EFFECTS OF SAMPLING DISTURBANCE ON DYNAMIC
PROPERTIES OF SATURATED SANDS
Carlos Funes∗
MEE09191

Supervisor: Akio ABE∗∗

ABSTRACTDisturbance effects on dynamic properties of saturated sand were evaluated in the laboratory by meansof cyclic undrained triaxial tests. For this purpose, undisturbed samples were subjected to deformationand liquefaction tests, the sand that constituted the undisturbed samples was kept and oven dried toreconstitute samples. From the shear modulus ratio-strain dependency curves it is shown that there isno significant effect between undisturbed and reconstituted samples for consolidation stresses of 150
and 300 kPa, anyhow as this parameter was reduced the effect turned evident. Damping ratio was also
object of analysis, and very slight effects were noticeable for a consolidation stress of 75 kPa. Benderelement tests were performed, and P-wave velocity for each sample remained fairly constant in spiteof changes in consolidation stress and disturbance condition, and for S-wave velocity the benderelement tests confirmed the results from deformation tests, the difference between both disturbanceconditions represented a minor divergence. Liquefaction tests were also carried out, when comparing
the shear modulus against the liquefaction potential value a very scattered plot was obtained, reflectingthat a large difference in liquefaction potential is present due to the significant change in soil fabric orinternal structure, even among undisturbed samples. A relationship representing disturbance effects ispresented in terms of a maximum shear modulus ratio and liquefaction potential ratio.
Keywords: Liquefaction, sampling disturbance, sand, undrained triaxial tests.

1. INTRODUCTION
During the first week in September of 2008, Tokyo Soil Research Co., Ltd. developed a full soil
investigation in Ooarai site located in Ibaraki’s east coast, and samples were retrieved using the in-situ
freezing method. At present, the in-situ freezing method is considered the most reliable technique toprovide undisturbed samples. In spite of all the benefits that this technique offers, its application
requires a strong economic investment and besides it’s very time consuming. Samples from thatproject have been preserved under proper refrigeration up to now, and they were available to be used
for the purpose of this study. This method is employed when such a soil investigation is reasonably
affordable. Small to medium size projects have to deal with traditional soil investigations that imply
the acquisition of samples in a disturbed condition, therefore lowering the quality of results to be used
to predict possible hazards or to recreate past events. Under or overestimation of results plays animportant role since this implies a higher cost or higher risk. Given that the cheapest mean to obtainsamples is remolding them from a totally disturbed and loose condition, against the fact that the in-situ
freezing method is now considered the most reliable technique to obtain undisturbed samples, thesetwo conditions are to be compared, to quantify the effects of disturbance on deformation
characteristics and for cyclic mobility properties, and specifically to determine the impact on theliquefaction potential. In this sense, samples for both states were prepared to carry out this study.

∗

Seprobia Soil Testing Laboratory, El Salvador.
Head of laboratory, Tokyo Soil Research Co., Ltd. & Lecturer of Tsukuba College of Technology, Japan.

∗∗− 73 −

2. DISTURBANCE EFFECTS ON SHEAR MODULUS-STRAIN RELATIONSHIP
Three were the consolidation stresses underanalysis, namely ɐǯ = 75, 150 and 300 kPa, andfor each of them an undisturbed and areconstituted sample were assigned. Before
analyzing both conditions together, acomparison of the effect of confining stress foreach state is to be made. Fig. 1 gathers the shear
modulus-strain dependency curves for theundisturbed samples and for the 10th cycle. As
expected based on the theoretical background,the shear modulus exhibits a significant growth
as the consolidation stress increases; for thisFigure 1. Effect of consolidation stress on thecase the maximum shear modulus for ɐǯ = 150
shear modulus-strain dependency relationshipkPa represents 1.55 times that presented for ɐǯ= 75 kPa. Go for ɐǯ = 300 kPa is 1.97 and 1.27
times that reported for ɐǯ = 75 and 150 kPa,
respectively. It can be also quantified that themodulus falls down to 50% of its initial value
for a level strain of 1ȿ10-3, 1.1ȿ10-3 and 1.6ȿ
10-3 for a consolidation stress of ɐǯ = 75, 150
and 300 kPa, respectively; and it falls down to
20% at around 4ȿ10-3, 4.5ȿ10-3 and 6.5ȿ10-3 forthe same set of consolidation stresses. As thecyclic loading steps progress and the strain
grows the shear modulus comes reduced, such
Figure 2. Modulus reduction grouped by strains
behavior can be better seen through Fig. 2
for undisturbed samples
where the shear modulus presenting the samestrain has been grouped in single tendencies.
This graph is intended to establish the relationship between the shear modulus for a given
consolidation stress and void ratio function. The expression to correlate Ooarai sand with theconsolidation stress has been determined as expressed by Eq. 1. The expression is calculated for astrain amplitude of 10-5 and therefore a level indicating the maximum shear modulus. Anyhow, otherstrains might be set to determine the shear modulus for that given level of strain. In fact the previous
proposed relationship differs from that proposed by Hardin and Richardt only by 2.9% indicating that
Ooarai sand deformation characteristics have a fair agreement with the original expression. The sameprocedure as used for the undisturbed samples was used and a relationship between the consolidationstress and maximum shear modulus was determined as presented by Eq. 2.
Go = 7200(2.17 − e)2 σ ' 0..5
1+ e

c

(1)

