Effects of Fines on Undrained Behaviour of Sands. Misko Cubrinovski 1 and Sean Rees 2

Transcription

1 Effects of Fines on Undained Behaviou of Sands Misko Cubinovski 1 and Sean Rees 2 1 Associate Pofesso, Depatment of Civil and Natual Resouces Engineeing, Univesity of Cantebuy, New Zealand; misko.cubinovski@cantebuy.ac.nz 2 PhD Candidate, Depatment of Civil and Natual Resouces Engineeing, Univesity of Cantebuy, New Zealand; sd4@student.cantebuy.ac.nz ABSTRACT: A seies of monotonic and cyclic tiaxial tests wee pefomed on a sand with fines souced fom Chistchuch, New Zealand. The sand was sieved and then mixed to give thee soils with diffeent fines contents. The undained tests wee used to examine the effects of fines on the stain softening behaviou unde monotonic loading and liquefaction esistance in cyclic loading. Two efeence states wee used as a basis fo evaluation of the effects of fines: the elative density and the steady state line within the state-concept famewok fo sand chaacteization. The addition of fines to the sand base caused downwad movement of the steady state line in the D -p' plane (e-p' plane), and this effectively inceased the potential fo stain softening o flow defomation. Samples pepaed at an identical elative density showed deceasing cyclic stength with inceasing fines content. Convesely, samples at an identical initial state elative to the steady state line showed inceasing cyclic stength with the fines content. INTRODUCTION Case histoies fom stong eathquakes and laboatoy studies show clealy that fines have significant influence on both monotonic and cyclic undained behaviou of sands (Lade and Yamamuo, 1997; Thevanayagam, 1998; Polito and Matin, 21). These effects ae paticulaly elevant when evaluating the potential fo stain softening o flow defomation duing monotonic loading and liquefaction esistance in cyclic loading. When compaing the liquefaction esistance of fines-containing sand with that of a clean sand, one encountes the poblem of a lack of pope basis fo compaison. Fo example, in the conventional pocedue fo liquefaction evaluation based on the penetation esistance (Youd and Idiss, 1998), finescontaining sands show geate liquefaction esistance as compaed to clean sands. In these citeia, howeve, the penetation esistance, e.g. nomalized SPT blow count N 1, is used as a basis fo compaison. Since the penetation esistance is affected by the gain size and fines content, these citeia do not show only the effects of fines on the liquefaction esistance but athe they depict the combined effects of fines on Page 1

2 σ 1 σ 3 Shea stess, 2 σ 1 σ 3 2 Limited flow No-flow Shea stess, No-flow Limited flow Flow Flow Mean effective stess, p' Shea stain, FIG. 1. Undained behaviou of sand in monotonic tests ε ε 1 3 both penetation esistance and liquefaction esistance. Thus, it is not staightfowad fom these chats whethe the fines pe se incease the liquefaction esistance o not. In ode to investigate the effects of fines in moe detail, a seies of undained tiaxial tests wee conducted on natual sands ecoveed fom a site in Chistchuch, New Zealand. In the intepetation of the effects of fines on undained behaviou both the elative density of the soil and the initial state of the soil elative to its steady state line (state-concept intepetation) wee used as a basis fo compaison. EFFECTS OF FINES ON UNDRAINED MONOTONIC BEHAVIOUR Backgound Thee types of esponse chaacteize undained behaviou of sands unde monotonic sheaing, as outlined in Figue 1. Vey loose sand shows fully contactive behaviou and stain softening until the steady state o citical state is eached at lage stains. The stain-softening phase of the esponse esembles flow-type behaviou and theefoe it is often efeed to as flow o flow defomation. If the density of the sand is somewhat highe, then the stain softening takes place ove a limited ange, and limited flow occus followed by stain hadening and steady state of defomation. Finally, in the case of medium dense and dense sands, a stain-hadening esponse associated with stong dilation towads the steady state is obseved. The undained behaviou of a given sand changes within the thee types of esponse outlined in Figue 1 and shows diffeent degees of stain softening and stain hadening depending upon the density of the sand and confining stess at the initial state, pio to sheaing. It is possible, theefoe, to distinguish between initial density-stess states that ae associated with stain-softening behaviou (flow defomation) and those entiely exhibiting stain-hadening behaviou in monotonic undained loading. In this context, Ishihaa (1993) defined a so-called initial dividing line (ID-line) that maks the bounday between flow and no-flow conditions in the e- p' diagam (void atio-mean effective stess diagam). As illustated in Figue 2, samples lying above the ID-line ae contactive and exhibit flow-type behaviou while all of the initial density-stess combinations lying below the ID-line exhibit stain-hadening behaviou upon monotonic sheaing unde undained conditions. Page 2

