Experimental Study of Micro Behavior of Hangzhou Soft Clay

Experimental Study of Micro Behavior
of Hangzhou Soft Clay
Jieqing Huang
Research Center of Coastal and Urban Geotechnical Engineering,
Zhejiang University, Hangzhou, Zhejiang, 310058, China
e-mail: [email protected]
Xinyu Xie *
Research Center of Coastal and Urban Geotechnical Engineering,
Zhejiang University, Hangzhou, Zhejiang, 310058, China
*Corresponding Author, e-mail: [email protected]
Jinzhu Li
School of Civil Engineering & Architecture, Ningbo Institute of Technology,
Zhejiang University, Ningbo, Zhejiang, 315100,China
e-mail: [email protected]
Wenjun Wang
School of Civil Engineering & Architecture, Ningbo Institute of Technology,
Zhejiang University, Ningbo, Zhejiang, 315100,China
e-mail: [email protected]
ABSTRACT
One-dimensional (1D) oedometer, mercury intrusion porosimetry (MIP) and scanning
electron microscopy (SEM) tests are performed on Hangzhou soft clay samples to research
the micro mechanical characteristics. Based on 1D oedometer tests, it can be found that the
secondary consolidation behavior of Hangzhou soft clay is obvious. Moreover, two Hangzhou
soft clay samples show the tertiary consolidation behavior. It can be found from MIP results
that the compression of soft clays preferentially occurs in the weaker interaggregate pores
instead of in the stronger intraaggregate pores during both primary and secondary
consolidation. It can be observed from SEM images that the arrangement of clay particles
becomes more orderly and the pores become smaller with developing consolidation.
KEYWORDS:
Hangzhou soft clay; one-dimensional oedometer test; mercury
intrusion porosimetry test; scanning electron microscopy imaging
INTRODUCTION
Natural rocks or soils, generally have certain microstructures developed during
depositional processes. Some scholars have demonstrated that microstructure is directly attributed
to properties of natural deposits [1-3]. Generally, there are three types of fabric associations in
soft clays: edge-to-edge(EE) flocculation, edge-to-face(EF) flocculation and face-to-face(FF)
aggregation [4-5]. Different types of fabric associations form different types of pores. In general,
the pores in soft clays can be divided into interaggregate pores and intraaggregate pores. The
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interaggregate pores refer to the space between the aggregates while the intraaggregate pores
refer to the space inside the aggregates [2,6]. Some scholars proposed the dual-porosity
hypothesis with associated delays of water expulsion [7-9]. This hypothesis assumes that primary
consolidation comes from the dissipation of the pore water pressure in the interaggregate pores,
and secondary consolidation occurs when water discharges from the intraaggregate pores. Many
efforts have been made to research the dual-porosity hypothesis. Nevertheless, this hypothesis has
been proven inadequate in some existing experiments [10-11]. The controversy regarding the
rationality of this hypothesis continues today.
Hangzhou soft clay is widely deposited on the plains of Hangzhou and it can cause the
excessive settlement of buildings because of high compressibility. Many scholars did research on
the macro properties of Hangzhou soft clay. The soften of normally consolidated Hangzhou soft
clay under cyclic loading was studied by cyclic triaxial tests with stress-controlled [12].
Comprehensive tests on Hangzhou intact soft clay were performed to obtain the soils’ critical
response to undrained dynamic stress paths under different combinations of principal stress
orientation [13]. Based on undrained cyclic triaxial tests, the cyclic behaviors of Hangzhou soft
clay subjected to initial shear stress were investigated [14]. Nevertheless, the micro behavior of
Hangzhou soft clay is yet to be investigated.
In this paper, a series of experiments using one-dimensional (1D) oedometer tests, mercury
intrusion porosimetry (MIP) and scanning electron microscopy (SEM) are performed on
Hangzhou soft clay samples to research the micro mechanical characteristics and reexamine the
dual-porosity hypothesis. Firstly, MIP and SEM tests are carried out on natural Hangzhou soft
clay samples to study the natural pore-size distribution and observe some representative pores and
clay particles. Afterwards, 1D oedometer tests are performed on Hangzhou soft clay samples.
Then MIP tests are performed to characterize the pore-size evolution of Hangzhou soft clay in the
process of consolidation. Finally, with the help of SEM, the microstructure evolution of
Hangzhou soft clay during consolidation can be observed clearly.
