![tunnel boston tunnel boston](http://imagestorage.nerail.org/photos/2002/11/21/2002112115121930521.jpg)
Its function is to support the earth above the roof of the tunnel temporarily, while the permanent concrete tunnel arch is being built below. shield is essentially a semi-circular steel frame weighing about sixty-five tons, the diameter of which is equal to the outside width of the tunnel, which is thirty-two feet four inches.The general method of tunneling adopted for this work was by means of a roof-shield, which is quite similar in character to the shields used in the tunnels already constructed in this vicinity! The Tremont Street tunnel of the original subway, the East Boston tunnel and the Dana Hill tunnel of the Cambridge subway. At one point, about sixty-five or seventy feet below the surface, some worn fragments of clam shells were found imbedded in the hard blue day. No serious difficulty resulted, however, from the ground-water, and this, together with the hardness of the ground, made it unnecessary to do any of the work in compressed air, although there was a provision in the contract for the work for doing so in case of necessity. Irregular streaks and pockets of sand, some containing water, were frequently encountered during the progress of the work. This tunnel forms the Boston end of the new subway lines connecting Harvard Square, Cambridge, with the business section of Boston.īeacon Hill is presumably a glacial formation, and is composed in general of a very hard mixture of blue and yellow clay and sand, together with numerous small stones and a few large boulders. The bottom of the tunnel, at the deepest point, is about one hundred feet below the top of the hill. THE most recent underground construction in the Boston district for the use of street railway traffic is the tunnel through Beacon Hill, which is now nearing completion This tunnel, which is a little less than half a mile in length, does not follow the street lines, but runs through the middle of Beacon Hill in a general northwesterly and southeasterly direction, connecting at its northwesterly end with an elevated structure which will cross the Cambridge bridge over the Charles River, and at its so utheasterlyerid terminates at present, in a station beneath Boston Common, under the Park Street station of the Tremont Street subway.