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.
The value of
, in [Figs. 17] and 18, is the average vertical unit pressure at the top of the tunnel. Experiments on grain bins lead to the inference that the pressure at the middle of the roof is greater than that at the sides, but no law of variation can be stated.
The lateral unit pressure on the vertical sides of the tunnel lining at the top is given by the equation for
, or by the corresponding diagram. The variation in this lateral pressure over the sides of the tunnel cannot be easily formulated, as so much of the weight of the earth, directly over the tunnel, has been transferred by a kind of arch action to the sides. Experience would better speak here.
[Table 5] gives the values of
and