166 philbrick. FOUNDATION PROBLEMS [Ch. 8 



be present in the form of coal, clay, or limestone mines. In the case of 

 structures requiring support only, it is not uncommon now to find the 

 site of the structure underlain by a coal mine which may have been 

 abandoned for many years or on which few or no data are available. 

 Such a mine may be accessible only under dangerous and adverse con- 

 ditions and not infrequently only through new shafts or entries. Such 

 mines may be grouted (Philbrick, 1948) or backfilled with stable, free- 

 draining materials as is common at present in the anthracite fields of 

 Pennsylvania. If sufficient data are available on the extent of mining, 

 the distribution of pillars, and the character and thickness of over- 

 lying materials, it may be feasible to disregard the mine entirely. If 

 the mine is on fire, it is wise to find another site rather than attempt 

 to put out the fire and utilize that site. 



Soluble Sedimentaky Rocks 



Much has been written on the foundation problems of soluble sedi- 

 mentary rocks, and feasible solutions to the problems of leakage control 

 have been developed (Lewis et at., 1941). Although tests of uncon- 

 fined 6-inch cores of the Ocala limestone have indicated compressive 

 strengths of as low as 64 pounds per square inch, in general the com- 

 mon limestones and dolomites are ample in supporting capacities both 

 in shear and in compression for a dam less than 300 feet in height. 

 The nature of the rock and the usual rough surface developed during 

 construction reduce to an academic consideration the problems of 

 failure by sliding. However, limestones may carry, close beneath the 

 base of the proposed structure, thin partings of shale which require 

 consideration of their properties in the design of the structure or ex- 

 cavation to below the shale. Certain limestones carry bentonite beds 

 which require similar investigation and treatment. It is generally as- 

 sumed that the rate of solution of limestone and dolomite is sufficiently 

 slow to have no bearing upon the safety of the structure. 



Chalk, such as the Niobrara in Nebraska (Happ, 1948) poses a dif- 

 ferent problem. Happ has described the fresh Niobrara as a fairly 

 dark gray, fine-grained, compact, brittle, soft chalky limestone cut by 

 thin horizontal layers of bentonitic clay ranging from a small frac- 

 tion of an inch to 4 inches in thickness, the majority being very 

 thin. It has an average dry weight of about 100 pounds per cubic 

 foot and a compressive strength ranging from about 250 to 2,400 

 pounds per square inch and averaging about 1,000 pounds per square 

 inch. The bentonitic clay layers in laboratory shear tests showed 

 values in cohesion ranging from 0.1 to 0.4 ton per square foot with 

 tan <£ ranging from 0.06 to 0.23. In this case the problem is not that 



