1874
Eads Bridge has many 'firsts'

The year marked completion of Eads Bridge over the Mississippi River at St. Louis. The world's first steel arch bridge, its three-arch spans--one 520 ft long and two of 502 ft--also made it the biggest bridge ever built. Eads was the first important steel structure of any kind in the world and involved the first significant use of pneumatic caissons.

The bridge was the first and only span designed and built by James B. Eads, who knew the Mississippi well from years of salvaging sunken steamboats. During the Civil War, he built 14 armor-clad, steam-powered gunboats for Union forces. After the war, St. Louis needed a bridge as a rail link with the east. Eads had a reputation for getting things done, and his bridge proposal was accepted over that of John Roebling and two other well-known engineers.

The bridge Eads built was supported by four piers founded on bedrock. For the west abutment, bedrock was reached through a cofferdam at 40 ft below water. Bedrock sloped steeply from west to east. Eads used pneumatic caissons on the west and east piers and the eastern abutment, where bedrock was 127.5 ft below the surface. The project generated the country's first cases of caisson disease.The three steel arches were each made up of four chrome steel 18-in. tubes, each made up of steel staves bound together by steel hoops. The river couldn't be blocked by falsework so Eads used tiebacks to cantilever the arches during construction.

1875
Record tunnel is on the mark

After 24 years and $10 million, the completed Hoosac Tunnel in northwest Massachusetts offered a new direct rail route between Boston and Albany. Boring through the Hoosac Mountain began in 1851 and eventually produced a 4.75-mile-long tunnel, the longest ever dug.

The project cost 195 lives but over the years introduced techniques that are in use today in hard-rock tunneling. Work began from both ends to excavate a 24-ft-dia tunnel by hand-drilling and black powder blasting. After 10 years, only 20% of the tunnel was dug. In 1865, contractors began using a compressed-air drill invented by Charles Burleigh. Four to six drills mounted on a carriage then created the first "jumbo." Nitroglycerin was adopted as a blasting agent in 1866.

Despite the new techniques, the tunnel was only one-third complete by 1868. At the mountain's east end, tunnelers met an unyielding combination of mica slate and quartz. At the western end, they faced a moist, crumbly mica schist that forced 2,500 ft of heavy brick tunnel lining. A further complication was groundwater intrusion--in some spots, at a rate of 3,000 gal per hour. A central shaft fire halted work for a year. The state hired contractor Shanley and Co. in an attempt to finish the job by 1874. The firm brought in 700 workers, assigning them to three shifts, six days a week. When tunnelers broke through late in 1874, their alignment was off by less than 1/2 in.