Effects of Soil Tension
Soil tension is the result of surface tension of water on soil particles in unsaturated soils. A suction pressure (negative porewater pressure from capillary stresses) is created that pulls the soil particles together. Recall that the effective stress is equal to total stress minus porewater pressure. Thus, if the porewater pressure is negative, the normal effective stress increases. For soil as a frictional material, this normal effective stress increase leads to a gain in shearing resistance. The intergranular friction angle or critical state friction angle does not change. Soil tension can be very large, sometimes exceeding 1000 kPa (equivalent to the pressure from about 100 m of water). If the soil becomes saturated, the soil tension reduces to zero. Thus, any gain in shear strength from soil tension is only temporary. It can be described as an apparent shear strength, ct.
In practice, you should not rely on this gain in shear strength, especially for long-term loading. There are some situations, such as shallow excavations in fi ne-grained soils that will be opened for a very short time, in which you can use the additional shear strength (apparent shear strength) to your advantage. Without soil tension, the soil would collapse and these excavations might need to be braced by using steel, concrete, or wood panels. With soil tension, you may not need these bracings because the apparent shear strength allows the soil to support itself over a limited depth (see Chapter 15), resulting in cost savings. However, local experience is required to successfully use soil tension to your advantage.
Peak shear stress envelope for soils
resulting from cohesion, soil tension,
Unsaturated soils generally behave like Type II soils because negative excess porewater pressure increases the normal effective stress and, consequently, the shearing resistance. In a plot of peak shear stress versus normal effective stress using shear test data, an intercept shear stress, ct, would be observed.
Effects of Cementation:
Nearly all natural soils have some degree of cementation, wherein the soil particles are chemically bonded. Salts such as calcium carbonate (CaCO3) are the main natural compounds for cementing soil particles. The degree of cementation can vary widely, from very weak bond strength (soil crumbles under fi nger pressure) to the bond strength of weak rocks. Cemented soils possess shear strength even when the normal effective stress is zero. They behave much like Type II soils except that they have an initial shear strength, ccm, under zero normal effective stress. In this textbook, we will call this initial shear strength the cementation strength.
In a plot of peak shear stress versus normal effective stress using shear test data, an intercept shear stress, ccm, would be observed (Figure ). The slope angle, jo, of the best-fi t straight line from shear test data is the apparent friction angle (Figure ). The shear strength from cementation is mobilized at small shear strain levels (,0.001%). In most geotechnical structures, the soil mass is subjected to much larger shear strains. You need to be cautious in utilizing ccm in design because at large shear strains, any shear strength due to cementation in the soil will be destroyed. Also, the cementation of natural soils is generally nonuniform. Thus, over the footprint of your structure the shear strength from cementation will vary.
Effects of Drainage of Excess Porewater Pressure:
You were introduced to drained and undrained conditions when we discussed stress paths in Chapter 8. Drained condition occurs when the excess porewater pressure developed during loading of a soil dissipates, i.e., Du 5 0. Undrained condition occurs when the excess porewater pressure cannot drain, at least quickly, from the soil; that is, Du 2 0. The existence of either condition—drained or undrained— depends on the soil type, the geological formation (fi ssures, sand layers in clays, etc.), and the rate of loading. In reality, neither condition is true. They are limiting conditions that set up the bounds within which the true condition lies.
The rate of loading under the undrained condition is often much faster than the rate of dissipation of the excess porewater pressure, and the volume-change tendency of the soil is suppressed. The result of this suppression is a change in excess porewater pressure during shearing. A soil with a tendency to compress during drained loading will exhibit an increase in excess porewater pressure (positive excess porewater pressure, Figure ) under undrained condition, resulting in a decrease in effective stress. A soil that expands during drained loading will exhibit a decrease in excess porewater pressure (negative excess porewater pressure, Figure 10.7) under undrained condition, resulting in an increase in effective stress. These changes in excess porewater pressure occur because the void ratio does not change during undrained loading; that is, the volume of the soil remains constant.
Effects of Cohesion:
The term cohesion, C, as used conventionally in geotechnical engineering, is an apparent shear strength that captures the effects of intermolecular forces (co), soil tension (ct), and cementation (ccm) on the shear strength of soils. In this textbook, we will separate these effects. Cohesion, co, represents the action
of intermolecular forces on the shear strength of soils. These forces do not contribute signifi cant shearing resistance for practical consideration and will be neglected. In a plot of peak shear stress versus normal effective stress using shear test data, an intercept shear stress, co, would be observed (Figure ) when a best-fi t straight line is used as the trend line.