The correct option is 1, 2, 3 and 4.
Explanation
Mass movements refer to the transfer of rock debris or soil down a slope under the direct influence of gravity. These movements are often triggered by specific "activating causes" that disturb the stability of slope materials by either increasing shear stress or decreasing shear strength.
Statement-wise Analysis:
- Statement 1 is Correct: Removal of support from below (undercutting) is a primary activating cause. This occurs naturally through river erosion or wave action, or artificially through excavation, leaving the material above unsupported and prone to collapse.
- Statement 2 is Correct: An increase in the gradient and height of slopes directly increases the gravitational force component acting parallel to the slope. Steeper slopes have higher shear stress, making them more susceptible to failure.
- Statement 3 is Correct: Overloading through the addition of materials (such as artificial filling, construction, or accumulation of debris) increases the weight on the slope. This added weight increases the shear stress, potentially overcoming the material's resistance to movement.
- Statement 4 is Correct: The removal of material or load from over the original slope surfaces (unloading) can lead to instability. This process often results in pressure release, causing the expansion of underlying rocks and the development of cracks or joints, which weakens the slope's structural integrity.
Key Takeaway:
Activating causes for mass movements include factors that alter the geometric or physical state of a slope, such as undercutting, steepening, overloading, and unloading, all of which disrupt the equilibrium between shear stress and shear strength.