Properties of Very Soft Clays a Study of Thixotropic Hardening and Behavior

Home , Effective stress, Shear strength (soil), Thixotropy

Soils and Foundations 2012;52(2):335–345

The Japanese Geotechnical Society

Soils and Foundations

www.sciencedirect.com journal homepage: www.elsevier.com/locate/sandf

Properties of very soft clays: A study of thixotropic hardening and behavior under low consolidation pressure

Sochan Seng, Hiroyuki Tanakan

Hokkaido University, Japan

Available online 29 March 2012

Abstract

In a study on the properties of very soft clays, bender element testing was used to evaluate thixotropic hardening behavior; that is, to measure the stiffness with resting time under constant volume and water content. A laboratory vane test, which measures the undrained shear strength of the materials, was also carried out for comparison purposes. To investigate the mechanism of the thixotropic phenomenon, a consolidation test with very low pressure was also performed in a cell equipped with bender elements. The most important findings from this study are as follows: (1) regardless of soil types, the effect of thixotropy was significant around the liquid limit state and less remarkable at the lower and higher ranges; (2) the shear modulus at the liquid limit after 24 h resting is around 200 kPa; (3) the correlation between the shear modulus and the undrained shear strength of very soft clays is similar to that of cement- treated soil proposed by Seng and Tanaka (2011); (4) the increment of the shear modulus developed in the thixotropy process appears to be noticeably higher than that in the secondary consolidation process. It is believed that these findings are very useful to establish a new theory for the consolidation of ground filled by very soft clays or dredged soils with extremely high water content as well as to understand the effects of ageing on the consolidation properties of natural soils. & 2012 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Thixotropic hardening; Bender element; Vane shear; Consolidation; Shear modulus; Shear strength (IGC: D05/D06)

1. Introduction disposal areas, but it is becoming increasingly difficult to find such disposal site due to the environmental problems A huge amount of dredged soils or very soft clays have they pose. To efficiently utilize these restricted deposit been generated from the maintenance of navigation chan- areas, the settlement of the ground deposited by these nels or the deepening of existing port facilities to cope with dredged soils with high water content needs to be precisely large ships. These dredged soils are normally dumped into estimated. In practice, the settlement caused even by self- weight is predicted based on the effective stress. The concept of ‘‘effective stress’’ has been widely accepted by

n geotechnical engineers and it is believed that geotechnical Corresponding author. parameters such as strength or stiffness can be simply E-mail address: [email protected] (H. Tanaka). correlated with effective stress. In this concept, these 0038-0806 & 2012 The Japanese Geotechnical Society. Production and parameters are independent of time, and the apparent hosting by Elsevier B.V. All rights reserved. time dependency during consolidation can be understood by changes in the effective stresses in the process of the Peer review under responsibility of The Japanese Geotechnical Society dissipation of the excess pore water pressure. On the other doi:10.1016/j.sandf.2012.02.010 hand, some behaviors opposite to the effective stress approach are also recognized, i.e., thixotropic hardening. Production and hosting by Elsevier The soil loses strength or stiffness by remolding, but recovers with time. It is not unreasonable to suggest that 336 S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 this factor plays an important role in the process of another approach, an oedometer test under very low settlement. Thus, this paper will try to understand the pressure was also carried out, equipped with the bender thixotropic hardening phenomenon of clays with high element to examine the development of soil stiffness due to water content. consolidation. The correlation between the stiffness and The term ‘thixotropy’ originally came from colloid the effective stress is established, and a discussion is made science, as presented in Mewis (1979), Barnes (1997), and on the changes in stiffness during secondary consolidation. Mewis and Wagner (2009), and as this term has come to be Using these test results, the increment of stiffness caused used in other fields its definition has changed somewhat by thixotropic behavior was evaluated. according to area of interests. Simply put, in geotechnical engineering, thixotropic hardening is, after softening by 2. Sample preparation remolding, the process of time-dependent return to a harder state under constant water content or volume The tests were run on very soft clays using samples conditions. The recovery of stiffness or strength may either obtained from three commercial powder clays and three fully or partially depend on material properties. The natural clays. The commercial clays were Fujinomori, mechanism of soil thixotropy has been explained in terms Kasaoka, and NSF clays. The natural clays were Ariake, of both microstructure and macrostructure in geotechnical Hachirogata, and Tokuyama clays. Their index properties engineering by, for example, Seed and Chan (1957), are summarized in Table 1. Large variation in the liquid Osipov et al. (1984), Mitchell and Soga (2005). Particu- limits of these clays was noted, ranging from low (48.6%) larly, the important role of thixotropy involving in geo- to very high (246.0%) values, which in effect allowed a technical engineering work was perfectly described by wide variety of materials with very different characteristics Mitchell (1960), which has inspired many researchers to to be investigated. Since all the clay particles were smaller pay attention to this topic. It is believed by most research- than 0.425 mm, however, it was determined that there was ers that thixotropic hardening occurs due to energy no sand content in these clays. imbalances which are induced by remolding or compac- Samples were prepared by mixing a soil with different tion. When soil is remolded, its strength or stiffness is water contents to vary its properties. Commercial clays reduced because of the dispersing structure caused by prepared by the dry powder condition were mixed with a repulsive forces. However, the flocculated interparticles water content lower than the target and kept for about one after remolding are gradually rearranged by attractive day before testing to let the soil particles adjust well and to forces and create a structure until an equilibrium state is ensure the homogenous distribution of water. Just before reached. testing, additional distilled water was added reaching order Some experimental data have been reported on this field, to achieve the final water content and the soils were mixed using the conventional destructive testing tools such as thoroughly to achieve uniformity. Natural clay samples, triaxial tests, unconfined compression tests, and laboratory which were stored under wet conditions, were prepared vane apparatus to measure strength hardening due to immediately after remolding with the target water content. thixotropy. These methods tend not to guarantee the same A prepared sample was poured into a mold with various testing conditions because different samples are used and a sizes for different purposes: a plastic cylindrical mold with relatively long period of time is required for preparing the a height of 10 cm and the diameter of 5 cm for the bender samples for testing, i.e., it is difficult to measure properties element test, a gallon bucket 20 cm in diameter and with a at a very early time, and the disturbance effect during the specimen height of 7–8 cm for the vane test, and a insertion of the vane blade makes it difficult to obtain consolidometer cell with a diameter of 10 cm and adjus- reliable results, for example. To overcome these draw- table height from 4 to 15 cm for the consolidation test. backs, a nondestructive bender element test, which allows Vibration was gently applied to drive the air bubbles from measurement to be taken very low strain levels, as small as the slurry specimen, and the sample was compacted when 105 (0.001%), is introduced in this paper to study the its water content was small and the sample was stiff. The thixotropic behavior of very soft clays, together with a uniformity of the sample was not well guaranteed in the vane shear test measuring the undrained shear strength. In latter case, i.e., inevitably the sample was partially