EXPERIMENTAL DETAILS
Sample preparations
Some undisturbed block samples of Hangzhou soft clay and Hangzhou stiff clay were
obtained in Zijingang Campus of Zhejiang University. Then oedometer tests were carried out in
the oedometer room in the same campus. Hangzhou stiff clay is selected herein for comparison
purposes. The basic properties of Hangzhou soft clay and Hangzhou stiff clay are summarized in
Table 1.
Table 1: Basic index properties of Hangzhou soft clay and Hangzhou stiff clay
Properties
Specific gravity Gs
Saturated unit weight γsat(kN/m3)
Natural water content w(%)
Natural void ratio e
Plastic limit wP(%)
Liquid limit wL(%)
Hangzhou soft clay
2.67
17.74
44.86
1.18
20.0
41.2
Hangzhou stiff clay
2.65
19.30
13.05
0.55
18.6
34.2
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Experimental procedures
1D oedometer test
The 1D oedometer tests are divided into two portions: multi-stage loading tests and singlestage loading tests. Multi-stage loading tests are performed on two Hangzhou soft clay samples.
The first loading is 25 kPa. Then the applied load is increased to 50, 100, 200, 400, 800 and 1,600
kPa. In the test of sample 1, each applied load is maintained for 24 hours. The test of sample 2
aims to observe the secondary consolidation behavior of Hangzhou soft clay under each applied
load. In this test the amount of time to allow the development of secondary consolidation is one
week. The information of multi-stage loading tests is summarized in Table 2. Sample 0 represents
the natural Hangzhou soft clay sample in this paper.
Table 2: Information of multi-stage loading tests
Sample number
Duration of each loading stage(hours)
Final loading(kPa)
0
0
0
1
24
1600
2
168
1600
In the single-stage loading tests, two kinds of samples are investigated: Hangzhou soft clay
and Hangzhou stiff clay. Hangzhou stiff clay is investigated for comparison purposes. Six
samples are investigated and they are divided into three sets. In each set, there is one Hangzhou
soft clay sample and one Hangzhou stiff clay sample and the two samples are applied with the
same load. The applied loads of three sets are 100 kPa, 200 kPa and 300 kPa respectively. The
consolidation is considered finished when the vertical settlement becomes completely steady. The
information of single-stage loading tests is summarized in Table 3. SOFT and STIFF represent
Hangzhou soft clay and Hangzhou stiff clay in this table respectively.
Table 3: Information of single-stage loading tests
Sample number
Kind of sample
3
SOFT
4
STIFF
5
SOFT
6
STIFF
7
SOFT
8
STIFF
Applied load(kPa)
100
100
200
200
300
300
After the 1D oedometer tests, the samples are carefully trimmed into small cubes. These
cubes are immediately put into liquid nitrogen and then they are freeze dried. The freeze-dried
cubes are then used for the MIP tests and SEM imaging.
MIP test
The MIP test is based on the principle that a non-wetting liquid cannot enter a porous
medium unless forced under external pressure. Considering the pores as cylindrical capillary
tubes, the intrusion pressure P and the pore diameter D are related as follows [15]:
D = −4 γ cos θ / P
(1)
where γ=surface tension of mercury and θ=the contact angle between clay minerals and
mercury.
The value of γ has been consistently reported to be 0.484 N/m [2, 16]. However, an accurate
value ofθ is difficult to obtain because this angle is intimately related to the properties of clay
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minerals. Diamond [17] determined a contact angle of 139°for montmorillonite and 147°for other
clay minerals. Penumadu and Dean [18] measured the contact angle of mercury with kaolin clay
and they found the advancing and receding contact angles are equal to 162°and 158°respectively.
Based on Diamond’s recommendation, the value of θ is equal to 147°in this study. The AutoPore
IV 9510 with a maximum intrusion pressure of 414 MPa is used in the MIP analysis and it is
shown in Fig. 1. The range of pore-size measurements is from 0.003 µm to 360 µm.
SEM imaging
Some representative cubes, after freeze-drying and gold coating, are selected for SEM
imaging. The thermal field emission SEM used in this research is shown in Fig. 2. Its type is
SIRION-100 and it is produced by FEI in Holland. The maximum magnification is 100000.
