Transactions . Scale in Fee^ZQ Fig. 2.—Transverse section op timbered_subway tunnel. walls. It will be noted that a line projected on a slope of 3^ to 1 from thefarthest decided break meets exactly the nearest upper corner of the drift, which is the point at which settlementmight be expected. The probableline of rupture shown is, of course,purely an assumption, there beingno possible way of survejdng it; j^etfrom observations of breaks in opencuts in similar material, it is knownthat rupture takes place along a curvedline, as shown, and that the breakoccurs at; a distance back from theedge
Transactions . Scale in Fee^ZQ Fig. 2.—Transverse section op timbered_subway tunnel. walls. It will be noted that a line projected on a slope of 3^ to 1 from thefarthest decided break meets exactly the nearest upper corner of the drift, which is the point at which settlementmight be expected. The probableline of rupture shown is, of course,purely an assumption, there beingno possible way of survejdng it; j^etfrom observations of breaks in opencuts in similar material, it is knownthat rupture takes place along a curvedline, as shown, and that the breakoccurs at; a distance back from theedge of the bank equal to half itsheight. It is a fair assumption, there-fore, that rupture will occur in asimilar manner through settlement above underground excavations whenthe relations existing between the breaks and the points of settlementbelow correspond. The center and side-wall drifts in Fig. 1 were driven. Fig. 3.—Sequence ofoperations. excavating H. G. MOULTON 333 in September, 1916. The shield passed this point in November, 1917,while the decided break in the surface was first noted on Nov. 23, 1917—or immediately after the passage of the shield broke up the arches in thehardpan over the side-wall drifts. Fig. 2 is sketched from a transverse section near the northern end ofthe contract. At this point a rather unique timbering method was 2 shows the timbering details, and Fig. 3, the sequence of opera-tions. The top heading was first driven and timbered with posts, caps,and knee braces. A sheeted trench was then sunk to subgrade, and,within this trench and under the timbers of the top heading, an umbrellasection of concrete was built to sustain the roof. Wall-plate drifts werenext driven and segmental sets placed to connect the umbrella sectionwith the wall plate. The wall plate was then underpinned on posts; itwas possible to excavate the remaining part of the material a
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
