10.3 Failure by heave
(1)P The stability of soil against heave shall be checked by verifying either equation (2.9a) or equation (2.9b) for every relevant soil column. Equation (2.9a) expresses the condition for stability in terms of pore-water pressures and total stresses. Equation (2.9b) expresses the same condition in terms of seepage forces and submerged weights. An example of situations where heave shall be checked is given in Figure 10.2.
- 1 excavation level (left); water table (right)
- 2 water
- 3 sand
(2)P The determination of the characteristic value of the pore-water pressure shall take into account all possible unfavourable conditions, such as:
- thin layers of soil of low permeability;
- spatial effects such as narrow, circular or rectangular excavations below water level.
NOTE 1 Where the soil has a significant cohesive shear resistance, the mode of failure changes from failure by heave to failure by uplift. The stability is then checked by using the provisions of 10.2 where additional resisting forces may be added to the weight.
NOTE 2 Stability against heave will not necessarily prevent internal erosion, which should be checked independently, when relevant.
(3) The measures most commonly adopted to resist failure by heave are:
- decreasing the water pressure below the soil mass subjected to heave;
- increasing the resisting weight.