Chapter 3 (Answers to Questions) (Soil Mechanics by David McArthy)

1. Clay is a soil material that possesses plasticity in the presence of water. What does the term plastic mean in relation to clay soils? Clay soil is plastic over a range of water content; that is, the soil can be remolded or deformed without causing cracking, breaking, or change in volume, and will retain the remolded shape.

2. What is the essential reason for the difference in behavior of natural clays and other soil types such as silts and sands? The reason for the difference in behavior between clay and silt relates to the difference in mineralogical composition of the soil types and particle shape. Silt soils are very small particles of disintegrated roc, as are sands and gravels, and possess the same general shape and mineralogical composition as sands and gravels which are nonplastic. The clay minerals, however, represent chemical changes that have resulted from decomposition and alteration of the original rock minerals. The effect is that their size and shape are significantly different from those of other types of soil particles.

3. Comment on the difference between the shape and size of clay particles compared to the other soil types such as silts and sands. Particles in sand, gravel, and boulder categories are considered as “bulky grain”, indicating that particle dimensions are approximately equal; that is, the dimension in length, width, and thickness direction would be of the same order of magnitude. Particles in the silt category, though classified as “fines” along with the clays, are still angular or bulky in shape and of the same mineralogical composition as the coarse-grained soils. Because of the mineralogical compositions, such as particles rarely break down to less than 2u in size.

4. What are the “building rocks” of most clay materials? Comment on the comparative length-width-thickness dimensions of a typical clay material. The building blocks, or constituent sheets, that combine to form most of the different types of clay mineral are the silica tetrahedral sheet and alumina octahedral. Although the thickness of a clay mineral sheet is limited, the dimensions in the length and width direction are not. As a result, the clay minerals have a flat, platelike shape (like an irregular sheet of paper), where the length and width can be several tens or several hundred times the thickness.

5. Referring to the attraction that typically exists between water and clay particles, what is adsorbed water? The water is held in the diffuse double layer is frequently termed adsorbed water or oriented water, to differentiate it from normal pore water, which is not oriented.

6. Describe what is meant by the dipole nature of water molecule. How is this related to adsorbed water and plasticity in a clay soil? The plasticity that clay soils possess is attributed to the attracted and held water. Water molecules are attracted because of their dipole structure. The unusual properties of plasticity possessed by clays result because of the unusual molecular structure and the common presence of water in soil deposits. Experiments performed with clay using nonpolar liquid in place of water have resulted in a "no-plasticity” condition similar to that noted for coarse grained sandy soil.

7. Why does the presence of water in a soil have so much greater an effect on clays than on sand or gravel? Because of the extremely small size of clay particles and the very high ratio of particle surface to particle mass, the forces of electrical charge have a profound effect on the behavior of particles coming in association with other particles and water present in the soil. And because of the manner of development or deposition, clay deposits almost always exist in the presence of some water.

8. In a general way, the strength of a coarse grained soil is related to the soil deposit’s structure (or particle orientation) and void ratio (or density). What is this relationship? For similar sized spherical particles, a condition with high void ratio is obtained from a loose arrangement of particles. A condition with a low void ratio is obtained from a dense arrangement of particles.

9. (a) Briefly describe the difference in a flocculent structure and a dispersed structure in clay soils. Clay deposits with flocculent structures will have high void ratios, low density and probably high water contents. The structure is quite strong and resistant to external forces because of the attraction between particles. In the dispersed condition, the particle structure would have a more parallel arrangement or orientation of particles than existed in the flocculent condition.

(b) What type of structure is most likely expected in a clay deposit that has resulted in a naturally occurring underwater environment? Attraction and contact between many of the clay particles is through an edge to face arrangement.

(c) What type of structure would be expected where clay had been used for a compacted fill on a construction project? Generally it loses some strength as a result of remolding. Subsequent to remolding and with the passage of time, however, the strength increases, though not back to the strength of the originally undistributed clay.

10. With regard to the thixotropy phenomenon in clay soils, what is it and what causes it? The phenomenon of strength-loss, strength-gain, with no changes in volume or water content is termed thixotropy. It has been defined as a “process of softening caused by remolding, followed by time-dependent return to the original, harder state.