SETTLEMENT CHARACTERISTICS OF RESIDUAL SOILS V.YathurshananUniversity of Moratuwa,Moratuwa,[email protected]
Kulathilaka, S.A.SUniversity of Moratuwa,Moratuwa Abstract: Srilanka have more residualsoils.so if we want construct on residual soils we must know aboutcharacteristics about residual soils.
Most of the countries have sedimentarysoils that’s why soil concepts develops for sedimentary soils. We do not doexperimental for residual soils widely. But in construction concept ofsedimentary soils use for residual soils.When weapplied tezaghi’s concept for residual soils we take 20mm thick sample forlaboratory testing but in field residual soil characteristics vary within smallareas.so this will affect our values, we draw the settlement versus square roottime graph will be continually curve.
And settlement taking place much fasterthan predicted. Because of residual soils are partially saturated soconsolidation settlement happed by air dissip/ation. When we draw settlementversus stress (log scale) there is virgin consolidation part is linear assedimentary soils.
so there is no reason for use conventional method using onresidual soils. (B.L.Thennekoon, 1998)Fromprevious research using by oedometer when we draw compression curve plottedusing log scale and linear scale they found for some soil same got linear curvehave very close to linear except to high stress. There is thus no increse incompressibitity with stress.the compressibility either remains constant ordecrease as stress level increase.
there is no longer trace, on the linearplots,of the prconsolidation pressure determined frome the log plots.there issimply no significant change of slope at the pressures identified aspreconsolidation pressures according to the standard casagrande construction. (D.Wesley, 2010)Keywords: residual soils, sedimentary soils, terzaghi’sconcept, oedometer 1. IntroductionProperties of soils are most important toconstruction.
If we understand main things on soil properties we can avoidconstruction failure. For this reason we have to study about soil properties. Thedifferences that have been identified between residual soils and sedimentarysoils are due to various reasons. Although these differences are underdiscussion, almost all the geotechnical concepts are developed by studyingsedimentary soils. (D.
Wesley, Behaviour and geotechnical properties of residual soils and allophane clays, 2009) This could becorrect if all soils that we need to deal with are sedimentary soils. Sincethat is not the case it is needed to identify the differences between these twosoil groups. When considering the settlement characteristics of soils thephenomena of consolidation plays a major role. So it is significant to analysethe consolidation test results accurately. The conventional method of analysingconsolidation test results has become irrelevant to residual soils due to thesedifferences. The facts causing those differences can be discussed as thevariations in the soil formation process, structure of soil particles, soilmineralogy and topographic influence2. Formation ProcessResidual soils are formed by chemicaland physical weathering from patent rock then lie on parent rock.
Sedimentaryrocks formed by chemical and physical weathering from parent rock thentransported by wind and river then deposited. But sedimentary soils areundergoing various additional processes such as erosion, hardening,cementation, primary consolidation, secondary consolidation, etc. (D.Wesley, Behaviour and geotechnical properties of residual soils and allophane clays, 2009) Figure 1: Soil formation process (D.Wesley, Behaviour and geotechnical properties of residual soils and allophane clays, 2009) Figure 2: Diagrammaticrepresentation of soil formation processes. (D.Wesley, Behaviour and geotechnical properties of residual soils and allophane clays, 2009) 3. Major difference betweensedimentary soil and residual soil3.
1 HomogeneityAfter formation of this soils. If weconsider homogeneous sedimentary soil is more homogeneous than residual soils.Because when forming of sedimentarysoils are undergoing various additional processes such as erosion, hardening,cementation, primary consolidation, secondary consolidation, etc. From thissedimentary became complex than residual soils. When sedimentary rocks areundergoing to various process, sorting process occurring on sedimentary rock sosedimentary rocks became homogeneous than residual soils.
(D.Wesley, Behaviour and geotechnical properties of residual soils and allophane clays, 2009) 3.2 Stress historyWhen formation of sedimentary soilsthere occurring loading and unloading process this process may create stresshistory on sedimentary soils.so we can identify over consolidated soil and normallyconsolidated soil. The abovediscussed concepts, stress history, virgin consolidation line, overconsolidation and normal consolidation cannot be applied to residual soils.
