Deep anchor near the failure plane 1

[FixedEarth]

There is a one level anchor drilled pier shored wall near a property line. Tieback anchors and Deadman anchors are not possible due to being outside the property line. Using another drilled pier as an anchor which derives its capacity from skin friction below failure plane is an option. Cantilevered pier would require too deep of an embedment.

Does anyone see any fundamental issue with this approach? See attached sketch. Thank you.

http://www.soilstructure.com/

 

[PEinc]

I don't like it. The shafts are too cose together. The rear shaft will be pushing on the front shaft. The rear shaft acts as a deadman. A deadman's passive wedge should not overlap the wall's active wedge.

http://www.PeirceEngineering.com

 

[FixedEarth]

I didn't realize about the passive overlap - thanks for pointing that out. I guess braced excavation is the only approach left for consideration?

http://www.soilstructure.com/

 

[RFreund]

PEinc has a good point. However, could the system act like a 'crib' type system where you are using the soil between the two "walls" to resist lateral pressure behind the rear "wall" (like a gravity wall). The only problem there is that I'm not sure how you connect the two "walls".

Braced excavation sounds about right, given the restraints.

EIT

http://www.HowToEngineer.com

 

[ameyerrenke]

No possibility of talking to the adjacent property owner to get permission to put the deadman on his property? I would think the adjacent property owner would be happy to ensure that the wall was stabilized correctly, since it would likely effect his property if it wasn't.

 

[FixedEarth]

RF;

The anchor piers would need more setback to work, so it is no longer possible. Seems Bracing every other drilled pier would work as you suggested.

ameyerrenke;

The client did not want to consider asking client for tieback permission. So this is approximately 28 ft x 25 ft garage. Above it will be a residence. Due to my current work load, the complexity of this job and the speed with which the owner wants to proceed, I had to let this project go to others. I am interested however in getting to a practical and buildable solution. It is very possible similar scenario will present itself next year.

The bracing solution will expose the struts and am just not a fan of that industrial look for a custom residence. I would like to hear from the members about this approach. I am thinking to drill a 30 inch diameter pier say every 7 foot on center reinforced with a W section. Then excavate about 8 ft down and place struts at the top of the pier. Then pour the concrete floor burying the struts. Then proceed excavation of the remaining 10 ft. Alternatively design the soldier beam and struts as a frame and still bury it in the concrete floor. Looking forward to your comments.

http://www.soilstructure.com/

 

[PEinc]

"...could the system act like a 'crib' type system where you are using the soil between the two "walls" to resist lateral pressure behind the rear "wall" (like a gravity wall)..."

Probably, but the two walls would need to be far enough apart that the mass between the two walls would be sufficient to prevent mass overturning and sliding. There may not be enough distance between the two wals to perform as a gravity crib wall.

http://www.PeirceEngineering.com

 

[aeoliantexan]

Could you connect each pair of piers with a cap or beam with a moment connection to create a "fixed head condition"? The stiffness of laterally-loaded piles is roughly doubled for a fixed head condition. If you use a continuous cap, you can stagger the piers to reduce their interaction.

 

[dcarr82775]

As PEInc notes, the piers are almost certainly too close together, the soil utilized for resistance will be doing double duty. Cap beam doesn't do you much good.

There are a couple of firms local to my area that will build what they call an A-frame wall out of micropiers. The front row is vertical and the back is steeply battered (in your picture down and to the right). They use a cap beam to tie them together. They do not fall down (often), but I have never been able to make the numbers work. Although 18ft is pushing it even for them.

if the soils are not too bad you ought to be able to just use tangent piers.

 

[FixedEarth]

aeoliantexan;

That is a great idea but I will need to do bracing in the perpendicular direction as well. Correct, a fixed head pier will deflect only half as much as the free headed pier. Thanks.

dcar82775;

The second drilled pier idea is now abandoned due to PEInc's comments. The soils are very dense Sandstone with no groundwater. I like the tangent piles ideas and think it would work as a cantilever.(or Secant piles for that matter) Thanks.

My two concerns with cantilver is this will deflect too much and am concerned about the corners. I anticipate lateral defelction of a free headed pier to be large for the 18 ft cut. Also I would have to provide a rigid type wall such that it will support the 2nd story. (only 2 kips/ft exterior load). In the past I have used cantilevered drilled piers to support a 16 ft cut with about 1.5% vertical steel bars. So the 18 ft wall is not that much of a stretch.

May be a counterfort retaining wall?

http://www.soilstructure.com/

 

[dcarr82775]

A little out of the ordinary, but you could run post tension the pier. If the strand is located properly it could counteract the pier deflection a bit. I did a design once, but it didn't get built. Probably too expensive, but it looked good on paper

 

[RFreund]

I have not designed a soil nail wall, but can an embedded soil nail wall be used. I'm assuming the soil nails are used to engage the weight of the soil directly behind the wall and do not need to extend beyond the failure plane.
Not sure how much soil is between.

EIT

http://www.HowToEngineer.com

 

[crcivil]

FixedEarth,

It would be worthwhile for you to try and ask for an easement permission. Once you get it (I don´t see why you wouldn't), your client will be happy to hear the good news and you can go on with a more practical solution by installing your tie-backs. I mean, talking doesn't hurt, does it?

 

[FixedEarth]

RF;
Soil nail is usually ideal but requires 20 - 25 ft nail length and this puts us in the neighbor's property.

crcivil;
For some reason, I got the feeling they wanted to stay inside their P/L lines. I agree, it would be easier at leat to discuss it.

Thanks to all. This project is on someone else's plate now. I think in the future, I would suggest a braced excavation or tangent piles.

http://www.soilstructure.com/