Open Caisson Calculator

Too low (will not sink): add kentledge, thicken the wall, or cut friction with a bentonite slurry jacket or water injection. Too high (sinks too fast): control excavation rate, brace at the cutting edge, and sink in stages. Caisson failures are usually not 'will not sink' but sudden sinking - one side undermined, the shaft tilts or breaks. Keep K near the low end and regulate by excavation rather than chasing speed.

The concrete formula is identical - outer outline volume minus inner outline volume - only a rectangle uses length × width for area. Sinking behaviour differs: a circle is uniform and sinks most steadily, while a rectangle concentrates stress at the corners and tends to tilt and jam, so rectangular caissons often add ribs and internal walls. This tool assumes a circle; for a rectangle replace the outer area with outer length × outer width.

Keep it in the 1.05-1.25 band. Below 1.05 there is not enough weight to get it down: add kentledge or flood the working chamber, reduce friction (a stepped cutting edge or a thixotropic slurry jacket), or cut the wall-soil contact area. Above 1.25 the caisson drops suddenly - one dig and it falls a metre - which causes over-run, tilt and cracking of the wall. There, control the dig rate and layer thickness, leave a soil berm under the cutting edge, and add friction collars if needed. Take the friction value from the ground investigation or a site test; the 20 kPa default is only a starting point.

A caisson wins on section size, stiffness and not needing heavy boring plant, and the shaft itself can be the permanent structure - a pump station, a manhole, an intake. It suits large plan dimensions (a few metres up to the teens) at depth, where there is room to build it in place and where the surroundings can tolerate the settlement it causes, and where boulders or hard layers would stop a drilling rig. It loses on control: tilt and sudden drop, settlement around it and a long programme. So dense urban basements go to diaphragm walls or secant piles; caissons belong to municipal pump chambers, jacking pits and bridge foundations.

Correct it by digging eccentrically. Do not load the high side. Dig more on the high side and leave a soil berm on the low side so the shaft rights itself under its own weight; light cases respond to local digging at the cutting edge or jetting to cut friction. Forcing the high side down with kentledge drives the cutting edge into the ground and triggers a sudden drop - by the time it is level it has usually over-run. Prevention is far cheaper: dig in symmetric layers, check verticality every 0.5-1 m, and start correcting at 1%.

The base plug is the most dangerous step. Seal immediately on reaching level - stopping there risks over-run or sand boiling. Underwater, the plug must be tremied continuously though one pipe: an interruption leaves a cold joint. Clean the base first: the thickness of sediment decides whether the plug bonds to the cutting edge. And check uplift before you pump out. If the plug is too thin or the water table is high, pumping lifts or punches through it and the pit floods with sand. Where uplift is short, lower the external water table, thicken the plug, or keep ballast water inside.