Owing to the very unstable character of the soil, considerable difficulty was experienced in building the masonry invert. The process adopted was as follows: Two parallel 12 × 12 in. timbers were first placed transversely across the tunnel, abutting against longitudinal timbers or wedges resting against the side walls. Short sheet piles were then driven into the tunnel bottom outside of these timbers, forming an inclosure similar to a cofferdam, from which the earth could be excavated without disturbing the surrounding ground. The earth being excavated, a layer of concrete 8 ins. thick was placed, and the brick masonry invert constructed on it. In less stable ground each of the above described cofferdams was subdivided by transverse timbers and sheet piling into three smaller cofferdams. Here the masonry of the middle section was first constructed, and then the side sections built. Where the ground was worst, still more care was necessary, and the bottom had to be covered with a sheeting of 1¹⁄₄-in. plank held down by struts abutting against the large transverse timbers. The invert masonry was constructed on this sheeting. Refuge niches 9 ft. high, 3 ft. wide, and 15 ins. deep were built in the side walls. =Accidents.=--In this tunnel, owing to the quick striking of the centers, it was found that the masonry lining flattened at the crown and bulged at the sides. This was attributed to the insufficient time allowed for the mortar to set in the rubble filling. Earth packing was tried, but gave still worse results. Finally dry rubble filling was adopted, with satisfactory results. There was necessarily some sinking of the surface. This resulted partly from the necessity of changing and removing of the timbers, and from the compression and springing of the timbers under the great pressures. The crown of the arch also settled from 2 ins. to 6 ins., due to the compression of the mortar in the joints. The maximum sinking of the surface of the street over the tunnel was about 18 ins.; it usually ran from 1 to 12 ins. Some damage was done to the water and gas mains. This damage was not usually serious, but it of course necessitated immediate repairs, and in some instances it was found best to reconstruct the mains for some distance. At one point along the tunnel where very treacherous material was found, the surface settlement caused the collapse of an adjacent building, and necessitated its reconstruction. CHAPTER XIV. THE FULL SECTION METHOD OF TUNNELING: ENGLISH METHOD; AMERICAN METHOD; AUSTRIAN METHOD. ENGLISH METHOD.