THE REASON FOR BEARING CAPACITY FAILURE IN FINE GRAINED SOILS
Ayfer Erken1, M.B. Can Ulker2, Z.Kaya3, B.Elibol4
ABSTRACT
In this study the effect of cyclic loads on monotonic shear strength has been studied on torsional,
triaxial and simple shear apparatus. Tests have been conducted on undisturbed fine grained soil
specimens. The plasticity index of soils is in the range from NP to 22 % for undisturbed soil
specimens. The existence of critical shear strain level, which is called as yield shear strain, where
softening starts, is determined from cyclic tests. The level of cyclic yield strain is 0.5 % for the
undisturbed soils. If soil undergoes a cyclic shear strain level below the cyclic yield strain,
reduction of monotonic strength of undisturbed specimens is limited but when cyclic shear strain
level is larger than yield strain monotonic strength decreases down to 40% of initial strength.
Introduction
After the 1999 Kocaeli disastrous earthquake, lots of structural damages and many casualties
have occurred as a result of ground failures generated due to softening silts and clays. Under
earthquake loads, the induced pore water pressure causes an increase shear strain at soft fine
grained soils. For this reason, it has a great importance to determine post cyclic strength of soils
in the areas having a seismic activity. To decrease structural damage related to foundation soils
the stress-strain relationships under cyclic loads and post cyclic shear strength properties of soil
stratums should be determined by conducting static and dynamic soil laboratory tests.
In this study, the cyclic and monotonic tests have been conducted on undisturbed soils by using
the cyclic simple shear apparatus, cyclic triaxial apparatus and cyclic torsional shear apparatus.
The cyclic behavior of silty and clayey soils was determined according to test results and postcyclic monotonic shear strengths have been evaluated after various numbers of cycles of
dynamic loading. Stress controlled dynamic tests were performed under different cyclic shear
stress ratios and monotonic tests have been conducted on reconstituted soil specimens
1
Prof. Istanbul Technical University, Civil Engineering Faculty, Geotechnical Division, 34469 Maslak-Istanbul,
Turkey. erken@itu.edu.tr, Fax:90-212-285 6557
2 Graduate Student, North Carolina State Univ., Department of Civil, Construction and Environmental Eng.
NC.,USA
3
Graduate Student Istanbul Technical University, Civil Engineering Faculty, Geotechnical Division, 34469 MaslakIstanbul
4
Graduate Student Istanbul Technical University, Civil Engineering Faculty, Geotechnical Division, 34469 MaslakIstanbul
1
controlling the strain rate in order to determine the pre-cyclic undrained static shear strength and
also the post-cyclic failure mechanism of fine grained soils.
To date, a comprehensive understanding of the cyclic behavior and post-cyclic shear strength of
non-plastic silts, the silty clays and clayey silts having low plasticity is lacking. The
determination of static undrained shear strength reduction plays an important role in this study
and the primary objective is to systematically determine the post-cyclic undrained shear
strengths of low plastic silty clays and clayey silts and compare the results with the pre-cyclic
condition. Also the effect of soil plasticity and the specimen type on the dynamic shear strength
and post-cyclic shear strength behavior of fine grained soils was determined. The observations of
the field conditions and damaged areas after the earthquakes showed that under cyclic loadings,
bearing failure occurs at low plastic silty and clayey soils due to rapid increase in shear strains
and limited amount of pore water pressures resulting from ground softening. The previous
studies made by several researchers agreed well with the field observations considering the
cyclic behavior of low plastic silty and clayey soil.
