CE 353 Geotechnical Engineering

Transcription

1 CE 353 Geotechnical Engineering Dr M. Touahmia 2 The Origin of Soil and Grain Size Lecture Outline: 1. Introduction 2. Rock Cycle and Rock Types 3. Weathering of Rocks 4. Origin of Soil 5. Mechanical Analysis of Soil Textbook: Braja M. Das, "Principles of Geotechnical Engineering", 7 th E. (Chapter 2). 1

2 Introduction What is a soil? Soil is defined as the uncemented aggregate of mineral grains and decayed organic matter with liquid and/or gas in the pores between the grains: (A) gas (mostly air); (B) solid particles (minerals); (C) liquid (water, contaminant liquid, etc.). Where did soil come from? Soils are formed by weathering of rocks. More specifically, the mineral grains that form the solid phase of a soil aggregate are the product of rock weathering. 2

3 Rock Types and Rock Cycle What is a rock? In Geology rock is defined as the solid material forming the outer rocky shell or crust of the earth. There are three major groups of rocks: 1. Igneous rocks: cooled from a molten state. 2. Sedimentary rocks: deposited from fluid medium; e.g., products of weathering of other rocks in water. 3. Metamorphic rocks: formed from pre-existing rocks by the action of heat and pressure. Apparently, the igneous rock is the one far more essential and intrinsic since the other two types are relative secondary in origin. 3

4 Rock Types and Rock Cycle Rock Cycle The final products due to weathering are soils 4

5 Rock Types and Rock Cycle Basic Mineralogy of Rocks Rocks are formed with minerals. What is a mineral? 1. A naturally occurring chemical element or compound formed by inorganic processes with an ordered arrangement or pattern for its atoms crystalline structure; 2. Possesses a definite chemical composition or range of compositions. 3. The main types of minerals are: metallic minerals; nonmetallic minerals; carbonate minerals; sulfate minerals; sulfide minerals; silicate minerals; oxide minerals; clay minerals. 5

6 Weathering of Rock The formation of soil happens over a very long period of time (thousands of years or more). In general, soils are formed by weathering of rocks. The physical properties of soil are dictated primarily by the minerals that constitute the soil particles and, hence, the rock from which it is derived. Weathering is the process of breaking down rocks by mechanical and chemical processes into smaller pieces to form soil or loose particles at or near Earth's surface. The weathering process occurs when rocks are exposed to the hydrosphere (water) and atmosphere (air). These weathering agents can change the physical and chemical characteristics of rocks. 6

7 Weathering of Rock Two types of weathering: 1. Mechanical/Physical Weathering: Physical disintegration or degradation of rock into smaller fragments without changing the chemical composition of the rock. Mechanical weathering processes include: Frost action/ice wedging: is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water. Abrasion: is the physical wearing down of rocks as they rub or bounce against each other. This process is most common in windy areas, under glaciers, or in stream channels. Exfoliation: is the peeling away of large sheets of loosened materials at the surface of a rock. Common in shale, slate, and mica. 7

8 Weathering of Rock 2. Chemical Weathering: Process by which a rock is broken down by chemical action resulting in a change in the composition of a rock. The main agents of chemical weathering are oxygen, rainwater, carbon dioxide, and acids produced by decaying plants and animals that leads to the formation of soil. Chemical weathering processes include: Oxidation occurs when oxygen interacts chemically with minerals. Hydration occurs when water interacts chemically with minerals. Carbonation occurs when carbon dioxide interacts chemically with minerals. 8

9 Transportation of Weathering Products The products of weathering may stay in the same place or may be moved to other places by ice, water, wind, and gravity. Soils that remain at their places of formation are called residual soils and soils that moved and deposited to other places are called transported soils. An important characteristic of residual soils is the gradation of particle size. Fine grained soil is found at the surface, and the grain size increases with depth. At greater depths, angular rock fragments may also be found. The transported soils may be classified into several groups, depending on their mode of transportation and deposition: 1. Glacial soils formed by transportation and deposition of glaciers 2. Alluvial soils transported by running water and deposited along streams 3. Lacustrine soils formed by deposition in quiet lakes 4. Marine soils formed by deposition in the seas 5. Aeolian soils transported and deposited by wind 6. Colluvial soils formed by movement of soil from its original place by gravity (e.g. landslides) 9

10 Soil-Particles Size The sizes of particles that make up soil vary over a wide range. Soils generally are called gravel, sand, silt, or clay, depending on the predominant size of particles within the soil. To describe soils by their particle size, several organizations have developed particle-size classifications. 10

