Shaft sinking advice

[renko]

Hello,
I'm designing a large storm holiding tank. The volume is 6000m3 (1585000 gallons). Its all under ground. The ground conditions are:
0-9m (0-29.5') soft SILT
9-14m (29.5'-46') soft CLAY
14-16m (46'-52.5') GRAVEL
on LIMESTONE BEDROCK
The ground is saturated up to surface level

The tank is to be approx 25m (82') diameter and 16m (52.5') deep.

My initial design was to use a secant pile wall down to bedrock, then excavate and build a RC tank in the void.

The contractor has suggested shaft sinking instead - I had considered this, but at Tender stage there was no local company with a 25m shaft capacity.

I like the idea, but my question is how do I stop the soft saturated soil 'running' from the outside to the inside of the shaft as it is being sunk. I know that we will have to pump out water, but what about a 'running silt'?

Apologies for the long question, any advice will be appreciated.

 

[DRC1]

Thats a pretty gosized shaft. Shgaft and cassion sinking was a technique that was popular up until the late 60's to70's when drilling technology got better. Typically the bottom ot the shaft is a steel cutting shoe and the shaft is built in lifts. The lift may be cast in place or precast. After the lift is built the material iside is excavated. After a certian amount of excavation, the shaft will settle. The excvation poceedes such that the settlement is uniform. When the shaft reaches the ground level, the next section is built. Running ground is reduced by excvating in the wet. Also since bearing capacity is generally low in such soils, it may be possible to leavea plug. Tilting of the shaft is a concern in running ground. Chemical grouting may be used to stablize the grond if necessary.
This is not just a dig and see what happens. It does require a lot of planning and engineering before and during construction. However, if the contractor has experience with the technique, it can be very effective.

 

[renko]

Thanks DRC1,

The contractor's method is as you've described, and they do have 30 years experience with this. I've discussed grouting and freezing to stabilise the ground, but due to the cost the contractor are opting not to go down that route, and accept the risk. They seem confident that they can excavate effectively.

When you say "Running ground is reduced by excvating in the wet" - do you mean that we shouldn't try to reduce the water table?

 

[DRC1]

Correct. By not drawing down the water table, the hydrostatic pressure inside and outside the shaft will be approximately equal. Thus the tendency of the water to flow into the shaft, brining soil withit, is reduced. Obviously as you excavate and remove soil, water will flow into the cassion, but the difference will be small and the flow will be controlable.