How Do You Build a Bridge Pier Underwater? Take the Water Away First

The enclosure is a temporary wall built around the area where work must happen, sealed against the bed below and extending above the water surface, which is then pumped out so the interior becomes dry. Workers, materials and machinery then operate on the bed as though the water were not there, and the enclosure is removed when the permanent structure is finished. Everything about the design follows from the fact that it holds back water it does not float in, so it must resist the pressure of the water outside with nothing balancing that pressure from within once the interior is dry.

Several forms are used depending on depth and ground:

• •Interlocking steel sheet piles driven into the bed with a hammer or vibrator
• •Braced internally with frames of steel struts across the opening
• •Double walls filled with earth, for larger or deeper enclosures
• •Circular cells of sheet piling filled with sand, which need no bracing
• •Earth banks for shallow water and small works
• •A concrete seal poured underwater across the base before pumping

Water pressure increases with depth, so the load on the wall is small at the surface and largest at the bed, and the total force grows with the square of the depth, which means doubling the depth quadruples what the structure must resist. That relationship is why these enclosures are cheap in shallow water and rapidly become impractical as depth increases. Water also pushes upward under the base, and if that upward pressure exceeds the weight of the material above it the floor lifts and the whole interior floods at once, which has killed people. Water seeping under the wall can wash out the soil supporting it, undermining the structure progressively.

The term is also used for something quite different, and mixing the two causes confusion in technical reading. In shipbuilding it names a narrow empty compartment deliberately left between two tanks, so that a leak from one cannot pass directly into the other and so that incompatible contents, such as fuel and drinking water, are never separated by a single plate. Those spaces are inspected regularly and are among the more unpleasant places on a vessel to enter, being confined, poorly ventilated and prone to accumulating gas. The same principle appears in fuel storage on land, where a void between two walls contains any leak and can be monitored for the presence of liquid.

Beyond a certain depth a different approach is used, in which a box open at the bottom is sunk to the bed and compressed air is pumped in to hold the water out rather than pumping water away. Workers enter through an airlock and dig from inside while the structure sinks under its own weight. The method built the piers of several famous bridges in the nineteenth century and was responsible for a great deal of suffering, since workers returning to normal pressure too quickly developed decompression sickness, which was not understood at the time and left many crippled or dead. The engineer of the Brooklyn Bridge was permanently disabled by it.

A temporary sealed wall around a work site, pumped dry, lets people build on a riverbed in the open air. Pressure grows with depth and the total force grows with its square, so doubling the depth quadruples the load and the method becomes impractical quickly. Upward pressure under the base can lift the floor and flood everything at once. Deep work instead used compressed air, which crippled workers before decompression was understood.