Inferior is always possible that some water may be forced into imperfect joints in the masonry and, if it be prevented from escaping at the lower side of the darn, have the full hydrostatic pres sure of time head in the reservoir. For this reason it is important that the water face of the (lain be made as nearly impervious as possible, and that the interior of the clam be drained so that any water passing into the masonry may escape without damage. It is evident that uplift of the interior of the masonry can exist only where continuous joints for considerable distances are filled with water under pressure.
If concrete be porous and its voids filled with water under hydrostatic pressure, no uplift occurs until the pressure becomes sufficient to over come the cohesive strength of the concrete. In properly constructed masonry- dams, it is usually unnecessary to consider the effect of uplift on sections above the base of the dam.
Upward Pressure on probability of uplift under the base of a dam depends upon the character of the foundation. Care ful attention should always be given to the determination of the character of the foundation material to considerable depths below the base of the dam. The kind of material of which the foundat ion is composed, and the existence of seams in the rock, or of strata of permeable material must be accurately investigated.
When the foundation is of solid rock without seams, if care be used in joining the base to the foundation and cut-off wall be used under the inner edge of the base, there is little chance of appreciable uplift under the base.
When the foundation is permeable and there is water against both sides, as is frequently- the case in dock walls, the full hydrostatic head is usually considered to act under the whole base. This is some what excessive, as it implies that the dam is floating upon a continuous surface of water. Probably two-thirds of this pressure would repre
sent about the maximum which could reasonably be expected in any case.
When the foundation is stratified horizontally, so that water may be expected to pass under the darn and escape below, a uniformly di minishing upward pressure from the inner to the outer edge of the base may be assured; the pressure at the inner edge being taken at about two-thirds the hydrostatic head above the darn, and that at the lower edge at zero.
The probability of upward pressure on the foundation should always be carefully investigated, and the section, where necessary, increased sufficiently to provide weight of masonry to overcome the overturning moment of this water pressure. This subject is very fully treated in the discussion of a paper by the late C. L. Harrison in the Transactions of the American society of Civil Engineers for December, 1912. Mr. Harrison's conclusions are: 1. For any stable darn, the uplift in the foundation cannot act over the entire area of any horizontal seam, and in the masonry it cannot act over the entire area of any horizontal joint.
2. The intensity of uplift at the heel of the dam can never he more, and is generally less, than that due to the static head. Also, this uplift decreases in intensity from the heel to the toe of the dare, where it will be zero if the water escapes freely, and will be that due to the static head if the water is trapped.
3. The uplift in the foundation should be. minimized by a cut-off wall, under-drainage, and grouting when applicable; and in the dam itself by using good materials and workmanship, and by drainage when advisable.
4. The design should be based on the conditions found to exist at each site after a thorough investigation by borings, test-pits, and otherwise, and modified if found necessary after bed-rock is uncovered.