An Investigation into Dynamic Behaviour of Reconstituted and Undisturbed Fine-grained Soil during Triaxial and Simple Shear

13 This study aims to evaluate the factors controlling the sensitivity of fine-grained soils to 14 seismic stresses and revise the criteria previously proposed by the authors to diagnose 15 liquefaction. To this end, dynamic tests have been performed on artificial mixes as well as 16 natural soils from a wide area of an earthquake devastated city (Adapazari) using two types 17 of dynamic testing. Studies have led to findings suggesting that the gray area between 18 susceptible and non-susceptible soils proposed by several investigators in the past can now 19 be dispensed with. Although physical properties of fine-grained soil supply sufficient 20 information for diagnosis, dynamic simple shear test is found to be a convenient and rapid 21 way to confirm the judgement. However, it has been seen that dynamic testing alone may not 22 be the last word in the determination of liquefaction, and physical properties should also be 23 addressed. Anomalies observed in test results are also discussed. Conclusions show 24 significant differences from existing proposed criteria in the literature.


INTRODUCTION AND BACKGROUND
The issue of the behaviour of fine-grained soils subjected to seismic loads is controversial.
Although some evidence concerning primary liquefaction and the cyclic mobility of silty/clayey soils have been presented in the literature (Ross et al. 1969), some observers remain skeptical to the failure of ground profiles dominated by fine-grained soil during earthquakes.In particular, possibility of triggering primary liquefaction in such soils is often questioned.
The frequently referred to Chinese Criteria on this subject for evaluating silts has been widely criticized because majority of soils used for identifying liquefiability were in fact clays of low plasticity (Bray and Sancio 2006).However, it will be seen in the following sections that the results of this study have confirmed the vulnerability of low plasticity clays (CL) as well as silts.
The study presented in this paper emanates from the observations of the authors during and after the destructive earthquakes in the city of Adapazari, Turkey in 1999.The results of ensuing research programs conducted on the topic are reported here (Önalp et al. 2007, 2010).
It is estimated through surface observations and subsequent laboratory testing that the ground in about 25% of the total area of the city indicated evidence of failure as depicted in Figure 1 (Bol et al. 2007).The vulnerable soil profiles were predominantly nonplastic silts (ML) and occasional silty sands (SM).The silts were found to contain up to 30% bentonitic clay.The amount of clay depends on the speed and the duration of the spring floods of the river Sakarya which inundated the city almost biannually and remained in the forms of lagoons and marshes for months.An early XIX th century traveler described the scene (Texier 1882), citing water buffaloes immersed in large slurry pools on both sides of the road as he was entering the town.Two large dams were constructed upstream in the late sixties which ended flooding.
Subsequently, desiccation of the top layers in the profiles throughout summers has been causing increasing overconsolidation effects for the past fifty years.

THE SCOPE
This paper gives an account of the results of two complementary research programs aimed at understanding the fundamental features of liquefaction in silty soil.The initial study was performed by determining the measurement of the dynamic properties of Adapazari silt reconstituted by mixing it with different percentages of bentonite and kaolinite.Its aim was to understand the influence of the type of clay mineral on its dynamic behavior (Arel et al. 2018).
The data obtained during testing of 'artificial' samples were then used to interpret the failure mechanism in natural soil.
Towards this aim, dynamic properties of the natural soils sampled (UD) strategically throughout the city at 57 locations were determined by laboratory testing while performing simultaneous in-situ tests (SPT, CPTu) at these locations.This paper intends to present a compendium of two research programs.

SEISMIC GROUND FAILURE
Primary liquefaction and cyclic mobility are two main modes of ground failure observed during earthquakes.In the early stress-based approach, initial liquefaction, nowadays referred to as cyclic mobility was described to be due to decrease in effective stress down to zero as a result of rapid rise in porewater pressure (Kramer 1996).That is, the residual strength of the soil is higher than that needed to maintain static equilibrium.The alternative approach was to adopt the strain-based model where soil deforms excessively, hence the name 'flow liquefaction'.In this case the residual strength of loose or soft soil is lower than that needed to maintain static equilibrium (Kramer and Elgamal 2001).Such attributes may have scientific and academic value, but it is often not possible to differentiate between the two events in the field.Attempting to simulate such field conditions by means of short-term laboratory dynamic testing thus deserves scrutiny.Nevertheless, the excess pore pressures measured in laboratory tests appear to be the more reliable variable in the process.Because those generated during a test are quite similar to those recorded in-situ during an earthquake.

