Table of Contents
Dams are constructed to retard the flow of water or debris; therefore natural forces are constantly at work to remove such obstructions. To circumvent nature and avoid expensive trouble with a dam requires thoughtful far-sighted planning, careful construction control and vigilant periodic inspections to detect and repair deterioration at its inception.
Although the factuality of the foregoing statements is apparent, all too many have failed to consider them in the past as will many in the future. “The economic, but safe construction of dams calls for the highest order of technical knowledge, practical experience and sound judgment.”
Special Geologic Considerations
A dam may be defined as an impervious membrane which is provided with some means of support and appurtenant facilities to permit the discharge of excessive quantities of flood water, as well as a means of utilizing the water from the reservoir, preferably without pumping. How this impervious membrane and its spillway and outlet works are constructed depends, or should depend, primarily on the local natural conditions, such as rainfall, foundations, abutments, available construction materials and the topographic conditions as they influence the location of the spillway and outlet works and stream diversion schemes. Arroyos have the unfortunate habit of flowing at the wrong time.
Where does geology come in? In true geologic fashion, the question will be answered by discussing something else first. If a dam is successful, it creates a lake or pond behind it. The fact that a successful dam creates a lake is all too frequently overlooked, so we will discuss the lake first.
The construction of a dam and the creation of a reservoir upsets nature’s delicate balance more than any other act of the mining or civil engineer. The weight of the dam is obvious to almost anyone, if he thinks about it, yet few even consider the infinitely greater weight of water in the reservoir behind the dam. Only an exceptional few reflect upon the greatest change of all, which is the unseen change in the surface of the underground water table. As the lake fills, the water table in the hills adjacent to the lake rises. Not infrequently, the amount of underground storage exceeds the amount of storage in the reservoir.
Masonry and Concrete Gravity Dams
Masonry dams were largely supplanted by concrete gravity dams early in this century. In this type of dam, if the base width is 84 percent of the dam’s height, the dam is theoretically safe from overturning or sliding. However, in most cases the designers have narrowed the base width to save concrete and depended upon grout curtains, drainage systems, sliding friction, etc. to keep the dam in place. Not all of these efforts have been successful. Some advocates of the 84 percent base width use California’s disastrous 1928 St. Francis Dam failure to demonstrate the soundness of their viewpoint. The higher central portions of that dam had a base width of about 84 percent and remained in place even after the narrower abutment sections slid and released a rush of reservoir water which swept them downstream causing great destruction of life and property.
Earth and Rock Fill Dams
The economic advantages of these types of dams continue to increase with the improvement in hauling and placing equipment. Also, a properly designed and constructed embankment requires less maintenance. At the present time, perhaps over 80 percent of the major dams in the United States are of this type, and the percentage is increasing. Perhaps well over 90 percent of the mining dams are of this type. In fact, the placer miners developed the first rock fill crib dams over a hundred years ago! Therefore, this discussion will be limited to the topographic, foundation, construction material and design concepts of earth and rock fill dams.
Flood waters are usually diverted around the dam embankment by means of a spillway which discharges, preferably, into an adjacent valley. A tunnel, of large diameter, or in some cases a pipe termed “the outlet works” is usually required for the diversion of stream flow and flood waters during the construction of the dam. This is subsequently utilized for the release of reservoir water with the valves on the upstream end to alleviate hydrostatic pressure. The cost of these two major appurtenant facilities usually ranges between 30 and 50 percent of the total cost of an earth or rock fill project—a fact that is frequently initially overlooked. The cost of access roads, operator’s quarters, security fences, etc. are also often unfortunate surprises. Careful consideration of the topography and subsurface conditions of alternative potential construction sites usually discloses advantageous conditions regarding these appurtenant facilities which results in appreciable reductions of the construction costs.
Importance of Dams to Mining
When one recalls the El Cobre disaster, and then begins to recognize the enormity of the maintenance problem, he is naturally discouraged. However, if miners were not a group of hopeless optimists, there would not be a mineral industry. It may therefore be concluded that Barnum’s profound statement regarding “one being born every minute” will continue to be valid.
Most of you will agree with that ancient sage who said, “Where a wealth of mineral abounds, little else is seldom found.” Those of you who have survived the development stage of a mine or a mill recognize the little-understood fact that water, next to air, is the most important material required for the support of human life. We can do without air for a few minutes; we can do without water for a few days. Moreover, in the mining industry we need even additional water for operating purposes. In mining areas the annual rainfall is usually far too scanty to permit mining and milling operations dependent upon stream flow. Therefore, the storage of water and the reclaiming, recirculation and reuse of water is an important part of most mine and mill installations. Storage and reuse is accomplished by means of dams which are usually cheaper than tanks.
Principals of Earth and Rock Fill Design
For many years, the writer maintained that a dam could be constructed of dead cats—if time and money were available. The cross section of the dam so designed illustrates the principal design features of the more common types of earth or rock fill embankments where free-draining material is available. Depending upon the circumstances, some have an upstream core and others are so impervious as to require drains.
The relatively impervious core is constructed of a blended mixture of powdered carbon and very fine bones. The carbon fills the voids between the very fine bones and provides a relatively water tight central portion for the dam. The very fine bones act as grit to provide some strength to the core material, and thereby reduce the detrimental fluid pressure.
The fine bones are placed in zones on both sides of the core to permit slow drainage and to prevent any washing out of the very fine core material. The medium-sized bones are placed in a thicker filter on each side of the fine bones to keep the fine bones from washing out. To utilize the remaining medium-sized bones, they are either blended with the larger bones or the thickness of the outer filter is increased. This blend is placed in the upstream and downstream shells which hold the core and filters in place.
The most dangerous condition in such a dam would occur if the reservoir was lowered rapidly enough to permit the water draining upstream from the saturated portions to burst the shell. Because of this, far more dams fail on the upstream side than on the downstream side.
A Well-Considered Future Development
One of the first expenses of any dam construction project is, or should be, the determination of the quality and quantity of construction materials and the subsurface conditions at all potentially feasible construction sites. Whether this expense is an investment which usually returns far more than its cost or an expenditure that leads to further and often larger expenditures depends on the manner in which this work is performed and on whether or not the investigator is allowed sufficient time to consider, reconsider, and finally select the appropriate scheme for the most economical utilization of the natural conditions and construction materials. Judgment is opinion and is the mental process by which one becomes aware of the differences and similarities in that which one perceives. It may be good or bad, depending upon the adequacy of the comparison of the facts and influences. Snap judgments are almost synonymous with poor judgments.