Water

The Water Height is measured from the bottom of the pillar, whereas the Pillar Height and Overburden Thickness are entered as absolute values. This is because water pressure is taken into account if the pillar is specified as "permeable" (Rigid and Elastic analyses only) therefore Water Height less than the Pillar Height can be specified to account for the effect of water pressure on effective vertical and lateral stress. The water surface is drawn on the model at the appropriate level. Any water above the pillar + overburden is referred to as "free water".

Depending on the relative water height, the permeability of the pillar, and the lateral stress option, various situations will arise.

Pillar Is Permeable

If Pillar Is Permeable is unchecked:

• Water Height < (Pillar Height + Overburden Thickness) will have no effect.
• Water Height > (Pillar Height + Overburden Thickness) will add a deadload due to the weight of the free water.

If Pillar Is Permeable is checked:

• Water pressure will reduce the lateral effective stress if the Lateral Stress Stress Type = Gravity.
• If the water height is more than about twice the Water Height , negative effective stresses will result, and a warning will be displayed when attempting to Compute.
• If Water Height > (Pillar Height + Overburden Thickness), and lateral effective stresses are not negative, then the weight of the free water will still add an extra deadload component to the pillar load.

Constant Lateral Stress

Water Height will have no effect on the lateral stress, since the user is already entering effective stresses. If Water Height > (Pillar Height + Overburden Thickness), then the deadload due to the weight of the free water will still be calculated.