Soil in drained vs undrained condition 3

[dccd]

One thing that takes a while for the budding geotechs to digest is the difference between undrained and drained parameters, and when to use what. Actually, it is simple and is common sense. When a saturated clay is loaded, it will not let the water out immediately (i.e. remains undrained) and that is when most of the failures occur. In the short-term, the clay can be treated as an undrained homogeneous material where we will not separate the grains and water. Here, we carry out the undrained analysis in terms of total stresses, using undrained shear strength cu (phi = 0).

Hi all, I came across this statement online. I just dont understand that why in undrained condition, soil failure will occur easily compared to drained condition ? Can someone help to explain it ??

 

[EireChch]

That statement is true for normally consolidated CLAYS, which typically have an undrained strength which is lower than its drained strength.

After Duncan and Wight "Soil Strength and Slope Stability" - As is typical for normally consolidated clays, the undrained strength is lower than the drained strength. This is due to the fact that the pore pressure increases and the effective stress decreases during undrained shear. For very heavily overconsolidated clays, the reverse is true: The undrained strength is greater than the drained strength, because pore pressure decreases and effective stress increases during undrained shear.

Duncan and Wight are referring to the reason in differing behavior being due to the change in pore pressure. In simplistic terms, the soil grains in normally consolidated clay are spaced apart. When they are sheared (or loaded) they can re organise themselves into a tighter packing. As they are trying to "re organise" quickly, the water can not escape and pressure is applied to the water in the soil, which leads to an increase in pore pressure. The term "re organising" is typically called contracting, so normally consolidated CLAY are contractive.

The opposite is true for heavily over consolidated CLAYs, as they have received a very large past pressure (pre consolidation stress) they have consolidated and the grains are tightly packed. When the are sheared (or loaded) they are already tightly packed and they need to "roll over each other" to try to arrange themselves in an even tighter packing. This "rolling over each other" means the soil grains are temporarily taking up more space (or volume). The "rolling over each other" is typically called dilating, so heavily over consolidated CLAYs are dilative. See below image showing the grains rolling over each other.

The water is still there, however there is now more pore space to occupy, the water is drawn into the pore space which causes a negative pore pressure.

 

[dccd]

Can you explain how does contractive and dilative soil related to the strength of soil ?

 

[EireChch]

Due to pore pressure. Contractive soil cause an increase in pore pressure when sheared. This reduces the undrained shear strength. Dilative soils cause an decrease in pore pressure, a decrease in pore pressure is always good news in geotechnical engineered. (well nearly always).

 

[fattdad]

somewhere behind the scene is the subject of critical void ratio.

I'm not sure I can easily contribute much more as this is a complicated topic that's seemingly an easy question.

f-d

ípapß gordo ainÆt no madre flaca!

 

[dccd]

I still dont get it , can you explain further ??

 

[LRJ]

The shear stress is equal to normal stress multiplied by the tangent friction angle. If you have lower pore pressure (due to positive excess pore pressure during shearing, i.e. contraction), then this will lower the normal stress, and lower the shear stress. The reverse is true for dilation (negative excess pore pressure). For fully drained conditions, the stress is constant, so the pore pressure does not change, and instead it is the volume of the material which contracts or dilates.

I suggest looking up critical state soil mechanics - it's a good framework to understand all of this. I think 'drained' and 'undrained' are less useful terms compared to 'constant stress' and 'constant volume'. The critical state framework, subsequently extended to the 'state parameter' for sands - though arguably the undrained shear strength is an equally valid proxy - helps one to visualise whether you're on the 'wet' or 'dry' side of the critical state line and therefore whether a material will contract or dilate when sheared.