Although the silts are classified as fine grained soils, as it has been also investigated from other
researchers, the cyclic behavior of them can be different from than that of clays. The difference
exists in the cyclic shear strength properties and the pore water pressure response during cyclic
loading depending upon the plasticity of fines. Pore pressures increase into a certain amount in
plastic silty clays whereas it can reach to the effective confining pressure in one cycle of loading
in saturated sandy silts and silty sands. The plasticity of fines is one of the major parameter that
affects the cyclic behavior of silty soils (Erken et al., 1994). Cyclic strength of undisturbed non
plastic silts is lower then that of plastic silts. Puri (1984) investigated the effect of plasticity
index to the cyclic undrained shear strength of normally consolidated silty soils conducting
cyclic triaxial tests on the reconstituted soil specimens. From the results, it was determined that
the shear stress ratio to cause 5 % double amplitude shear strain increases significantly with the
increase in the plasticity index at the same number of cycles. El Hosri (1984) determined the
effect of plasticity index on the cyclic behavior of undisturbed silty soils. As it has been observed
from the normalized shear stress ratio-plasticity index relationship, the cyclic strength increases
with the plasticity index starting from a certain value of 5 %. Zhu and Law (1988) reported the
similarity of the cyclic behavior of silty soils to that of silty sands from the results of the cyclic
tests conducted on undisturbed and reconstituted specimens having a constant initial void ratio.
Additionally, they have concluded that the shear deformations increase rapidly when the pore
water pressures reach the 80 % of the effective confining pressure. Ansal and Erken (1989)
presented an existence of the cyclic yield strength level having a critical cyclic shear strain level
above which soils undergo rapidly large deformations at every number of cycles.
Konrad and Wagg (1991) investigated the effect of the degree of shear deformation on the pore
water pressure response from the undrained cyclic tests performed using anisotropically
consolidated clay-silt mixtures having a clay content ranging from 0-40 %. Liu (1992) also
studied the effect of plasticity index on the cyclic behavior of soft silty soils by obtaining the
strength curves from regression analyses. He emphasized the difference in the strength of clean
fine sands and the silty soils having plasticity index of between 4 and 40 %. The clean sands
have the maximum cyclic shear strength and the difference between the test results of sands and
silts could become more than 40 %.
2
Sandoval (1989) and Prakash and Sandoval (1992) obtained relatively different results from the
previous researches. They pointed out the decrease in the liquefaction resistance of silts with the
addition of clays having a plasticity of between 2-4 %. Guo and Prakash (1999) also
demonstrated the similar behavior of silts with clays. In general, the previous studies and the
critical examination of relevant data from the literature suggest that there is a threshold plasticity
index value of clay fractions below which the liquefaction resistance decreases and beyond,
which the liquefaction resistance starts increasing.
Another important part of this study deals with the post-cyclic strength of reconstituted and
undisturbed soft silty soils. Several researchers have investigated the post-cyclic behavior of the
fine grained soils since the beginning of the 1980s. These studies include the loss of static
undrained shear strengths of softening soils lying under the foundations of structures. In fact, it
would be relatively safe to tell that the soils subjected to residual strains from cyclic loadings
have zero post-cyclic strength (Koester, 1992). On the other hand, it was determined from the
previous studies that the static undrained shear strengths of fine grained soils decrease
significantly after the application of cyclic shear stresses.
Finn (1982a) and Ramanujam et al. (1978) evaluated the change in the residual undrained shear
strength of the soil through the shear surface after the earthquake loads. They determined that the
stability analysis of the soil should be made at the limit state condition with a confining pressure
correction. In another research, monotonic undrained strain controlled triaxial shear tests have
been conducted and the post-yield shear strength was determined. This strength was found as the
minimum value that the soil could experience in the undrained condition.
The post-cyclic strength of soils was also studied by Marcuson et al. (1990), Seed et al. (1989),
Vasquez and Dobry (1989) and Castro et al. (1989). In order to determine the residual strengths
of the soil specimens taken from the hydraulic fill of San Fernando Dam, several laboratory tests
were performed. Then the test results were compared with the field approach obtained from the
standard penetration values. It was concluded that the actual field approach was determined
having a wide range of residual strengths considering the actual laboratory curve.
Lee and Focht (1975) determined the post-cyclic static strength reduction of fine grained soils.
