Crawl space converstion to basement w/ (n) retaining wall

[mcr07]

Hello Eng-Tip Forums

I'm currently working on a residential project (Los Angeles CA), in which a crawl space with interior sloping grade is intended to be transformed into a basement (8ft tall). I'm sure this type of conversion has been done before.

I'm designing the cantilever retaining wall, (see attached pdf) and my construction constrain is that it's not possible to demolish the (E) SOG on the exterior side (there's currently a patio and a second story bedroom above it). So the idea is to try to build the retaining wall from the inside of the crawl space. We have proper soils report, with the majority of the project being founded in sandstone bedrock.

I'm considering the design of a cast in place retaining wall against the (E) wall footing. I'm properly providing epoxy dowels to the (E). However, I have a question in regards to a point load coming from the patio/2nd bedroom framing. It's a footing founded adjacent to the (E) wall footing. I haven't run the retaining wall numbers yet; but is a surcharge point load of approximately 10kips something of a concern? Reason is, it'd be preferable not to have to demolish around this (E) spread footing and lower it to the (N) retaining wall footing level. If the (N) retaining wall is designed for this surcharge, is there anything else I should be considering?

I appreciate any comments.

 

[msquared48]

Well, I would do two things differently.

1. I would underpin the current footing at 4 to 8 foot centers with concrete piers along the length of the wall. This will remove the surcharge load from the new basement wall from the existing footing.

2. Then I would make this a non-yielding basement wall configuration rather than an "L" shaped retaining or yielding wall as you have shown. The "L" shaped wall is very inefficient and can overstress the soil. You will need to establish a sound connection to the existing floor diaphragm to make this work, and verify the capacity of the floor diaphragm to carry the soil load.

Mike McCann
MMC Engineering

 

[mcr07]

Thanks msquared48;

1) when you suggest underpinning the current footings, do you mean also to underpin the isolated spread footing that carries the point load?
2) I'm guessing you're referring to design the retaining wall as a "constrained" condition and analyze that the reactions of the wall can transfer across the floor diaphragm?

Thanks again for your prompt feedback.

 

[msquared48]

1. Yes, I would underpin that too.

2. Yes.

Mike McCann
MMC Engineering

 

[mcr07]

Thanks msquared48. Sounds good

 

[XR250]

I have done it the way Mike has suggested, but many times, we just shore up the house and replace the foundation wall entirely. Attaching it to the floor system can be a challenge as Mike pointed out, but you end up with a lot more room for not that much more cost.

 

[mcr07]

Thanks for the tip ExcelEngineering. However, the owner wants to save in cost and he doesn't mind if he loses 12" of space along the perimeter. The contractor on board considers this a good amount of savings if we can design something that is cast against what' existing without having to jack the entire house and replace what's existing.

Are there any comments regarding casting a (N) concrete retaining wall against an (E) wall footing?

 

[EngineeringEric]

You will get a surcharge, try looking at "boussinesq bulb of pressures" or others. Boussinesq is a great start and shows things in a simple graphic way.

Theoretically, you can design your wall and footing to resist all the loading as you have it. You may want to looking into soil anchors at the top to be installed. this will help handle your loads and help the deflection at the top of the wall.

 

[XR250]

@EngineeringEric, mcr07;
If he underpins the existing footing, there is no surcharge from the footing gravity load, but still a surcharge from the back fill ABOVE the new retaining wall. That needs to be accounted for.

 

[mcr07]

ExcelEngineering:

I wanted to point out that the purposes is to build the wall from the inside. Hence, no demo from the exterior side which would require a back fill will be done. However, the existing SOG from the patio would exert some surcharge. I'm underpinning all the existing wall footings that are next to the proposed retaining wall.