THE METHOD
A drilling program of 15 CPT soundings and 15 boreholes was implemented to sample soils from several districts of Adapazari where ground failure was observed as well as those clayey silt and clay sites where no ground failure had been reported.The results of the simultaneous dynamic triaxial (CTX) and simple shear (DSS) tests on the UD samples were then analyzed with reference to the physical/mechanical properties of the soils to define failure conditions.
The findings of the previous mineralogical study conducted on reconstituted samples of Adapazari silt were used as reference to establish upper and lower 'envelopes' to gain a better understanding of the behaviour of natural soils during shaking (Donahue et al. 2007).The rise of excess porewater pressures and ensuing deformations were then evaluated to prognose failure.

TESTING RECONSTITUTED SAMPLES
This initial part of the study on reconstituted samples was implemented to support and evaluate the findings of the study to be carried out on UD samples.Adapazari silt contains 5 to 30 percent clay, which is mainly montmorillonitic (Donahue et al. 2007).200 kg sample was collected from a liquefaction site to prepare its various blends.It was thoroughly mixed with excessive amount of water in a tank and the supernatant drained after 30 minutes to remove the clay fraction.The process was repeated until most of the clay component was eliminated.The washed silt was then air dried and mixed with increasing percentages of bentonite and kaolinite.The samples mixed with bentonite classified as CI (intermediate plasticity clay) through CH (high plasticity clay).Those samples mixed with kaolinite indicated symbols ML (low plasticity silt) and CL (low plasticity clay).This suggested that the kaolinite samples might eventually represent the mixes that are prone to liquefaction, and bentonite samples constituting the resistant group.
All reconstituted samples were consolidated to a pressure (c) of 100 kPa (Ishihara 1993), representing the characteristic depth of liquefaction in Adapazari and then tested in triaxial as well as simple shear conditions at a frequency f = 0.5 Hz. and cyclic stress ratio (CSR) 0.35 at this c.CSR=0.35corresponds to an earthquake of Mw=7.5.
Excess 'peak' pore pressures (uw,max) generated during a test are instantaneous measurements of response and are directly influenced by the deviatoric stresses during the test.To eliminate stress induced effects 'residual' pore pressure (uw,res) is used.It is defined as the value of the deviator stress as it crosses the zero level during a loading cycle (Idriss and Boulanger 2006).
Its value may differ up to 30% from that of uw,max depending on the type of soil.
The results were plotted as residual excess pore pressure ratios (ru,res) as well as peak values against liquid limit, plasticity index, clay fraction, in-situ water content and average size of the samples.An overwhelming number of failed samples plotted above an ru,res of 0.7 which was defined as the 'liquefaction line'.The line intersects the curve corresponding to a liquid limit of 40 (37 for percussion).The plasticity index is also a reasonable indicator of failure at IP <19 but with a smaller R 2 of 0.8.It is noteworthy that all kaolin blends failed, regardless of their clay content (CF< 2 m).This 'calibration' using homogeneous and samples of prescribed composition showed that liquefaction initiates at a residual pore pressure ratio of 0.7 corresponding to a liquid limit of 40.The kaolinite samples which exhibit low plasticity, reached failure conditions invariably at N ≤ 20 loading cycles whereas the bentonite samples showed remarkable resilience and most blends did not fail readily, despite the fact that all samples were tested in normally loaded state.N = 15 cycles was preferred to indicate failure because it is representative of a quake of Mw = 7.5 magnitude experienced in Adapazari.

TESTS ON UNDISTURBED(NATURAL) SAMPLES
The samples were tested in a cyclic triaxial system (CTX) manufactured by Wykeham Farrance in accordance with ASTM D5311 and in the dynamic simple shear device (DSS) manufactured by Geocomp complying with ASTM D6528 conditions.Same equipment and testing methods were used for the samples (Önalp et al. 2010).Results were listed in a table comprising all physical and dynamic findings.They will be provided to researchers wishing to utilize them in their studies.these ultimate values.σn,DSS,ult may be viewed as the inverse of the pore pressure, but more sensitive to changes in the effective stress.Using the critical liquid limit value of 40, Figure 5 indicates that this value corresponds to a n,DSS,ult of 15 kPa, a possible threshold of failure.provide the correct answer all the time and a check on physical properties would be prudent.

