Arching in soil and its requirements

Arching in soil is the transfer of pressure from a yielding part of a soil mass to the adjacent non-yielding (stationary) parts through the shear resistance of the soil. When one part of the support moves away, the soil above tries to move with it; the shear stresses along the boundaries of the moving mass hold it up, so the pressure on the yielding part falls and the pressure on the neighbouring stationary part rises.

   ground
   ~~~~~~~~~~~~~~~~~~~~~~~~
   | stationary |  moving  | stationary |
   |  (stress   | (stress  |  (stress   |
   |  increases)| reduces) |  increases)|
   |____________|__________|____________|
                 yielding support

Essential requirements

1. Relative movement: one part of the soil support must yield (move or deflect) relative to the neighbouring part; arching does not occur in a rigid, unyielding support.
2. Shear strength in the soil: the soil must be able to mobilise shear resistance (friction and/or cohesion) along the boundaries of the moving mass; therefore it works best in dense sand, and in clay in the short term.
3. Sufficient depth of soil cover: a sufficient height of soil above the yielding part, so that the arch can form (arching fails if the cover is too thin).
4. Limited movement: the yielding must be small. Large movement breaks the arch, and the soil then loses its supporting effect.
5. Stable adjacent parts: the adjacent soil and supports must be stiff enough to resist the extra load transferred to them.

Examples: pressure on tunnel lining, buried conduits, braced cuts, flexible retaining walls and piles in soil.