648 whitmoee. MILITARY GEOLOGY [Ch. 34 



and problems of construction in unfamiliar environments, especially the 

 tropics, the sub-Arctic, and the Arctic. 



Concerning the use of substitute materials, little more need be said. 

 Once the necessity for such substitution is recognized, it is obvious that 

 granite, despite its obvious shortcomings, can be used as road metal 

 in place of basalt; that ordinary surface loam can be used as binder 

 instead of clay or can be stabilized with some substance such as bitu- 

 men to serve, say, as an emergency airstrip surface. The geologic 

 principles used in locating such substitute materials are the same as 

 those applied in the search for the best possible ones. 



It is the strange environment that now, as during the war, presents 

 the greatest challenge to the geologist, engineer, and soil scientist who 

 are trying to predict terrain conditions in foreign areas of operation. 

 All three made predictions and calculations, based on training and 

 experience in temperate zones, which did not apply elsewhere. It was 

 at first taken for granted, for instance, that the moisture content of 

 tropical clays could be reduced by working and reworking with earth- 

 moving equipment. This proved to be far from true, because the clays 

 in question were friable or "lateritic"; when worked carefully they are 

 well-drained, but excessive working turns them to a morass. Another 

 example, seen on Leyte, was the weathered condition of the interior of 

 pebbles in the high terrace gravels that rendered them useless for 

 construction (Putnam, unpublished manuscript) . 



Because of the depth of weathering in the tropics, interpretation of 

 geology from aerial photographs is likely to be difficult on a physio- 

 graphic basis alone, especially wfien such a control as knowledge of the 

 stratigraphic column of part of the area is lacking. For this reason, 

 vegetation has become important in the search for construction mate- 

 rials in the tropics (U. S. War Department, 1944a) . 



Coral is another tropical material in the use of which American 

 engineers had little experience until World War II. Even now much 

 remains to be learned about its properties as construction material, 

 for coral differs widely in hardness, toughness, texture, and case-hard- 

 ening properties. These affect not only its use as a building material, 

 but also its properties as a subgrade or foundation. 



Coral sand and gravel and crushed coral were used extensively during 

 the war for the foundation of airfields and other structures. As base 

 material and foundation rock, coral was poor to good in proportion to 

 the clay content ; a thick overburden of weathered coral also decreased 

 its usefulness for these purposes. Massive coral requires blasting and 

 crushing; therefore, because of the demand for speed in construction, 

 the softer portions of the coral deposits were used, and thereby more 



