Backpacking behing Lagging

[STEVE97]

Is it a common practice to show a louver and backpacking behind timber lagging in a soldier pile retention system? We specify to replace any lost soil behind the lagging before installation of the next board to have solid soil behind the lagging. I was told however that it should be backpacked with a drypack mixture and to provide a 1 1/2" louver between the boards and that my original detail was incorrect. Was my original detail showing the replaced soil wrong?

Thanks all

 

[born2drill]

I have seen that detail many times on drawings that come across my desk, but never done it that way. In practice my experience was to always backfill with soil as the boards were stacked up to complete a lift.

It is a good idea to use louvers because 1. it allows you to inspect behind the lagging to check for voids and 2. it allows the system to be free draining. You may need to pack straw behind the boards if fines washing out are a concern.

What I don't understand is if you drypack behind the lagging, I would think the wall would tend to build up water behind it and you can only see the drypack if you try to look through the louver. Doesn't that defeat the purpose of having the louvers in the first place? And are systems with these details designed to resist additional hydrostatic loading from rain events?

No one has ever answered these questions to my satisfaction, so I'm eager to hear anyone else's take on this...

 

[STEVE97]

BorntoDrill,

Thanks for the response. I understand your concern with the drypacking and I agree. As far as the site I am working on now it will have a dewatering system. The transit authority that we are working for insists that the lagging needs to be drypacked so that no soil failure will result behind the lagging. I have argued my case with them but they have the final approval so I am stuck. I just wanted to hear other opinions on this issue. I have always seen it done as you have described and never with a drypack mixture.

 

[born2drill]

If it's the NYC MTA, you're gonna lose your argument no matter what it is. They don't care about anything except doing it their way, and nothing happens without their approval. I've got plenty of horror stories.

I suppose they think that packing behind the lagging will prevent settlement of their structure due to loss of ground. They should probably be more worried about it settling when they turn on the dewatering system.

Also, even though there will be dewatering, does your sheeting system design consider buildup of water from rain events at all?

 

[DRC1]

I have never heard of dry packing behind lagging. Soldier beam and lagging does allow some ground movement, and there is always a chance for a run. Further if the area is to be dewatered, it should be dewatered in advance of the excavation. I would be afraid a lot of drypack behind the foundation might prevent adequate drainage behind the lagging. Obviously I do not know all the details, but my initial reaction is that this may not be the best application for soldier beam and lagging.

 

[cvg]

I assume backpacking is done by hand? If so, I would hazard a guess that it would be difficult to get a continuous layer of mortar without any cracks or gaps that would stop water infiltration from moving behind the boards. In addition, the water will move freely through the soil immediately behind the mortar layer anyhow and hopefully you have a weeping zone at the base of the excavation to let it out.

 

[STEVE97]

When we designed the sheeting system we neglected hydrostatic pressure because the water would seep through the gaps in the lagging and that the site was to be dewatered. In any case the TA is forcing us to add a detail to our drawing showing a one bag cement to sand mixture behind the lagging. I have argued my point with them and have got nowhere. They will not approve the drawings until this detail is shown. They are worried about settlement behind the sheeting and effecting there structure. Now if water is present behind the sheeting we have a hydrostatic pressure that the sheeting was not designed for.

 

[InDepth]

I've looked into this issue and there are several solutions in literature:

1) Pump Slurry behind lagging in lifts - Slurry is porous enough to allow water. If you really want to be rigourous, performa a permeability test and very with your geotech what hydrostatic pressure if any builds up.

2) Provide Salt Hay between Louvres - Old technicque, but still works like filter fabric.

3) Provide filter fabric - This will require some sequencing when installing lagging, but can be done. You will need to select an appropriate permeability and look into fines passing (clay soils / clogging).