Building into a hillside 1

[mijowe]

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?

 

[HouseBoy]

One question that comes to mind is: How are you(they) going to make the excavation? In other words, what provisions will you have for the initial excavation during construction of the retain structure (and until that structure is functioning)? This answer may help guide your decision.
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.

 

[mijowe]

The temporary earth retention system will be H-piles with wood lagging and tiebacks, this is the most common sheeting and shoring in the DC area. The sheeting and shoring contractor has given an add alternate to make his system permanent. This involves attaching his H-pile to my below grade concrete wall, as well as providing a higher protective coating for his tiebacks. They also design their temporary system for an earth pressure of 25H while increasing this for a permanent system of 50H.

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.

 

[HouseBoy]

OK, that is common around Cincinnati Ohio also. I would think using the H piles as perminant (with the higher loading and additional corrosion protection) would be the best way to go. Unless the building is cast in place concrete or a hybrid with some precast, I would think that depth is going to be tough to deal with any other way.
Drainage mats on the face of the lagging and a more durable cast wall "linked" to the H piles sounds likely too.

 

[hokie66]

I think you have the options covered. It is either a permanent retention system, or a temporary one with the building designed to resist the earth pressure. I am sure the permanent retention system will be more economical, provided gaining the permission is not too expensive. This can vary depending on location, but permanent tiebacks for retention walls such as you describe are in place all over the world. You may want to make a case to the authorities based on making the best use of the resources.

 

[DRC1]

Bracing a side hill cut is difficult. You end up with large loads on one side of the brace and need to get rid of them on the cut side which usually does not have the geometry to develop significant passive pressue, as the wedge daylights too quickly. Tiebacks are generally the best option by far. Single bore multiple anchor tiebacks are now avialble which significantly shorten the bonded length. Further, if you are below the utilities, an easement from the municipality should not be difficult.
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.

 

[msquared48]

What about using soil nailing - usually doesn't have to penetrate as far back as a tieback system.

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

 

[mijowe]

Two foundation contractors have looked at the project. They have proposed tie backs as short as 40' and as long as 65'. There are streets on the two high sides of the building. Property lines across the street vary from about 50' to 75'.

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?

 

[HouseBoy]

What is the expected construction material for the building floors and interior walls?
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.

 

[mijowe]

Its a cast in place concrete building. 340' x 200' are the legs of the triangle. I tried to put the lateral loads into moment framse and the deflections were too great. I have one long side basement wall, but at the point end of the triangle i lose all stiffness because the building is not deep.

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

 

[HouseBoy]

Doesn't the height of the hillside become less as you get out to the "point end of the triangle"?

I think I'm going to need a picture in order to understand the problem better. (sorry)

 

[mijowe]

The grade drops pretty quick at both tips of the triangle. It does this with frestanding retaining walls outside of the building foorprint. In any case i do not get much relief with the grade change.

 

[hokie66]

In th absence of permanent tiebacks, you would have to approach this with shear walls internally, and as you say they would be substantial. Externally, you would probably have to use inclined tiebacks/tension piles under your lowest level. Not a pretty picture.

 

[BigH]

Just thinking - (sometimes it IS dangerous) - have you considered the use of cantilevered secant pile walls of large dia drilled caissons? This may be stiff enough to minimize the deflection - or you can reinforce with the shear walls like Hokie666 intimates. You can maybe incorporate them into your permanent walls sort of like "plastering". BTW - what is your deflection criterion since is seems that the back of the walls are carrying a road?

 

[DRC1]

I think BigH is on to something. Again no matter how stiff you make the house, the down slope side must resist the upslope side. I think the 50H & 300H you are using is for level backfill. For a slope your active will be somewhat higher and your passive significantly lower. Also again bracing does not address global stability of the hillside which could be a significant factor.

 

[mijowe]

The grade at both the high and low side is level beyond the building, so i am not supporting a slope or using passive for bearing on the low side at a down slope.

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.

 

[Teguci]

For the first floor, transfer the load through the diaphragm and into the opposite wall. Make sure to design the short wall for the passive pressure resistance.

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.