158 philbrick. FOUNDATION PROBLEMS [Ch. 8 



Permeability 



Water may move through the intergranular pore spaces in sedi- 

 mentary rocks as well as through channels along bedding planes, 

 fractures, joints, faults, and solution channels. Two general founda- 

 tion problems arise from permeability of the rock: uplift and leakage. 

 Uplift is the buoyant effect of water on a submerged foundation. 

 Leakage is the passage of water through a foundation and is of primary 

 importance in the determination of the economic feasibility of a proj- 

 ect. 



Uplift has been the subject of considerable thought and discussion 

 recently, much of which has been summarized by Harza (1947) and 

 others with the result that it is commonly assumed in the design of a 

 structure with a submerged foundation that the entire base of the 

 structure is subject to uplift pressure, regardless of the type of rock 

 upon which it is founded. There remain then to be determined the in- 

 tensity of the uplift at any point in the foundation and a means of re- 

 duction of that pressure. The pressures may be closely determined by 

 the construction of a flow net. Construction of a grout curtain will 

 reduce the leakage but not the uplift pressure. The reduction of the 

 uplift pressure can be effectuated only by provision of relief. This is 

 usually in the form of nearly vertical drainage wells or drain holes 

 drilled in the foundation rock downstream from the grout curtain, as 

 close to the zone of maximum pressure as feasible and connected to a 

 collector system in the basal portion of the dam. The direction of these 

 holes, their size, spacing, and depth should be dependent upon the 

 type and structure of the foundation rock and its post-construction 

 characteristics, because the methods of construction and type and 

 extent of foundation treatment may modify or obliterate the necessity 

 for draining certain elements of the foundation rock. 



Leakage beneath structures has been encountered in varying degrees 

 in projects located on all types of sedimentary rocks. Leakage can be 

 reduced to a negligible minimum by finding and cutting off stratifica- 

 tion, fracture, and solution channels that would allow water to by-pass 

 the structure. The prime purpose of leakage reduction is to prevent 

 the escape of water for whose storage the structure was built and upon 

 which storage the economics of the project depend. The safety of the 

 structure should not be endangered by the free passage of water through 

 its foundation. Although limestone is only slowly soluble, it, like any 

 other rock, is subject to mechanical erosion. Theoretically, then, it 

 might be permissible to reduce leakage in a limestone foundation to a 

 point where velocities of flow are mechanically non-erosive, if the 