Table 1 Physical property of soil samples.

Soil types Plastic limit (%) Liquid limit (%) Plasticity index Particle density (g/cm3)

Fujinomori 21.3 48.6 27.3 2.69 Kasaoka 27.5 62.0 33.5 2.61 NSF 29.0 55.0 26.0 2.76 Ariake 31.4 72.5 41.1 2.62 Hachirogata 96.5 246.0 149.5 2.43 Tokuyama 40.0 110.6 70.6 2.62 S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 337 unsaturated. The mold with the specimen for the thixo- It should be pointed out that measured Vs is affected by tropy test was wrapped in thin plastic film, and then kept the existence of the mold or the consolidometer cell, but in a high humidity box to prevent water evaporation. Since their rigidity is much larger than that of soft clays, so that the changes in water content in each specimen after the test the arrival time of the shear wave should not be inter- were found in the range of less than 1% compared to the rupted by the existence of a mold or a consolidation cell. initial water content, this method was considered effective. For the thixotropy test, the bender elements were The water content within the specimens was distributed embedded in the specimen for the monitoring period such that there was a slightly difference between the top, without repeated insertions in order to avoid disturbance middle, and bottom parts. If the specimen was not well- caused by insertion. saturated, the lower portion had a higher water content and vice-versa for the saturated condition. The elapsed 3.2. Vane shear test time for thixotropy was judged to be just after the sample was poured into the mold. The increase in the undrained shear strength during thixotropic hardening was also confirmed by the vane shear test. Since conventional triaxial or unconfined tests 3. Testing methods cannot be performed on such low strength soils as those targeted in the present study, the vane shear test was 3.1. Bender element test considered an appropriate test. Undrained shear strength (su) is calculated as follows: A pair of bender elements is required for both the 2T thixotropy and consolidation tests. For the former test, a ¼ ð Þ su 2 3 parallel type 10 mm in length, 10 mm in width, and 0.5 mm pD ðH þðD=3ÞÞ in thickness was used as the transmitter and a series type where T is the measured torque at peak, D is the vane with length of 13 mm, a width of 10 mm, and a thickness diameter, and H is the vane height. of 0.5 mm was the receiver. The transmitter element was The vane diameter and height used in this experiment assembled with a light acrylic cap while the receiver was were 20 and 40 mm, respectively. The shear rate of the attached to a brass pedestal where the protrusion length laboratory vane apparatus was constant at 61 rotations per was approximately 7 and 9 mm, respectively. As for the minute. All tests were carried out for the same sample latter test, only the series type was employed and equipped created by a gallon bucket, as already mentioned. When in the consolidometer cell with a 6 mm protrusion length. the predetermined time came, the vane blade was inserted It is important that the protrusion length is sufficient to at a point away from the previous tested points not to be allow the bender element to penetrate into the soil sample influenced by the disturbance caused by the previous tests. to generate or receive shear waves. As input signals, both To attain accurate results, two different maximum capa- sine and rectangular waves have been alternatively used cities of load cells, 1 and 20 N, were alternately used with wide ranges of frequencies in accordance with according to the shear strength of the material. material stiffness, in order to attain a clear output wave- form. Generally, high frequency waves are required for the 4. Thixotropic effect measurement testing stiff soils and lower frequency waves are required for the less stiff soils. 4.1. Shear wave velocity and shear modulus by bender Methods for determining the arrival time and travel element test length has been investigated by many researchers, for example, Viggiani and Atkinson (1995), Kawaguchi et al. Fig. 1 shows an example of increase in the shear wave (2001),andYamashita et al. (2009). Considering those velocity (Vs) with time, measured by the bender element previous researches, the ‘‘start-to-start’’ method for deter- test on a specimen made from Kasaoka clay with a 60.6% mining the arrival time (Dt) and ‘‘tip-to-tip’’ method for water content. determining the travel distances (Dd) of the shear wave In this example, an input wave with 1 kHz of frequency were adopted in this study. The shear wave velocity (Vs) and710 V of amplitude was adopted, as indicated on the can be calculated from Eq. (1): top of the figure. The waves at the lower section of the Dd figures are the received signals with black triangles which V ¼ ð1Þ s Dt define their arrival time. Since the water content of the specimen was almost equal to the liquid limit state, the Shear modulus (G) can be derived from Vs, through the received shear wave signals at the beginning of the shear wave propagation in an elastic body theory, as measurement were difficult to identify because of their shown below: low amplitude and frequency. P-waves clearly appeared G ¼ rV 2 ð2Þ since they could propagate through the liquid. As time s proceeded, the shear waves’ arrival times were faster, where r is the density of the soil specimen. indicating that the shear wave velocity increased. The 338 S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345

10 Input Signal 4000 0 A1 (79.8%; 1.10) -10 Sine 1 kHz 3500 F1 (40.9%; 0.84)

Voltage (V) Voltage F2 (52.5%; 1.08) K1 (62.3%; 1.00) 3000 K2 (60.6%; 0.97) 1.5 hrs K3 (52.0%; 0.84) (kPa) 2500 K4 (62.1%; 1.00) G 2.2 hrs K5 (46.0%; 0.74) 2000 N1 (54.0%; 0.93) 2.7 hrs N2 (48.6%; 0.86) P-waves H1 (241.3%; 0.98) 19.5 hrs 1500 T1 (134.3%; 1.21) T2 (149.3%; 1.35)

23.7 hrs Shear Modulus, 1000

44.4 hrs 500 93.8 hrs 0 116.8 hrs 0.1 1 10 100 1000 Time (hr) 142.0 hrs 800 03691215 Time (ms)

Fig. 1. Measurement of thixotropic hardening by bender element test for 600 Kasaoka clay with 60.6% of water content. (kPa) G

400 increase in Vs can be assumed to correspond to an increase in stiffness, which reflects the thixotropic phenomenon. The received shear waves became much clearer with high amplitude and frequency after a certain time, while the Shear Modulus, 200 P-waves seemed to decay. The same testing method was conducted on other soils under various water content conditions, and the shear 0 0.1 1 10 100 1000 modulus (G) derived from Vs with elapsed time is illu- Time (hr) strated in Fig. 2(a). An enlarged scale for high water content samples with low G magnitude is shown in Fig. 2. (a) Variation in G with time at rest and (b) enlarged scale. Fig. 2(b). The symbols of A, F, K, N, H, and T represent Ariake, Fujinomori, Kasaoka, NSF, Hachirogata, and to a wide range of water contents. Generally, the relations Tokuyama clays, respectively, with sample number. The at different times seem to be parallel each other, but if number in the brackets indicates the water content (w) and these relations are carefully examined, the ranges of G at the normalized water content by the liquid limit (w/wL). It various observed times are somewhat different by w/wL, can be seen from the figures that the G values for all i.e., the increment of G to time at around wL is the largest, conditions increases with time even at over limit liquid while at smaller water contents than wL, say, when w/wL is states. Also, G builds up nearly in proportion to time on a less than 0.8, the tendency of G increasing against time logarithmic scale, but this magnitude clearly depends on becomes smaller. To confirm this trend clearly, the incre- the type of soil and the amount of water content. mental G based on G24 is plotted against the w/wL ratio in Since G increases in time, G at 24 h (G24) was used a Fig. 3(c). Fitting curves are drawn by excluding the representative parameter for the identification of the three encircled data. This tendency described above can characteristics of various soil types and the influence of be recognized in Fig. 3(c): the thixotropic effect on water content. G24 is plotted in Fig. 3(a) against the G is prominent at around wL, and at lower and higher normalized w/wL, for different types of materials. A clear water contents the thixotropic hardening becomes less correlation between the two parameters can be observed remarkable. with minor scatters. Obviously, G decreases as the normal- As pointed out by Mitchell (1960), water content has ized w/wL increases, meaning that wL defines the magni- been used as a primary parameter to evaluate the magnitude of G regardless of the type of soil. In this case, the G24 tude of soil strength gained by thixotropy, which results value at wL appears to be around 200 kPa. from the alteration of internal soil structure between By extruding data of Hachirogata clay, because of its dispersion and flocculation. The range of water content strange point in the G24 and w/wL relation shown in between the plastic limit (wP) and the liquid limit (wL) Fig. 3(a), the correlations of hardening G and normalized affecting the thixotropy has been discussed by many w/wL at not only 24 h but also various times are plotted in researchers, for example Skempton and Northey (1952) Fig. 3(b) to examine the effect of thixotropy corresponding and Seed and Chan (1957). They suggested that thixotropic S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 339

increase in the thixotropic effect. However, Skempton and 10000 Ariake Northey showed a consistent increase in the thixotropic Fujinomori effect only from wP until wL, then over the wL value, some Kasaoka NSF samples continued to increase while it dropped off in

(kPa) 1000 Hachirogata others. Mitchell (1960) conducted the experiment on 24 Tokuyama G compacted clays in the range of w from 0.36 to 0.90 of wL and found that thixotropy had the greatest effect at 100 values of w between about 0.56 and 0.74 of wL. Therefore, to our knowledge, an explanation of the w effect in the range over wL has yet to be provided in the literature. Between the ranges of w/wL from 0.75 to 1.35 in this 10 experiment, the effect of thixotropy was found to increase from 0.75 to 1.0 of w/w ratio, which is consistent with

