Shear strength of soil increases with increase in cohesion, angle of internal friction, and normal stress on soil.

1. Cohesion of Soil:
Cohesion is the internal molecular attraction between soil particles. It represents the shear strength of soil when there is no interparticle friction. Cohesion is mainly dependent on the type and composition of soil. When the cohesion of soil increases, the shear strength of the soil also increases. This is because cohesive forces between soil particles resist shear deformation and provide additional strength.

2. Angle of Internal Friction:
The angle of internal friction is a measure of the resistance to sliding or shearing between soil particles. It represents the shear strength of soil due to interparticle friction. When the angle of internal friction increases, the shear strength of the soil also increases. This is because a higher angle of internal friction means that more force is required to induce shear deformation between soil particles.

3. Normal Stress on Soil:
Normal stress is the stress acting perpendicular to the plane of shear. It is the pressure exerted by the weight of the soil or any other external load. When the normal stress on soil increases, the shear strength of the soil also increases. This is because the increase in normal stress increases the contact forces between soil particles, leading to a greater resistance to shear deformation.

Overall Explanation:
The shear strength of soil is influenced by both cohesive and frictional forces between soil particles. The cohesion and angle of internal friction are soil properties that determine the shear strength of soil. The cohesion represents the attraction between particles, while the angle of internal friction represents the resistance to sliding or shearing between particles.

The normal stress on soil, which is the pressure exerted by external loads or the weight of the soil itself, also plays a significant role in determining shear strength. The increase in normal stress leads to a greater contact force between soil particles, increasing the resistance to shear deformation.

Therefore, as all three factors - cohesion, angle of internal friction, and normal stress on soil - increase, the shear strength of the soil also increases. It is important to consider these factors in geotechnical engineering to ensure the stability and safety of soil structures such as foundations, retaining walls, and slopes.