(D.Wesley, Behaviour and geotechnical properties of residual soils and allophane clays, 2009)3. Structure and MineralogyStructure and mineralogyare used for identify soil is sedimentary or residual (D.Wesley, Geotechnical Engineering in Residual soils, 2010).
when we consider thestructure there are 2 types micro structure and macrostructure Which can be seen by naked eyes called as macrostructure.Which cannot see naked eyes called microstructure.Mineralogy is depend onparent rocks. Most of the residual soils are micro – structured up to variousstates.
The existenceof such various microstructures induces a sense of different compressionbehaviours and presence of a yield pressure of soils. (D.Wesley, Geotechnical Engineering in Residual soils, 2010)4. Review of methodologiesThere are several methods are using fordetermine the settlement characteristics of residual soils. Experiments arecarried out for determine rate of consolidation to determine parameters such ascoefficient ofconsolidation (Cv), compression index (Cc), recompression index (Cr),coefficient of volume compressibility (mv), etc. For measure these parameterssettlement and time will be used.
Undisturbedsoil samples will be collected from the fields to conduct laboratory tests.Results will be compared with tests conducted on remolded soil samples.Consolidation tests will be carried out using both Oedometer apparatus and RoweCell apparatus (with pore pressure measurements). The sample size will bedifferent in these two tests.
Since the variation of soil characteristics ofresidual soils is larger within aselected small area, the representativeness of Oedometer sample isquestionable. The Rowe Cell apparatus will be used as the solution whichaccommodates larger, more representative samples. Fordetermine coefficient of consolidation. These methods are using· Taylor’s method· Casagrande method· velocity method · Hyperbola method (1) The Uzvs. Tv plot proposed by Terzaghi can be used to find Tv.Figure3: Variation of Uz with Tv and depth Figure 4: log time plot casegrande’s methodWe can calculate t50 fromthis graph. Then we can calculate Cv.Figure 5: root time versus settlement graph, Taylor’s methodFrom this graph we can calculate t90 from this we can calculate Cv.
Coefficient of volume compressibility (mv) (2) 4.1 Rowcell testAccording to Rowe and Barden (1966), thefirst consolidation cell that was used by Frontard in 1910, was a metalcontainer with a perforated base and was loaded by means of a piston. In 1919Terzaghi devised a single drainage cell which was called an Oedometer. It doesnot facilitate sufficient control over drainage and measurement of porepressure. Also it does not facilitate to test larger samples which is arequirement for testing non – uniform soils.
After further researches andmodifications, the Rowe cell was introduced as a new consolidation cell.Diaphragm loading, pore pressure measurements, horizontal drainage andminimized wall friction are the main features that were leading to presentdesign of Rowe cell. Load is applied by means of hydraulic pressure actingacross a rubber diaphragm in order to achieve uniformity of loading. (K.H.Head, 1985) Figure6: Basic configuration of the Rowe Cell Eight different drainage types can be performed byusing the Rowe cell as shown in Figure 7. Basically the four drainage types aresingle drainage, double drainage, inward drainage and outward drainage.
Rowe& Barden (1966) also noted that the type of loading, either flexible platenwith a uniformly distributed load (free stain) or rigid platen (equal strain)can be adapted according to the nature of sample to be tested. The importantfactor in all these arrangements is that full control of drainage, initial porepressure and back pressure can be achieved (K.H.
Head, 1985). Figure4: Different types of drainage and loading conditions in Rowe cellUsuallyproperties of residual soils are varying vastly within a small area and therepresentativeness of Oedometer sample has become questionable. Since Rowe cellapparatus accommodates much larger samples to be tested, adopting Rowe cellwould be an acceptable solution for the matter of representativeness. Pressureis directly applied via a hydraulic system which makes much accurate stressingother than stressing in terms of loads. Also the Rowe cell can perform eightdifferent loading and drainage types resulting this test more flexible thanOedometer test. (K.H.