Building into a hillside
Building into a hillside
(OP)
I have a somewhat triangular building being built into the side of a hill. Two sides of the triangle have 4 levels of below grade construction, while the third (hypotenuse) leg is only buried one level. I have been recommending a permanent earth retention system to deal the unbalanced earth pressures. I feel like the economics work as compared to bracing the building internally for these earth pressure, however there may be some problems with easements for the permanent tiebacks.
My at rest pressure is 50H, and my levels ar about 9.5' each. My passive pressure is 300H. The magnitude of the loads get large pretty quick, an i am running out of ways to resist the forces.
Anybody with experience or ideas on dealing with this?
My at rest pressure is 50H, and my levels ar about 9.5' each. My passive pressure is 300H. The magnitude of the loads get large pretty quick, an i am running out of ways to resist the forces.
Anybody with experience or ideas on dealing with this?






RE: Building into a hillside
That issue asside, I have used vertical rock anchors to tie down the heal of the wall footing and to increase sliding resistance.
Where is the site?
What type of soil are you dealing with (above and below the excavation)?
Easement for horizontal tiebacks may be needed.
I have also seen drilled pier walls installed prior to excavation then lagging installed as the excavation is made (top down). Still, horizontal tiebacks may be needed until the building is in place to provide lateral resistance.
In this case they might be temporary. Otherwise, permanant tiebacks maybe more difficult to get approval on adjacent property.
RE: Building into a hillside
In a temporary system the tie back can be abandon after my floors are in place so special permits are not required. For the permanent system the tieback cannot be damaged, so a easement will be required so that no future construction will damage them. In my case they will be under a street, and they get pretty deep so there will not be utility conflicts. Its possible, however as you said the easements may be hard to obtain.
RE: Building into a hillside
Drainage mats on the face of the lagging and a more durable cast wall "linked" to the H piles sounds likely too.
RE: Building into a hillside
RE: Building into a hillside
Aside from active and passive pressures, the global stability of the cut should be checked. Replacing a large amount of soil with a house will significantly lower the pressure near the base of the slip circles which could cause instability. Tiebacks would address this problem where bracing would not.
RE: Building into a hillside
Remember though, with soil nailing, it is normal to get some vertical settlement in the soil mass in order to load the nails. This will show at the top of the excavation, but it does reach a limiting, stabilizing settlement.
How close is the nearest structure?
Mike McCann
MMC Engineering
RE: Building into a hillside
I am happy with both proposals for permanat sheeting and shoring. I have been asked to compare the added cost of the Permanat system to bracing the building internally. Without doing a complete design i want to make sure i have accounted for all the costs. Quick estiamtes for shear walls have not been too promising. The passive pressure for the one buried level on the low side only rpovides 25% of the at rest pressure from the high side. What else can i add to resist the load?
RE: Building into a hillside
What is your "shearwall" material?
What is the approx size of the building (in plan)?
I'm surprised the low side only provides 25% of the resistance needed. Even if that problem can be solved, I think the floor construction materials will make a difference in the ability of the building to resist lateral forces.
RE: Building into a hillside
as far as my active / passive comparison i have :
Active @ 50H 37' deep: (50x37'x37')/2 = 34.3 kips per ft of wall
Passive @ 300H not including the top two feet and 10' deep: (300x2'+300x10')x8' / 2 = 7.2 kips per ft of wall
RE: Building into a hillside
I think I'm going to need a picture in order to understand the problem better. (sorry)
RE: Building into a hillside
RE: Building into a hillside
RE: Building into a hillside
RE: Building into a hillside
RE: Building into a hillside
The global stability is a good point. I will pass that responsibility onto the geotech. They already know what we are building and understand the site, but i am not sure if they considered it.
RE: Building into a hillside
For the balance of the three stories, design the wall as a cantilever retaining wall. Bottom of the wall will be 24 to 30 inch thick.
For the remainder of your lateral pressure, use battered drilled piers under all of your foundations.
At this point, recognize that this will never be a good idea and go to a permanent tied back sheeting and shoring system.