JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ENGINEERING GEOLOGY Previous Question Paper (March - 2022 - R18)

Weathering is the physical disintegration and chemical decomposition of rocks at or near the Earth's surface. Its impact on civil engineering projects is profound, as it alters the physical, mechanical, and chemical properties of rock masses, directly influencing their stability, permeability, and durability.

1. Importance for Dams:

For dam construction, the foundation must be competent enough to bear the immense load of the structure and resist seepage. Weathering creates a regolith or soil cover that must be excavated to reach sound, unweathered bedrock. If weathering is deep or uneven, it can lead to differential settlement, causing cracks in the dam body. Furthermore, chemical weathering along joints and fractures can create pathways for water, increasing the risk of piping and foundation failure.

2. Importance for Reservoirs:

The primary concern for reservoirs is water tightness and stability of the slopes. Weathering increases the porosity and permeability of rocks, which can lead to significant water loss through leakage. Additionally, intense weathering of slopes surrounding a reservoir reduces the shear strength of the rock mass, significantly increasing the probability of landslides or slope failures when the reservoir is filled, which can cause catastrophic overtopping of the dam.

3. Importance for Tunnels:

In tunneling, weathering dictates the choice of excavation method and the intensity of support required. Highly weathered zones act as weak, unconsolidated material that lacks self-supporting capacity, necessitating immediate and heavy lining or steel rib support to prevent collapse. Weathering also complicates groundwater management; weathered zones are often saturated and act as aquifers, leading to water inflow problems during construction. Mapping the depth of the weathering profile is essential to predict the transition between stable rock and unstable, weathered material, ensuring the safety of the tunnel face.

The failure of major civil engineering structures is often linked to inadequate geological investigations. Two prominent case studies illustrate the critical role of geological factors in structural stability:

1. The Malpasset Dam Failure (France, 1959)

The Malpasset Dam was an arch dam constructed on a foundation of gneiss. The primary geological drawback was the presence of a thin, sub-horizontal fault zone that was not adequately identified during the site investigation. When the reservoir was filled, the high water pressure penetrated the joints and the fault plane, leading to a loss of shear strength in the rock mass. This resulted in the catastrophic sliding of the foundation rock, causing the dam to collapse. This failure highlighted the necessity of detailed mapping of structural discontinuities such as faults, joints, and shear zones before dam construction.

2. The Vaiont Dam Disaster (Italy, 1963)

The Vaiont Dam was a high arch dam built in a narrow gorge. The geological failure here was not of the dam itself, but of the reservoir slopes. The mountain slopes consisted of alternating layers of limestone and clayey marl, which were prone to sliding when saturated. Despite warnings, the reservoir was filled, causing the pore water pressure to increase significantly within the rock mass. This triggered a massive landslide of approximately 260 million cubic meters of rock into the reservoir. The resulting displacement wave overtopped the dam, destroying the town of Longarone downstream. This case demonstrates that geological surveys must extend beyond the immediate dam site to include the stability of the entire reservoir basin.

Lessons Learned:

• Comprehensive site investigations must include deep drilling and geophysical surveys to detect hidden structural defects.
• Geological mapping should identify potential planes of weakness, such as bedding planes, faults, and joints, which can act as failure surfaces.
• Hydrogeological conditions, including groundwater flow and pore water pressure, must be evaluated as they directly influence the shear strength of rock masses.
• The stability of the surrounding terrain, not just the foundation footprint, is critical for the safety of large-scale projects.