To determine which type of soil requires deeper excavation to avoid structural damage due to its swelling and shrinkage behavior, we need to understand the characteristics of each soil type mentioned.

1. Sandy Soil: Sandy soil consists mainly of sand particles. It has good drainage properties and does not swell or shrink significantly with moisture content changes. Hence, it does not require deeper excavation to mitigate structural damage.
2. Black Cotton Soil: Black cotton soil is known for its high content of clay mineral called montmorillonite. This type of soil experiences significant swelling when wet and shrinkage when dry. The extensive volume changes can lead to structural damage if not properly managed, necessitating deeper excavation and other foundation precautions.
3. Gravelly Soil: Gravelly soils have larger rock fragments and are well-draining. Like sandy soil, gravelly soil does not exhibit significant swelling or shrinkage. Thus, it does not need unusually deep excavation.
4. Loamy Soil: Loamy soil is a mixture of sand, silt, and clay. It has moderate drainage properties and some capacity for minor swelling and shrinkage but not to the extent of black cotton soil.

Based on the characteristics, Black cotton soil is the type of soil that requires deeper excavation to prevent structural damage due to its swelling and shrinkage behavior.

Conclusion: The correct answer is Black cotton soil as it is prone to significant swelling and shrinkage, posing a risk of structural damage without proper excavation and foundation techniques.