Underpin Design with Rakers / Soil Anchors

[dcjones]

I am designing traditional pit underpinning for a new addition with basement in an existing building. The building is a lightly loaded one story structure with a 2 ft wide concrete wall footing about 3 ft below existing grade. The new basement's bottom of footing will be 10 ft below the building's slab (7 ft below bottom of existing footing). The new basement wall's proximity to the existing building is along a slope and varies from 0' to 7' away.

The soils are an elastic silt with an internal friction angle of 25 degrees. The geotech. engineer has provided an EFP of 50 psf and 3,000 psf allowable bearing for the pins with this soil.

Unfortunately, after doing the calculations for the pins the soil is not able to stabilize the pins without a tie anchor or rakers. Due to the lightly loaded structure there is not enough weight on the pins to control the sliding / overturning with the relatively large lateral loading.

I would like to use rakers instead of tiebacks to limit mobilization of equip for soil anchors and avoid drilling below the building. Although the concern with rakers is the interference with the new wall and footing due to them being required to go through the new foundation wall.

Does anybody have experience using rakers instead of tie backs? Lateral force would be roughly 20 kips for rakers at 8' o.c. Would they be cheaper then soil anchors?Estimate 15 (rakers/tiebacks req'd)

I am also looking for details for the raker to underpin / deadman connections if anybody has some input would be helpful.

Thanks.

 

[fattdad]

Your equivalent fluid density is likely incorrect and too liberal. Earth pressures for walls that are not free to rotate, must be based on at-rest earth pressure. At rest earth pressure is often taken as Ko= 1-sin(phi), which would return a value of 0.58. Equivalent fluid pressure is that value times the moist unit weight, which could be 130 pcf. I'd be using 75 pcf in other words.

(actually if the clay is overconsolidated there is a further adjustment to the at-rest earth pressure.)

Now, we really don't know the Ko value. If you jam a dilatometer in the ground you could measure Ko directly.

I'll let others discuss rakers v. tiebacks.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

If you are designing and need help, I suggest contacting an experienced contractor or two such as those that put ads in Civil Engineering or Engineering News Record and get a price for what they would do and aske for a confidential description of their plan.. There is nothing that can substitute for experience.

 

[dcjones]

Fatdad,

Thank you,

You are correct the 50 psf was for the active pressure which I was hoping would work w/o anchors. The at rest is 73 psf (EFP) and is what I used to get the 20kips lateral for each tie back/raker.

 

[PEinc]

Raker braces would be temporary lateral support for the underpinning piers. You would need to provide permanent support for when the raker braces are removed. This permanent support would be the new addition's foundation wall. The underpinning would support the existing building vertically and would eliminate any lateral surcharge pressure from the existing adjacent building. However, there would still be permanent lateral soil pressure from the soil beneath the existing building. The new foundation wall would need to be supported at its top, usually by the addition's first floor. The base of the new foundation wall would need to be supported by either 1) sufficient embedment of its footing below the new basement slab subgrade, 2), by the new basement slab that would be poured against the foundation wall, or 3) by the addition's dead load causing friction resistance at the bottom of the new wall footing.

Generally, raker braces are cheaper than tieback anchors, especially for small quantities. However, tiebacks are usually preferred because they make other work operations cheaper (no raker removal, no wall penetrations, easier excavation, etc.). However, you may be able to find a small, helical anchor contractor who could be economical compared to using raker braces. The tiebacks for the underpinning could be either temporary (but remain in place) or they could be permanent so that the new foundation wall needs to support vertical loads only.

I suggest that you call a company such as Ram Jack for ideas.

http://www.PeirceEngineering.com

 

[dcjones]

PEinc,

Thanks yes the new basement wall is designed as a foundation wall and supported by a concrete first floor and basement slab.

If I were to use helicals, do they usually get set just after the pit excavation (through the approach pit)? And then the underpin concrete poured around them?

 

[PEinc]

Installing the helical anchors first would probably interfere with digging the pits. When I have installed hollow stem auger tiebacks in underpinning piers, I dug and shored the pits. Then, I installed a corrugated pipe sleeve in each pier needing a tieback and then poured the pier concrete, leaving a corrugated hole that was big enough to fit the auger (or your helices). Then, I dry packed the underpinning piers. Next, we dug down to the tieback grade and installed the tiebacks thru the piers. Since the tiebacks can't be grouted or concreted to the piers, you should have a smooth plastic sleeve over the portion of the tieback shaft or tendon that passes thru the pier so that when you fill in the corrugated sleeve with concrete, the underpinning pier does not bond to the tieback. Once the tiebacks are installed sleeved, and concreted in the piers, the tiebacks can be stressed and locked off against the piers.

Smaller diameter tieback anchors could be drilled thru the piers and then be grouted into the soil behind the underpinning but that would probably cost more than having a little contractor screw in helicals.

http://www.PeirceEngineering.com