Minimum Clearance of Drilled Shaft from Tunnel Wall 1

[molerat2210]

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.

 

[miningman]

Well I'm looking forward to reviewing the theoretical answers to this one, but personally I'd be a lot more concerned about the effect of ground vibrations generated during the boring of the 30 inch hole, on an existing egg shell structure

 

[MiketheEngineer]

Me too!!

 

[BigHarvey]

Don't forget to check if the tunnel is able to withstand the fresh concrete pressure when pouring the pile.
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.

 

[cvg]

I think a thorough understanding of the geology is critical to answering this question

 

[molerat2210]

The engineer slowly paced the office in his tweed jacket with his pipe in hand. As he passed the great oak bookshelves his mind pondered the great works of Terzaghi, Meyerhoff, Bowles, Vesic, et. al. After careful thought he came to the answer: 6 feet.

(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.

 

[miningman]

Molerat, I hope youre not serious at 6 feet. If the geology was good competant hard rock, the minimum required pillar distance is 12.5 feet. However the presence of water and something less than competant rock, makes me re iterate my previous warnings, plus drilling deviation most certainly is a factor, and how do you know that the tunnel is EXACTLY where you think it is. I hope your are not relying on 100 year old plans

 

[molerat2210]

Miningman, I appreciate your thoughtful comments. I am serious about 6 feet. My support for 6 feet of clearance comes from 2 sources:

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.

 

[miningman]

Good luck, and dont take this the wrong way, but you need to get out into the field more often.

 

[cvg]

can you install struts and bracing in the tunnel before you drill?
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.

 

[molerat2210]

Thanks for everyone's good advice. I will increase clearance to 7.5 feet in case caisson driller forgets to plumb the rig and help avoid any ground loss effects on the tunnel walls. I shall also leave to comfy confines of my cubicle and make a pilgramage to the field to hear the wisdom of the caisson drillers.

 

[dicksewerrat]

What is being conveyed by the tunnel? If sewage, then some indepth testing should be done. That tunnel may have been 10-12 inches in '06 but probably closer to 5-9 inches now. You have to go in and take pictures, soundings, measure cracks etc. as if you were going to rehab the tunnel.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[fattdad]

I don't think that 3B group effects criterion has any bearing on this problem. While I'm not sure the answer, I'll attempt to think through my approach. . .

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!

 

[molerat2210]

fattdad,

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.

 

[dicksewerrat]

I can't believe the owner doesn't want to know the tunnel condition not even taking into account the work around it. Has anyone thought of what you are going to do if the tunnel is impacted? Once it goes, it is late to start planning. Where is this tunnel anyway.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[molerat2210]

Dick,

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.

 

[miningman]

Molerat, I have to ask this. Are you implying that if, in the highly unlikely event that your geotechnical evaluations / recommendations result in a less than satisfactory end product to the client........ lets assume that for whatever reasons the sewer tunnel collapses... then your response will be something to the effect " Well the building is still standing , therefore I have no responsibiliy for the client's predicament??".

I would enjoy being an expert witness for the client's lawyer in the event that this project ended up in court.

 

[fattdad]

From the drive home, I have done some reconsideration on my original thought process. Considering that it's granular soil, I'd likely divide the friction length into 10 ft sections and calculate the frictional resistance along each 10-ft section. That would be a function of Ko and the coefficient of interface shear (delta). Of course it'd increase with depth so each 10 ft section would have increasing capacity. Then I'd apply that force at the D' for each 10-ft "joint" of pile and run a series of Boussinesq equations (horizontalx2 and vertical) for each of these loads.

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!

 

[Ingseven]

The way you describe the procedure (drilled shaft) plus the bearing capacity of "dense to very dense sand" supporting the shaft makes it unlikely that any load will be transfered to the tunnel if the shaft is 6ft appart. Do the math and confirm but I would not expect effects on the tunnel. Also, A 30" shaft will not deviate from the vertical as easy as a borehole. In worst case consider 5% of the length for vertical deviation.

 

[dicksewerrat]

Baltimore sewers are not the best in the world. We have done a lot of work on them. Even the smaller shallower ones show bad compaction during construction. Granted that in 1906 they used good practices, I'd still want to look at it if I was the owner.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com