Geo-grid Retaining Wall 2

[kslee1000]

I saw a new retaing wall construction and noticed there was no foundation (concrete), nor reinforcing. The void blocks were placed directly on soil, and the contractor said it was a "geo-grid" system that no foundation required. Is his statement correct?

 

[cvg]

do a google search for MSE wall

 

[fattdad]

The block face should be placed on a leveling pad, usually about 2 ft below the finished toe grade and consisting of aggregate.

As cvg points out you will learn alot if you look up MSE (mechanically stablized earth) or SRW (segmental retaining wall) design.

f-d

¡papá gordo ain’t no madre flaca!

 

[kslee1000]

fattdad:

Thank you. You answered my question. I tried search with so many unrelated topics returned - not much help there.

 

[concretemasonry]

If you are referring to segmental retaining wall units (SRWs), look at the testing and research reports provided on-line by the various international licensors. The results are the proven installation specifications from the licensor of the system.

The "void blocks" were not placed on "soil". They were probably placed on compacted base at a prescibed distance below the finished exposed toe of the wall as determined by the engineer if it was tall enough to require geo-grid and engineering. - Most standard design plates from munipcipalities, counties and DOTs do not show any concrete for typical gravity wall applications.

Usually, no concrete footing is recommended or suggested to maintain agreement with the wall design.

I have seen walls in the U.S. and many other countries (Australia, Spain, Taiwan, China, Canada, Mexico, etc.) that were designed by professionals. The most impressive was a 10km stretch of road with walls varying from 5' to 40'. - Also, just a mile from my home there is a commercial development with many engineered walls from 1' to 30' including "tiered" landscaping all without concrete footings.

Dick

 

[kslee1000]

This application is a revier front embankment with potential to be under the water during flood stage. At the time, only 2 layers of blocks were placed. Along length of the wall, 50% blocks were on soil, and the other half were on gravel. Is this a good practice, or wise choice of type of wall?

 

[MSEMan]

MSE walls are often used in river wall applications - BUT: You need to carefully evaluate the potential for scour and provide appropriate scour protection if this is appropriate. In addition you need to assess how the qualities of the in-situ soils will change when wet.

Im hoping that the backfill is a manufactured, fricitonal and free draining material. IF the condition you describe is accurate it seems like an unhappy situation. I'm particularly worried about a potential sliding failure of the wall under rapid draw down conditions where the blocks are founded on soil. The situation will be much worse and potentially more complicated if a high quality free draining backfill isn't being used.

With all due respect, I'd suggest you get an experienced geotech to look at all of these issues before you go much further.

 

[kslee1000]

MSEMan:

Thank you very much for the valuable insight you provided. It appears in-line with one of my concerns - soil errosion/washed away beneath/behind the block wall, causing instability at the base. Judging from the poor ground preparation, I don't think there is an engineer on this job, but manufacturer/contractor representatives.

From replies of others and online reading, I sense this type of wall can endure and survive large displacement/settlement because of its flexibility. Can it survive without anything blow the blocks (the blocks hanging on the backfill, holding in place by the geogrid)?
This may be a silly question, just curious.

 

[cvg]

no, it probably won't survive without some type of toe protection to mitigate potential scour. A hydraulic analysis / scour analysis including local scour and long term degradation should be done to provide the data for designing the erosion protection.

 

[kslee1000]

CVG: Thanks.

 

[msucog]

perhaps the wall designer responsible for the design should address the issues. if there's no designer, then the contractor should address these issues since they are responsible for what they are doing. however, a geotech should be consulted for their opinion and contracted to perform some amount of exploration and testing as required by the designer.

the owner should be contacted first to see what the heck they think they bought, how they expect it to perform, and how much they think it is supposed to cost.

 

[escrowe]

Here's one free opinion--

Potential nightmare here. Several jurisdictions I have worked in
have curtailed the use of MSE block walls in SWM applications due to these concerns.

In this case the usual aggregate leveling pad
should be replaced with a concrete foundation,
bearing below the potential scour elevation.

The soil backfill reinforcement (geogrid) schedule, length and spacing
should be designed for the undrained hydrostatic condition.

Passive earth resistance (Kp) at the toe
should be ignored for sliding analysis.

