Module 1 - Soil Formation PDF

# CE 35A/L - GEOTECHNICAL ENGINEERING I (SOIL MECHANICS) ## MODULE 1: Soil Formation Prepared by: Engr. Victor Czar A. Austria Faculty, CE Department College of Engineering, Architecture and Technology Palawan State University ## Introduction This module has the following sections and correspon...

# CE 35A/L - GEOTECHNICAL ENGINEERING I (SOIL MECHANICS) ## MODULE 1: Soil Formation Prepared by: Engr. Victor Czar A. Austria Faculty, CE Department College of Engineering, Architecture and Technology Palawan State University ## Introduction This module has the following sections and corresponding icons: | Icon | Section | Description | |---|---|---| | | Title | It shows the specific topic of the module. | | | Overview | The topics in this module are included in this section. | | | Lecture Proper | A brief debate on the lectures is given in this section. It helps you explore new ideas and capabilities. | | | Practice Problems | It involves questions or an expression that sets out the concepts and wordings that you learned from real-life circumstances. | | | Assessment | It is a job aimed at evaluating your mastery in acquiring learning skills. | | | Supplementary Knowledge | In this segment you will improve your awareness or experience through the lectures as an additional practice. | | | Answer Key | This contains answers to all activities in the module. | | | References | This is a list of all sources that this module uses for creation. | ## Overview ### Course Description: This course deals with soil formation and identification. Engineering properties of soils. Fundamental aspects of soil characterization and response, including soil mineralogy, soil-water movement, effective stress, consolidation, soil strength and compaction. Use of soils and geosynthetics in geotechnical and geo-environmental applications. Introduction to site investigation techniques. Laboratory testing and evaluation of soil composition and properties. ### Course Outcomes: At the end of this module, students will be able to: * Perform the tests using the apparatus/equipment and senses on the properties/types of soil ### Intended Learning Outcomes: At the end of this module, the students will be able to: * Categorize soil according to its formation. * Explain the characteristics of soil. * Relate mineralogy of soil to its behavior. * Describe the special soil categories. * Use your senses in describing soil types. ### Program Outcomes: * An ability to apply knowledge of mathematics, physical sciences, engineering sciences to the practice of civil engineering. * An ability to design and conduct experiments, as well as to analyze and interpret data. * An ability to design, build, improve, and install systems or processes which meet desired needs within realistic constraints. * An ability to recognize, formulate, and solve civil engineering problems. * An understanding of the effects and impact of civil engineering projects on nature and society, and of the civil engineers' social and ethical responsibilities. * Specialized engineering knowledge in each applicable field, and the ability to apply such knowledge to provide solutions to actual problems. * An ability to effectively communicate orally and in writing using the English language. * An ability to engage in life-long learning and an acceptance of the need to keep current of the development in the specific field of specialization. ### Time Frame: This module can be covered in two (2) weeks. ### Module Overview: This module is designed to introduce the basic concepts of soil in terms of soil formation, categories, properties, consistency limits, behavior, mechanics through lectures and laboratory works. You will end each lesson by submitting your written report describing the categories of soil, quantifying its properties and consistency limits and the application of soil moisture and density to actual practice. ## Lecture Proper ### Lecture 1.1 : Fundamental Role of Soil 1. **To a geologist** : the material in the relative thin zone of the Earth's surface within which roots occur formed as the products of past surface processes. 2. **To a pedologist** : it is the substance existing on the surface which supports plant life. 3. **To an engineer**: it is a material that can be * built on: foundations of buildings, bridges * built in: basements, culverts, tunnels * built with: embankments, roads, dams * supported: retaining walls Focusing then on the role of soil to engineers, you can now identify the specific function soil played in the pictures given to you. #### 1.1.2 What you should know about soil Before moving on to the soil categories, it is essential for you to know why soil differs from other construction materials. Below discusses the complicating characteristics of natural soil deposits: * Soil does not possess a linear or unique stress-strain relationship. * Soil behavior depends on pressure, time and environment. * The soil at essentially every location is different. * In nearly all cases the mass of soil involved is underground and cannot be seen in its entirety but must be evaluated on the basis of small samples obtained from isolated locations. * Most soils are very sensitive to disturbance from sampling, and thus the behavior measured by a laboratory test may be unlike that of the *"in-situ"* soil. The image that follows shows the