Question

what pressure allows fine beach sand to have good bearing capacity at low tide on flat area near water?

Answer 1

On flat area near the water, we have capillary zone;

The effect of the capillary is the same as that of surcharge , it helps in increasing effective stress.

Stresses	Point A	Point B	Point C
Total Stress ()	0
Pore Water Stress (u)		0
Effective Stress ( = )

[Note: If the soil would have been saturated due to groundwater table instead of capillary water, effective stress in the soil would have decreased by due to increased pore water pressure].

As effective stress increases shear strength also increases, as;

i.e.

and hence the increased effective pressure allows the fine beach sand to have the good bearing capacity of the soil in the flat area near the water at low tide.