EFFECT OF SAMPLE PREPARATION ON STEADY STATE

Naeini, Seyed Abolhassan Baziar, Mohammad Hassan,Associate member ASCE 2

ABSTRACT Undrained Triaxial compression tests were performed on reconstituted

samples of Ardebil sand to evaluate the effect of the specimen preparation procedure on the position of the steady state line under monotonic loading. Tests were performed on saturated samples by three distinctive methods of sample preparation, namely, "wet tamping", "water sedimentation", and "undercompaction ". In all cases, the loosest state of structure are tried to be obtained The weakest samples were those formed by water sedimentation method because of non-uniformity of the sample. The steady state line of specimens prepared by water sedimentation method lies under those of wet tamping and undercompaction methods The test results indicate that the normalized steady state strength of a given material tends to vary to some extent depending upon the fabric formed by different modes of deposition, Based on the data presented, it is found that the liquefaction behavior of specimens tested in triaxial equipment may be significantly affected by the method of sample preparation.

INTRODUCTION

Steady state in cohesionless soils represents a condition of deformation at constant volume, pore pressure, velocity, shear and mean normal effective stress. Steady state is an outgrowth of Casagrande's (1936) concept of critical

(I)Cnllege of Engineering, Imam Khnmeini International University, Qazvin, Iran.

(2)College of Civil Engineering, lran Univ. Of Science and Technology, Tehran, lran. Associate Professor

16

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void ratio for characterizing the flow - slide behavior of sands. Further investigation of undrained response and liquefaction of saturated soils were based on monotonic triaxial tests (Castro et al 1982,1987,1992, Poulos et al, 1985. Ishihara 1993, Zlatovic and lshihara 1994, BaziarandDobry 1995, Verdugo and lshihara 1994, Chemeau et al 1997, Mitchell and Seed 1996)

The results from these work have generally confirmed that the steady state line of saturated sand is influenced primarily by the factors such as: density, drainage condition, level of confining stress and effective stress path. However , other researchers have pointed out that the undrained steady state strength is not only dependent on the void ratio of the soil mass, but is also affected by the effective consolidation stress (Alarcon- Guzman et al 1998). Studies by De Gregorio (1990) indicated that the dry pluviationtechnique

provides a lower location of the steady state line in the e - ~ a plane than the

moist tamping and moist vibration sample preparation method .Experimental results pressented by Marcuson et al ,(1990) indicates that the steady state line is somehow affected by the sample prepartion method. They found that the steady state line obtained from the test specimens, prepared by means of the remolded continuously wet pluviated technique was slightly below the steady state line obtained from that of the specimens prepared by moist tamping. In addition , Dobry and his Co- workers have tested specimens of silty sand prepared by sedimentation under water. This procedure results in a layered sample that is not uniform. Their results have indicated that the steady state line is strongly affected by the sample preparation procedure (Vasquez an Dobry 1988, Baziar 1991 , Baziar and Dobry 1995 ).

The work presented here, are the results of undrained -strain controlled monotonic triaxial tests on saturated samples compacted to a density by different sample - preparation techniques such as:" wet tamping", "water sedimentation", and "undercompaction" to study the effect of sample preparation on the position of steady state line.

TESTING MATERIAL

Ardebil sand collected from a liquefied zone located in the state of Ardebil is used throughout this study (Baziar et al 1998). Individual particles are subrounded and the predominant minerals are feldspar and quartz . Maximum and minimum void ratios are obtained I 16 and 0.625, repectively . The gradation is uniform with a coefficient of uniformity of 1.87. The particle size distribution curve along with selected index properties of the samples are shown in Fig (1) and Table 1.

SAMPLE PREPARA T/ON

The specimen preparation procedure for triaxial strength testing most commonly described in the literature requires the sample to be saturated and

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then densified to the required density by some means (I,ee and I:ithm 1()60, Lee and Seed 1967, Finn Picketing and Bransby 1071 ).

Fig I- ( ; rain Size I)isl,'ilmli~m (:urve for Al'del)il S,'md

All of the specimens in this shtdy are prepaled by tluee distinctive methods of sample preparation , namely , "wet tamping", "water sedimentation",and "undercompaclion". In all cases, the Ioo,~e,~t state of sl~uclure are tried to be obtained The basic requiremenls for the two methods are, firstly to obtain homogeneous sample with tmiform distribution of w~id ratio, and secondly to be able lo prepare samples of the lowest possible dry density (highest porosity).

In wet tamping method, six equal pre~veighed portions of sample are mixed wilh tie- aired waler al a water contellt of 9% . The memblane is

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GEOTECHNICAL MEASUREMENTS 19

streched to the inside face of a split mould which is attached to the base pedestal of the cell.

