Note Return to text — EXPERIMENTS ON RETAINING WALLS AND PRESSURES ON TUNNELS.

A simple way of deriving these extreme values is given in the Appendix. The values of , and have been taken here the same as those used by Mr. Meem, in framing his table of pressures,[Footnote 13] which may be supposed to embody, in part, practical experience. The results found from Figs. 17 and 18 by the writer, for a depth of covering of several hundred feet, are uniformly much larger than those given by Mr. Meem. Are they too large for safety? 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. The results are submitted for discussion, in the hope that engineers will give their experience relative to the pressures realized in the timbering of tunnels, particularly through sand or earth not thoroughly consolidated.