Go = 10600(2.17 − e)2 σ ' 0..4
1+ e

c

(2)

Fig. 3 shows both modulus ratio-strain dependency curves for undisturbed and reconstituted
sample under ɐǯ = 75 kPa, and so does Fig. 4 and 5 for ɐǯ = 150 and 300 kPa, respectively. From Fig.
3 it is visible that disturbance causes a narrow gap between both conditions, indicating a substantial
difference in shear modulus. To obtain a correction function, the hyperbolic model was used and aunique relationship for each condition was established to better fit the laboratory results, thereafter
determining a correction function in terms of the strain. For ɐǯ = 300 kPa the difference comes to beless significant and it can be concluded that no major disturbance effect is found for such a
consolidation stress. Anyhow, and in spite of the slight difference a correction function has been alsoestablished for this case. In consideration of this behavior where the effects of disturbance are in− 74 −

relation to an isotropic consolidation stress, shallow depths are prone to present more disturbanceeffects than deeper layers. For instance, the consolidation stresses of 75 kPa and 150 kPa correspondto a depth of 6.4 m and 15.2 m, respectively. Hence, for a more detailed assessment of the soil profile
behavior when reconstituted samples are meant to be used, deformation characteristics may becorrected through a correction function, for this specific case, Ooarai sand, and Fig. 6 might be used.

Figure 3. Normalized modulus-strain
dependency curve for ı’c = 75 kPa

Figure 4. Normalized modulus-strain
dependency curve for ı’c = 150 kPa

Figure 5. Normalized modulus-strain
dependency curve for ı’c = 300 kPa

Figure 6. Effects of disturbance on shear
modulus-strain relationship3. DISTURBANCE EFFECTS ON DAMPING RATIO-STRAIN RELATIONSHIP
Shear modulus ratio-strain relationship (G/Go – Ȗ) is of primary importance for the analytical study ofsoil response, when acceleration, velocity and displacement are to be examined. Nevertheless, anotherproperty of the same importance is the damping ratio-strain relationship (D – Ȗ). To check theinfluence of disturbance on the damping properties Fig. 7 shows the damping ratio-strain dependency
curves for both the undisturbed and reconstituted state under a consolidation stress of 75 kPa. Due tothe relative small consolidation stress, the initial behavior is very erratic and the final stage represents
a large degradation, therefore it is considered for this study that only the points in the range from 1ȿ
10-4 to 1ȿ10-3 are representative for this curve in particular. In such a manner a function for this range
has been determined for both conditions and thereafter a correction function as well, which is shown in
Fig. 10. This time the hyperbolic model was not employed due to the lack of agreement, the simple useof least squares was used instead. It also has to be said that the variation is small though it wasconsidered that it needs to be corrected. Unlike the previous case, for the consolidation stresses of 150
kPa and 300 kPa no variation in damping ratio is detected between the undisturbed and reconstituted
samples. For ɐǯ = 150 kPa the agreement is found until a shear strain of around 8ȿ10-4, after this point
the large degradation splits the convergence, and it was caused by an abrupt fall in the reconstituted
sample due to a big change in the cyclic loading, so similar consequences are reflected for thedamping ratio characteristics. For ɐǯ = 300 kPa the agreement is notorious for the whole strain range,− 75 −

slightly diverging after a strain of 2ȿ10-3 due to the large degradation itself. The same as for the
shear-strain relationship, it seems from the results that the disturbance effects on the damping
ratio-strain relation get vanished as the consolidation stress grows. It has to be highlighted that thistendency (present in both the shear strain and damping ratio-strain dependency curves), where thedisturbance effects grow weaker as the consolidation stress is raised, has a base upon few data, 6
samples, 2 assigned to each of the three selected consolidation stress values, one undisturbed and one
reconstituted sample.

Figure 7. Damping ratio-strain
dependency curve for ı’c = 75 kPa

Figure 8. Damping ratio-strain
dependency curve for ı’c = 150 kPa

Figure 9. Damping ratio-strain
dependency curve for ı’c = 300 kPa

Figure 10. Correction function for
damping ratio under ı’c = 75 kPa

4. DISTURBANCE EFFECTS ON THE SHEAR AND LONGITUDINAL WAVE VELOCITY
Fig. 11 shows the S-wave
arrivaltime
foranundisturbed
andreconstituted sample undera consolidation stress of
300 kPa. Unexpectedly the
S-wave arrival time for thereconstituted sample isshorter than that presented
by the undisturbed sample,
Figure 11. Shear wave velocity comparison against disturbance condition
checking the height foreach specimen it was found that the velocity for the undisturbed sample was 252 m/s and for thereconstituted sample 257 m/s. It roughly can be said that both conditions report the same shear wave
velocity, taking into consideration also that the density after consolidation was higher for thereconstituted sample than for the undisturbed state, 1.378 and 1.350 g/cm3, respectively. S-wave− 76 −

velocity for ɐǯ = 150 kPa represented a small variation, 218 m/s for the undisturbed state and 212 m/s
for the reconstituted sample. P-wave velocity was also revised for disturbance effects, for this case thesamples subjected to ɐǯ = 150 kPa were compared, as well as the consolidation stress effect it was
found no significant alteration of the P-wave velocity. The P-wave velocity for each of the 5 samples
remains fairly constant in spite of changes in consolidation stress and disturbance condition.

