This year marks the 125th anniversary of the opening of the 7.6km long Hoosac Tunnel in north-western Massachusetts. The tunnel is on the railway route connecting Boston on the Atlantic seaboard of the US and Troy on the Hudson River in New York State. Its place in the history of tunnelling was assured when both nitroglycerine explosive and compressed air rock drills were used during the latter stages of its excavation. These two innovations were to revolutionise drill and blast hard-rock tunnelling. Swedish industrialist Alfred Nobel was soon to make nitroglycerine safer to handle by mixing it with kieselguhr (a diatomaceous earth formed from the silica-impregnated shells of unicellular organisms) to produce dynamite, while compressed air drills were to reign supreme until the late 1970s when modern hydraulic rock drilling machines were developed.
The idea of a tunnel through Hoosac Mountain was first mooted in 1825 when the feasibility of constructing a canal from Boston to the Hudson River to connect with the 584km long Erie Canal was being considered. But with the advent of railways that project was abandoned. By 1842 a railway through southern Massachusetts had been constructed as far as the east bank of the Hudson River opposite Albany. However, interest in the northern route along the line recommended for the canal continued, and construction of a railway began in the early 1840s. By 1848 it had reached Greenfield. That year a charter was granted to the Troy and Greenfield Railroad for the construction of a railway to the Massachusetts-Vermont border, including the Hoosac Tunnel.
Geological opinion at the time correctly believed that Hoosac Mountain was composed of metamorphic rock – mica schist with some gneiss and occasional quartz veins. Some limestone was observed near the west portal. The rock dipped steeply, striking at right angles to the tunnel alignment and it was considered that little masonry lining would be required.
Based on the preliminary geological appraisal, A F Edwards, the first chief engineer of the Troy and Greenfield Railroad decided a tunnel was feasible. He estimated the cost would be $1,968,557 and the work would take 1,556 working days, although he calculated this could be reduced to 1,054 working days by using two shafts. Edwards also felt that further time reductions were possible if a tunnel boring machine proved to be effective. The tunnel cross-section proposed by Edwards has the sloping floors to the side headings or sidings are hardly practical.
Slow beginnings
In 1854 the Massachusetts General Court agreed to lend the Railroad Company $2M to help with construction of the tunnel. Contracts were let with E W Serrel & Co in 1855 and 1856 and with Herman Haupt & Co also in 1856. Some work was done but all three contracts fell through.
In 1858 another contract was signed with Haupt under which the contractor agreed to complete the Troy and Greenfield railway including the Hoosac Tunnel, receiving $2M in state bonds, $900,000 in railway company mortgage bonds and $1.1M in cash. Work progressed until 12 July 1861 when the state engineer refused to certify an instalment payment to the contractor and construction ceased.
At that stage 731m of 4.3 x 1.8m of top heading had been driven. Some 649m had been enlarged by two 1.8m high benches to full single-track dimensions of 4.3m x 5.5m at the east end of the tunnel. This heading, using traditional manual tunnelling methods, had progressed an average 15m/month, at an estimated cost of $164/m, or $6.54/m3. The west shaft had been sunk to grade and some 150m of tunnel excavated.
The State in charge
In 1862 an Act was passed by the Massachusetts legislature “for the more speedy completion of the Troy and Greenfield Railroad and the Hoosac Tunnel”. Under this Act a three-strong commission was appointed to oversee the project. One of its first acts was to send Charles S Storrow to Europe to study contemporary methods of tunnelling, including the works on the Mont Cenis rail tunnel in the Swiss Alps.
They also appointed Thomas Doane as chief engineer and recommended that the work on the tunnel be undertaken by the state and that the bore should be enlarged to double track. Work on the enlargement of the already excavated section of tunnel began in October 1863 and was completed in March 1865 at a cost of $13.92/m3.
A new heading was then begun near the bottom of the tunnel, driving in three shifts of eight hours. Each shift consisted of five holders and five strikers under a foreman. Five 35mm diameter holes were drilled and blasted every two hours, producing an average advance rate of 0.75m/day.
With the introduction of the Burleigh drill in November 1866, progress in 1867, at 362m, was more than twice that in 1866, even though the heading had been progressively enlarged from 4.6m x 2.1m to 7.3m x 2.4m. By the time the state ceased work in 1868, the heading had been advanced 1,610m from the east portal and the tunnel was virtually complete for 803m.
Sinking of the 8.2m x 4.6m elliptical central shaft was begun in December 1863; by September 1864 it was 22m deep and had reached 71m at the end of the following year. Drill holes 0.9m deep were made by hand; by the end of 1866 the shaft had reached a depth of 120m.
