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Oedometer test

From Wikipedia, the free encyclopedia

Two oedometers at the University of Cambridge

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.

Oedometer tests are designed to simulate the one-dimensional deformation and drainage conditions that soils experience in the field. The soil sample in an oedometer test is typically a circular disc of diameter-to-height ratio of about 3:1. The sample is held in a rigid confining ring, which prevents lateral displacement of the soil sample, but allows the sample to swell or compress vertically in response to changes in applied load. Known vertical stresses are applied to the top and bottom faces of the sample, typically using free weights and a lever arm. The applied vertical stress is varied and the change of the thickness of the sample is measured.

For samples that are saturated with water, porous stones are placed on the top and bottom of the sample to allow drainage in the vertical direction, and the entire sample is submerged in water to prevent drying. Saturated soil samples exhibit the phenomenon of consolidation, whereby the soil's volume changes gradually to give a delayed response to the change in applied confining stresses. This typically takes minutes or hours to complete in an oedometer and the change of sample thickness with time is recorded, providing measurements of the coefficient of consolidation and the permeability of the soil.

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  • CE 326 Mod 10.5a Consolidation test
  • Consolidometer Test for Calculation of Void Ratio | Soil Mechanics
  • CONSOLIDATION TEST SOIL MECHANICS 2013

Transcription

Hello this is Professor Kitch. Welcome to this webcast which is the first of two on section 10.5. This webcast covers the consolidation lab test After this webcast, you should be able to describe the lab consolidation test Explain why we generally use a semi-log plot of strain versus the log of effective stress as the stress-strain curve for soils, rather than the arithmetic stress-strain curve used for nearly all other materials. Once you have the strain versus log-sigma-prime curve you should be able to identify the recompression curve, virgin curve and rebound curve You need to be able to derive the relationship between strain and change in void ratio and explain why we can plot consolidation data as either strain or void ratio versus the log of effective stress You should be able to define the preconsolidation stress and explain why we always see one in our laboratory data And finally, you should be able to perform Casagrade method to estimate the preconsolidation stress from lab data We discussed the basic consolidation process in the previous section. Specifically we discussed how a change in total stress immediately generates an excess pore pressure and that as that excess pore pressure dissipates, the soil consolidates and the stress is transferred from pore pressure to the soil skeleton in the form of effective stress. We�re now starting sections that will allow you to compute the magnitude of consolidation settlement. In understanding the magnitude of consolidation settlement, it helps to start off by looking at the 1 dimensional consolidation test. In the field, if we have a compressible layer, shown here with a thickness of H-naught. And that layer is covered by a wide areal fill we�ll have one dimensional consolidation and experience a consolidation settlement of delta-c To understand what�s happening in the field we can take a sample of soil from location A, transport it to the lab, and put a specimen from that sample in a device that allows for only one dimensional consolidation. We can then place a load, P, on the specimen and it will consolidate one dimensionally. If we measure what happens in the lab, we should be able to use that data to predict the field settlement. This test is called the one dimensional consolidation test, or the oedometer test. Three are two main types of consolidometers, the floating ring and the fixed ring. The only one we�ll discuss here is the floating ring consolidometer, which is the most common type. In the center is a short cylindrical soil specimen about the size and shape of a hockey puck. It�s usually about three quarters to one inch thick and 3 to 4 inches in diameter The specimen is contained in a confining ring which is made of metal, is very stiff compared to the specimen, and therefore prevents any lateral strain, thereby ensuring that we have one dimensional consolidation. There are porous stones placed on the top and the bottom of the specimen to allow drainage of pore water out of the specimen. With porous stone on the top and bottom, the water can drain out from the center of the specimen in both directions. A loading cap is place on top to distribute applied forces over the entire area of the specimen And a dial gage or digital device is used to measure the vertical displacement. We place a load, P, on the top of the specimen and the loading cap distributes the load evenly over the specimen The final effective stress that is applied, is then P over A minus the pore pressure, u. Since the specimen is so thin the average head of pressure is an inch or less so we can assume the pore pressure is zero and the final effective stress is just