920 EXPLORATION GEOPHYSICS 



Spillway and Outlet Diversion Tunnels. — The low saddle on the 

 left abutment should be considered for the spillway location. Here a 

 spillway would rest on hard andesite, but it should be lined. The only 

 stripping necessary would be the removal of the surface weathered zone of 

 about 5 feet. 



From a geological standpoint the outlet tunnel could be placed on either 

 side of the river. The rocks of both abutments should prove suitable for 

 tunneling. 



Dike No. 1. — The abutments of this dike section are fairly hard 

 andesite, and the foundation rock is probably breccia. The overburden 

 has a maximum thickness of ± 84 feet and consists of light gravels, sands 

 and silts. 



Dike No. 2. — This dike section is similar to the other except that 

 the left abutment is a breccia. The overburden is similar but shallower, 

 ± 55 feet. 



Construction Materials 



Impervious embankment material is plentiful. Graded gravels, sands 

 and silts 3 to 6 feet deep cover the valley floor, while deeper deposits 

 (±15 feet) are to be found on the terraces above the flood plane. 



Free draining material (gravels and cobbles) can be found in the river 

 channel beneath the top soil. The terrace area between the main river and 

 the Rio Grande Canal to the east of the dam site should also be considered. 

 Here gravels underlie only 2 to 3 feet of the top soil, and the deposits 

 should be satisfactory for aggregate. They are within one mile of the site 

 of the dam. 



Riprap can be quarried from any of the adjacent andesite or breccia 

 hills. 



Results of Seismic Survey 



The maximum depth to the bed rock of andesite or breccia was 136 

 feet, as determined by seismic refraction traverses at the Del Norte Dam 

 Site. However, bedrock may be deeper than this near the center of the 

 present flood plane, which was not accessible for geophysical work. Depths 

 to bedrock were also determined in the dike sections. The maximum depth 

 at Section No. 1 was 84 feet and at Section No. 2, 55 feet. The geology 

 and the geophysical depth points are shown in the two cross-sections which 

 form one continuous line in Figures 571 and 572. 



It is of note that no drill control was available to check the bedrock 

 depths predicted. They appear to be reasonable, although even one drill 

 hole would have removed any uncertainty in this respect. The geophysical 

 survey is therefore of a reconnaissance nature, although velocity deter- 

 minations in the exposed outcrops of bedrock material in work of this kind 

 are often useful for control. 



