Civil Engineering Department College of Engineering Course Soil

Civil Engineering Department College of Engineering Course: Soil and Rock Mechanics (CE 260) Lecturer: Dr. Frederick Owusu-Nimo

• What is Soil Mechanics? o It is a discipline in Civil Engineering that applies principles from engineering mechanics (Basic Mechanics) and Elementary fluid mechanics to determine or predict the behavior of soil. • Why Soil Mechanics? o Whatever structure we build, the ultimate foundation or support is the Earth o Need to study the behavior of the soil and its interactions with the structure, to ensure its stability and safety of the people who use it.

Soils as Foundation Material What supports these structures?

Soils as Foundation Material • Proper functioning of these structures require well designed foundations o A foundation is the material, which provides satisfactory and economical support for the structure. o Foundation includes both the soil under the structure and intervening load carrying members

Soils as Foundation Material Location of competent soil material Shallow Foundation Deep Foundation

Soils as Construction Material Earth dam Road pavement Use of Soil as construction material requires o o o Selection of appropriate soil type (Why? ) Selection of appropriate method of placement Control of actual placement

Soil in Slope and Excavations Important to carry out stability analysis of excavations and slope of embankments to prevent failures and disasters

Earth Retaining and Underground Structures • Design against lateral or earth pressures on retaining walls • Design against pipe crushing and pipe sag due to foundation settlement or failure Braced Excavations- Subjected to lateral stresses Buried Pipeline – subjected to stresses from overburden

Solution of Soil Engineering Problem Solving any soil-engineering problem requires • Knowledge in soil mechanics and engineering geology • Experience (including case studies) • Economics (to allow selection of best solution among possible ones)

Formation of Soils • Important to determine engineering properties of soils • Engineering properties of soils influenced by how the soil is formed • Knowledge of the soil forming process greatly facilitate engineering classification of soils • How are Soils formed? o Soils are formed by disintegration of rocks through the process of weathering • Physical (Mechanical) Weathering • Chemical Weathering

Physical Weathering • Break down of parent rock by mechanical processes such as abrasion, expansion and contraction • Factors o Temperature changes o Freezing and thawing of water in cracks o Rainfall, wind and sediment transport • Soils produced maintain the same chemical composition (crystal structure) as the parent rock

Chemical Weathering • Involves break down of rock atomic structure, and the changing of its mineral from one form to another • It results from reactions of rock minerals with: o Oxygen (Oxidation), o Water (Hydrolysis), o Alkaline/acidic materials dissolved in water (Dissolution)

Chemical Weathering • These processes can o Lead to ‘rusting’ of iron rich rocks o Increase the volume of materials e. g. clay mineral absorbing water and expanding o Dissolve part of rock matter creating voids in them e. g. caves in rocks • Physical weathering also enhances chemical weathering • Soils produced have their physical and chemical characteristics modified

Types of soils based on rock type Rock Type Rock Soil Formed Igneous Rocks Granite Silty Sands, Sandy Silts with some clay Basalt Clayey Soils Shale Clays and silts Sandstone Sandy Soils Limestone Fine grained soils Gneiss Schist Silty sand soils Slate Clayey Soils Marble Fine grained Soils Quartzite Coarse grained soils ( Sand gravels) Sedimentary rocks Metamorphic rocks

Types of soils based on method of formation • Sedimentary soils (Transported Soils) o Refers to soils formed at one location, transported and deposited at another location o Agents of transportation • • • Wind Water Gravity

Formation of Soils • Residual soils o Soils formed and accumulated at same location (They are not transported) • Residual Soil Profile? o Parent rock entirely altered in mineral form near the surface o Alteration lesser at greater depth

Constituent of Soils Primary rock minerals o Pieces broken from parent rock o Sizes greater than 0. 002 mm Clay minerals o Mainly products of chemical weathering o Particle sizes less than 0. 002 mm o Basic clay minerals • Kaolinite • Illite • Montmorillonite