equal to P over A When the load P is placed on the cap, the specimen starts to consolidate and we measure the vertical displacement, delta-z, versus time When the sample is finished consolidating, we can measure the final vertical displacement, delta-zf We can then compute the vertical strain as the change in height over the initial height or delta-z over H-naught We can then incrementally increase the vertical stress several times, measuring the vertical deflection with each load. From the data measured from each load increment we can create a table of vertical effective stress versus vertical strain. Once we have our table of strain versus effective stress, we can plot a stress-strain curve for our soil. You�re probably used to seeing such data plotted as stress versus-strain, but in geotechnical engineering we generally plot the data as strain versus stress. We also generally plot stress increasing downward because when soils compress they move down. As you can see from the curve shown, soil stress-strain behavior is highly non-linear. The portion of the curve from A to B to C is the initial loading curve for the soil. Notice that the amount of strain from zero to 200 is more than that from 200 to 400 which is more than that from 400 to 600. This indicates that the soil is always getting stiffer as it consolidates. This makes since. As the soil consolidates, the soil particles get more and more tightly packed and as a result, the soil skeleton gets stiffer and stiffer. Interestingly, if we plot the strain versus the log of effective stress, the stress-strain curve becomes much simpler as shown on the right. In this semi-log space, the curve is very close to being composed of three straight lines We call the portion from the beginning of the plot to the first brake, from A to B, the recompression curve. The next portion of the curve from a bit past B to C is called the virgin curve. and the final portion, which occurs during unloading of the specimen, is called the rebound curve. It may not be readily apparent, but we can also plot the curves as void ratio as a function of effective stress rather than strain versus effective stress. Notice that the void ratio decreases as the soil consolidates�which makes sense since the void volume decreases as the soil skeleton compresses. The deformation of the soil during consolidation, can be expressed either as strain or a change in void ratio. Let�s use our phase diagram to determine the relationship between void ratio change and strain. On the left we have our initial conditions. For convenience, we�ll assume the volume of the voids is 1 unit. Recalling that the void ratio is the equal to the volume of voids divided by the volume of solids When the volume of solids is equal to 1, the volume of the voids will simply be equal to the void ratio, e. In this case it will be e-naught, the initial void ratio. Now, after loading and consolidation, there will be a change in volume of the soils, and all of the change will be in the void space, and therefore the change in volume will be equal to delta-e. If our soil is in one dimensional compression, then the change in void ratio must come only from vertical compression Therefore, we can write that the vertical strain, epsilon-z will be equal to the change in length of our soil element over the original length or delta-l over l-naught If we look at the initial phase diagram on the left, we�ll see that the original length of the element is 1 plus e-naught. And from the phase diagram on the right, we see that the change in length is delta-e Therefore the vertical strain epsilon-z is equal to delta-e over one plus e-naught. That�s why we can plot our consolidation curve either as either strain versus log-sigma-prime, or void ratio versus log-sigma-prime. Epsilon-z and delta-e are directly proportional to one another and the proportionality constant is 1 over 1 plus e-naught. So let�s look at how the stress-strain curve for soils gets its characteristic shape in semi-log space To do this let�s go back a few hundred thousand year, just a few minutes in geologic time, and consider a point A just below the bottom of a Pleistocene ocean during a depositional period. At this point the soil is under some very small effective stress shown as point 1 on our semi-log stress-strain curve. As the deposition continues the effective stress increases, the soil at point A consolidates and undergoes strain. On our strain versus log effective stress plot it travels alone a line from point 1 to point 2. If the deposition continued, the stress path of the soil would continue down the dotted line shown. However, it doesn�t. The deposition at this location stops, and we enter the Holocene epoch. Neanderthals go extinct, homo sapiens get bigger brains, the stone age gives way to the bronze age then the iron age. Finally, the industrial revolution happens along, we start spewing billions of tons of CO2 and other pollutants into the atmosphere. A English teacher, a history teacher, and writer, open a coffee house in Seattle featuring a two tailed mermaid in their logo. Three decades later barista turned Starbucks CEO Howard Schultz, shows up at the empty lot above point A and decides to build yet another Starbucks because the nearest one is over 200 feet away. Schultz hires your geotech firm to do a site