In fairly loose foundation soil, which failure mode is expected?

In fairly loose foundation soil, the expected failure mode is foundation settlement. Loose soils tend to have a lower load-bearing capacity and may experience significant compressibility. When a structure is placed on such soil, the weight can lead to excessive settlement as the soil grains rearrange and compact over time under the load.

Foundation settlement occurs due to changes in the soil structure and the gradual consolidation of the soil, which may lead to differential and overall settlement in the structure. This condition can manifest as uneven floor surfaces or cracking in walls and foundations, indicating that the soil below is compressing under the weight of the structure.

The other failure modes are less relevant in this context. For instance, bending failure generally pertains to materials under flexural stress, such as beams or slabs, while punching shear failure typically occurs in rigid structural elements transferring loads to columns or foundations in very localized areas. Sliding failure involves horizontal shear forces overcoming friction, typically seen in slopes or retaining walls, and isn't the primary concern with loose soils supporting a structure.

Foundation settlement is the most characteristic response of loose soil under load, making it the expected failure mode in such conditions.