—In describing the construction of the roof arch, mention was made of the stone filling employed between the back of the masonry ring and the ceiling of the excavation. The spaces behind the side walls are filled in a similar manner. The object of this stone filling, which should be closely packed, is to distribute the vertical and lateral pressures in the walls of the excavation uniformly over the lining masonry. As the masonry work progresses, the strutting employed previously to support the walls of the excavation has to be removed. This work requires care to prevent accident, and should be placed in charge of experienced mechanics who are familiar with its construction, and can remove it with the least damage to the timbers, so that they may be used again, and without causing the fall of the roof or the caving of the sides by removing too great a portion of the timbers at one time.
Thickness of Lining Masonry.
—It is obvious, of course, that the masonry lining must be thick enough to support the pressure of the earth which it sustains; but, as it is impossible to estimate these pressures at all accurately, it is difficult to say definitely just what thickness is required in any individual case. Rankine gives the following formulas for determining the depths of keystone required in different soils:
For firm soils
d = √(0.12 r2s),
and for soft soils,
d = √(0.48 r2s),
where d = the depth of the crown in feet, r = the rise of the arch in feet, and s = the span of the arch in feet. Other writers, among them Professor Curioni, attempt to give rational methods for calculating the thickness of tunnel lining; but they are all open to objection because of the amount of hypothesis required concerning pressures which are of necessity indeterminate. Therefore, to avoid tedious and uncertain calculations, the engineer adopts dimensions which experience has proven to be ample under similar conditions in the past. Thus we have all gradations in thickness, from hard-rock tunnels requiring no lining, and tunnels through rocks which simply require a thin shell to protect them from the atmosphere, to soft-ground tunnels where a masonry lining 3 ft. or more in thickness is employed. [Table II]. shows the thickness of masonry lining used in tunnels through soft soils of various kinds.
The thickness of the masonry lining is seldom uniform at all points, as is indicated by [Table II]. [Figs. 46 and 47] show common methods of varying the thickness of lining at different points, and are self-explanatory.
