2

I have built two soldier pile retaining walls (where you slot the boards between I-Beams you sink into the ground) with good success - but that success was due to:

  • sinking the piles at least their above ground height (ie. 5 feet above and 5 feet below)

  • very generous "deadmen" going backwards under the retained material

But now ... we have a much bigger wall to build - 80 feet long and running from 4 feet tall out to a corner that is 12-13 feet tall. Soil samples show 8 feet of "creeping" soils and our wall engineers tell us they want us 9 feet PAST the creeping, so a 12 foot corner requires a 12+17 (let's call it 30) foot H beam. And that's just one of them ...

So that's getting really expensive and complicated and the engineering firm said to just forget the soldier pile wall and switch to a segmental retaining wall - just the plain old versa-lock pre-fab blocks that everyone uses (although we'll probably use "big blocks" that are 200-500 pounds each ...

My question is this:

Apparently a segmental retaining wall is placed right on grade. As simple as that. You just level your grade and start stacking.

That sounds great, but I don't understand how that can be that simple ... if we have creeping soils and we need to go 9 feet past them and everyone (including myself) is scared to death of not drilling pier holes deep enough ... how can we go from that to just tossing them right on grade and calling it a day ?

Why won't the grade move just like the creeping soils move ? Why won't the combined weight of all that wall (which will probably be canted backward 10-15 degrees ?) push the soil underneath down and out ?

What am I missing here ? When we switch to building with concrete legos, basically, why do we suddenly not care at all about the dirt underneath the wall ?

Thank you.

  • For the versa- lock blocks, were you planning on removing grade behind the wall, installing the tie back netting than putting soil back? Or simply stacking the blocks with no net interacting with the soil? – represton May 24 at 19:27
  • The wall is at the bottom of a gentle grade, so right now there is nothing behind it ... so that will allow us to fill in behind the walls, step by step, as we build. Yes, we are planning on perhaps 3 (?) 4 (?) layers of geo-grid as we build it up ... – user227963 May 24 at 19:32
  • I'm surprised this type of wall wasn't the first recommendation. The geo-grid not only ties the wall back, but it keeps the soils from creeping. The reason why the piles had to be so deep was because it was a cantilever sticking out of the ground. Now that you don't have that, you don't really have to worry about it. It seems like your shoring engineer just gave a bad initial recommendation. – represton May 24 at 19:42
  • 1
    Big blocks 200 to 500 lbs each!? Where would you get such blocks? I see Keystone and VersaLok walls made of 80 lb blocks in higher walls than you are talking about. – Jim Stewart May 24 at 19:46
  • @Jim Stewart: versa-lok makes "Brute" (big) and "Bronco" (enormous) blocks - google those two ... – user227963 May 24 at 20:17
1

The soil below the wall is only supporting the weight of the wall. The segmental block will have layers of geo fabric every 8" or so. Basically there is not lateral force on the wall. Since no lateral the foundation is simple, and the weight of the those big blocks is nearly the same of the soil that used to be there, bad soil has some 1000-1500 psf capacity.

You have an engineering firm, talk to them. They will talk to you about these forces for free, we actually get excited to!

| improve this answer | |
  • OK, so if we scrape away the top soil and make a good, flat layer of dirt and maybe use a vibratory rammer and attempt to compact it even more, we basically have a footing that will hold the weight of the wall ... OK. Reading the rest of your comment, CAN YOU CLARIFY - you are saying geo fabric is every 8 INCHES ? So in a 12 foot wall we are doing something like 18 courses of geo-grid ? Just curious ... thanks. – user227963 May 25 at 5:12
1

We are concerned with the soil directly below a retaining wall AND we’re concerned with the type of soil that abuts a retaining wall.

There are 3 types of retaining walls: 1) cantilever, 2) gravity, and 3) counterfort and buttressed.

1) You have been building a cantilever type retaining wall. There are two types: the type where you put something down deep in the ground and it resists any force pushing it over. (This type resists forces by keeping the portion in the ground from “swinging” through the soil. The type of soil is very important because the area of the member in the ground is the only thing resisting the overturning.) The other type is a concrete or masonry retaining wall is connected to a concrete footing. (The wall extends into the air and resists any force that is pushing it over by its connection to the footing. This type of retaining wall has far more contact with the soil.)

2) Gravity retaining walls are based on the weight of the wall, (whether it’s concrete, sand bags, or whatever,) pressing down more than the force pushing against the wall. The taller the wall, then obviously the heavier the wall, and therefore the more likely it is to sink into the soil. (Soil has voids that can compact under pressure AND some soils are stronger than other soils.)

3) Counterfort and buttressed retaining walls are a combination of the cantilever and gravity retaining walls. These walls can be built 40’ - 60’ tall...like Medieval castles. ...but that’s another discussion.

ALL retaining walls start by preparing the soil where the retaining wall will bear. You indicate that “it seems like you can just lay the blocks on top of the ground”. This is not true and becomes more important the taller the wall is built.

In ALL retaining walls, the topsoil is removed and usually a compacted gravel base is installed to help distribute the weight of the wall to the soil.

In gravity retaining walls, the variable is the size (and weight) of the blocks that are stacked up. The higher the wall, then the more force it needs to resist, so the bigger the blocks need to be. A good engineer can calculate the force of the soil pushing on the wall (called the angle of repose) and then can calculate the weight of the gravity retaining wall required.

Summary:

The taller the retaining wall, then the greater the bearing is on the ground (soil). If the bearing value of the soil isn’t high enough, then the retaining wall (cantilever or gravity) needs to be wider to distribute the force to the soil so it doesn’t sink into the soil.

Also, the force of the soil pushing on the wall could cause the wall to slide sideways. So, if the first course of blocks are recessed into the ground, the ground will help keep it from sliding.

Therefore, the soil characteristics are very important.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.