CEEN 341 - Lecture 17 - Time Rate of Settlement

This lecture introduces how getoechnical engineers estimate the length of time it will take for primary consolidation to occur. The Terzaghi 1D consolidation equation is presented and used to develop isochrones, which can be used to predict the amount of excess pore water pressure at a depth of interest in the clay layer at time = t (i.e., uz), the degree of consolidation at a depth of interest in the clay layer at time = t (i.e., Uz), and the average degree of consolidation over the entire clay layer (i.e., Uavg). The coefficient of consolidation is introduced, and two methods (the logarithm of time and the square-root of time methods) are introduced to measure it from a consolidation test at particular load increment. Finally, two techniques to speed up the process of primary consolidation are introduced: the use of prefrabicated vertical drains (PVDs, or "wick" drains), and the use of surcharge loading.
Learning Objectives:
1) Explain the difference between uz, Uz, and Uavg.
2) Compute uz, Uz, and Uavg for a given soil and load using Terzaghi's 1-D consolidation theory.
3) Obtain cv from a time rate consolidation curve using the square root of time method and/or the logarithm of time method.
4) Describe two ways through which engineers accelerate primary consolidation settlements in practice.
Critical Questions to Consider:
1) What is Hdr? Why does it have such a large impact on the time rate of consolidation (hint: think of the equation for the time factor, Tv).
2) What is the difference between uz, Uz, and Uavg? Be sure to be comfortable with these different parameters and know how to compute them!
3) Why is the initial increase in the pore pressure (i.e., u0) always equal to the applied induced stress in the soil layer (i.e., delta_sigma)?
4) What is the difference in the coefficient of consolidation (i.e., cv) for an overconsolidated soil versus a normally consolidated soil? Why is it important to make sure that you are using the cv value that corresponds to the stresses that the soil will actually be feeling in the ground?
5) Describe the two methods that engineers commonly use to accelerate primary consolidation settlement. How does each of these methods work? Be prepared to explain the concepts.

[@ahamadromen]

Thank you very much for such nicely explaining the fundamentals. Really appreciated it !!

[@mghlakhine]

Thank you very much.

[@yousefbaytam6213]

thank you very much sir you're actually a life saver.

[@nicolasramirez3944]

Thank you for these excellent lectures!

[@JegusGrist]

You're very good at teaching. Thank you!

[@abduljabbarkhan9107]

Sir, I have a situation: first layer is a clay with thickness H1 and coefficient of consolidation Cv1, the second layer is also a clay layer with thickness H2 and coefficient of consolidation Cv2, the third layer is a sand layer with thickness H3, the fourth layer is again a clay layer with thickness H4 and coefficient of consolidation Cv4. How can I apply Terzaghi's solution t = Tv/Cv*hd^2 for such situation?

[@farilala8480]

Thank you so much...well explained and easy to follow. you are amazing 💓

[@abduljabbarkhan9107]

Say, there are four layers below a square footing. Up to the depth of footing the soil is clay.
First layer: clay soil, thickness H1, coefficient of consolidation: Cv1
Second layer: clay soil, thickness H2, coefficient of consolidation: Cv2
Third layer: sandy soil
Fourth layer: clay soil, thickness H4, coefficient of consolidation: Cv4
The fourth layer is followed by impermeable rock.
How would you apply Terzaghi's classical formula , t = Tv/Cv*hd^2 to this problem? (hd = drainage path)
To be more specific, how can we calculate time required for 50%
consolidation of entire soil system? What will be the values of Cv and
hd for the first layer?

[@victorcaraballo5457]

Hi. I think Hdr for the first layer has to be Hdr = Ht, because we can consider that the second layer is "impermeable" (to simplify) and first layer is single-drained. Another way to do it is consider Hdr = H1+H2 for the first layer, and find a way to get an "average" for Cv1 and Cv2 to use it for Tv equation for the first layer. That's only my opinion.
For second and fourth layer we can consider only single-drained system, so Hdr = Ht.
I hope you get the answer soon. That's an interesting question.

[@mohammedashraf3813]

u Are sooooo great thankkkkk you hope to make more lessons (postgraduate student in National technical University of Athens )

[@mehulkatakiya4892]

Sir, at 14:56, you are showing the area covered with a green mark on left of u(z) line, won't that area be right of u(z) line?

[@aliabdulrazaq512]

100% amazing teaching thank you .....

[@burakceylan9409]

kralsın

[@mehulkatakiya4892]

SIr, how OC soil can have a higher value of k than NC soil?

[@jacksonzheng4288]

if you check the consolidation curve, OC part has larger void ratio than NC part

[@bishoomaser]

Where is lecture 16 ??

[@alxms10]

Hi
Is there no Lecture 16?

[@shahofheartz]

Lecture 16 and 22 Please?

[@rayoaklight]

lecture 16 and 22 please sir