foundation on soft soil

[mary787]

I need to build a segmental wall on soft soil having N value of ZERO from 0 till 10 m deep near a monsoon drain . How to have a foundation of 100KN/M2 by replacing the soft soil with crusher rock & how deep to replace ?
I have suggested to client to replace 1.2m of soft soil
A) with bottom layer 0.6m of 200mm size crush rock wrap with membrane
B) Also with top layer 0.6m of 50mm crush rock on top wrap with membrane .
Any suggestion & calculation formula to convince my client ?

 

[ishvaaag]

You say N is 0 which is then truly a soil of poor strength, and this to the significant depth -to almost any construction- of 10 m. I frankly if facing such problem would have to think what to do seeing what the intent is and what we have to consider in the site. People that to this site comes to answer and facing problems of poor soils normally targeted to road works is usually happy in recommending geotextile help for the works. I coming from architectural works acknowledge the usefulness of these but am two orders of magnitude less prone to recommend such things. Hence, in my view you have a difficult problem, mere replacement in less than the soft soil depth is not a solution in my view, even if it partially can be considered or even proven to be so.

Then your application is a wall. For a building one surely soon would go with piles to competent layers, maybe piled mats or even vessel foundations, or foundations balanced with the weights. But for a significant wall (not merely a 1 m tall thing, in which the problem would be the same but less untreatable) a floating foundation itself doesn't make sense if the wall itself is not buttressed against anything to be stable and inmobile enough as to provide the demanded serviceability. Hence, the vertical stabilization by flotation if used would surely need horizontal (and/or vertical) supplement by piles to competent soil able to sustain the whole horizontal forces. I would (gladly!) consider any contribution of the standing ground layers as long I would find them truly of use by my technical interpretation.

 

[SlideRuleEra]

Investigate a "Floating Foundation" which displaces it's weight in "mud". Most foundation textbooks cover the basics.

 

[Focht3]

If my math is right,

100 KN/m² = 100 kPa ~ 1 tsf

You can't have a "net zero load" foundation - the applied load is too high for the depth in question. I suspect the answer may be wick drains or stone columns to improve the site - or a pile foundation to provide support. Please describe the soil (type, water content, other properties, thicknesses) so that we can give you some better suggestions and comments -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

This sounds like my site presently - we are using wick drains to gain shear strength - but rather than N values, what are the approximate undrained shear strengths? 15kPa??, 10kPa?? No way will the ground, even with some treatment of 2m of replace hold up a wall of more than 3m or so with any reserve of safety. I couldn't really recommend it here as, well, it's never been used to my knowledge here, but if you are in the States or Europe, try geofoam as the material to be used. This applies a no-load situation. You can use a facing panel with geogrids or metal strips between blocks to hold the facing in place - it will make it look like an MSE wall.

 

[mary787]

Thanks alot for all the advices , concerning BigH on wick drain to build the soft soil shear strength ,
Question ; WHAT IS A WICK DRAIN DRAIN & HOW IT CAN BUILD UP THE SHEAR STRENGTH ?

Appreciate your precious infor

 

[ishvaaag]

With such a soft soil I also would start to think about the likelihood of soil liquefaction, and deep surfaces of failure. No matter how nice is the set structure, if the soil flows under it -even if standing- it will be no nice retention structure.

 

[Mudcake]

Wick drain is a good idea if the construction schedule allows the new fill to sit for a while while the underlying soils consolidate. What about something like gepiers? Depending on your height, the geopiers may not need to extend to the base of soft soils (you need to do a stability anaysis to figure out the minimum geopier depth)

 

[npthao121]

Dear all,
This is a actuall case:
I have a chance to visit a site which is in the same condition like yours, 11 m of weak OH soil, about 4m of surcharge by well compacted sand. Fence (by clay brick) is supported by shallow foundation placed directly on the soil. The fence high is 2.3m as initial design. one and half year later, the fence is only 1.3 m high, 1 m already disappeared.

In this site, main & heavy structures are supported by pile foundation but the others are just supported by shallow foundation with some small woodden pile (the length is about 5 m, meaningless). Probably, the designer though fence is just miscellaneous simple structure. At the present this site still settle down, and the owner is calling for bid to treat this consolidation settlement.

After the experience getting from this site, the other sites neighbour to it applied wick drain or piled foundation for almost structure (fence, small building etc). of course the cost is increased.

 

[BigH]

Mary787:

Wick drains are prefabricated vertical drains (of similar operation to the older sand drains). You can install these units quickly using a crane-mounted "stitcher". What you are doing is using vertical drains to reduce the drainage path of "escaping" water during consolidation.

As for increasing the undrained shear strength, as soil is compressed by loading (hopefully preloading), water in the soil is forced out to the vertical drains. As the void ratio is thus decreased, there is an increase in the undrained shear strength accordingly.

You can check out the US Army manual on settlements. Also, check out Terzaghi, Peck and Mesri (p323); Coduto (Geotechnical Engineering) (p672, Tchebotarioff; Bowles (Focht3 - I tried not to reference this - I really did). You can also do a search on "wick drains" under

http://www.rediff.com

or other search engines. There is a lot of reference out there on the net.

 

[Focht3]

[blue]BigH[/blue]:
Yeah, right!

[blue]mary787[/blue]:
Strength and moisture content are inversely related (in a general sense) - as moisture content decreases the shear strength increases. The soil's density will also increase as the moisture content decreases - as [blue]BigH[/blue] indicated. You should expect the ground to settle as the drainage occurs -

Do you have a geotechnical engineer for your project?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ishvaaag]

Mary says monsoon area, so the flooding -more or less- is likely to be recurrent. Hence I don't think that consolidation by just draining maybe of midterm help, even if if available would be great.

I think for areas like this that surely are inhabited to such extent as to require public works treatment precisely by the richness brought by the seasonal rains, any modern policy must demand the usefulness of what built to stand for some time, and this demands the use of technologies able to cope with the recurrent monsoons. Hence anything to be dissolved each year should be more or less dismissed. Floating structures, anchors between them, piling for most works of reasonable value and cost of replacement plus bridging between these and the others, is in my view the way to go.

 

[mary787]

mary787
I wish to thank all of you who have provided very good informations with regards to my question on soft soil foudation .

Thank You
GOD BLESS YOU

 

[Focht3]

[blue]ishvaaag[/blue]:

I understand your concern for "things washing away" in monsoon areas. But sometimes it is cheaper to rebuild (several times) than it is to use a structural approach (to do things "right&quot in constructing 'minor' structures.

I would not expect a wall bearing on improved soil to simply melt away. Done properly, it will last for many years - perhaps decades. But I do understand and appreciate your concerns -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.