“[Referring] to [Fig. 20], it is evident that the maximum limit of
would be realized if the weight of any horizontal lamina is entirely held up by the friction and cohesion of the sides; * * *”
* * * * * * * * *
“[As seen], such a state is not exactly realized, but is practically true for great depths.”
Fig. 23.
Referring to the last quotation, the writer would go further and say that if the assumption is true that the spaces above a tunnel are considered as a series of horizontal layers dependent on the natural coefficient of friction and cohesion (not added to by pressure) to hold them up, that it would appear to be far preferable to calculate always on full pressure to the top than to assume that some of these strata may be sustained by what would appear to the writer to be largely chance conditions.
It would appear that the author has considered cohesion and friction only as normally found in exposed faces, and as they would be developed between contiguous vertical columns of earth through which pressures were transmitted laterally; and, in tunnels as against vertical faces, he does not appear to have given sufficient weight to the essential factor that cohesion and friction, combined into what the writer has previously termed “cohesive friction,” are increased by the pressure in some definite relation to it.
If, for example, on a tunnel section,