Helical cassed pull down pile as a soilder pile? 2

[Nuccio]

I have a job that I am doing the concrete work and excavation and underpinning. In one corner there will be a 17' deep cut along an existing building and front sidewalk. The building will be jacked up and supported on steel so we can excavate and build new foundations. But the front corner along the sidewalk will need to be shored. The site engineers want no vibration because of the existing structure are 100+ years old rubble foundation not in good shape. The boring for that location shows fill for the first 4-5 feet N Value 12- 17, then brown clayey silt with trace of fine sand from 5-10 feet N Value 11-10, then Grey clayey silt with trace of fine sand from 10-30 feet with N Values 10-12. Ground water was at 22 feet. I have evolved an engineer to come up with some designs. I was think if it is possible to use a 1 3/4" helical pile with a 10"12"14" helices as the lead and then add an displacement plate to install a 6" Sch. 40 steel casing to be grouted solid. So I would like to install these piles as solider piles 6' OC with wood lagging total dept.35 feet so you will have a 17'6" embedment and 17'6" pile sticking up out of the cut. What I am trying to eliminate is using tiebacks or al east use a little as possible so we avoid the utilities in the street.

 

[PEinc]

I would not do that if you paid me - a lot! A 17' cut is almost always too high to cantilever (no tiebacks or braces). This would be especially true for Chance Anchor "soldier beams," even with the added 6" pipe. This vertical anchor is very flexible. The 6" pipe is weak. Even if you could design that type of wall, the "soldier beam" deflections would be excessive. I'm not sure that you could develpoe the required passive resistance from a combination of a 6" pipe and an 1.75" square shaft lead section. As the 10", 12" and 14" helices cut through the soil, they will disturb and loosen the soil that you want to support the 6" pipe. No good.

If you need to minimize vibrations, use drilled in HP or WF soldier beams, tiedback or braced as required. Or, you could drill in larger diameter pipe soldier beams. However, the wall would still need lateral support.

It would also be nice to hear a few more details of your underpinning system.

 

[PEinc]

Sorry, "develpoe" should have been "develop."

 

[Nuccio]

Well we can go with smaller helices enough just to pull the pile down. I figured tie backs were necessary but I would like to go with the min. need to ovoid all the utilities in the street and under sidewalk (water, gas, storm main) I am guessing by adding the pipe pile as a solider beam it would be better than just soil nail the hole cut. We could go the soil nail route but am concerned with the utilities and n values seem to be low.
The underpinning along the existing building was to been done with a traditional pit excavation concrete and dry pack. The site eng deemed the rubble wall with no footing was unstable so they decided to support on one and the end with timber cribbing placed inside the basement and timber cribbing placed outside with steel I beam carrying the load so we can demo an rebuild to the new deeper elevations needed for the new building to be built after.
I am equipped to do helical piles and helical soil nail also with some mods. My rock drill attachment can be modified to do titan bars for tie backs if necessary. So that why we are trying to do the work in house if possible if not it will be subbed out to the approved contractor.

 

[DRC1]

I agree with PE inc. Typically for a 17 ft cut you will have 12 inch H-piles and fairly good tie loads. Unless you tie every pile you will need a waler system. Tying the lagging into the Atlas/Chance piles will not be easy. Also if you are deeling with rubble foundations, you are probably more concerned about stiffness than strength as even minor deflctions can be a problem. Depending on the locations of the structures, soil nailing may be an option. Granted N values are not the best, but they seem okay for soil nailing. One thing that struck me as odd was that watter was at 22 ft, yet there were a couple of clayey layers. You need to be sure that that is not a mistake, as water in the excavation will ruin your day.
The underpinning of rubble wall sounds dicey. Granted, I have not seen it, but working with rubble is never fun. If it is very good shape you may be able to do what you are talking about, however more commonly a series of short concrete beams spliced together under the rubble and that beam is supported.
Finally, this sounds like a job for a shoring contractor and engineer wiyh experience in this sort of work. This is not the job to move into this type of work.
Question; Did Atlas & Chance merge?

 

[Nuccio]

Yes Chance and Atlas are one now. The steel supporting the structure will be placed under the 1st floor houlding one side and both end up. So we can demo the rubble wall and rebuild. The helical pull down piles will also be grouted solid. These are just ideas I gave the engineer to look at.with the round pile we can weld a section of angle to the side for sucuring the lagging and we would use whalers to hold the tiebacks. Accords to the boring in this area the water table is at -17' we have an open trench along the side of the building 12' away that is shored and it step up toward the rear of building to -10' and I am getting water coming in from the higher elevation. The building to the rear we already underlined to the elevation of -10' and we hit water at about -8'.

 

[PEinc]

In order to do any underpinning or soil nailing, you need to control (eliminate) the water seeping in over the clayey layers. As DRC1 said, the blow counts and soil descriptions aren't bad for soil nailing. However, soil nailing is not an acceptable substitute for underpinning in most situations.

 

[Nuccio]

There will be no underpinning envolved like I said the building will be supported by tempary steel and cribbing. There is shoring need from the front of building to sidewalk and along the sidewalk need to have accesss to the new basement for construction puposes. Yes I am still trying to confine the GC that dewatering is needed for the construction process. They still just want to through a pump at bottom of exacavtion and consider it done

 

[PEinc]

You said that the building has a rubble wall and was unstable enough that conventional pit underpinning was not possible. I have underpinned many, many rubble stone buildings. These foundations scare many engineers. Usually, the foundation is not as bad as some think. If properly underpinned by an experienced contractor, supporting the building with concrete pit underpinning could be easier and cheaper than needling the building as you describe, removing the original rubble foundation, and then replacing the foundation with a new one. Without seeing the rubble foundation, it is hard to tell if it really is as unstable as you say.

 

[Nuccio]

I agree that underpinning would have been more economical. But when I tell you the building is in bad shape the walls a bulging in many places the rubble can be removed by hand it has no motar in the joints you can pick any stone and just pull it out. We dug several test pits and it is a loose. The only semi good spot was at the chimeny because it was a rubble footing and brick chimeny. We pressure grouted it and underpinned that with pit excavations because they could not support that with steel.