Lesson 2 Geotech PDF

PHASE RELATIONSHIPS a) SOIL ELEMENT IN NATURAL STATE b) BASIC DEFINITION AND PHASE RELATIONS Note: SATURATED -​ if all voids are filled with water -​ Fully saturated (S=100%) and partially saturated DRY -​ if all voids are filled with air -​ Unsaturated (S=0) SOIL -​ is composed of...

PHASE RELATIONSHIPS a) SOIL ELEMENT IN NATURAL STATE b) BASIC DEFINITION AND PHASE RELATIONS Note: SATURATED -​ if all voids are filled with water -​ Fully saturated (S=100%) and partially saturated DRY -​ if all voids are filled with air -​ Unsaturated (S=0) SOIL -​ is composed of solids, liquids, and gases. SOLID PHASE -​ may be minerals, organic matter, or both. VOIDS -​ the spaces between the solids (soil particles). ​ Water is often the predominant liquid and air is the predominant gas. ​ The soil water is called the porewater and plays a very important role in the behavior of soils under load. ​ The physical parameters of soils are influenced by the relative proportions of each of these phases. Vs = volume of solids Vw = volume of water Va = volume of air V = total volume of the soil Vv = Va - Vw —> U=δ𝑤. ℎ Ws = weight of solids Ww = weight of water Wa = weight of air is negligible (Wa = 0) W = total weight of soil WEIGHT –VOLUME RELATIONSHIP 1.​ WATER CONTENT -​ The ratio of the amount of water (Ww) in the soil (Ws) and expressed as a percentage. -​ The water content of a soil is found by weighing a sample of the soil and then placing it in an oven at 110 + 5 °C until the weight of the sample remains constant. For most soils, a constant weight is achieved in about 24 hours. The soil is removed from the oven, cooled, and then weighed. 2.​ VOID RATIO, e : -​ The ratio of the volume of void space to the volume of solids. The void ratio is usually expressed as a decimal quantity: 3.​ SPECIFIC VOLUME, V’ : -​ The volume of soil per unit volume of solids:. This equation is useful in relating volumes. 4.​ POROSITY, n : -​ The ratio of the volume of voids to the total volume: Porosity is usually expressed as a percentage: -​ Porosity and void ratio are related by the expression: -​ Coarse-grained soils: The maximum and minimum porosities would be 48% and 26%, respectively. -​ This is equivalent to maximum and minimum void ratios of 0.91 and 0.35, respectively. -​ The void ratios of real coarse-grained soils vary between 1 and 0.3. -​ Clay soils often have void ratios greater than 1. 5.​ SPECIFIC GRAVITY, Gs: -​ The ratio of the weight of the soil solids to the weight of water of equal volume: -​ where w = 9.81kN/m3 is the unit weight of water. The specific gravity of soils ranges from approximately 2.3 to 2.8; the lower range (2.3 to 2.5) are for silt particles with traces of organic material. For most problems, Gs can be assumed, with little error, to be equal to 2.7. -​ Two types of container are used to determine the specific gravity for soil particles less than 4.75 mm (No. 4 sieve). One is a volumetric flask (at least 100mL) that is used for coarse-grained soils. The other is a 50-mL density bottle (stoppered bottle) that is used for fine-grained soils. -​ The container is weighed and a small quantity of dry soil is placed in it. The mass of the container and the dry soil is determined. De-aired water is added to the soil in the container. The container is then agitated to remove air bubbles. When all air bubbles have been removed, the container is filled with de-aired water. The mass of container, soil, and water is determined. The contents of the container are discarded and the container is thoroughly cleaned. De-aired water is added to fill the container and the mass of the container and water is determined. -​ Let M1 be the mass of the oven-dried soil, M2 be the mass of the container and water, and M3 be the mass of the container, oven-dried soil, and water. The mass of water displaced by the soil particles is M4 = M1 + M2 − M3, and Gs = M1/M4 6.​ DEGREE OF SATURATION, S : -​ The ratio of the volume of water to the volume of voids, often expressed as a percentage. -​ If S = 1 or 100%, the soil is saturated. If S = 0, the soil is bone dry. It is practically impossible to obtain a soil with S = 0. 7.​ UNIT WEIGHT, :  -​ The weight of soil per unit volume (bulk unit weight). -​ The weight of soil can be expressed in terms of the weight of soil solids, the moisture content and the total volume: SPECIAL CASES: 1.​ Saturated UnitWeight (S = 1): 2.​ Dry UnitWeight (S = 0): 3.​ Effective or Buoyant UnitWeight: RELATIONSHIP AMONG UNIT WEIGHT, VOID RATIO, POROSITY MOISTURE CONTENT, AND SPECIFIC GRAVITY  ​ Considering the limits of unit weights for soils, we will use the ratio of the soil's unit weight to that of water, which for saturated soil is (Ysat/Yw). This ratio is a dimensionless quantity that will be labeled as Rd. Rd -​ indicates how much soil is heavier than water per unit volume termed as the unit weight ratio or density ratio. 8. RELATIVE DENSITY, (Dr): (Compaction behavior of soil) -​ : This is an index that indicates the degree of packing between the loosest and densest possible state of coarse-grained soils as determined by experiments  -​ Where emax is the maximum void ratio (loosest condition), emin is the minimum void ratio (densest condition), and e is the current void ratio. Similarly, relative density can be written as: -​ Relative density can be defined in terms of porosity : 9. DENSITY INDEX, (Id): -​ This is a similar measure (not identical) to relative density: 10. SWELL FACTOR, (SF): -​ This is a ratio of the volume of excavated material to the volume of in-situ material (sometimes called borrow pit material or bank material): PHYSICAL STATES AND INDEX PARAMETERS OF FINE- GRAINED SOILS -​ BASIC STATES OF SOIL (Fine-Grained)  ​ The physical and mechanical behavior of fine-grained soils is linked to four distinct states: solid, semisolid, plastic, and liquid, in order of increasing water content. SOLID SEMISOLID - Visible cracks appear PLASTIC - Deforms without visible cracks LIQUID - Flows like