Fine grained soil Behaviour based on permeability

[Geosubhtech]

Greetings to all.

The project location is majorly dominated by Sandy SILT with very less clay content (ML-CL). Silt is everywhere more than 55% and clay is less than 18%. The direct shear tests (DST) shows a C-phi behaviour of soil. The cohesion ranges from 5kPa to 10kPa and Phi ranges from 27 deg, to 30 deg.. But if i see according to SPT 'N' the DST results vary too much. For example in refusal N>100 also the DST shows a C- Phi of 10kPa and 30 deg.
And at some locations the Soil is in borderline classification of CLAY having low plasticity (CL) the undrained triaxial tests (UUT) had been conducted. The results were 55kPa (C) and 11 deg. (phi).

The in-situ permeability tests in
CL shows 5.57 x 10^-5 cm/s
ML-CL shows 4.125 x 10^-5 cm/s

So based on the permeability values how will be able to decide the behaviour is drained or undrained, So i can rule out the undrained tests and consider the CL layer drained parameters.

The foundation type is Pile foundation.
Ground water table is at 40m below Ground level.

Thanks your thoughts and advices.

 

[r13]

I think this blog may provide some help. Link

 

[r13]

Your in-situ permeabilities are relatively low, or moderate to be accurate. See info below.

Also the quote:

- Dikes without any impermeable clay core may be built from soils having a coefficient of permeability less than
K = 1 x 10-4 m/s;
- Pond bottoms may be built into soils having a coefficient of permeability less than K = 5 x 10-6 m/s.

 

[GeoEnvGuy]

Based on your description you have 40 m of soil before the groundwater table and the foundation is piles. I would assume you have 40 m of drained soils before having to check both the drained and undrained foundation soils.

Are you considering end bearing or friction fitting the piles?

 

[Geosubhtech]

Thank you retired13 for the references.

Geoenvguy:

Considering mostly the skin friction only, as end bearing have many issues like base cleaning and all.

 

[geotechguy1]

How long are the piles?

 

[Geosubhtech]

The piles were 29m length as preliminary analysis.

 

[geotechengineer1]

So for 29m piles with a water table at 40m, how much excess pore water pressure will be generated during loading

 

[Geosubhtech]

to be very specific, The client want to have a option for recharge wells in that urban area for recharging the GWT. So if the GWT rises then the soil will get saturate. SO i may need to think of analyzing with undrained analysis. If they rule out the option of raising the GWT, then the soil is partially saturated and if i conduct the tests at natural moisture content (Direct shear tests, UU triaxial tests) i will get a apparent cohesion due to the silty dominance and low saturation. Correct me if i am wrong.

So what is your view on following which analysis (drained or Undrained) ?