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# Craig's Soil Mechanics 7th Edition

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range of water content. Soil compaction 27 PROBLEMS 1.1 The results of particle size analyses and, where appropriate, limit tests on sam- ples of four soils are given in Table 1.5. Allot group symbols and give main and qualifying terms appropriate for each soil. 1.2 A soil has a bulk density of 1.91Mg/m3 and a water content of 9.5%. The value of Gs is 2.70. Calculate the void ratio and degree of saturation of the soil. What would be the values of density and water content if the soil were fully saturated at the same void ratio? Figure 1.14 Moisture condition test. Table 1.5 BS sieve Particle size Percentage smaller Soil E Soil F Soil G Soil H 63mm 20mm 100 6.3mm 94 100 2mm 69 98 600mm 32 88 100 212mm 13 67 95 100 63mm 2 37 73 99 0.020mm 22 46 88 0.006mm 11 25 71 0.002mm 4 13 58 Liquid limit Non-plastic 32 78 Plastic limit 24 31 28 Basic characteristics of soils 1.3 Calculate the dry unit weight, the saturated unit weight and the buoyant unit weight of a soil having a void ratio of 0.70 and a value of Gs of 2.72. Calculate also the unit weight and water content at a degree of saturation of 75%. 1.4 A soil specimen is 38mm in diameter and 76mm long and in its natural con- dition weighs 168.0 g. When dried completely in an oven the specimen weighs 130.5 g. The value of Gs is 2.73. What is the degree of saturation of the specimen? 1.5 Soil has been compacted in an embankment at a bulk density of 2.15Mg/m3 and a water content of 12%. The value of Gs is 2.65. Calculate the dry density, void ratio, degree of saturation and air content. Would it be possible to compact the above soil at a water content of 13.5% to a dry density of 2.00Mg/m3? 1.6 The following results were obtained from a standard compaction test on a soil: Mass (g) 2010 2092 2114 2100 2055 Water content (%) 12.8 14.5 15.6 16.8 19.2 The value of Gs is 2.67. Plot the dry density–water content curve and give the optimum water content and maximum dry density. Plot also the curves of zero, 5 and 10% air content and give the value of air content at maximum dry density. The volume of the mould is 1000 cm3. 1.7 The in-situ dry density of a sand is 1.72Mg/m3. The maximum and minimum dry densities, determined by standard laboratory tests, are 1.81 and 1.54Mg/m3, respectively. Determine the density index of the sand. REFERENCES 1 American Society for Testing and Materials Annual Book of ASTM Standards, Vol. 04/08, Hitchin, Herts. 2 British Standard 1377 (1990) Methods of Test for Soils for Civil Engineering Purposes, British Standards Institution, London. 3 British Standard 5930 (1999) Code of Practice for Site Investigations, British Standards Institution, London. 4 British Standard 6031 (1981) Code of Practice for Earthworks, British Standards Institu- tion, London. 5 Collins, K. and McGown, A. (1974) The form and function of microfabric features in a variety of natural soils, Geotechnique, 24, 223–54. 6 Department of Transport (1993) Earthworks, in Specification for Highway Works, HMSO, Series 600, London. 7 Grim, R.E. (1962) Clay Mineralogy, McGraw-Hill, New York. 8 Parsons, A.W. and Boden, J.B. (1979) The Moisture Condition Test and its Potential Applications in Earthworks, TRRL Report 522, Crowthorne, Berks. 9 Rowe, P.W. (1972) The relevance of soil fabric to site investigation practice, Geotechnique, 22, 193–300. 10 Wagner, A.A. (1957) The use of the unified soil classification system by the bureau of reclamation, in Proceedings of the 4th International Conference of SMFE, London, Vol. 1, Butterworths, London, pp. 125–34. References 29 Chapter 2 Seepage 2.1 SOIL WATER All soils are permeable materials, water being free to flow through the interconnected pores between the solid particles. The pressure of the pore water is measured relative to atmospheric pressure and the level at which the pressure is atmospheric (i.e. zero) is defined as the water table (WT) or the phreatic surface. Below the water table the soil is assumed to be fully saturated, although it is likely that, due to the presence of small volumes of entrapped air, the degree of saturation will be marginally below 100%. The level of the water table changes according to climatic conditions but the level can change also as a consequence of constructional operations. A perched water table can occur locally, contained by soil of low permeability, above the normal water table level. Artesian conditions can exist if an inclined soil layer of high permeability is confined locally by an overlying layer of low permeability; the pressure in the artesian layer is governed not by the local water table level but by a higher water table level at a distant location where the layer is unconfined. Below the water table the pore water may be static, the hydrostatic pressure depending on the depth below the water table, or may be seeping through the soil under hydraulic gradient: this chapter is concerned with the second case. Bernoulli’s theorem applies to the pore water but seepage velocities in soils are normally so small that velocity head can be neglected. Thus h ¼ u �w þ z ð2:1Þ where h is the total head, u the pore water pressure, �w the unit weight of water (9.8 kN/m3) and z the elevation head above a chosen datum. Above the water table, water can be held at negative pressure by capillary tension; the smaller the size of the pores the higher the water can rise above the water table. The capillary rise tends to be irregular due to the random pore sizes occurring in a soil. The soil can be almost completely saturated in the lower part of the capillary zone but in general the degree of saturation decreases with height. When water percolates through the soil from the surface towards the water table some of this water can be held by surface tension around the points of contact between particles. The negative pressure of water held above the water table results in attractive forces between the particles: this attraction is referred to as soil suction and is a function of pore size and water content. 2.2 PERMEABILITY In one dimension, water flows through a fully saturated soil in accordance with Darcy’s empirical law: q ¼ Aki ð2:2Þ or v ¼ q A ¼ ki where q is the volume of water flowing per unit time, A the cross-sectional area of soil corresponding to the flow q, k the coefficient of permeability, i the hydraulic gradient and v the discharge velocity. The units of the coefficient of permeability are those of velocity (m/s). The coefficient of permeability depends primarily on the average size of the pores, which in turn is related to the distribution of particle sizes, particle shape and soil structure. In general, the smaller the particles the smaller is the average size of the pores and the lower is the coefficient of permeability. The presence of a small percentage of fines in a coarse-grained soil results in a value of k significantly lower than the value for the same soil without fines. For a given soil the coefficient of permeability is a function of void ratio. If a soil deposit is stratified the permeability for flow parallel to the direction of stratification is higher than that for flow perpendicular to the direction of stratification. The presence of fissures in a clay results in a much higher value of permeability compared with that of the unfissured material. The coefficient of permeability also varies with temperature, upon which the viscos- ity of the water depends. If the value of k measured at 20 �C is taken as 100% then the values at 10 and 0 �C are 77 and 56%, respectively. The coefficient of permeability can also be represented by the equation: k ¼ �w K where �w is the unit weight of water, the viscosity of water and K (units m 2) an absolute coefficient depending only on the characteristics of the soil