Heavy clay soils are a challenge, they are hard to break up, easily block the pipes and air lift pumps commonly used for this application show unstable production rates. Extensive testing showed BELL dredge pumps can overcome these issues. The main challenges for using dredge pumps to sink caissons to be overcome are in fine tuning the cutter configuration (how many teeth of what size should be placed at what interval), how to move the pump at 60m depth efficiently along a production grid, and how to cut the clay underneath the caisson rim.

The equipment most commonly used in China for the sinking of caissons for bridge foundations is an air lift pump. Compressors inject compressed air into the well. The compressed air mixes with the liquid causing the air-water mixture to be less dense than the rest of the liquid around it and therefore displacing it upwards through the discharge pipe carrying away solids in its stream. This works relatively well for less compacted soil layers. For harder sand or clay layers a cutter needs to be added. China Harbor Second Engineering Company decided to look for alternative equipment to improve this methodology and did rigorous testing of the BELL dredging equipment at the Oujiang Bridge Project. The 67.5m high, 70x63m caisson consisted of 30 cells.

At the Oujiang bridge project extensive testing of the BELL pumps proved they can reach the specifications listed by the manufacturer. Mixture concentration rates are consistently high, water depth of 60 meters is no issue, lifting the mixture 20 meters is possible without reducing the production rate, and a stable solid mixture is produced during operation. The client trialed both the single as well as the double cutter. 

The double cutter preformed significantly better as the surface area of the double cutter is much larger, almost doubling the area that can be cut at one time. An additional benefit is that because of the reverse action of the double cutter the torque forces on the hoisting mechanism are also much reduced.

The main challenge in excavation work is to keep production constantly high. There is some uncertainty involved when operating the cutter at depth, which areas have been cleared and which haven’t, how to move the pump efficiently along the grid to make sure the cutters are bringing up clay, how to design a suitable system to cut the clay at a 45 degree angle along the edges of the caisson so it will sink straight down, how to clean debris from the caisson so it doesn’t get tangled in the cutter. Our experience following this project is that the dredge method is the best choice for non-ideal circumstances.

The configuration we suggest for caisson dredging application is the BELL 200 or 250, outfitted with double cutter head to maximize surface area of the pump, and powered by a BELL power pack. The double cutter works in combination with high pressure water jetting, we recommend using the BELL water jet pump for best results. These models will handle 800-1250m3 of mixture per hour, with a 30% solid content (240-375m3/ hour).

The benefits of choosing a hydraulic dredge pump with hydraulically operated cutter head over an air lift are:

·       Mixture concentration rates are consistently high, water depth of 60 meters and lifting the mixture 20 meters is no issue

·       Fewer pumps necessary, reduced operator teams

·       Better for avoiding tilting caissons, and a solution to correct the position once tilting has occurred

·       The double cutter is designed for soil hardness of 4 Mpa, it is suitable for harder soil layers, cutter teeth can be engineered to different dimensions and hardness