Which test method is most directly linked to assessing the mechanical properties of soil?

The direct shear test is the primary method used to evaluate the mechanical properties of soil, specifically its shear strength, which is critical for stability assessments in geotechnical engineering. This test determines how the soil behaves under stress by measuring the force required to shear the soil along a defined plane, thus providing crucial data about the soil's strength and behavior under load.

By applying a controlled force to a soil sample and monitoring the deformation until failure, the direct shear test allows engineers to quantify parameters like internal friction angle and cohesion. These parameters are essential for predicting how soil will respond in various engineering applications, such as in the design of foundations, slopes, and retaining structures.

In contrast, the settlement test primarily deals with how much a structure may sink under its load over time, focusing more on consolidation properties rather than mechanical strength. The moisture content test measures the amount of water present in the soil, which affects its physical properties but does not directly measure mechanical behavior. Lastly, the permeability test assesses the ability of soil to transmit water through its pores, which influences drainage and stability but does not provide insights into mechanical strength directly.