Kitkakulma

Kitkakulma, in geotechnical engineering commonly translated as the friction angle, is a parameter that describes the shear resistance arising from interparticle friction in soils. It is a component of the Mohr–Coulomb failure criterion, where the shear strength tau is expressed as tau = c' + sigma' tan(phi'), with c' as the effective cohesion, sigma' the effective normal stress, and phi' the effective friction angle. The friction angle reflects how readily particles slide past one another and, together with cohesion, governs stability under loading.

The friction angle is typically determined in laboratory tests. Direct shear tests measure the shear and normal stresses at failure along a predefined plane, yielding phi' values. Triaxial compression or extension tests provide peak and residual friction angles under controlled drainage and varying confining stresses. Field approaches include back-analysis from slope performance or certain in-situ tests, though laboratory results are most common for design purposes.

Several factors influence kitkakulma. It depends on soil type, density or relative compaction, moisture content, loading history, and confinement level. In granular soils, phi' tends to increase with density and particle angularity, while saturation or softening can reduce effective friction. Some soils exhibit distinct peak and residual friction angles due to particle rearrangement or breakage.

Typical ranges vary by soil class. Loose sands may exhibit roughly 25–35 degrees, dense sands 35–45 degrees, and gravels higher still; clays generally have low friction angles where cohesion dominates. The term phi' is essential in assessing slope stability, bearing capacity, and earth pressures for foundations and retaining structures.