How is the friction angle of soil estimated?

The friction angle of soil is crucial for understanding its shear strength and stability when used in construction or excavation projects. The common and effective method for estimating the friction angle involves utilizing Standard Penetration Test (SPT) results. SPT tests provide empirical data based on the resistance encountered when a standard split-barrel sampler is driven into the ground. The number of blows required to achieve a specific penetration depth is recorded, and from this data, correlations can be made to estimate the soil's friction angle.

Using SPT tests is especially advantageous because it allows for in-situ assessment, meaning it can reflect the actual conditions of the soil at the specific site, rather than relying solely on laboratory samples that may not represent the entire area's geological variability. Thus, SPT tests provide a practical approach to estimating the friction angle based on directly obtained field data.

In contrast, while laboratory testing and observational methods can contribute to understanding soil properties, they often require additional steps or assumptions that may introduce variability. Empirical data derived from field tests, such as SPT, directly correlate to real-world conditions and provide a more reliable estimation of the friction angle.