The wall should be backed with a vertical aggregate drainage layer
wrapped in a non-woven geotextile, with a daylighted drainage conduit.

 

[cvg]

escrowe, I would agree with your analysis. Not to say it couldn't be done, but there may be a better way than a MSE wall...

 

[msucog]

i don't blame the type of wall, i blame the folks that design them (not all designers) for not properly informing the clients of the expected performance based on the design, risks associated with using certain soil types, additional liability to the owner, costs, etc. instead, these particular designers attempt to defer liability to others such as the geotechnical testing firm and/or the client and/or the contractor. so they forsake the client's interest in an effort to sell a "cheaper" product for a quick profit and hope for minimal/no liability. many folks are returning to cast in place walls or are forced to resort to implement measures to increase the reliability of the structure because the sites contain very marginal or poor conditions for this type of structure. in other words, the walls are often "designed" using a cook book recipe with disclaimers putting the liability on others to make the site and soil satisfy the design instead of designing the wall to work on the site.

i have actually seen a wall "designed" to go along a creek (within the natural floodplain) but the "designer" made absolutely no considerations for groundwater either in the foundation or backfill areas...but they did put notes all over the plans and in the specs that it was the owner's and owner's geotech problem. of course, that same designer also said that the geotech should provide all the specific design values for them to input in to their software (so they wanted to get paid to run a $400 software program is what it boils down to--while the geotech ends up taking the fall if the thing fails). go figure...

i would also agree with escrowe...also use free draining material (crushed stone) above where the water might exist.

 

[kslee1000]

Thanks for the valuable information/thinkings from all of you. I wasn't quite sure about MSE system before, now I understand better after all of these. Thanks again.

 

[BigH]

For Reinforced Earth (Trademark) walls, RECO only guarantees the internal stability of the wall. The external stability (sliding, bearing, etc.) is in the hands of the designer (MSEMAN - advise if I am wrong on this). So may other designs. One needs to be careful and be fully aware of the contractual requirements/limitations of a particular wall system - who designs what? - who is responsible for what? etc. BTW, I was involved in RE Walls in India that underwent 1100mm of settlement during construction - and maintained their integrity.

 

[escrowe]

BigH / msucog, the ability to withstand large total
and differential settlements is an attractive
quality of MSE. And the potential for an
inexperienced contractor to screw up the
installation is likely the scariest!
But I may be biased...

In addition to external stability, the geotech
should also complete a global slope stability analysis;
the MSE wall is superfluous if the reinforced zone
is swept away by a land slide or mobilized
by a deep rotational failure.

 

[msucog]

btw, the wall designer should be responsible for the global/slope stability analysis...it's foolish for the geotech to be responsible for this since the results will be a function of the wall design itself.

 

[BigH]

escrowe - I believe that you confirmed my point. The global stability is part of the design. The internal stability is from the "Wall System" supplier (e.g., RECO) unless the system supplier is simply supplying the materials and leaves it all in the hands of the designer. You are correct - as I indicated, I've been involved with a number of walls that settled 800 to 1100 mm during construction! You are correct in that you need an experienced supervisor/foreman for the contractor to follow. In my cases, RECO (India) did the supervision for the general highway contractor.

Who does the overall global stability (not the internal stability) - it rests with the designer but he probably will have the geotech, as part of the team, do it - as per normal slope stability analyses for embankments, cut slopes and the like.

 

[msucog]

i, as the team geotech on most instances, will not perform the global stability analysis for the wall designer just because the wall designer says that i'm supposed to do it (especially when they disclaim all their liability on me). if the owner contracts me to include this in my scope of services prior to beginning the project, then that is a different story. however, the geotechs are being made the scape goats more often than not...so no thank you. we'll do the analysis for the walls we design but i'm not doing another designer's job so that i can take his liability especially when very "marginal" soils are used or when they use parameters/s.f. that we feel are not appropriate. i'd just prefer to not be associated with the projects where i'm going to be someone's punching bag down the road.

i believe it's best to have the designer contract their own geotech to give them whatever they want...that relieves the owner and owner's geotechnical testing firm from taking liability that should not be theirs...it should be the designer's liability since it's their design.