Geogrid segmented Concrete Wall Question 1

[msquared48]

A friend has asked me to look into a horizontal mesh reinforced segmented concrete retaining wall - 11 to 12 feet max retention - that is has laterally spread at the top by 1 to 2" from the paving from excessive hydrostatic pressure due to a gap between the paving and top of the wall allowing water to filter down. There is a drainage channel at the top of the wall that is supposed to to catch the runoff from the sloping backfill and divert it elsewhere. The walls from this block system arte shown as 2X2X4' and detailed as a gravity wall system by the supplier's catalog, with no geogrid. There was a design by another engineer 10 years ago showing that the wall is OK for the original design loads that included 2 feet of surcharge for a sloping backfill and a geogrid layer at 2' vertical intervals. Now they want to add a HS truck road surcharge. Lots going on here...

My question and concern has to do with the geogrid reinforcing and the overload from the excessive hydrostatic pressure - the geogrid has obviously stretched and my concern is that it may be close to failure, if not at least locally failed, at the upper levels. How much can this material be stretched before failure? The length of the embedded geogrid layers is 15 feet according to the reports furnished. I am unfamiliar with the different types (patterns, thicknesses, grades, etc.) of geogrid material, so any enlightenment to ask further questions will be appreciated.

Controlling the hydrostatic pressure is a must, but the wall will never return to the original geometry. I am of the opinion to excavate the top portion of the backfill and examine the condition of the geogrid. If it is OK, then reconstruct the excavated section and proceed. Otherwise, remove and replace with a new wall. Seems like this scenario is like a soil nailed wall where the wall has to deflect in order to load the geogrid and gain lateral stability.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

Less likely the geogrid stretched an more likely the geogrid slid through the layers of fill IMO. Geogrid has a very steep modulus of elasticity by design so that the wall doesn't "creep" during it's service life. However, I'm sure they can and do stretch somewhat before failure so maybe you're right and the geogrid is heavily stressed. Typically you'll get around 5,000 lb/ft of allowable tension in a typical geogrid but it depend highly on the manufacturer and the "grade" of geogrid.

I've done a few geogrid retaining walls and controlling moisture pressure behind the blocks is definitely critical. I usually spec a foot of well draining gravel the full height of the wall with a underdrain at the bottom.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[oldestguy]

Anchorage to the wall is another possible failure zone.

 

[dcarr82775]

If the upper layers were put in with slack, you would need movement to mobilize the grid.

 

[concretemasonry]

In my opinion,a geo-tech engineer with experience in the many different type of soil reinforcement. should be brought in.

There is a wide range of fabrics, patterns and type of reinforcement be it an open mesh or a tighter/stiffer fabric the looks like it is perforated.

As a registered engineer working for a concrete products company that made concrete wall systems, I quickly realized that the soil reinforcement that had been used for decades is not just something that comes off the roll and is buried.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[TehMightyEngineer]

Right there with you dick. Thankfully the company I work for now realized a while ago that MSE walls are houses of cards that depend on a carefully detailed installation for the geogrid and backfill to work successfully.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[JedClampett]

Are you positive the Geogrid was even put in? Whenever I specify one of these type walls, I'm worried that they put the blocks in, but forget the tiebacks (geotextile). It really looks insignificant to your common construction worker.

 

[msquared48]

Yes, it was installed, and even further back than specified.

In thinking on this, I think I will pass on the job as there are too many "possibles" when I described the problem to my wife. Too much liability to accept the existing wall.

Sometimes you have to listen to yourself...

Mike McCann, PE, SE (WA)

 

[darthsoilsguy2]

i'm a little confused and am having trouble following the narrative...
you made a statement about the wall plans showing a gravity wall and another wall design showing grid...
it is common rule of thumb to design a wall for 2' overburden to size grid for traffic loads, but we have a backslope that disappeared i guess... was that backslope cut away?... was another wall added to remove the backslope?... you say the gap is at the top of the wall and excessive hydrostatic pressure may be in play, but wouldn't the water drain down to the base of wall where you would see the excessive water pressure.... or are we talking about lots of water in the upper region creating a quick-condition or lubing the soil up to pullout the grid?... a property I checked out opened up on the top when the house next door burned down and most of the fire water routed to the top-of-wall.

if it really is the blocks at the top grid course... seems like it should be easy enough to go take a shovel remove the topsoil in that gap and probe into it to see if it is stiff and to see the soils... you might find the soils above the grid are all loose and help you explain why they aren't really gripping the grid. you should be able to make a pothole big enough to see the grid and guess what it is if you know grids and probe through a few holes to check out the compaction of the layer below and look for continuity of the drainage stone behind the block between lifts.

 

[msquared48]

The manufacturer's catalog shows a wall without geogrid, but the engineer of record adapted geogrid to the wall. That, too, is a worry for me, even with the calculations.

Regarding the wall drainage, the specifications called for a full height, 1 foot wide, section of pervious backfill behind the wall. The wall section only showed a rock liked drain at the bottom and a drainage channel at the top. I have a REAL problem with that difference, considering the deflection seen at the top.

Regarding the a lateral movemeny, I hand no indication where the movement starts on the wall.

Mike McCann, PE, SE (WA)

 

[msquared48]

Crap. My phone will not allow me to edit the above post. I'll have to correct it later...

Continuing... the movement could be global sliding, rather than rotational. That is another BIG unknown.

Mike McCann, PE, SE (WA)

 

[msquared48]

For the record though, I found out from the other engineer that they were not going to accept the liability for the wall either. Gee... you think?

Mike McCann, PE, SE (WA)

 

[dicksewerrat]

When they installed the geogrid, they may have forgotten to install the proper sized aggregate above and below the grid to make it work.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[darthsoilsguy2]

the simplest approach (apart from not taking the project on) is to install survey points at the top and middle and monitor it over time to a common permanent reference.... and perhaps correcting the drainage issue that may or may not be contributing if it isn't too big a deal.

 

[EngineeringEric]

Big soil anchors at given spacing with a system of tie back walers...

 

[oldestguy]

Mike: Having walked off a few jobs like this, I look back and it never hurt me.
Actually it helps your reputation.