Tied-Back Earth Retaining Wall

[SH43]

Hi,
I am designing a number of retaining walls. Some of them are as high as 9m (30ft). On the heel side, there are 3 or 4 storey buildings that give me the opportunity to tie-back the wall at top to the basement or ground floor slab.

Rotation of bottom of the wall around the tie point is checked in conjunction with the Sliding stability, and shear keys will be provided in many cases. However I am not sure that Overturning stability of the wall around the toe will be a valid mode of failure that should be investigated. Any help, reference or idea will be very much appreciated. Thasks.

 

[ChipB]

Can you provide a sketch of what you are trying to do?

 

[SH43]

Yes, Please see the sketch attached.

 

[ChipB]

Careful with that tie back. I know you are talking about tying back to a building, but unless that building is a good distance away, you are almost trying to pick yourself up by your shoestrings. That same building is going to be surcharging your soil and increasing your soil pressure at the heel side.

You can download an old version of Enercalc (v5.8) from their website. With that, you can design a restrained wall, with an adjacent footing. Whatever it tells you the restraining force is, you need to multiply it by 1.5.

I also recommend looking at NAVFAC 7.01 and 7.02. You should be able to do an internet search for these documents. Especially, you need to review Chapt 3 of 7.02, in the area where they explain the tiebacks and the effect of their location distance from the wall. The current "equivalent" of 7.02 is not the same.

At 30 feet in height, your most economical solution would likely be a segmental retaining wall.

 

[civilperson]

Can the building take the horizontal load that the tie produces?

 

[PEinc]

At 30' high, the most economical solution could be a tiedback soldier beam and lagging wall with a cast-in-place facing. That could depend on (1) getting permission to install permanent anchors under the building, (2) the buildings being far enough away and shallow enough to allow anchors to be installed under the buildings, and (3) the buildings being far enough away or on deep foundations so that the buildings are not being supported by the tiedback wall. Another option could be a stronger, but much more expensive, tiedback, secant pile wall that could be designed for the building surcharges.

In your sketch, the proposed concrete wall will be a cantilevered wall which will deflect when backfilled prior to installing the tie rods. Therefore, you may want to consider whether or not at-rest pressure is more than necessary.

I would not recommend a segmental retaining wall if you have to install temporary sheeting to build the segmental wall. Again, how far back are the existing buildings? Is there enough room between the buildings and the proposed new wall for installation of the geogrid wall reinforcement and temporary, tiedback sheeting?

 

[SH43]

Thanks all for your replies:
"ChipB", thanks for the references. I will go through them.

"civilperson", the buildings will be designed for the tie force.

"PEinc", the site will be undergoing significant earthwork for stabilizing soil including grouting and using geotextile. I have been given the soil parameters to use for earth pressure.

The buildings, in the case of tallest walls, are only about 0.2H away (H height of the wall)which is very close. In the case of shorter walls, I have distances ranging from 0.5H to 2H.

Type of the walls have been agreed on long before I got involved in the project. I have no choice there. I just have to make it work. Nothing is built yet.

Thanks again!

 

[civilperson]

Counterforts is an option for tie back of wall face.

 

[PEinc]

So, the buildings will be built after the wall?

 

[NS4U]

I think you need to check stability of the wall rotating about the tie...

 

[kslee1000]

Consider the building as "deadman" and check the required passive zone to develop the anchorage force. Any Geo-Tech text book will help. Your building needs to be designed for the passive pressure as well. Make sure the building won't slide, nor rotate excessively. The tie back solution has its drawbacks for long distance applications (relaxation, creep...), also, any thoughts on losing some or all the tie backs due to later development, are both wall and the building will still be safe?

 

[STVU]

Have you considered a soil nail wall? Earlier post mentioned Soldier Beam with Tieback-you would probably need two levels of tiebacks-maybe one at -7ft and the other at -20 ft or so.

As for references, Caltrans has soil nail publications and FHWA used to have tieback wall designs.

 

[PEinc]

I would not use soil nails if the wall is close to the buildings unless the buildings are on deep foundations. Soil nails are passive elements which need movement to pick up the load. Movement is the last thing you need if supporting a building.

If the 30' high wall is supporting the buildings, you probably will need two rows of tiebacks, as STVU said, unless the wall is very stiff, such as a secant pile wall or the wall in your sketch. If you need two rows of tiebacks and if the buildings are shallow, the upper row of tiebacks may be able to be attached to the buildings but the lower row would probably need to be drilled and grouted anchors which may need to be installed under the buildings.

SH43, you really did not provide any information about the buildings or the soils.