Researchers indicated that due to cyclic loading, failure occurs as a result of softening behavior
and the deformations occurred when the liquefaction takes place at the foundation soils. The
cyclic triaxial test results showed a 20 % decrease in the static strength of fine grained soils.
Also, the pore water pressures obtained after the cyclic tests decreased to a certain value in the
static triaxial tests.
Materials and Experimental Procedure
To determine the reason bearing capacity failure of fine grained soils cyclic and post cyclic
monotonic tests have been conducted on reconstituted and undisturbed soil specimens.
Undisturbed soil samples were obtained from boreholes drilled in the cities of Adapazari,
Erzincan –Eksisu (1992 earthquake site) and Izmir located on Aegean Sea shore cost. The index
properties of the reconstituted and undisturbed soil specimens can be seen in Table 1. The
plasticity index of soils varies between NP and 22 %.
3
Table 1. Index properties of soils tested
WL IP
(%) (%)
FC
(%)
n
(kN/m3)
Group
7.0-7.5
49
NP
98
15.7
ML
E18
4.0-4.5
36
10
62
17.46
ML
Erzincan-Eksisu1
E20
4.0-4.5
36
10
62
17.95
ML
Erzincan-Eksisu1
E25
10.0-10.5
55
22
99
15.5
MH
Erzincan-Eksisu1
E37
5.5-6.0
31
NP
83
17.36
OL
Adapazari2
S3-1
10.5-11.0
-
NP
53
-
ML
Adapazarı2
S3-3
10.5-11.0
-
NP
39
-
SM
Adapazarı2
S4-1
3.0-3.5
-
NP
56
17.0
ML
Adapazarı3
DO2
3.5-4.0
37
13
95
19.0
ML
İzmir3
DSO2
9.0-9.5
34
8
63
18.6
ML
İzmir3
DSO1
5.0-5.5
31
9
55
18.7
CL
Izmir3
DSO3
5.0-5.5
31
6
52
17.9
ML
İzmir3
DO3
9.0-9.5
31
5
80
17.37
ML
Field
Sample
No
Depth
(m)
Erzincan-Eksisu1
E06
Erzincan-Eksisu1
1
Cyclic simple shear test
2
Cyclic triaxial test
3
Cyclic torsional shear test
In this study, soils were tested using the pneumatic cyclic torsional shear apparatus, cyclic
triaxial apparatus and cyclic simple shear apparatus. The cyclic and static tests have been
conducted hollow cylinder specimens in torsional shear apparatus. The stress-strain relationships
and the shear strength properties of the soil specimens can be determined under isotropic and
anisotropic conditions by applying sinusoidal earthquake loads with a frequency range of 0.01
Hz at torsional and triaxial systems and 1 Hz at simple shear system. Then, the cyclic tests were
stopped at the desired loading cycle and monotonic undrained tests were conducted at 0.50
mm/sec loading rate in torsional system and 0.20 mm/sec loading rate in triaxial system.
4
Undisturbed soil specimens were prepared using an outer mold to hold the cylindrical specimen
and the inner parts of the specimens were carved with a special screw by applying torque. The
inner radius of undisturbed hollow cylinder specimens is 1.50 cm, the outer radius is 3.50 cm and
the height of the specimens is approximately 14.00 cm each. The size of specimens are in simple
shear apparatus is 3 cm high and 7 cm diameter in simple shear apparatus and 5 cm diameter and
10 cm high in triaxial apparatus. The specimens were consolidated isotropically to an effective
stress of 100 kPa. All the undisturbed specimens were exhibited a saturation ratio of greater or
equal to 0.96. After the consolidation process, the loading was applied cyclically in dynamic
tests and monotonically in static tests.
Testing program
The testing program consisted of three test series, each designed to investigate a particular effect
of the undrained cyclic and monotonic behavior of the undisturbed silty and silty clay soil
specimens. Series 1 included only the cyclic simple shear tests conducted on Erzincan - Eksisu
specimens with a plasticity of NP to22 %.
