Drained and Un-drained Condition

[Nr Ahmad]

Hi, I am working for a Transmission line Project (Civil Part), Our project Route traverses mainly through a soil of CL-ML type (fine grained soil), The ground water level at site is mainly 10-30 meter below the intended foundation depth (3.75 m), the soil at foundation level is expected to remain unsaturated due to the low water table, in such a situation is it OK to use the drained condition or do analysis for Drained and Un-drained both and the small value i should consider?

 

[oldestguy]

If you never have high ground water, YES. However,any storm situation with plenty of rain usually will be when there i high wind also. That's when they blow over.

 

[GeoEnvGuy]

Turn field vanes or do uu tests to determine your undrained shear strength. This will give you the justification for assuming a drained failure mode.

 

[BigH]

if the natural condition is that the water level is so low (and presuming that is historic), it probably will not be possible to turn vanes. Allowable bearng pressure will be based on settlement rather than shearing strength. Any idea of the overconsolidation ratio? Check this out vs applied loading.

 

[Nr Ahmad]

@Oldestguy, Thank you,
Our project route has been designed for a wind speed of ~100 km/h, normally there will be Wind speed around ~30-35 Km/h once or twice per week, You mean if any time during the project's life time, it may suffer from the max wind (100KPH) and the foundation bed will be saturated at the same time due to heavy rains or storms, in this circumstance the soil will behave in Undrained Condition[/b.
How likely this scenario maybe (max wind + foundation bed saturation at 3.75m due to storm/rains)?

 

[Nr Ahmad]

@GeoEnvGuy,
The soil is in Stiff to Hard Condition (SPTN 10-Above 32)Not sure if Turning Vanes will be possible, Tri-Axil tests not available in our area, How this could help in justifying the drained failure mode?

 

[Nr Ahmad]

@ BigH, There is a report of a Parallel T-line around 300M from the current route, it shows that the soil is Over-consolidated, they have carried out Direct Shear tests in CD mode, the C value is around 3-5 KPA, and PHI around 20-30deg.
The allowable tolerance for differential settlement is given as 20mm for Substation elements and 25mm for Tower footings.

 

[EireChch]

GeoEnv - If you twist a vane in an unsaturated soil you will get a reading however this will not be an undrained shear strength. Taking a sample, saturating and doing a UU will give you a Cu.

You should be able to push a vane into a material with an undrained shear strength of up to 200kPa.

What is your OCR? Do you have access to consolidation testing?

If your soil is over consolidated then your drained strength will likely govern as your bearing capacity calculations should consider Phi of 20-30 deg and c' of 0kPa.

However, given you have limited access to lab testing, groundwater is within 10m of the foundation at some location, I would do a drained and undrained analysis and take the lowest bearing capacity.


In saying all that, settlement will govern so you need to be able to take shelby tubes samples and do consolidation tests. If not, you will have to assume some conservative Mv values for normally consolidated soil.

Good luck.

 

[GeoEnvGuy]

If the material is truly in a stiff condition based on spt results any geotech book will have a table showing you the undrained shear strength of stiff clay is between 100 and 200 kpa. It will also say that estimation is not very accurate and as per my previous post you should confirm with field vanes and/or uu tests.

What is the shear strength of a drained material with a friction angle of 30 at 3.75 metres depth, less than 50 kpa.

So you want to be able to justify that the undrained strength greatly exceeds the drained strength. Which will indicate the most likely failure mechanism will be a slowly deforming drained failure.

However as most practitioners are aware the bearing capacity of a structure on cohesive material is almost exclusively based on limiting settlement. Which in most cases limits the foundations maximum dimensions to achieve a tolerable settlement.