Conservation of Historic Stone Buildings and Monuments (1982) / Chapter Skim
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Deformation and Fracture of Rock
Deformation and Fracture of Rock
Pages 87-107
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From page 87... ...
Him pressure or chemical activity of these pore fluids decreases strength. The strength of rock increases dramatically as confining pressure increases.
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From page 88... ...
Plastic flow occurs at higher stresses, and the complete curve describes a hysteresis loop with permanent strain sp.
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From page 89... ...
Elastic materials are those that behave elastically under both hydrostatic and deviatoric stresses, whereas ductile materials undergo plastic deformation at deviatoric stresses greater than the yield stress. When the load is increased to a sufficiently high level, a specimen eventually fractures.
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From page 90... ...
Rock-forming minerals are hard and brittle because of the low symmetry of their crystallographic structures. Dislocations cannot move easily in these complicated structures, so plastic flow is almost completely inhibited.
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From page 91... ...
The cooling of a rock after it has solidified creates thermal stresses that are sufficient to fracture the crystalline framework. Likewise, microcracking can result from the decrease in stress that a sample experiences when it moves from its origin at depth to the surface or from the increases in stress that occur during tectonic deformation.
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From page 92... ...
. \ / \ \ \ UNDER CONFINING PRESSURE FIGURE 3 A schematic description of the effect of confining pressure on cracks and pores.
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From page 93... ...
A rule of thumb is that the effect of cracks on elastic properties depends on the total surface area of the crack phase bind is independent of its volume) , whereas the effect of pores is in direct proportion to the volume of the pore phase.
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From page 94... ...
Cracks under changes in hydrostatic pressure merely open and close. However, one side of a suitably oriented crack can slide relative to the other side when the applied stresses contain a
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From page 95... ...
The elastic properties of the constituent minerals do not vary with temperature to an appreciable extent until the temperature reaches a level of approximately half the melting temperature (in keIvins) ; these temperatures are well above those that most building stone is subjected to
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From page 96... ...
under normal circumstances. Temperature changes of 20° C to 30° C, however, are sufficient to cause measurable thermal cracking in competent granites,4 thereby changing their compliance.
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From page 97... ...
The stress at which fracture occurs depends not only on the type of rock and its previous history but also on factors such as the types of stresses that are applied, the degree of saturation and pressure of the pore fluid, and the rate at which the load is applied. Temperature change over the range experienced in building construction is not a factor of major importance in the discussion here.
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From page 98... ...
98 CONSERVATION OF HISTORIC STONE BUILDINGS FIGURE 7a A photornicrograph of Westerly granite at a stress well below the fracture. strengths Note that cracks have begun to extend and that the.
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From page 99... ...
Deformation and Fracture of Rock 50~ FIGURE 7b A photomicrograph of Westerly granite on the verge of fractured Note that fracture on the microscale is brittle, although the stres~strain curve has the characteristics of a ductile material.
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From page 100... ...
- Griffith's theory, although it describes in broad outline how the strength of rock depends on the types of stresses applied, does not accurately describe the fracture process itself.6 ~2 Griffith visualized that fracture in compression occurs as it does in tension: Cracks are inactive until- the fracture stress is reached, when the largest crack grows across the specimen, separating it into two pieces. The stressstrain curve for a rock sample in Figure 8 shows that this mode!
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From page 101... ...
Theoretical analysis and laboratory experiments show that pore pressure can be handed effectively by using the "law of effective stress." The effective stress for fracture is given by the applied stress less the pore pressure. The law of effective stress requires that all combinations of applied stress and pore pressure that produce the same effective stress must have the same effect on fracture.
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From page 102... ...
102 _~0 5 ~ 20 15 1 10 ~0 o CONSERVATION OF HISTORIC STONE BUILDINGS O Pp= 100b O Pp = 300b O Pp= 7.00b o 2 4 As—Pp. kb 6 FIGURE 9 Compressive strengths for samples with different pore pressures all fall on the same curve when plotted in terms of effective stress.l1 to
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From page 103... ...
The fracture strength of samples of a quartzitic shale is shown in Figure 10. Saturating rock with a fluid having low surface tension increases its Toad-ca~rying capacity.
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From page 104... ...
A w x I c ._ .O A) o O ~ 0 0.04 0.08 0.12 0.16 POUN DALS PE R FOOT 1 1 1 1 I I I ~ I I 1 1 0 10 20 30 40 50 60 70 DYNES PER CENTIMETER SURFACE TENSION ~ OF IMMERSION LIQUIDS AT 20 C (68° F ~ FIGURE 10 Compressive strengths of samples saturated with various fluids.
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From page 105... ...
0 0.5 1.0 1.5 2.0 SPECIMEN SIDE LENGTH, m 105 2.5 3.0 FIGURE 11 The compressive strength of rock is found to be smaller for larger samples 15,16,17 under compressive stress. A large crack in a large sample that is closed under confining pressure acts like a collection of small cracks.
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From page 106... ...
Wild, The influence of moisture content in the compressive strength of rock, Rock Mech. Sympos.
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From page 107... ...
.C. Zienkiewicz, Rock Mechanics in Engineering Practice, John Wiley, London, 1968.
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