JNTU B.Tech III ENGINEERING GEOLOGY Previous Question Paper (August - 2022 - R13)

The failure of a dam due to geological reasons is often linked to the inadequate assessment of the foundation conditions or the reservoir rim. The primary geological causes for failure include:

1. Foundation Instability: Failure occurs if the dam is constructed on weak, compressible, or unconsolidated materials such as thick layers of clay, silt, or loose alluvium, which cannot support the structural load, leading to differential settlement or shear failure.
2. Seepage and Piping: If the foundation rock is highly fractured, jointed, or porous (e.g., limestone with solution cavities), water can percolate through the foundation. This leads to internal erosion or 'piping,' where fine particles are washed away, creating voids that eventually cause the dam to collapse.
3. Faults and Shear Zones: The presence of active or inactive faults, shear zones, or crushed rock zones beneath the dam axis creates planes of weakness. These zones can act as conduits for water or result in differential movement during seismic events, compromising structural integrity.
4. Slope Instability at the Reservoir Rim: Geological instability in the reservoir area, such as the presence of unstable cliffs or landslide-prone slopes, can lead to mass wasting. A large landslide into the reservoir can trigger a massive displacement wave, causing overtopping and subsequent failure of the dam.
5. Inadequate Grouting and Cut-offs: Failure to properly seal permeable geological formations through grouting or cut-off walls allows high-pressure water to bypass the dam, increasing uplift pressure at the base, which can destabilize the structure.
6. Seismic Activity: Dams located near active geological faults are susceptible to earthquake-induced ground shaking. This can cause liquefaction of foundation soils or structural damage to the dam body, especially in earth-fill dams.
7. Unfavorable Dip and Strike: In rock foundations, if the bedding planes or joints dip downstream (parallel to the valley slope), the dam may slide along these planes due to the hydrostatic pressure exerted by the reservoir.

Weathering is the natural process of physical disintegration and chemical decomposition of rocks at or near the Earth's surface due to exposure to atmospheric agents like water, oxygen, temperature fluctuations, and biological activity. It transforms solid rock into regolith or soil without significant transport of the material.

The importance of weathering in civil engineering projects, particularly for dams and tunnels, is critical for assessing site stability, permeability, and construction feasibility:

For Dams:

1. Foundation Stability: Weathered rock has lower compressive strength and higher compressibility. Deep weathering zones may require extensive excavation to reach competent, unweathered bedrock to prevent differential settlement of the dam structure.
2. Permeability and Seepage: Weathering increases the porosity and permeability of rock masses by widening joints and fractures. This can lead to significant seepage losses under the dam, potentially causing piping or uplift pressures that threaten the structural integrity of the reservoir.
3. Grouting Requirements: Heavily weathered zones often necessitate extensive pressure grouting to seal voids and improve the rock mass quality before dam construction can proceed.

For Tunnels:

1. Excavation Hazards: Tunnels passing through deeply weathered rock zones are prone to instability, roof falls, and squeezing ground conditions. These zones often require immediate support systems like shotcrete, rock bolts, or steel ribs.
2. Groundwater Inflow: Weathered rock acts as a conduit for groundwater. Intersecting highly weathered, fractured zones during tunneling can result in sudden, high-pressure water inflows, which complicate excavation and increase costs.
3. Lining Design: The degree of weathering dictates the thickness and reinforcement requirements of the tunnel lining. In areas of intense chemical weathering, the potential for aggressive groundwater to corrode concrete linings must also be evaluated.