Influence of Clay Content
It was found that the liquefied samples plotted below a clay content of 20%, when the components of all UD samples are shown on the Ferret's triangle (Figure 9).This supports the finding that soils with clay fraction of less than this are most likely vulnerable (Figure 7c). 14

Influence of Natural Water Content
The liquidity index IL of a fine-grained soil is known to be a reliable indicator of its mechanical behaviour.However, dynamic testing showed that it has no relationship with the pore pressures generated.Alternatively, if liquid limit is a significant indicator of dynamic performance, ratio of the in-situ water content to the liquid limit was expected to be an indicator of liquefaction, as proposed by several investigators.Again, the relationship appears to be non-definitive (Figure 10).Same finding appeared for water contents after the test (wf).
Figure 10.The relationship between in-situ water content and porewater pressure

Influence of Average Size
The average size D50 of a soil reflects mechanical behaviour of a soil as well as its several index properties.It also represents the characteristic pore size of the soil.However, tests on UD samples did not show a reliable correlation with the pore pressures during shaking as can be seen from Figure 11 with unacceptably low R 2 .
Figure 11.The influence of average size on pore pressure (all samples)

Evaluation of Parameters
All foregoing discussion showed once more that behaviour of soils are notoriously unpredictable.Common knowledge dictates that fat clays do not fail/liquefy under normal conditions.Nevertheless, a look at the bar charts in Figure 12 shows that several samples of high and intermediate plasticity reached the state of failure under intense shaking.
Furthermore, all samples of low plasticity clays liquefied.This finding suggests that referring to dynamic laboratory tests alone may be misleading as physical properties can be.It is speculated that another variable, the pore/microstructure may be involved in the process.This requires further research.
. The literature and the test results from this study show that dynamic behaviour of a finegrained soil cannot be identified by a single property.Factors contributing to its vulnerability to seismic acceleration have been defined by five values.These are the liquid limit, plasticity index, clay content, average grain size and water content in-situ.A multiple linear regression analysis performed on these variables indicated that D50 and wn have relatively low correlation coefficients.Accordingly, they were not included in further analyses.The maximum residual pore pressure is expressed as a function of three variables: liquid limit (wL,cone), plasticity index (IP) and clay content (CF) resulting in Equation ( 3) with an R 2 of 0.517 as illustrated in Figure 13.(5) if the soil was nearly elastic as in the case of dense sand (Vucetic and Dobry 1991).Assuming fine-grained soil as an elastic material with a Poisson ratio  of 0.3, one would expect a 30% increase in the DSS readings compared to the z,CTX readings.This was not found to be the case as illustrated by Figure 14 where shear strain in DSS and axial strain in CTX are compared, because there is near proportionality.It was thus decided to assume equality of z and  in the following stages.Figure 15 illustrates the relationship between the consistency and strains at N15 for artificial mixes where wLcone = 40% has shown that it is a reasonable limit where deformations are at the acceptable Double Strain Amplitude (DSA) of 5%.Some bentonite CTX samples showed higher deformations suggesting the effect of high plasticity and void ratio.The trend was confirmed when test results for natural and artificial samples were plotted together.Figure 16 shows that soils with liquid limit higher than 40% are likely to deform below a limit such as 10% DSA that will prevent ground failure by excessive settlement.The

20
When the two outstanding variables, liquid limit and clay fraction were plotted against strain the surface represented a reasonable relationship, as illustrated in Figure 17.The failure surface created in Figure 17 is a surface where the data are concentrated up to the 40% liquid limit value.The zone of the surface which contains liquid limit values above this, is the region that required extrapolation due to insufficient data.The failure surface requires improvement for values above the limit of wL,cone > 40 % and should be used with caution.

DISCUSSION
A comprehensive study on probable behaviour of ground under dynamic conditions was carried out both on reconstituted and undisturbed and samples of Adapazari fine-grained soils.The results for the reconstituted samples support those of UD samples, although majority of the UD samples were found to be overconsolidated with OCR values rising to unprecedented levels.There is a clear transition from vulnerable to resistant when the relationship is represented by a sigmoid function.Several properties have been proposed by previous researchers (liquefaction liquid limit, plasticity index, clay content, average size and in-situ water content) as contributing to liquefaction .However, the results of this study show that the influence of average grain size and natural water content on failure are not significant.
It may be worthwhile to quote that there are several cases recorded in Adapazari where buildings at sites of high plasticity clay that exhibited excessive settlement on clay including toppling of the superstructures, although no sign of liquefaction was observed.

CONCLUSIONS
It has been confirmed by this study that dynamic behaviour of fine-grained soil cannot be identified by a single property.Factors contributing to its vulnerability to seismic acceleration can be prescribed by three values, these being the liquid limit, plasticity index and clay content.Surprisingly, in-situ water content, average grain size and the overconsolidation ratio have not been found to contribute to the process.
The direct simple shear test is effective as the triaxial test to study dynamic behaviour, but simpler and quicker to perform.The ultimate normal stress measured in the DSS test is an additional advantage to diagnose sensitivity to shaking.
The excess residual porewater pressure can now be predicted within reasonable range of confidence.Those soils reaching a pore pressure ratio of 0.75 using equations ( 2) and (3) can be deemed to be vulnerable during earthquakes of magnitude 7.5 or bigger.
Based on past visual experience and laboratory studies, it can be stated that a soil under the groundwater table can be declared as "non-liquefiable" if all the limits below are satisfied: Liquid limit (percussion) wL > 35 Plasticity Index IP > 14

