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  • AASHTO classification system — A classification system developed by the American Association of State Highway and Transportation Officials that rates soils relative to their suitability for road embankments, subgrades, subbases, and basis.
    Atterberg limits — Water contents at which soil changes engineering behavior; the most important ones in classification are the liquid limit and plastic limit.
    Boulders — Rock particles larger than 9 to 12 inches or 200 to 300 mm.
    Clay — Fine-grained soil that exhibits plasticity.
    Coarse grained — Soils that are retained on a No. 200 sieve.
    Coarse fraction — In the Unified Soil Classification System, that portion of a soil sample retained on a No. 200 sieve.
    Cobbles — Rock particles smaller than a boulder but larger than 3 inches (75 mm).
    Coefficient of curvature — A mathematical parameter, D2
    30/(D60D10), used as a measure of the smoothness of a gradation curve.
    Coefficient of uniformity — A mathematical parameter, D60/D10, used as a measure of the slope of a gradation curve.
    D10 size — The grain size, in mm, for which 10% by weight of a soil sample is finer.
    Effective grain size — Another name for the D10 size.
    Fat clay — Highly plastic clay; clay with a liquid limit greater than 50.
    Fine fraction — In the unified soil classification system, that portion of a soil sample passing a No. 200 sieve.
    Fine grained — Soil passing a No. 200 sieve.
    Grain-size analysis — The determination of the relative proportions of soil particles of each size in a soil sample, performed by passing the sample over a nest of sieves.
    Grain-size distribution curve — A plot of percent finer or coarser versus soil-grain size. Grain size is plotted on a logarithmic scale.
    Granular material — In the AASHTO classification system, soil with less than 35% passing the No. 200 sieve.
    Gravel — Soil or rock particles smaller than 3 inches but retained on a No. 4 sieve (Unified Soil Classification System) or on a No. 10 sieve (AASHTO system).

    Lean clay — Clay with low plasticity; clay with a liquid limit less than 50.
    Liquid limit — The water content above which soil behavior changes from a plastic solid to a viscous liquid.
    Median grain size — The grain size for which one-half of a soil sample, by weight, is larger and half is smaller.
    Nest of sieves — A stack of sieves of different sizes, having the largest opening on the top and progressing downward to successively smaller openings.

    Peat — A highly organic soil, dark brown to black in color, with noticeable organic odor and visible vegetable matter.
    Plastic limit — The water content above which the soil behavior changes from a brittle solid to a plastic solid.
    Plasticity — The ability of a soil, when mixed with water, to deform at constant volume.
    Plasticity index — The difference between the liquid and plastic limit.
    Sand — Soil particles retained on the No. 200 sieve that pass the No. 4 sieve (Unified Soil Classification System) or the No. 10 sieve (AASHTO system).
    Shrinkage limit — The water content at which further reduction in water content does not cause a further reduction in volume.
    Sieve analysis — A grain-size analysis using a nest of sieves.
    Silt — Fine-grained soil having a low plasticity index or not exhibiting plasticity.
    Unified Soil Classification System — A descriptive classification system based on Casagrande’s airfield system and now standardized by ASTM D 2487-93.

    Buoyant unit weight — The apparent unit weight of a submerged soil, obtained as the total unit weight
    minus the weight of water.
    Compaction mold — A metal mold, typically 1/30 ft3, used to determine the density of compacted soil.
    Degree of saturation — The ratio of the volume of water to the volume of void space in a sample of soil.
    Density — The mass per unit volume of a soil or one of its components.
    Dry density — The ratio of the mass of solids to the total volume of a soil sample.
    Dry unit weight — The ratio of the weight of solids to the total volume of a soil sample.
    Effective unit weight — Another term for buoyant unit weight.
    Porosity — The ratio of the volume of void spaces to the total volume of a soil sample.
    Saturated — The condition in which all of the void spaces in a soil are filled with water and the volume
    of air is zero.
    Saturated unit weight — The unit weight obtained if a soil sample is saturated by adding water at  constant total volume.
    Specific gravity — The ratio of the density of a material to the density of water; usually refers to the
    specific gravity of soil solids.
    Total unit weight — The total combined weight of solids and water in a unit volume of soil.
    Unit weight — The ratio of the weight of a material to its volume.
    Void ratio — The ratio of the volume of void space in a soil sample to the volume of solid particles.
    Water content — The ratio of the weight of water to the weight of solids of a soil sample.

