×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Geo-grid Retaining Wall
2

Geo-grid Retaining Wall

Geo-grid Retaining Wall

(OP)
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?

RE: Geo-grid Retaining Wall

do a google search for MSE wall

RE: Geo-grid Retaining Wall

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!

RE: Geo-grid Retaining Wall

(OP)
fattdad:

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

RE: Geo-grid Retaining Wall

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

 

RE: Geo-grid Retaining Wall

(OP)
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?

RE: Geo-grid Retaining Wall

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.  

RE: Geo-grid Retaining Wall

(OP)
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.

 

RE: Geo-grid Retaining Wall

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.

RE: Geo-grid Retaining Wall

(OP)
CVG: Thanks.

RE: Geo-grid Retaining Wall

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.

RE: Geo-grid Retaining Wall

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.   

RE: Geo-grid Retaining Wall

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...

RE: Geo-grid Retaining Wall

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.

RE: Geo-grid Retaining Wall

(OP)
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.  

RE: Geo-grid Retaining Wall

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.

RE: Geo-grid Retaining Wall

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.       

RE: Geo-grid Retaining Wall

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.

RE: Geo-grid Retaining Wall

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.   

RE: Geo-grid Retaining Wall

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.

RE: Geo-grid Retaining Wall

You (the owner or the EOR who may (likely) be hiring the wall designer) need to have this clear at the beginning.  If this was a cantilever retaining wall, would not the geotechnical engineer confirm the stability of the wall against bearing and global stability?  The wall is designed by the EOR or his structural team member. It is typically the EOR (and I am thinking along the lines of the highway design engineer) that chooses the wall locations and needs to ensure that his choice of walls has solid bearing.  If it were an embankment and not a wall, wouldn't the design engineer be responsible for the stability of the embankment?  Clearly, the wall designer (like RECO) must ensure that his MSE wall behaves as a unit without failures internally - but how is he responsible for where the wall is situated and on what soil it is sitting?  If the ground needs to be modified (preloaded, piled, etc), I don't see it as his position - but would be the position of the designer (as it would be if it were a cantilever retaining wall).  Maybe our experiences are just different.  However, whichever way goes it should be clearly pointed out at the beginning of the project - so each party knows his responsibility. I once saw a designer specify an RE wall (not an MSE wall) on soil and try to wash his hands of the fact that the first 6 m of soil had undrained shear strengths of less than 20 kPa - and the walls were specified to be 10 m high.  No wall design, no soils information, nor ground modification in the design - just that a certain type of wall was specified for the contractor at a specific location.  Is the contractor responsibile? His wall designer responsible?  I think that the EOR is responsible for ensuring that the foundations will support whatever wall is placed.

RE: Geo-grid Retaining Wall

Just to be clear,
by 'global slope stability analysis' I mean that the proposed wall
and reinforced earth zone should be considered in the context
of the larger slope, should a larger slope exist.  
A wall constructed at the toe of an existing slope (for example)
may be structurally sound and adequately resistant to sliding and overturning,
but still vulnerable to a deep slumping failure or landslide
originating and/or terminating well beyond the limits
of the area considered in the wall design.  
To the extent that construction of a wall changes
the geometry of a portion an existing slope,
a global stability study may be critical.  

This sort of study, which should include test borings and say,
a few iterations of PCSTABLE,
is usually done by the project geotech, or some other masochist.  
It would definitely be an extra for the wall designer, IMO.   

RE: Geo-grid Retaining Wall

i see bigh's position and it's not unreasonable depending on the contractual agreements and predetermined scopes of services. the mse designer is designing a geotechnical structure...if they cannot evaluate their own structures, then they should not be designing the things. the geotechnical firm is typically "reduced" to simply being the geotechnical testing firm and performing the testing as requested by the designer and then providing all the test results, boring data, etc to the designer for their assessment. and again, this goes back to certain designers i have dealt with the choose to completely disclaim any all liability for the wall, wall design, performance, etc. on to the owner and owner's geotech. additionally, when they ask for the specific parameters that they should directly input to their software, i will always refuse. if i am going to give them that, i'll just cut out the middle man and provide the design myself. if these certain designers would take a reasonable approach, then i would be willing to cooperate...and i do cooperate with other designers that do their job and accept their responsibility. when a sizable wall goes south, the geotech can easily be stuck with many hundreds of thousands of dollars in litigation related fees regardless of what they did or did not do on the job.
so have a pre-project meeting that includes the geotech, designer, civil, and owner to discuss who does what and what is expected.

