Causing failure prior to actual construction of Embankment

[1967pradyot]

An embankment of 3.0m height is to be built on a thick (10m from ground level) marine clay layer whose N=0.It is failing in stability (deep-seated), bearing capacity and naturally the settlement is also abnormally high - so a combination of ground improvements are required.
I am thinking to construct the embankment upto 6.0m i.e preloading with 3.0m surcharge over 3.0m height and wait for 3-4 months and let the embankment fails in shear. After that the entire embankment will be removed and new construction of 3.0m high embankment will be taken up.
Is this a technically correct approach?

 

[VAD]

Are you constructing a bridge approach fill or roadway embankment. If approach fill wick drains could have been used to accelerate consolidation and strength gain of soft marine clay with staged construction. The use of stone columns would have been another approach. It would appear that the undrained strength of the clay is less than or around 150 lbs/ft2. Failure would have resulted from too rapid loading of the fill.

The approach you indicate is the displacement technique which can be and has been used. Some of your surcharge can become part of your embankment. If the material was peat or muskeg I would tend to go along with the approach as the displaced material could be re utilized. With soft clay this could pose a problem but I am not sure in your case if it would.

Depending on the extent of construction and cost considerations one could examine other options such as leaving clay in place and modifying ground using deep soil mixing etc.

It seems to me that you have examined the alternatives.

On one project I witnessed muskeg being displaced as fill was dozed in (quite a nice peel). The original intent was not to displace the muskeg but it occurred as the bottom of the peat was fairly competent and fill advanced with a D9. In hindsight it could have been the contractor's intent.

Fill was brought in as the dozer moved ahead. Some failures occured in the new embankment and the concern was whether peat was trapped in isolated areas. This was the case as was suspected. I think you anticipate this hence your decision to completely remove surcharge and displaced material.

I would appreciate learning a bit more of the project if you do not mind to see if there are other alternatives. However, if failure has already occurred as you have noted then your approach seems plausible.

 

[BigH]

I've used displacement techniques before - in Guyana - to displace 40 ft of tailings slimes. There is a very good description of the method (emanating, I believe from Michigan practice in 1940s) in Tshebetarioff's book - near last chapter. Basically, you advance a finger fill with a "raised" end. Applying more fill at the end, and with weight of dozer, you cause slips to occur and the sand fill sinks down and displaces your soft layers. While it certainlly will not remove all such thickness, it should replace enough to give you a stable "deck". Another approach that I've heard used was to use dynamite to blast the soft clay and thoroughly remould it. Then use large stone to displace and make your pad - see Canadian Geotechnical Journal (back in late 1960s) for Rainy Lake Causeway construction (Matich and Stermac, I believe). I'll find out actual details for you when I get to office. They used this method to remould some 40 ft of clay. This is a major causeway up near Thunder Bay Ontario. Basically, though, you are wanting to get a "floating pad" onto the clay layer. To handle side effects and to keep shear failures low - use a small stabilizing berm (say 1m) and extend out 15m each side of embankment. I needed to do this on a causeway in northern Quebec. This will minimize the potentials for slips, but not settlments. For this, wicks are probably the best bet. Hope the ramblings are of some help. Best to you.

 

[VAD]

BigH:

Just curious.

What year were you involved in Guyana. I remember John Huntley (I hope I have his last name correct, it has been a long time) from Guyana graduating at Queens U, Canada with a mining degree in 1975/76 undertaking an undergrad thesis on displacement methods related to work at the bauxite mine. I was doing graduate work at the time and had some discussions on his topic. I probably still have a copy of his work. Have to do a search. He had the blasting method as well as a technique. At that time a lot of work was being done in Guyana by Geocon monitoring tailings dams etc.

I spent the summer of 69 in the GED locating pipelines in the bauxite and alumina plants.

 

[BigH]

VAD - I admit it! I'm an old Geoconite! One of the ones that remembers well 14 Haas Road (where the cabbies hated to hear as it was too close to the airport). I was in Guyana in Summer of 1977. Hinds was the head of the alumina plant then - hadn't become PM yet. Neville Winter was my main contact there along with a guy named Lachmansingh. I loved it there - as a youngster; I played BBall in Linden - yes, I was a "white guy who could jump" at that time; taught the local run and gunners how to put in hip checks, use hand checks, etc; had good success against the Georgetown Cuban coached teams. My displacement was for decant towers on the Highway Dyke. Had some great pictures - but don't know where they are now - slides, and either with Fred or ????? Dr. Holubec was looking after the project at the time for Fred Matich - but Ted Jurgens had been there in 1973 or so (now with Kilborn). Fred used the blasting method on the Rainy Lake Causeway. Also, you might not know, he did some work with sunken ships in the St. Lawrence shipping channel and in New Orleans. Basically, they sunk the ships by excavating on one side of ship, causing a slide, then go to the other side, cause slide, come back. To and fro failures. Techniques developed by the geotechs in the 50s and 60s were, when I look back, outstanding engineering. Small world! Take care -

 

[VAD]

BigH:

Thanks for info. Small world indeed. Should have an XM on that. I am sure that you partied a lot in Wismar and Mac City.

