dry-packing when underpinning

[brianthegiant]

The general wisdom when dry packing, seems to be to have a 3-4" gap between the top of the old concrete and top of the old wall, is there a compelling reason you cant go thinner - say 2 inches?

If you trench under the wall on both sides, is it possible to ram the cement in from both sides and get a dense enough fill, or would I need to block up one side (eg with a stack of bricks) to ram against?

Thanks for any tips!

 

[brianthegiant]

Also - I know this is probably also a dumb question, but I'm wondering why you can't leave a much smaller gap and pack it with a layer or two of slate as you would above a Lintel?

 

[SLTA]

When underpinning, you want to get a solid and tight connection under the old wall to the new foundation. A layer or two of slate may fall out over time, with slight movement of the footing not mirrored in the wall. I think the 3-4 inches is to leave room for someone to actually get the grout in there, under the entire wall, with no air pockets. You could send the grout from both sides, but again, that leaves more of a chance for missing some areas. I also highly recommend non-shrink grout.

Good luck!

Please remember: we're not all guys!

 

[MacGruber22]

For what it is worth, I prefer flowable grout. After a lot of field visits, I am not convinced the workmanship of most people is good enough to hand pack grout, particularly in large cavities. It is usually way too dry or sopping wet. AISC recommends flowable grout be used for larger base plates, so I apply the same logic to underpinning large areas. Every major concrete/grout manufacture has non-shrink or shrinkage compensating flowable grout.

"It is imperative Cunth doesn't get his hands on those codes."

 

[brianthegiant]

Thanks guys, very helpful.

Yes it looks like non-shrink grout is widely available here in the uk as a dry powder.

http://gbr.sika.com/dms/getdocument.get/60789a95-de17-3c9b-a7c4-00b7811aef23/SikaGrout 212.pdf

I was at first quite keen on this solution, but the detail looks problematic -

it says it should be applied in formwork with some pour hopper arrangement to give 150-200mm grout head
(so does that also mean you need some type of vibrator to ensure it flows into all the corners?)
Or is that overkill & it could be applied with a mortar gun?

It also says the surfaces should be blast cleaned and wetted for 6 hours before use. I'm not keen on using that quantity of water around the foundations during underpinning, since the structural settlement I'm fixing is likely in part due to a leaky drain and soft soil.

I would normally go over suraces with a wire brush & spray lightly with water a few times with a pump action sprayer, which with the low ambient temperature here is enough for bricks to stay damp for hours if not days.

 

[hokie66]

It depends on the thickness of the wall you are grouting, but dry packing a 2" gap is not difficult to do if you are conscientious. A bit tedious, but it sounds like you are doing it yourself, so you are in full control of the workmanship.

 

[brianthegiant]

Yes, the golf ball test for water content in the older underpinning thread sounds straightforward enough.

http://www.eng-tips.com/viewthread.cfm?qid=232821

I have some experience pushing cement into joints from Helifix installation (

http://www.helifix.co.uk)

I think I'll need to put a form on one side of the wall to pack against, I guess shuttering ply or similar screwed to the wall is going to be more secure than messing about with a heavy stack of bricks.

 

[MotorCity]

No reason 2" won't work, I've seen spaces less than an inch dry packed. I would suggest blocking one side and packing from the other. Unless you have a really wide space to fill, you will probably knock out grout on one side if you attempt to pack from both sides. The stiffer the grout, the easier it will be to dry pack.

 

[Once20036]

I`ve always utilized a 3-4" drypack layer so that a 2x4 could be easily used to pack the grout into the back of the cavity, for a thick wall accessible from one side only.
I won't have any objections to a thinner layer if the in situ conditions allowed for it.

 

[moltenmetal]

I'm not a mason, nor a structural engineer- but when I underpinned the 38' long wall of my house, I used a nominal 2" gap and dry-packed using non-shrink grout because that is what is recommended on the detail that was supplied by the building department (i.e. what the building inspector inspects against). There have been zero settlement issues so far- we're only a decade in but I think that more or less means it'll be good...The end of a piece of 2x4 on edge still makes a really good packing tool.

One key is to scrub or pressure wash the underside of the existing concrete before setting the forms for the underpinning footing, when you have room to do this, to dislodge soil.

3-4" clearance might make the job more comfortable, but if the grout mix isn't at perfect moisture content (i.e. at all too wet) you will get a poor result. Working with the dry packing mix was not easy for an amateur, and I can imagine the temptation to do the work wrong would be very tough to resist if you were being paid by the job.