Reinforced Soil Wall on clay 4

[andyz22]

We have designed a 5 mt Reinforced soil wall wall on a soft foundation during construction of leveling pad due to groundwater fluctuation water level raised much above ground level and diffcult to work with.

SBC of soil we require is 19 t/m2 (190kPa).The soil strata consists of Clay over 6m below ground.

even after dewatering has been done by suction What will be the economical solution in such conditions to ensure the BearingCapacity and permissible differential settlement

thanks

 

[BigH]

It is one thing to give the SBC required, but another to get any reasonable answers without providing data as to the undrained shear strength of the clay. Is the leveling pad "permanent" or "temporary"?

I worked on a job outside of Calcutta where we had 6 m of soft clay (Su in the order of 18-20 kPa). We had 11 m high RECO retaining walls for bridge approach to build. We ended up using wick drains and then stage loadings of the RECO wall. We were able to place about 4 m of RECO at the initial stage. Of course, we ended up with about 400 mm of settlement. In the end, we were able to successfully build the RECO wall with about 900 to 1100 mm of total settlement during construction without any distress to the wall. We did have a couple of problems develop on other walls but this was due to slightly different (weaker) areas within the approach and other non-engineering reasons.

It can be done. As noted above, the settlement of the fill caused increases in the undrained shear strength of the clay and hence increased the allowable bearing capacity. The wick drains allowed the settlement so that, in effect, the allowable bearing capacity (shear) approximated the allowable bearing pressure (serviceability).

 

[andyz22]

Thanks BigH for response,

leveling pad is going to permanent

Mostly of soft clay material exist.Typical soft clay properties can be considered.

can anyone help me on the maximum size of particle allowable in the Reinforced/backfill and the gradation specifications

Thanks!

 

[BigH]

We used river sand for ours . . . some specify a near road subbase gradation. Do a web search - you will find such as:

http://reinforcedearth.com.my/new/download/specification.pdf

(page 5)

 

[fattdad]

I consider "soil bearing pressure" for reinforced soil walls irrelavent. I mean, how do you gauge the soil bearing pressure? The reinforced zone (i.e., the length of the geotextile extending back from the face) represents the "footing" size and it's big - mobilizing stresses to some prominent depth, i.e., for a 15.5 ft tall wall, the grid length would be about 11 or 12 ft (using 0.7) and the stresses would exend to 30 or 40 ft.

I look at these more from the global stability perspective, which is akin to the failure mode of "bearing capacity," but more in line with wall desing requirements.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

I don't think it is irrelevant, fattdad.

You are correct in that it one has either a global stability (shear failure) problem to deal with (which we had in India) and also the settlement question. A clay of Su = 20 kPa (given a simple 2xSu as the allowable bearing capacity (for ball park) gives about 40 kPa (at SF=3) or 2 m or so or, if looking at a global stability point of view (SF=1.5) about 4 m. (see example in Lambe and Whitman). In our case, we used 4 m as the first height in the stage loading. Settlements were in the order of 400 mm for this first stage.

So, how could we build a wall of 9 to 10 m high on a soft clay and not try to figure out how much load could be put on the supporting clay for the stage loading without causing a shear failure or potentially distressing settlement (i.e., settlement exceeding that which would permit the walls to remain "straight")? In our case, 4 m was all that could be constructed in a single stage.

One has to decide if 400 mm of settlement would cause too much distortion of the wall. In our case, we had walls undergoing 1100 mm of settlement and the RECO remained absolutely vertical without distortion of the panels. (again, we had a few problems but these were isolated - we had some 20 walls).

 

[fattdad]

Ah, but BigH it would fail by global stability too, eh?

Sure, I'm somewhat splitting hairs, but I'm pretty sure, we'd end up at the same place. What I react to (unfavorably) is folks with their DCP "confirming" a bearing pressure on a leveling pad as if that's somehow "telling." It's not (well at least to me).

Back to work!

f-d

¡papá gordo ain’t no madre flaca!

 

[andyz22]

BigH and fattdad
Thanks for d valuable inputs

Global Stability is important
The answer is yes,It's the priority i feel too

the working platform is wet,the foundation failure will takes place due to the super imposed load here the external stability along with global going to be critical

i agree with BigH with out knowing how much stress is coming on to the foundation the soil must be competent enough to resist the load

During construction the settlement of the structure starts taking place its a tough job to control

We haven't come to a conclusion yet
Thanks for the interactive discussion

 

[BigH]

As both fattdad and I have pointed out - if the clay is soft, you will not be able to achieve a safe bearing capacity (or allowable bearing pressure) without some form of ground improvement technique. Strongly suggest the use of pvd wick drains and surcharge loading as a suitable measure. Likely much cheaper than the use of stone columns or other techniques.

 

[edstimator]

Check into compaction grouting. it is usually prohibitively expensive but you might find a hungry operator

 

[BigHarvey]

hungry operator you said ! edstimator your way of thinking is a pity for the profession ( apart from the fact that compaction grouting is a ground improvement method that works mainly in cohesionless soils and not in clays ! ).
Please meditate the following :

An old prospector shuffled into the town of El Indio, Texas leading a tired old mule. The old man headed straight for the only saloon in town, to clear his parched throat.


He walked up to the saloon and tied his old mule to the hitch rail.

As he stood there, brushing some of the dust from his face and clothes, a young gunslinger stepped out of the saloon with a gun in one hand and a bottle of whiskey in the other.

The young gunslinger looked at the old man and laughed, saying, "Hey old man, can you dance?"

The old man looked up at the gunslinger and said, "No son, I don't dance... never really wanted to."

A crowd had gathered as the gunslinger grinned and said, "Well, you old fool, you're gonna dance now!" and started shooting at the old man's feet.

The old prospector, not wanting to get a toe blown off, started hopping around like a flea on a hot skillet.

Everybody standing around was laughing.

When his last bullet had been fired, the young gunslinger, still laughing, holstered his gun and turned around to go back into the saloon.

The old man turned to his pack mule, pulled out a double-barreled 12 gauge shotgun and cocked both hammers.

The loud clicks carried clearly through the desert air. The crowd stopped laughing immediately.

The young gunslinger heard the sounds too, and he turned around very slowly.

The silence was deafening. The crowd watched as the young gunman stared at the old timer and the large gaping holes of those twin 12 gauge barrels.

The barrels of the shotgun never wavered in the old man's hands, as he quietly said;

"Son, have you ever kissed a mule's ass?"

The gunslinger swallowed hard and said, "No sir... but...but I've always wanted to."

There are a few lessons for all of us here:

*Don't be arrogant.
*Don't waste ammunition.
*Whiskey makes you think you're smarter than you are.
*Always make sure you know who is in control.
*And finally, don't screw around with old folks; they didn't
get old by being stupid.

With your mentality don't be surprised if one of these days a hungry operator makes you kiss his ass when you are in trouble.

 

[edstimator]

Really? I think it is perfectly fine to offer whatever it is worth to you and fits in your budget. He's a grown up and can decide for himself.
I offered that suggestion knowing that the last job i used a compaction grouter on was for a school and ran $350,000.00 us
Nest bid up was twice that. The contractor apeared to be happy and as far as I know he made money on the job. I know we did. If we had not found a "hungry" contractor the job would have been scrapped due to budget issues. No one would have worked that job.
In your world somehow that is better?
If this guy pays high bid for compaction grouting it isn't going to happen either so I guess that's better than offering a budget number and haveing a guy reverse engineer his way into a job.

Not saying I'm right but I don't have any trouble sleeping when I put sixty guys to work