The writer, in the course of his experience, has met with or been
interested in the solution of many problems similar to the following:

What difference in timbering should be made for a tunnel in ordinary,
normally dry ground at a depth of 20 ft. to the roof, as compared with
one at a depth of 90 ft.?

What difference in timbering or in permanent design should be made for a
horizontally-sheeted shaft, 5 ft. square, going to a depth of 45 ft. and
one 25 by 70 ft., for instance, going to the same depth, assuming each
to be braced and sheeted horizontally with independent bracing?

What allowance should be made for the strength of interlock, assuming
that a circular bulkhead of sand, 30 ft. in diameter, is to be carried
by steel sheet-piling exposed around the outside for a depth of 40 ft.?

What average pressure per square foot of area should be required to
drive a section of a 3 by 15-ft. roof shield, as compared with the
pressure needed to drive the whole roof shield with an area four times
as great?

To what depth could a 12 by 12-in. timber be driven, under gradually
added pressure, up to 60 tons, for instance, in normal sand?

What frictional resistance should be assumed on a hollow, steel,
smooth-bore pile which had been driven through sharp sand and had
penetrated soft, marshy material the bearing resistance of which was
practically valueless?