3 Void atio, e e o Flow Limited flow FLOW Initial dividing line Steady state line No-flow NO-FLOW Mean effective stess, FIG. 2. Chaacteization of monotonic undained behaviou based on state concept p' This illustates the essence of the state-concept intepetation of sand behaviou whee a paticula type of behaviou is associated with the initial e-p' state of the soil elative to a efeence state. As depicted in Figue 2, the pojection of the steady state line in the e-p' plane is elatively close to the initial dividing line, and theefoe, it could be used as its appoximation when identifying whethe the sand behaviou upon monotonic undained sheaing is of stain-softening o stain-hadening type. Two featues of the steady state line make this line a paticulaly suitable efeence state. Fistly, the steady state line is unique fo a given sand, and theefoe, the position of the steady state line is uniquely defined in the e-p' plane. Secondly, any soil sample subjected to monotonic sheaing will eventually each the steady state of defomation; in othe wods, the ultimate e-p' state will end up on the steady state line iespective of the initial e-p' state of the soil. Hence, the ultimate level of dilation/contaction o excess poe wate pessue development in monotonic undained sheaing is pactically defined by the initial e-p' state of the soil elative to the steady state line. All initial states above the steady state line will show contactive behaviou o positive excess poe wate pessue. The contactive tendency o excess poe pessue level will incease with the distance fom the steady state line. Note that initial states with a void atio geate than e o will end up at p' = and exhibit zeo steady state stength upon monotonic undained loading. The position and slope of the steady state line of sandy soils ae affected by a numbe of factos including the gain-size chaacteistics, fines content and gain shape of sands. Figue 3 summaizes the effects of gain-size composition and fines content on the steady state line of sandy soils. Hee, the cicula symbols show published data on 52 sandy soils obtained fom tiaxial compession tests on econstituted samples of clean sands and sands with non-plastic fines (Cubinovski and Ishihaa, 2). The figue also includes ecently summaized data by Jeffeies and Been (26) and esults fo the thee soils tested in this study. Note that D o indicates the elative density coesponding to e o (see Figue 2) and hence defines the theshold elative density below which soils exhibit a zeo steady state stength. Figue 3 shows that the position of the steady state line in the e-p' plot o D -p' plot is Page 3

4 Relative density, D,o (%) Cubinovski & Ishihaa (2) e - e max min e - e max o D = o e - e x 1 max min D,o (%) 1 5 * Open symbol = clean sand, Solid symbol = F > 5% C -5 Cubinovski & Ishihaa (2) Jeffeies & Been (26) This study Fines content, F (%) C FIG. 3. Position of the steady state line of sandy soils as a function of: Void atio ange, (e max -e min ); Fines content, F C elated to the gain-size composition, as epesented by the void atio ange (e max - e min ), and fines content, F C. As the fines content o (e max -e min ) inceases, the steady state shifts downwads in the e-p' plot o the elative density D o coesponding to e o, defined in Figue 2, inceases. This implies that, at a given elative density, loose samples containing fines ae moe contactive and show geate potential fo stain softening than clean sands. This featue of the undained behaviou of sands was futhe investigated by testing samples of Chistchuch soils with diffeent fines content using conventional tiaxial compession tests. Mateials Tested Thee soils wee tested in this study all based on a host sandy soil obtained fom a single laye at the Fitzgeald Bidge Avenue site in Chistchuch, New Zealand. We will call the host mateial as well as the soils deived fom it Fitzgeald Bidge Mixtue (FBM) and will distinguish between them based on the fines content, as follows. The host soil was ceated by mixing undistubed samples obtained fom the Fitzgeald Avenue Bidge site. The initial soil mixtue ceated in this way was a silty sand containing appoximately 1% fines by weight (), with the fines detemined to be non-plastic. This mateial was dy-sieved afte testing to isolate the clean-sand base and ceate the second mateial that happened to have 1% fines by weight (). Finally, the thid soil was poduced by eintoducing the fines to the clean-sand base, and ceating a silty sand with appoximately 3% fines by weight (FBM-3). Mateial popeties of the tested soils ae summaised in Table 1 while thei gain-size cuves ae shown in Figue 4. Note that e max and e min Table 1. Popeties of the tested soils Mateial F C (%) G s e max e min e max - e min FBM Page 4

