Retaining wall sliding problem

[qcjr]

Hello everyone,

This is my first post so bear with me please . Also, I am really sorry about using metric units (Canadian engineer here).

We are designing a new building 4 m away from an existing one. Since the ground level is higher than the basement level (slab on grade) of the new building, we need a retaining wall for the new building. The new building has two levels: basement and ground. I will give you some numbers so you can figure it out:

Soil level: 10.00 m
Ground slab level: 10.00 m
Bottom of ground level footing: 7.90 m
Basement slab level: 6.50 m
Bottom of basement level footing: 5.75 m

Distance from existing building wall to ground level wall: 4.00 m
Distance from existing building wall to basement level wall:
7.00 m

As you can see, the wall closer to the existing building (ground level wall, 4 m away) is actually a standard footing wall (with soil on both sides).

The second wall (basement wall, 7 m away) is a retaining wall (soil on only one side), and is also deeper than the first wall.

The ground level slab is presumably supported by both walls (along with the building's steel frame).

As far as construction goes, I am assuming the contractor will dig, build the retaining (basement) wall first, fill back to bottom of the footing (ground) wall, build the second wall and fill again (including the portion between the two walls).

So, do you understand?

My problem is, after calculation of at-rest earth pressure and horizontal forces equilibrium, all this stuff slides (security factor less than 1 even).

Question #1: Is it actually better (safer) to assume no soil between both walls, above the ground-level footing level (7.90 m)? Since I consider this problem at-rest, is the passive resistance developed by the smaller wall necessary?

Question #2: How would you prevent this wall from sliding (apart from keys or footings enlargement)?

Sorry for long post. Thanks for your help!

 

[csd72]

qcjr,

Never apologise for using metric units, some countries have moved onto better things while some others......

I think you need to post a sketch (look in the FAQs to find out how) as I just tried to sketch it out and I still dont understand you.

Oh and welcome to eng tips.

 

[qcjr]

csd72,

Thanks for the reply. I just learned about posting a sketch, I will do it.

Stay tuned..!

 

[qcjr]

Ok, try #1 for the upload. Let me know if you can see it/figure out the problem.

http://files.engineering.com/getfil...35-4d48-a7a7-829951786f62&file=retaining1.JPG

 

[csd72]

Just put the left hand wall on a grade beam over piles and avoid the issue entirely. Otherwise if you stick to strip footings then it just needs to be taken down to below the line of influence of the retaining wall (usually taken as 45 degrees) or the frost depth whichever is greater. You can then design the retaining wall as if everything was soil.

Be careful of where the neighbors footing is with respect to your excavation and observe the line of influence as noted above.

The top footing would not slide as it has soil on both sides of equal height, the lower footing would not slid as it will bear on the basement ground slab.

 

[qcjr]

Thanks for the help!