What is Consolidation of soil ?
Soil mass is formed of tiny loose particles in which lots of voids are created in between. In such voids either air or water gets occupied unless external pressure or load for compaction is applied to it.
So whenever the load is applied over soil mass, there is a distinct decrease in volume of soil mass. This decrease in volume can cause by three reasons :
- Compression of soil particles and water itself (This is very rare and doesn’t occur in small magnitude of stress we talk in soil mechanics)
- Compression and expulsion of air voids (This generally leads only compaction of soil from unsaturated soil)
- Expulsion of water (This is important and major factor for consolidation of soil)
Consolidation in soil mechanics is a process that occurs in saturated or partially saturated soil(even if it’s partially saturated, at initial consolidation its get compacted and forms saturated soil) under the application of steady static load or pressure for a long time with a decrease in original soil volume.
As water expels out the soil particles under pressure tends to roll and slide for rearrangement to give a compacted structure, thus resulting decrease in volume. In consolidation, most of the volume change is generally governed by this mechanism rather than compression of soil mass itself.
The consolidation is a major reason to form the settlement in the civil structure. Thus it is very important factor for Geotechnical engineer and Civil engineers to know about the consolidation of soil at certain places. And also, the consolidation helps to increase the effective stress in soil and remove all the pore water pressure.
Moreover, the consolidation rate and magnitude are different for various types of soil. This might be clearly projected on the discussion of different stages of consolidation explained below:
Stages of consolidation (Mechanism)
The initial consolidation occurs when the load is applied to the partially saturated soil. In this, the air voids present in partially saturated soil are compressed and expel out easily on application of load this decrease the volume of soil and bring soil to saturated state.
The further steady static load can lead to the compression of very few weak solid soil mass particles whose volume changes are not much significant.
A) Primary consolidation
After the initial consolidation, a further decrease in volume occurs. This time it is due to the expulsion of water from voids. The phenomenon or mechanism is not much simple and faster than the earlier stage.
After full saturation, the static steady load applied is now taken up by the water in form of pore water pressure. The question may arise ‘Why not by soil particles’ and the answer for that us water is almost incompressible in comparison to soil. So even soil mass gets compressed water cannot and takes up all pressure. (This phenomenon is beautifully explained by the Terzaghi Spring analogy model).
Now on excess pressure in the water, the hydraulic gradient is developed and thus water starts to flow forming capillaries and release out. The pressure is now transferred to soil and thus closes capillary voids resulting increase in effective stress and a decrease in volume.
The rate and magnitude of decrease in volume depending upon the permeability of the soil. Thus their rate are different for various types of soil i.e. in fine-grained soil(like clay) consolidation occurs for long time and coarse-grained soil (like sandy loam) it takes comparatively shorter time due to high permeability.
The primary consolidation is simply known as “Consolidation” over a large scale.
B) Secondary consolidation
Even after the complete removal of hydrostatic pressure, the reduction in volume continues at a very slow rate. This additional reduction in volume after primary consolidation is called secondary consolidation.
The definite reason for it is unknown, however, it is supposed ti cause due to rearrangement of solid particles and adsorbed water release.
Consolidation Test of soil
A consolidation test is generally a method to study the compressibility of saturated soil at the lab. The apparatus used for this test is known as Consolidemeter or Oedometer, which consist of loading device(plate), a cylindrical container called Consolidation Cell and a steel cylindrical ring to keep sample .
The cylindrical steel ring is approximately 25mm high and 75mm in diameter (according to ASTM D2435).
However, various dimensional apparatus are also available accordingly to the specific codes or standards. The loading device facilities to load the weight through load plate along with the arrangement of collar and nut and bolts, so that load are exactly loaded on center and causes axially loaded conditions.
The dial gauze over load plate helps to measure the decrease in volume(height). Filter stones are placed (along with filter paper) at top and bottom of sample to allow the water to escape from both end. The mould assembly is required to connect with water reservoir at same height so that soil is confirmed to be saturated all the time.
The inside surface of cell must be kept Smooth and polished to reduce the friction so that the condition of zero lateral strain on sample is achieved while loading. The porous stone are saturated either by boiling them in distill water for 15 min or keeping them submerged in water for 4-8 hrs.
Several loading procedure are available; incremental loading, constant rate of strain and constant gradient, which gives their distinctive graph characteristic for study of consolidation. Among them, the most widely accepted method is incremental loading because of its easy procedure.
At initial, the pressure of about 5KN/m² (for soft soil 2.5KN/m²) is applied. The load is kept at least for 24hr or till there is no change in the thickness of the sample. The dial gauge under initial setting pressure is noted.
Then, the load are stared to increase with the first incremental load to give a pressure of 10KN/m². The dial gauge reading are now taken at distinctive interval of time till 24hrs (Generally, 0.25,1,2.25,4,6.25,9,12.25,16,20.25,25,36,49 minutes and after that 1,2,8,10 and 24hrs)
The primary consolidation is supposed to complete within 24 hrs. The next incremental loads are applied. It is usually taken as double to previous load in each increase (i.e. 20,40,80,160etc.) till desired maximum load intensity is reached. The maximum load intensity is actual loading n soil due to structures that are supposed to cause on the actual site.
After consolidation under the final load, the incremental process is completed. Now the load is reduced to 1/4th of the final load and allowed to stand for 24 hrs under proper water supply.
The sample takes up water and swell. The load is further reduced to 1/4th if last reduced load and again left for swelling. Each respective load and swelling reading on the dial gauge are noted. And finally reduced to 10KN/m² at last and taken record at 24hrs.
The noted record and observations are used in calculations to find out the results. The results we expect from the above test is;
- Dial gauge reading time plot
- Final void ratio – effective stress plot
- Final void ratio – log effective stress plot
- Unloading and reloading plot
The result gives require information on the consolidation of the soil of the site.
Consolidation in Geology
In geology, consolidation, however, means differently than in soil mechanics. The process and mechanisms are almost similar but in geology it takes up long period of time and involves large static pressure for long time. The resulting products are solid hardened sedimentary rocks at the end.
In fact, the whole process of formation of consolidated rock from sediment deposit is uniquely terms as Lithificationprocess.
The process involves similar mechanism as consolidation i.e. Compaction and elimination of air space by soil grain, removal or expulsion of fluid and lastly cementation, filling and compaction of voids by dissolved material (Only in consolidation in geology). A very good example of consolidation in geology can be distinctly obtained in rock “Conglomerate”.
The process of consolidation can be exactly analogue to the Sponge with complete absorption of water and pressurizing it to static load to remove all water.
The decrease in in-depth or volume is the consolidation that has occurred. The same analogue can be used for cleared visualization consolidation of soil consolidation and to explain the layman person about the consolidation process.
Moreover, Terzaghi has adopted the Spring analogy model to explain the mechanism of primary consolidation.
Compaction and Consolidation
Compaction and consolidation are the topics that are often confused with each other. The whole process are totally different and the only similarity they got is the reduction in volume of soil mass.
Compaction is the process of reduction of volume of soil mass by decreasing air voids and compacting it to optimal water content by the action of dynamic loading. Compaction is an instant process accomplish for a certain level of stability in soil. The compaction doesn’t ensure that there won’t be a further decrease in volume and settlement in structure.
While consolidation is very slow process occur naturally ( not by action of mechanical vibratory loading) due to long term static load in which not only air voids are eliminated but entire water from saturated soil is expel out causing the compact molecular arrangements of soil particles.
The fully consolidated soil ensure no settlement effect. Thus sometimes for mega structure, the field is pre loaded with the static load for long time and then only after that construction are initiated over it.
I hope this article on “Consolidation of soil” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha