Which property of soil is crucial for determining its stability under load?

The shear strength of soil is a fundamental property used to evaluate its stability under load. Shear strength reflects the soil's ability to withstand loads without failing, which is vital in engineering and construction applications. It is influenced by the soil's cohesion and internal friction angle, which together determine how well the soil can resist sliding and shearing forces.

In practical terms, when loads are applied to a soil mass, the soil needs sufficient shear strength to avoid failure, such as sliding or collapse. Engineers assess shear strength to design foundations, slopes, and various structures to ensure they will remain stable under anticipated loads.

Though the other properties listed—plasticity index, particle density, and saturation level—are important in various contexts, they primarily influence other characteristics of soil rather than directly indicating stability under load. The plasticity index, for example, helps to understand the workability and compressibility of soil, while particle density can affect the overall weight and compaction of the soil. Saturation level indicates how much water the soil contains, which can also affect its strength. However, none of these properties alone directly measure the soil's resistance to shear stress, which is why shear strength is the most crucial property for assessing soil stability under load.