 

[RHTPE]

Having been involved in a similar project involving a 3 story university classroom building with the exact same starting conditions, I offer these considerations for you:

1. What plumbing lines exist and how are you going to deal with them?
2. Same for any HVAC equipment that may exist in the crawl space.
3. Electrical?
4. Are there interior bearing walls that need to be considered?
5. How is the contractor going to place concrete into the necessary forms for the new foundation wall, as there will be little room at the top of the wall?
6. Does the contractor understand the forces involved in constructing forms for a one-sided wall during the concrete placement?
7. Does everyone understand that one-sided wall formwork often fails if one does not understand how to properly construct it?

Once the cost for all of the above has been tallied, you might consider comparing against:

A. Involving a building mover who is going to be experienced at jacking a house off its foundation (including the 2nd floor bedroom).
B. Get a quote on the cost of the needed perimeter excavation.
C. Re-evaluate the cost for demolition of the old foundation plus construction of the new full-height new foundation, keeping in mind that the house can be raised a foot or more above the required formwork.
D. Include the costs associated with the above mentioned utilities.
E. Include the cost for reconstructing the patio.
F. Tally the total cost and compare to the your proposed method.

You may also want to factor in the time it is going to take. The crew that we used to support the classroom building installed all necessary cribbing and support beams in 2-1/2 days. It'll likely be about the same or less for a residence. Remember that the cribbing must be founded at an elevation that won't be undermined by the foundation excavation.


Ralph
Structures Consulting
Northeast USA

 

[EngineeringEric]

Excel,

I only meant that there is a surcharge the way it was drawn, and i can see the hesitation on trying to underpin that if it can avoided. People look at the cost of using #7 bars instead of #5 as insignificant, but the application of underpinning can be costly and added liability.
-
But we are in agreement of the effects of the patio load and pavers and more.

 

[XR250]

@Mcr0&, Engineering Eric

Sorry, did not look at pic. I thought you were stopping the wall at the existing footing - That is what I usually do to save money. As long as all the surcharge is accounted for , there is no need to go higher unless you are bracing against the floor system. How are you going to keep the existing FDN from sliding inwards while constructing the new wall?

 

[PEinc]

How are you planning to retain the 10 feet of lateral soil pressure while you build this retaining wall? Usually with less than 10 feet of retained earth, the concrete pier underpinning, if thick enough, does not need any lateral support. However, with a lightly loaded building such as this residence, the underpinning should need lateral bracing, both temporarily and permanently. The small building's dead load and the weight of the underpinning may not be sufficient to prevent overturning and sliding. If you try to excavate and build any reasonable length of the retaining wall in your sketch, you are taking a serious risk of causing a collapse.
msquared48 is correct. You should underpin the existing wall footing and then plan on providing both temporary and permanent lateral support for the underpinning and any wall you may try to build in front of the underpinning.

http://www.PeirceEngineering.com

 

[mcr07]

Thanks everyone for the replies. I understand the means and methods for this retaining walls ain't easy. Honestly, I'm not a 100% sure how the shoring would be. In my mind, the digging will sectional and the patio/2nd bedroom beam load going to the 4x6 in the sketch be shored from the outside. However, i understand the concerns of the (E) footing sliding due to the soil pressure. The length of the wall that needs to be retained is about 15'. Its connected at the end to perpendicular foundation walls.
I've been also considering this situation, and perhaps design the retaining wall so it spans horizontally? My active pressure is 35pcf and at rest is 53pcf. 15ft doesn't seem to unreasonable to me, when there's no penalty in terms of thickness of wall.

However, I'm sure that creating a basement from a crawl space digging from the interior had to be done by somebody at some point. I was hoping I could get some insight on that. Appreciate your comments so far. It's been really valuable.

 

[dicksewerrat]

How you hold up the new bldg. walls is the primary consideration here, but how are you going to remove the existing sandstone without shaking the bldg.?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[RHTPE]

I for one would appreciate an update when you finally determine how this will be done.

Even better, an update on how it all went after it's done.

This kind of work is far more interesting than constructing a new building from scratch.


Ralph
Structures Consulting
Northeast USA