hatch for approach 
from submarine alternative spoil removal 
ve lock in small shaft 
VE spoil removal seawater 
lock 
ee 
= Lai a. 
sediment 
eae a 
main entry 
small diameter shaft 
lateral | 
excavation | 
lateral 
excavation 
Figure 61. Spoil removal schemes. 
Some notes on the important physical characteristics and weaknesses 
of rocks, some observations on how mechanical penetrators probably function, 
novel drilling methods which have been considered, and possible additional 
approaches which appear to have merit are discussed below. 
Walsh (1965) gave a thumbnail sketch of the physical nature of hard 
rock. Paraphrasing his remarks may provide a worthwhile understanding of 
the nature of the material. Under photomicrographic examination, rock 
appears similar to many metals. It has grains and grain boundaries just as 
do the metals, and little difference is seen upon superficial examination. 
However, upon close examination, differences are seen. A network of fine 
cracks on the order of 1 mm long by 0.001-0.0001 mm wide can be seen 
within the grains, along the grain boundaries, and across the grains and 
boundaries. These cracks are probably caused by extreme anisotropy of 
the individual grains. The rock is solidified deep in the earth at very high 
pressure and upon release of this confining pressure when the rock is brought 
to the surface and to atmospheric pressure, stresses high enough to produce 
the fine crack network are generated. The crack network interconnects to 
78 
