Retaining Structure Chapter One General Review Shear strength

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Retaining Structure Chapter One General Review: Shear strength Dr. Omer Nawaf Maaitah

Retaining Structure Chapter One General Review: Shear strength Dr. Omer Nawaf Maaitah

Shear strength Dr. Omer Nawaf Maaitah

Shear strength Dr. Omer Nawaf Maaitah

 • Equation (7. 2) is generally referred to as the Mohr–Coulomb failure criteria.

• Equation (7. 2) is generally referred to as the Mohr–Coulomb failure criteria. The significance of the failure envelope can be explained using • Figure 7. 1. If the normal and shear stresses on a plane in a soil mass are such that they plot as point A, shear failure will not occur along that plane. • Shear failure along a plane will occur if the stresses plot as point B, which falls on the failure envelope. A state of stress plotting as point C cannot exist, since this falls above the failure envelope; shear failure would have occurred before this condition was reached. Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Critical void ratio Dr. Omer Nawaf Maaitah

Critical void ratio Dr. Omer Nawaf Maaitah

Sand • 1 - In dense and medium sands, shear stress increases with shear

Sand • 1 - In dense and medium sands, shear stress increases with shear displacement to a maximum or peak value m and then decreases to an approximately constant value c at large shear displacements. This constant stress c is the ultimate shear stress. • For dense and medium sands the volume of the specimen initially • decreases and then increases with shear displacement. At large values of shear displacement, the volume of the specimen remains approximately • constant. • 2. For loose sands the shear stress increases with shear displacement to a maximum value and then remains constant. • For loose sands the volume of the specimen gradually decreases to a certain value and remains approximately constant thereafter. Dr. Omer Nawaf Maaitah

Friction angle Dr. Omer Nawaf Maaitah

Friction angle Dr. Omer Nawaf Maaitah

Triaxial Test • Three conventional types of tests are conducted with clay soils in

Triaxial Test • Three conventional types of tests are conducted with clay soils in the laboratory: • 1. Consolidated drained test or drained test (CD test or long term, or effective stress analysis). • 2. Consolidated undrained test (CU test). • 3. Unconsolidated undrained test (UU test, short term analysis, quick test, at the end of construction ). Dr. Omer Nawaf Maaitah

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Dr. Omer Nawaf Maaitah

General comments on the friction angle of granular soils • The soil friction angle

General comments on the friction angle of granular soils • The soil friction angle determined by the laboratory tests is influenced by two major factors. The energy applied to a soil by the external load is used both to overcome the frictional resistance between the soil particles and also to expand the soil against the confining pressure. The soil grains are highly irregular in shape and must be lifted over one another for sliding to occur. This behavior is called dilatency. Hence the angle of friction can be expressed as Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

 • must be some degree of interlocking even when the overall volume change

• must be some degree of interlocking even when the overall volume change is zero at very high strains. • Effect of angularity of soil particles. Other factors remaining constant, a soil possessing angular soil particles will show a higher friction angle than one with rounded grains because the angular soil particles will have a greater degree of interlocking and thus cause a higher value of [Eq. (7. 8)]. • Effect of rate of loading during the test. The value of friction in triaxial compression tests is not greatly affected by the rate of loading. For sand, Whitman and Healy (1963) compared tests conducted in 5 min and in 5 ms and found that friction decreases at the most by about 10%. Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Shear strength of cohesive soils Dr. Omer Nawaf Maaitah

Shear strength of cohesive soils Dr. Omer Nawaf Maaitah

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Stress path Dr. Omer Nawaf Maaitah

Stress path Dr. Omer Nawaf Maaitah

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Dr. Omer Nawaf Maaitah

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Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

CAPILLARITY • The phenomenon in which water rises above the ground water table against

CAPILLARITY • The phenomenon in which water rises above the ground water table against the pull of gravity, but is in contact with the water table as its source, is referred to as ‘Capillary rise’ with reference to soils. The water associated with capillary rise is called ‘capillary moisture’. The phenomenon by virtue of which a liquid rises in capillary tubes is, in general, called ‘capillarity’. • All voids in soil located below the ground water table would be filled with water (except possibly for small pockets of entrapped air or gases). In addition, soil voids for a certain height above the water table will also be completely filled with water. This zone of saturation above the water table is due to capillary rise in soil. Even above this zone of full saturation, a condition of partial saturation exists. The zone of soil above the water table in which capillary water rises is denoted as the ‘capillary fringe’. Capillus literally means hair in Greek, indicating that the size of opening with which the phenomenon of capillarity is connected or related, is of this order of magnitude. Dr. Omer Nawaf Maaitah

 • The principle of capillary rise in soils can be related to the

• The principle of capillary rise in soils can be related to the rise of water in glass capillary tubes in the laboratory. When the end of a vertical capillary tube is inserted into a source of water, the water rises in the tube and remains there. This rise is attributed to the attraction between the water and the glass and to surface tension which develops at the air-water interface at the top of the water column in the capillary tube. • The surface tension is analogous to a stretched membrane, or a very thin but tough film. The water is “pulled up” in the capillary tube to a height, dependent upon the diameter of the tube, the magnitude of surface tension, and the unit weight of water. • The attraction between the water and capillary tube, or the tendency of water to wet the walls of the tube affects the shape of the air-water interface at the top of the column of water. For water and glass, the shape is concave as seen from top, that is, the water surface is lower at the centre of the column than at the walls of the tube. The resulting curved liquid surface is called the ‘meniscus’. The surface of the liquid meets that of the tube at a definite angle, known as the ‘contact angle’. This angle, incidentally, is zero for water and glass (Fig. 5. 12). Dr. Omer Nawaf Maaitah

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Dr. Omer Nawaf Maaitah

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Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah

Dr. Omer Nawaf Maaitah