Casing Oscillator effect on side friction

[Alexander87]

Hey Guys,

Does anyone have any references that quantify the impact of oscillating steel casing on side friction for a drilled shaft, particularly for overconsolidated silts (MH) and clays (CL/CH) with sporadic sand and gravel lenses.

I believe FHWA does not delve into this particularly deeply other than saying it may actually improve unit friction values but our understanding is it can adversely impact unit friction values.

Also does anyone have any rules of thumb/methodologies for ensuring and verifying shaft base cleanliness when the shaft has been excavated using a spherical grab and will tip out in the aforementioned stiff clay/silt?

Thanks!

 

[oldestguy]

There are rules for OKing the base of excavated caissons, generally at least 3 feet diameter. My memory may not be fully correct, but no more than 3 inches of water, at least half of area undisturbed. I've done some checking from above with a long rod, but it's pretty crude. Best bet is get someone down there. Case the hole, secure the guy with a harness and rope from above, test for oxygen. Is that allowed?

 

[Alexander87]

Hey oldestguy, these will be constructed over-water in the wet so there is no possibility of entering the shaft itself to verify. I'm assuming for now the contractor will airlift the base for accumulation and we will verify pre-pour with sounding weight. I'm assuming a SID would be difficult to use if there is a spherical end bearing surface at base but I'm not sure what other methodologies are out there for this particular case.

Any idea how a spherical end bearing surface caused by the grab affects axial end bearing capaity?

 

[oldestguy]

Spherical vs flat, I'd suspect insignificance difference for same area (horizontal plane). Can any impact or over-loading be done? For a large job, even a load test may be in order to accept the technique. Tremmi placement of concrete maybe difficult and a load test on typical completed piles might check this as being OK. As to effect of soil type because of vibrations, that also may need checking by later load test since silts can become weak because of disturbance.