Shear Modulus at 24hrs, L Skempton and Northey (1952), but over wL, the thixo- 1 tropic effect decreases. However, the upper boundary of 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 the water content without any thixotropic effect has not Normalized w/w L been established even at 1.35 times wL. Suthaker and Scott (1997) carried out tests on the thixotropic strength of oil 10000 sand ﬁne tailings which contained extremely high w of Resting Time 5hrs more than 8 times of wL, and proved that the thixotropic 10hrs effect was still there; nonetheless, the properties of this soil 1000 24hrs differ considerably from marine clays. In the case of 48hrs Tokuyama dredged clay, the natural water content con- 120hrs (kPa) tains only 1.20 times of wL, at which point thixotropic G 192hrs effect is still pronounced. From this ﬁnding, it can be 100 assumed that thixotropy plays an important role in altering the properties of natural soil. Fig. 4 shows the relationship between normalized G with

Shear Modulus, 10 G24 and the logarithm of elapsed time for different clays. As can be seen in the ﬁgure, tested samples may be classiﬁed into three groups in accordance with their w/wL ratios, i.e., less than 0.95, between 0.95 and 1.15, and 1 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 greater than 1.15. The relation at around wL shows the Normalized w/w highest gradient to elapsed time, followed by the greater L water content group, while the low water content samples 2.4 showed the smallest increment of G against time. G /G 2.2 10 24 G /G 48 24 w w 2.0 / L < 0.95 G /G F1(40.9%; 0.84) 1.8 120 24 3.0 K3 (52.0%; 0.84) 24 1.6 K5 (46.0%; 0.74) N1 (54.0%; 0.93) G/G 1.4 2.5 N2 (48.6%; 0.86) 0.95

24 2.0 F2 (52.5%; 1.08) G

0.8 / K1 (62.3%; 1.00) Normalized

G K2 (60.6%; 0.97) 0.6 K4 (62.1%; 1.00) 1.5 w/w > 1.15 0.4 L T1 (134.3%; 1.21) 0.2 T2 (149.3%; 1.35) 1.0 0.0 Normalized 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 Normalized w/w L 0.5

Fig. 3. (a) Relationship between G and normalized w/wL,(b)G and normalized w/w at different resting times, (c) Normalized G/G versus w/w . L 24 L 0.0 effect at or close to w can be very low or cannot be 0.1 1 10 100 1000 P Time (hr) observed. The ﬁndings of most investigations presented in the literature were that increasing water content caused an Fig. 4. Relationship between normalized G/G24 with time. 340 S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345

4.2. Undrained shear strength by vane test steeper gradient were recognized than those in Fig. 3(a). At wL, the magnitude of su24 varies from about 1 to 2 kPa, Soil stiffness is found to increase due to the thixotropic with values somewhat lower than those suggested by phenomenon as mentioned in the above section. The Wood (1990), who did not consider the thixotropic hard- increase in the strength caused by the thixotropy was also ening effect and recommended 2 kPa of su at wL. measured with a laboratory vane test on ten different specimens, as depicted in Fig. 5. It is interesting to note 4.3. Comparison between shear modulus and undrained that only some restricted conditions show a slight increase shear strength in su with time, unlike thixotropically gained G illustrated in Fig. 2(a) and (b). The constant s with time is observed u The relationship between thixotropic hardening G and su for some samples with a higher water content than the at various elapsed times is shown in Fig. 7 together with liquid limit, but as previously demonstrated, G did increase that obtained from the cement-treated soil (CTS) material even at these conditions. This interesting ﬁnding will be proposed by Seng and Tanaka (2011). They found that the discussed in the following section in more detail. G and su relation of CTS can also be applicable to most of The values of s measured at 24 h (s ) are plotted with u u24 the natural clays found throughout the world with su normalized w/wL in Fig. 6, in the same manner as G24 varying from 10 to 150 kPa. It is observed in Fig. 7 that at shown in Fig. 3(a). An almost linear correlation between very high water content corresponding to low initial s in the logarithm scale and w/w ratio was also u24 L strength, su remains constant until a certain time, unlike identiﬁed, but more remarkable scatters and a slightly G. After G reaches a certain value, su starts to increase and the relation of G and su seems to approach the same line as 11 CTS. Indeed, Seng and Tanaka (2011) reported that even F1 (40.9%; 0.84) 10 F2 (52.5%; 1.08) CTS material behaves in a similar way when the strength K1 (62.3%; 1.00) of the CTS is extremely small. However, when the strength 9 K2 (60.6%; 0.97)

(kPa) of the CTS is greater than 1 kPa, the G and su correlation

u K4 (62.1%; 1.00)

s 8 K5 (46.0%; 0.74) for each sample forms a linear function, unlike very soft 7 N1 (54.0%; 0.93) H1 (241.3%; 0.98) clays which show a monotonic increase. Both behaviors, 6 T1 (134.3%; 1.21) the constant values and slow increases of su, are quite T2 (149.3%; 1.35) 5 interesting and might be associated with either viscosity or 4 the strain rate effect, which appears to be an important 3 factor governing soil strengths especially when material remains soft. Further investigation is necessary, however, 2

