For the foregoing reasons, and particularly from observations on the effect of earth pressures on wooden timbers used as bracing, the writer believes that, ordinarily, the theoretical earth pressures computed by Rankine and Coulomb are not realized by one-half, and sometimes not even by one-third or one-quarter in trenches well under-drained, rapidly excavated, and thoroughly braced.

J.C.  MEEM, M. AM.  SOC.  C. E. (by letter).—­The writer has been much interested in this discussion, and believes that it will be of general value to the profession.  It is unfortunate, however, that several of the points raised have been due to a careless reading of, or failure to understand, the paper.

Taking up the discussion in detail, the writer will first answer the criticisms of Mr. Goodrich.  He says: 

“The writer believes that, in the design of permanent structures, consideration of arch action should not be included, at least, not until more information has been obtained.  He also believes that the design of temporary structures with this inclusion is actually dangerous in some instances.”

If the arching action of earth exists, why should it not be recognized and considered?  The design of timbering for a structure to rest, for instance, at a depth of from 200 to 300 ft. in normal dry earth, without considering this action, would be virtually prohibitive.

Mr. Goodrich proceeds to show one of the dangers of considering such action by quoting the writer, as follows: 

“About an hour after the superimposed load had been removed, the writer jostled the box with his foot sufficiently to dislodge some of the exposed sand, when the arch at once collapsed and the bottom fell to the ground.”

He fails, as do so many other critics of this theory, to distinguish the difference between that portion of the sand which acts as so-called “centering” and that which goes to make up the sustaining arch.  The dislodgment of any large portion of this “centering” naturally causes collapse, unless it is caught, in which case the void in the “centering” is filled from the material in the sustaining arch, and this, in turn, is filled from that above, and so on, until the stability of each arch is in turn finally established.  This, however, does not mean that, during the process of establishing this equilibrium of the arch stresses, there is no arching action of any of the material above, but only that some of the so-called arches are temporarily sustained by those below.  That is, in effect, each area of the material above becomes, in turn, a dependent, an independent, and finally an interdependent arch.

If Mr. Goodrich’s experience has led him to examine any large number of tunnel arches or brick sewers, he will have noted in many of them longitudinal cracks at the soffits of the arches and perhaps elsewhere.  These result from three causes: 

First.—­In tunneling, there is more or less loss of material, while, in back-filling, the material does not at first reach its final compactness.  Therefore, in adjusting itself to normal conditions, this material causes impact loads to come upon the green arch, and these tend to crack it.