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  • If only relationships (e.g., void ratio or unit weight) are given, the quantity of soil is indefinite and only
    other relationships can be calculated. Nevertheless, it is convenient to solve such problems using a phase
    diagram and assuming one fixed weight or volume value. That quantity of solids, water, or soil is “brought
    to the paper” and used to calculate corresponding quantities of components. Although any one quantity
    can be assumed to have any value, the following assumptions simplify calculations:

  • Example 15.4

    Assume that a compaction mold having a volume of 1/30 ft3 was filled with moist soil. The total weight
    of the soil in the mold was found to be 4.10 lb. The soil was oven dried and its weight after drying was
    3.53 lb. The specific gravity of solids was known to be 2.70. Water content, void ratio, porosity, degree
    of saturation, total unit weight, and dry unit weight must be determined. A phase diagram is shown in
    Fig. 15.5, with the known quantities in bold. The weight and volume of water are calculated as

    Ww = W – Ws = 4.10 – 3.53 = 0.57 lb
    Vw Ww gw = § = 0.57 § 62.4 = 0.00913 ft3

    The volume of solids is

    If the same soil now becomes saturated by the addition of water at constant total volume, the saturated water content and saturated unit weight can be calculated as follows. The new volume of water is the entire void volume, 0.01238 ft3. Multiplying this value by 62.4 lb/ft3, the new weight of water is 0.77 lb. The water content at saturation is then

    Note that the dry unit weight does not change if water is added without changing total volume.

    Example 15.5 (SI units)

    A soil sample has a volume of 2.5 liters (2.5 x 10^(–3) m3) and a total mass of 4.85 kg. A water content test
    indicates the water content is 28%. Assuming that the specific gravity of solids is 2.72, it is desired to
    determine the total density, total unit weight, dry density, dry unit weight, void ratio, porosity, and degree
    of saturation.
    A phase diagram is shown in Fig. 15.6, with known values shown in bold.


    Va = V – Vs – Vw = 0.00250 – 0.00319 – 0.00106 = 0.00005 m3

    The total density is

    r = M / V = 4.850 kg / 0.00250 m3 = 1940 kg § m3

    The total unit weight is

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  • Weight-volume problems may be divided into two categories: those where there is a defined quantity of soil, and those where the quantity of soil is not defined and it is only desired to make conversions among relationships. The solution to problems of the first category is discussed first; discussion of the second category follows. Problems of the first category can be solved in four steps:
    1. A blank phase diagram is sketched and known weights, volumes, and unit weights are entered on the diagram.
    2. Known volumes are multiplied by their respective unit weights to obtain weights. Known weights are divided by unit weights to obtain volumes. Where values of some relationships are given, additional weight and volume values are calculated using the definitions of Eqs. (15.6), (15.7), (15.10), and (15.11). To numerically balance the phase diagram it is recommended that all calculations be carried to at least four significant digits.

    3. Multiplication and division horizontally across the diagram and addition and subtraction vertically along the sides is continued until all weights, volumes, and unit weights are determined and found to numerically balance.
    4. All desired values and relationships can now be calculated from the completed and checked diagram.

  • A soil sample has a total unit weight of 125 lb/ft3. It is desired to find its total unit weight in kN/m3 and its density in kg/m3. Although the problem can be worked using a chain of conversion factors, a simpler approach is to consider that the unit weight and density of the soil sample have a constant ratio to the unit weight and density of water. Placing the unit weight or density of water in any system of units in both the numerator and denominator of a fraction forms an equality. (It is assumed that the problem is on the planet Earth and a mass of 1000 kg weighs 9.81 kN!) Thus,

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