Undrained Shear Strength, to conﬁrm this presumption. 1 An alternative explanation for the constant su until 0 certain times may be that it can be attributed to the order 0.1 1 10 100 1000 Time (hr) of the strain level, as presented by Seed and Chan (1957) and Mitchell (1960). They investigated the thixotropic Fig. 5. Variation in su with time. 10000 10 CTS (Seng and Tanaka, 2011) 72 F1 (40.9%; 0.84) 120 24 F2 (52.5%; 1.08) 10 (kPa) 0.5

u24 K1 (62.3%; 1.00) 1000 s K2 (60.6%; 0.97) K4 (62.1%; 1.00) 144 (kPa) 227.5 24 96 K5 (46.0%; 0.74) G 168 18 48 F1 (40.9%; 0.84) N1 (54.0%; 0.93) 4 24 H1 (241.3%; 0.98) 100 20 288 F2 (52.5%; 1.08) T1 (134.3%; 1.21) 5 0.5 K1 (62.3%; 1.00) 1 72 120 T2 (149.3%; 1.35) 24 K2 (60.6%; 0.97) 168 11 120 96 2 K4 (62.1%; 1.00) 0.5 10 24 K5 (46.0%; 0.74) Shear Modulus, 10 0.5 N1 (54.0%; 0.93) H1 (241.3%; 0.98) T1 (134.3%; 1.21) T2 (149.3%; 1.35) 1 0.1 1 10 20 Undrained Shear Strength at 24hrs, 0.1 Undrained Shear Strength, s (kPa) 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 u Normalized w/w L Fig. 7. Relationship between G and su of very soft clays and CTS (after Seng and Tanaka, 2011). Numbers with symbols indicate the elapsed Fig. 6. Relationship between su and normalized w/wL. time (h). S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 341 strength of compacted clays using triaxial compression 2, 4, 8, 16, and 20 kPa. Using the same input sine wave tests and reported that the strength ratio measured at signal with 1 kHz frequency as depicted in Fig. 1, the smaller strains appears much greater than at large strains. arrival time of the received waves become faster as the The strain level measured by the bender element test, consolidation pressure is increased. In this case, the travel which was adopted in this study, is less than 0.001%, while distance decreases with settlement; however, the measured the strength obtained by vane test corresponds to large Vs was found to increase. Interestingly, no compressional strain, where the progressive failure to attain the peak waves appear in front of the shear waves in Fig. 1, but the strength may destroy weak bounds created by thixotropy. large magnitude of the reflected waves, as encircled in the Another reason for the constant su may be the insertion of figure, was identified in accordance with the decreases in the vane blade disturbs the samples, and therefore the arrival time. validity of the results. When a specimen is too soft, the Fig. 9(a) and (b) presents the relationships between the 0 insertion of vane blade might destroy the particles bound- void ratio (e) and the vertical effective stress (sv) for ing created by cement or thixotropy. Kasaoka and Hachirogata clays, which were prepared with two different initial water contents (wi). As can be seen 5. Shear modulus measurement under low consolidation from the figures, the specimens with higher wi (for instance, pressure K.C2 and H.C2) show larger void ratios at each loading step than those with lower wi. This indicates that e cannot 0 Soil samples, namely the Kasaoka, NSF, and Hachir- be determined by sv alone since it is influenced by wi. This ogata clays, were mixed with different initial water contents and consolidated under very low pressures as small as 2–20 kPa. Bender elements equipped inside the cell were 2.2 used to monitor the shear modulus developed during the 2.1 K.C1 (63.2%) consolidating process, focusing mainly on G at the end of K.C2 (129.6%) primary consolidation (EOP) and after EOP, i.e., during 2.0 the secondary consolidation. In this paper, the time at 1.9

EOP was obtained by the Taylor method, i.e., the root e 1.8 time method. 1.7

5.1. Shear modulus measurement at EOP Void Ratio, 1.6

1.5 Fig. 8 shows a typical example of the shear wave velocity 1.4 (Vs) measurement at EOP, for Kasaoka sample with 129.6% of initial water content, at ﬁve loading levels: 1.3

1.2 0 2 4 6 8 1012141618202224 10 Vertical Effective Stress, ' (kPa) Input Signal v 0 Sine 1 kHz 8.5

Voltage (V) -10 H.C1 (319.0%) 2 kPa 8.0 H.C2 (446.0%)

7.5

4 kPa e 7.0

8 kPa 6.5 Void Ratio, Ratio, Void

6.0 16 kPa

5.5 20 kPa 5.0 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 81012 ' Time (ms) Vertical Effective Stress, v (kPa)

0 Fig. 8. Shear wave velocity measurement under consolidation process for Fig. 9. Relationship between e and sv: (a) Kasaoka clay and (b) Kasaoka clay with 129.6% of initial water content. Hachirogata clay. 342 S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 tendency, which has been reported by many researchers, e 0 0 G ¼ 41; 600 0:67 sv0:5 ðin kPaÞ becomes prominent when sv is very small. Nevertheless, 1þe the shear modulus at EOP (G ) of the same clay appears EOP ðShibata and Soelarno; 1978Þð5Þ to be approximately equal for all tested soils, as illustrated in Fig. 10(a) and (b) for Kasaoka and Hachirogata clays, 1:5 0 G ¼ 5000e sv0:5 ðin kPaÞðShibuya and Tanaka; 1996Þ ð6Þ respectively. It may be concluded that G is mainly governed by pressure, and the effect of void ratio on G 2 0:2 1þOCR0:5 0:6 G ¼ 20; 000w0:8 OCR s0 0:8 can be ignored. L 3 3 v The estimation of the G value from fundamental para- ðin kPaÞðKawaguchi and Tanaka; 2008Þð7Þ meters is quite a challenging topic in the geotechnical 0 0 engineering ﬁeld and its formulation has been extensively where p is the mean effective stress; sv is the vertical proposed by many researchers. The following are examples effective stress; OCR is over consolidation ratio. (Hardin and Black, 1969; Shibata and Soelarno, 1978; Eq. (4) proposed by Hardin and Black (1969) is well Shibuya and Tanaka, 1996; Kawaguchi and Tanaka, known, but this formula was originally derived for prediction 2008): ofcleansandandfoundtobeoverestimatedforverysoft clays used in this study. As for Eq. (5), if the test results from ð2:97eÞ2 the present study under very low consolidation pressures, i.e., G ¼ 3270 p00:5 ðin kPaÞðHardin and Black; 1969Þ 1þe under large e are applied to this formula, the calculated G ð4Þ becomes negative. Formulae using faction of e, including Shibuya and Tanaka (1996), is not suitable because G is not affected by measured e, as previously mentioned. The 3600 formula proposed by Kawaguchi and Tanaka (2008) is one of a few equations without using the e parameter, and 3200 showed a successful applicability to both reconstituted and in situ soils. However, they did not prove its validity to the