investigation. Having read about the Kansai airport and knowing settlement might be an issue, you make a boring at the site and take an undisturbed sample back to the lab. In the process of taking the sample, the soil is unloaded and it follows the stress path form point 2 to point 3. Notice that the soil does not travel back along the original compression line from point 1 to point 2. This is because soil is not elastic and undergoes unrecoverable plastic strain during consolidation. So in unloading it�s stiffer than in loading. We now take a specimen from our field sample and place it in a consolidometer and perform a 1-D consolidation test. During the initial part of the test, the stress path for the soil closely follows the rebound curve from point 3 to 4. But when it gets near the original consolidation curve at point 4, the stress path turns down and follows the original depositional consolidation curve or virgin curve to point 5 The consolidation test continues with unloading during which the soil travels along the rebound curve from point 5 to 6. However, realize that the only information you will have is the laboratory strain versus log effective stress curve shown here. Due to plastic strain during consolidation, it will have a recompression curve, a virgin curve, and rebound curve. And the point where the recompression and virgin curves meet is the preconsolidation stress, sigma-c-prime. If we have a high quality specimen our testing is done very carefully, we�ll get a strain versus log effective stress plot with distinct breaks between different segments of the curve. And it will be very easy to identify the recompression curve, the virgin curve, and find the preconsolidation stress. However, we don�t always have a high quality sample. As samples get more disturbed due to poor sampling techniques or poor handling, the lab test specimen quality is lower and the strain versus log effective stress plots tend to soften and don�t show a clear break between recompression and virgin loading. This can make it difficult or impossible to determine the preconsolidation stress accurately. If the specimen is of relatively good quality and tested properly, but doesn�t show a distinct preconsolidation point, as shown here, We can use the method developed by Casagrande to estimate the preconsolidation stress. The first step in the Casagrande method is to locate the point of maximum curvature on the strain versus log effective stress plot. From this point you then draw a horizontal line and a line tangent to the lab data curve you then determine the angle between these two lines and draw a third line bisecting that angle. Next you go to the lower end of the virgin curve and draw a line tangent to the low end of the curve. Now locate the point where this extension of the virgin curve intersects the bisector line. That point is the location of the preconsolidation stress. This method is based on Cassagrande�s considerable experience, but there�s no theory behind it. It is however, a consistent method and if followed carefully, will ensure some consistency in estimating the preconsolidation stress. However if the specimen is highly disturbed such as the one show in red, there is now way of accurately estimating the preconsolidation stress. Schmertmann extended Cassagrande�s method into a procedure to reconstruct an estimate of the field compression curve from a moderately disturbed laboratory curve. This figure shows a typical lab data curve and the corresponding reconstructed field curve using Schmermann�s method. The procedures are outlined on pages 439 & 440 of your text. It is important to note that the reconstructed field curve lies outside of the measured lab data and therefore constitutes an extrapolation of the lab data. Extrapolation is always a dicey endeavor and the accuracy of curves reconstructed by this method is questionable. The best approach is to get high quality samples and use good lab techniques so you get a lab curve that clearly show the preconsolidation stress and require little correction. So let�s summarize Soils have nonlinear stress-strain curves and their stiffness increases during consolidation. We generally plot these curves as strain versus the log of effective stress or void ratio versus the log of effective stress. Plotting in this semi-log space makes the curves more linear On unloading soil always exhibits plastic strain and unloads on a curve stiffer than the loading curve We use the laboratory consolidation test to determine our stress-strain curve Lab tests will always show both a recompression and virgin curve, because the field sample is unloaded during the sampling process The point where the recompression and virgin curves meet is the preconsolidation stress and it represents the greatest effective stress the soil has ever experienced. If we have high quality tests with high quality specimens, we can easily draw tangents to the recompression and virgin curves and locate the preconsolidation stress. When the lab data are moderately disturbed, you should use Cassagrande�s method as a consistent way to estimate the preconsolidation stress. And finally, we can use Schmertmann�s method to reconstruct field compression curves for lab tests on disturbed samples. However, it�s better to get a high quality sample and generate a high quality lab curve.

