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19 PRESSURE OF RESERVOIR , , , , , I I I I I , , RESISTANCE BY ABUTMENTS AND FOUNDATION FIGURE 3.~ OVERHEAD VIEW OF ARCH DAM The view above shows how the pressures from the reservoir tend to flatten the arch and how the action is resisted by the abutments and foundation. The failure of an arch dam can occur if one abutment releases its support. A concrete arch dam usually requires less material than a concrete gravity dam. But the arching action requires a very suitable founda- tion and abutment area. 3.6-3 SPECIAL FOUNDATION TREATMENT Like earth or rockfill dams, a concrete dam has special provision for controlling seepage under the dam. The most common method is pressure grouting a line of holes into the foundation and abutments before the dam is constructed. The cement grout will fill most voids or fractures in the rock. The sketch below illustrates the difference between the flow paths with and without grout treatment. ....._ ____ "I ----- .. Without TreRtment FIGURE 3....5 GROUTED CUTOFF This method is not limited to concrete dams, but can also be used to control seepage under an earth or rockfill dam. If a dam is designed and constructed to perform safely the functions of (I) holding back water, (2) resisting movement, and (3) maintaining shape, it can have a long, successful life span. 3.7 SPILLWAY A spillway is required for a dam to allow excessive inflows to pass the reservoir site without overtopping the dam. The spillway may be in natural terrain, exca- vated in rock, or constructed of concrete. A spillway can be located on either abutment or con- structed as part of the dam. Sometimes a natural drain- age channel in the reservoir is used to carry floodwater safely around the dam. The location is selected based on the topography, size of expected storm, and eco- nomics. The adequacy of an existing spillway requires evalua- tion by a professional engineer and is beyond the scope of this manual. The owner can, however, inspect the spillway for damage and deterioration. Chapter 10 lists the signs to look for and indicates whether the damage requires maintenance or the attention of an engineer. 3.8 OUTLET SYSTEM The primary purpose of an outlet system is to provide for controlled release of the water from the reservoir behind the dam. Upon demand, the outlet system releases water downstream for irrigation or other uses. The system is also used to lower the reservoir in an emergency or for maintenance and repair of the dam and appurtenant structures. The size of the outlet system is determined by the rate ofthe demand for use ofthe water. The adequacy ofthe outlet requires evaluation by a professional engineer. The owner should, however, inspect the outlet system for damage and hazardous conditions. Chapter 9 pre- sents a list of important signs the owner should look for to identify situations which may endanger his dam. 3.9 SUMMARY This chapter applied the principles of Chapter 2 in explaining the behavior of dams and how the design fulfills the purpose of holding back water. The remainder of the manual specifically addresses the inspection and maintenance of earth dams. However, the similarities between the different types of dams should enable the owner to apply his knowledge to inspection and maintenance of other types of dams.