(kPa) 2800 low conﬁning stress conditions dealt with in this study. To EOP

G 2400 examine the applicability of their equation to G measured 2000 under low consolidation pressures, the comparison is made as illustrated in Fig. 11. The consistent results are identiﬁed for 1600 Hachiragata and NSF clays, though they somewhat over- estimate the value of G for Kasaoka clay. 1200 Since GEOP is the shear modulus after the dissipation of 800 the excess pore water pressure at a constant effective stress, K.C1 (63.2%) it is attained or increased due to changes in the effective 400 K.C2 (129.6%) 0 stress. By referring to the relations between sv and GEOP in 0 Fig. 10(a) and the relations between G and time in 0 2 4 6 8 1012141618202224 Fig. 2(a), the process of the thixotropic hardening can be Vertical Effective Stress, σ' (kPa) 0 v replotted as sv building up with time. This relation for Kasaoka clay is shown in Fig. 12, as an example. 2600 2400 8000 2200 2000 7000 (kPa) Shear Modulus at EOP, 1800 EOP 6000 G 1600 1400 5000 1200 4000 1:1 1000

800 3000 600

Shear Modulus at EOP, 400 H.C1 (319.0%) 2000 (Kawaguchi and Tanaka, 2008)

200 H.C2 (446.0%) G Kasaoka 1000 Hachirogata 0 0 2 4 6 8 10121416182022 NSF 0 σ' Vertical Effective Stress, v (kPa) 0 10002000300040005000600070008000 G (This study) 0 Fig. 10. Relationship between GEOP and sv: (a) Kasaoka clay and (b) Hachirogata clay. Fig. 11. Comparison of G with Kawaguchi and Tanaka (2008). S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 343

28 constant values of suction for intact clays at about Suction K (49.8%; 0.80) 26 K1 (62.3%; 1.00) 20 min over 1 h of measurements. From these experimental 24 K2 (60.6%; 0.97) results, it might not be reasonable to conclude that the K3 (52.0%; 0.84) 22 K4 (62.1%; 1.00) thixotropically gained stiffness is completely caused by the 20 K5 (46.0%; 0.74) increase in the suction, though the possibility that the 18 suction created by particles rearrangement may be partly 16 contributed to the increase in G due to thixotropy 14 phenomenon cannot be denied. 12 10 5.2. Shear modulus measurement during secondary Effective Stress (kPa) 8 consolidation 6 4 The development of G during secondary consolidation 2 (GSC) is also an interesting topic, and has been discussed 0 for decades by researchers, including Anderson and 0.1 1 10 100 1000 Woods (1976), Athanasopoulos and Richart (1983), Time (hr) Lohani et al. (2001), and Howie et al. (2002). Most Fig. 12. Relationship between effective stress and time. researchers showed that the value of GSC increases until a point where it is almost linearly correlated with logarithm of time. Using data from Anderson and Woods Suthaker and Scott (1997) pointed out the importance of (1976) and Lohani et al. (2001), the increment of G is the consolidation effect caused by the self-weight of SC plotted in Fig. 13. The pressures applied in their study sediments for increasing the long-term strength of fine varied from 35 to 450 kPa, which is a considerably larger tailing material. Given that particle density is 2.61 g/cm3 range than those in the present study. The G /G for and assuming that the specimen is fully saturated, then the SC EOP Anderson and Woods’s data was derived from the square vertical effective stress at 5 cm depth, which is equivalent of V /V ratio (where V and V are the shear to the middle point of the mold for measuring G by the sSC sEOP sSC sEOP wave velocity during secondary consolidation and at the bender element test in the present study, is 0.3 kPa for EOP, respectively) by assuming that the soil density at w/w at 0.8 and 0.2 kPa for w/w at 1.2. The order of these L L EOP is equivalent to that during secondary consolidation. self-weight consolidation pressures are far smaller than s0 v The gradient of G /G ranged between approximately values in Fig. 12. Therefore, although the authors do not SC EOP 0.05 and 0.30 per log cycle of time, regardless of the soil completely deny the possibility of the involvement of self- type and initial water content. Mitchell and Soga (2005) weight, the consolidation effect during the thixotropic summarized the variation of DG/G ratios (where DG is hardening process on the measured G values can be 1000 shear modulus increase in every 10-fold time increase and neglected in this study. G is shear modulus at 1000 min) of clay and sand As briefly explained in the introduction, there is a well- 1000 materials from many researchers. The ratios varied from known hypothesis that thixotropic hardening is caused by the rearrangement of soil particles to a stable condition (for example, Mitchell, 1960). During this process, despite 1.7 the absence of changes in the confining stress, Day (1955) Anderson and Woods (1976) 35 kPa and Mitchell (1960) suggested that negative pore pressure 1.6 69 kPa or suction is built up within the soil causing increase in 138 kPa effective stress. Ideally, the meaning of suction and 1.5 EOP

effective stress should be the same under the fully saturated G / conditions. To conﬁrm these results, a suction test using a SC 1.4 ceramic disc was conducted following the method pre- G sented in Horng et al. (2010). It can be seen from Fig. 12 1.3 that within 120 h of monitoring, the Kasaoka sample with