Work continued well during 1867, until a fire broke out at the shaft on 19 October when the work there was under the direction of Dull, Gowan and White. The fire, caused by the accidental ignition of naptha gas used to light the shaft, killed 13 workers at the bottom of the 178m deep shaft, and every structure at the shaft was destroyed.
Work did not start in earnest from the west shaft until June 1865, at which time the bottom east heading was increased in size from 3.4m x 1.5m to 4.6m x 1.8m. By the end of 1866 a further 265m of heading had been driven. The first experiments with nitroglycerine were carried out here in August and October 1866 by Colonel Schaffner, but an inflow of water of around 1,150 litre/min stopped the work.
In 1867 some work was carried out by Dull, Gowan and White and the east heading was relocated in the crown and increased in size to 7.3m x 2.4m. In 1868 the state introduced drilling machines in June and adopted nitroglycerine for general heading excavation in August. A drift connection was also made to the west end to allow water to gravitate out of the tunnel. When work by the state ceased in 1868 the main heading had been extended 490m from the West Shaft.
At the west end of the tunnel, progress was slow because interest charges could be saved by delaying expenditure until construction was critical for tunnel completion. Several trial pits were sunk to define ground conditions better and a supplementary shaft was sunk 80m west of the west shaft to improve alignment, and to be used as a pumping shaft.
In spring 1866 Frost, to everyone’s surprise, agreed to excavate the west end under contract, constructing 53m of enclosing structure in the open-cut area and 61m of full-size tunnel lining underground within a year. The following year he agreed to construct a further 152m full-size tunnel lining as well as an adit from the westward heading from the west shaft to allow water to drain freely from the workings. At this time an estimated 3,200 to 4,500 litre/min water was being pumped from the west shaft. This adit was complete in October 1868. Rock sufficiently strong to serve as a footing for the lining was encountered 244m to the east of the west portal.
The work under state control had proved very expensive, although allowance must be made for inflation during the American Civil War and for the considerable research and development activities undertaken at the tunnel.
The Shanlys in charge
In June 1868 Walter and Francis Shanly of Canada received a letter from Benjamin H Latrobe Jr, consulting engineer to the tunnel, inviting tenders to complete the tunnel. Their bid found favour and Walter – a businessman and Member of Parliament in Montreal – negotiated with the state of Massachusetts. It was agreed on 24 December 1868 that F and W Shanly, Contractors, would build the remainder of the Hoosac Tunnel by March 1874 for $4,594,268.
The contract originally called for a lump sum deposit of $500,000 and reimbursement of 80% of earned income for the duration of the project; this was changed so that, instead of a deposit, Shanlys were to perform $500,000 worth of work. They were to receive monthly payments at 80% of the schedule of rates summarised in Table 1. The stringent rates of progress in Table 2 were also agreed (given in the original imperial units).
The Shanlys began work early in 1869. Nitroglycerine and blasting caps were used. The shaft reached full depth in August 1870. East and west headings were driven from the central shaft initially by hand and then with drilling machines. But no sooner were the machines introduced than the west heading encountered a water-bearing zone. The Shanlys abandoned work there until the connection to the east-end heading had been made so that the water could be allowed to drain along the tunnel to the east portal. Breakthrough occurred on 12 December 1872 with a difference in alignment about 8mm horizontally and vertically.
With drainage established, the west heading was then attacked and monthly advance rates of 40m to 56m were achieved until breakthrough on 27 November 1873: alignment differed by 14mm horizontally and 41mm vertically.
At the east end of the tunnel the Shanlys had begun work in April 1869 and started to use a small locomotive to remove muck from the enlargements. The tunnel was advanced using a 7.3m x 2.4m bottom heading with two 1.8m overhead enlargements. Gunpowder was used in the heading as an economy measure and nitroglycerine in the enlargements; three eight-hour shifts with 15 men each worked the east heading six days per week. Output increased to 51m per month as work progressed.
At the west end of the tunnel work overcame flooding in October 1869 which took a month to clear, a major failure in the crown in March 1870, bringing down 1,100m3 of ground, and the entire length of 746m from the west portal to the west shaft was lined with brickwork by early 1873.
Success at last
Although the Shanlys pressed ahead and the tunnel had holed through in November 1873 they were apparently not interested in finishing the project. They reached a provisional settlement with the state of their contract on 22 December 1874, although a considerable amount of enlargement and lining remained to be done. Substantial open-cut excavation was also needed to the west of the west portal. B N Farren was contracted on 19 November 1874 to complete the work.
Farren laid up to a million bricks monthly and completed the remaining 1,500m of lining by 30 June 1876. The tunnel was opened for business the following day.
The Hoosac Tunnel had taken 21 years to complete. The cost of construction was put at $10M. In addition the interest paid on debt during construction was $3.3M.
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