Heel-Less Basement Retaining Wall

[FEM4Structures]

An old residential structure with failed basement wall (structural cracks all around). Any ideas on how to reinforce the retaining structure such as an L-shaped wall with Toe lying on top of basement slab? Sliding becomes an issue...any ideas...Much appreciated.

Thanks,

FEM4Structures

 

[XR250]

What about using solider beams?

 

[BAretired]

You could build a wood stud wall inside the existing basement wall. I have done that on a number of older residences. The studs must be anchored top and bottom and capable of resisting the soil pressure.

BA

 

[FEM4Structures]

I like your idea. Do you know of a design document which can explain the calculations?

Thanks,

FEM4Structures

 

[FEM4Structures]

The retained height will be 8 feet. I have experience with typical wood framing however I will highly appreciate if you can refer me to a reference material which may have such calcs.

Thanks,

FEM4Structures

 

[XR250]

I have done the wood stud method as well using 2x8 or 2x10

 

[dhengr]

Any good text on wood framing and wood design and construction should help you with this problem. If you don’t have that text you should probably not be doing this kind of design and construction repair. Maybe throw in a soils text and a bldg. code too. Also, some knowledge and study of the cracking pattern and any moisture problems should be factored into the solution, because sometimes they indicate the cause of the problem.

 

[BAretired]

It is a bit unusual to have as much as eight feet of retained backfill on old residential basement walls. The walls I have fixed have usually been six or seven feet high with about four or five feet of fill on the exterior, allowing for basement windows. The basement walls were constructed of brick or very old and poor quality concrete with rounded aggregate...the sort of stuff where you could almost pull chunks out with your bare hands. Sometimes they had deflected inward as much as three inches under the pressure of the clay backfill which is predominant in my locale.

The calculations are pretty straightforward if the design pressure is known. I have always assumed that the existing wall is incapable of resisting any lateral pressure but is capable of carrying gravity load, subject to site confirmation.

PWF material is recommended for the bottom plate which will be fastened to the existing grade slab. Studs may be untreated unless there is a lot of water coming through the old walls in which case they should be PWF material too. The top of studs must be adequately anchored to the floor system to resist the horizontal reaction due to earth pressure and the floor system should be checked to ensure its adequacy. Sometimes, extra bridging is required between joists where walls are parallel to joists.

After building the stud walls and fully anchoring them top and bottom, wedges or shims are installed in the variable gap between the studs and the existing wall to transfer earth pressure directly to the studs.

In your case, with an eight foot height of retained earth, you should obtain a soil report to provide recommended design values of lateral pressure which vary according to soil type.

If there is no weeping tile around the exterior of the footing, exterior grading must ensure that rainwater and snow melt will flow away from the foundation. Alternatively, a drainage tile system inside the existing basement wall may be required.

BA

 

[AELLC]

FEM4Structures,

I am assuming you are a recent University graduate, and don't have any experience with wood, which is very common.
I could help if you don't have the time at work to research all this, and you are not able to seek advice from co-workers.

This is merely wood studs in flexure, with no axial load, and Load Duration factor = 0.9 because of soil pressure, correct?

Give me height of stud wall, pressure distribution of soil, and wood species and I could run a quick check on my "software".

 

[FEM4Structures]

All,
Thanks for responses...Much appreciated

Thanks,

FEM4Structures

 

[FEM4Structures]

The stud height will be 8 ft with 7.5 ft soil retained behind the basement.
Active Moment will be 3,400 #.ft per ft. To be on conservative side, I can use DFL 4x4 16" O/C with sheathing on both sides. What should be the sheathing selection given that wall is exposed to insitu soil and moisture. Also, don't have numbers on existing roof beam. How would one conservatively deal for connecting studs to the roof beam? Recommendation highly appreciated.

Most of my experience is in concrete and steel structures but don't want to use concrete retaining wall solution. Just want to have confidence in the wood solution.

Thanks,

FEM4Structures

 

[BAretired]

The stud height will be 8 ft with 7.5 ft soil retained behind the basement.
Active Moment will be 3,400 #.ft per ft. To be on conservative side, I can use DFL 4x4 16" O/C with sheathing on both sides. What should be the sheathing selection given that wall is exposed to insitu soil and moisture. Also, don't have numbers on existing roof beam. How would one conservatively deal for connecting studs to the roof beam? Recommendation highly appreciated.

I don't think 4x4@16 o/c is a good choice. How did you arrive at 3400#' per ft? Doesn't sound right. Sheathing can't be installed on the outside and is not required on either side. Wall is not exposed to soil and moisture if it is inside the existing wall.

Roof beam? Don't you mean main floor joists? Provide a cross section.

BA

 

[FEM4Structures]

I was using 8 ft retained height.
For 7.5 ft, I have following numbers
Active force = 0.5 x 120 #/ft3 x 0.333 x 7.5ft ^ 2 = 1124 # per ft @ 2.5' above base
Active Moment = 2810 3.ft per ft

It is floor joist. Please ignore the typing brevity as I was typing on smartphone.


Thanks,

FEM4Structures

 

[FEM4Structures]

How would one mitigate with severely cracked wall if there are chances of mass seeping (during rainfall) into basement without sheathing.

Thanks,

FEM4Structures

 

[BAretired]

You are using an equivalent fluid pressure of 40 pcf so the pressure at the base is 7.5*40 = 300psf.
W = 300*7.5/2 = 1125# @ 2.5' from bottom (5.5' from top).
Rtop = 1125*2.5/8 = 352#
Mmax is at 0.5 + x where 40x[sup]2[/sup]/2 = 352
so x = 4.19'
Mmax = 352*(0.5 + 4.19*2/3) = 1160#'/' which is less than half the value you reported.

Please confirm my numbers.

BA

 

[BAretired]

How would one mitigate with severely cracked wall if there are chances of mass seeping (during rainfall) into basement without sheathing.

You must provide an adequate drainage system if it isn't there now. Exterior grading could be done to guide water away from the foundation or you could provide a weeping tile system connected to a positive drain.

Sheathing can't be installed between the new studs and the existing wall. Even if it could, it wouldn't prevent water from entering the basement.

BA

 

[FEM4Structures]

I am attaching my calcs for the studs spaced at 16" O/C. I have maximum bending moment for the studs = 1.16 k.ft per ft x 1.33 ft = 1.55 kip. ft / stud.

Please confirm if you don't agree with the number.

Thanks,

FEM4Structures

 

[FEM4Structures]

I agree with your numbers. I multiplied it with the stud spacing.

Thanks,

FEM4Structures

 

[AELLC]

40 psf/f looks low. I use 52 because it is in-situ pressure. Instead of 4x4, why not 3x6 pressure-treated.

 

[XR250]

Make sure you check deflection and long term creep. 4x4 seems kind of thin. I usually end up with 2x8 min. which is significantly stiffer.
Also, PT lumber (and all modern lumber in general) seems to creep alot more than the NDS would lead you to believe.