Shoring options for new basement construction

[shaunco]

We are getting close to breaking ground on our new house (in Los Angeles) and have been working with a few shoring/excavation contractors (that have in-house engineers) for our basement dig. I will give some background info before I give the problem.

Our lot is nearly flat (about 1' change from highest point to lowest point) and there are no surprises in the soil (no groundwater, non expansive [EI=00] - "slightly silty sand; brown, medium dense, damp", consistent at depth, dry density of 119pcf). The soil report calls for shoring that can withstand 25pcf active earth pressure and 250pcf (500pcf for temporary) passive fluid pressure. Lateral resistance coefficient of friction is 0.40. The basement requires about 13' of excavation.

The problem is that the west wall of the basement (which is actually the down-sloped driveway and west wall of the underground garage) is on the property line (8" concrete wall with 4" heel, heel touches the property line). The neighbor to the west is about 7-8' from the PL, so it is possible for us to place the shoring in their yard, but not ideal. The two engineers that have come back have both come back with the same suggestion, which is 12" soldier beams with lagging. I have two issues with this. First is that we need 18" room for form work and water-proofing work, which means that the shoring is 2.5' in to the neighbor's yard. Second is that this shoring is permanent, which is a problem since it will likely be in the neighbors yard. Although if the shoring was temporary, that would essentially ruin all the hard work of the drainage/gravel/backfill guys when the 12" of shoring was yanked out.

I have looked at sheet pile... it seems like we *might* be able to get away with 8" sheet pile, saving 4", at a huge price increase (even with Skyline cold-formed) - which doesn't seem worth it.

So, my question to the board is: does anyone have any other ideas?

Thanks!

 

[dirtygeo]

My first question is how did you get approval to have your house right at the property line? Usually there is a set-back requirement to avoid this kind of problem.

Second thought is that you had better be on very friendly terms with your neighbour. No matter where the shoring ends up being, the contractor will almost certainly be all over their property during construction.

Third, if you want to build out right to the property line you don't have too much choice. You need to install shoring to support the excavation with no lateral movement so that your neighbours house is not effected. Soldier pile and timber lagging is likely the most effective option, although would depend on the actual soil conditions. If you have two local engineers giving you the same answer (are they working sperately or together?) then it is likely to be your best option.

The shoring can be installed right at the property line and still accommodate your waterproofing, etc. Don't need formwork as the choring serves as your outside form. You say groundwater is not a problem, so simply place a dimple board against the shoring with a weeping tile line at the bottom (geofabric encased and cover with clear stone), then form the inside face and pour concrete.

This seems unusual for residential construction as the construction costs for shoring are quite high relative to the cost of the house.

 

[dirtygeo]

Sorry, another thought.

You say the neighbour is 7 to 8 feet from the property line. What depth are their foundations at? If they have a basement level then the founding level is likely deep enough that you would be able to open cut and not effect the foundation, as long as you can get permission to excavate onto the neighbours property.

 

[kslee1000]

Why is the shoring permanent? Since you intended to form both faces of the wall, there should be adequate space to get the forms and the shoring out.

 

[oldestguy]

Is your roof going to overhang the PL?

In our neck of the woods, that is infringing on that property and might be a problem with some neighbors.

Also, here any loss of support to the neighbor is the neighbor's task to deal with, until the depth of excavation is over 10 feet.

No point in risking these legal things unless you can get a waiver in writing ahead of time.

 

[shaunco]

Sorry for being so slow to respond to everyone...

My first question is how did you get approval to have your house right at the property line? Usually there is a set-back requirement to avoid this kind of problem.

The city I am in, and most neighboring cities allow underground non-living space to touch the property line (such as my underground garage). Underground living space must follow the same setbacks as above ground.

All of the above ground space follows proper setbacks.

The shoring can be installed right at the property line and still accommodate your waterproofing, etc. Don't need formwork as the choring serves as your outside form. You say groundwater is not a problem, so simply place a dimple board against the shoring with a weeping tile line at the bottom (geofabric encased and cover with clear stone), then form the inside face and pour concrete.

This is the same conclusion that I came to with the shoring/excavating engineer today. He called it "incorporated shoring", where the soldier beams end up becoming part of the retaining wall.

You say the neighbour is 7 to 8 feet from the property line. What depth are their foundations at? If they have a basement level then the founding level is likely deep enough that you would be able to open cut and not effect the foundation, as long as you can get permission to excavate onto the neighbours property.

The neighbor is single story, no basement. They were willing to let me put the shoring on their lot, but now I don't need to.

Why is the shoring permanent? Since you intended to form both faces of the wall, there should be adequate space to get the forms and the shoring out.

The shoring is permanent because the basement cut is 135' x 48' x 14' (L x W x D, with the W facing the street). With soldier beam and lagging style shoring, this calls for either 14"x74lb beams at 45' length to be vibrated/pounded in or 12"x65lb beams, also at 45', to be placed in drilled holes, plumbed, and concrete filled (about 8'). Either way, the list of potential things that can go wrong when the crane, sitting on the street (135' away), is trying trying to pull out the back 10 (or so) 45' beams is way too long... and I seriously doubt any shoring contractor would be willing to take that liability to recover $30k worth of steel.

Also, now that the beams are incorporated in the wall, this is also a moot point.

Is your roof going to overhang the PL?

.
Nope. This is just for the driveway that leads to the underground garage. The above-ground house is about 12' from the property line for the front 75' and about 5' from the property line for the back 40' (5' is the minimum setback ... well 10% of lot width, not less than 3' and need not be more than 5').

Thanks for all the feedback... this issue has been resolved.