Reinforced and Plain Concrete Footing:

Introduction:
In civil engineering, footings are structural elements that are used to transmit the load of a building or structure to the underlying soil. They provide support and stability to the structure above. Footings can be made of reinforced concrete or plain concrete, depending on the requirements and design of the structure.

Importance of Thickness:
The thickness of a footing is an important consideration as it determines the load-bearing capacity and stability of the structure. The thickness at the edge of a footing is particularly crucial as it is responsible for distributing the load from the structure to the soil.

Minimum Thickness:
According to standard engineering practices and codes, the minimum thickness at the edge of a reinforced or plain concrete footing resting on soils is specified. In this case, the correct answer is option 'C', which states that the minimum thickness should not be less than 15 cm.

Reasoning for the Correct Answer:
The minimum thickness requirement for footings is determined based on various factors, including the type of soil, the size and weight of the structure, and the expected loads. The minimum thickness ensures that the footing can adequately distribute the load to the soil without excessive settlement or failure.

Considerations:
The thickness of the footing may vary depending on the design requirements and soil conditions. In some cases, a thicker footing may be required to accommodate larger loads or unstable soil conditions. The thickness at the edge is particularly important as it is subjected to higher stress concentrations.

Conclusion:
In reinforced and plain concrete footings resting on soils, the minimum thickness at the edge should not be less than 15 cm. This requirement ensures the stability and load-bearing capacity of the footing, preventing excessive settlement and failure. It is important to follow these guidelines to ensure the safety and longevity of the structure.