5 % Passing FBM Paticle size (mm) FIG. 4. Paticle size distibution of the tested soils wee obtained fo all soils using the Bitish standad pocedues fo minimum and maximum densities of sands (BSI, 22). Wheeas one should be cautious when applying these pocedues to soils with elatively high fines contents, Cubinovski and Ishihaa (22) pesented evidence based on 3 natual soils that the pocedues stipulated by the Japanese Geotechnical Society fo clean sands (JGS, 2) povide easonably consistent e max and e min values fo sands with a fines content of up to 3%. Each of the thee soils equied e-use duing the monotonic and cyclic tiaxial testing. This was due to the limited amount of mateial available. The tested soil was died ovenight at 14ºC and then gently gound to allow a etun to a homogenous mateial. This soil was placed in a sepaate containe and only e-used once the oiginal mateial mixtue had been exhausted. An effot was made duing these steps to etain as much mateial (paticulaly fines) as possible. Paticle size distibutions taken of the e-used mateial vaied insignificantly fom the oiginal gain-size cuves shown in Figue 4. Results fom Monotonic Tests In total, 28 undained monotonic tests in tiaxial compession wee pefomed on the thee mateials ceated fom the Fitzgeald Bidge soils. All specimens wee pepaed using the moist-tamping method by fistly pepaing the sand to a moistue content of appoximately 9%, placing the soil in 6 equal layes, and applying an appopiate amount of tamping fo the taget specimen density. The specimens had conventional dimensions of appoximately 5mm in diamete and 1mm in height. All specimens had CO 2 pecolated though them befoe satuation fo vaying amounts of time, with the amount of time inceasing with the fines content. The samples wee isotopically consolidated to an initial mean effective stess of 1 kpa, except fo two tests in which p' = 2 kpa was used. The tests wee conducted in a stain-contolled manne at an axial stain ate of.3 %/min. Figue 5 shows stess-stain cuves and effective stess paths fo each of the thee soils tested. In this case, all thee specimens exhibited eithe limited flow o flow defomation. The stess-stain elationships shown in Figue 5 give good indication of how fines can affect undained behaviou of sand. All specimens peak at appoximately the same stess level, aound 8kPa to 9kPa, but the steady state Page 5

6 Deviato stess, q (kpa) D = 28% e =.847 D = 5% e =.892 FBM-3 D = 5% e = Axial stain, ε (%) a Deviato stess, q (kpa) FBM-3 D = 5% e =.693 D = 28% e =.847 D = 5% e = Mean effective stess, p' (kpa) FIG. 5. Stess-stain cuves and effective stess paths obseved in monotonic undained tests on sands with diffeent fines content stesses vay significantly. The clean sand mixtue () has the highest stess at steady state, being close to q = 55kPa, whilst having the lowest elative density of D = 5%. The specimen pepaed using the FBM-3 mixtue has the lowest stess at steady state, with q = 5kPa, yet is the densest specimen with a elative density of D = 5%. These esponses demonstate that the addition of fines to the clean sand base inceased the potential fo flow defomation duing undained monotonic loading. The sand with 3% fines (FBM-3) shows geate dop in the shea stess duing the stain softening at a elative density of D = 5% than the clean sand () at a elative density of D = 5%. This esult is in ageement with findings fom pevious studies summaized in Figue 3. Anothe way to conside these effects is to depict the position of the steady state line fo each of the tested soils in the e-p' plot o even moe suitably in the D -p' plot, as shown in Figue 6. Hee, the steady state condition of a given test is defined by the Void atio, e FBM-3 Relative density, D (%) FBM Mean effective stess, p' (kpa) Mean effective stess, p' (kpa) FIG. 6. Effects of fines on the position of the steady-state line: e-p' plane; D -p' plane Page 6

7 state at 4% stain. The downwad shift in the steady state line as the fines content inceases can be clealy seen in Figues 6 and ; the steady state line fo is at elative densities of % to 1% while the coesponding elative density fo FBM-3 is 6% to 7%. In view of the state-concept intepetation of sand behaviou intoduced peviously, the downwad shift in the steady-state line with inceasing fines content in effect implies that fines tend to incease the flow potential and make the sand moe contactive duing undained monotonic loading, at least when the elative density is used as a basis fo compaison. EFFECTS OF FINES ON UNDRAINED CYCLIC BEHAVIOUR Results fom Cyclic Tests In addition to the monotonic tests, undained cyclic tiaxial tests wee pefomed on the thee tested soils. In total 43 cyclic tests wee conducted including thee seies of tests on, fou seies of tests on and two seies of tests on FBM- 3. Each of these test seies was used to detemine the liquefaction esistance cuve fo a selected elative density of the espective soil. All specimens wee pepaed using the peviously descibed moist-tamping technique and wee isotopically consolidated to an initial mean effective stess of p' = 1 kpa. Figue 7 shows typical effective stess path and stess-stain esponse obseved in one of the cyclic tests. Fo each soil, fou o five specimens wee pepaed at a nealy identical elative density (void atio), and then the specimens wee subjected to cyclic stesses at diffeent cyclic stess atios (CSR) allowing liquefaction esistance o cyclic stength cuves to be geneated fo each test void atio (elative density). Liquefaction esistance cuves detemined in this way ae shown in Figue 8 whee the cyclic stess atio is plotted against the numbe of cycles equied to cause 5% double amplitude stain. As expected, fo each soil, the liquefaction esistance inceases with the elative density of the soil. The effects of fines on the liquefaction esistance ae not appaent in these plots. In ode to scutinize these effects, in what follows the liquefaction esistance of soils, and FBM-3 is examined using two efeence states: the elative density of the sand and the steady state line espectively. 1 D = 62%, CSR = D = 62%, CSR =.334 Deviato stess, q (kpa) 5-5 Deviato stess, q (kpa) Mean effective stess, p' (kpa) Axial stain, ε a (%) FIG. 7. Stess path and stess-stain cuve obseved in a cyclic test on Page 7

8 Cyclic Stess Ratio, CSR = q/2p'.6 c D = 5-1% D = 28-31% D = 59-62% Numbe of cycles to 5% DA stain, N C Cyclic Stess Ratio, CSR = q/2p' c (c) c CSR = q/2p' Numbe of cycles to 5% DA stain, N C D = 36-4% D = 44-46% D = 57-6% D = 67-68% Numbe of cycles to 5% DA stain, N C FBM-3 D = 5-51% D = 68-7% FIG. 8. Liquefaction cuves fo the tested soils at diffeent elative densities: ; ; (c) FBM-3; The cyclic stength data is e-plotted in Figue 9 whee cyclic stess atios at 5 cycles and 15 cycles, CSR Nc=5 and CSR Nc=15 espectively, ae plotted against the elative density of the soil. These plots allow compaison of the liquefaction esistance of soils with diffeent fines content, at the same elative density. Clealy, at any elative density within the ange consideed, the clean sand shows highe cyclic stength than the sand containing 1% o 3% fines. In othe wods, when the elative density is used as a efeence fo compaison, the liquefaction esistance of FBM soils deceases with inceasing fines content. The state concept appoach has aleady been shown to be useful in descibing the undained monotonic behaviou of sand. Paametes such as the state paamete (Been and Jeffeies, 1986; Roscoe and Pooooshasb, 1963) and state index (Ishihaa, 1993) quantify the initial state of the soil elative to the steady state line, whilst taking into account combined effects of density and stess on sand behaviou. Theefoe, it is inteesting to examine how this concept woks in descibing the undained cyclic behaviou of sands, and in paticula the effects of fines on the liquefaction esistance. The most staightfowad way to assess the suitability of the state concept in Page 8

9 .4.4 CSR at N C = FBM-3 CSR at N C = 5 CSR at N C = FBM-3 CSR at N C = Relative density, D (%) Relative density, D (%) FIG. 9. Cyclic stess atio equied to cause 5% DA stain in 5 cycles and 15 cycles as a function of elative density descibing undained cyclic behaviou is to compae the cyclic esponse fo initial states lying on the steady state line itself. Both the state index (I s ) and the state paamete (ψ) have a constant value along the steady state line, I s = 1 and ψ = espectively, and this suggests that the monotonic behaviou fo all initial states lying along the steady state line should be simila. This is eadily appaent fo an undained test because in such a case the initial state pio to sheaing and the ultimate state achieved upon monotonic sheaing will be identical. In othe wods, the poe pessue due to the contactive tendency and dilation will be identical when a sample with an initial state at the pojection of the steady state line is subjected to undained monotonic sheaing. Figue 1 shows the cyclic stength cuves fo the thee tested soils, fo specimens with initial states on the steady state line. Hee, all samples had an initial mean Cyclic Stess Ratio, CSR Initial e-p' states at the Steady State Line FBM-3 D = 68-7% I s = ψ = D = 5-1% I = D s = 36-4% ψ = I s = ψ = Numbe of cycles to 5% DA Stain, N C FIG. 1. Cyclic stength cuves fo specimens with initial states on the steady state line Page 9

10 CSR / CSR Nc= D = 59-62% I s = ψ = D = 28-31% I s = ψ = D = 5-1%.5 I s = ψ = Numbe of cycles to 5% DA Stain, N C CSR / CSR Nc= FBM-3 D = 68-7% I s = ψ = D = 5-51% I s = ψ = Cycles to 5% DA Stain, N C FIG.11. Cyclic liquefaction cuves fo and FBM-3 nomalised by CSR Nc=15 effective stess of p' = 1kPa and quite diffeent elative density of D =5-1%, 36-4% and 68-7%, fo, and FBM-3 espectively, in accodance with the position of the steady state line of these soils shown in Figue 6b. Figue 1 shows clea effects of fines on the liquefaction esistance of samples having identical initial states elative to the steady state line. Thee is a small incease in the cyclic stess atios fo the sand containing 1% fines () as compaed to the clean sand (), and futhemoe a significant incease in CSR as the fines content eaches 3% (FBM-3). Clealy, the liquefaction esistance of FBM soils inceases with the fines content when initial states on the steady state line o moe geneally when the state concept is used as a basis fo compaison. As the specimens wee all pepaed using the same technique, effects of fabic ae most likely not significant and any diffeences in fabic would be due to the effects of fines content on fabic. In ode to examine the effects of elative density, state of the soil and fines content on the shape of the liquefaction esistance cuve, the cyclic stess atio was nomalized by the CSR at 15 cycles (CSR Nc=15 ), as shown in Figue 11 fo and FBM-3. The clean sand () cuves show an incease in the liquefaction esistance with inceasing density o coesponding change in the state measue, in the egion below 1 cycles. The nomalised cuves fo (not shown) wee simila to the cuves. In the case of FBM-3, howeve, the effects of density o state of the soil on the shape of the cuve ae not ponounced, though one should acknowledge that the ange of densities/states consideed was elatively small fo this soil. Futhe testing both at low densities high above the steady state line and at high densities well below the steady state line ae needed to quantify the effects of fines in this egad. CONCLUSIONS A seies of monotonic and cyclic tiaxial tests on FBM soils wee used to investigate the effects of non-plastic fines on undained behaviou of sand. The key findings can be summaized as follows: Page 1

11 The steady state line shifts downwads in the e-p' (D -p') plot as fines ae added to a clean-sand base. This effectively inceases the flow potential of the finescontaining soil, as thee ae moe initial states in the e-p' plane above the steady state line showing contactive behaviou associated with stain softening. When the elative density (D ) is used as a basis fo compaison of cyclic behaviou, the cyclic stength deceases with inceasing fines content. At a given elative density, the liquefaction esistance of the clean sand () was the highest while the sand with 3% fines (FBM-3) showed the lowest esistance. (c) The liquefaction esistance of FBM soils inceases with the fines content when initial states on the steady state line o moe geneally the state concept is used as a basis fo compaison. (d) The shape of the cyclic stength cuve of FBM soils is affected by the elative density and initial state of the soil. The extent of these effects, in tun, depends on the fines content. REFERENCES Been, K., and Jeffeies, M. G. (1985). "A state paamete fo sand." Geotechnique. 35 (2): BSI (22). BS :199 Methods of tests fo soils fo civil engineeing puposes. Pat 4: Compaction-elated tests. Cubinovski and Ishihaa (2). Flow potential of sandy soils with diffeent gain compositions," Soils and Foundations, 4 (4): Cubinovski and Ishihaa (22). Maximum and minimum void atio chaacteistics of sands, Soils and Foundations, 42 (6): Ishihaa, K. (1993). "Liquefaction and flow failue duing eathquakes." 33-d Rankine Lectue, Geotechnique. 43 (3): Japanese Geotechnical Society (2). Test methods fo minimum and maximum densities of sands, Soil Testing Standads. (in Japanese). Jeffeies, M. G. and Been, K. (26). Soil liquefaction: a citical state appoach. Taylo&Fancis, New Yok. Lade, P.V. and Yamamuo, J. (1997). Effects of nonplastic fines on static liquefaction of sands. Canadian Geotechnical Jounal, 34, Polito, C.P. and Matin II J.R. (21). Effects of nonplastic fines on the liquefaction esistance of sands. ASCE Jounal of Geotechnical and Geoenvionmental Engineeing, 127 (5), Roscoe, K. H., and Pooooshasb, H. B. (1963). "A fundamental pinciple of similaity in model tests fo eath pessue poblems." Poceedings of the 2nd Asian Regional Confeence on Soil Mechanics, Vol. 1, Tokyo: Thevanayagam, S. (1998). Effects of fines and confining stess on undained shea stength of silty sands. ASCE Jounal of Geotechnical and Geoenvionmental Engineeing, 124 (6), Youd, T.L. and Idiss, I.M. (1998). "Liquefaction esistance of soils: summay epot fom the 1996 NCEER and 1998 NCEER/NSF Wokshops on Evaluation of LiquefactionResistance of Soils " Jounal of Getoechnical and Geoenvionmental Engineeing, 127 (4): Page 11