Chapter 2: Geotechnical Properties of Soil

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1 Part 1: Geotechnical Properties and Exploration of Soil Chapter 2: Geotechnical Properties of Soil Introduction This chapter reviews the basic geotechnical properties of soils. It includes topics such as: grain-size distribution, plasticity, soil classification, hydraulic conductivity, effective stress, consolidation, and shear strength parameters. Grain-Size Distribution The grain-size distribution is required to properly classify a soil. There are two types of soil grains which are: (1) coarse-grained soil: sieve analysis. (2) fine-grained soil: hydrometer analysis. Sieve Analysis Two parameters can be determined from the grain-size distribution curves of coarse-grained soils. These coefficients are: (1) the uniformity coefficient, C u, and C u = D 60 D 10 (2) the coefficient of gradation, C z, or coefficient of curvature, C c, C z = C c = (D 30) 2 (D 60 )(D 10 ) Engr. Yasser M. Almadhoun Page 1

2 Hydrometer Analysis This test is based on the principle of sedimentation of soil particles in water. The largest diameter of the soil particles still in suspension at time, t, can be determined by Stokes law: D = 18η L (G s 1)γ w t NB: The sieve and hydrometer techniques may be combined for a soil having both coarse-grained and fine-grained soil constituents. Engr. Yasser M. Almadhoun Page 2

3 Size Limits for Soils The following organizations have attempted to develop the size limits for gravel, sand, silt, and clay on the basis of the grain sizes present in soils. Classification system Unified AASHTO Grain size (mm) Gravel: 75 mm to 4.75 mm Sand: 4.75 mm to mm Silt and clay (fines): < 0.075mm Gravel: 75 mm to 2 mm Sand: 2 mm to 0.05 mm Silt: 0.05 mm to mm Clay: < 0.002mm Weight Volume Relationships As mentioned earlier, soils are three-phase systems consisting of solid soil particles, water, and air (or gas). Based on the separation of the three phases, the volume relationships can then be defined. Void ratio, e e = V v V s Porosity, n n = V v V n = e Engr. Yasser M. Almadhoun Page 3

4 Degree of saturation, S S(%) = V w V v Moisture content, w w(%) = W w W s Moist unit weight, γ γ = W V = W s + W w V Dry unit weight, γ d γ d = W s V = G sγ w V = G sγ w (1 + w) = G sγ w = γ 1 + w Saturated unit weight, γ sat γ sat = W s + W w V s + V v Moist density,ρ ρ = G sρ w (1 + w) Dry density,ρ d γ d = G sρ w Saturated density,ρ sat ρ sat = ρ w(g s + e) = G sγ w + eγ w Relative Density In granular soils, the degree of compaction in the field can be measured according to the relative density which defined as: D r (%) = e max e e max e min γ d γ d(min) D r (%) = { γ d(max) γ d (min) } γ d(max) γ d Engr. Yasser M. Almadhoun Page 4

5 Atterberg Limits If the soil is gradually dried, it will behave like a plastic, semisolid,or solid material, depending on its moisture content. The limits that separate these states are known as Atterberg limits which refer to the following three limits: Liquid limit (LL): the percent moisture content at which the soil changes from a semiliquid to a plastic state. Plastic limit (PL): the percent moisture content at which the soil changes from a plastic to a semisolid state. Shrinkage limit (SL): the percent moisture content at which the soil changes from a semisolid to a solid state. Plasticity index (PI): The difference between the liquid limit and the plastic limit of a soil. PI = LL PL Soil Classification Systems These systems are used to divide soils into groups and subgroups based on common engineering properties, such as the grain-size distribution, liquid limit, and plastic limit. The two major classification systems presently in use are: (1) AASHTO System, and (2) ASTM System. Engr. Yasser M. Almadhoun Page 5

6 Hydraulic Conductivity of Soil This test aims at determining how much water is flowing through a soil per unit time. v = ki i = h L Effective Stress The total stress at a given point in a soil mass can be expressed as: σ: total stress, σ : effective stress, and u: pore water pressure. σ = σ + u Critical hydraulic gradient Under such a situation, soil stability is lost. This situation generally is referred to as boiling, or a quick condition (for zero effective stress). i = i cr = γ = G s 1 γ w i cr : critical hydraulic gradient (varies from 0.9 to 1.1, with an average of 1.0 for most soils, particularly sandy soils), and γ : effective or submerged unit weight of soil. Engr. Yasser M. Almadhoun Page 6

7 Consolidation Refer to your Textbook and refresh your knowledge about. Shear Strength Refer to your Textbook and refresh your knowledge about. Unconfined Compression Test Refer to your Textbook and refresh your knowledge about. Engr. Yasser M. Almadhoun Page 7