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?

 

[BigH]

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.

 

[escrowe]

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.

 

[msucog]

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.

 

[BigH]

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.

 

[escrowe]

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.

 

[bmike]

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.

 

[cvg]

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.

 

[bmike]

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

 

[MickityMike]

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!

 

[darthsoilsguy2]

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.

 

[msewalls]

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 ....

 

[msewalls]

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!