Figure 1: Mercury intrusion porosimetry
Figure 2: Thermal field emission scanning
Electron microscopy
EXPERIMENTAL RESULTS
Microstructure of natural Hangzhou soft clay
Fabric associations and pore types in soft clay
Wang and Xu [11] classified the pores in soft clays into four categories: Type A, Type B,
Type C and Type D. This classification is applied in this paper. Type A pores are the small
intraaggregate pores between the clay particles in the aggregates. Note that the intraaggregate
pores mentioned in the dual-porosity hypothesis are referred to as Type A pores [9,19,20]. Type
B pores are the large intra-aggregate pores and they can have a wide range of sizes [11]. In
natural soft clays, there are EE or EF associations between clay particles. Type C pores are
formed within this weak structure. Type D pores lie in the structure formed by aggregates, so they
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are stronger than Type C pores. It is worth mentioning that Type D pores are the interaggregate
pores mentioned in the dual-porosity hypothesis.
The conceptual view of soft clays is shown in Fig. 3. Four types of pores can be clearly
observed. It can be seen that a clay particle is of a plate shape and several particles are gathered in
a group, forming an aggregate, together with interlayer water. Based on SEM results, the length
and width of a clay particle are generally within the range of 1~10µm and the thickness of a clay
particle is approximately 1µm.
EE flocculation
Type C pore
EF flocculation
FF aggregation
Type D pore
Type B pore
Type A pore
(a) Microscopic structure of soft clays
1µm~10µm
clay particle
¡Ö
0.1µm
Type A pore
(b) an aggregate of clay particles
Figure 3: Conceptual view of soft clays
Pore-size distribution
MIP tests are performed on natural Hangzhou soft clay (sample 0) and the test results are
shown in Fig. 4. It can be observed that the pore size distribution of natural Hangzhou soft clay
has two sharp peaks: the major peak located at a diameter of about 0.1 µm and the minor peak
located at a diameter of about 100 µm. It is clear that the major peak represents the intraaggregate
pores and the minor peak represents the intera-ggregate pores. According to the aforementioned
classification of pores, the major peak indicates the Type A pore and the minor peak indicates the
Type D pore. It is evident that Types A and D pores are the dominant pores in natural Hangzhou
soft clay. It is worth mentioning that MIP analysis does not give the measurement of pore sizes,
but of pore entrance sizes. Delage and Lefebvre [2] pointed out that MIP analysis may give
smaller average values for relatively large pores.
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Figure 4: Pore size distribution of natural Hangzhou soft clay
Representative pores
The pores in natural Hangzhou soft clay samples are observed by performing the SEM tests.
The SEM images in Fig. 5 provide details of several representative pores. Fig. 5(a) shows a
special pore which is composed of two Type D pores: an outer pore and an inner pore. It is shown
that both of the outer and inner pores are formed by aggregates. The sizes of these two pores are
approximately 1.5µm×1.5µm and 0.5µm×0.75µm respectively. The pore shown in Fig. 5(a) is a
nested pore and it is named Type D-D pore in brief. The pore shown in Fig. 5(b) is also special
because it is formed by an individual clay particle and some aggregates. As mentioned, Type C
pores and Type D pores are formed by clay particles and aggregates respectively. This special
pore is named the composite pore. A representative Type B pore is shown in Fig. 5(c) and its size
is approximately 1µm×3µm.
The aforementioned classification of pores is based on the structure of pores and it is strict.
Nevertheless, it can be found from Fig. 5 that the majority of actual pores in Hangzhou soft clay
are irregular. Sometimes it is difficult to classify an actual pore into a certain type.
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(a) Type D-D pore
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(b) Composite pore
(c) Type B pore
Figure 5: SEM images of pores in natural Hangzhou soft clay
Representative clay particles
Fig. 6 presents three SEM images of clay particles in natural Hangzhou soft clay. Fig. 6(a)
shows the top view of aggregated structures composed of hundreds of clay particles. It can be
observed that the clay particles have a wide range of sizes and they are arranged unevenly. Fig.
6(b) shows several close-ups of dispersed clay particles. The clay particle in the lower left portion
of this figure seems like a rectangle and its size is approximately 1.5µm×2.5µm. In order to
measure the thickness of clay particle, we observe the side view of aggregated structures. It can
be seen from Fig. 6(c) that the thickness of clay particle is about 0.1 µm and the width of Type A
pore is about 0.1 µm too.
