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January-February 2004: Preparation for Seal Pours, Pier 37
March, 2004: After almost two years of excavation and concrete pours, Pier 37 has reached its final or "founding elevation" below the bottom of the river. The top of the caisson is now below the water line inside the coffer dam. The Pier 37 caisson is 200 feet (61m) tall from bottom to top.
Before the bottom of the caisson can be sealed with concrete, it must be free of loose material. A water jet is used to blast loose material from the caisson floor, a layer of very hard clay. The 12-inch (30cm) diameter jet pipe reduces to a one-inch opening that increases water pressure, as does the nozzle on a garden hose -- only with much more force!
Loose material is vacuumed from the bottom of the caisson floor with an airlift, a pneumatic vacuum with a pipe two feet in diameter and over 200 feet (61m) long.
The airlift in use. Note the water being removed from the caisson (arrow).
After vacuuming, sonar is used to check for anomalies on the caisson floor. Each of the caisson's 24 dredge wells are checked.
Engineers watch the sonar image on a computer monitor. They carefully map the features they see in each dredge well and in the excavated space beneath the caisson. The hard clay floor should be relatively flat, of sound material, and clean of any loose material.
The box shape of the caisson and the outline of dredge wells can be seen in this sonar view.
Results of the sonar scan are checked by divers. Here, a diver prepares for a descent of 200 feet (61m): 60 feet (18m) from the surface to the mud line, then another 140 feet (43m) to the bottom.
A "diving stage" carries the diver into the water, provides a safe haven in case of an emergency and offers comfort to the diver on the ascent from the bottom.
The underside of the diving stage canopy provides a supply of breathing air and a pocket of airspace where divers can blow air out their helmets.
The diver rides the diving stage to into the water, then exits. The diving stage is lowered to the bottom to ensure there are no obstructions. The diver then makes the 4-minute descent to the bottom of the caisson.
A last check of equipment, and the diver begins his descent. The water is so murky, his helmet light disappears almost immediately. At the bottom, visibility is 6 to 12 inches. The diver gathers information by feeling his way around the chamber and by aiming his helmet cam.
Topside, the diver's depth and breathing mixture is monitored in the dive trailer. The rate of his descent and return to the surface are carefully controlled. Pressure at the bottom is 100 psi, -- almost 7 times the air pressure at the surface and 7 times the pressure against the diver's lungs. The diver can spend a maximum 30 minutes at the bottom.
The dive supervisor talks to the diver through a communications link. At a 200-foot (61m) depth, the diver is breathing 85 percent helium, and his voice sounds high and thin on the intercom. Video from the helmet cam appears on a monitor.
The ascent from the bottom takes 40 minutes and the diver gradually changes from his "bottom mix" of helium and oxygen to oxygen only. After exiting the water, the diver is escorted to a decompression chamber by two attendants.
The diver has five minutes to get from a 40-foot dive depth into the decompression chamber. Symptoms of decompression sickness or "the bends", painful and very dangerous, can begin soon after.
The diver spends 90 minutes in decompression, and breathes oxygen for three 30-minute cycles, followed by short intervals of regular air. The support team regulates the air mixture and watches the diver through a window.