In spring, roads begin to thaw from both the top and bottom. As ice in the pavement melts, water saturates the road's base and sub grade. These layers lose strength. As thawing continues, melt water becomes trapped in the upper sub-grade, unable to drain away through the frozen soils below. Pavements weaken considerably in a very short time under these conditions. Without adequate support, a pavement surface deflects too much under trucks carrying heavy loads, developing cracks or ruts. these then let more moisture penetrate, worsening the cycle until the road fails completely.