Unstable zone of sand-silt mixture using statictriaxial testsN. J. SaharePost Graduate Student, (Geotechnical Engineering Department), College ofengineering Pune-411005, Maharashtra, India. Email: [email protected]
com, ContactDetails- 9404824292N. T. ChaudhariPost Graduate Student, (Geotechnical Engineering Department), College ofengineering Pune-411005, Maharashtra, India.AbstractPresentwork deals with the identification of unstable zone of sand with fines contentusing static triaxial tests.
A series of undrained monotonic triaxialcompression tests were conducted on saturated samples of clean sand withvariation in silt content as 0 %, 15 %, 25 % and 35 %. Total 24 tests were carriedout on specimens of 50 mm in diameter and 100 mm in height with three differentconfining pressures of 60 kPa, 120 kPa and 240 kPa. The specimens were preparedat 30 % and 50 % relative densities using moist tamping method of samplepreparation. It was observed that the limiting fines content and relativedensity played an important role in deciding the undrained behavior of amixture of sand and silt. Furthermore, it was observed that liquefactionresistance of soil decreased with an increase in silt content until thelimiting fines content was reached.
However, a further increase in the siltcontent beyond the limiting silt content increased the liquefaction resistance.Keywords:- sand-silt mixture, liquefaction,triaxial test, limiting fines content, unstable zone.1 IntroductionInstabilityhas been observed to occur for saturated loose sand under undrained conditions(Lade andPradel, 1990; Leong et al.
, 2000) and for saturatedmedium to dense sand under strain-controlled conditions (Chu, 1991; Chu et al.,1993; Chu & Leong, 2001). The term instability refers to a behaviour inwhich large plastic strains are generated rapidly owing to the inability of asoil element to sustain a given load or stress. In recent years, instabilityhas been considered as one of the failure mechanisms that lead to flow slidesor collapse of granular soil slopes in a number of case studies (e.g. Kraft etal., 1992; Lade, 1993; Hight et al.
, 1999; Olson et al., 2000). It has beenestablished by Lade & Pradel (1990) that instability occurs when the stressratio at the onset of instability is above the instability line.Thebehavior of silty sandy soils such as hydraulic fills, landfills or alluvialdeposits not clearly known during earthquake. Therefore, a thorough understandingof unstable behavior of silty sand is needed.
The main focus of the presentwork is to conduct static triaxial tests to study undrained behavior of sandsilt mixture. Isotropically consolidated undrained triaxial tests are performedon 50 mm × 100 mm sample size with varying fines content. Tests are conductedat two different relative densities at three confining pressures.2 Experimental InvestigationThe clean sand used in theseexperiments was silicasand obtained locally and has been classified as SP accordingto the unified soil classification system (USCS). Silt used in this study is non plastic and obtained from quarry dust.Figure 1 indicates grain size distribution curves of clean sand-silt andvarious sand-silt mixture.
The index properties of clean sand and silt areshown in Table 1.Table 1: Index properties of sand and silt G D50 ?max ?min emax emin Cu Cc Sand 2.416 0.28 15.
16 13.98 0.745 0.609 2.24 0.
85 Silt 2.751 0.06151 15.70 12.65 1.175 0.753 3.6714 1.
1633 The tests are conducted on clean sand and mixture of sand-silt at 30% and 50% relative densities for three confining pressures of 60 kPa, 120 kPa and 240 kPa with varying silt content as 15%, 25% and 35%. Total twenty four tests wereconducted on 50 mm ×100 mm sample size where 50 mm is diameter and 100 mm isthe height of the sample (H/D = 2). Detail test program is as shown in Table 2.Fig. 1: Soil gradation curveTable 2: Test program Soil Fines Content (%) Relative Density (%) Confining Pressure (kPa) No. of tests Sand 0 30 60, 120, 240 3 50 60, 120, 240 3 15 30 60, 120, 240 3 50 60, 120, 240 3 25 30 60, 120, 240 3 50 60, 120, 240 3 35 30 60, 120, 240 3 50 60, 120, 240 3 Total 24 Moist tamping method wasadopted to prepare a sample of clean sand and mixture of sand-silt to performtriaxial tests on the cylindrical specimen of size 50 mm × 100 mm. In thismethod, a known quantity of soil for achieving particular density was mixedwith 5 % of water by weight added in soil and placed in a split mould inlayers. In order to achieve particular density tamping was done for each layerwith the help of a hammer.
Fig.2 shows details of triaxial test instrument.3 Results and Discussion3.1 Clean sand specimenFig. 3 shows thedeviator stress-strain behaviour of clean sand for 60 kPa, 120 kPa and 240 kPaconfining pressures for 30% relative density. It was observed that for cleansand specimen, stress-strain relation becomes curved at very small strains andachieve a peak at a strain of about 3% for effective confining pressure 120 and240kPa. The resistance of soil then gradually decreases until this test wasarbitrarily stopped at a strain of 20%.
It is also seen that as the confiningpressure increases from 60 kPa to 240 kPa peak value of deviator stressincreases from 66.9993 to 262.1799 kPa and steady state has been achieved atlarge percentage of strain (20% strain). Fig.
3: Deviator stress vs axial strain response of clean sand specimen (Relative density = 30%) Fig. 4: Excess pore pressure vs axial strain response of clean sand specimen (Relative density = 30%) Typical graph ofincrease in excess pore pressure with axial strain during shearing is shown inFig. 4. For clean sand it is observed that peak value of pore pressure wasreached within strain of about 2% to 3% and remain constant at large percentageof strain (about 15% axial strain) for all confining pressures. Further it isalso observed that as confining pressure increases, peak pore pressure also increases.P’-q plot forclean sand specimen for all confining pressures are shown in the Fig. 5.