Each portion of the slightly moist sand is strewned to a predetermined height in six lifts. At each stage of the lifts, tamping is applied lightly with a small flatbottom tamper. Because of capillary effects between particles, the moist sample can be placed at a very loose state well in excess of the maximum void ratio of the dry sample, which is determined by the procedures stipulated in the ASTM standard test method

In water sedimentation method, a predetermined amount of sample is dumped in to a mould filled with water to a certain height by using a funnel with an opening of 0.3 cm in diameter. A time of 30 minutes is allowed, until the material has completely settled . The soil was allowed to fall from the funnel which is kept 1-3 mm above the water level. This stepwise deposition is repeated six times to construct a complete test sample. This porcess creates a layered specimen.

For undercompaction , the specimens are constructed by compacting wet materials in layers to a selected percentage of the required dry unit weight of the whole specimen. Undercompaction is used since it is generally recognized that when a typical soil is compacted in layers, the compaction of each succeeding layer can further densify the soil below it. The undercompaction method uses this fact to achieve a uniform specimen by compacting the lower layers to a lower dry unit weight than the desired final unit weight by a predetermined amount which is defined as "precent of undercompaction (Uni)".

Desired dr3.' unit weight - Dr',' unit ~eight of layer used during compaction U n i =

Desired d~' unit weight

This value of (Uni) is linearly varied from a maximum at the bottom layer to zero at the top layer of the specimen, l f the value ofundercompaction has been selected appropriately, a uniform dry unit weight (void ratio) throughout the specimen will be achieved (Ladd 1978). Therefore, the key point in this method of specimen preparation is the selection of the ofundercompaction value for the bottom layer

All test specimens prepared by undercompaction method had an initial water content of 9% and were compacted with 4% undercompaction (Uni = 4%) value in six layers in a split compaction mould attached to the triaxial cell (see Ladd 1978). All samples were 508 mm (2in) in diameter and 101.6 mm (4in) height.

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TESTING TECHNIQUE

Once a sample was formed by any method &sample preparation, a vacuum of 35 Kpa was applied to produce confining pressure.Then the forming mold was dismantled . The specimen height and diameter were measured at several heights on the specimen to compute its void ratio. The triaxial cell was then assembled and filled with deaired water. Carbon dioxide was run into the specimen for 45 minutes and then the sample was saturated with deaired water. The cell and back pressures were then continuously increased, while maintainnig a difference of 35 Kpa, until the desired back pressure was achieved.

A B-value greater than 0.97 was typically obtained at a back pressure of 200Kpa . The volume change was recorded during isotropic consolidation from 100 to 485 Kpa so that the void ratio and relative density at the end of consolidation could be calculated When the specimen was fully consolidated (which required approximately 60 minutes), the drainage valves were closed, and the sample was subjected to monotonic loading. The axial deformation and pore pressure in the specimen were recorded during loading. Tests were conducted with strain control at a rate of 1%/minutes.

TEST PROGRAM

To investigate the effects of sample preparation on steady state line of a silty sand, a total of nine isotropically consolidated, undrained monotonic compression triaxial tests (CIUC) were performed using Ardebil sand, as listed in Table 2.

Specimens were prepared by using each of the three methods, as described earlier, and the steady state line (SSL) was obtained Ill a CIUC test, a continuous increase of pore pressure when the axial stress is increased, indicates that the specimen is contractive, while a decrease in pore pressure build up indicates that the specimen is dilativeln all ClUe tests performed here, the specimens were contractive

In all tests, the consolidation E,, pressure varied from 250 to 660 Kpa It is noted that during consolidation, the nine specimens experienced significant reductions in their overall void ratio. As listed for Test 9 in Table 2, the largest decrease was from 1.1 lto 0.905.

TEST RESULTS

The results of the liquefaction tests performed on the Ardebil Sand, using three different sample preparation techniques are presented in Table 2.

Figs. 2 to 4 illustrate the deviatoric stress and induced pore pressure in terms of the axial strain, and also the effective stress paths for different methods of sample preparations.

Fig. 2 shows the stress-strain behavior for the Ardebil sand for specimens prepared by watre sedimentation In these tests,the specimens were isotropically

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GEOTECHN ICAL MEASU REMENTS 21

M M

tS-

,k

~,~

d ~ Z J

~ m

~ >,

~ S c5 s 6 6 - o <5

~ o c~ >: o~ c~ , . .

