LibraTech:
you can try the technical report IF-99-025 "Drilled shaft: construction and design methods" published by FHWA or the EM-1110-2908 "Rock Foundations" by the U.S. Army Corp of Engineers.
A comment on always "weak" rock, as in soil clasification there are several methods for instance USCS or AASHTO, in rock classification there is also some guidelines to clasify rocks, I mean, I'm not geologist but if I can remmeber well, in fact the term soft rock already exist and is used to clasify rocks in field according to its hardeness, the criteria is something like
very soft- can be deformed by hand
soft - can be scratched by finger nail
Moderately hard- can be easily scratched with a knife
hard - can be scratched with dificulty with a knife
very hard- can not be scratched with a knife
Now "weak" rock seems to relative for me, for insatnce someone who is only familiar with a region where igneous rocks are the outcropping rocks, is acostum to deal with "strong" rocks or may be heavy or hard rocks? Lets say this rocks have RQD values higher than 90 and compressive strengths higher than 7000 psi, wow! that's a "strong" rock for someone who live in a reagion where sedimentary rocks outcrop, let say shale with RQD values ranging from 30 to 60 and compressive streghts of abouth 2800 psi that seem to a "weak" rock, well to avoid those interpretation clasification methods give the following descrptions
The first rock may be clasified as:
Basalt, dark gray,of excelent quality (based on RQD) slighlty weathered,hard or very hard (based on Hardeness), fine grained, vesicular perhaps, massive I'll say, moderatly to slightly fractured. Perhaphs this rock could be referred as sound or intact rock not as hard or strong rock
The second rock can be calsified as:
Shale, greenish gray, very poor to fair quality (again RQD), maybe moderately to slightly weathered, soft or mederately hard (Hardeness too),very fine grained, medium to thick bedded, highly to moderately fractured, dipping, moderately to highly weathered joints, in this case this rock usaully is referred as weathered, fractured or soft rock
In the other hand, soft sand it is in fact an abomination