Normalized Lohani et al. (2001) 49.8% (w/wL ¼0.80) of water content showed a ﬂuctuated 1.2 200 kPa increase of suction to a maximum value about 5 kPa, 450 kPa which is signiﬁcantly lower than the 14 kPa attained by the 1.1 135 kPa present thixotropy test for a sample with a water content 300 kPa of 52.0%. In addition, when water content was added to 1.0 62.6% (w/wL ¼1.0), no change in suction was observed. 1 10 100 1000 However, the reliability of the ceramic disc method used in Normalized Time/Time at EOP this study for longterms measurements is still required for Fig. 13. Relationship between normalized GSC/GEOP with time by EOP further research, since Horng et al. (2010) obtained from other reported data. 344 S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345

0.05 to 0.25. It can be concluded that the GSC/GEOP and stress, the volume is changing, while in the thixotropic DG/G1000 ratios are somewhat similar. condition, the volume change does not take place. There- The experimental results of the secondary consolidation fore, without experimental conﬁrmation, it might be under low pressure in this study are shown in Figs. 14 and considered reasonable to assume that the increase rate of 15, for Hachirogata and NSF clays, respectively. Again, hardening G owing to secondary consolidation should be the average gradients of GSC/GEOP were approximately greater than that of thixotropy due to the decreasing void 0.25 for Hachirogata clay and 0.15 for NSF clay. The ratio during secondary consolidation. However, the results Hachirogata clay had a higher gradient. Kokusho (1987) from this study show that when the average incremental suggested that the ratios increased with increases in the gradient of normalized thixotropic G/G24 against time is plasticity index. As seen in Table 1, the plasticity index for compared with that of the general tendency of GSC/GEOP, Hachirogata clay is greater than that for NSF clay. It can the increment stiffness due to thixotropy is much higher be therefore concluded that even at very low pressures, the than that during the secondary consolidation. This can be gradient during the secondary consolidation is the same as explained in terms of a rearrangement of interparticles that under the conventional stress range. within the soil microstructure. From the explanation of the It should be noted that the even though the condition of thixotropic mechanism provided by Mitchell (1960) men- the secondary consolidation is under a constant effective tioned beforehand, Anderson and Woods (1976) proposed the mechanism of increase in soil stiffness during second- 1.5 ary consolidation as the energy imbalance within soil structure took place at the end of primary consolidation, then particles rearrange with time until an equilibrium 1.4 state, resulting in a stronger bound or greater rigidity. However, based on the results presented in this study, the small gradient of incremental G which undergoes second- EOP

G ary consolidation might be caused by pressure which

/ 1.3

SC constrains the particle from being arranged freely, unlike G under the unconﬁned stress condition (thixotropy). In 1.2 H.C1 (319%) addition, the applied pressures which act as induced energy 2 kPa to the soil mass from the beginning of consolidation

Normalized 4 kPa process are not removed, leaving a slight imbalance in 1.1 8 kPa energy caused by the dissipated pore water pressure, 16 kPa meaning that a small amount of stiffness is left to be 20 kPa gained. Another possibility is that the continuous change 1.0 in volume during secondary consolidation may well 1 10 100 destroy the interparticle bounding, and, as a consequence, Normalized Time/Time at EOP the rate of increase in G is noticeably low.

Fig. 14. Normalized GSC/GEOP and time by EOP for Hachirogata clay. 6. Conclusions

The experimental results of thixotropic phenomenon 1.8 measured for various clays are presented and compared N.C2 (105.2%) 1.7 with values obtained by consolidation tests at low pres- 2 kPa sures. The main conclusions which can be drawn are as 4 kPa follows: 1.6 8 kPa 16 kPa EOP

G 1.5 (1) The bender element test is a powerful tool and an / 20 kPa SC appropriate method for evaluating the thixotropic G 1.4 hardening stiffness of very soft clays, since it is able to detect even small changes in G with an extremely 1.3 small strain. Normalized (2) G at 24 h for w is around 200 kPa. 1.2 L (3) Regardless of the type of soil, thixotropy affects the 1.1 clay most strongly at around the liquid limit state and becomes less remarkable at lower and higher water 1.0 contents. 1 10 100 1000 (4) The correlation between G and su for very soft clays Normalized Time/ Time at EOP is analogous to that of cement-treated soil proposed