At the second series tests to determine the dynamic shear strength and post cyclic monotonic
strength of the undisturbed silty soil specimens obtained from Adapazari city, cyclic axial loads
were applied till the failure condition of DA 5 % shear strain occurred in specimens. The
monotonic loading that was applied at 0.20 mm/sec loading rate lasted till the soil specimens
exhibited a shear strain of 10 %.
Series 3 also included the cyclic tests followed by static loading of low plastic silty soils in order
to determine the effect of cyclic shear stresses on the undrained static shear strength of
undisturbed fine grained soils in torsional apparatus. The cyclic torsional loads were performed
under the shear stress ratios within 0.170 and 0.327 in silty soils with a plasticity index ranged
from 5 to 13 %. The monotonic loading that was applied at 0.50 mm/sec loading rate lasted till
the soil specimens exhibited a shear strain of 10 % (Table 2).
Table 2 Cyclic test properties of undisturbed specimens
Test
No
wn
(%)
wn/wl
IP
(%)
nc
(kN/m3)
B
(%)
d/c'
N
(=2.5%)
E06
36
0.73
NP
15.7
95
0.183
4.5
E18
36
1
10
17.46
95
0.353
5
E20
36
1
10
17.95
100
0.249
40
E25
44
0.8
22
15.5
95
0.249
100
E37
31
1
NP
17.36
95
0.200
20
S3-1
37
-
NP
96
0.280
8.5
5
S3-3
32
-
NP
95
0.210
23.5
S4-1
22
-
NP
96
0.320
19
DO2
37
0.99
13
19.0
100
0.327
1
DO3
19
0.61
5
17.37
98
0.208
760
DSO1
30
0.97
9
18.7
96
0.240
10
DSO2
29
0.85
8
18.58
96
0.227
3
DSO3
28
0.90
6
17.9
98
0.170
20*
* The test is continued till 1 % DA strain
Cyclic Behavior of Undisturbed Silty Soils
In this part of the study, cyclic simple shear tests were conducted on the undisturbed and
reconstituted silty specimens. Fig. 1 shows the cyclic test result of the non-plastic soil specimen.
Here, the relationships between shear strain versus number of cycles and pore water pressure
versus number of cycles are presented. It is obviously seen that with the increasing number of
cycles, shear strains increase rapidly whereas pore water pressures stay limited which results
from soil softening in reconstituted silty specimen. So, when the pore water pressure ratio
reached nearly 0.8, shear strains have already exceeded the failure limit of 5 % DA. The other
specimens present similar behavior.
As the compressive character of the water is neglected, pore water pressure occurs and the
effective stresses decrease significantly. This decrease would be much more in the low plastic
range because the pore pressures increase much more in low plastic silty soils. The contractive
behavior of the low plastic silty soil seems to be same as the condition of clean fine sands under
cyclic loading (Ishihara, 1993; Verdugo and Ishihara, 1996; Lade and Yamamuro, 1997).
Fig. 2 shows the cyclic behavior of low plastic silty soil samples obtained from Adapazari city
after 1999 Kocaeli earthquake. The sample was taken from 3.5 m depth and included 56% of
non-plastic fines. Cyclic axial strain increases suddenly after 11th repetition of cyclic axial stress
ratio of d/2c=0.320. Pore pressure ratio is around 0.7 at this number and reaches around 1.0
at DA =6%. The sample having 53 fines, obtained from 11 m depth, presents similar behavior
under d/2c= 0.280 (Fig. 3).
Cyclic behaviors of undisturbed soils obtained from the cities of Adapazari and Izmir are
compared in Fig. 4. The plasticity index of the undisturbed specimens having fines content
varying from 52 to 95% is in a range between 5 and 13 %. As it is seen from Figure 4, the
increased cyclic shear stress ratio causes to reach the failure limit of double amplitude shear
strain of 5 % in lower cycles of loading. Shear strains increase rapidly at first few cycles and the
undisturbed soil specimen loses its cyclic strength following the test. When the ratio of w n/wl
exceeds 0.90 undisturbed soils become more collapsible due to strain softening within 20 cycles
with a pore pressure ratio in range 0.52 to 0.60 (Table 2). Whereas the soil having a ratio of
6
wn/wl =0.61 reaches the failure state of deformation at 750 cycles of loading with a limited
excess pore pressure ratio of 0.52. The silty specimen with a plasticity of 13 % has considerably
more cyclic strength compared to specimens with lower plasticity index under smaller cyclic
shear stresses.
