Sand hogs at work in the bottom chamber of a caisson.
Crossing the Hillsborough River to-day it is hard to realize that the bridge over the river was, at the time, one of the biggest engineering projects ever to be attempted in the history of the province, second only to the building of the Prince Edward Island Railway in the early 1870s. When the decision was made to put a bridge across the Hillsborough River linking Charlottetown with southern Queens and Kings Counties one of the big unknowns was the nature of the river bottom and the extent that it could carry the bridge piers. While soundings had been taken, that gave information only as to the depth of the water. A more important question was how deep the underlying mud lay until bedrock or a firm bottom capable of bearing the weight of the bridge could be reached.
Excavating the bottom from the surface was impossible except in the shallows where dredges could reach the mud and remove at least a portion of it. The contract called for 10 feet of mud to be removed and then pilings of from 30 to 65 feet to be driven into the bottom. The masonry piers were then built on top of the pilings extending four feet below the low water level. It was initially planned that all but the four central piers would be constructed in this way. However in the end almost half of the piers had to be constructed using another technique.
Nearer the channel the dredging approach was impossible and the problem was exacerbated by the water depth, tidal flow and the river currents. The answer lay in sinking caissons, an approach most famously used in the construction of the Brooklyn Bridge from 1869-1883.
The cluttered deck of the caisson with airlocks and piping. The floating powerhouse can be seen in the background.
A caisson is basically a bottomless box. With top and sides sealed it was weighted down until it reached the bottom of the harbour and dug into the mud. The boxes themselves, built in a construction yard at Connolly’s Wharf, weighed over 500 tons. The hollow box was sealed except for two air locks, one for men to enter and leave and one to remove the mud and rocks quarried from the bottom. In order to keep the water from seeping into the caisson the digging chamber was pressurized with air pumped in to keep the water out. As the caisson was lowered into the mud the courses of stonework for the actual piers were built on the top of the caisson adding hundreds of tons to the weight and driving the box downwards. An iron lip helped the box cut into the soft bottom. The chamber itself was about fifty feet square and six feet deep.
Sand hogs used hand and muscle techniques to move the mud and rocks in the river bottom.
The actual digging was done by hand and the work had become so specialized that it was handled by crews of men known as “sand hogs”, although for the Hillsborough River job “mud hogs” may have been more appropriate. The workers travelled from job to job arriving on P.E.I. in September of 1902. About 30 men did the digging on the P.E.I. project. No Islanders were involved in the digging which was the most dangerous part of the bridge building enterprise.
Sandhogs entering the air lock at the top of the caisson.
The danger increased with the pressure which rose as the hole in the river bottom became deeper. While most of the caissons went down from 40 to 60 feet the deepest of the piers for the Hillsborough bridge went to a depth of 94 1/2 feet and the men worked under a pressure of 43 lbs. per square inch. This pressure drove air, especially nitrogen, into the bloodstream and could cause damage to muscles and organs and even result in a painful death. Today pressure workers such as deep-sea divers can mitigate some of the dangers through gradual decompression but in 1902 little was known about what was known as “the bends.” On emerging from their shifts, which were shortened as the depth increased, the workers often had to be carried to the hospital where they tried to recover in time for their next shift. The danger of increased depth was recognized by heightened pay as the structure sank into the river. Depending on the composition of the river bottom the descent could be agonizingly slow, sometimes only inches per week.
On the surface a large barge housed a coal-fired powerhouse which ran a generator providing electricity for the electric arc lights both in the digging chamber and on the surface of the 24 hour-per-day operation. It also ran an air compressor plant pumping air into the chamber, increasing the pressure as the chamber dug itself into the mud, or rather as it was dug into the mud by the workers in the chamber who used drills and pick and shovel to remove the mud from the edges and center of the chamber and gradually lower it toward the bedrock. In addition the barge also housed a “hospital” where the workers recovered after their time in the high-pressure environment.
Service barge housing the powerhouse, generator and air compressor as well as the “hospital”. Beside the service barge a coal barge can be seen and on the other side of the cribwork a derrick barge and mud barge are tied up. The construction required almost a dozen barges, tugs and steamers.
Some of the debris was removed through the creation of a slurry of mud and water which was siphoned to the surface through the high pressure in the chamber, essentially a vacuum in reverse. In other cases larger rocks and gravel had to be hoisted through an air lock. When the desired depth was reached and the caisson rested on the bedrock the digging chamber was filled with concrete. The courses of the stone piers were then laid on top until a height of ten feet was reached and the piers were readied for the steelwork.
Hillsborough Bridge construction photos. Click on any image to enlarge.
Large airlock for removal of debris
Filling the digging chanber with concrete after bedrock is reached
There would have been significant pressure on the walls of the caissons at a depth of 90 feet.
Launching one of the caissons from Connoly’s Wharf
The walls of the caissons were three feet thick
This posting is one of a series about the building of the Hillsborough Bridge. The story of how a used railway bridge came to be used for the Murray Harbour Branch of the P.E.I. Railway can be found here. A posting about the building of the piers and the mysterious disappearance of one of the bridge workers is found here.
The Public Archives and Records Office of Prince Edward Island has several collections with construction photos of the Hillsborough Bridge. Some of the best are found in Accessions 3909, 3961, 4190 and 4261.