typical stress-strain diagram of soil. ### Lecture 1.2 : Categories of Soil #### 1.2.1 Weathering Process * **Mechanical weathering** - refers to physical disintegration resulting from the effects of wind, rain, running water, ice and frost wedging, tectonic forces The following image shows the effect of ice and frost. * **Chemical and solution weathering** - rock decomposition due to chemical reactions in the rock that occurs from exposure to atmosphere, temperature changes, water, or other materials. The images that follow show the types of chemical weathering and land formation due to chemical weathering. #### 1.2.2 Geologically, soil is classified as: 1. **Residual or Sedentary Soils** - have formed from the weathering of rock or accumulation of organic material and remain at the location of their origin. Primarily a result of chemical and solution weathering. 2. **Transported Soils** - materials that have been moved from their place of origin. * **Gravity and wind transported** * **Gravity** - capable of transporting aggregate particles on limited distances only; either downhill or mountain slopes (colluvial deposits) * **Wind** - moves small particles rolling or carrying them - known as aeolian deposits The image that follows shows soil formed by gravity. ##### Examples of wind transported soil: 1. **Sand dunes** - good source of sand for some construction purposes but may not be highly suitable for all construction purposes if of uniform size and rounded. The image that follows shows sand dunes. 2. **Loess (lo'is)** - deposits of wind-blown silts laid down in a loose condition that has been retained because of particle-bonding or cementing minerals - poor foundation material since settlement or subsidence results if subjected to excessive water and severe ground vibrations. The image that follows shows loess. 3. **Volcanic ash ** - has mineral characteristics of igneous rocks - greatly affected by weathering agents. The image that follows shows volcanic ash. * **Glacial deposits**: * moving sheets of ice * present in the polar region The image that follows shows soil formed from glacial movement. ##### Effects of glacial formation: * level of sea is lowered by as much as 125-150m * tremendous weight of glaciers causes the land beneath to depress ##### Characteristics: * excellent foundation support * good construction materials if composed of coarse particles but if containing large percentage of silt and clay materials, it is relatively difficult to handle and compact. * **River deposits** : moving considerable volume of soil by carrying the particles in suspension or by rolling, sliding and skipping them along the river bottom - known as alluvial deposits. The image that follows shows river deposits. ##### Example of River Deposits: 1. alluvial fans - gravel and sand for construction purposes 2. natural levee - source of gravel and sand 3. flood plain - fine-grained soil 4. meander bends - deposits are made of coarse soils 5. ox-bow lake - filled with fine-grained soil, poor foundation site 6. delta - if made of coarse soils it provides good foundation support and soils for construction use * **Lacustrine deposits**: * soil formations remaining at the location of former lake areas * weak, compressible and make poor foundation The image that follows shows soil formation from lake. * **Marine clay deposits**: * soil deposits carried by flowing water to seas and oceans * weak and compressible therefore poor foundation material The image that follows shows marine deposits formed from flowing water. * **Beach deposits**: * predominantly sand materials * marine sands are rounded and smooth and of uniform size * corrosion potential due to salinity may affect their usefulness for certain construction purposes * ideal for waterfront and marine structures because excavation is uncomplicated and transportation economical The image that follows shows beach deposits. * **Swamps and Marsh deposits**: * are developed in stagnated areas where limited depths of water accumulate, or where periodic inundation and drying occurs because of fluctuations in the groundwater level and vegetation has a chance to grow. * soils are of high organic content, soft and odoriferous * weak and highly compressible The image that follows shows swamps and marsh deposits. ##### Example of Swamps and Marsh deposits: 1. **Peat** - partially decomposed vegetation and is normally spongy and relatively light. 2. **Muck** - geologically older than peat, fully decomposed vegetation and is relatively dense. * **Sanitary (solid waste) Landfill**: * relates to the technique of using burial methods for disposing of solid waste resulting from human activities. The image that follows shows a sanitary landfill. ### Lecture 1.2.3 : How Soil is Classified by Engineers 1. **Coarse - grained particles**: individual particles are large enough to be distinguished without magnification. * **Boulder** - a rock fragment, usually rounded by weathering or abrasion, with an average dimension of 305 mm (12") or more * **Cobble** - a rock fragment, usually rounded or semi-rounded, with an average Dimension between 75 mm and 305 mm (3" and 12") * **a. Gravel**: * 200 mm - 2.00 mm - general limit * 75 mm - 2.00 mm - for highway engineering * **b. Sand**: * 2.00 mm - 0.075 mm * shape: rounded, sub-angular or angular The image that follows shows coarse grained particles. 