Hi, can a clay layer with a Cu va

[aj_k]

Hi,
can a clay layer with a Cu value of 40kpa have a unit weight of 18kn/m2.
I have received a geotechnical investigation report with the above-mentioned condition.
When I checked with the Eurocode specification tables, it shows that,
1. if the Cu value is 40kpa, then the unit weight should be 15kn/m2.
2. otherwise if the unit weight is 18kn/m2, then the Cu value should be 80kpa.
and I cannot understand this correlation with Cu =40kpa and unit weight = 18kn/m2.
This soil layer is 2.0m to 4.2m depth from the ground level. The soil in this 2.2m thickness is a mixture of sensitive fine-grained and silty clay and clay.
The water table is at 1.45m below the ground level.

 

[PEinc]

Geotech reports almost always are very conservative when "recommending" soil properties. It is very, very common for the geotech to recommend low cohesion, low friction angles, and high unit weight. This makes for a conservative design (and therefore less risk to the geotech). If the geotech's recommendations are just recommendations, then I use what I believe to be correct based on my experience and my review of the geotech report. However, be prepared to defend your soil properties to the reviewing engineer.

http://www.PeirceEngineering.com

 

[fattdad]

Seems a confusing post. If you have an undisturbed sample and use it to measure undrained strength in the laboratory, you'd also have a sample to measure density in the laboratory. If these two laboratory tests give the data you suggest, then who cares about codes, correlations and the like? You did one better, you actually got the design values!

Good job!

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

Short answer yes it is possible to have an undrained shear strength of 40kpa and a unit weight of 18 kn/m3.

The easy way to check the geotechnical report is to look at the borehole logs and lab testing. The unit weight of the saturated clay is a function of the moisture content measured insitu. The undrained shear strength is a function of shear vane testing, triaxial testing or correlation from atterberg limits or SPT testing.

 

[EireChch]

Agree with f-d, unit weight would be available if you have done a triax test.

When I have no testing in a Clay layer, other than Cu measured indirectly (i.e. SPT), i use the below. For your 40kPa you would be in the low to medium cat. A unit weight of 18-19 would be ok with me.

Fig 1 is from BS8002 - code of practice for foundations.

PEinc - a high unit weight would be unconservative for bearing capacity and pile end bearing in sand as higher confining pressures.

 

[BridgeSmith]

The values such as that, which are given in the AASHTO code are conservative default values that can be used if data about the on-site soils are not available. I would expect the Eurocode to be similar.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

EireChch, I agree with what you said about the heavy unit weight. I was thinking about non-gravity wall design, not bearing capacity for footings or for bearing piles. I should not have assumed that the original post was for a particular application.

http://www.PeirceEngineering.com

 

[aj_k]

Thank you for all your reactions.