Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910 / The New York Tunnel Extension of the Pennsylvania Railroad. / The North River Tunnels. Paper No. 1155

by Bertram Henry Majendie Hewett; W. L. Brown

In the silt the pressure was much lower than the hydrostatic head at the crown, but if it became necessary to make an excavation ahead of the shield, for example at the junction of the shields, the air pressure required was about equal to the weight of the overlying material, namely, the water and the silt, as the silt, which weighed from 97 to 106 lb. per cu. ft. and averaged 100 lb. per cu. ft., acted like a fluid.

TABLE 29.— Monthly Progress of Shield-Driven Tunnel Work.

Month	North Manhattan.				South Manhattan.				North Weehawken.				South Weehawken.				Average progress per shield lin. ft. per month.
	Number of rings erected.		Station of leading ring.	Lin. ft. for month.	Number of rings erected.		Station of leading ring.	Lin. ft. for month.	Number of rings erected.		Station of leading ring.	Lin. ft. for month.	Number of rings erected.		Station of leading ring.	Lin. ft. for month.
	For month.	To date.			For month.	To date.			For month.	To date.			For month.	To date.
1905
May	26	26	200 + 83.7	63.7													15.9
June	26	52	201 + 49.0	65.3					24	24	260 + 76.6	59.3	12	12	260 + 70.0	30.0	38.6
July	28	80	202 + 19.2	70.2					12	36	260 + 46.6	30.0	15	27	260 + 32.4	37.6	34.4
Aug	26	106	202 + 84.3	65.1					15	51	260 + 09.1	37.5	16	43	260 + 07.4	25.0	31.9
Sept	21	127	203 + 36.8	52.5	31	31	200 + 96.4	76.4	1	52	260 + 06.6	2.5	18	61	259 + 47.2	60.2	47.9
Oct	25	152	203 + 99.4	63.6	45	76	202 + 09.2	112.8	10	62	259 + 81.5	25.1	20	81	258 + 97.2	50.0	62.9
Nov	31	183	204 + 76.9	77.5	31	107	202 + 86.5	77.3	29	91	259 + 09.0	72.5	39	120	257 + 99.7	97.5	81.2
Dec	59	242	206 + 24.6	147.7	34	141	208 + 71.8	85.3	46	137	257 + 94.0	115.0	77	197	256 + 07.1	192.6	135.1
1906
Jan	94	336	208 + 59.8	235.2	27	168	304 + 39.4	67.6	77	214	256 + 01.4	192.6	73	270	254 + 24.6	182.5	169.4
Feb	78	414	210 + 54.9	195.1	64	232	205 + 99.6	160.2	133	347	252 + 68.6	332.8	165	435	250 + 11.7	412.9	275.2
Mar	56	470	211 + 95.2	140.3	96	328	208 + 39.9	240.3	142	489	249 + 13.3	355.3	111	546	247 + 34.0	277.7	253.4
April	119	589	214 + 93.0	297.8	84	412	210 + 59.1	210.2	32	521	248 + 33.3	80.0	78	624	245 + 38.9	195.1	195.7
May	129	718	218 + 15.7	322.7	70	482	212 + 25.3	165.2	121	642	245 + 30.6	302.7	2	626	245 + 33.9	5.0	198.9
June	218	936	232 + 60.9	545.2	140	622	215 + 75.5	350.2	162	804	241 + 25.3	405.3	157	788	241 + 41.1	392.8	423.4
July	155	1,091	227 + 48.5	387.6	82	704	217 + 80.7	205.2	113	917	238 + 42.4	282.9	118	901	238 + 45.9	295.2	292.7
Aug	145	1,236	231 + 11.2	362.7	134	838	221 + 15.8	335.1	138	1,055	234 + 97.1	345.3	140	1,041	234 + 95.8	850.1	348.3
Sept	89	1,325	233 + 34.1	222.9	168	1,006	225 + 35.8	420.0	55	1,110	233 + 59.5	137.6	177	1,218	230 + 52.8	443.0	305.9
Oct					105	1,111	227 + 98.6	262.8	1	1,111	233 + 57.0	2.5	94	1,312	228 + 16.8	236.0	125.3
Nov					7	1,118	228 + 16.8	18.2	9	1,120	233 + 34.1	22.9					10.3

A ½-in. air line was taken direct from the working chamber to the recording gauges in the engine-room, which enabled the engine-room force to keep a constant watch on the air conditions below. To avoid undue rise of pressure, a safety valve was set on the air line at each lock, set to blow off if the air pressure rose above that desired. The compressor plant was ample, except, as before described, when passing the gravel section at Weehawken.

Records were kept of the air supply, and it may be said here that the quantity of free air per man per hour was in general between 1,500 and 5,000 cu. ft., though in the open gravel where the escape was great it was for a time as much as 10,000 cu. ft. For more than half the silt period it was kept between 3,000 and 4,000 cu. ft., but when it seemed proved beyond doubt that any quantity more than 2,000 cu. ft. had no beneficial effect on health, no attempt was made to deliver more, and on two separate occasions for two consecutive weeks it ran as low as 1,000 cu. ft. without any increase in the number of cases of bends.

The amount of CO₂ in the air was also measured daily, as the specifications called for not more than 1 part of CO₂ per 1,000 parts of air. The average ranged between 0.8 and 1.5 parts per 1,000, though in exceptional cases it fell as low as 0.3 and rose to 4.0. The air temperature in the tunnels usually ranged from 55° to 60° Fahr., which was the temperature also of the surrounding silt, though at times, in the earlier parts of the work when grouting extensively in long sections of the tunnel in rock, it varied from 85° to 110° Fahr.

Grouting.—Grout of one part of Portland cement to one part of sand by volume was forced outside the tunnel lining by air pressure through 1½-in. tapped and plugged grout holes formed in each segment for this purpose, wherever the ground was not likely to squeeze in upon the metal lining as soon as this was erected. That is to say, it was used everywhere up to the river line; between river lines it was not used except at the New York bulkhead wall in order to fill voids in the rip-rap, and at the point of junction of the shields where the space between the metal lining and the shield skins outside it was grouted. Cow Bay sand was used, and it had to be screened to remove particles greater than 1/10 in. in diameter, which would choke the valves. For later grouting work, namely, in the top of the concrete lining inside the metal lining, Rockaway Beach sand was used. This is very fine, and did not need screening; it cost more, but the saving of screening and the non-blocking of valves, etc., resulted in a saving.

The grout was mixed in a machine shown in [Fig. 2, Plate XLI], which is a view of the grouting operation.

The grout pipes were not screwed directly into the tapped hole in the segments, but a pipe containing a nipple and valve was screwed into the grout hole and the grout pipe screwed to the pipe. This prevented the waste of grout, enabled the valve to be closed and the grout pipe disconnected, and the pipe to be left in position until the grout had set. In the full rock section, 20 or 30 rings were put in without grouting; then the shield was stopped, the last two or three rings were detached and pulled ahead by the shield, a masonry stop-wall was built around the outside of the last ring left in, and the whole 20 or 30 rings were grouted at one time. In the landward silt and gravel each ring had to be grouted as soon as the shield had left it, in order to avoid the flattening caused by the weight coming on the crown while the sides were as yet unsupported. The grout was prevented from reaching the tail of the shield by plugging up the space with empty cement bags, assisted by segmental boards held against the face of the leading ring by U-shaped clamps, fitting over the front circumferential flange of the ring and the boards, and tightened by wedges. The air pressure varied between 70 and 100 lb. per sq. in. above normal.