Jawaharlal Nehru Technological University Hyderabad B.Tech III ENGINEERING GEOLOGY Previous Question Paper (October - 2020 - R13)

Weathering is the process of disintegration and decomposition of rocks at or near the Earth's surface due to atmospheric agents. It is broadly classified into physical, chemical, and biological weathering.

1. Physical (Mechanical) Weathering: This involves the physical breakdown of rocks into smaller fragments without changing their chemical composition. Key processes include:

• Thermal Expansion and Contraction: Repeated heating and cooling in arid regions cause differential expansion of minerals, leading to rock fracturing.
• Frost Wedging: Water enters rock fractures, freezes, and expands, exerting pressure that splits the rock.
• Exfoliation: The peeling off of outer rock layers due to pressure release or thermal stress.
• Salt Crystal Growth: Salt crystallization in pores exerts pressure, causing granular disintegration.

2. Chemical Weathering: This involves chemical reactions that alter the mineral composition of rocks, typically occurring in humid and warm climates. Key processes include:

• Hydration: Minerals absorb water, increasing in volume and causing structural stress.
• Hydrolysis: The reaction of minerals with water, often converting silicates into clay minerals.
• Oxidation: Reaction with oxygen, particularly in iron-bearing minerals, leading to rust-like staining and weakening.
• Carbonation: Reaction with carbonic acid (formed by CO2 and water), which dissolves minerals like calcite in limestone.

3. Biological Weathering: This is caused by living organisms. Plant roots penetrate rock fractures, widening them through mechanical pressure. Burrowing animals expose fresh rock surfaces to atmospheric agents, while organic acids produced by decaying vegetation accelerate chemical decomposition.

Weathering Products: The end result of these processes is the formation of regolith and soil. Physical weathering produces angular rock fragments, scree, and talus. Chemical weathering produces stable secondary minerals such as clay minerals (kaolinite, montmorillonite), iron oxides, and soluble salts, which eventually form the inorganic component of soil profiles.

Soil erosion significantly compromises the structural integrity, functionality, and longevity of hydrological structures such as dams, reservoirs, canals, and embankments. The adverse effects are primarily driven by the transport and deposition of sediment, which alters the hydrological regime and physical stability of these assets.

The key adverse effects include:

• Reservoir Sedimentation: Erosion in the catchment area leads to the transport of silt and clay into reservoirs. This reduces the effective storage capacity of the reservoir, limiting its ability to provide water for irrigation, power generation, or flood control. Over time, this silting can lead to the premature decommissioning of the structure.
• Structural Instability and Scouring: High-velocity water flow carrying abrasive sediment particles causes scouring at the base of spillways, piers, and abutments. This mechanical erosion weakens the foundation, potentially leading to structural failure or collapse.
• Clogging of Hydraulic Components: Excessive sediment load can clog intake structures, trash racks, and drainage galleries. This restricts water flow, reduces the efficiency of turbines in hydroelectric plants, and prevents the proper functioning of pressure relief systems, which can lead to dangerous pore-water pressure buildup.
• Canal Silting: In irrigation networks, eroded soil deposited in canals reduces the cross-sectional area, decreasing the discharge capacity and requiring frequent and costly desilting operations to maintain design flow rates.
• Increased Maintenance Costs: The cumulative impact of erosion necessitates continuous monitoring and expensive remedial measures, such as dredging, bank stabilization, and the construction of check dams to trap sediment before it reaches the primary structure.
• Environmental and Safety Risks: Uncontrolled erosion can lead to the destabilization of dam embankments or slopes surrounding the reservoir, increasing the risk of landslides into the water body, which may generate waves that overtop the dam, posing a severe threat to downstream safety.

To mitigate these effects, integrated watershed management practices, such as afforestation, contour bunding, and the construction of silt-trapping basins, are essential to minimize sediment yield at the source.