Etymology

The word "oedometer" (/iˈdɒmɪtər/ ee-DO-mi-tər, sometimes /ˈdɒmɪtər/ oh-DO-mi-tər) is derived from Ancient Greek οἰδέω (oidéō, "to swell") and the noun oídēma meaning 'swelling',[1] which is also used in English with the same meaning, as oedema.[2]

This should not be confused with the similar-looking but unrelated word "odometer", derived from Ancient Greek ὁδός (hodos, "path") which refers to a device to measure the distance travelled by a vehicle.[3]

History

Consolidation experiments were first carried out in 1910 by Frontard. A thin sample (2in thick by 14in in diameter) was cut and placed in a metal container with a perforated base. This sample was then loaded through a piston incrementally, allowing equilibrium to be reached after each increment. To prevent drying of the clay, the test was done in a room with high humidity.[4]

Karl von Terzaghi started his consolidation research in 1919 at Robert College in Istanbul.[4] Through these experiments, Terzaghi started to develop his theory of consolidation which was eventually published in 1923.

The Massachusetts Institute of Technology played a key role in early consolidation research. Both Terzaghi and Arthur Casagrande spent time at M.I.T. - Terzaghi from 1925 to 1929 and Casagrande from 1926 to 1932. During that time, the testing methods and apparatuses for consolidation testing were improved.[5] Casagrande's contributions to the technique of oedometer testing includes the "Casagrande method" to estimate the pre-consolidation pressure of a natural soil sample.[6] Research was continued at MIT in the 1940s by Donald Taylor.[7]

Both the British Standards Institute and the ASTM have standardised methods of oedometer testing. ASTM D2435 / D2435M - 11 covers oedometer testing by incremental loading. ASTM D3877, ASTM D4546 and AASHTO T216 also provide related procedures for conducting other similar tests for determination of the consolidation characteristics of soils.[8] BS 1377-5:1990 is the relevant British Standard for oedometer testing; the wider BS 1377 series also provides background information and best-practice advice on sample preparation for various geotechnical investigations.[9] There are also two ISO standards on oedometer testing: ISO 17892-5:2017 on incremental loading oedometer tests;[10] and BS EN ISO 17892-11:2019 covers various methods of soil permeability testing, including oedometer tests on saturated samples.[11]

Equipment

Two disassembled oedometers at the University of Cambridge

An oedometer is fundamentally made out of three components: a "consolidation cell" to hold the soil sample, a mechanism to apply a known pressure over the sample, and an instrument to measure the changes in the sample's thickness.[12]

The equipment required to perform an oedometer test is sometimes called an "oedometer test set". A typical inventory of an oedometer laboratory includes:[13]

  • 1 x Bench
  • 3 x Oedometers
  • 3 x Cells, either 50mm or 63.5mm, or 75mm
  • 3 x Dial gauges, either analogue, or digital
  • 1 x Weight set

The consolidation cell is the part of the oedometer that holds the soil sample during a test. At the centre of the consolidation cell is a sample ring where the soil sample is held. The sample ring is typically shaped like a cookie cutter, with a sharp edge on one side, so the ring can be used to cut out a sample slice of soil from a larger block of natural soil. Two slices of porous stone, which fit snugly into the sample ring, provide water drainage to the soil sample while confining it mechanically. These components all fit in a larger cylinder, which has grooves to ensure alignment of the components, and provides water supply and drainage to external plumbing. A rigid loading cap sits on top of the soil sample to apply compressive loads to the soil.[12][14]

The loading mechanism of the oedometer applies a known compressive load, and therefore a known compressive stress since the diameter is fixed, to the soil sample. Most oedometers achieve this with a lever arm and a set of free weights: the free weights provide a known gravitational load, and the lever arm multiplies and transmits the load to the soil sample.[15]

Testing procedures

Schematic drawing of the incremental loading frame developed by Alan Bishop

There are many oedometer tests that are used to measure consolidation properties. The most common type is the incremental loading (IL) test.[16]

Sample preparation

Tests are carried out on specimens prepared from undisturbed samples. A stiff confining ring with a sharp edge is used to cut a sample of soil directly from a larger block of soil. Excess soil is carefully carved away, leaving a sample with a diameter-to-height ratio of 3 or more. Porous stones are placed on the top and bottom of the sample to provide drainage. A rigid loading cap is then placed on top of the upper porous stone. For saturated soil samples, it is important to submerge the entire sample ring in water to prevent the sample from drying out.[16]

Incremental loading

This assembly is then placed into a loading frame. Weights are placed on the frame, imposing a load on the soil. Compression of the sample is measured over time by a dial indicator. By observing the deflection value over time data, it can be determined when the sample has reached the end of primary consolidation. Another load is then immediately placed on the soil and this process is repeated. After a significant total load has been applied, the load on the sample is decreased incrementally. Using a load increment ratio of 1/2 provides a sufficient number of data points to describe the relationship between void ratio and effective stress for a soil.[16]

Results

Coefficient of volume compressibility.

Oedometer tests provide engineers with very useful data about the soil being tested.

