Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1/24/2017 1. Origin of Soils and Rocks (Das, Chapter 2) 1 1/24/2017 Introduction • The mineral grains that form the solid phase of a soil aggregate are the product of rock weathering. • The physical properties of soil are dictated by the size, shape, and chemical composition of the grains, and hence the rock from which is derived. • Rocks are compact, semi-hard to hard mass composed of one or several minerals. 2 1 1/24/2017 110401336-Geotechnical Engineering 1/24/2017 On the basis of their mode of origin, rocks can be divided into three basic types: • • • Igneous rocks Sedimentary rocks Metamorphic rocks 3 1/24/2017 1. Igneous Rocks Formed by solidification of molten Magma ejected from deep within earth’s mantle. Magma has originated well below the surface and ascended towards the surface, and has crystallized as solid rock either on the surface or deep within the earth's crust as temperature fell. 4 2 1/24/2017 110401336-Geotechnical Engineering 1/24/2017 2. Sedimentary Rocks Sedimentary rocks are widely spread over the surface of earth. They constitute about 75% of the area of the continents. Development Weathering reduces the exposed rock mass to fragmented particles which can be more easily transported more easily by wind, water, and ice. When dropped by the agents of transportation, they are called SEDIMENTS. Sediments are typically deposited in layers or beds termed STRATA. When compacted and cemented together (a process called LITHIFICATION), they formed sedimentary rocks. 5 1/24/2017 The process through which sediments are converted into sedimentary rocks is called DIAGENSIS. It includes the following phases: 1. Cementation Water percolating through the voids (or pores) between the particles of sediment carries mineral matter which coats the grain and acts as cement that bind them together. 2. Compaction The weight of top layers compacts sediments and expels water out. 3. Crystallization Sometimes grains of sediments are joined together due to crystallization of some of their constituents due to pressure. 6 3 1/24/2017 110401336-Geotechnical Engineering 1/24/2017 3. Metamorphic Rocks Metamorphic rocks are formed if a rock is subjected to increase in temperature, pressure, or both, to such degree that a new TEXTURE or possibly a new MINERAL composition is produced. The process of METAMORPHISM. change is referred to as The original rock may be igneous, sedimentary or metamorphic. 7 1/24/2017 Rock Cycle The formation cycle of different types of rock and the processes associated with them. This is called the rock cycle. Sedimentary Rock Metamorphic Rock 8 4 Igneous Rock The rock cycle is important in any discussion of soil formation, especially if we know that soils are nothing but disintegrated rocks. 1/24/2017 110401336-Geotechnical Engineering 1/24/2017 2. FORMATION OF SOILS 9 Soil: some definitions 5 Soils are formed from rock, loose unconsolidated materials (may be transported), or organic residues. • The word ‘soil’ means different things to different people but basically it may be defined as the solid material on the earth’s surface that results from the interaction of weathering and biological activity on the soil parent material or underlying hard rock. • 110401336-Geotechnical Engineering 1/24/2017 Weathering This refers to the breakdown and decomposition of • rocks, soils and minerals into smaller pieces through contact with the atmosphere, biota and waters. Weathering include comprise of mechanical and • chemical weathering Weathering I. Mechanical Weathering: It is the physical disintegration of the original rock mass into smaller particles without any change in the chemical composition. Mechanical Weathering Processes: 1. 2. 3. 4. Unloading or pressure release (e.g. uplift, erosion) Thermal expansion and contraction (thermal stresses) Frost action Biological and Organic effects (e.g. the growth of plant roots) 12 6 110401336-Geotechnical Engineering 1/24/2017 Weathering Photos of Mechanical Weathering: 13 II. Chemical Weathering 1. It involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils and minerals 14 7 110401336-Geotechnical Engineering 1/24/2017 Rates of Weathering of Rocks Fine-grained rocks weather more slowly than coarse-grained rocks of the same mineral composition. Igneous rocks (excluding certain volcanic rocks that weather rapidly) and quartzite are the most resistant. 15 Parent Rock Residual soils ~ in situ weathering (by physical & chemical agents) of parent rock Transported soils ~ weathered and transported far away by wind, water, ice, gravity. 16 8 110401336-Geotechnical Engineering 1/24/2017 B. Transported Soils These are soils which were formed from rock weathering at one site and are now found at another site. The transporting agent may be: 1.Water (Principal transporting agent) 2.Glaciers 3.Wind 4.Gravity Transported soils are very important in engineering because nearly all major cities are located, at least in part, on flood plains, deltas, and coastal plains. 17 The transported soils may be classified into several groups, depending on their mode of transportation and deposition: 1. Glacial soils — deposition of glaciers 2. by transportation and Alluvial soils —transported by running water and deposited along streams 3. Lacustrine 4. Marine soils —formed by deposition in quiet lakes soils —formed by deposition in the seas 5. Aeolian 6. formed soils —transported and deposited by wind Colluvial soils —formed by movement of soil from its original place by gravity, such as during landslides 18 9 110401336-Geotechnical Engineering