11 Soil-Particles Size Gravels are pieces of rocks with occasional particles of quartz, feldspar, and other minerals. Sand particles are made of mostly quartz and feldspar. Silts are the microscopic soil fractions that consist of very fine quartz grains and some flake-shaped particles that are fragments of micaceous minerals. Clays are mostly flake-shaped microscopic and submicroscopic particles of mica, clay minerals, and other minerals. Soils can be divided into cohesive and non-cohesive soils. Cohesive soil contains clay minerals and posses plasticity. Non-cohesive means the soil has no shear strength if no confinement. Sand is non-cohesive and nonplastic. Furthermore, gravel and sand can be roughly classified as coarse textured soils, wile silt and clay can be classified as fine textures soils. 11

12 Mechanical Analysis of Soil Mechanical analysis is the determination of the size range of particles present in a soil, expressed as a percentage of the total dry weight. Two methods generally are used to find the particle-size distribution of soil: 1. Sieve analysis: for particle sizes larger than mm in diameter, and 2. Hydrometer analysis: for particle sizes smaller than mm in diameter. 12

13 Sieve Analysis Sieve analysis is used to determine the distribution of the larger grain sizes. The soil is passed through a series of sieves with the mesh size reducing progressively, and the proportions by weight of the soil retained on each sieve are measured. There are a range of sieve sizes that can be used, and the finest is usually a 75 m sieve. Sieving can be performed either wet or dry. Because of the tendency for fine particles to clump together, wet sieving is often required with fine-grained soils. 13

14 Sieve Analysis Sieve test procedure: 1. the total mass of the soil sample (ΣM) under sieve test; 2. determine the mass of soil retained on each sieve and the pan at last (i.e., M 1, M 2, M 3,. M n, and M p ). 3. the sum of soil mass retained on each sieve plus the mass in the pan should be equal to the total mass (ΣM= M 1 + M 2 + M 3 +M n + M p ). 4. determine the cumulative mass of soil retained above each sieve, for the ith sieve we have M 1 + M 2 + M 3 +.+M i. 5. the mass of soil passing the ith sieve is ΣM (M 1 + M 2 + M Mi ) 6. the percent of soil passing the ith sieve (percent finer) is: 14

15 Sieve Analysis The results are plotted on semi-logarithmic graph paper with percent finer as the ordinate (arithmetic scale) and sieve opening size as the abscissa (logarithmic scale). This plot is referred to as the particle-size distribution curve. 15

16 Hydrometer Analysis Hydrometer analysis is based on the principle of sedimentation of soil grains in water. When a soil specimen is dispersed in water, the particles settle at different velocities, depending on their shape, size, weight, and the viscosity of the water. For simplicity, it is assumed that all the soil particles are spheres and that the velocity of soil particles can be expressed by Stokes law, according to which: 16

17 Hydrometer Analysis From the Stokes equation, rearranging the factors we can get : With where G s is the specific gravity of the soil particle, we get: With the use of the SI units and choose g-sec/cm 2 for viscosity η, and 1 g/cm 3 for the density of water ρ w,and the length L in cm, and time t in minute, and D in mm, we can get: 17

18 Hydrometer Analysis Since both viscosity and specific gravity of soil particles are temperature dependent, so does parameter K. The values of K as function of specific gravity and temperature are given in table (ASTM 2004): 18

19 Hydrometer Analysis In the laboratory, the hydrometer test is conducted in a sedimentation cylinder usually with 50 g of ovendried sample. The soil is mixed with water and a dispersing agent, stirred vigorously, and allowed to settle to the bottom of a measuring cylinder. An hydrometer is used to record the variation of specific gravity with time. By knowing the amount of soil in suspension, L, and t, we can calculate the percentage of soil by weight finer than a given diameter. where L 1 : the length of the hydrometer stem L 2 : the length of the hydrometer bulb V B : volume of the hydrometer bulb A : cross-sectional area of the sedimentation cylinder 19

20 Particle-Size Distribution Curve A particle-size distribution curve can be used to determine the following four parameters for a given soil: Effective size (D 10 ): This parameter is the diameter in the particle-size distribution curve corresponding to 10% finer. 60 Uniformity coefficient (C u ) defined as: Cu where D 60 diameter D10 corresponding to 60% finer. D Coefficient of gradation (C c ) defined as: C c D 2 30 ( D D ) S 75 Sorting coefficient (S 0 ) expressed as: is an another parameter for measuring uniformity, used mostly by geologists,. 0 D D 25 20

21 Particle-Size Distribution Curve Curve I represents a type of soil in which most of the soil grains are the same size. This is called poorly graded soil. Curve II represents a soil in which the particle sizes are distributed over a wide range, termed well graded. A wellgraded soil has a uniformity coefficient greater than about 4 for gravels and 6 for sands, and a coefficient of gradation between 1 and 3 (for gravels and sands). A soil might have a combination of two or more uniformly graded fractions. Curve III represents such a soil. This type of soil is termed gap graded. 21

22 Particle-Size Distribution Curve Example of particle-size distribution curves 22