shallow foundation on the week soil

[npthao121]

Dear all,
I’d like to know your advices in the following case:
Soil profile:

1st layer: Sandy clay (CL-ML), 2.2m thick. SPT = 8
Wn=21.2, Wl=22.0, Wp=16.4, Ip=5.6. Void ratio e = 0.853, saturated degree S = 66%
DSS test gives: phi=24o, C=0.099 kg/cm2.
Sieve analysis gives: gravel (7%), sand (75%), silt (9%), clay (11%)

2nd layer: Organic silts (OL), 3.0m thick. SPT = 5
Wn=33, Wl=41.2, Wp=22.3, Ip=18.9. Void ratio e = 1.07, S = 93%
DSS test gives: phi=10, C=0.143 kg/cm2
Sieve analysis gives: gravel (1%), sand (32%), silt (25%), clay (42%)

3rd layer: Sandy clay (CL-ML), thickness varies from 3.0 – 8.5m. SPT = 9
Wn=20.4, Wl=21.8, Wp=15.8, Ip=6.0. Void ratio e = 0.71, S=76%
DSS test gives: phi=24, C=0.098
Sieve analysis gives: gravel (9%), sand (74%), silts (6%), clay (11%).

4th layer: Organic silts (OL), thickness varies from 2.5 – 6m. SPT = 7
Wn=22.9, Wl=28.2, Wp=16.8, Ip=11.4. Void ratio e = 0.724, S=85%
DSS test gives: phi=15, C=0.166
Sieve analysis gives: gravel (3%), sand (52%), silts (21%), clay (24%)

End of boring log.

Question: Could shallow foundation be applied for this case? The contact pressure at bottom of footing is approximately 10 ton/m2 (this is the two stories building). What potential problem may occur?

Shear strength parameters are not so bad; but, consolidation settlement of the foundation seems to be occur (however, consolidation test is not available). Also, water content of the 1st & 3rd layer is high (near the liquid limit) so it may cause some problem, doesn’t it?

Pile foundation is hardly accepted due to budget limit. Moreover, soil layers are not good enough for pile. In case pile foundation is needed, the borehole must be drilled more until reaching the good layer.

I feel inconvenient with shallow foundation placed on the 1st layer. I intend to use foundation and make some more boreholes deep enough to get data. However, there are some constraints of money and time.

All comments are appreciated. Thank in advance

 

[BigH]

A number of questions and comments to be clarified:
1. How do you call layer 1 and 3 a sandy clay? Your grain size analysis shows that the layer is 75% sand and only 15 to 20% is fine sized. This gives it a USCS "S" symbol - as the PI is between 4 and 7, you would use SM-SC as the USCS.
2. How do you classify layers 2 and 4 as organic silt? Actually layer 4 should be classified as a "sand" using symbol SC. Layer 2 is a fine grained and should be judged as CL according to USCS. Did you do any organic content? I usually found organic silts to be somewhat "spongy" on tactile examination.
3. Where is the water table? It should be high if the sand, trace silt, trace clay has such a high natural moisture content.
4. What type of structure are you trying to found? A house? A oil storage tank? A muti-story office building? A steel mill's walking beam furnace? A nuclear power plant? This makes a bit of difference.
5. None of the layers reach out and grab me as bearing stratum for piles - would then need to be friction piles although, it might be possible to put expanded base piles higher up in Layer 3. Depending on the compressibility of layer 4, piles might need to be spaced to act as independent piles rather than a pile group - I did this on a job in Northern Ontario - had a 3m layer of sand between two very soft clay layers - used timber piles bearing in the middle for 20 tons/pile; for school addition - worked well - but my N values were in the mid to high 20s not at 9.
6. Get some more information on consolidation testing if you can and confirm the "organic" leaning you have. Void ratios are not that high - too low in any of the layers to be considered "organic" in my view.
7. Money is a constraint so advantageous use of DMT or piezocone would be out. Too bad; might consider a pentest though to help delineate layers and possible thinner seems of less or more competent material.
8. In the end, especially since money on the investigation is a constraint, use conservative engineering judgment. If they can't pay you for the investigation, why are you going to stick out your neck on "cutting edge" recommendations. Give them something that will work but be conservative.

 

[npthao121]

Thank for your quick response.

I've got these soil information from the soil investigation report. I just take it out without review, too bad, right? I agree that it is a mistake of soil clasification.
The water table is 0.6m from the ground surface.
The project is a two story office building. As the structural design, what i have to do is to design the foundation for the building. Project is budgeted with shallow foundation solution. Now with this kind of soil, this solution is to be on question.
If no room for shallow foundation, pile foundation would be selected. If this is a case, I will ask the owner to make some more boreholes or do CPT to reasonable depth.
However, I'd like to hear your comments about the shallow foundation on this kind of soil. Is it possible?

 

[fndn]

As mentioned, I would do an organic ignition test on layer 2. Also you have in layers 1 and 2 significant non palstic soils and you're getting moderate PI results. That coupled with the very low cohesion results makes me suspect the Atterberg test results are high. This leads me to think of the foundation soils as non palstic when performing settlement analysis.

So with high water table and low blow counts, I would prefer to remove and replace layers 1 and 2 and then bring in crushed rock as replacement to 0.6 m below the existing grade. This would give us competent bearing material, and the high water table would be in the rock voids and would be of little concern. With this 4.5 m compacted rock we could increase the allowable bearing pressure to 2.0 kg/cm^2.

Another option- if the owners could use a basement space, you could directly excavate to the top of layer 3. This would give us partially compensated foundation (the weight of the new building is partly offset by the removed soils from the basement box area) and we would avoid the cost of the crushed rock. However, we will have to still keep the allowable bearing pressure near 1.0 kg/cm^2 and perform settlement analysis and do 1 more deep boring going down to at least 10 m below the top of the third layer-this will reveal or eliminate any deep seated settlement issues.

