GeoGrid Limitations

[surfviper]

Hi,

We are looking at options for replacing a retaining wall. Some of the options include the use of "GeoGrid" or similar products.

The questions is, do these products create problems/limitations later, such as for:
• Planting trees
• Setting fence posts
• Changing landscape shape
at a later time where the GeoGrid has been placed...?

Are there any other down-the-road considerations we should know?

Many thanks!!

 

[PEinc]

1. Yes.
2. Talk to the contractor and wall designer. Basically, if you build a geo-grid reinforced retaining wall, the area occupied by the geo-grids is restricted to underground modifications. Also, as with any retaining wall, there are limitations on the magnitude of surcharge that the wall can support. For example, if a wall is designed for a 100 psf surcharge, you shouldn't be able to place a 4' deep above ground swimming pool behind the wall. 4' x 62.4 pcf = 249.6 psf which is greater than 100 psf.

 

[surfviper]

Thanks, PEInc -- I am obviously asking a newbie question out of my area, but your answer makes sense to me -- many thanks.

 

[msucog]

if you're dealing with a sizable wall, i suggest you read the fine print if you're involved with the project. in my part of the country, the plans are typically riddled with disclaimers and somewhat fuzzy assumptions (the soil parameters used for the design are not typically to this region). and based on the language, the liability is usually dumped back on the owner and testing firm. the testing firm should be there to run the tests (and at the frequency) as specified by the designer. i have yet to find a designer accept responsibility for their design and they refuse to provide the testing protocol. the projects i cross them on are typically rather large projects so if it's a small wall, then the scenario may not be similar. also, different areas of the country may see different practices. as you can tell, i have a rather negative opinion of the designers/designs i've seen. i like the walls but the practices used by most here are dishonest in my opinion. i do believe there is only one designer out of many that the engineers in our office have a positive opinion of. good luck.

 

[DarthSoilsGuy]

we've had the manufacturer supplied wall engineer incorporate a primary stormwater member, a sewer manhole, a sewer line, and fence posts into their design for the same 14' wall we put on our Civil plan. Another time i have required planting beds. This was done in the submittal process.

i wrote a retaining wall specification for my company that requires both a Design submittal and a Material submittal. Anything we have concerns about we include in the Design submittal requirements.

The fencepost item commonly comes up b/c codes require fences often. this should not be new to anybody, but must be asked for or you might not get it.

Planting beds need to be really well thought out on your side with the Landscape Arch. Consider the root ball and root path in plant selection. The wall engineer will probably go no further than considering the planting bed as a load and a region for no grid.

i don't know what you mean by changing landscape shape.

MSUCOG - when i worked in your region (NC,SC,VA right?), it was my impression that the reinforced and even retained soil parameters increased every year. some of the wall engineers i work with have not been infected with this virus in the Northeast. i put it right up there with more specs for 98% STD compaction, 100% STD compaction, or any modified compaction that isn't for transportation work or industrial. it would seem to me that upping the numbers is a knee-jerk reaction (and trend) by specifiers to problems of never having the specified compaction (in untested areas of course) in the 1st place. but, that is a rant for another post.

 

[msucog]

the problem i see (and eluded to) is that:
1. the plans/specs are written by the designer very favorably for the designer and implies complete liability for everything to the owner and their testing firm

2. the material assumptions are not "typical" for sites in this area. the owners are not made aware of this on the front end until they've bought in to the design because the wall is going to save them money. the designer gives very general specs for most things and specific specs for others. then, they get wishy washy when it's pointed out that their design is unrealistic for what will likely be encountered. for example, the gradations almost never work here and are usually far from even being close. i mean, it's rare that i run across soils that have less than 35% fines. and if it happens to be marginal, it's because it's got so much mica that it's throwing off the gradation. then when it happens that all that exists on the site contains 50%+ fines, they'll (in a very vague way and after many attempts to dump it off on someone else) say it's okay to use. and the assumed strength parameters always differs from what was suggested in the preliminary subsurface exploration for the general site (nothing to do with the walls which are usually located along/in creeks--i.e. much crappier conditions than the rest of the site).

3. the designer never indicates or even suggests the cost of testing to the owner that is required to satisfy their specifications. and the testing that is noted as required is so vague that it's completely unrealistic for anyone other than the designer to clarify what they want tested and when they want it.

4. the designer refuses to provide a testing protocol. if i (as a testing firm) calls up and asks a structural engineer what kind of testing he wants on his concrete, he'll say something like "slump, temp, air, unit weight, compressive strength for every 100cy. compressive strength testing at 1-7 days, 2-28 days, and 1 reserve". even when we do corner them in a meeting and ask which sample to run strength and index testing on, they won't even provide testing parameters. for instance, triaxial shear testing is reported at 15% strain. i have no idea if that strain is appropriate for the wall (actually, i'm sure it is not in most cases since the owner will not accept a wall that moves enough to reach 15% strain). i've asked which strain they want results reported at and they tell me to pick it. i then turn around and tell 'em i'll give it at 5, 10 and 15% and let them decide which to use. i have actually had designers tell me to specify what's needed since i'm the geotechnical engineer...HELLO! THE WALL DESIGNER SHOULD BE A GEOTECHNICAL ENGINEER AND IS THE ONLY WHO KNOWS WHAT IS APPROPRIATE FOR THEIR DESIGN!

i could keep going on and on but i do not want to over run the thread with negative comments that stray too far from the question at hand. i guess i would suggest looking at the long term liability that will be accepted based on the walls plans and specs. compare that to the "complete" cost of the wall design, construction, exploration, testing, etc. as far as the overall look/feel of the walls, i like the look of the things and think they're great when the correct assumptions are used, when the owner knows what he's buying in to, and when the designer actually works with others on the project instead of playing CYA while throwing everyone else under the bus. however, this is not good for most designers because this will usually eliminate the apparent/advertised cost effectiveness of the walls versus concrete walls. concrete is tried and true and reliable without the owner getting twisted up in the complicated details of the soils.

if i were the owner and it were my wall and the designer put off all responsibility on me, i'd fire them and start going through new wall designers until i found one that was reasonable...or just use concrete. all in all, it's my opinion that the wall designer should include exploration, design, testing, and complete oversight of the construction in their package. that way, they're responsible for what they're getting paid for.

(i'll get back off my soap box now for a while).

 

[DarthSoilsGuy]

msucog,
you haven't explored emoticons well.
may i suggest:






another thing i was thinking about..
The blocks play a major part too.

For complicated walls with a lot of junk in them, we specify "Large Modular Segmental Retaining Wall Units" i define them as "Wall unit weight/coverage exceeds 150-pounds/vertical sq. ft" (Note:It is my own definition that i made up for the spec. there may be a generally accepted definition out there already.

as far as i can tell, the SRW market is made up of blocks that the wall builders can place manually (keystone, versalok, ridgerock, etc...) and they fall under the "SMALL Modular SRW Units" definition i set up. There are also concrete cast blocks that must be placed by machine (somewhere between 0.5 and 1.5 cubic yards per block). google "Redi-Rock" for example.

The large blocks have a greater SFF (Safety Factor when F###ed up) in my opinion. so i prefer to specify them when junk gets thrown in the wall.

 

[PEinc]

This thread has wandered way off the original subject.

 

[DarthSoilsGuy]

wudn't me