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.