Forall confining pressures, contractive behavior has been observed.Fig. 5: p’ vs q response of clean sand specimen(Relative density = 30%)Similar observation have been observed for specimensof clean sand with relative density 50%.3.
2 Specimen ofmixture of sand-siltFig. 6 showsdeviator stress vs axial strain of specimen of sand with 15% silt contentprepared at relative density of 30% and for all confining pressures. It isobserved that deviator stress increases as confining pressure increases from 60kPa to 240 kPa. However peak value of deviator stress was varied from 62.0795kPa to 116.6601 kPa. All the specimens achieve steady state at the large strain.Typical graph ofincrease in excess pore pressure with axial strain during shearing for sandwith 15 5 silt content is shown in Fig.
7. It is observed that peak value ofpore pressure was reached at a strain of about 5% and remain constant at largepercentage of strain for all confining pressure. Fig. 6: Deviator stress vs axial strain response of specimen of sand with 15% silt (Relative density = 30%) Fig.
7: Excess pore pressure vs axial strain response of specimen of sand with 15% silt (Relative density = 30%) P’-q for sandwith 15% silt content prepared at relative density of 30%for all confiningpressure is as shown in Fig. 8. It is observed that all samples showcontractive behavior. Excess pore pressure developed in sand with 15% silt ishigher than that of clean sand, whereas peak deviator stress developed in sandwith 15% silt is less than that of clean sand.Fig. 8: p’ vs q response of specimen of sand with15% silt (Relative density = 30%)Similar observations were madefor 50 % relative density of sand with 15% siltas well as for loose and mediumdense specimens of sand with varying silt content of 25% and 35%.3.3 Effect ofconfining pressureThe variationsin peak deviator stress and excess pore pressure generation with increase in confiningpressure for clean sand as well as sand with silt content of 15%, 25% and 35%.
It is observed that as the confining pressure increases deviator stress as wellas peak pore pressure increases for all the cases studied.Similar behaviour is observed for specimens prepared with 50 % relativedensity. Howeverpeak value of deviator stress for 50 5 relative density is higher than that of30 % relative density.3.4 Effect ofsilt contentSmall fines content can affect undrained behaviour of clean sandconsiderably, therefore study is needed to check the effect of fines content onundrained behaviour. The effect of fines content on peak deviator stress ofspecimens prepared at loose and medium dense state (i.e.
relative density 30 and 50%) with increase in silt content from 15%, 25% and 35%at various effective confining pressures is as shown in Figs. 9 and 10.As the fines content increase, peak deviator stressdecreases up to limiting fines content after that peak deviator stressincreases. Addition of fines reduces value of peak deviator stressbefore limiting fines content because fines do not participate in capacity ofcarrying load. Addition of fines beyond the point of limiting fines contentchanges behaviour of soil from sand dominated to silt dominated. Formaintaining same relative density, soil needs to be compacted densely inspecimen with fines content more than limiting fines content.
So, load carryingcapacity increases for sand with 35% silt content. Similar behaviour isobserved for specimens prepared at medium dense state (50 %). (Fig. 10). Fig. 9: Peak deviator stress vs silt content (Relative density = 30%) Fig.
10: Peak deviator stress vs silt content (Relative density = 50%) 3.5 ResidualstrengthIn order tocheck slope stability of liquefied soil masses knowledge of residual strengthis important. When loose and medium dense sandy soils are subjected toundrained loading beyond the point of peak strength, the undrained shearstrength declines to near constant value over large deformation. Conventionallythis strength is called the undrainedresidual strength. Castro and Polous (1985) determined theresidual strength of silty sand using following equation: …..eq 1 Where, Su :residual strength ….
.eq 2 ?s : inter granular friction angle …..eq 3 qs : deviator stress (at 20% strain) 3.6 UnstablezoneIn order to classify the liquefaction behavior of soil,Pathak and Dalvi (2011) have established unstable zone plotted between Kfline and peak pore pressure line on effective stress path plot. Unstable zonehas been obtained by plotting effective stress path for all three confiningpressures for each relative density for each percentage of fines content.
Kfline is plotted using p’ and q values corresponding to peak deviator stressvalueand line is joined with origin. p’ and q values arecalculated by peak effective major principal stress (?1′) and correspondingeffective minor principal stress (?3′) using following equations:q = ?1′- ?3′ …..eq 4 Where, ?1′ :effective major principal stressp’ = …..eq 5 ?3′ : effective minor principal stressPeak porepressure line is plotted using p’ and q values corresponding to peak pore pressure point.
Unstable zone for clean sand and clean sand with 15%, 25% and 35% silt is shown in Fig. 11(a),(b), (c) and (d)respectively. It is observed that as fines content increase upto limiting finescontent,unstable zone becomes wider. Further increase in fines contentbeyond limiting fines content, narrows down the unstable zone. This may be aneffect of excess pore water pressure, as fines content increased to limitingfines content (25%) excess pore water pressure increases. (a) (b) (c) (d) Fig. 11: Variation of unstable zone with increase in fines content at relative density 30% Similar trend of unstable zoneobtained for specimen prepared at relative density 50% as that at relativedensity 30%.
4 ConclusionFrom the present study following conclusions are made:As the confining pressure increases peak deviator stress and excess pore water pressure increases for all the specimens.Peak deviator stress increases as the relative density increases at all the confining pressures.For both loose and medium dense specimens, peak deviator stress decreases as fines content increases up to limiting fines content. Further increase in fines content increases the peak deviator stress value.As the confining pressure increases, residual strength also increases.Residual strength decreases as fines content increases up to limiting fines content and after that it increases as fines content increase.Unstable zone of sand-silt mixture widens as fines content increases up to limiting fines content. Further increase in fines content narrows down the unstable zone for both loose and medium dense specimens.
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