The bunker sand should stay slightly moist all the time to avoid a major change in weight and subsequent wash-out when rain starts. With Capillary Concrete, some ground moisture is always being fed up to sand as moisture evaporates from sand, effectively delaying a complete dry-out of sand until ground moisture is depleted. This is the reason the patented Capillary Concrete system avoids wash outs better than any other bunker liner product.