Fig. 15. Normalized GSC/GEOP and time by EOP for NSF clay. by Seng and Tanaka (2011). Additionally, similar S. Seng, H. Tanaka / Soils and Foundations 52 (2012) 335–345 345

behavior is recognized at very low strengths, where su Day, P.R., 1955. Effect of shear on water tension in saturated clay, appears constant while G increases. Annual Reports I and II. Western Regional Research Project, W-30. (5) The increment of the shear modulus under secondary Hardin, B.O., Black, W.L., 1969. Vibration modulus of normally consolidated clay. ASCE 95 (SM6), 1531–1537. consolidation is relatively low compared with that Horng, V., Tanaka, H., Obara, T., 2010. Effects of sampling tube developed during the thixotropic process. It is sug- geometry on soft clayey sample quality evaluated by nondestructive gested that the difference may due to the different methods. Soils and Foundations 50 (1), 93–107. constraint pressures between the secondary consolida- Howie, J.A., Shozen, T., Vaid, Y.P., 2002. Effect of ageing on stiffness of tion and thixotropic condition, i.e., the free arrange- very loose sand. Canadian Geotechnical Journal 39 (1), 149–156. ment of soil particles is prevented under the former Kawaguchi, T., Mitachi, T., Shibuya, S., 2001. Evaluation of shear wave travel time in laboratory bender element test. In: Proceedings of the condition, while under the thixotropic condition, the 15th ICSMGE 1, 155–158.

soil particles can be freely arranged. Alternatively, the Kawaguchi, T., Tanaka, H., 2008. Formulation of Gmax from reconsti- continuous changes in volume during secondary con- tuted clayey soils and its application to Gmax measured in the ﬁeld. solidation destroy the particle bounding. Soils and Foundations 48 (6), 821–831. Kokusho, T., 1987. In-situ dynamic properties and their evaluations. In: Proceedings of the 8th ARC on SMFE, Kyoto 2, 215–240.

Lohani, T.N., Imai, G., Tani, K., Shibuya, S., 2001. Gmax of fine-grained From the present study, the importance of the thixo- soils at wide void ratio range, focusing on time-dependent behavior. tropic hardening phenomenon for understanding soil Soils and Foundations 41 (5), 87–102. behavior with a high water content is clear. It is well Mewis, J., 1979. Thixotropy—a general review. Journal of Non-New- known that the void ratio or water content in situ is much tonian Fluid Mechanics 6 (1), 1–20. Mewis, J., Wagner, N.J., 2009. Thixotropy. Advances in Colloid and larger than that predicted by laboratory consolidation tests 0 Interface Science 147–148 (1), 214–227. based on the e-logp relation at the EOP (for example, Mitchell, J.K., 1960. Fundamental aspects of thixotropy in soils. Tanaka et al., 2004). If we consider the secondary Journal of the Soil Mechanics and Foundations Divisions, ASCE 86 consolidation effect, in situ void ratio should be smaller (3), 19–52. than that predicted from the e-logp0 relation at the EOP. Mitchell, J.K., Soga, K., 2005. Fundamentals of Soil Behavior. John To cope with such inconsistency, Burland (1990) has Wiley & Sons, Inc., New Jersey. Osipov, V.I., Nikolaeva, S.K., Sokolov, V.N., 1984. Microstructural proposed a concept of sedimentation consolidation line changes associated with thixotropic phenomena in clay soils. Geó- (SCL), considering the cementation or fabric effect. technique 34 (2), 293–303. Further studies on thixotropy are expected to provide an Seed, H.B., Chan, C.K., 1957: Thixotropic characteristics of compacted important key to understanding such a phenomenon. clays. In: Proceedings of the American Society of Civil Engineers, vol. 83 (SM4), pp. 1–35. Seng, S., Tanaka, H., 2011. Properties of cement-treated soils during the Acknowledgments initial curing stages. Soils and Foundations 51 (5), 775–784. Shibata, T., Soelarno, D.S., 1978. Stress–strain characteristics of clays This research was supported by the Program for Pro- under cyclic loading. JSCE 276, 101–110. moting Fundamental Transport Technology Research Shibuya, S., Tanaka, H., 1996. Estimate of elastic shear modulus in from the Japan Railway Construction, Transport and Holocene soil deposits. Soils and Foundations 36 (4), 45–55. Skempton, A.W., Northey, R.D., 1952. The sensitivity of clays. Geótech- Technology Agency (JRTT). nique 3 (1), 30–53. Suthaker, N.N., Scott, J.D., 1997. Thixotropic strength measurement of References oil sand fine tailings. Canadian Geotechnical Journal 34 (6), 974–984. Tanaka, H., Kang, M.S., Watabe, Y., 2004. Ageing effects on consolida- Anderson, D.G., Woods, R.D., 1976. Time-dependent increase in shear tion properties –Based on the site investigation of Osaka Pleistocene modulus of clay. Journal of the Geotechnical Engineering Division, clays. Soils and Foundations 44 (6), 39–51. ASCE 102 (GT5), 525–537. Viggiani, G., Atkinson, J.H., 1995. Interpretation of bender element tests. Athanasopoulos, G.A., Richart, F.E., 1983. Effect of creep on shear Geótechnique 45 (1), 149–154. modulus of clays. Journal of the Geotechnical Engineering Division, Wood, D.M., 1990. Soil Behavior and Critical State Soil Mechanics. Press ASCE 109 (10), 1217–1232. Syndicate of the University of Cambridge, USA. Barnes, H.A., 1997. Thixotropy—a review. Journal of Non-Newtonian Yamashita, S., Kawaguchi, T., Nakata, Y., Mikami, T., Fujiwara, T., Fluid Mechanics 70 (1/2), 1–33. Shibuya, S., 2009. Interpretation of international parallel test on the Burland, J.B., 1990. On the compressibility and shear strength of natural measurement of Gmax using bender element. Soils and Foundations 49 clays. Geótechnique 40 (3), 329–378. (4), 631–650.