Ch. 8] CONCLUSION 167 



of providing effective cutoff of pre-existing water channels as in lime- 

 stone terranes, but rather of designing a structure on soft rock cut by 

 seams of softer rock, the latter being subject to disintegration upon 

 exposure and to expansion upon release of load with the removal of 

 overlying materials. 



Some structures have been built on gypsum and on salt, and others 

 are being considered for sites containing these rapidly soluble sedi- 

 ments. The problems concern the geologic dating of collapse of caverns 

 leached in the gypsum, the determination of the present water table, 

 and the effect of the increased height of the water table on the rate of 

 solution of the soluble sediments. Where it has been proved that the 

 gypsum is effectively cut off by impermeable beds from the additional 

 hydrostatic head, reservoirs have been successfully constructed and 

 operated over gypsum. For salt the situation is similar. The general 

 rule in considering foundations containing rapidly soluble sediments 

 is that another site is probably better. However, if no other site is 

 available, the site should be considered as one on which a flexible 

 structure capable of withstanding some settlement should be designed 

 because there are as yet available insufficient data to warrant an as- 

 sumption of the rate of solution of these sediments. The laboratory 

 rate of solution of gypsum in a bath may be highly misleading, or it 

 may be approximately the rate of solution in situ. 



CONCLUSION 



In many cases it may be more economical and equally desirable to 

 design for the worst rocks in the foundation, even though this may 

 mean an apparently more expensive structure, than to attempt to raise 

 the values of these rocks or to remove them and found on stronger 

 materials or choose another site where better rock may be present 

 closer to the ground surface. It should be recognized that foundation 

 treatment (the all-inclusive term applied to "monkeying" with a foun- 

 dation) can be most astonishingly expensive even when compared with 

 the cost of a major project. On the other hand, it may be considerably 

 cheaper to invest a large sum in improving foundation conditions at a 

 site than to reduce the magnitude of the structure and build other 

 smaller supplementary structures, particularly in the case of a coor- 

 dinated river development scheme or in the case of a high bridge. The 

 economic treatment of foundation problems is intimately related to 

 the general engineering and economic features of the entire project, 

 and they may dictate the general permissible scope of the foundation 

 treatment. 



