Asking about CBR Test 3

[tonytianlei]

My company is designing a refinery .A geotechnical company is responsible for the bearing capacity test of the subsoil. But only CBR Test is done. We got mass shallow foundations to be designed and constructed. We think the CBR test is not reliable, because there are some week layer under the foundation bottom. Do Plate bearing capacity test needed, before our design and construction work?

 

[EireChch]

What are mass shallow foundations?

CBR or PLT is not suitable for design of foundations. You need to investigate to at least 2-4 times the width of your foundation, take samples and run laboratory tests.

A CBR/PLT test doesnt tell you anything about the soils below 1m and I would be wary of the capability of a geotechnical engineering company that tells you this is suitable for foundation design.

 

[Ron]

@EireChCh....exactly!

 

[tonytianlei]

Thank you very much Mr EireChch，Sorry,I made a wrong expression.I mean There are a lot of foundations which bottom level is around -2.0m. By geotechnical report,the soil under the foundation bottom is very soft.
We got the geotechnical report of the whole site.We can calculate the bearing capacity by Hansen formula.Is the calculation result reliable?
If the bearing capacity calculated is not enough.We have to take some measures to enhance the soil. We also have to test the enhanced soil proving the capacity meet our need, right?
thank you very much, hoping for your reply.

 

[EireChch]

The Hansen, Vesic, Meyerhoff method are all "reliable" methods. However these methods tell you the bearing capacity based on shear failure.

You need to assess the settlement beneath your foundations, if it is above your settlement limit then you need to improve the soil. Dynamic compaction, stone columns etc are all potential solutions. CBR/PLT is again not suitable to confirm acceptance of ground improvement for the reasons stated above. You need post improvement CPTs, zone load testing etc.

To be honest and I dont mean to insult, I dont think you have the knowledge to confirm what is right or what is wrong. You need to pass this to a geotech, or hire an external consultant to confirm what the other geotech consultant is proposing is appropriate.

 

[tonytianlei]

Thank you very much for your reply
I’m new of the geotechnical region. I’m majored in structural engineering. In my country geotechnical engineering and structural engineering is divided. The geotechnical method or knowledge system is different from the Euro or American system. I have no one around me to ask.
I’m now in charge of contacting with geotechnical company and construction company abroad. So geotechnical knowledge is need.
In the former reply, you mean calculation according to geotechnical report is reliable. And I also have to calculate the settlement beneath the foundations?
Can you recommend some books or articles about geotechnics for me? thank you.

 

[EireChch]

You're in over your head. I think you need to speak to your boss and tell them that you havent got the experience for such a task.

A good place to start is Das - Principle of Geotechnical Engineering.

Good luck.

 

[tonytianlei]

Thank you for your advice

 

[fattdad]

you don't have the data you need. CBR is used to design pavements, so you have what you need for pavement design. You don't have what you need for effective foundation design.

Simple as that!

f-d

ípapß gordo ainÆt no madre flaca!

 

[tonytianlei]

Thank you for your information @fattdad

 

[BigH]

I would also check out Tomlinson's book on Foundation Design and Construction.

Totally agree with fattdad that the CBR is for pavement. However, do not get caught relying on plate load tests for foundations. Most tests are using 300 mm plates which only extend to 600 mm or so below the foundation - for foundations of 1000 mm or more this is clearly inadequate. Case in point. In India on a job in the Calcutta clay plain, the soil is very soft to depths of 6 m or so - then firm. We had numerous bridge ramp foundations (RE Walls)to 11 m in height. In one location, in a "city", the structural engineer had a plate load test done and said - wow, 350 kPa bearing!!! I would not agree as the area had 2 m of long term fill over the very soft soil and insisted that the pvd drains and stage loading was required (in his view, not). Anyway - I "won" and with 4 m of fill placed for the first stage, we had 400 mm of settlement. Imagine what would have happened with out pvd and stage loading - in the middle of a street in the commercial district.

 

[fattdad]

. . . also to point out, plate load tests mobilize unsaturated bearing ratio, which we know is liberal (i.e., a greater value) compared to the saturated laboratory value.

. . . also to agree with BigH! Large foundations place strength demand to a much greater depth than the plate load test would ever penetrate. Dynamic cone penetrometers have similar issues.

f-d

ípapß gordo ainÆt no madre flaca!