Undrained Shear Strength

[rizwanh]

Guys,

I'm a mechanical engineer, with very limited knowledge on soil. i have to design a structure for pipeline crossing on a VERY SOFT CLAY.

Problem is, from survey report there is no Su recorded in the bore holes. going downstrem to the next bore holes, there is an Su recorded, at 0.7m depth, about 3.9kPa. Going upstream, recorded Su is about 1.7kPa.

Therefore, i assume linear relationship between the two points and got a 2.5kPa at the crossed pipe. However, this value is not suitable for concrete design as it will be very heavy in mid air.

My question is:

1). What is the range of Su for a VERY SOFT,GREENISH CLAY? I know the max is about 12kPa? I'm looking at something that is official, and written. Braja M.Das didn't give any range in his book. I need the min value for conservativeness purpose.

2). Does assuming a linear relationship between two points is acceptable? can it be used as a based of arguments?

Thanks..

 

[fattdad]

You need more data. The linear relationship that you are considering doesn't seem appropriate. If anything, I'd take the lower value and design with a safety factor.

You may need to design the support as a pier, in which case you'd need data to a greater depth also.

You also need to consider the tendency for settlement under the design loading condition. You may not achieve foundation failure, per se, but have excessive settlement that would otherwise affect your design.

I'd also question the data. 3.9 kPa is less than 100 psf, which is a real low value for shear strength - sounds more like a cohesion intercept.

Do you have any Atterberg limits, natural moisture contents, percent sand data? Can you inform how the undrained shear strength was originally determined? What soil layers were found below the soft clay?

Drive some piles!

f-d

¡papá gordo ain’t no madre flaca!

 

[mudman54]

you probably won't find the appropriate information is a textbook, because too many unqualified engineers take soils parameters and apply them improperly. some very simple testing might give what you need, such as moisture content testing and atterberg limits. the color you describe makes me think perhaps it is a residual soil derived from the local bedrock. get back to us with some more information and we can better help you review the recommendations of your geotechnical consultant.