.o

o .~ ch ~ .-

o ~

c~

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22 GEOTECHNICAL MEASUREMENTS

F.C.=tO % ,Effective Stress :a)-400, b)-280, c)-1~0 (l<p~)

Fig 2 - Monotonic Triaxial Test on Ardebil Sand

( Water Sedimentat ion Mefllod )

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F.C.=10% ,Effective Stress :a)-485, b)-275, c)- 100 (Kpa)

Fig 3 - Monotonic Triaxial Test on Ardcbil Sand

(Undrecompaction Method )

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F.C.=10% ,Effective Stress :a)-385, b)-280, c)- 245 (Kpa)

Fig 4 - Monotonic Triaxial Test on Ardebil Sand

(Wet Tamping Method )

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GEOTECHNICAL MEASUREMENTS 25

consolidated to 300-600 Kpa. As seen in Figure 2 Test (b), the shear stress q, as well as the pore pressure, increased up to a strain of about 1%. At this axial strain, the specimen failed with peak strength of about 196 Kpa. At an axial strain of about 3%, the deviatoric stress and pore pressure remained constant up to an axial strain of 15%. Both curves of stress and pore pressure in Figure 2 show that the soil were contractive during the test

Fig 3 shows the results of CIUC test for undercompaction method In these tests the specimen were isotropically consolidated to 250-660 Kpa. As seen in Figure 3 test (b), the shear stress and the pore pressure increased up to a strain of about 1%. At this axial strain, the specimen failed with peak strength of about 120Kpa, and at 6 to 11% axial strain exhibited almost the same strength and pore pressure.

In Fig 4 test (b), at strain of 1% , the specimen failed with maximum strength of about 125Kpa, and at 7% strain exhibited almost the same strength and pore pressure up to an axial strain of 11%. In these tests, the specimens were isotropically consolidated to 295-550 Kpa

The results of nine CIUC tests on Ardebil sand showed that they were contractive.That is, the peak strength and steady state strength are distinctively different The development of large strains at steady state for water sedimented specimens was about 4% to 7%, while for the same kind of tests ,this value is 6% to 7% for undercompaction and wet tamping methods respectively The observed differences between large strains in water sedimentation and two other methods is due to the non-uniform packing of the particles in each layer when water sedimentation is used

The values ofotu~ and Cu.~ for three methods of sample preparation are listed in Table 2. These values for undercompaction and wet tamping methods

are almost the same ( Ctus = 2564 ~ and 25.17 ~ respectively), while for water sedimentation is 27.35 ~ Undercompaction and wet tamping methods produces almost an isotropic and homegeneous fabric. Whereas, the method of water sedimentation roughly duplicate the natural or artificial sedimentation processes through water, and the fabric produced is anisotropic and layered. This method likely represents to some extent hydraulic fill process in the field.

To compare the SSL's of this three sets of tests values of steady state strength versus the initial confining pressure for different method of sample preparation are plotted in Fig 5. The residual strength increased from undercompaction to wet tamping and from that to water sedimentation. The test data arranged in Fig5 indicates that the normalized steady state strength of a given material tends to vary to some extent depending upon the fabric formed by different modes of deposition (see Baziar and Dobry (1991) , Ishihara (1993) and zlatovic (1994)). This is considered as a manifestation of the effect of the fabric created by the different methods of deposition in preparing the test specimens.

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26 GEOTECHNICAL MEASUREMENTS

Fig 5 - Variation Of Undrained Steady State Strengthd ( Sus )With

Effective Vertical Consolidatioin Pressure For Ardebil Sand

Fig 6 - Comparision of Steady Slate Line For Three Methods :

a- Undercompaction

b- Wet Tamping

c- Water Sedimentation

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GEOTECHNICAL MEASUREMENTS 27

Fig 6. Shows the relationship between the void ratio and the log of the effective mean stress, as the samples deformed at large strains. The steady state line of samples prepared by water sedimentation method lies under that of wet tamping and undercompaction methods. In another words, for a given effective mean stress, the sample prepared by water sedimentation is more liquefiable than the sample prepared by two other methods.

CONCLUSIONS

From the study described in the preceeding pages, the effects of the method of sample preparation on the liquefaction characteristics of saturated Ardebil sand under strain-controlled monotonic triaxial test conditions my be summarized as follows:

1) The liquefaction behavior of reonstituted saturated specimens tested in triaxial equipment will be significantly affected by the method of specimen preparation.

2) Because of the same grain size distribution of the samples in this study, the slopes of the steady state lines formed by different techniques were all about the same, which is consistent with previous data (castro et al 1982, poulos 1981 , 1985) ; and is an important point that counts in practical selection of steady state strength

3) The undercompaction mothod is able to make looser sample than the other two method of sample preparation

4) The steady state line of samples prepared by water sedimentation method lies under that of wet tamping and undercompaction methods.

5) The steady state strength for wet tamping and undercompaction were roughly the same, but tends to vary for water sedimentation.