 

[1967pradyot]

Thank you VAD and BigH for your valuable suggestions.

Actually I have already done the calculations with band drain and staged construction. But I am not sure about the efficacy of stone column in such very soft marine clay formation.

Frankly speaking, this is the first time probably I will be doing this displacement technique. So, I really need the references for analysis and how actually it works.
Thanks once again

 

[VAD]

Here you go :

Treatment of Soft Foundations for Highway Embankments NCHRP Synthesis of Highway Practice No 29.

Treatment of Problem Foundations for Highway Embankments NCHRP Synthesis No 147.

No 147 is an update to No 29.

I read the Rainy Lake Causeway that Big H is talking about but cannnot put my hand on the paper. He may be able to provide the Journal reference otherwise run a search or check the Canadian Geotechnical Society journals around 1960's.

Just as a thought, have you looked at using hogfuel, sawdust encapsulation as lightweight fill. This technique works well also as the materials have a low density. You can possibly use 2m of sawdust or blend of hogfuel and sawdust and a metre of clay on top. As well geofoam etc are techniques re use of lightweight fill. However, there is a cost as well and performance depends on loading conditions, intent of structure etc but they have been used successfully Dr. Horvath of Manhattan College has done work in this area and you will probably obtain references using civldept@manhattan.edu.

Best of luck. Let us know how you finally proceed, and results.

 

[BigH]

VAD - Don't you just love the name "hog fuel". I had a major laugh when I first moved to VCR in 81 and I heard it! They did most of the Burnaby Highway on hog fuel and also did a lot of work on the Lulu Island part of New Westminster in encapsulate hog fuel too - back in 85 or so.

I will look up Fred's paper - it is at the office; don't have it here. Will post, enshala, later this week; decided it is time, after a year, to take the family to Calcutta and stay in a luxury hotel - one with full time aircon!!

As I indicated, too, earlier, check out Tschebetarioff's book - he has a nice section near the end on this; also I saw something in a more recent book but don't have handle on the book - just the chapter (given to me). Michigan also used this technique and it was almost known as the Michigan Method!! (1946)

Best to all.

Howard

 

[1967pradyot]

Thanks a lot once again VAD and BigH for your advises.

But BigH, you are really coming to Calcutta - basically I from Calcutta (Kolkata) but based at New Delhi.

 

[BigH]

No, 1967pradyot - I AM in Calcutta! Actually just spent 5 glorious days at Oberoi Grand not thinking of work - but I live in Kharagpur.

 

[BigH]

1967Praydot: Send me your co-ordinates and I'll courier you over some information. Send data to bohica@indiatimes.com (or whatever their back-end is - you probably know).

 

[alwyn511]

@ VAd and BigH:

it's good to note that u have been involved in a project in Guyana. I'm from Guyana (Georgetown) and would like some details of what was actually done on the project.

@ VAd: where can i locate the paper: "Treatment of Soft Foundations for Highway Embankments NCHRP Synthesis of Highway Practice No 29.", from

 

[dirthead]

Looks like I missed the crux of this thread by a couple of months, but I'd like to respond to some of the earlier posts. The only soil info 1967pradyot gives us is that he has marine clay with N=0. VAD was pretty quick to assume that this means the undrained strength is <150 psf -- an exceptionally low value. (I should probably admit to knowing next to nothing about soil conditions in Calcutta so somebody give me a good swat if I'm off base here.) If the strength really is that low then yes, I could see how a displacement method might work (although you won't need to wait 3-4 months for it to fail 1967pradyot). I still say might because I don't know the depth of the deposit -- is there a more competent layer nearby? Usually this is the case with organic deposits, which is one reason why displacement methods work well.

Correct me if I'm wrong 1967pradyot, but it sounded like you were telling us the 3m embankment by itself might have been nearly stable, but when you analysed it with a surcharge (to take care of the settlement) you got an FoS<<1. If this is the case then you should do whatever it takes to AVOID causing a deep seated failure. Once you fail the foundation you'll be dealing with a remolded zone of even lower strength beneath your embankment. Depending on the properties of the clay, it could take months or even years for the pore pressures to dissapate. There are many methods you could use here, which VAD and BigH touched on -- i.e. stabilizing berms, lightweight fills, ground improvements (deep mixing or stone columns). Depending on the timeline for you project, maybe one of the cheapest is to use staged construction with peizometers to monitor the strength gain. Use wick drains if you need speed things up.

So am I too late?

 

[BigH]

NEVER too late to add good comments &quot;dirthead&quot;. My personal snaps for your comments. VAD and I were probably reminscing a bit. IF any are interested, I can send the paper by e-mail regarding displacement in Guyana - send requests to bohica@indiatimes.com