Soldier Pile and Lagging Settlement 1

[blindpig]

How common is long-term settlement above the line of soldier pile and wood lagging supported walls? In several instances I have observed a narrow but deep trough of settlement that forms years after the cut-and cover structure has been back filled. These were support-of-excavation walls where the concrete structure is likely to have been cast directly against the wood lagging. I suspect the settlement is caused by voids that exist in the soil placed as backfill behind the lagging boards, due to a lack of compactive effort, but the experience of others in this regard would be appreciated.

 

[PEinc]

Your suspicions are probably correct about the improperly backfilled lagging boards. When lagging is installed, the excavation is supposed to be made only to the back face of the lagging so that the lagging is in tight contact with the dirt. However, this is rarely done properly in the field. It is common for the excavation to be made to several inches (and sometimes feet) behind the lagging. When this happens, it is necessary to properly backfill the lagging as it is being installed - not from the top after the lagging is all in place. The problem with backfilling lagging is that, if not done properly, the backfill will run out the bottom when you dig the next lower lift.

Lagging is supposed to be installed with approx. 1.5 inch louver spaces between the boards. This space allows inspection for voids and allows additional fill to be shoveled in if necessary. The louvers also allow a space to stuff hay or filter fabric if necessary to prevent soil loss when water is seeping through the lagging.

My experience is that everyone claims to know how to install lagging. Actually, very few know how to do it properly.

 

[GeoPaveTraffic]

PEinc makes some very good points. Another thing that could be causing some of the settlement is that clean rock is most commonly used to backfill behind the lagging. With time soil could be migrating into the voids in the clean rock.

 

[PEinc]

I have designed and built hundreds of soldier beam and lagging walls, both permanent & temporary. The only time any were backfilled with crushed stone was when the wall was a low, cantilevered wall with precast concrete lagging AND in a fill situation. Otherwise, you risk the chance (probability?) of the stone falling out from behind the bottom of the lagging when a lower lift is excavated for a higher wall.

 

[blindpig]

Around here, the backfill consists of the spoil resulting from the excavated lift (normally 4 feet or less in height) that is then manually tossed back behind the lagging with shovels as the lagging is placed. I suppose the degree to which the void is filled depends upon the dedication of the laborers performing the task. And even if a given lagging bay is completely filled, as PEinc points out, the work progress in stages from the top down, introducing the possibility of movement and settlement of the previously backfilled levels above. Perhaps I have answered my own question, but given the circumstances of how the walls are constructed, it seems that some degree of settlement above and behind the lagging boards should be the rule, rather than the exception. And it would appear that in some circumstances, the voids take 30 years to finally reach the surface.

 

[PEinc]

Settlement behind the wall due to backfilling of lagging is not "the rule." You are not supposed to excavate behind the rear face of the lagging. The more you do, the worse your problems can be.

If you need to backfill behind lagging, the fill needs to be packed into place, usually tamped with the top of a sledge hammer. Also, when you set the bottom (or first) lagging board in any 4 or 5 foot lift, you need to tightly pack and nail some extra lagging pieces behind the bottom board. This packing should tightly fill the space behind the bottom board and prevent the above backfill from dropping down when the next lift is excavated.

 

[LCruiser]

You can expect drifting into the holes augered for the piles if not backfilled correctly - and sometimes even if they are. Ensuring the backfill is with a good noncohesive material to the bottom of the piles can help.

 

[PEinc]

I disagree with LCruiser. Soldier beam drill hioles should not be backfilled with non-cohesive (cohesionless) material. If so, the material can run out when lower lifts are excavated for lagging. Then, the soldier beams will not be in good contact with the dirt behind the beams. The beams will be pushed back excessively when tieback anchors are stressed. You will not get arching action on the lagging. Drilled soldier beams should be backfilled with low-strength, lean mix concrete or flowable fill (or sometimes with structural concrete under certain conditions).

 

[LCruiser]

You are describing the optimal situation. However, keep in mind that with your method, you will lose the piles. That will increase the cost of the excavation. from about a nickel per lb. (~rental rate) to the price to purchase the steel, which is about 35 cents per lb (ENR today). When you consider a common H-Pile is 76 lbs per foot, the price for shoring goes up rapidly.

 

[PEinc]

OK, by "lose the pile" you mean it will not be extracted and reused. Yes, that is correct. I have rarely seen jobs where the soldier beams were pulled after the site was backfilled. On most jobs, the GC or CM does not want to pay someone to return to pull out the piles. Sometimes, when the soldier beams were installed by the GC, the GC may try to remove them when done. That's rarely done in the areas I work. Again, I do not recommend filling soldier beam drill holes with cohesionless material that can fall out when you excavate down and install lifts of lagging.

 

[LCruiser]

I suspect you have rarely seen it because you *specify* it to be irretrievable by your backfill requirements. Look at the economics. You pull it and it's yours, cheap. Virtually as good as new. I've pulled miles of it - and it's much cheaper than buying new. Could be the site conditions are different between our types of projects though. I'm talking about primarily water transmission structures - deep down in a busy street, for example, or other tunnels.

 

[PEinc]

Economics are important but not as important as performance. A more economical job is very good, but a poorly performing job should not be the result.

I work on all types of support projects: major urban buildings, highways, bridges, subways, water treatment facilities, utility trenches, etc. Controlling movements is very important. Losing backfill from around drilled soldier beams can cause problems. In the many states I have worked over many years, pulling soldier beams is the exception, not the rule.

 

[LCruiser]

One definition of an engineer is somebody who can do for one dollar what anyone can do with ten. Performance is easy. It's economic performance that's important.

If you spec cementitious material for backfill so they can't be pulled, then of course in your experience pulling them would be the exception.

However, we won several jobs by the margin of the salvage value of the H piles. Thinking back, it was mostly backfilling the holes with sand immediately after pulling the beams, and we used 8' x 12' plates instead of lagging whenever possible as well.

 

[PEinc]

LCruiser,

I don't know whom you work for, but design-build specialty companies who build sheeting walls nation-wide don't usually pull soldier beams, backfill drill holes with sand, or use 8' x 12'steel lagging plates. If this was the better way to do things, I'm sure these contractors would do so. But they don't. And that's my final answer.

 

[cap4000]

PEinc,

The ease by which you explain things so precisely continues to amaze me. Are you from the New Jersey area where I am a PE. I would like to be able to contact you if I ever come across difficult underground site conditions. As land in NJ has become very scarce, it would great to know a guy like you that could "transform it" overnight the way you seem to do so professionally on this website. Nice Job.

 

[PEinc]

Well gee whiz! Thanks, cap4000. I'm just outside Philadelphia, near Phoenixville. I do a lot of work in NJ.

Google "Peirce Engineering, Inc."

 

[jheidt2543]

Nice friendly discussion of two differing view points. It is good to remember we are all prisoners of our experiance. That is what makes a site like this so valuable, it gives us all a chance to find out how the other guy does it.

In my part of the country, construction labor and equipment rates have got to the point where it is actually cheaper to leave the piles in place in most, but not all instances.

As an aside, there was a time when reusing face brick was done quite frequently, but now the labor to clean them is more than the cost of buying new, so it is rarely done any more.

 

[PEinc]

Exactly, jheidt2543!