Plumbing Repair Under Slab on Grade

[Mule]

How do you guys go about repairing a trench where a plumber has dug underneath a foundation to replace a sewer line?

The trench is about three or four feet wide and an average of three feet deep for a length of about thirty-five feet. House built back in the 60's so I doubt that there isn't any beams under the load bearing walls.

I'm worried about not getting the proper compaction/backfill under the foundation to support the load bearing portions of the foundation.

Any ideas?

 

[steveray]

We use a product up here called "flowable fill" comes in a ready mix truck, supposed to be 98+% compaction, in a former life I used it a a state university for anchoring thrust blocks for large steam lines and to expedite road patching jobs to get them back together as quickly as possible. I will look for a more technical name. Hope this helps!

 

[steveray]

http://www.flowablefill.org/performance.htm

Try that site Mule!

 

[Mule]

Thanks Steve, I'll look into that and determine if this will work in this situation.

 

[Mule]

Anything code wise for this type of fix??????

 

[Mark K]

The code doesn't directly address this situation. The flowable fill is a reasonable solution for most situation.

If there is a structural engineer or geotechnical engineer on the project they should be consulted.

 

[Glennman CBO]

Wouldn't that be a "CLSM" material (controlled low strength material). See 1803.6 and 1805.1, 2006 IBC.

 

[Mark K]

Lean mix concrete, flowable fill, and CLSM are all terms refering to a mix of aggregates and cementitious material. One common problem is that if the mix is to strong it can be difficult to impossible to remove in the future is you need access to the pipes in the future. CLSM can be specified with an upper bound on the strength that is low enough that it can be removed without too much difficulty. All products will likely work in the situation described since they are stronger than the removed soil and the loads from the building appear to be light.

While of value to the owner, I do not see that the ability to easily remove the material in the future is a code issue. The effective use of CLSM requires some knowledge regarding the consition where it will be used, how to specify properly, and the ability of the concrete supplier to provide it. It may be difficult for all of this to come together when there is no engineer on the job and the project is in a rural location.

 

[Paul Sweet]

He should also put a sleeve around the pipe in case the house settles any more, especially if this is in an expansive soil area.

 

[Mule]

The plumber has proposed "hand tampering". Just don't know about that one!

 

[pwood]

we have used a 2 sack pea gravel mix in these situations to bring up to grade before pouring the finish floor, easily removed later and provides 1500 p.s.f. soil bearing value in most applications.

 

[Mark K]

The problem with hand tampering is that it is hard to control and worthless unless they do a good job.

1,500 psf = 10.4 psi which any concrete mix can easily exceed. The issue is settlement not strength.

I believe that sleeves are over rated especially in existing construction. With rare exceptions in order for the foundation to settle the surrounding ground needs to settle also. In addition I suspect that what ever you or mother nature fills the space between the pipe and the sleeve with will be strong and stiff enough to cause the pipe to settle with the foundation.

When the pipe is installed after the concrete is poured the space around the sleeve can give the plumber some tolerance to set the pipe to the right slope.

 

[Mule]

pwood,

They didn't cut into the foundation. It's trenched under the foundation.

 

[pwood]

mule,

flowable fill is the ticket. i worked for fema after the northridge earthquake in 1994 and they would recommend a high pressure grouting to fill in areas where liquification had created voids under foundations.