A Member of

http://www.civilvillage.com

 

[BigH]

fndn gives some good suggestions. I like the basement idea - to offset the building load with the basement soil removal - this is done quite a bit (original case history of a sugar mill in Guyana back in the early 1960s is a case in point). The big problem, though, will be the necessity to dewater the site in order to construct the basement. This will especially be tricky if there are nearby buildings or other structures that may be present and affected. With only a two story structure, you might be able to achieve compensation with a high crawl space (about 1m high or so). This would be much easier to handle than a full blown basement.
With respect to my colleague's suggestion to dig out Layers 1 and 2 - this would be about a 6m excavation; again, there will be groundwater issues - might be mitigated to some degree if you can open cut at shallow slopes. Too, 6 m of excavation and replacement with crushed rock will be quite expensive - too expensive.
With some thought and care, you might be able to found a normal raft foundation - 2 storey building is not all that heavy. You can dig out and replace a couple of meters of fill and replace - or you might want to consider a grillage raft foundation (closely spaced strip footings along with slab would act as a raft). Consideration of surcharge loading for a while can mitigate the settlement issue (with the soils you've indicated it shouldn't be too long). You should grill the geotechnical engineer as to if there are any coarse layerings in Layers 2 and 4. I would think that these deposits likely have thin sand beds within that would assist in reducing surcharge loading time. Surcharging is done all the time in coastal New Jersey and in Richmond British Columbia.
The bottom line is that I've seen and worked with much worse. It is the toys that you are willing to play with and the costs that can be accepted. You should now be totally confused; but dispair not. Sit down, pull out your pipe and gain an overall perspective; it'll come to you as to the best of the advice here along with your own experience to get the right foundation for the job!

 

[fndn]

Well done BigH-the raft foundation and the dewatering are certainly good reminders.

A Member of

http://www.civilvillage.com

 

[dmoler]

Come on, all that testing to get all this data for a 2-story building? How did you get all that data with SPT tests? Are you really a grad student trying to get help on a homework problem?

 

[jjeng2]

Dmoler,
Interesting...Ive never seen a soil report that detailed and have done buildings much larger than 2 stories. But..where I am almost all soil reports are the same. The reports I get have a bearing capacity, recommended foundation type, lateral soil load guidelines, IBC classification, and maybe some boring logs with blow counts if lucky.

 

[BigH]

dmoler - depending on the country you are in - the information given may very well be provided in the soils report. Here in India for many structures, they do almost all the bloody tests you can think of; testing is part of the investigation charges so it is expected to be and is included in the reports. But, I agree . . .

 

[VAD]

Seems that you and the geotech enginer need to go together on site with a back hoe and take a good look at the subsoils. 90 % of the answer lies in observation, otherwise
you may end up spending a lot of money with solutions that may not benecessary.

Forgetting the test results and looking at the SPT only,you may wish to use some screw piles below your footing. These piles can be screwed to 8 m depth quite easily in the given conditions.

Regards

 

[dmoler]

Please dont tell me that people go to the trouble to determine the amount of clay and silt (hydometer tests), phi and cohesion (triaxials, and on SPT samples, highly suspect), saturation and void ratios (wont even go there) for organic silts!...(Sobbing) and not even do an organic content test on them? (sigh)

Well, some foundation alternatives come to mind for this. You could excavate the first and second layers, and then fill in with granular material (first layer could be stockpiled and reused). A geogrid reinforcing the fill would be required (see tensar website (dimension software for footings helps some with this method, but be careful using geogrids and this software)). Might be risky without knowing organic content and compression indexes of the lower organic silt layer.

If you dont like the geogrid, you could preload the building pad area with fill; say, three times the building design load, to compress the layers. Monitor the settlement until it reduces enough to declare it ok. Takes time though, probably 6 months or more (organic content again would be helpful).

I dont think helicals would work, since the lower layer may compress. Maybe adjustable helicals, but probably not.

 

[BigH]

sorry dmoler - but there are places that do! The spec says that the following tests are to be done; they are done or the Quantity Surveyor withholds payment. Kid you not!

 

[dmoler]

Must be nice to have such funding for testing. Cut-throat competition has left many places I have worked with minimal fees and the basics are only done. Government jobs usually have more, but for a 2-story building...('nuf said)

I suppose I still have trouble believing that the soil is organic silt. The liquid, plastic limits, and natural moisture contents dont correspond well with that designation. Also, for organic silt, to get a PI is difficult. The ASTM testing standard for that is very, very difficult to perform and I found it difficult to understand. For starters, the LL and PI goes off the charts, and the procedures call for a convoluted oven drying procedure. Not to get off topic further (too late, haha), but it is hard, if not impossible, to get testable samples of organic silts for triaxals with SPT. Shelby tubes would work, but only with a double wall sample retaining system so the sample does not to get sucked out on the withdrawl. In any event, not as useful info as a simple organic content correlation with natural moisture to get compression indexs and then N value data.

I would certainly go back and auger down at the boring locations and proceed deeper to see if a decent bearing strata is somewhere within economical piling range.

P.S. I just found out about this web site recently, and really like it. Its more oriented to practical problems than academic problems, and that was why I reacted negatively to this problem. Reeks of homework.

 

[BigH]

I agree with you on the organic aspects - I raised it in my first post of 26Jan05 - points 2 and 6. Some people probably see a thin root or a non-fibrous rootlet and think it makes the whole stratum organic. Work with amorphous peat and you'll see organic!
I had organic silt on a harbour job in Oshawa Ontario once - we got the material out with normal shelby tube - did a triaxial test on it; interestingly, very high effective friction angle. You could smell the organic and see the seashells!
Chou and