Like snrub said, for sealing, the clay needs to be very well compacted and on the front side to seal. The water pressure, though not high, will push against the seal and the weigh of the rest of the Dam provides a solid structure. On the back side, the water would simply go through the Dam soils, lift the clay, and run under it, or find a weak spot and push through. Since the clay needs to be/remain slightly moist and elastic to seal, it must have a solid structure behind to support it.
