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
RE: Geo-grid Retaining Wall
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
Thank you. You answered my question. I tried search with so many unrelated topics returned - not much help there.
RE: Geo-grid Retaining Wall
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
RE: Geo-grid Retaining Wall
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
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
RE: Geo-grid Retaining Wall
RE: Geo-grid Retaining Wall
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
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
RE: Geo-grid Retaining Wall
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
RE: Geo-grid Retaining Wall
RE: Geo-grid Retaining Wall
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
RE: Geo-grid Retaining Wall
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 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
RE: Geo-grid Retaining Wall
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
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
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
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
RE: Geo-grid Retaining Wall
RE: Geo-grid Retaining Wall
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
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
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
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!