Compaction & Consolidation: Discussion of Odometer Test Essay
Discussion The process of consolidation is often confused with the process of compaction. Compaction increases the density of an unsaturated soil by reducing the volume of air in the voids. Meanwhile, Consolidation is a time-related process of increasing the density of saturated soil by draining some of the water out of the voids. According to Karl Terzaghi “consolidation is any process which involves decrease in water content of a saturated soil without replacement of water by air. In general it is the process in which reduction in volume takes place by expulsion of water under long term static loads.
It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil. Consolidation is generally related to fine-grained soils such as silts and clays. Coarse-grained soils (sands and gravels) also undergo consolidation but at a much faster rate due to their high permeability.
Saturated clays consolidate at a much slower rate due to their low permeability. Generally Compression of saturated soil consists of two successive phases, namely the primary and secondary consolidation phases. The settlement is defined as the compression of a soil layer due to the loading applied at or near its top surface. The total soil settlement of a soil layer consists of three parts: Immediate or elastic compression Compression due to primary consolidation Compression due to secondary consolidation
Primary consolidation This method assumes consolidation occurs in only one-dimension. Laboratory data is used to construct a plot of strain or void ratio versus effective stress where the effective stress axis is on a logarithmic scale. The plot’s slope is the compression index or recompression index. The equation for consolidation settlement of a normally consolidated soil can then be determined to be: Secondary compression Secondary compression is the compression of soil that takes place after primary consolidation.
Even after the reduction of hydrostatic pressure some compression of soil takes place at slow rate. This is known as secondary compression. Secondary compression is caused by creep, viscous behavior of the clay-water system, compression of organic matter, and other processes. In sand, settlement caused by secondary compression is negligible, but in peat, it is very significant. Due to secondary compression some of the highly viscous water between the points of contact is forced out. Secondary compression is given by the formula
The magnitude of consolidation can be predicted by many different methods. In the Classical Method, developed by Terzaghi, soils are tested with an Oedometer test to determine their compression index. This can be used to predict the amount of consolidation. An Oedometer test is a kind of geotechnical investigation performed in geotechnical engineering that measures a soil’s consolidation properties. Oedometer tests are performed by applying different loads to a soil sample and measuring the deformation response.
The results from these tests are used to predict how a soil in the field will deform in response to a change in effective stress. Also Oedometer tests are designed to simulate the one-dimensional deformation and drainage conditions that soils experience in the field. To simulate these conditions, rigid confining rings are used to prevent lateral displacement of the soil sample. Porous stones are placed on the top and bottom of the sample to allow drainage in the vertical direction.
In the experiment, we applied several loads to our specimen in order to obtain the required set of gauge readings for us to plot graphs. First, the void ratio and consolidation pressure are calculated to plot the e-log p graph. There are two methods to determine the value of Cv (which is the parameter used to describe the rate at which saturated clay or other soil undergoes consolidation, or compaction, when subjected to an increase in pressure). The first method is by using the Casagrande (log time) method. Cv= 0. 97(H/2)2/t50 where H is the specimen height Based on the experiment, Cv is 1. 104 X 10-3 The second method is Taylor and Merchant method or root time method For the root time method, Cv = 0. 848 (H/2)2/t90. In this method, the value of Cv is 11. 94 X 10-3. Between these two values we choose the highest amount which is 11. 94. Volume of compressibility, mv = ? H/ (H0.? p). mv is the compression of a clay per unit thickness, due to a unit increase of effective stress, in the load range exceeding pre-consolidation stress From the experiment, mv= 3. 3 x 10^-3 Next, coefficient of permeability is determined using the following formula: Kv= cv. mv.? w From the experiment, Kv value is 0. 355 The compression index (Cc ) (which represents the slope of the void ratio versus logarithm of effective pressure beyond maximum past effective stress) can be determined by calculate the gradient of the curve of void ratio versus logarithmic of pressure (e-log p curve) using the following formula: ? C? _c=(e_2-e_1)/? log(??? P_2/P_1 )? Cc = 0. 0409
Swelling index (Cs) (which represents the slope of rebound curve of void ratio versus logarithm of effective pressure) is determined from the graph and is equal to 0. 0049 Cs = 0. 0049 By the way there are some errors that might happen during the experiment. The most probable error is parallax error. The eyes of observer might not be parallel during the dial gauge reading. Plus, the time interval between readings is very short, so the readings recorded are low in accuracy. The next error is maybe because of the way the experiment is conducted.
Theoretically, consolidation process takes a long time, Sometimes consolidation can take years. This is especially true in saturated clays because their hydraulic conductivity is extremely low, and this causes the water to take an exceptionally long time to drain out of the soil. But this experiment only takes 5 days so the clay might not consolidate completely. In this experiment also I learnt that time is one of the factors that influence the consolidation characteristics of the soil and it is depended on the path of water outflow.
This path is equal to the thickness of the consolidated layer in case of only one direction outflow (upwards or downwards) or half of the thickness in case of both directions outflow (upwards and downwards). Real time factor is evaluated according to the following formula: where:cv-consolidation coefficient t-real time H-drainage path Time factor of build duration is influenced by duration of load action. When the whole loading is introduced at the beginning of stage, build time is equal to zero.
When load linearly increases during stage duration, then build time is equal to the time of stage duration. Time factor of build duration is calculated by formula: where:cv-consolidation coefficient tc-build time Advantages and disadvantages of the test: Advantages: – Gives first estimate of creep/consolidation parameters and the “vertical” pre? consolidation stress directly. Disadvantages: – Time consuming compared to CRS tests –Only average settlement parameters for large stress increments – Ideally back calculation with mathematical model is needed (FEA)