484 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
Fig. 7 
the rock condition was sufficiently sound so that no lining would be required. 
This evaluation was proved to be correct during subsequent drilling and con 
struction. Fig. 10 indicates a proposed crossing of Dry Strait in Alaska. Tides 
in this area reach up to 22 feet. The photo taken in low tide shows the channel 
(dotted lines) which must be bridged to allow ship traffic to pass; the re 
mainder of the crossing can be a causeway. This project was a joint effort of 
Knorle Graef Bender Associates and Aero Service Corporation of Philadelphia. 
Fligh water table and seepage conditions can frequently be assessed by the 
photo interpretation technique. In fig. 2, the vegetation in the area of the 
beaver dams indicates the presence of a high water table. 
A hnal special problem to be considered is troublesome soils such as dif- 
ficult-to-compact silts, volcanic ash and saline soils. These frequently can be 
assessed by photo interpretation by their erosion pattern. Volcanic ash calls 
for special engineering considerations including the necessity of importing 
borrow material for embankment construction. It can be identified on the 
photos by its special weathering pattern. 
Concluding remarks 
It has been proved beyond doubt that photo interpretation can be used to 
great advantage in the various stages of route location. The authors firmly