ed the bracing of a large part of
the load, the ordinary loose material being held in place by the arching
or wedging together of the 2-in. by 3-ft. sheeting boards in the roof,
arranged in the form of a segmental arch. The material above this roof
was coarse, sharp sand, through which it had been difficult to tunnel
without losing ground, and it had admitted water freely after each rain
until the drainage of a neighboring pond had been completed, the men
never being willing to resume work until the influx of water had
stopped.

The foregoing applies only to material ordinarily found under ground not
subaqueous, or which cannot be classed as aqueous or semi-aqueous
material. These conditions will be noted later.

[Illustration: FIG. 5.]

[Illustration: FIG. 6.]

The writer will take up next the question of pressures against the faces
of sheeted trenches or retaining walls, in material of the same
character as noted above. Referring to Fig. 2, it is not reasonable to
suppose that having passed the line, _R F J_, the character of the
stresses due to the thrust of the material will change, if bracing
should be substituted for the material in the area, _W V J R_, or if, as
in Fig. 3, canvas is rolled down along the lines, _E G_ and _A O_, and
if, as this section is excavated between the canvas faces, temporary
struts are erected, there is no reason to believe that with properly
adjusted weights at _W_ or _W_{2}_, an exact equilibrium of forces and
conditions cannot be obtained. Or, again, if, as in Fig. 5, the face,
_P Q_, is sheeted and rodded back to the surface, keying the rods taut,
there is undoubtedly a stable condition and one which could not fail in
theory or practice, nor can anyone, looking at Fig. 5, doubt that the
top timbers are stressed more heavily than those at the bottom. The
assumption is that the tendency of the material to slide toward the toe
causes it to wedge itself between the face of the sheeting on the one
hand and some plane between the sheeting and the plane of repose on the
other, and that the resistance to this tendency will cause an arching
thrust to be developed along or parallel to the lines, _A N_, _B M_,
etc., Fig. 2, which are assumed to be the lines of repose, or curves
approximating thereto. As the thrust is greatest in that material
directly at the face, _A O_, Fig. 6, and is nothing at the plane of
repose, _C O_, it may be assumed arbitrarily that the line, _B O_,
bisecting this