Design of solid/hollow blocks as retaining wall

[MSUK90]

Dear all,
Below is the attached picture for the design I pursue. Basically, client wants to make a boundary wall in front of existing wall due to some issues. Now he doesn't wants to use RCC for design and insists to use the Solid Block work to support soil horizontal pressure. I am not much into design of block work for horizontal loading. Require some suggestions or references.
Spacing between boundary wall columns will be 3m c/c. Can I design my wall to span horizontally between the supports?
Please suggest the solutions that I can provide.

 

[r13]

What is the material of columns? I'll provide another bond beam at the level of fill, and tie the bond beams to the columns. More important is, as your new footing is lower than the existing, how do you purport to support it during excavation? To whom the existing wall belong to?

 

[MSUK90]

What is the material of columns? I'll provide another bond beam at the level of fill, and tie the bond beams to the columns. More important is, as your new footing is lower than the existing, how do you purport to support it during excavation? To whom the existing wall belong to?

a) Concrete columns will be used at 3m c/c. Size of column will be 200mmx200mm, reinforcement(initial)-4Y12 bars on corners and 8mm diameter stirrups at 200mm c/c.
b) I will attach a picture with your suggested solution, tell me if you were saying the same thing. In such case, do I need to design my solid block work as horizontally supported or I can ignore the design totally?
c) The existing structure will be supported through sheet piling while excavating for the new one.
d) The existing wall also belongs to the same owner.

 

[r13]

Yes. Now I'll design the lower wall spanning vertically between the beams, and grout solid with reinforcing steel. You might not need to do any thing on the upper wall, but to check its stability for lateral load.

If there is sheet piling during excavation, why not put the footing at same level?

 

[MSUK90]

Now I'll design the lower wall spanning vertically between the beams, and grout solid with reinforcing steel. You might not need to do any thing on the upper wall, but to check its stability for lateral load.

Thank you. Was thinking the same.

If there is sheet piling during excavation, why not put the footing at same level?

I think the main reason they want to make new wall is because the existing wall is exposed fully above the ground level with its footings(due to difference in level along the boundary wall length). That's why need more depth and excavation to new ones.

 

[r13]

I see. Good luck then.

 

[PEinc]

Do you know anything about the subsurface conditions? Any bedrock, boulders, ground water? Can sheet piling be driven deep enough? Is the existing wall retaining soil? No retained soil is shown on your sketch.

http://www.PeirceEngineering.com

 

[MSUK90]

Do you know anything about the subsurface conditions?

Yes. The subsurface is silty sand with bearing capacity of around 120kPa.

Any bedrock, boulders, ground water?

No bedrock or boulders. Ground water not found in till 6m depth from EGL. However, the soil will be compacted and a 20cm road base layer will be used below the footing with 10cm PCC.

Is the existing wall retaining soil?

No, existing wall is not a retaining wall. Please read my last comment for the probable reason for this newly proposed wall.

 

[BAretired]

I'm not so sure about the new tie beam location until we establish the geometry of the whole wall a little better. You can't build the new foundation as low as you are indicating. You must maintain a slope of 1:1 from the existing footing. Depending on the soil type, that could be closer to 1.5H:1V or even 2H:1V.

What is the cap on the existing wall? Precast concrete? I suppose your client will want to match that with the new wall.

Quote (PEinc)
Is the existing wall retaining soil?

No, existing wall is not a retaining wall. Please read my last comment for the probable reason for this newly proposed wall.

If the existing wall was not a retaining wall before, it will be afterwards because you are adding fill on the inside. Where is grade level outside the existing boundary wall? And what is the clear distance between walls?

BA

 

[MSUK90]

You can't build the new foundation as low as you are indicating. You must maintain a slope of 1:1 from the existing footing. Depending on the soil type, that could be closer to 1.5H:1V or even 2H:1V.

That means I have to move my footing to the left? How did you define those slopes? Based on the failure cone?

What is the cap on the existing wall? Precast concrete?

Those are CIP concrete beams. Size 200mmx200mm, 2Y12 top and 2Y12 bottom bars.

If the existing wall was not a retaining wall before, it will be afterwards because you are adding fill on the inside. Where is grade level outside the existing boundary wall? And what is the clear distance between walls?

Some of these are answered in attached picture. The clear distance between walls is almost 1m.

 

[BAretired]

Below is the attached picture for the design I pursue. Basically, client wants to make a boundary wall in front of existing wall due to some issues. Now he doesn't wants to use RCC for design and insists to use the Solid Block work to support soil horizontal pressure.

I have no idea what the issues are and you are under no obligation to tell me, but so far as I can see, there is no need for a second boundary wall. However, if a second wall is deemed desirable, there is no need for it to be as deep as you show it. The foundation could be placed at the same level as the existing and still be hidden from view as it would be well below interior grade. This would make imminent sense if cost is a consideration.

Using sheet piling to do the excavation is a needless waste of money. Using cast-in-place concrete columns at 3m o/c and CIP beams is expensive and unnecessary unless you are doing it for aesthetic reasons. With a lower height of retained fill, reinforced masonry should be perfectly adequate to do the job. I can't read your green dimensions, but it looks like the difference in grade each side of the new wall is less than 1000mm, a piece of cake for reinforced masonry.

That means I have to move my footing to the left? How did you define those slopes? Based on the failure cone?

Why? The footing does not have to shift left. The slopes are based on the natural angle of repose of the soil under the existing footing.

If the existing wall was not a retaining wall before, it will be afterwards because you are adding fill on the inside. Where is grade level outside the existing boundary wall? And what is the clear distance between walls?

You did not respond to that comment. It should be self evident that the existing boundary wall is not retaining soil now, but will be retaining soil after the new wall is built unless you plan to raise the grade outside the existing wall.

Before starting to fret about how to reinforce the new wall, it is better to determine where its foundation ought to be.

BA

 

[r13]

BA has raised a important point, make sure the existing wall can retain the new backfill, which wasn't there before. Also, good drainage is required for area enclosed by the walls. Stability of the existing wall throughout the construction is a focus effort to be achieved.

 

[BAretired]

Another possibility is to omit or lower the compacted sand fill between the two walls, then tie them together with a slab or deck at the level of the existing tie beam.

BA