2. **Fine-grained particles**: particles are so small that its size cannot be distinguished by the naked eye * **a. Silt**: * 0.075 mm - 0.002 mm * has a smooth texture * it possesses little or no cohesion * shape: rounded --- little or no cohesive property * flaked shape * **b. Clay**: * derived from chemical weathering * 0.002 mm - 0.001 mm * soil can be remolded or deformed without causing cracking, breaking, or change in volume and will retain the remolded shape. * texture is smooth * cohesive and plastic when wet * shape: flaked-shape or needle shape Note: Silt and clay are determined based on the plasticity and non-plasticity of the material. ##### Engineering Properties: * Have poor load-sustaining qualities. * Highly impermeable, thus has poor drainage characteristics. * They will compress under the action of a sustained load. * They will change in volume and strength due to change in water content. The image that follows shows fine grained particles. 3. **Organic Soils**: those soils particularly fine-grained which contain small fragments of decomposed vegetation and decayed animals. * **a. Muck**: * a relatively dense fibrous soil containing a more oxidized organic matter * geologically older than peat * **b. Peat**: * a fibrous soil containing partially decomposed vegetation * normally spongy and relatively light. * **c. Organic Silt or Organic Clay**: * contains finely divided organic matter. The image that follows shows organic soil. ##### Engineering Properties: * Change in volume very considerably under comparatively light loads. * Have very poor load-sustaining qualities. ### Lecture 1.3 : Factors Affecting the Behavioral Properties of Soil Soil's behavior is affected by several factors. Among these are: #### 1.3.1. Size | Particle Size Classification | Diameter of Particle, mm | US Standard Sieve Passing | US Standard Sieve Retained | |---|---|---|---| | Boulder | > 305 | 3" | | | Cobbles | 75 - 305 | No. 10 | No. 10 | | Gravel | 75 - 2.00 | No. 200 | No. 200 | | Sand | 2.00 - 0.075| | | | Silt | 0.075 - 0.002 | Cannot be separated by sieving. Size is determined by hydrometer test. | | | Clay | 0.002 - 0.001| | | | Colloid | less than 0.001 | | | The following image shows a visual representation of the size differences of gravel, sand, silt, and clay. #### 1.3.2. Particle Shapes * **a. Bulky grain**: particle dimensions are approximately equal developed into angular, sub-angular and rounded shape * angular particles possess better engineering properties * sand, gravel and silt possess this shape * **b. Flaky or platelike shape**: extremely thin compared to length and width * clay minerals possess this shape * **c. Needle-like grain**: shape similar to a needle The following image shows a visual representation of the different shapes of soil. #### 1.3.3. Soil Structure Refers to the pattern of arrangement of the soil particles * **a. Single-grained**: * gravel, sand and silt * actual soil deposits are made of accumulations of soil particles having at least some variation in particle size The following image shows a visual representation of single grained soil. * **b. Honey-combed**: * sand or silts * possible for soil with particle mass relatively great compared to the surface * developed for grains settling slowly in quiet waters or a loosely dumped moist soil * this soil structure is incapable of providing great support when subjected to external loading * it breaks, bends and rearranges under load The following image shows a visual representation of honeycomb structure. * **c. Flocculated (edge to face arrangement)**: * for clay particles * has high void ratios, low density and high-water contents, quite strong and resistant to external forces because of the attraction between particles * clay settling out in saltwater solution tends to a structure more flocculent since saltwater acts as an electrolyte in which repulsion between particles is reduced. * clay settling out in fresh water produces a structure where some parallel orientation of settled particles occurs * clay particles in ponds and wetlands where organic decay is taking place are highly flocculent * **d. Dispersed or oriented structure**: developed when clay soils are reworked or remolded by the transportation process (glacial action or human activities) The following image shows a visual representation of flocculent and dispersed structures. #### 1.3.4. Thixotropy * The phenomenon of strength loss-strength gain * Is the process of softening caused by remolding followed by a time-dependent return to harder state * Affects clay minerals that adsorb large quantities of water, ex. Montmorillonite ##### Benefits: * soil structures (dams, highway embankments) and disturbed foundation soils become stronger with the passing of time * ##### Problems: * construction sites may be quickly transformed into a mire of mud when construction equipment travels across the area, making handling of equipment and materials very difficult. * piles driven in clay soils meet considerable resistance if driving continues after a one-or two-day wait. #### 1.3.5. Mineralogical Composition: * This factor affects the soil type true clay or clay minerals * Almost all clay minerals are crystalline that are capable of developing cohesion and plasticity #### 1.3.6. Plasticity: * Term applied to fine-grained soils (particularly clays) to