98 THE KANSAS CITY BRIDGE.

is formed of loose stones or coarse gravel, the pressures of sand and
water remain distinct, each substance transmitting its own pressure, and the
gravel alone producing friction; if the material is a water-tight clay, the
weight of the water above is equivalent to that of any other load, increas-
ing definitely the pressure of the clay and causing additional friction ;* if, how-
ever, as is commonly the case, the material be something between these
extremes, a fine sand, a silt, or mixture of the two, the water neither penetrates
the whole with perfect freedom nor remains as a weight on the top of a sub-
stance which it does not enter; its action is therefore dependent on capillary
attraction and matters which cannot be measured precisely ; and while the total
pressure would not differ materially from that in either of the two preceding
cases, the portion of that pressure which is transmitted by the sand, and which
alone produces friction, would be somewhat greater than in the case of the
gravel and less than with the clay. This could be better guarded against by
an empirical allowance than measured by exact computation ; in estimating fric-
tion, accordingly, the calculations were made by the formula given above, but
the value given to w was the full weight of the saturated sand, and not its sub-
merged weight alone ; this, undoubtedly, gave excessive results, but as no
allowance was made for the portion of the pressure of the superposed water
transmitted by the sand, this discrepancy was a little less than might at first be
supposed.

The coefficient of friction of wet Kansas City sand upon itself was ascer-
tained by experiment to be about .8; the least observed was .725, which cor-
responds very nearly to a = 54°; this, substituted in the above formula, gives

P = 0.1298 h?2 w. (0.)

Substituting for w the immersed weight of the heaviest sand weighed, or 69.5
pounds :—

P = 9.02 h?. (c.)
And if w be made the full weight of a cubic foot of such sand saturated,

P = 17.13 22. (d.)

eee

* In this case the actual and not the submerged weight of the clay must be used in computation ; bus
the cohesion of the particles of clay is so great that this formula would give very excessive results,