Ch. 10] 



SPILLWAY STRUCTURE 



185 



cated on expansive formations such as the Bear Paw shale encountered 

 at Fort Peck Dam presents a problem for further soil mechanics re- 

 search. At Fort Peck the expansion of the shale foundation under 

 the spillway chute and approach slabs due to release of load in the 

 spillway cut has been extensive. Maximum upheaval in the center of 

 the chute has been over 12 inches. Upheaval of the gate structure, 

 which was constructed on caissons to a depth of 40 feet, has been nil. 



2,100'±to river at El.315± 

 500' ±- 



Backfill 



Fig. 5. Saddle spillway, Wappapello Dam. Foundation is hard, blocky limestone. 



Infrequent operation. 



200'+- 



El.592.0- 



50 '—I 0.5% > } > 



-To river 16,000' + at El.390±- 

 •1,000'± 



Fig. 6. Saddle spillway, Blakely Mt. Dam. Foundation is firm shale, highly 

 folded. Infrequent operation. 



Chute spillways are often founded on weak rock. Drainage under 

 the chute section and stilling basin similar to that supplied for soil 

 is generally required. Usually it is possible to anchor the slabs into 

 the rock, thereby decreasing the chute and stilling-basin slab thick- 

 nesses over those required for soil. The upstream cutoff is usually 

 in the form of a concrete key. 



Saddle spillways without concrete pavement in the chute section 

 are employed extensively where flow occurs infrequently. This type 

 of spillway varies in design with foundation conditions and elevation 

 of foundation rock in the saddle. At the Wappapello Dam (Fig. 5) 

 a small ogee concrete section with a flip-up bucket was employed. At 

 Blakely Mountain Dam (Fig. 6) a 50-foot paved section with a cutoff 

 upstream and downstream forms the control. There are numerous 

 cases where the rock itself is adequate for the control structure and 

 no concrete weir or paving is required. There are also a few cases 