indicate the soils' (plus included water's) ability to flow or be remolded without raveling or breaking apart. #### 1.3.7. Intereffect with Water (Clay and Water): * The forces of electrical charge have a profound effect on the behavior of particles coming in association with other particles and water (or other fluids) present in the soil. * Clay particles have very high ratio of particle surface to particle mass compared to coarse particles. * Groundwater is rarely pure, it contains dissolved gases, minerals and other compounds in solution or suspension. With groundwater, rocks and minerals will break down into cations (positively charged ions) and anions (negatively charged ions). * Since clay minerals have net negative charges present on the surface, it attracts cations (potassium, sodium, calcium and aluminum). Because of the net positive charge of the cations, they in turn attract negative charges possessed by the negative tips of water molecules. Thus, significant water becomes "bonded" to the clay. * The farther from the particle surface, though, the weaker the attraction becomes. The image that follows shows visual representation of adsorbed water and cations in a diffuse double layer surrounding clay particles. #### 1.3.8. Diffuse double Layer: * The distance from the clay particle surface to the limit of attraction * Water bound to soil particles because of the attraction between electrical charges existing on soil particle surfaces and (dipole) water molecules ### Lecture 1.4: Special Soil Categories #### 1.4.1. Collapsible Soil * Refers to the category of soil deposits that experience significant decrease in volume when exposed to water, typically found in arid regions. * Predominantly silt-size particles but granular deposits that include considerable gravel can be collapsible The image that follows shows an example of collapsible soil. #### 1.4.2. Expansive Clays * Clay soils that experience significant volume expansion in the presence of water and shrinks upon drying. Clays including montmorillonite or illite minerals are especially noted for their volume-change characteristics. * Volume change is related to the thickness and mobility of the water film adsorbed onto or surrounding the montmorillonite particle. The image that follows shows an example of expansive soil. ##### Negative Effects: * Capable of lifting slabs and heavy structures. * Excessive lateral thrusts on retaining wall structures. * Ground settlement due to shrinkage may result to serious damage. ##### Beneficial Uses: * General grout in preventing leakage from reservoir. * For plugging leaks in tunnel construction. * As drilling mud in connection with soil borings and oil and gas wells- it prevents flocculation and facilitates the removal of the drill cutting of the rotary drill. * Used as backfill for slurry trench walls. * For clarification of beer and wine. #### 1.4.3. Formation * Extreme disintegration of the parent material, alkaline environment and semiarid climate (absence of enough water resulted in the accumulation of magnesium, calcium, iron, sodium and magnesium ions), enhancing the formation of expansive clays. #### 1.4.4. Methods of identifying expansive clays * Obtaining soil samples in the proposed site and apply soil tests for evaluating the clay's potential for expansion #### 1.4.5. Methods used to stabilize expansive clays: * **How to protect soil from variations in moisture content**: * **Chemical stabilization**: it uses an additive to reduce the inclination to attract or lose moisture ex. lime slurry mixing. * **Replacement**: applicable only where limited area or depth requires protection against soil volume changes. #### 1.4.6. Dispersive Clay: * Clay soils that deflocculated in still water and erode when exposed to a low-velocity flow of water. A clay-pore water system that has a high concentration of sodium ions tends to have high dispersity. The image that follows shows an example of dispersive soil. ##### Identification: * Areas show steep erosion gullies and eroded tunnels, though not always. * Embankments similarly experience the development of gullies and tunnels. * Piping of earth dams (this condition develops quickly when fresh water replaces saltwater). #### 1.4.7. Formation * Dispersion of clay soils is attributed to the presence of cations in the soil pore water. * Repulsive forces generally decrease as the concentration of ions increases. * However, repulsion increases as the quantity of sodium ions increases. #### 1.4.8. Measures to counter erodibility of dispersive clay: * Can be reduced through the use of hydrated lime or aluminum sulfate admixtures, 1-2% by weight. * Use of soil transition-filter zones that border the dispersive clay can be designed to control erosion and seal concentrated leaks. * Use of geofabrics as an erosion control. #### 1.4.9 Testing for Dispersive Clays * The relationship between the presence of ions and susceptibility to deflocculation * Atomic absorption spectroscope or flame photometer - instruments used to determine ion concentrations. ##### Simple qualitative tests using standard laboratory equipment: * **Soil Conservation Service Dispersion Test or the Double Hydrometer Test**: * % dispersion = % finer than 0.005 mm in plain water test / % finerthan 0.005 mm in conventional hydrometer test * A % dispersion > 35% indicates a dispersive clay