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(a) Top view of aggregated structures
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(b) Dispersed clay particles
(c) Side view of aggregated structures
Figure 6: SEM images of clay particles in natural Hangzhou soft clay
Microstructural evolution of Hangzhou soft clay in
response to consolidation
1D oedometer results
Multi-stage loading tests are performed on two Hangzhou soft clay samples. As noted, for
sample 1 and sample 2, the amounts of time to allow the consolidation development under each
applied load are one day and one week respectively. The e-logp curves (e=the void ratio and
p=the consolidation stress) of the two samples are plotted in Fig. 7. The difference between the
void ratios of the two samples under each applied load shows the amount of secondary
compression. It can be observed from Fig. 7 that the above difference is 0.1 under 25 kPa load
and it becomes 0.2 under 1600 kPa. This implies that the accumulative secondary compression
increases with increasing applied load.
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Figure 7: Oedometer compression curves
MIP results
As mentioned, MIP tests were performed on sample 1 and sample 2 after consolidation. The
pore-size distribution curves of samples at three different stages, i.e., the initial stage (sample 0),
the stage after primary consolidation (sample 1) and the stage after secondary consolidation
(sample 2), are jointly considered in Fig. 8. It can be observed that the amount of the minor peak
apparently decreases during both primary and secondary consolidation. However, the major peak
remains almost unchanged during consolidation. This observation implies that the aggregated
structure that encloses Type A pores moves as a whole during consolidation. Considering the
characteristics of the Types A and D pores, the Type D pores are more collapsible than the Type
A pores because the EF or EE contacts have much smaller areas compared with those at the FF
contacts and small contact areas can lead to a higher stress concentration. It can be found that the
compression of soft clays preferentially occurs in weaker Type D pores during both primary and
secondary consolidation. Hence, the secondary consolidation is fundamentally the same as the
primary consolidation.
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Figure 8: Pore size distributions of Hangzhou soft clay samples at different
consolidation stages
SEM results
Fig. 9 shows the SEM images of sample 0, sample 1 and sample 2. The magnification of
these three SEM images is 5000. It can be observed from Fig. 9(a) that in the natural sample the
arrangement of clay particles is disorderly and there are many large Type D pores. It can be found
from Fig. 9(b) that after primary consolidation large pores disappear but medium and small pores
still exist in the sample. It can be seen from Fig. 9(c) that after secondary consolidation the
arrangement of clay particles is much more orderly than before. At this time, the majority of
pores are Type A pores.
(a) Initial stage (sample 0)
(b) Stage after primary consolidation (sample 1)
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(c) Stage after secondary consolidation (sample 2)
Figure 9: SEM images of Hangzhou soft clay samples at different consolidation stages
Microstructural evolution of Hangzhou soft clay in response
to secondary consolidation
1D oedometer results
Single-stage loading tests are performed on three Hangzhou soft clay samples and three
Hangzhou stiff clay samples. The experimental results are shown in Fig. 10. It can be observed
that the final settlement of Hangzhou soft clay sample is much larger than that of Hangzhou stiff
clay sample under the same load. Moreover, the settlement of Hangzhou stiff clay sample is
increasing approximately linearly all the time without secondary consolidation behavior.
Nevertheless, the settlement of Hangzhou soft clay sample increases quickly at first and then
increases slowly after the first inflection point. In conclusion, the settlement of Hangzhou soft
clay is large and the secondary consolidation behavior of Hangzhou soft clay is very obvious.
(a) 100 kPa
(b) 200kPa
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(c) 300kPa
Figure 10: Time-deformation curves of Hangzhou soft clay samples and Hangzhou stiff
clay samples under different consolidation pressures
For the Hangzhou soft clay samples under 100kPa and 200kPa loads (sample 3 and sample
5), the settlement rates increase after the second inflection point. It can be observed that the clay
exhibits secondary and tertiary consolidation. Ichikawa et al. [20] found that bentonite shows the
tertiary consolidation. They considered that the chemical properties of soils are drastically
changed if multicharged anions are infiltrated in the last stage of consolidation. They thought that
the above mechanism causes the change of permeability and then causes the tertiary
consolidation. Nevertheless, their opinion was not based on experimental results.
