Soil Behavoir 4

[Rafael Benavides]

Good nigth everyone

MY name is Rafael, Im from Mexico.

One question, hope someone can comment about.

A sand with more that 12% of fine soil (SM) would be considered a cohesive soil and behave like one?

This is to see if I should work with a cohesive of friction soil, in order to use cohession of internal friction angle.

Thanks fro your comments.

 

[geotechguy1]

12% bentonite - sure

12% kaolinite - maybe not

 

[GeoEnvGuy]

I think you need to examine the soil using the dilatancy test and roll test and make the decision based on the outcome.

 

[cvg]

SM is silty sand, not clayey sand. it is probably non plastic or very low plasticity. cohesion would be very low

 

[BigH]

OPer didn't indicate as to the purpose of the material (shallow foundation, deep foundation, .. . .)

As cvg noted, SM is a sand some silt if fines less than 20% or silty sand. I would consider this a cohesionless soil and ignore any slight or apparent cohesion that you might consider toying with.

 

[Rafael Benavides]

Thanks for all your comments, very appreciated.

The things is this.

For the design of drilled piers to be done near the coast line.

We made a boring hole down to 25m.

THe soils that came out in the SPT samples, after lab analysis was 80% sand 20% fine soil, in which at least half the depth was mud and the second half was clay, both of low compresibility. So the classification was SM and CL.

One guy considered the soils as cohesive because of the high % of fines. And obtain c values from the Sowers and Sowers book in which the author gives values of c depending on the N SPT values even for CM and SC type soils.

And caclulated the point and friction capacity.

Other guys did the same, but consideriong the soil and cohesionless and worked with the angle of internal friction, which usually result in higher values of the bearng capacity. He did this saying that even the 20% of fine soil, it is a SM type soild and there for neglected the cohesion.

Using the cohesion, a low value as stated in Sowers Sowers book, you stay in the safety side since you get lower Qa values.

I consider this better since in either case (c of friction angle) you are working with values obtain from corelations and not obtain from triaxial tests. The uncertanty is higher.

Appreciate your comments.

Regards from Mexico

Rafael