The bridge's "legs" that support the piers and piling cap or footing under the water or ground line; a deep foundation unit implanted in the earth by putting new concrete in a drilled hole with steel reinforcing. The capacity of drilled shafts is determined by the surrounding soil and/or rock. If soft, dig down twice the diameter of the shaft; if hard, dig down only once the diameter of the shaft. The leg must be long enough to reach below the frost line and must be driven far enough into the ground so that it will not be affected by seasonal freezing and thawing. If the top of the leg freezes, it won't affect the bearing pressure on the pylon any more than if it was covered with snow. But if the bottom of the leg freezes, it could cause the center of gravity of the structure to shift and possibly damage it. Freezing and thawing of the soil under a bridge may also have an effect on the structural integrity of the bridge. As the temperature changes, the amount of water in the soil expands and contracts, which can cause the soil to dry out or flood, which can lead to erosion. If a drilled shaft becomes saturated with water, it can cause the bridge to collapse. The weight of the traffic on the bridge can force even more water into the shaft, causing the bridge to sink further into the mud.