1148 Subsurface Geologic Methods 



tentative locations and elevations of the proposed tunnels and was re- 

 quested to determine whether the tunnels could be kept exclusively in the 

 andesite. 



The solution of the problem lay in determining the regularity of the 

 lower surface of the andesite body. It was reasoned that if the andesite 

 had been emplaced as an extrusive flow during the deposition of the 

 surrounding sedimentary series, the lowest surface would be roughly 

 regular and little or no drilling would be necessary to establish its posi- 

 tion with respect to the tunnel lines. On the other hand, if the andesite 

 was intrusive in the sedimentary series, the lower surface could have any 

 configuration whatever; hence the relationship of the lower surface of 

 the andesite to the tunnel line could be determined only by extensive 

 drilling. Field studies were inconclusive, but petrographic analysis of 

 thin sections of the andesite and the adjacent sediments demonstrated 

 that the andesite was intrusive. The additional drilling could not be 

 eliminated from the exploration program. 



Columbia Basin Landslides 



Grand Coulee dam impounds Lake Roosevelt, which is 150 miles in 

 length. The lake fills a gorge of the Columbia River, which during glacial 

 times was dammed by ice and received lake sediments that were deposited 

 in bars to thicknesses of hundreds of feet. These sediments, known as 

 the "Nespelem formation," were subsequently trenched and now form 

 terraces, which stand high above the lake's surface. Some large pre- 

 historic landslides had occurred before the reservoir was filled, but new 

 slides occurred in considerable numbers after the lake was impounded, 

 especially when the lake was drawn down from a high to a lower level. 



Farms, highways, and a railway would be endangered if slides 

 occurred at certain locations on the reservoir rim. To determine the extent 

 to which such areas should be condemned and reserved from use, it was 

 necessary to classify the entire reservoir rim in terms of relative landslide 

 potentialities. A geologic survey classified the different kinds of materials 

 exposed on the edges of the reservoir, measured and mapped the old and 

 new slides in both plan and profile, identified different types of Nespelem 

 silt, and obtained samples for laboratory study. 



Although the immediate engineering problem of how much land and 

 which land to remove from use was quickly solved, the laboratory studies 

 are continuing in the interest of analyzing further the mechanics of land- 

 slides in general. Petrographically, the Nespelem sediments are ultrafine 

 rock flour of fluvioglacial origin; the particles are clay-size, but mineral- 

 ogically they are mainly quartz and feldspar. The permeabilities, angles 

 of internal friction, apparent cohesions, pore pressures, and related prop- 

 erties have been determined for the various facies of the formation. 



One aspect of the Nespelem sediments has intrigued our analysts. 



