This page is dedicated to explaining how the Hoosac Tunnel was actually "dug".

    When The Hoosac Tunnel was chartered, it was expected to take about 4 1/2 years to finish. Geologists said that there would be no major obstacles, no water problems, and no weak earth. None of this turned out to be true. In the end it took 23 years, 195 lives and estimates between 17 and 21 million dollars (at the 1874 value).

    The original plan for digging the Hoosac Tunnel was to start digging at an angle from the east and west sides of the mountain and pray that the 2 unaligned adits meet each other in the middle. Such a plan is risky however, for there were 3 dimensions where a fatal error could occur. First one tunnel could end up too high or low. Second one portal could go off in one direction too far, thus making for a hard turn inside the mountain. Third one tunnel could go too far, and the other could meet one for instance 1000 feet back from the heading, making that 1000 feet a waste of labor. This plan was thankfully abandoned and never implemented, if it were then aligning such things as the Central Shaft, and to a lesser extent the West Shaft would have been nearly impossible.

    When digging commenced the contract called for a height of 20' 6", and a maximum width of 24' with a base width of 23'. However as late as 1861, that is 10 years after digging began the actual width was only to be 18' tall and 14' wide. Widening would have to be done later.

Tunnel Profile as seen in 1861, only wide enough for one set of track

Tunnel profile as contracted, wide enough for 2 sets of track.

    Digging began at both ends early on. At first a steam powered rock borer was demoed on the east side, but it failed (learn more on the east portal page). This machine promptly failed and was abandoned. Work would have to progress by the old method of drilling and blasting. Two men would work as a team, one holding a star drill, the other swinging a 20 pound double jack sledge hammer at the drill. After striking the drill the drill holder rotated the bit and it would be struck again. The star drill would slowly bore into the rock until the desired depth was achieved (usually between 2 and 3 feet). After a number of holes were drilled the holes would be filled with black powder. Legend has it that the newest worker, or the fastest was given the task of igniting the powder while the others kept their distance. The blast would break the rock and the rock or "spoil" could be removed in a process that we all know as mucking. In general the most effective method of blasting at the heading was found to be the center cut method, where the center would be blasted out in a V shape, and then they would work out towards the sides.

Pictures From Building Big by David Macaulay. These pictures depict the drilling and black powder loading process.

    This process worked well and fine on the east side of the mountain where the rock was solid Rowe Schist. However on the west side where limestone and other weak stone existed the process was not so easy. Subterranean pressure from a thrust fault ground a lot of the stone a few hundred feet in from the portal into a nightmarish weak "porridge stone" which was completely incapable of holding itself up. This section would later be lined with brick but not until after the sections were held up with timbers during construction. The West Shaft was sunk 2447 feet from the west portal to allow for 2 more headings, one heading towards the west portal and the other... you guessed it, heading towards the east.

    The method of digging at each side of the mountain was varied somewhat. On the east side work was staggered so that the bottom 6 feet of the tunnel were drilled and blasted first. A few hundred feet behind the heading another team drilled and blasted another 6 feet off of the roof known as the "stope" and shortly after that another group blasted the remaining stope until the required height was achieved. On the west side because of the weaker stone the exact opposite method was applied. The top 6 feet would be drilled and blasted, and then another 6 feet would be removed from the floor or the "bench". Benching would continue until the desired height was achieved.

Method used on the west portion of the tunnel. Notice the big contraption used for mining carts to dump spoilage into lower mining carts. [Click to enlarge]

Another picture depicting the west side method. This contraption has a special ramp instead of a dump.

Method used on the east side. This is a little easier to manage I would think, because you could just let the spoil drop to the floor. [Click to enlarge]

Another picture depicting the east side method. This depicts how the whole operation may have looked. [Click to enlarge]

    Now as you can imagine drilling by hand was difficult, dangerous, and expensive. A new method of drilling was developed in Europe that fit the Hoosac engineers wishes for a faster approach perfectly. A Fitchburg man by the name of Charles Burleigh adapted the European pneumatic drill into a compressed air drill aptly named the Burleigh Drill. By 1866 these drills were implemented at Hoosac. On the east side a water powered compressor building was built, on the west side steam powered compressors were built. On the east end air was compressed to 65 PSI, and carried to the heading by two 8 inch cast iron pipes. On the west end at the West Shaft 5 steam engines capable of running 2 drills each ran the drills at 240 strokes per minute. The drills were mounted on carts usually 4 per cart for ease of use. The air released as exhaust from the drills brought fresh air into the heading which was of great relief to those working there! Generally the drills broke down often and were not on the whole reliable, however after their implementation, progress increased from 50 feet a month to 150 feet a month. It is speculated that our next topic is at least partially responsible for the increase in productivity as it was implemented about the same time as the drills.

Pictures depicting Burleigh Drill Carts [Click to enlarge]

    Black powder was not powerful enough to break some of the harder granite and quartz that was being encountered during the tunneling. An alternative would need to be found. And that alternative was nitroglycerin. A new factory was built near the West Shaft in 1867 and operating by January 1867. Nitroglycerin was very unstable except while frozen. One pound of Nitroglycerin had the explosive power of 13 pounds of black powder. Nitroglycerin was used almost exclusively at the west heading and at the Central Shaft. Black powder was still used at the east heading for enlargement, but nitroglycerin was used to blast at the heading.

    The biggest challenge at the west side was supporting the demoralize rock which was completely incapable of supporting itself. In 1866 B.N. Farren won the contract to build a 500 foot long 6-8 course thick brick tube to support the weak stone (it would ultimately be 883 feet long). First a 8 brick thick invert was built at the bottom of the tunnel. This invert was packed tightly and given proper drainage. Then a brick arch was built using the invert and wooden beams as support. Finally any space between the top of the arch and the stope was filled in.

Picture depicting the profile of the tube. The invert is the portion at the bottom shaped like an elongated U.

    After the tunnel penetrated far enough into the mountain to get away from the weakest stone a regular arch was sufficient and an invert was no longer necessary. In the end 7573 feet of the tunnel would be brick lined, I would say at least 6500 feet of the brick lining is west of Central Shaft. The brick arching was completed in 1872. In all over 2 million bricks were used in the the construction of the tunnel. The kiln for making the bricks was located up the hill from the West Portal. Today the 883 feet of brick tubing is lined with a steel barrier because shifts in the earth have caused parts to collapse.

This picture depicts the Brick Arched portion of the tunnel. The left half shows what it looked like with timber supports, the right side shows it complete.

    When completed the Hoosac Tunnel measured 20' Tall and 24' Wide with a base of 23'. Just as it was originally contracted. It has a grade of 26 2/5 feet per mile heading towards the center. This pitch is there to allow water to flow out without the need for pumping.

    For those interested, the actual geologic composition of the mountain is (from east to west):

Rowe Schist
Stamford Granite Gneiss
Metamorphic Conglomerate
Hoosac Schist
Stockbridge Limestone

Pictures

Two sections: Historical and Contemporary. Most can be clicked to enlarge.

Historical

An actual picture of 2 men drilling at the heading. You can see the drill bits as well as the rain suites that they wore. The rain suites were necessary because of the huge amount of water that leaked into the tunnel from deep water pockets and springs. Courtesy North Adams Public Library!

A pen and ink of workers at the heading. Notice the rain suites, carts and drills.

A pen and ink depicting the actual blasting. I don't think that they were actually this close when blasting occurred but I could be wrong.

Profile of the east end. Supports were not necessary here because the stone was hard enough to support itself.

Contemporary

CLICK HERE - Large image archive of November 2005 excursion to Central Shaft.