentation 344
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Chapter 1 Rock Types and Stratigrap
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Chapter 1 several tens of metres bu
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Chapter 1 Figure 1.4 Distribution o
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Chapter 1 Figure 1.5 Lapilli near C
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Chapter 1 Figure 1.7 (a) Ropy or pa
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Chapter 1 Figure 1.8 Columnar joint
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Chapter 1 Figure 1.9 Pegmatite vein
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Chapter 1 Figure 1.10 Thin section
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Chapter 1 Figure 1.11 An old quarry
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Chapter 1 a b Figure 1.13 (a) Gneis
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Chapter 1 and the types of country
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Chapter 1 A variety of minerals suc
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Chapter 1 metasomatic activity is c
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Chapter 1 derived by partial intras
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Chapter 1 character (are they irreg
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Chapter 1 the top. Individual grade
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Chapter 1 Figure 1.20 Thin section
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Chapter 1 Montmorillonite [(Mg,Al)
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Chapter 1 Figure 1.22 Salt teepees
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Chapter 1 The extent and regularity
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Chapter 1 the lowest bed in the upp
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Table 1.1. The geological timescale
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Chapter 1 The principal way in whic
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Chapter 2 Geological Structures The
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Chapter 2 (b) Figure 2.2 (a) Block
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Chapter 2 (b) Figure 2.4 (a) Types
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Chapter 2 Figure 2.6 Chevron fold i
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Chapter 2 direction of extension. T
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Chapter 2 Figure 2.9 Types of fault
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Chapter 2 Figure 2.11 (a) Repetitio
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Chapter 2 side of a fault are of di
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Chapter 2 Figure 2.14 Geometric ori
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Chapter 2 As joints represent surfa
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Chapter 2 (several metres). The int
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Chapter 2 Strength of Discontinuous
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Chapter 2 Table 2.5. Classification
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73 Figure 2.17 Discontinuity survey
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Chapter 2 Figure 2.19 Representatio
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Chapter 3 Surface Processes All lan
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Chapter 3 humid regions more than d
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Chapter 3 Figure 3.3 Honeycomb weat
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Chapter 3 less soluble than limesto
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Chapter 3 Figure 3.4 The slake-dura
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Chapter 3 Because weathering brings
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Chapter 3 Figure 3.6 Approaches to
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Chapter 3 Figure 3.7 Valley bulging
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Chapter 3 Internal slides are usual
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Chapter 3 Figure 3.8 A classificati
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Chapter 3 Figure 3.10 Block diagram
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Chapter 3 direct factor in causing
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Chapter 3 Figure 3.14 (a) Trellised
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Chapter 3 Throughout its length, a
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Chapter 3 Figure 3.17 (a) Paired ri
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Chapter 3 Figure 3.18 Component par
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Chapter 3 In the early stages of ri
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Chapter 3 Karst Topography and Unde
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Chapter 3 Figure 3.22 Appearance of
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Chapter 3 the surface throughout th
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Chapter 3 Figure 3.25 Drumlins, nea
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Chapter 3 barrier, the threshold, o
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Chapter 3 Most melt water streams t
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Chapter 3 Other small ridge-like ka
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Chapter 3 of perennially frozen gro
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Chapter 3 Wind Action Wind erosion
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Chapter 3 Figure 3.32 Buttes and me
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Chapter 3 areas in which there is e
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Chapter 3 commonly falls in both in
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Chapter 3 Figure 3.35 Terminology o
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Chapter 3 disrupts their pattern of
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Chapter 3 is extended out to sea. B
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Chapter 3 (a) (b) Figure 3.37 (Cont
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Chapter 3 Table 3.2. Average beach
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Figure 3.38 Hurst Castle Spit with
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Chapter 3 windspeeds of approximate
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Chapter 3 Free oscillations develop
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Chapter 4 Groundwater Conditions an
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Chapter 4 Figure 4.1 Map of part of
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Chapter 4 buried upper surface of a
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Chapter 4 Table 4.2. Soil suction p
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Chapter 4 The factors affecting the
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Chapter 4 Table 4.3. Some examples
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Chapter 4 Permeability and porosity
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Chapter 4 Flow through Soils and Ro
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Chapter 4 General Equation of Flow
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Chapter 4 and k h H / k + H / k + H
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Chapter 4 Figure 4.8 Standard piezo
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Chapter 4 velocity of the upward se
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Geological mapping frequently forms
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Chapter 4 As groundwater moves from
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Chapter 4 Figure 4.9 Drillhole pack
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Chapter 4 Figure 4.10 Yield drawdow
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Chapter 4 Figure 4.11 Flow net bene
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Chapter 4 in sedimentary rocks. The
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Chapter 4 Figure 4.12 Gravel-packed
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Chapter 4 In addition, the fracture
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Chapter 4 to another as groundwater
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Chapter 4 Figure 4.14 An example of
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Chapter 4 Many of the VOCs are liqu
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Chapter 4 Table 4.7. Composition of
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Page 426: Chapter 5 Table 5.2.—Cont’d. In
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Page 458: Chapter 5 Table 5.12. Particle size
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E n g i n e e r i n g G e o l o g y
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252 Table 5.22. Rock type classific
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272 Table 5.33. Some physical prope
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274 Table 5.34. Geomechanical prope
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280 Table 6.1. Some properties of B
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282 Table 6.2. Some properties of B
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372 Table 7.7a. Excerpts from the e
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374 Table 7.7b. Key to the engineer
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390 Table 8.1. Modified Mercalli Sc
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412 Figure 8.15 (a) (b) (a) Rip-rap
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488 Table 9.4. The rock mass rating
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490 Table 9.6. Classification of in
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532 Table 9.8. Typical compaction c
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I n d e x coastal protection, 410,
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I n d e x grout, 526, 548 groutabil
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I n d e x rebound, 513 recumbent fo
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