A foundation is called shallow if its depth, is

ANS:D - equal to its width

1. One-fourth of its width: This option suggests that a foundation is considered shallow if its depth is one-fourth of its width. In this scenario, the depth of the foundation is relatively small compared to its width, indicating a shallow foundation design. Shallow foundations are typically used when the soil is strong enough to support the structure within a relatively shallow depth.
2. Half of its width: This option implies that a foundation is shallow if its depth is half of its width. Similarly, the depth is relatively small compared to the width, indicating a shallow foundation design. Half the width as the depth is still relatively shallow and suitable for supporting structures in stable soil conditions.
3. Three-fourth of its width: This option suggests that a foundation is shallow if its depth is three-fourths of its width. Again, the depth is relatively small compared to the width, indicating a shallow foundation design. Even with a slightly greater depth compared to the previous options, the foundation is still considered shallow.
4. Equal to its width: This option implies that a foundation is shallow if its depth is equal to its width. In this case, the depth is equal to the width, which indicates a shallow foundation design. While it may seem less shallow compared to the previous options, it still qualifies as a shallow foundation due to the relatively small depth compared to other types of foundations.

In civil engineering and construction terminology, a shallow foundation refers to a type of foundation that is characterized by its relatively small depth compared to its width. The depth of the foundation is typically limited to a few times its width, and in some cases, it may even be equal to its width. Here's why a foundation with a depth equal to its width is still classified as shallow:

1. Soil Bearing Capacity: Shallow foundations are typically used when the soil immediately below the structure has sufficient bearing capacity to support the loads transmitted by the structure. When the soil is strong enough to provide adequate support within a shallow depth, there's no need to excavate deeper for the foundation.
2. Stability: Shallow foundations are suitable for stable soil conditions where there are minimal risks of settlement or instability. By keeping the foundation close to the ground surface, the structure's stability can be ensured without the need for extensive excavation.
3. Construction Convenience: Foundations with depths equal to their widths are relatively easy to construct compared to deeper foundations. There's less excavation required, which reduces construction time, labor, and costs.
4. Design Considerations: Structural engineers design shallow foundations based on factors such as soil properties, structural loads, and settlement criteria. If the soil can adequately support the structure within a depth equal to its width, then a shallow foundation design may be deemed appropriate.
5. Risk Management: Shallow foundations can be advantageous in areas prone to high groundwater tables or where there are concerns about nearby structures or utilities. By keeping the foundation closer to the surface, the risks associated with groundwater infiltration or interference with underground infrastructure can be minimized.

Overall, while a foundation with a depth equal to its width may seem less shallow compared to foundations with shallower depths, it still falls within the classification of shallow foundations due to its relatively small depth relative to other types of foundations. The decision to use a shallow foundation with this depth-to-width ratio is based on a combination of soil conditions, structural requirements, construction feasibility, and risk management considerations.