Consolidation settlement is a time dependent settlement that occurs in clay soils, due to applied loads from above.
As we wrote before, settlement of soils in general has three stages:
- The immediate settlement as the load is applied, which is elastic settlement,
- the gradual, slow consolidation settlement while water is escaping through soil pores,
- and the secondary compression due to rearrangement of soil skeleton structure only.
Sands settle elastically, which we have talked about before, and for them consolidation settlement is negligible. For clays on the other hand, apart from elastic settlement consolidation settlement is also important, which are the second and third items in this list. The consolidation process we will talk about right below is the second one, which is the settlement that takes long time, while the water is squeezed out of clay soil which has very low permeability.
When a heavy load is placed over fine grained soils such as clay, water in the soil gradually, slowly exits the pores in the soil, which causes those pores to contract, which causes overall decrease in soil volume. As the soil can not move laterally, the volume decrease can be only because of reduction of the soil height, in other words, settlement. It is similar to pushing a piston which has a spring and water in it.
Soil strength 21
In this figure, the water in the piston represents the water in soils pores, and the spring represents the force that the soil skeleton takes. At first the piston is in balance. Then, an extra load is placed on it. At first, the extra load is entirely taken by water. But then, as water escapes from the soil slowly, which is represented by water exiting the open valve at the top of the piston, more and more of the extra load is is carried by the spring, and, the spring deforms, up to a point that all of the extra load is now taken by soil skeleton. So, when new balance is reached, the soil volume is now less than the original, in other words, settlement of soil has occurred. If we were to place further extra load on the piston, more water would exit, and further soil settlement would occur, and so on.
This consolidation process may take months or even years. To make the process faster, sometimes gravel or prefabricated drains can be placed in these soils, so that water can escape quicker.
Through consolidation equations we can determine the amount of settlement under given load for a certain soil and also the time it takes for a certain amount of settlement to occur. But we will not go into the details of how to calculate those settlements here, as that would be too detailed for the purpose of this book.
And what if you were to take away that extra load? Then the spring would push back the piston upwards, which means, the soil volume will increase again. But this can not bring back the soil to its original height again, it can recover only part of it. This is because some permanent particle rearrangement has already taken place when the extra load was first applied and we let the full consolidation take place under that load. This permanent rearrangement of particles can not be recovered, and that is why the piston can not go up to its original height even if the extra load was to be entirely removed. This partial recovering of soil of its existing height is called heaving of soil.
Soil strength 22
But in real life when can this load removal occur? It can happen when we excavate an area. After we complete our excavation, the new excavated surface, which had more soil on top, or load in the past, was in balance, as it had completed all of its consolidation for previous load. Now, as the soil on top of it is gone, the soil will push back, which causes heaving (or swelling) of soil. The soil in such state is called “Overconsolidated”. The degree of overconsolidation, called Overconsolidation Ratio, shortly written as OCR, is a parameter frequently used in soil mechanics.
Overconsolidation ratio is defined as:
OCR = maximum effective stress in the past / current effective stress
Writing with proper symbols:
σ’o stands for maximum effecitive stress in the past.
σ’ stands for current effective stress.
Note that in soil mechanics, to indicate effective stress parameters, we add the apostrophe sign near the symbols (such as the stress symbol or the angle of internal friction symbol). We use effective stress, which is the soil skeleton stress, because that is what matters for strength and settlement after water is gone. Water just takes on the pressure temporarily, before it escapes through pores.
σ’o is also called “preconsolidation pressure”, which means the same thing. So it is the stress existed for the soil, prior to the excavation or removal of stress above the soil.
We will not go into further details of overconsolidation here, but the reader should be aware that overconsolidation is an important concept in soil mechanics, and it causes significantly different or even reverse behaviour in soils.
The most fundamental graph when discussing consolidation settlement would of course be the plot of void ratio versus effective stress, which is e vs. log σ’. We just plot effective stress σ’ by taking its logarithm for convenience, as the graph would be difficult to draw and read otherwise, so do not be confused by it, in the figure below. A typical graph looks as below:
Soil strength 23a
Void ratio, e, is a volume related property as you remember from section “phase relationships” that we introduced before. So this e vs. log σ’ graph basically shows the relationship of volume change with increasing or decreasing stress on the soil. So in other words, since consolidation is all about settlement of clay soil when we apply load on it, and since settlement also means decrease of soil volume, when we express void ratio versus effective stress in soil, we are basically showing, describing, what is happening to the soil during a consolidation process. This is why this graph is the first thing we plot and look, whenever we talk about consolidation in soils.
As can be seen in the figure above, at first the stress increase does not cause much volume change, and the curve goes flatter. In other words, a lot of stress increase is needed, to cause a little volume change at first. This is a good thing, from construction point of view, when we see that the loads we add do not cause much volume change. The reason that curve is flat here, is because, clay soils have “memory”, and if it had been subjected to higher stresses in the past, it does not change volume easily when a stress lower than in the past is applied on it. It first “waits” that the stress increase reach to that highest point in the past. This is exactly the overconsolidated condition, that we described in previous page.
Then gradually, every increase in stress cause more and more volume change, and finally it becomes a flat, down sloping line, which means it goes linearly now, in direct proportion now. That flat line is called the virgin compression line. The point where virgin curve starts, is where our new stress reaches the maximum past stress. And as soon as we go beyond the maximum past stress, now the curve is flat and steeply sloping downward, which means and every increase in stress cause a lot of volume change. But how can a past stress be more than the current stress in the past? It can happen by human acitivity such as excavation of soil above. It can happen by geological processes, where in the past there was thicker soil on top, and now a thinner layer exis, such as glaciers or water eroding the soil on top, or seismic movements. It canalso happen due to sampling process of soil specimen from field, when we take out a core of soil from natural ground, it does not have the overburden anymore, that existed on it for many millions of years.
Now let’s look at what happens to the curve, if we were to release the load we introduced, and then reloaded the sample and then release it again, and then reload it again, and so on (by the way all this can be done in an oedometer test in the lab, so we obtain the graphs above and below in the lab with oedometer test, also called consolidation test).
The figure below is the same thing with the previous one, except, in this one we also show releasing of load and the loading it back. So this is how the curve looks, when we perform such a cycle:
Soil strength 23b
As can be seen in the figure above, this figure is basically the repetition of the previous figure in fact. First we start loading as in the previous figure, first soil settles less, then it reaches virgin line, and starts to deform a lot. Then, different than the preivous figure, we do something extra here, and release the load. As you can see, the volume goes up again (soil swells), and the stress falls. But does it go up all the way back to original? No. It can only recover some of it. Then we start loading again and a next cycle begins, exactly looking like the preivous one in shape, with the only difference that the stresses are higher here and volume is smaller. So again, at first the curve is flat like before, not a lot of deformation since we had higher stress from previous cyce, but, the moment we reach to that previous maximum stress level, now the curve joins to virgin curve and it just continues on it, as we now compress the soil for the first time with this much stress. Then we release the load again, so the soil swells a little again, and we strart loading, again the deformation is not much at first, but as soon as we reach the preivous maximum stress, again the soil enter back into virgin line and the cycles can be continued like this, up to a certain practical limit.
In the next post of this series, we will discuss “Slope Stability”