DS Uplift Capacity with Socket

[RJ62]

I'm having a debate in the office on whether the soil-drilled shaft side friction for uplift is mobilized before the uplift load is entirely resisted by the rock socket.

The major drilled shafts are a 72" with 610 kip uplift and a 54" with 475 kip uplift. The shafts go through 40 to 45 feet of silt (ML) with sand. The rock is hard, intact, granodiorite. Probes using an air-track rig indicate no or very few fractures or seams.

The question is, as the uplift load is applied, is the skin resistance in the soil mobilized (partially, fully) before the side friction of the concrete and rock socket comes into play? Is there a depth (distance from top of socket) at which point the soil side friction is discounted and the entire uplift load is carried by the socket?

Has anyone seen papers discussing this? Thank you.

Jefferys

 

[dmoler]

The FHWA Drilled Shaft Manual has some good methods and charts for estimating the mobilized shaft friction vs. movement. Emperical, and based on large amounts of data, and generalized, of course.

Based on my projects where I have installed vibrating wire strain gauges to determine this type of thing, the data usually shows that the movemement mobilizes the skin friction fully in the middle depth area, and less near the rock socket top. Also, the skin friction for the shaft top portion, say from ground surface to 10 or 15 feet, could go from fully mobilized and then reduce much less to the residual friction load resistance as loading is applied and the shaft moves.

It sounds like the rock would have a very high modulus. Once the movement reaches the rock, the movement/load increments gets much smaller, since mobilizing rock shear strength only takes very small shaft movement relative to silt/sand shaft shear strength mobilizing movement. Subsequently, the silt/sand shear strength has not fully mobilized extending from the top of the rock socket to some distance above the socket, probably 5 or so feet, but that depends on certain silt/sand soil constitutent variables and modulus values for the shaft and soil. This portion might not ever develop full mobilized strength, due to limited rock socket shaft portion movement directly below.

 

[RJ62]

dmoler,

Thanks for the lead. I've been looking through that publication. It seems to treat the soil ressitance only and rock socket resistance alone but nothing tying the two together. But I'm still looking. I know it is a strain compatibility problem and that the soil resistance should fall off to zero several shaft diameters above the rock, but I haven't found any research on the subject.

I'll keep looking through the FHWA drilled shaft manual. Thanks again for the response.

Jefferys