What factor contributes to the stability of a shallow foundation?

Soil compaction plays a crucial role in the stability of a shallow foundation because it directly influences the load-bearing capacity of the soil beneath the foundation. When soil is properly compacted, it reduces the air gaps between particles, increasing the density and coherence of the soil matrix. This leads to improved resistance against settling and shear failure when loads are applied.

In the context of shallow foundations, adequate soil compaction prevents excessive settlement and maintains the foundation's integrity over time. A well-compacted soil can better distribute the loads from the structure above, limiting differential settlement and ensuring uniform performance.

Other factors like the depth of the foundation, use of pilasters, and width of footings also influence stability but to varying degrees and in specific contexts. However, the immediate impact of soil compaction on the load distribution and overall performance of the foundation makes it the most critical factor among the options presented.