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  2 Soil and Rock Properties 2.1 INTRODUCTION In contrast to building materials such as steel and concrete, which can bespecified and obtained with desired properties, soils cannot be ‘‘designed’’.They are just there, they already exist at the construction site, withproperties which are adequate or inadequate for the work to be done. Thereare alternatives for dealing with poor, inadequate soils. These include:1. bypassing them, by either moving to a different site, or by carryingthe foundation loads to better soils at greater depths2. remove the poor soils and replace with better ones3. redesign the structure and its foundations to impose lighter loads4. treat the soil to improve its characteristics.In assessing a specific problem, better sites are more and more often notavailable in the desired location, removal of large volumes of poor soil is notfeasible economically for deep deposits, and redesign of the structure to suitthe soil may defeat the purpose of the structure. Soil improvement methodsare often the only viable alternatives.Prior to improving a soil we must know its existing properties so thatwe can evaluate the potential for improvement, determine the degree of  Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  improvement required and measure the results of the improvement methodsused. 2.2 VOID RATIO AND POROSITY All soils and many rocks consist of solid particles and voids. In soils, thevoids are filled with air and water, are distributed in a more or less uniformmanner, and are almost always interconnected. This is also true of sandstone and siltstone. In solid rock there are no uniformly distributedvoids. In a large mass of ‘‘solid’’ rock, the void spaces, if any, are due tofractures and fissures of various sizes, and are also filled with air and water,and sometimes with sand, silt and clay. In soluble minerals such aslimestone, solution cavities of various sizes are found, both filled and empty.The void ratio of a soil, e, is defined as the quotient of the volume of voids divided by the volume of solids in a soil mass:e ¼ V v = V s  ð 2 : 1 Þ Figure 2.1 is a pictorial representation of the void ratio. Void ratios forgranular soils range from 0.4 to 0.8. Clays and clayey soils will range from0.5 to 1.5 or 2.0. Soils with high organic content, peats and mucks, may havevoid ratios as high as 4 or 5.Porosity, n, is defined as the quotient of the volume of voids divided bythe total volume of a soil (or rock) mass. Thus, for any soil or rock theporosity must be less than one.n ¼ V v = V  ð 2 : 2 Þ F IGURE  2.1  Relationships to define void ratio and porosity. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  2.3 DENSITY AND RELATIVE DENSITY The density of a soil or rock is expressed as its weight per unit volume. Insoils work, it is general practice and more convenient to use dry densities,that is, dry weight per unit volume.Probably the most important index to the behavior of granularmaterials (sands and coarse silts) is the  relative density . The volume of voidsin a soil mass will vary with the different possible arrangement of grainswithin the mass. The void ratio cannot be greater than the value that wouldcause the individual grains to lose contact with each other, and natural soilsdo not approach this value too closely. The least density a sand can have isapproximately equal to the density resulting when the dry sand is pouredslowly into a container with minimum free fall. This is considered to be itsminimum possible density. If the container is now vibrated gently until itsreducing volume stabilizes, the sand is now considered to be at its maximumdensity. The relative density, D R , is a comparison of the natural density of asoil with its loose and dense states. The comparison is made in terms of thevoid ratio:D R : e max  ee max  e min ð 2 : 3 Þ where ‘e’ is the natural, or in place void ratio. For surface and shallow soils,field density tests define the natural void ratio. For deep deposits values areinferred from other tests. Table 2.1 shows shows relationships between T ABLE  2.1  Descriptive Terms for DensityDescriptive term Degree of densityLoose 0 to 1/3Medium 1/3 to 2/3Dense 2/3 to 1Results of Standard Penetration TestsDescriptive term No. of blows per footVery loose 0 to 4Loose 4 to 10Medium 10 to 30Dense 30 to 50Very dense 50 and over Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  descriptive terms for density and Standard Penetration Test (SPT) data.These numbers should be used as guides only, since blow count will varywith grain shape and size, and also with grading. (Granular soils of the samegeneral classification, such as fine sands, may have a wide spread of grainsizes. The range of grain sizes is defined by a factor called the  uniformitycoefficient , C U :C U  ¼ D 60 = D 10  ð 2 : 4 Þ Where D 60  and D 10  are the grain sizes of which 60 % and 10 % of the soil isfiner.)Density and relative density are important factors in determining theneed for, and possible effectiveness of, various soil stabilization methods,since both of these properties are related to shear strength, settlement andpermeability. Some typical properties for natural soils are shown inTable 2.2. Specific Gravity The  specific gravity , G, of any substance is the ratio of the dry weight of agiven volume of that substance divided by an equal volume of water. Forsoils, which may contain many different minerals, specific gravity is anaverage value of the various particles composing the soil.Over 1000 minerals have been identified as rock constituents. Sincesoils are formed from rocks, many of these can also be identified in soils.The specific gravity of different soil minerals raages from as low as 2.3 to ashigh as 5.2. Most soil masses consist of minerals ranging from 2.4 to 3.0, andthe specific gravity of soils will almost always fall within this range. Quartzhas a specific gravity of 2.65, and the many granular soils consisting mainlyof quartz particles will thus have a specific gravity very close to this value.Clay minerals are generally heavier than quartz, and specific gravities of 2.9are not uncommon.Table 2.3 lists densities and specific gravities of common rocks andminerals. Water Content Most soils have some amount of water in the soil voids. The water may begravitational, capillary, or hygroscopic. The basis for distinction is the forcethat influences the water behavior. Stabilization methods are concernedmainly with gravitational water, generally present beneath the soil surface inareas requiring stabilization. The topography of the surface below whichwater is continuous is called the  water table  or the  phreatic line . Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.

dke78_Ch3

Jul 23, 2017
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