Clay Fraction CF > 19
Excessive deformations recorded during laboratory testing suggest that it may not be reflecting the actual response of the ground in-situ.
It is recommended that the criteria proposed here be checked for less adverse shaking

Figure 1 .
Figure 1.Ground failure map of Adapazari (black lines delineate administrative districts)

Figure 2
Figure 2(a) and Figure 2(b) illustrate the consistency of the natural samples (points) compared with those of the reconstituted samples that were cleaned of their clay component and blended with kaolinite and bentonite (lines).The activity of montmorillonite imparts high plasticity to the silt.Almost double the plasticity index (IP) value was measured with bentonite samples compared to those of kaolinite at a clay fraction (CF) of 15%, the typical CF value for natural Adapazari silt.The full blue line representing natural Adapazari soils plots between the two lines developed for the reconstituted samples albeit closer to the bentonite line.This is possibly due to their similar clay mineral contents.The natural samples exhibit significantly lower R 2 values (0.68) due to their different sand contents and type of

Figure 2 .
Figure 2. The influence of clay content on the consistency of reconstituted and natural Adapazari soils (a) Liquid limit by fall cone (b) Plasticity index (points represent natural samples)

Figure 3 .
Figure 3. Behaviour of reconstituted Samples (a) Liquid limit (b) Plasticity index

Figure 4 .Figure 5 .
Figure 4.The progress of the measured normal stress during DSS test (a) ML silt (b) CH clay

Figure 6
Figure 6 illustrates examples from the testing.Figure 6(a) from a CTX shows that the porewater pressure ratio has not reached unity after 200 cycles of loading.Conversely, Figure 6(b) shows that failure was reached at 15 cycles.

Figure 6 .
Figure 6.Results from dynamic tests on (a) CH sample (b) ML sample

Figure 7 .
Figure 7.The effect of consistency and clay content on excess pore pressures generated (a) liquid limit (b) plasticity index (c) clay content An interesting coincidence appeared when the curves for the artificial mixes and natural samples were superposed.Figure 8 illustrates this feature.It can be observed that the intersection point indicates ru,res = 0.7 and wL,cone = 40%, establishing a reference for the evaluation of natural samples.Same intersection was found to apply to the plasticity index.

Figure 8 .
Figure 8.Comparison of results for natural and artificial samples

Figure 9 .
Figure 9. Position of the samples on the classification chart

Figure 15 .
Figure 15.Deformations of the artificial samples natural samples demonstrate higher resistance to shaking in general, possibly due to OCR values higher than unity.The unexpected behaviour of clayey samples (8) deforming less as consistency increased, requires further investigation.

Figure 16 .
Figure 16.Deformability as a function of liquid limit (all samples)

Figure 17 .
Figure 17.Failure surface with reference to deformation number at peak pore pressure OCR: Overconsolidation ratio ru: Pore water pressure parameter ru,res: Residual pore water pressure ratio ru,res,N15: Residual pore water pressure ratio at 15 : Liquid limit by cone wL,perc: Liquid limit by percussion (Casagrande) γ: Deformations in DSS εz: Deformations in CTX σc: Preconsolidation pressure σn: Normal stress σn,DSS,ult: Ultimate normal stress at DSS τ: Shear stress REFERENCES

Table 1 .
Table-I shows 17 characteristic results from the database of 57 samples.Characteristic properties of Adapazari soils (UD samples) It can be seen from Columns 1 and 2 that almost all the UD samples were collected from 168 below the groundwater table.Column 3 lists the number of cycles required to reach peak 169 porewater pressure during CTX and DSS tests.Maximum and residual porewater pressures170 recorded are listed in Column 4 and Column 5.An interesting feature of the DSS test is the progress of the vertical stress measured as the 172 number of cycles increases.The n,DSS decreases steadily with the number of cycles because 173 the porewater pressure is rising.It was found that for a sample which liquefied readily, the 174 normal stress rapidly dropped to zero, whereas a sample which did not fail sustained the Abbreviations: GWL: Groundwater level; Nuwmax: cycle number at uw,max; uw,max: max.pore pressure; 164 uw,res: residual pore pressure; σn,DSS,ult: ultimate normal stress in DSS; wL,cone: liquid limit by fall cone; 165 Ip: plasticity index; IL: liquidity index; Sr,: degree of saturation; CF: clay fraction; σc: preconsolidation 166 pressure; OCR: overconsolidation ratio; LIQ?: judgement on liquefaction; NL: normally loaded 167 stresses and was employed as an extra tool to check on failure.Column 6 in Table-I lists