Shear strength of particles determines their angles of repose. Factors affecting shear strength include:
  • Properties of particles: Size and Angularity
  • Water content

    Particle Size and Angularity

    Greater angularity of particles will result in more inter-granular friction and interlocking of particles, contributing to greater shear strength and angle of repose. The following pictures show the interlocking of particles of different roundness.

    Particles with greater angularity interlock better with each other, resulting in higher intergranular friction.
    Particles that are rounder interlock less tightly, resulting in less intergranular friction.
    Spherical particles such as marbles do not interlock and therefore have extremely low intergranular friction.

    Water Content and Cohesion

    Water content affects the cohesiveness of particles. If water is added to particles such as sand, water coating the grains would tend to bind them together by its surface tension, giving rise to greater internal cohesion, and therefore shear strength. However, if water is added to completely saturate the pore spaces, the pore water would act as a lubricant between grains and the pore pressure would force the grains apart.

    Dry sand — pores filled with air. Particles interlock with each other. Angle of repose is smaller than when the sand is wet.
    Wet sand — grains covered with a layer of water. Water tension would increase the cohesiveness of the grains, resulting in higher shear strength and angle of repose.
    Sand saturated with water — lubrication by water encourages downslope movement of particles. Water pressure in the pores would make the particles buoyant, reducing its shear strength and angle of repose.