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REFERENCES

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2) Baziar. MH. (1987): "Influence of Testing Technique on The Steady State Line of a Sand," Master Thesis, Department of Civil Engineering, Rensselaer Polytechnic Institute, Troy .N.Y.

3) Baziar, M H (1991): "Engineering Evaluation of Permanent round Deformations Due to Seismically - Induced Liquefaction," Doctoral Thesis, Rensselaer Polytechnic Institute, U S A

4) Baziar, M. H .and Dobry, R. (1991):"Liquefaction Ground Deformation Predicted From Laboratory Tests," Proc. Second International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics.

5) Baziar, M H , and Dobry R. (1995): "Residual Strength and Large- Deformation Potential of Loose Silty Sands , "JG.T. ASCE, Vo1.121, No.12, PP. 896 - 906

6) Baziar, MH., Razeghi, HR., and Neaini, A.H. (1997): "Determination of Liquefaction Potential by Monotonic Triaxial Test" Research Report, lran University oF Science and Technology.

7) Baziar, MH., Naeini, AH, and Ziaie Moayed, R. (1998): "Geotechnical Damaje During the 1997 Ardebil Earthquake in Iran"Proceedings &The First IRAN - JAPAN Workshop on Recent Earthquakes in lran and Japan, PP. 137-146

8) Casagrande, A. (1963): "Characteristics ofCohesionless Solis Affecting The Stability of The Slopes and Erath Fills "Journal of The Boston Society of Civil Engineering.

9) Castor ,G.(1969) : " Liguefaction and Cyclic Deformation of Sands, A Critical Review" Harvard Soil Mechanics Series ,No. 88

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11) Castro, G, Poulos, S.J. France, J.W., and Enos, J.L. (1982): "Liquefaction Induced by Cyclic Loading", Geotechnical Engineering Inc. Report Submited To National Science Foundation.

12) Castro, G., Keller, T.O. and Seed, H.B. (1992): "Steady State Strength Analysis Of Lower San Fernando Dam Slide", Journal Of Geotechnical Engineering, ASCE, Vol. 18, NO. 3, pp. 406 - 427

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GEOTECHNICAL MEASUREMENTS 29

13) De Gregorio, VB. (1990): " Loading Systems, Sample Preparation, and Liquefaction, "Journal of Geotechnical Engineering, Vol 116, No.5, PP 805 - 82l

14) Dobry. R (1991):'" Soil Properties and Earthqueke Ground Response, '" Guest Lecture, Proceedings Tenth European Conference on Soil Mechanics and Foundation Engineering, Florence, ltaly.

15) Finn , W D L ,Pickering, D J and Bransby, P.L ,(1971 ) : " Sand Liquefaction in Yriaxial and Simple Shear Tests " Journal of The Soil Mechanics and Foundation ASCE, Vol. 97, No. Ss Proc .Paper, 8039, PP. 639 - 659

16) lshihara, K. (1993): "Liquefaction and Flow Failure During Earthquakes", The 33 rd Rankine Lecture, Geotechnique, Vol. 43, No.3, PP. 351 -4 t5

17) Ladd, R.S. (1978): "Preparing Test Specimens Using Undercompaction", Geotechnical Testing Journal, GTJODJ, Vol., No.l, PP. 16 - 23

18) Lee, K L , and Seed, H.B., (1967): "Dynamic Strength of Anisotropically Con Solidated Sand", Journal of The Soil Mechanics and Foundations Division, ASCE, Vol 93, No. SM5, PP. 169- 190

19) Lee, K L, and Fitton, J A , (1969): " Factors Affecting The Dynamic Strength of Soil", in Vibration Effects on Soils and Foundation, ASTM, Stp 450, Philadelphia, PP. 71-95

20) Marcuson, Ill, W.F., Hynes, ME, and Franklin, A.G. (1990): " Evaluation and Use of Residual Strenght in Seismic Safety of Embankments", Earthquake Spectra, 6, PP. 529-572

21) Poulos, S.J., Castro, G., France, J.W., (1985): "Liquefaction Evaluation Procedure", Journal of Geotechnical Engineering, Vol. l 11, No. 6, PP. 772 - 792

22) Verdugo, Rand Ishihara, K. (1996): "The Steady State of Sandy Soils", Japanese Geotechnical Society, Vol. 36, No.2, PP. 81 - 91

23) Vasques - Herrera, A and Dobry, R. (1988) : " The Behavior of Undrained Contractive Sand and Its Effect on Seismic Liquefaction Flow Failure of Earth Structures, " Department of Civil Engineering, Rensselaer Polytechnic Institute, USA.

24) Zlatovic, S. and lshihara, K, (1994): "The Loosest State of Silty Soils", Normalization, Proc. 49 Th Japanese Conference On Civil Engineering, Sapporo. 3, PP. 332 - 333

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