7
Shear Starin (%)
6
4
2
0
-2
Shear Strain (%)
0
20
40
60
80
Effective Stress (kPa)
100
120
6
5
4
3
2
1
0
-1
-2
-3
0
10
20
30
40
50
30
40
50
Time (Sec)
Pore Pressure Ratio
1
0.8
0.6
0.4
0.2
0
0
10
20
Time (Sec)
Fig. 1. Cyclic undrained test results of the undisturbed silty soil specimen, IP=NP d/c=0.183.
8
Axial Shear Starin (%)
10
8
6
4
2
0
-2
-4
-6
-8
-10
Axial Shear Strain (%)
0
20
40
60
80
Effective Stress (kPa)
100
120
8
6
4
2
0
0
2
4
6
Number of Cycle (N)
8
10
0
2
4
6
Number of Cycle (N)
8
10
Pore Pressure Ratio
1
0.8
0.6
0.4
0.2
0
Fig.2. Cyclic undrained triaxial test results of the undisturbed silty soil specimen, IP=NP
d/2c=0.320.
9
Axial Shear Starin (%)
4
2
0
-2
Axial Shear Strain (%)
-4
0
24
48
72
Effective Stress (kPa)
96
120
0
5
10
15
Number of Cycle (N)
20
25
0
5
10
15
Number of Cycle (N)
20
25
4
3
2
1
0
Pore Pressure Ratio
1
0.8
0.6
0.4
0.2
0
Fig.3. Cyclic undrained triaxial test results of the undisturbed silty soil specimen, IP=NP
d/2c=0.280.
10
6.0
Test No IP(%)
DSO2 8
DSO1 9
DO3 5
DSO3 6
DO2 13
Shear Strain (%)
5.0
4.0
 d/ c
0.227
0.24
0.208
0.17
0.327
3.0
2.0
1.0
0.0
1
10
100
1000
Number of Cycles, N
1
Test No IP(%)
DSO2 8
DSO1 9
DO3 5
DSO3 6
DO3 13
Pore Pressure Ratio
0.9
0.8
0.7
0.6
 d/ c
0.227
0.240
0.208
0.170
0.327
0.5
0.4
0.3
0.2
0.1
0
1
10
100
1000
Number of Cycles, N
Fig. 4. Pore water pressure and shear strain behaviors of undisturbed soils
As shown in Fig. 2 and Fig. 3, there is a critical cyclic shear strain level for every soil that the
cyclic shear strain increases suddenly due to the softening of soil when cyclic shear stress is
large enough to produce pore water pressure and shear strain (Erken et al., 2005). As shown in
the figures, the intersection of the straight part of curve gives the cyclic yield shear strain level as
cr = 0. 5 % for the undisturbed fine grained soils with the plasticity index in a range from NP to
22. Above the cr, the shear deformations increase rapidly exceeding the failure limit of 5 % DA.
Ansal and Erken (1989) also determined around  0. 50 % of the cyclic yield shear strain level
for one-dimensionally consolidated kaolinite clay having a plasticity index of 40. Erken and
Ulker also used reconstituted fine grained soil specimens to obtain the relationship between
plasticity index and the number of cycles at cr. As shown in Fig. 5 the number of cycles at cr
increases by increasing plasticity index.
