Underpinning using CMU

[jpyeron]

Our underpinning foundation subcontractor is advocating use of 12" block as the underpinning material (dry packed at the top 2”). Are there any opinions, questions or observations from experience contractors that I should address with the sub?

Background:

The underpinning is to be continuous. The existing foundation is a 16” rubble stone without a footing. The clay soil support greater than 4 tons/sq ft. There will eventually be an 8” thick concrete vault (per DCID 6/9 4.1.1) wall laminated in front of the underpinned foundation and rubble stone foundation.

 

[PEinc]

Never saw that done and I've done hundreds of underpinning projects. You really do not give enough information for a helpful answer.

For instance, how deep is the underpinning from bottom of existing wall to excavation subgrade? If more than about 4 feet, according to OSHA, you would need to shore the excavation to make it safe for the masons. Also, if the underpinning and excavation are more than about 4 feet, you may need to laterally support the masonry wall, either temporarily or permanently.

I have a presentation paper about underpinning problems on my web site. I suggest you review it before proceeding.

http://www.PeirceEngineering.com

 

[msquared48]

I assume the 12" block is to be grouted? It better be!
How is it to be reinforced?
Will it be as isolated pads, or a strip footing?
If a strip footing, there will need to be some horizontal reinforcing. How would that be done? Bond Beam block used at bottom?

Questions, Questions, Questions...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jpyeron]

My apologies for not providing enough information, I will try to supplement what was missing.

The depth is approximately 9’.
Shoring is to be used in the excavated areas.
The excavation is interior for a vault installation.
The existing foundation is laterally supported temporarily against the opposing foundation 12’ away.
The vault ceiling and walls (8” reinforced concrete) is to provide permanent lateral support.
I have read your presentation, good illustrations of rubble stone foundations.
The block will be filled and vertical rebar inserted, except for the cap course.
The block will have segmented lateral wire, tied when the adjacent section is worked.
The block will also have concrete anchoring wire on its face to mate with the with the pored vault’s rebar.
The footing will be poured as pads, adjacent and interconnected by rebar.
Vertical rebar will penetrate the surface of the pad one block height, blocks are to be recessed into the footing 1”.

 

[jpyeron]

To Mr. Peirce,

When you said “Never saw that done and I've done hundreds of underpinning projects.”, did you mean that as it should never be done for reasons x, y, & z? Or more of I have never seen it and have no basis to evaluate it?

 

[PEinc]

In my opinion, Using reinforced CMU blocks is a very unusual way to underpin a building. I would never try that method. 9' is relatively deep, unsafe, and illegal unless you shore the excavation. Shoring the footings will not hold the soil in place beneath the footings. Review the pit shoting details in my presentation.

Although I have not seen details of the CMU underpinning method, I don't like it for the following reasons:
1. Sounds expensive compared to regular unreinforced concrete underpinning piers.
2. Concern for shoring the excavation
3. Location of the CMU wall inside a shored excavation (Is any backfill required behind the CMU's? If not, is the CMU wall properly centered on a prpoerly sized footing?)
4. Placement of vertical resteel could be difficult under an existing footing.
5. Do you intend to install any horizontal reinforcement? If so, splicing it between underpinning sections will be difficult.
6. Concrete underpinning usually has no resteel.
7. Pouring a concrete footing and possibly a top bond beam will cost more than just filling a sheeted underpinning pit with unreinforced concrete.
8. The CMU wall will be much thinner than the width of the existing footing. There could be eccentricity to consider.
9. What happens if you encounter any column footings along the wall?
etc., etc., etc...........

http://www.PeirceEngineering.com

 

[PEinc]

Try this link for more underpinning info.


www.PeirceEngineering.com

 

[jpyeron]

[tab]>In my opinion, Using reinforced CMU blocks is a very unusual way to underpin a building.

That was my reaction as well. 35 cubic yards of concrete vs. 1050 blocks is the same price. Then you have mortar, grout and rebar.

[tab]> 9' is relatively deep, unsafe, and illegal unless you shore the excavation.
Shoring the footings will not hold the soil in place beneath the footings. Review the pit shoting details in my presentation.

Shoring of the soil is to be done on the project. No ifs, ands, or butts about it.

[tab]> 1. Sounds expensive compared to regular unreinforced concrete underpinning piers.
I am sure I can confirm that once I get the final proposal in my hands. Although the prospective sub is claiming it saves money.

[tab]> 3. Location of the CMU wall inside a shored excavation (Is any backfill required behind the CMU's? If not, is the CMU wall properly centered on a prpoerly sized footing?)

He indicated that any backfill needed would be done at each block course.

[tab]> 4. Placement of vertical resteel could be difficult under an existing footing.

The rebar does not reach the top course, the block slide over it, as explained to me.

[tab]> 5. Do you intend to install any horizontal reinforcement? If so, splicing it between underpinning sections will be difficult.

Agreeing with you, but it was indicated as easy with experience.

[tab]> 6. Concrete underpinning usually has no resteel.

That is a cost saving, compared to the block, as I see it.

[tab]> 7. Pouring a concrete footing and possibly a top bond beam will cost more than just filling a sheeted underpinning pit with unreinforced concrete.

There was no mention of a to bond beam.

[tab]> 8. The CMU wall will be much thinner than the width of the existing footing. There could be eccentricity to consider.

Regardless of CMU or pored concrete, there will be an 8” reinforce concrete wall tied into the underpin structure. See attached diagram.

[tab]> 9. What happens if you encounter any column footings along the wall?

There should not be any column footings here, in fact there are no footings under the rubble stone at all.

 

[Nuccio]

Where is the job located? Can you you use other types of shoring sheet piles, H piles and lagging? Has your underpinning contractor ever done this before? Is he a underpinning contractor or a mason? How much room or tolarance do you have to work with to get your valut in?

 

[PEinc]

Backfilling CMU "underpinning" as you set the block courses will not assure that the CMU wall is strong enough to resisit the compacted backfill. The backfill load will increase before the mortared block reached sufficient strength. In addition, the blocks and rebars will also not be strong enough soon enough.

I assume the CMU underpinning will be built in small sections along the existing wall so as not to undermine too much of the wall.

IMHO, this is a bad underpinning method!

http://www.PeirceEngineering.com

 

[jpyeron]

I am going to reject the proposal for CMU underpinning.

Thanks everyone for the advice and critical thinking.

In regards to the space for the vault, we are short by 13 inches to meet the "minimum" design and have the space functional. We are asking for permission to modify the specs from the authority. In otherwords room to play at all.