Points were established at Station 220 + 00, which is the point of intersection for the curve on the original center line of the tunnel, and also at Station 220 + 23, where the intersection of the track center line comes in the North Tunnel. As it was desirable to have the survey stations not much more than 300 ft. apart, so as to obtain clear sights, other stations were established so that the distances between survey stations were at about that interval.
For studying changes of shape in the tunnel, brass "diameter markers" were inserted at each survey station in the concrete lining at the extremities of the vertical and horizontal axes. These were pieces of brass bar, ⅜ in. in diameter and 6 in. long, set in the concrete and projecting ⅝ in. into the tunnel, so that a tape could be easily held against the marker and read.
For obtaining the tidal oscillation of elevation of the tunnel, recording gauges are attached to the invert of the tunnel at each of the five permanent bench-marks referred to above in such a way that the recording pencil of the gauge is actuated by the rod of the permanent bench-mark. A roll of graduated paper is driven by clock-work below the recording pencil which thus marks automatically the relative movement between the moving tunnel and the stable rods. These have shown that in the subaqueous part of the tunnel there is a regular tidal fluctuation of elevation, the tunnel moving down as the tide rises, and rising again when the tide falls. For an average tide of about 5 ft. the tunnel oscillation would be about ⅛ in. Before the concrete lining was placed, there was a tidal change in the shape of the tunnel, which flattened about 1/64 in. at high tide. After the concrete lining was placed, this distortion seemed to cease.
The general design and plan of the work have been described, and before giving any account of the contractor's methods in carrying it out, [Table 22], showing the chief quantities of work in the river tunnels, is presented.
Methods of Construction.
The following is an account of the methods used by the contractor in carrying out the plans which have already been described. First, it may be well to point out the sequence of events as they developed in this work. These events may be divided into six periods.
1.—Excavation and Iron Lining: June, 1903, to November, 1906;
2.—Caulking and grummeting the iron lining: November, 1906, to June, 1907;
3.—Surveys, tests and observations: April, 1907, to April, 1908;
4.—Building cross-passages and capping pile bores: April, 1908, to November, 1908;
5.—Placing the concrete lining: November, 1908, to June, 1909;
6.—Cleaning up and various small works: June, 1909, to November, 1909.
The tunnels were under an average air pressure of 25 lb. per sq. in. above normal for all except Periods 5 and 6, during which times there was no air pressure in the tunnels.
All the work will be described in this paper except that under Period 3 which will be found in another paper.
Period 1.—Excavation and Iron Lining, June, 1903, to November, 1906.—[Table 23] gives the chief dates in connection with this period.