Minimum Clearance of Drilled Shaft from Tunnel Wall
Minimum Clearance of Drilled Shaft from Tunnel Wall
(OP)
I am proposing a axially-loaded 30-inch diameter drilled shaft adjacent to a tunnel wall. The drilled shaft is designed for end bearing of 5 tsf. The length of the drilled shaft will be 50 feet. The bottom of the tunnel is at a depth of 40 feet and the height of the arched tunnel is 10 feet. The tunnel wall is composed of unreinforced concrete constructed in 1906 (i.e., fragile eggshell).
In reality, the loading of the drilled shaft will include side friction and end bearing with more side friction after construction and shifting to end bearing over time.
What is the minimum clearance of the drilled shaft from the tunnel so that zero stress from side friction is imposed on the tunnel wall?
Thanks for reading.
PS: Not considering bond break since extra cost.
In reality, the loading of the drilled shaft will include side friction and end bearing with more side friction after construction and shifting to end bearing over time.
What is the minimum clearance of the drilled shaft from the tunnel so that zero stress from side friction is imposed on the tunnel wall?
Thanks for reading.
PS: Not considering bond break since extra cost.





RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
One of the other aspects is deviation during drilling. An ACP pile might end up straight into the tunnel while a cased rotary pile might not depending upon the clearance between the two structures.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
(excerpt from upcoming geotechnical suspense novel)
PS: geology is medium dense to very dense SP and SM and hard CL. Groundwater at 30 feet. Probably relict shoring close to the tunnel.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
1)AASHTO Highway Bridges 4.6.6.4 say 3B center-to-center spacing between adjacent drilled shafts. This requirement seems intended to avoid disturbance of side friction or green concrete of adjacent shafts.
I think this spec is a valid argument in my favor since if AASHTO considers no loss of skin friction of adjacent pier at 3B then in my case there should be no stress placed on the tunnel wall using the same requirement (let me know if my argument is shaky).
2)FHWA Drilled Shafts Manual, Appendix B, Axial Group Effects. This is a collection of data and hypothesis that seems to indicate that 6 feet is about right. I am using the same logic that there would be no stress on the wall at this distance due to side friction from the shaft.
I accept your valid points about construction tolerances, plumbness of the rig, disturbance caused by drilling and lack of as built info. I am relying on plans for 1906. There are manholes so it is possible to enter and verify location prior to drilling.
I imagine the shafts will be contructed by auger drilling and steel casing. The sand and clay soils are dense/hard but should be OK for augering without much heave to side of the hole.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
assuming 1906 concrete is "eggshell" is not necessarily a reasonable assumption without some evidence to back it up. But I would do an engineering evaluation first, testing, as-builting, determine thickness, strength and condition of the lining and do a structural analysis. I would also see if you can find a sucker to drill the hole and ask them to guarantee this. With sand, especially interbedded with clay and high groundwater I would be very concerned about loss of ground, but maybe you have some geotechnical information you can rely on that says otherwise.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
Ignoring all end bearing conventional wisdom would say apply the vertical load of the shaft group at the depth D', which is equal to 2/3D where "D" is the length of the shaft, i.e., 50 ft. That makes D' equal to 33.3 ft or 6.7 ft above the bottom of the tunnel (3.3 ft below the top of the tunnel).
Now you can run some numbers through the Boussinesq equations to see the affect. Don't forget to double the horizontal loads that you calculate as there is no soil on the other side of the egg-shell tunnel.
You'll probably end up at 7.5 ft or so. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
Thanks. I had not considered Boussinesq "onions" starting at D'. That makes a lot of sense. I had been thinking of the problem as the side friction causing shear strain to adjacent soils. That may still be true. But, your method provides a means to analyze a horizontal stress on the tunnel wall above the bottom of the shaft.
Dick,
Tunnel is combined sewer. But, we a looking of a safe clearance where it can be argued that there is no impact. Entry and testing could be done but an expensive and hazardous affair that is better avoided, if possible.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
It is the job of the geotechnical designer to assess impact to adjacent facilities or verify that there is no impact. I can support my claim that there will be no impact from drilled shaft next to this tunnel. This job site is in Baltimore which has its fair share of underground facilties, much of which pre-dates modern construction. The assumption of egg-shell/sensitive condition is only an assumption since no re-bar, etc. Knowledge of actual condition would only serve to prove that tunnel wall is better than assumed. In which case grade beam span over tunnel could be reduced. But, as it stands now the span is about 36 feet which is fine. If the tunnel failed and ground collapsed then drilled shafts will support building including any lateral load.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
I would enjoy being an expert witness for the client's lawyer in the event that this project ended up in court.
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
If you need an answer of zero, you'd likely need to sleeve the most proximal piles.
Don't worry about the lawsuit. Just use good engineering and stay within the lines.
f-d
¡papá gordo ain't no madre flaca!
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
RE: Minimum Clearance of Drilled Shaft from Tunnel Wall
Richard A. Cornelius, P.E.
WWW.amlinereast.com