5. EFFECTS OF DISTURBANCE ON CYCLIC MOBILITY CHARACTERISTICS
Liquefaction potential was meant to berelated with the maximum shear modulus,
separately for the undisturbed andreconstituted state, and then comparing
both conditions to quantify the disturbanceeffect. Fig. 12 shows how scattered theactual outputs are; in fact according to thisdata, liquefaction potential for thereconstituted samples comes reduced as Go
increases. Anyhow, when comparing bothconditions for each case, it can beconcluded that the large difference inliquefaction potential is due to the
Figure 12. Go against liquefaction potential
significant change in soil fabric or internal
structure, brought by disturbance of the original underground condition. Fig. 12 also contains thecomparison of results between the undisturbed condition and reconstituted samples. A secondary axis
(on the right side) represents the relationship of maximum shear modulus for the reconstituted samples
over the maximum shear modulus for undisturbed samples. This tendency seem to be away fromprevious studies, but also it must be taken into consideration that the number of samples is still
considered little for this type of investigation to determine a complete and reliable trend of disturbanceeffects on liquefaction characteristics. However, a trend curve was established and it is shown in Fig.
12. Shear modulus ratio between undisturbed andreconstituted samples represents direct disturbanceeffects on the modulus, and it can be symbolized as
GoR/GoU. Just like for the modulus case, aliquefaction potential ratio between both
disturbance conditions would reflect disturbanceeffects over liquefaction potential. For both,
modulus ratio and liquefaction potential ratio, an
undisturbed condition comes identified by arelationship equal to 1, so in this sense an ideal
undisturbed state would be defined when bothratios are equal to 1. Fig. 13 shows the dataobtained evaluated under both ratios, and eventhough a clear tendency is not possible from thevery few points, only three, a correlation
Figure 13. Disturbance effect on the liq. potential
representing disturbance effects on liquefaction
potential was determined as expressed in Fig. 13.

6. CONCLUSIONSDisturbance effects are reflected during the consolidation process of samples. As the consolidationstress reduces, reconstituted samples not only show a higher amount of settlement in comparison with− 77 −

undisturbed samples, but also an increase in rate at which consolidation occurs.ʳ From the shear
modulus ratio-strain dependency curves it’s shown that there is no significant effect between theundisturbed condition and the reconstituted samples for consolidation stresses of 150 and 300 kPa.
Anyhow, as this parameter was reduced the effect turned evident as for the case of a consolidationstress of 75 kPa, for which a correction function was determined.ʳDamping ratio was also object ofanalysis, and the results showed a similar tendency than that presented by the loss of shear, where veryslight effects were noticeable for a consolidation stress of 75 kPa only.ʳ
Bender element tests were conducted on samples subjected to deformation tests. P-wave
velocity for each of the samples remained fairly constant in spite of changes in consolidation stressand disturbance condition, and for S-wave velocity the bender element tests confirmed the results fromdeformation tests, the difference between both disturbance conditions represented a minor divergence.ʳ ʳ
Liquefaction tests were also carried out, when comparing the shear modulus against theliquefaction potential value a very scattered plot was obtained, reflecting a large difference inliquefaction potential due to the change in soil fabric or internal structure, even among undisturbed
samples. However, a trend curve was determined for the relationship between the maximum shear
modulus of undisturbed samples against the ratio of undisturbed over reconstituted maximum shear
modulus, so as to quantify disturbance effect as follows: (GoR/GoU) α Ǧ0.0239GoU + 2.4787.
Maximum shear modulus ratio and liquefaction potential ratio were determined for each pair ofsamples, undisturbed and reconstituted. Even though a clear tendency was not possible from the very
few points, they were only three, a correlation that represents disturbance effects on liquefaction
potential was determined as follows: (LpR/ LpU) = 0.32(GoR/GoU) + 0.47. The number of samples is
considered little for a study of this kind when disturbance effects on liquefaction are under analysis.
More pairs of comparison (undisturbed and reconstituted samples) presenting a wider range of initialshear modulus are necessary to establish a clearer trend curve, therefore a more reliable model
quantifying the effects of disturbance can be determined.
It’s highly recommended to refer to the complete Master Report for a wider and better
understanding of this study.

AKNOWLEDGEMENT
Special acknowledgement is addressed to Dr. Akio ABE for his invaluable guidance and supervision
to the present study, and who in the kindest manner made possible to conduct the necessary laboratory
tests in Tokyo Soil Research Co., Ltd., Tsukuba division.

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