Which factor does not affect the shear strength of soil?

The shear strength of soil is influenced by several factors, including moisture content, soil type, and the applied load. Moisture content directly affects the cohesion and frictional resistance within the soil, which are integral to its shear strength. Soil type is crucial as different soils exhibit varying properties, such as particle size, shape, and distribution, which ultimately affect how they respond to shear stress. The applied load is another key factor, as increasing the load can lead to failure if the shear strength is surpassed.

On the other hand, while purity may play a role in certain contexts, it is not a primary determinant of shear strength in the same way the other factors are. Shear strength is more significantly influenced by the composition and characteristics of the soil rather than its purity. Therefore, purity does not have the same level of impact as the other stated factors, making it the correct answer to the question.