Soil, Lec 2, Residual soil.pptx

Soil Mechanics Experiment

1. CT-234 : Soil Mechanics (THEORY) BSC Civil Engineering Technology Third Semester (Fall-2022) Lecture 02 Residual & Transported Soils By: Engr. Kamran Khan Email: [email protected] 2

2. Residual Soils

3. Residual Soil When the rate of weathering is more than the rate at which the weathered materials are carried away by transporting agents, result in residual soils. An important characteristic of the residual soil is the gradation of the particles. Fine grained soils are found near the surface of the earth and grain size increases with depth. At greater depth relatively course grained and angular particles are found.

4. Boring log of a Residual Soil derived from granite When hard rocks such as granite undergo weathering most of the materials are likely to remain in place. These soil deposits generally have a top layer of clayey or silty clay material , below which are silty or sandy soil layers . These layers in turn are generally underlain by a partially weathered rock and then bedrock .

5. Transported Soils

6. A. Gravity Transported Soil Gravity can transport soil in Three ways . Creep: Residual soils on a natural slope can move slowly downward, and this is usually referred to as creep . Landslide: Sudden and rapid downward soil movement is called as a landslide . Mud flows: In this case, highly saturated loose sandy soils on relatively flat slopes move downward like a viscous liquid and come to rest in a more dense condition. The soil deposits formed by Creep, landslides and mudflows are Colluvial soil.

7. B. Alluvial Deposits Loose, unconsolidated soil mass, which has been eroded , reshaped and redeposited in a non-marine environment by water are called Alluvial deposits. Divided into two major categories: (1) Braided-stream deposits (2) Meandering Belt Deposits

8. 1. Braided-Stream Deposits Braided streams are high-gradient, rapidly flowing streams that are highly erosive and carry large amounts of sediment. Because of the high sediment load, a minor change in the velocity of flow will cause sediments to deposit. The deposits formed from braided streams are highly irregular in stratification and have a wide range of grain sizes. Braided Stream

9. Braided-Stream Deposits (CONTD…)

10. 2. Meandering Belt Deposits Meander Belt: The valley area in which a river/stream meanders (curve back and forth) is referred to as the meander belt. As the stream meanders within its belt, it produces a suite of deposits, consisting of sediments characteristics of point bars, natural levees, oxbow lakes & Backswamp deposits. 

11. 1. Point bar deposits In a meandering river, the soil from the bank is continually eroded from the points where it is convex in shape and is deposited at points where the bank is concave in shape form point bar deposits. Usually consist of sand and silt-size particles

12. 2. Oxbow lake: Sometimes, during the process of erosion and deposition, the river abandons a meander and cuts a shorter path. The abandoned meander, when filled with water, is called an oxbow lake . Meandering Belt Deposits (CONTD…)

13. Meandering Belt Deposits (CONTD…) 3. Natural levees: During floods, rivers overflow low-lying areas. The sand and silt-size particles carried by the river are deposited along the banks to form ridges known as natural levees . Levee: A n embankment built to prevent the overflow of a river. A ridge of sediment deposited naturally alongside a river by overflowing water.

14. 4. Backswamp deposits: Finer soil particles consisting of silts and clays are carried by the water farther onto the floodplains. These particles settle at different rates to form what is referred to as Backswamp deposits. Often highly plastic clays. Meandering Belt Deposits (CONTD…) A  floodplain  is the relatively flat area that borders a stream which is periodically flooded with water during high flow periods.

15. Composite Diagram

16. C. Lacustrine Deposits In arid regions , streams carry large amounts of suspended solids. Where the stream enters the lake , granular particles are deposited in the area forming a delta . Some coarser particles and the finer particles (silt and clay) that are carried into the lake are deposited onto the lake bottom in alternate layers of coarse-grained and fine-grained particles are called Lacustrine deposits .

17. Drift: A general term usually applied to the deposits laid down by glaciers. A  moraine  is material left behind by a moving glacier. This material is usually soil and rock. Glaciers transport all sorts of dirt and boulders that build up to form moraines. Moraines are stretching for kilometers on the Earth. Moraines only show up in places that have, or used to have, glaciers.  Glaciers are extremely large, moving rivers of ice.  Glaciers shape the landscape in a process called glaciation.  Glaciation can affect the land, rocks, and water in an area for thousands of years. That is why moraines are often very old. D. Glacial Deposits

18. Types of Moraines 1. End moraines form at the terminus of the glacier. 2. Terminal moraine: Ridge that marks the maximum limit of a glacier's advance. 3. Ground moraine: A ground moraine is made of sediment that slowly builds up directly underneath a glacier by tiny streams. When a glacier melts, the ground moraine underneath is exposed. 4. Outwash: The sand, silt, and gravel that are carried by the melting water from the front of a glacier are called outwash. 5. Lateral moraines: The sides of a valley glacier accumulate large quantities of debris from the valley walls. When the glacier melts, this material is left behind as ridges called Lateral moraines. 6. Medial moraine: Medial moraines are formed when two glaciers meet. If a glacier melts, the medial moraine it leaves behind will be a long ridge of earth in the middle of a valley.

19. E. Aeolian Soil Deposits Wind is also a major transporting agent leading to the formation of soil deposits. When large areas of sand lie exposed , wind can blow the sand away and redeposit it elsewhere . Deposits of windblown sand generally take the shape of dunes . As dunes are formed, the sand is blown over the crest by the wind. Beyond the crest, the sand particles roll down the slope. The process tends to form a compact sand deposit on the windward side , and a rather loose deposit on the leeward side , of the dune. Dunes exist in the Arab countries, at the southern coast of California, and at various places along the coasts of Washington.

20. E. Aeolian Soil Deposits (CONTD…) Typical properties of dune sand: 1. The grain-size distribution of the sand at any particular location is surprisingly uniform. 2. The general grain size decreases with distance from the source, because the wind carries the small particles farther than the large ones. 3. The relative density of sand deposited on the windward side of dunes may be as high as 50 to 65%, decreasing to about 0 to 15% on the leeward side.

21. F. Organic Soil Found where the water table is near or above the ground surface. The presence of a high water table helps in the growth of aquatic plants that, when decomposed, form organic soil. Found usually in coastal areas and in glaciated regions. Characteristics: 1. Natural moisture content range 200 to 300%. 2. Highly compressible. 3. Exhibits large amount of settlement in secondary consolidation.

22. T h a n k s