Coefficient of volume compressibility

derivied from Oedometer test

Applied to layer in field (see pictures)

Consolidation Properties

  • Preconsolidation pressure σ'p[17]
    • The effective stress that marks the boundary between stiff and soft deformation response of a soil to loading
    • Usually indicative of high loadings in the past from glaciers or eroded layers
  • Recompression Index CR = Δe/Δlogσ'v[18]
    • How the soil will change volume (settle) under loads less than the preconsolidation pressure
    • Can be used to approximate swelling due to unloading
  • Compression Index CC = Δe/Δlogσ'v[18]
    • How the soil will change volume (settle) under loads greater than the preconsolidation pressure
  • Duration of Primary Consolidation tp[19]
  • Secondary Compression Index Cα = Δe/Δlogt[19]
    • How the soil will change volume (settle) under a constant loading

See also

References

  1. ^ Liddell, Henry. "οἴδ-ημα". A Greek-English Lexicon. Tufts. Retrieved 8 December 2019.
  2. ^ "oedometer | Definition of oedometer in English by Oxford Dictionaries". Oxford Dictionaries | English. Archived from the original on April 6, 2019. Retrieved 2019-04-06.
  3. ^ "odometer | Definition of odometer in English by Oxford Dictionaries". Oxford Dictionaries | English. Archived from the original on April 6, 2019. Retrieved 2019-04-06.
  4. ^ a b Bjerrum, Laurits; Casagrande, Arthur; Peck, Ralph; Skempton, Alec. (1960). From Theory to Practice in Soil Mechanics. (p44) John Wiley & Sons, Inc.
  5. ^ Bjerrum, Laurits; Casagrande, Arthur; Peck, Ralph; Skempton, Alec. (1960). From Theory to Practice in Soil Mechanics. (p6-7) John Wiley & Sons, Inc.
  6. ^ "Coefficient of Earth Pressure at Rest", Geotechnical Correlations for Soils and Rocks, John Wiley & Sons, Inc., 2018-06-01, pp. 73–75, doi:10.1002/9781119482819.ch8, ISBN 9781119482819
  7. ^ Taylor, Donald W. (1942). Research on Consolidation of Clays. Massachusetts Institute of Technology
  8. ^ "ASTM D2435 / D2435M - 11 Standard Test Methods for One-Dimensional Consolidation Properties of Soils Using Incremental Loading". www.astm.org. Retrieved 2019-04-07.
  9. ^ "BS 1377-5:1990 - Methods of test for soils for civil engineering purposes. Compressibility, permeability and durability tests – BSI British Standards". shop.bsigroup.com. Retrieved 2019-04-07.
  10. ^ "BS EN ISO 17892-5:2017 - Geotechnical investigation and testing. Laboratory testing of soil. Incremental loading oedometer test". shop.bsigroup.com. Retrieved 2019-04-07.
  11. ^ "BS EN ISO 17892-11:2019 Geotechnical investigation and testing. Laboratory testing of soil. Permeability tests". shop.bsigroup.com. Retrieved 2019-04-07.
  12. ^ a b Sjursen, Morten Andreas; Dyvik, Rune. "Lab Test - Oedometer Test" (PDF). Norwegian Geotechnical Institute. Retrieved 2019-04-14.
  13. ^ "Front Loading Oedometer Test Set". www.cooper.co.uk. Cooper Research Technology. Retrieved 5 September 2014.
  14. ^ "Floating Ring Consolidation Cell". www.humboldtmfg.com. Retrieved 2019-04-14.
  15. ^ "Soil Consolidation - Oedometers". www.pcte.com.au. Retrieved 2019-04-14.
  16. ^ a b c Terzaghi, Karl; Peck, Ralph; Mesri, Gholamreza (1996). Soil Mechanics in Engineering Practice (3rd Edition). (Article 16.9) Wiley-Interscience
  17. ^ Terzaghi, Karl; Peck, Ralph; Mesri, Gholamreza (1996). Soil mechanics in Engineering Practice (3rd Edition). (Article 16.4) Wiley-Interscience
  18. ^ a b Terzaghi, Karl; Peck, Ralph; Mesri, Gholamreza (1996). Soil mechanics in Engineering Practice (3rd Edition). (Article 16.6) Wiley-Interscience
  19. ^ a b Terzaghi, Karl; Peck, Ralph; Mesri, Gholamreza (1996). Soil mechanics in Engineering Practice (3rd Edition). (Article 16.7) Wiley-Interscience
This page was last edited on 28 January 2023, at 07:54
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