When the settlement becomes completely steady, we open the specimens exhibiting the
tertiary consolidation and find some changes. First, all of the samples have an intense rotten
smell; Second, the natural color of Hangzhou soft clay is light gray, but some dark-gray-colored
stains are found at the surfaces of the samples; Third, the erosion of the filters is severe and many
small holes appear on the filters. According to the above phenomena, we conjecture that humus
exists in the natural Hangzhou soft clay. The permeability of soils changes when the humus
causes a series of chemical reactions in the last stage of consolidation. Unfortunately, further
chemical or biological analyses are not performed on the samples to investigate the inner
components of humus and the mechanism of the chemical reactions. The relationship between
humus and tertiary consolidation is yet to be investigated.
The ending times of primary consolidation tp under different applied loads are judged with the
Casagrande Method. Afterwards, the secondary compression indices Cα during two different
periods under different applied loads are calculated. The calculation results are summarized in
Table 4. It can be seen that tp decreases with increasing applied load and Cα decreases with
increasing time under each applied load.
Table 4: Secondary consolidation parameters of Hangzhou soft clay samples
Parameters
tp(minutes)
Cα(tp-1000 minutes)
Cα(1000-10000 minutes)
100
70
0.01303
0.00690
Applied load (kPa)
200
300
55
50
0.01140
0.00998
0.00514
0.00427
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MIP results
The MIP analyses were carried out on the three Hangzhou soft clay samples after secondary
consolidation. The experimental results are shown in Fig. 11. It can be observed that the minor
peak gradually decreases but the major peak remains almost unchanged with increasing applied
load. This implies that the compression of soft clays mainly occurs in weaker Type D pores
instead of the stronger Type A pores.
Figure 11: Pore size distributions of Hangzhou soft clay samples after
secondary consolidation
Even though the observations in Fig. 8 and Fig. 11 can not verify the dual-porosity
hypothesis, this hypothesis should not be completely abandoned. The expulsion of water from the
intraaggregate pores still can account for the deformation of soft clays during secondary
consolidation. Nevertheless, the expulsion of water from the interaggregate pores is much more
significant than that from the intraaggregate pores.
SEM results
Fig. 12 presents the SEM images of the three Hangzhou soft clay samples after secondary
consolidation. The magnification of the SEM images is 5000. Jointly considering the three SEM
images, it can be easily observed that with increasing applied load, the arrangement of clay
particles becomes more orderly and the pores become smaller and smaller.
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(a) 100 kPa (sample 3)
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(b) 200 kPa (sample 5)
(c) 300 kPa (sample 7)
Figure 12: SEM images of Hangzhou soft clay samples after secondary consolidation
CONCLUSIONS
In this paper, the 1D oedometer, MIP and SEM tests are carried out on Hangzhou soft clay
samples to investigate the change in macromechanical and microstructural behavior during
consolidation. The following conclusions can be made:
1) In natural Hangzhou soft clay, types A and D pores are the dominant pores. The majority
of actual pores are irregular and it is difficult to classify an actual pore into a certain type.
2) Hangzhou soft clay samples all exhibit the secondary consolidation behavior clearly. Two
Hangzhou soft clay samples show the tertiary consolidation behavior. The relationship between
humus and tertiary consolidation is yet to be researched.
3) The compression of soft clays preferentially occurs in the weaker interaggregate pores
instead of in the stronger intraaggregate pores during both primary and secondary consolidation.
This implies that the secondary consolidation is fundamentally the same as the primary
consolidation.
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ACKNOWLEDGEMENTS
The writers would like to thank Dr. Jie Song in the research and teaching lab for civil and
hydraulic engineering in Zhejiang University for her help in the 1D oedometer tests. We would
also like to thank Dr. Qun Pu in the state key laboratory of chemical engineering in Zhejiang
University for her help in the MIP tests. Dr. Mingyong Yuan in the analysis and measurement
center of Zhejiang University is acknowledged for his help in the SEM imaging.
The financial supports from National Natural Science Foundation of China under approved
No. 51378469, Zhejiang Provincial Natural Science Foundation of China under approved No.
Y1111240 and Natural Science Foundation of Ningbo City, China under approved No.
2013A610196 are gratefully acknowledged.
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