Depth of Excavtaion Around a Drilled Shaft

[EngineerX]

Hi,

I just finished the design of drilled shafts for a bridge. A utility company expects to excavate a maximum of 11', sometime after the shafts are constructed. They would like to know how close to the shaft they can excavate without undermining the stability of the shaft. Does anyone know of any references, papers or existing standards I could use? The bridge will be built in New Jersey.

The shaft is about 30' deep, and socketed a depth of about 8' into bedrock. The lateral loads control the design.

Thanks.

 

[BigH]

How long will the excavation be open? What kind of soils do you have (sand/relative density, clay/consistency? You also indicate that lateral loads govern - are the loads "high" due to earthquake? . . . other live loads. I would, as a study, see what effect my design would have if I removed the earthquake bit and then see what the loads are. If open for a very short time, then there is that "risk" you would take neglecting seismic effects. Give these additional facts and you might find a better results from your query.

 

[EngineerX]

Thanks for your response.

The soil to the top of bedrock is mainly coarse grained, with a unit weight of about 120 pcf. The analysis was done for service, seismic and wind load, but for now I am considering only the effect of service loads. Excavating 11', results mainly in excessive deflection, the compressive, tensile stresses remain below allowable, and the shear stress can be handled by providing more reinforcement. But my main question is this: Is there an influence zone beyond which the removal of soil will have no effect on the behavior of the shaft? I haven't been able to find any reference. I know finite difference analysis could answer the question, but it can be rather time consuming, for now I just need something 'quick and dirty'. I'm using L-Pile in my analysis, and it doesn't allow me to simulate the effect of soil removal at a distance, as far as I know.

 

[Focht3]

EngineerX:

Let me try to answer your last question first: if the excavation is stable and at least 17 feet from the pier shaft, then the overall effect should be minor. This assumes the trench is shored - and that a trench box is not allowed. Shoring protects against excessive lateral movement; trench boxes only protect the workers. The use of a shored excavation will help reduce the risk of damaging movements in the remote circumstance of a design earthquake occurring while the trench is open.

Treat the soil between the trench and pier as though you have a slope stability problem - check with your project geotech to see what he / she would consider a stable slope in this material. Draw a 'slope' into the ground from the shaft/ground surface toward the trench; if the trench is above this line, you're probably okay.

I have assumed the trench parallels the bridge; if this assumption is wrong, then let us know.

I'm bothered by your statement that deflections are excessive if the upper 11 feet of soil is removed: are the piers independent, without fixity at the top of the pier shafts? I would not expect a granular soil at the surface to offer enough lateral resistance to be of real benefit for the first 5 to 10 feet to affect the deflection profile of the pier. What are your design loads?

 

[EngineerX]

Thanks for your reply. It is most helpful. The design loads are 674KN 8492 KN and 1173 KN-m for horizontal, vertical and moment. There is fixity at the top of the piers, but in L-Pile the boundary condition used is for Shear and Moment, not Shear and Slope, w/ Slope = 0 for a fixed pile. I plan to refine the calculations later. Do you have a reference for the 17 feet quoted above, or know of any supporting literature I could provide with my calcs?

Thx.

 

[Focht3]

No reference for the 17 feet; comes out of stability calcs done for similar problems (it's a 1.5:1 H:V slope.)

If you have fixity at the top of the column, you can't apply a moment as well. Re-run your analyses with the proper end conditions, and I suspect your deflection problem will go away...

 

[EngineerX]

Thanks a lot, and best wishes!!