soil. * **Crumb Test**: A small sample of soil (10 mm or less in diameter) preserved at the natural water content is placed in a beaker of distilled water. The reaction in terms of a colloidal cloud around the crumb is an indication of the soil's tendency to disperse; no cloud indicates a nondispersive material. * **Pinhole Test**: The test soil is compacted, and distilled water is then made to flow through a 1-mm-diameter hole in the sample. The test can be set up in the permeameter apparatus used for permeability determinations, using pea gravel filters at the ends of the test sample. With a dispersive clay, the hole erodes and the flowing water discharge is colored, whereas for a nondispersive soil the hole does not enlarge and the water discharge remains clear. ### Lecture 1.4.4. Laterites * A category of residual soil formed from the weathering of igneous rock in tropical regions that, through the process of its formation, will include high concentrations of iron and aluminum sesquioxides with low concentrations of silica. The image that follows shows an example of lateritic soil. ##### Identification/characteristics of laterites: * Frequently reddish in color but not always. * Typically contain only meager concentrations of nutrients necessary for productive farming. * Deposits may be found in hard or cemented state (due to presence of free iron oxide), particularly in areas where vegetation is sparse or has been removed. * Is gap-graded, existing with prevalence of gravel-sized and clay materials but having limited sand-and silt-sized particles. * Found to be unstable; during handling and testing or excavation and placement, physical properties change particularly soils from damp region than from arid region. ##### Measures to Counter Negative Effects: * Protect the material from percolating or migrating water. * Protect it from the effects of heavy repetitive loading. * Properties can be improved by the use of common admixtures such as cement and lime. ### Lecture 1.4.5. Permafrost * Permanently frozen ground located in the northern regions of the earth. * He depth of frozen soil is on the order of 300 - 1000 meters. * Surface zone of soil thaws in summer and refreezes in winter, this soil depth that is subjected to seasonal cycle of thawing and freezing is termed active zone The image that follows shows a visual representation of permafrost. ##### Solutions to Counteract Effects on Structures Built on Permafrost: * Foundations should be installed in the permanently frozen earth that exists below the active zone. * Foundations must be designed to ensure that heat is not conducted through the permafrost that provides the support: insulating materials could be necessary along the vertical perimeter of the foundation and at the base. * The floors and subsurface floors that would be in contact with the ground must be insulated, artificially cooled, or separated to prevent building heat losses from thawing the permafrost *Presence of tundra, the surficial blanket, acts as an insulating material limiting the depth of summer thawing. ##### Beneficial Effects of the Presence of Tundra * Roadways are now constructed on embankments of gravel, which acts as a non-frost-susceptible insulating pad to keep the underlying soil frozen and stable. * It has kept sloped and hilly areas stable against slides. ### Lecture 1.5. Liquefaction: * Loss of strength occurring in saturated cohesionless soil exposed to shock or vibrations when the soil particles momentarily lose contact. The material then behaves as a fluid. * Vibration or shock waves may result during earthquake, explosions or operation of some types of machinery. The following image shows a visual representation of liquefaction. ##### Effects of Liquefaction: * Structures may experience significant vertical or lateral movements. * Unsupported earth slopes tend to slide. #### Tests Identifying Sand Deposits Susceptible to Liquefaction * Soil boring test * Penetration resistance testing * Relative density testing #### Conditions Encouraging Liquefaction: * Deposits of uniform sands are considered more susceptible than well-graded sands. * Fine sands are more susceptible than coarse sands. * Severity of shocks passing through the deposit #### Measures to Eliminate or Reduce Liquefaction Occurrence: * Drainage to remove the saturation condition * Soil densification-compaction procedure ## Practice Problems 1. What is Soil Mechanics? 2. What are the fundamental roles of soil? 3. Explain the characteristics of natural soil deposits. 4. Discuss the categories of soil from the standpoint of a geologist and an engineer. 5. What is the effect of shapes on the behavior of soil? 6. Describe the diffuse double layer of clay soil. 7. Discuss the special soil categories by giving solutions to counter its negative effects. 8. What are the conditions necessary for liquefaction to take place? ## Assessment Assessment for this module will be scheduled by the instructor. ## Answer Key Answer key for this module will be provided on the next module. ## References * McCarthy, D. F. (2001). Essentials of Soil Mechanics and Foundations: Basic Geotechnics, 6th Edition. New Jersey: Prentice Hall. * Das, Baja; Sobhan, Khaled. (2014). Principles of Geotechnical Engineering. Cengage Learning * Budhu, Muni. 2011. Soil Mechanics and Foundation, 3rd Edition. USA. John Wiley and Sons. Inc.