JNTU B.Tech III EARTH AND ROCKFILL DAMS AND SLOPE STABILITY Previous Question Paper (February/March - 2022 - R16)

The failure of earthen dams typically occurs due to three primary categories of causes: hydraulic, seepage, and structural failures.

1. Hydraulic Failures: These account for approximately 40% of failures. They include overtopping, where water flows over the dam crest due to inadequate spillway capacity, leading to erosion of the downstream face. Wave action can also erode the upstream face, while toe erosion occurs due to high-velocity discharge.
2. Seepage Failures: These occur due to uncontrolled water movement through or under the dam. Piping is the most critical, where seepage creates a path (pipe) that removes soil particles, eventually leading to a breach. Sloughing occurs when the downstream face becomes saturated and unstable.
3. Structural Failures: These involve the shear failure of the dam body or foundation. Factors include sliding of the upstream or downstream slopes due to improper design, high pore water pressures, or foundation settlement leading to cracks. Earthquake-induced liquefaction is also a significant structural cause.

Earth dams are classified based on their construction method and cross-section design:

1. Homogeneous Embankment Type: Constructed entirely of a single type of soil (usually clayey). It is simple but requires a flat slope to prevent seepage issues.
2. Zoned Embankment Type: Consists of a central impervious core (clay) to prevent seepage, flanked by outer shells of more pervious material (sand/gravel) to provide structural stability.
3. Diaphragm Type: Features a thin central core (diaphragm) made of concrete, steel, or timber, supported by earth or rockfill on both sides.

Quality control measures are essential to ensure the integrity of the dam:

• Compaction Control: Soil must be compacted at the Optimum Moisture Content (OMC) to achieve maximum dry density. Field density tests (e.g., core cutter or sand replacement method) are mandatory.
• Material Selection: Strict testing of borrow pit materials for permeability, shear strength, and plasticity index.
• Instrumentation: Installation of piezometers to monitor pore water pressure and settlement gauges to track foundation behavior during and after construction.
• Filter Design: Proper grading of filter zones (drainage blankets/chimney drains) to prevent internal erosion while allowing seepage to escape safely.