RE: Geo-grid Retaining Wall

escrowe - I think that msucog and myself were/are well aware of what you meant by global stability - it mirrors our understanding.  We agree (may I say, msucog!!) is not the wall design (sliding, overturning, toe bearing) but looking at the overall basal foundation and how the wall acts with up/down slopes.

RE: Geo-grid Retaining Wall


All good points!  Sorry for my pedantry (BigH)
I was thinking of other readers possibly not experienced
with the terminology.  
------

And might I add that certain wall designers
are not alone in asking geotecs for more and more
data, recommendations, meeting time, etc...
beyond the scope of their contracts.  

You know, I'm sure that most clients can read
and understand a proposal,
but I'm conviced that some choose not to.  
And I have had a number of 'run-ins' with folks
(owners, contractors, even other consultatnts)
who believe that once the geo engineer has submitted
even the most preliminary report based on limited data,
that the geo engineer is then somehow responsible
for every subsequent foundation and soils-related issue
on the entire site, not to mention the specific project.   

RE: Geo-grid Retaining Wall

A functional MSE wall can be constructed with no facing material at all, if you can accept some minor ravelling at the outward face.  

The "footing" serves to do little more than make sure your facing units are lined up and levelled when you start.  It's primarily aesthetic.  

RE: Geo-grid Retaining Wall

for a river front embankment wall which would be inundated by floodwater from time to time (as stated by the OP),  building the wall without facing and without a footing could be a serious mistake if there is any erosive force on the wall due to flowing water.   

RE: Geo-grid Retaining Wall

Missed the bit about the river - I won't argue with you there.

RE: Geo-grid Retaining Wall

1. Design crushed stone keyway (foundation) with base below scour elevation.

2. As backfill material for reinforced zone, use 1" clean rock. Wrap this entire zone in synthetic filter fabric.  This way your wall will be freely draining and hydrostatic pressure will not be an issue.

3. Have a healthy snack!

RE: Geo-grid Retaining Wall

instead of tiny MSE blocks take a look at the large concrete blocks... something in the area of 2'x4'x3'.  they're better for taking environmental damage and staying put when getting a beatdown.

if it doesn't need to be pretty, your local concrete company may have some "waste blocks" that they make out of old and rejected concrete.

RE: Geo-grid Retaining Wall

OP, you will for sure have issues, especially on a river front.

BigH - What's your deal?
1. Last time I checked this was a topic about Geo-grid Retaining Wall (which Reinforced Earth (Trademark) walls, RECO) as far as I know do not and have never done Geo-Grid Walls?
2. There are many other MSE Wall Companies out there as well, you do not mention them only RECO?
3. Pretty sure all MSE wall companies say they are responsible for their wall and internal stability only. They all need to know what the Wall is going to sit on, they design walls, they do not do soil analysis.
4. I know RECO and most other MSE Wall companies WILL DO THE GLOBAL ANALYSIS... but you have to pay them for it. They are all in the business of designing walls, not doing global stability analysis!

sorry everyone else, just can't stand when someone bashes a company for no good reason and it's not even on topic ....

RE: Geo-grid Retaining Wall

OP - his statement is technically correct, but it really depends on the application. You can use Geo-Grid without any facing to help in some applications.
However, This does not apply to you.
I worked on a project for a dam and we used an MSE Wall for it, you need to:
1. Be sure your foundation is not going to erode.
2. You need to be sure the foundation can hold your wall (i.e. global stability)
3. After the flood situation, what kind of draw down of the water do you expect? (i'd say this is the most crucial for whatever you do). the pressure of the water draining in my dam project was "the main" design factor.
If you take this into account, an MSE Wall will work fine.
Good Luck!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources