CYCLIC TRIAXIAL TESTING OF FULLY AND PARTIALLY SATURATED SOIL AT SILCHAR

Transcription

1 4 th Intenational Confeence on Eathquake Geotechnical Engineeing June 25-28, 27 Pape No CYCLIC TRIAXIAL TESTING OF FULLY AND PARTIALLY SATURATED SOIL AT SILCHAR Sanjay PAUL 1, Ashim Kanti DEY 2 ABSTRACT Silcha is located in the southen pat of the state of Assam at Noth East India as shown in figue 1. It is seismically bounded by thee faults and hence, eathquakes of magnitude 5. on Richte scale and above ae vey fequent in this pat of Assam. Although, the soil at Silcha is pedominantly clayey in natue, layes of silty sand ae available nea banks of Baak Rive and its tibutaies. These layes contain wate and ae susceptible to liquefaction. It is known that the natue and distibution of eathquake damage is stongly influenced by the dynamic popeties of soil. The dynamic popeties include vaiation of shea modulus and damping atio at diffeent stain levels caused by a dynamic load. A study has been made on detemination of shea modulus and damping atio of Silcha soil unde diffeent stain levels using cyclic tiaxial testing appaatus. It is obseved that, the shea modulus deceases and damping atio inceases with incease in stain level. Relationship of modulus eduction with effective confining pessue, elative density and stain levels have been obtained. The liquefaction analysis of Silcha soil has also been caied out. Keywods: Shea modulus, Damping atio, Stain level, Liquefaction, Cyclic tiaxial testing appaatus INTRODUCTION The Noth-East India is located in seismic zone V, the highest zoning in the seismic zoning map of India and is expeienced with fequent eathquakes; almost all of them ae above magnitude 5. on Richte. Silcha is seismically bounded by Dauki fault in the noth-west, Sitakunda Teknaf fault in the south-west and Indo-Myanma subduction zone in the east. Although, the soil at Silcha is pedominantly clayey in natue, layes of silty sand ae available nea banks of Baak Rive and its tibutaies. These layes contain wate and ae susceptible to liquefaction. Thin layes of satuated fine sand have also been found at vaious depths in the hilly aeas which actuate sliding of ovelying layes duing eathquakes. Fo example, wide spead liquefaction occued afte Cacha eathquake in 1984 due to which many fissues and cacks wee fomed in the flat gound followed by sand and wate ejection. A two minute temo was felt in the ealy hous on 6 th August, 1988 (Indo-Myanma bode eathquake, 1988) and lage scale landslides, fissues, sand boiling wee epoted. It is known that the natue and distibution of eathquake damage is stongly influenced by the dynamic popeties of the soil. The dynamic popeties include vaiation of shea modulus and damping atio at diffeent stain levels caused by an eathquake. 1 Reseach Schola, Depatment of Civil Engineeing, National Institute of Technology, Silcha, India, sanjaypaul_76@yahoo.com. 2 Assistant Pofesso, Depatment of Civil Engineeing, National Institute of Technology, Silcha, India, akdey@nits.ac.in.

2 REVIEW OF LITERATURE The natue and distibution of eathquake damage is stongly influenced by the esponse of soils to cyclic loading. Eathquake loading is andom in natue and the andom loading may be expessed as an algebaic sum of diffeent cyclic loadings. This esponse is contolled in lage pat by the mechanical popeties of the soil. Fo many impotant poblems paticulaly those detemined by wave popagation effects, only low levels of stain ae induced in the soil. Fo othe impotant poblems, such as those involving the stability of masses of soil, lage stains ae induced in the soil. The behavio of soils subjected to dynamic loading is govened by what have come to be populaly known as dynamic soil popeties. Following types of dynamic loading may aise: (a) Hamonic Loading; (b) Peiodic Loading; (c) Random Loading; and (d) Tansient Loading. Since the eathquake loading is of andom in natue, it becomes necessay to detemine the equivalent numbe of significant unifom stess cycles of hamonic natue. The tem equivalent means that the effect of the iegula stess histoy on a given soil deposits should be same as the unifom stess cycles. Suppose τ max is the maximum shea stess in an iegula stess-time histoy of an eathquake loading. Based on laboatoy data, it has been found (with a easonable degee of accuacy) that the aveage equivalent unifom shea stess τ av is about 65 % of the maximum shea stess (Afte Das, 1983). G tan τ c τ γ c G sec γ Figue 1. Location Map of Silcha Figue 2. Secant shea modulus (G sec ) (Afte Kame, 1996) Dynamic Soil Popeties The majo soil popeties that need to be ascetained in soil dynamics and geotechnical eathquake engineeing ae: (a) Shea stength against a pemissible stain; (b) Dynamic moduli, Young s modulus, shea modulus, bulk modulus, and constained modulus; (c) Poisson s atio; (d) Damping; and (e) Liquefaction paametes: cyclic sheaing stess atio, cyclic defomation, and poe-pessue esponse. Out of these, two impotant popeties namely Shea modulus and Damping atio ae defined as follows:

3 Shea Modulus Unde a symmetical cyclic loading, a typical soil element is subjected to stess-stain defomations in the fom of a hysteesis loop as shown in figue 2. In geneal tems, two impotant chaacteistics of the shape of a hysteesis loop ae its inclination and its beadth. The inclination of the loop depends on the stiffness of the soil, which can be descibed at any point duing the loading pocess by the tangent shea modulus, G tan. G tan vaies thoughout a cycle of loading, but its aveage value ove the entie loop can be appoximately by the secant shea modulus G τ = c sec (1) γ c whee τ c and γ c ae the shea stess and shea stain amplitudes, espectively. Thus G sec descibes the geneal inclination of the hysteesis loop. Damping Ratio The beadth of the hysteesis loop as shown in figue 2 is elated to the aea, which as a measue of enegy dissipation, can be conveniently descibed by the damping atio W 1 A D = (2) D loo p = 2 4πW S 2π Gsecγ c whee, W D, W S, A loop, G sec and γ c ae the dissipated enegy, maximum stain enegy, aea of hysteesis loop, secant shea modulus and shea stain amplitude espectively. Measuement of Dynamic Soil Popeties The measuement of dynamic soil popeties is a citical task in the solution of geotechnical eathquake engineeing poblems. A wide vaiety of field techniques like Seismic Reflection test, Rayleigh Wave test, Seismic Coss-Hole test, Cone Penetation test, Pessuemete test, etc. and laboatoy tests like Resonant Column test, Ultasonic Pulse test, Piezoelectic Bende Element test, Cyclic Tiaxial test, etc. can be pefomed based on small o lage stain of the soil specimen. Thee ae a few model tests, namely, Shaking Table test, Centifuge test, etc. The choice of a test depends upon initial stess conditions and anticipation of actual cyclic loading conditions at field. Cyclic Shea Stain The cyclic shea stain ( γ ) is defined as: γ = ( 1+ µ ). ε (3) whee, µ = Poisson s atio, which is taken as.4 fo dy sands and.5 fo satuated sands (RaviShanka et. al., 25) ε = Axial stain, which is expessed as a atio between defomations of sample to its oiginal length. SOIL SAMPLING AND CHARACTERIZATION Dy epesentative sand samples wee collected fom the diffeent locations of Silcha town. It was obseved that the physical chaacteistics of the samples ae moe o less unifom at all the places. Hence the samples wee mixed in the laboatoy to make a single specimen sample. Figue 3 shows a

4 typical gain size distibution of the specimen sample. Table 1 gives the othe physical popeties of the sample. Pecent Fine (%) Tested Sand Paticle Size (mm) Figue 3. Gain Size Distibution of Specimen Soil Table 2. Physical Popeties of Sand Sample G s e max.822 e min.57 D 1 (mm).19 D 3 (mm).26 D 6 (mm).35 TESTING EQUIPMENT USED IN THE PRESENT STUDY A cyclic tiaxial testing equipment (figues 4 and 5) has been used in the pesent study to measue the dynamic soil popeties at high stain levels. The equipment is make of HEICO, India with diffeent sensos, including a load cell to monito the axial load, an LVDT to measue the vetical displacement, thee tansduces to detect the chambe pessue, poe pessue and volume change. The tiaxial cell is built with low fiction piston od seal along with a 1 KN sevo contolled submesible load cell. The loading system consists of a load fame and a hydaulic actuato capable of pefoming stain-contolled as well as stess-contolled tests (by sevo hydaulic loades) with wide ange of fequency.1 Hz to 1 Hz. The diffeence between the axial stess and the adial stess is called the deviato stess. In the cyclic tiaxial test, the deviato stess is applied cyclically, eithe unde stess-contolled conditions (typically by pneumatic o hydaulic loades) o unde stain-contolled conditions (by sevo hydaulic o mechanical loades). Cyclic tiaxial tests ae the most commonly pefomed with the adial stess held constant and the axial stess cycled at diffeent fequencies. In the pesent analysis, 38 mm diamete and 76 mm high samples wee pepaed and placed between top and bottom loading platens and suounded by a thin ubbe membane. The specimen was subjected to vetical and hoizontal stesses; hence the pincipal stesses wee always vetical and hoizontal. SAMPLE PREPARATION Dy sand specimens of above mentioned size with elative densities 4%, 5%, 6 % and 7% wee pepaed by pouing the pe weighted amount of dy sand though a funnel whose spout was placed at the bottom of the membane lined split mould. The funnel was slowly aised along the axis of symmety and the mould was filled with soil compacted in thee layes to achieve the desied density. Tests wee caied out on both satuated and unsatuated soil samples. Fo full satuation, distilled wate unde a back pessue was used keeping the cell pessue little above the back pessue. Afte satuation Skempton s poe pessue paamete (Skempton,1954), B = u/ σ c, was checked, whee, u = cyclic pessue and σ c = ise in cell pessue. Fo all the tests, unde full satuated conditions, B value was kept as.95. Afte the satuation, the sand specimens wee consolidated unde a pe-defined isotopic stess condition.

5 Fo patial satuation of the soil, the samples wee satuated to diffeent degee of satuation. This was done by pouing a measued volume of wate though a standpipe into the sand samples Some idle time was allowed fo fee movement of wate thoughout the sample. Figue 4. Typical Tiaxial Appaatus (Afte Kame, 1996) Figue 5. Cyclic Tiaxial Testing Machine CYCLIC LOADING AND DATA ACQUISITION Cyclic loading in the vetical diection was applied diectly on the soil samples using stain-contolled method. The tests wee caied out as pe ASTM (1996) with diffeent stain, elative densities, confining pessues and degee of satuations at a sinusoidal hamonic loading of fequency.1 Hz and 1. Hz. Fo fully satuated sand samples the tests wee conducted with diffeent shea stains (γ ) fom % to % at diffeent elative densities fom 4 % to 7 % ( f = 1. Hz). Fo unsatuated sand samples the tests wee conducted fo diffeent stain amplitudes (γ ) fom.54 % to 4.73 % at diffeent elative densities fom 4 % to 8 % ( f =.1 Hz) at diffeent degees of satuation. RESULTS AND DISCUSSION Evaluation of Dynamic Popeties and Liquefaction Potential of Fully Satuated Soil Figues 6 shows the elationship between dynamic popeties vesus shea stain fo satuated sand samples at elative densities of 4 %, 5 % 6 % and 7 % unde a confining pessue of 1 kpa and at a fequency of 1 Hz. It is evident fom the figues that, shea modulus educes and damping atio inceases fo any elative density with the incease of shea stain. At a paticula shea stain the G value emains highe fo a dense sample and the D value emains highe fo a loose sample when confining pessue emains constant.. Figue 7 shows the elationship between G and γ and D and γ espectively fo satuated sand samples at confining pessues of 5 kpa, 1 kpa, 15 kpa and 2 kpa unde a elative density of 6 % and at a fequency of 1 Hz. It is evident fom the figues that, shea modulus value educes and the damping value inceases with the incease of shea stain. At a paticula shea stain the G value emains highe fo highe confining pessue and the D value emains highe fo lowe confining pessue when elative density of sample emains constant.

6 G ( kpa) f = 1. Hz ' = 1 kpa RD = 4 % RD = 5 % RD = 6 % RD = 7 % D ( %) f = 1. Hz ' = 1 kpa RD = 4 % RD = 5 % RD = 6 % RD = 7 % γ (%) (a) γ ( %) (b) Figue 6. Vaiation of (a) Shea Modulus (G) and (b) Damping Ratio (D) with Shea Stain (γ) fo a constant confining pessue ( = 1 kpa) G ( kpa) f = 1. Hz RD = 6 % σs' 3 ' = 5 kpa s' = ' = 1 kpa s' = ' = 15 kpa s' = ' = 2 kpa 1 γ ( %) 1 D ( %) f = 1. Hz RD = 6 % s' = ' = 5 kpa s' = ' = 1 kpa s' = ' = 15 kpa s' = ' = 2 kpa 1 γ ( %) 1 (a) (b) Figue 7. Vaiation of (a) Shea Modulus (G) and (b) Damping Ratio (D) with Shea Stain (γ) fo a constant elative density (RD = 6 %) Figue 8 shows the vaiation of deviato stess (σ d ), axial stain (ε) and poe pessue atio ( u ) with time fo the sand at a stain-contolled cyclic test. It is evident fom the figue that the poe wate pessue builds up steadily duing application of cyclic shea stain and eventually appoaches a value equal to the initial confining pessue. Fo the sample tested at 4 % of RD and 1 kpa confining pessue, the cyclic poe pessue atio becomes 1 % afte 3 cycles at % shea stain, which is known as liquefaction. It can be seen that the incease in poe wate pessue esults in a coesponding decease in the deviato stess, which finally becomes constant. Such a state of the specimen is ecognized as liquefaction which is a state of softening poduced suddenly with complete loss of shea stength o stiffness. A typical hysteesis loop duing a cyclic tiaxial test in the pesent study is as shown in figue 9. It is obseved in the figue that, as the cyclic load is applied the aea of hysteesis loop educes due to loss of shea stength of the soil sample.

7 σ d (kpa) ε (%) u 2-2 f = 1. Hz RD =4 %; =1 kpa; γ =2.961 % 1 2 Time (sec.) (b) Time (sec.) (c) -1 N L = Time (sec) Figue 8. Vaiation of (a) deviato stess, (b)axial stain and (c) poe pessue atio with time (a) Deviato Stess (kpa) f = 1. Hz RD =4 %; =1 KPa; γ =2.961 % Cycle 1 Cycle Axial Stain (%) Figue 9. Typical Hysteesis Loops u f =.1 Hz RD = 7 %.2 N = 1 N = 5 N = γ (%) Figue 1. Typical Hysteesis Loops 6. RD = 4 % Eff. Conf. Pess. : 6. Cyclic Shea Stain (%) kpa 1 kpa 15 kpa 2 kpa Numbe of Cycles fo Initial Liquefaction Figue 11. Cyclic Resistance Cuve fo constant Relative density Cyclic Shea Stain (%) Eff. Conf. Pess. = 1 kpa RD = 4 % RD = 5 % RD = 6 % RD = 7 % Numbe of Cycles fo Initial Liquefaction Figue 12. Cyclic Resistance Cuve fo constant Confining Pessue

8 Evaluation of liquefaction potential can be done eithe by cyclic stess method o cyclic stain method. In stain-contolled tests, thee is a pedictable coelation between cyclic shea stain, numbe of cycles, and poe-wate pessue buildup; this coelation is much less sensitive to factos, such as elative density and fabic than compaable esults obtained fom stess-contolled tests (Ladd et.al., 1989). In the pesent analysis, stain contolled tests wee conducted and a theshold shea stain of appoximately.15 % has been obtained. Figue 1 shows the theshold shea stain value fo a test sample. Figues 11 and 12 epesent the cyclic esistance in tems of cyclic shea stain vs. numbe of cycles fo initial liquefaction fo constant confining pessue and constant elative density espectively. Evaluation of Dynamic Popeties of Patially Satuated Soil Typical vaiation of deviato stess, axial stain and poe pessue ise against time is shown in figue 13. The stain levels wee inceased afte evey 1 cycles. It can be seen that poe pessue inceases gadually and then deceases suddenly. Simila esults wee obtained at all confining pessues and elative densities of sand samples. Vaiation of poe pessue with numbe of cycles fo diffeent degee of satuations ( ) is shown in figue 14. It can be seen that fo all the cases, except = 1 % poe pessue builds up initially, eaches a maximum value and then dops suddenly. It was obseved that most of the unsatuated sand samples failed in shea along a diagonal plane. Typical vaiation of shea modulus and damping atio with shea stain at diffeent confining pessues is shown in figue 15. It is obseved that iespective of confining pessue, shea modulus deceases and damping atio inceases with stain. σ d (kpa) ε (%) u (kpa) ε =.4 %.8 % 1.2 % 1.6 % 2. % 2.5 % 3. % 3.5 % f =.1 Hz R.D. = 4 % = 1 KPa = % Time (sec) Figue 13. Typical esults of a stain-contolled cyclic tiaxial test

9 u (kpa) S =. % S = % S = % S = 83.6 % S = 1. % f =.1 Hz R.D. = 6 % = 1 KPa G ( kpa) = Cycles Figue 14. Change in poe pessue ( u) with numbe of cycles fo diffeent degee of satuations ( ) 5 kpa = 1 kpa = 15 kpa = 2 kpa f =.1 Hz RD = 4 % = % γ ( %) (a) D ( %) = 5 kpa = 1 kpa = 15 kpa = 2 kpa f =.1 Hz RD = 4 % = % γ ( %) (b) Figue 15. Vaiation of (a) shea modulus (G) and (b) damping atio (D) with shea stain (γ) at diffeent effective confining pessues Vaiation of shea modulus with diffeent degee of satuation is shown in figue 16 (a). It can be seen that with incease in degee of satuation shea modulus inceases, eaches a maximum value at an optimum degee of satuation and then deceases shaply. This optimum value of degee of satuation coesponding to maximum value of shea modulus may aise due to the following factos : (a) Development of mico factue of the soil samples duing dying ( Mainho et. al. 1995) due possibly to abupt changes in wate content, o (b) Reduction of the capillay effects duing shea defomation, which involves paticle displacement and paticle otation ( Mendoza and Colmenaes, 26). Vaiation of damping atio with diffeent degee of satuation is shown in figue 16 (b). Damping atio educes with the incease of degee of satuation. Figue 17 shows the compaison of hysteesis loops of sand samples at diffeent degee of satuations. Fom the figue it is obseved that, as the degee of satuation inceases the hysteesis loop aea educes. It epesents that damping atio educes with the incease of degee of satuations, which is again clea fom figue 16 (b).

10 Figue 18 shows the p q plot fo diffeent values, whee, ' ' ( σ 1 + σ 3 ) p ' = (4) 2 ( σ ) 1 σ 3 q = (5) 2 It is seen that, change in poe pessue duing cyclic load application is not constant fo loading and unloading opeations. This change is moe pominent fo highe degee of satuation. G ( kpa) f =.1 Hz ε =.4 % RD = 6 % = 1 kpa Cycle No. = S ( %) D ( %) f =.1 Hz ε =.4 % RD = 6 % = 1 kpa Cycle No. = S ( %) (a) (b) Figue 16. Vaiation of (a) shea modulus (G), (b) damping atio (D) with σ d ( kpa) f =.1 Hz RD = 6 % = 1 kpa Cycle No. = 62 S =. % = % = % = 83.6 % = 1. % Aea of Hysteesis Loops in kpa : A L at. % = A L at % = A L at % = A L at 83.6 % = 9.8 A L at 1. % = ε ( %) Figue 17. Compaison of hysteesis loops at diffeent degee of satuations

11 2 1 = % 2 1 = % q (kpa) -1 q (kpa) p (kpa) p (kpa) q (kpa) (a) =. % (b) = % 2 S = % 1 S = 83.6 % q (kpa) p (kpa) p (kpa) q (kpa) (c) = % (d) = 83.6 % = 1 % p (kpa) (e) = 1. % Figue 18. Vaiation of stess-path fo diffeent degee of satuations (a) (e) at f =.1 Hz, R.D. = 6 %, = 1 kpa CONCLUSIONS Following conclusions can be dawn fom the pesent study on local soil : Fo full satuated condition Unde full satuation, local soil behaves simila to aleady epoted behavio of soil like eduction of shea modulus with incease in shea stain and incease in damping atio with incease in shea stain. Howeve, following additional points wee noted : (a) At a paticula shea stain the G value emains highe fo a dense sample and the D value emains highe fo a loose sample when confining pessue emains constant. Again at a paticula shea stain the G value emains highe fo highe confining pessue and the D value emains highe fo lowe confining pessue when elative density of sample emains constant. (b) When a cyclic load is applied on the soil, poe wate pessue builds up steadily and eaches initially applied confining pessue depending on the magnitude of cyclic shea stain as well as on the density of the soil. (c) The amplitude of cyclic shea stain govens the liquefaction esistance of a soil chaacteized by the cyclic stain appoach. A theshold stain of appoximately.15 % is obtained in the pesent analysis.

12 Fo unsatuated condition (a) With incease in numbe of cycles, poe pessue inceases gadually and then deceases suddenly. The samples failed in shea along a diagonal plane. (b) Iespective of confining pessue, shea modulus deceases and damping atio inceases with stain. (c) The ise of poe pessue with numbe of cycles is moe fo low elative density. (d) With the incease in degee of satuation, the shea modulus of soil inceases, eaches a optimum value and then deceases at a paticula stain level. (e) Damping atio educes with the incease in degee of satuation, as the aea of hysteesis loop inceases with the incease of the degee of satuation at a paticula stain level. (f) The change in poe pessue duing cyclic load application is not constant fo loading and unloading opeations. This change is moe pominent fo highe degee of satuation. REFERENCES ASTM Designation: D (Repoduced 23). Standad Test Methods fo the Detemination of the Modulus and Damping Popeties of Soils using the Cyclic Tiaxial Appaatus, Annual Book of ASTM Standads, ASTM Designation: D (Repoduced 24). Standad Test Methods fo Load Contolled Cyclic Tiaxial Stength of Soil, Annual Book of ASTM Standads, Das, B.M. Fundamentals of Soil Dynamics, Elsevie Publications, New Yok, Ladd, R.S., Doby, R., Dutko, P.,Yokel, F.Y. and Chung R.M. Poe-Wate Pessue Buildup in Clean Sands Because of Cyclic Staining, Geotechnical Testing Jounal, Vol. 12, Issue 1, HEICO. Manual fo Cyclic / Static Tiaxial test System, (Confoming to ASTM D ). Kame, S.L. Geotechnical Eathquake Engineeing, Pentice Hall, New Jesey, l996. Mainho, F., Chandle, R. and Cilly, M. Stiffness Measuements on an Unsatuated High Plasticity Clay Using Bende Elements. Poceeding 1 st Intenational Confeence on Unsatuated Soils. Vol. 2, pp , Mendoza, C.E. and Colmenaes, J.E. Influence of the Suction on the Stiffness at Vey Small Stains. Poceeding 4 th Intenational Confeence on Unsatuated Soils, pp , 26. Pakash, S. Soil Dynamics. McGaw-Hill Book Company, RaviShanka, B., Sithaam, T.G. and Govinda Raju, L. Dynamic Popeties of Ahmedabad Sands at Lage Stains, Poceedings, Indian Geotechnical Confeence-25, Ahmedabad, INDIA, pp , 25. Skempton, A.W. The Poe Pessue Coefficients A & B, Geotechnique, 4, pp , 1954.