“[In answering this question], it must be remembered that, of the weight of earth directly over the tunnel, all has been transferred to the sides that it was possible to transfer, for the coefficients of friction and cohesion given. We know scarcely anything of the cohesion coefficients, so that the value assumed,
lb. per sq. ft., may not be near the truth. Certainly it must appear plain from this discussion that the values of
and
must be better known, for all kinds of earth, before reliable results can be attained.”
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“[In reality], an arch or dome of the earth should be considered in place of the horizontal stratum, but the result is the same, because the same vertical forces act in either case.”
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