11
40
Plasticity Index (%)
35
30
25
20
15
R (Erken et al., 2005))
10
DS O1 (Erken et al., 2005)
5
DS S T (Erken et al., 1995)
0
1
10
100
1000
Number of Cycles, N  cr
Fig. 5. The relationship between plasticity index and the number of cycles at cr (Erken et al.,
2005)
Post-cyclic Undrained Monotonic Strength of Undisturbed Fine Grained Soils
During 1999 Kocaeli earthquake, one of the most important phenomenon related to ground
failure was bearing capacity failure developed as a result of shear strength reduction due to soil
softening. It is necessary to determine the effect of earthquake loads on monotonic shear
strength for the design of structure to mitigate earthquake effects. In this part of the research, the
static torsional tests have been conducted after ending the cyclic tests on undisturbed specimens
for a certain number of cycles at constant shear stress amplitude. Fig. 6 shows the monotonic
shear strength results obtained from three undisturbed silty specimens loaded cyclically at
different stress ratios. The plasticity index and fines content values of the specimens are between
6-9 % and 52-63 % respectively. The cyclic shear strain was increased up to  0.09 % at the end
of 20 cycles of application of the cyclic shear stress level of 0.170. The post cyclic ultimate
monotonic strength value is obtained as 40 kPa at 10 % shear strain. However the failure state
achieved during cyclic loading significantly influenced the monotonic undrained strengths. After
a shear strain of  2.95 %, shear strength decreased to 35 kPa and strain potential of 5.04 %
caused the monotonic strengths to decrease to 25 kPa.. The cyclic strain amplitude of  0.09 %,
pore water pressures increase from 29 kPa to 45 kPa. However, pressures decrease from 55 kPa
to 14 kPa at the cyclic shear strain history of  2.95 % and from 78 kPa to 50 kPa at the static
test having at the cyclic shear strain history of  5.04 %. It is obviously seen that cyclic shear
stress history directly affects the saturated and soft undisturbed silty soils.
12
Pore Water Pressure (kPa)
100
DSO2
80
c=5.04%
60
DSO3
c=0.09%
DSO1
c=2.95%
40
20
0
0
2
4
6
8
Shear Strain %
10
12
14
60
Shear Stress (kPa)
DSO1
c=2.95%
DSO3
c=0.09%
50
40
30
20
DSO2
10
c=5.04%
0
0
2
4
6
8
Shear Strain %
10
12
14
Fig. 6. Undrained monotonic shear strength and pore water behavior of undisturbed silt samples
after cyclic loading.
Conclusion
On the basis of the tests reported in this paper regarding the cyclic and the post-cyclic monotonic
behaviors of fine grained soils by using cyclic tests, the followings were found:
The results of cyclic undrained torsional shear tests conducted on both reconstituted and
undisturbed low plastic silty and silty clay soils show that shear strains increase rapidly and
steadily until the failure limit described as 5 % DA shear strain level, but excess pore water
pressures increase only a limited amount whereas the pore water pressures are equal to effective
confining pressure when cyclic axial strains in triaxial test system and cyclic shear strains in
simple shear system exceed DA5%.
There is a critical cyclic shear strain level triggering excessive increase in shear strain. As the
yield cyclic shear strain level is obtained around cr =  0.5 % for undisturbed fine grained soils.
13
The number of cycles at cr increases by increasing plasticity index of reconstituted and
undisturbed fine grained soils.
The monotonic undrained shear strengths of undisturbed specimens decrease with the cyclic
shear stress history and the reduction depends on the previous number of cycles of loading at the
same cyclic shear stress amplitude. However, this reduction is significant when the soil specimen
exceeds a certain yield strain level under the same shear stress amplitude prior to static undrained
test and reaches nearly 40 % in silty soil.
While the previous cyclic stress history affects the post-cyclic strength, the influence of other
factors like effective confining pressure, relative density, aging, structure etc. on the post-cyclic
monotonic shear strength should also be determined with a comprehensive study.
Acknowledgments
The research reported herein has been supported by the Graduate Research Department of
Istanbul Technical University and Ulker Geotechnical Engineering Company. The supports are
strongly appreciated.
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