Geologic Techniques in Civil Engineering 1147 



here reported is based: namely, that small gravity differences may reflect 

 a density variation in the material beneath a particular station. The 

 density value used in the Bouguer reduction, which gives the corrected 

 gravity stations on a profile line the smoothest character, is the average 

 density of the materials above the datum plane. 



Malheur Siphon 



The Malheur siphon is a feature of the irrigation system in the state 

 of Oregon. It is approximately 23,000 feet long, and the portion under 

 consideration consists of welded steel pipe 80 inches in diameter, sup- 

 ported at intervals by concrete piers. Within three years after comple- 

 tion the piers were being displaced laterally and vertically, the displace- 

 ment being of such magnitude that the siphon kinked in one place and 

 threatened to fall from its support in other places. The field geologist 

 reported that shales of the Payette formation formed the foundation of 

 the structure, and he obtained undisturbed samples for study by the 

 petrographic and earth-materials laboratories, where it was found that 

 the shales contained bentonite and would swell conspicuously when wet. 

 Further field studies disclosed that the shales continued downward to 

 great depth, and the laboratory undertook to determine the swelling 

 pressure so that a calculation could be made of the depth at which the 

 swelling pressure would be equaled by the superincumbent load; any 

 remedial measures to be adopted would necessarily have to be applied 

 down to this depth. It was found that the swelling pressure equaled the 

 weight of the empty siphon plus sixty feet of overburden, or the full 

 siphon plus thirty feet of overburden. Inasmuch as the topography would 

 not permit gravity drainage to such depths, it was decided to investigate 

 the possibility of cutting off the source of the water that was satuiating 

 the shales. Observation holes were drilled over the surrounding area to 

 permit a detailed analysis of the ground water. A contour map of the 

 water table disclosed the direction of ground-water flow and indicated that 

 ground-water recharge was occurring through leakage from an adjacent 

 canal. The materials laboratories designed an impervious asphaltic lining 

 for the canal, and after its installation no further displacement of the 

 siphon has been reported. Opportunities for this kind of field-laboratory 

 cooperation are becoming more frequent and important with the use of 

 consolidated materials as foundations. 



Palisades Dam Site Intrusive 



At Palisades dam site in southeastern Idaho, preliminary drilling 

 disclosed in the left abutment a tilted tabular mass of andesite 300 feet 

 thick, underlain and overlain by beds of clay, silt, and conglomerate. 

 Preliminary designs contemplated three large tunnels in the left abut- 

 ment, two for outlet works and one for a spillway. The geologist was given 