a viscous liquid Point A -​ original liquid state. As the soil dries, its water content reduces and so does its volume. Point B -​ the soil becomes so stiff that it can no longer flow as a liquid. -​ The boundary water content at point B is called the liquid limit; it is denoted by LL. -​ As the soil continues to dry, there is a range of water content at which the soil can be molded into any desired shape without rupture. Point C -​ the water content at which the soil changes from a plastic to a semi-solid is known as the plastic limit, PL. -​ The soil at this state is said to exhibit plastic behavior: the ability to deform continuously without rupture. -​ But if drying is continued beyond the range of water content for plastic behavior, the soil becomes a semisolid. -​ The range of water contents over which the soil deforms plastically is known as the plasticity index, PI: ​ As the soil continues to dry, it comes to a final state called the solid state. At this state, no further volume change occurs because nearly all the water in the soil has been removed. Point D  -​ Shrinkage Limit, SL; the water content at which the soil changes from a semisolid to a solid. -​ The shrinkage limit is useful for the determination of the swelling and shrinking capacity of soils. -​ The range of water content from the plastic limit to the shrinkage limit for which the soil behaves as a semisolid is called the shrinkage index (SI): CONSISTENCY OF SOILS ATTERBERG (Albert Atterberg, 1911) -​ developed a method to describe the consistency of fine-grained soils with varying. 4 Basic States/ Behavior of the Soil a. Solid b. Semi-solid c. Plastic d. Liquid Shrinkage Limit (SL) - Solid and Semi-solid Liquid Limit (LL) - Plastic and Liquid Plastic Limit (PL) - Semi-solid and Plastic Index parameters: a. Plasticity Index b. Liquidity Index c. Shrinkage Index Atterberg Limits 1.​ LIQUID LIMIT (LL) – moisture content after 25 blows or moisture content at which a soil changes from the liquid state to the plastic state. Casagrande Cup – Liquid Limit device -​ The liquid limit is determined from an apparatus that consists of a :semispherical brass cup that is repeatedly dropped onto a hard rubber base from a height of 10mm by a cam-operated mechanism. Casagrande (1932) developed this apparatus. 2.​ PLASTIC LIMIT (PL) -​ moisture content in percent at which the soil crumbles, when rolled into threads of 3.2mm (1/8”) in diameter. -​ The plastic limit is determined by rolling a small clay sample into threads and finding the water content at which threads of approximately 3mm diameter will just start to crumble. Two or more determinations are made, and the average water content is reported as the plastic limit. Atterberg Limits (Consistency Relationship) Liquidity Index (LI) -​ relative consistency of a cohesive soil in the natural state. -​ ratio of the difference in water content between the natural or in-situ water content of a soil and its plastic limit to its plasticity index. Soil consistency -​ or simply consistency is analogous to viscosity in liquids and indicates internal resistance to forces that tend to deform the soil. The internal resistance may come from inter-particle forces (cohesion or adhesion), cementation, inter-particle friction, and soil suction. Terms such as stiff, hard, firm, plastic, soft, and very soft are often used to describe consistency. Consistency changes with water content. A measure of consistency is provided by the consistency index defined as: ​ For soils with a particular mineralogy, the plasticity index is linearly related to the amount of the clay fraction. He coined a term called activity (A) to describe the importance of the clay fractions on the plasticity index. The equation for A is: Swelling Clays Expansive Clays PLASTICITY CHART– proposed by Casagrande. A-LINE → separates the inorganic clays from the inorganic silts. U-LINE → approximately the upper limit of the relationship of the PI and LL. 3.​ SHRINKAGE LIMIT (SL): Fine-grained -​ Moisture content (in percent) which the volume of the soil mass ceases to change. -​ Soil shrinks as moisture is gradually lost from it. With continuing loss of moisture, a stage of equilibrium is reached at which more loss of moisture will result in no further volume change. ​ The shrinkage limit is determined as follows: A mass of wet soil, M1, is placed in a porcelain dish 44 mm diameter and 12 mm high and then oven-dried. The volume of oven-dried soil is determined by using mercury to occupy the vacant spaces caused by shrinkage. The mass of the mercury is determined, and the volume decrease caused by  shrinkage can be calculated from the known density of mercury. The shrinkage limit is calculated from: Where: M1 is the mass of the wet soil, M2 is the mass of the oven-dried soil, w is water content (not in mass of mercury percentage), V1 is the volume of wet soil, V2 (= mass of mercury/ density of mercury) is the volume of the oven- dried density of mercury soil, and g is the acceleration due to gravity (9.81m/s2). KEY POINTS: 1.​ Fine-grained soils can exist in one of four states: solid, semisolid, plastic, or liquid. 2.​ Water is the agent that is responsible for changing the states of soils. 3.​ A soil gets weaker if its water content increases. 4.​ Three limits are defined based on the water content that causes a change of state. These are the liquid limit the water content that caused the soil to change from a liquid to a plastic state; the plastic limit the water content that caused the soil to change from a plastic to a semisolid; and the shrinkage limit-the water content that caused the soil to change from a semisolid to a solid state. Water contents at approximately these limits are found from laboratory tests. 5.​ The plasticity index defines the range of water content for which the soil behaves like a plastic material. 6.​ The liquidity index gives a qualitative measure of strength. 7.​ The soil strength is lowest at the liquid state and highest at the solid state.