Transcript The Liquefaction Resistance and Maximum Shear Modulus of

The Liquefaction Resistance and Maximum
Shear Modulus of Frozen Samples
Yao-Chung Chen
Department of Construction Engineering
National Taiwan University of Science and
Technology
Taipei, Taiwan
Pu-Shang You
Department of Civil Engineering
De-Lin Institute of Technology
Taipei, Taiwan
INTRDUCTION


The 1999 Chi-Chi earthquake caused a
lot of liquefaction damages in western
Taiwan. Our National Science Council
sponsored a 3-year integrated research
to study the liquefaction.
We are in charge of carrying out in-situ
freezing and sampling to recover high
quality soil samples and to investigate
their dynamic properties.
INTRDUCTION(CONT.)


The liquefaction resistance and
maximum shear modulus of frozen
samples were obtained by
performing cyclic triaxial tests and
resonant column tests, respectively.
The laboratory testing results were
compared with the in-situ testing
results.
FREEZING AND SAMPLING



Descriptions of Test Site
The town of Yuanlin is selected as the
test site for the research.
It was subject to some of the most
severe liquefaction observed from the
1999 earthquake.
Descriptions of Test Site


Soil conditions in the relatively flat
portions of Yuanlin generally consist of
Holocene alluvium to depths of 50-60 m
overlying older sedimentary deposits.
Groundwater occurs at depths between
0.5-7 m.
Descriptions of Test Site


The test site had sand boils at the
surface and tilting buildings.
Preliminary investigation shows that the
ground condition consists of interlayered silty sands and silty clays.
Descriptions of Test Site

The zone between depths 10 to 15m
consists mostly of sandy soils; therefore,
it is selected as the target zone of
freezing and sampling.
Geometry of Freezing and
Sampling System


In-situ freezing was carried out by using
liquid nitrogen flowing through four
freeze pipes which were arranged in a
square pattern with 80 cm spacing.
Four sample boreholes were advanced
and cased to the depth of 9.5m prior to
the freezing process.
Fig. 1 Layout of freezing and sampling boreholes
Freezing Process
Fig. 2
Changes of ground temperatures with time.
Freezing Process


only No. 1 and No. 2 showed rapid
decrease of ground temperatures and
the ground became frozen after two
days.
sample holes S-3 and S-4 probably did
not reach ground frozen condition . it
was decided to carry out sampling work
from sample holes S-1 and S-2.
Freezing Process


Coring of in-situ frozen samples was
undertaken utilizing continuous coring
technique with NX core barrel. Core
runs were 1m in length.
The frozen samples were transported to
the laboratory in a refrigerator at -20℃.
CYCLIC TRIAXIAL TEST
RESULTS AND ANALYSES


Physical Properties of Frozen Samples
Frozen samples obtained from borehole
S-2 were trimmed for cyclic triaxial
testing. A total of nine samples were
trimmed.
Table 1 Physical properties of frozen samples for cyclic triaxial tests
No.
CFS1
CFS2
CFS3
CFS4
CFS5
CFS6
CFS7
CFS8
CFS9
Depth (m)
14.2
14.4
14.6
14.8
15.0
11.5
12.5
13.0
13.5
Gs
2.72
2.72
2.73
2.73
2.72
2.72
2.71
2.71
2.71
γd (g/cm3)
1.50
1.40
1.49
1.37
1.30
1.33
1.45
1.41
1.35
FC(%)
54.8
50.7
59.4
54.5
56.6
51.7
57.3
53.9
16.0
LL
27.4
21.0
PL
18.1
17.2
PI
9.3
3.8
Type
CL
SM
Fig. 3 Particle size distribution curves of frozen samples for cyclic triaxial tests
Liquefaction Resistance of
frozen samples


Undrained cyclic triaxial tests were
performed in a cyclic triaxial test
apparatus.
All samples were consolidated at
98.1kPa effective confining pressures
before they were subjected to
undrained cyclic loading.
Liquefaction Resistance of
frozen samples

The liquefaction resistance is defined as
the cyclic stress ratio required causing
soil to reach initial liquefaction at a
certain number of cyclic loading.
Table 2 Summaries of liquefaction test results of frozen samples
Specimen No.
γd
(g/cm3)
FC (%)
Number of Cycles
Cyclic stress ratio
CFS1
1.50
55
4
0.299
CFS2
1.40
51
14
0.266
CFS3
1.49
59
24
0.288
CFS4
1.37
55
86
0.250
CFS5
1.30
57
14
0.181
CFS6
1.33
52
20
0.205
CFS7
1.45
57
10
0.231
CFS8
1.41
54
11
0.210
CFS9
1.35
16
8
0.322
Fig. 4 Liquefaction test results of CL frozen samples
Liquefaction Resistance of
remolded samples


Two types of remolded samples with
fines contents equal to 16% and 55%
were prepared by both dry tamping (DT)
and moist tamping (MT) methods.
The samples were consolidated at
98.1kPa effective confining pressures
before they were subjected to
undrained cyclic triaxial loading tests.
Liquefaction Resistance of
remolded samples

The remolded specimens were prepared
and consolidated to have dry densities
equal to 1.40 and 1.60 g/cm3 for CL
samples, and 1.40 g/cm3 for SM
samples.
Fig. 5 Liquefaction test results of CL remolded samples (γd=1.40g/cm3)
with comparison of the results of frozen samples.
Fig. 6 Liquefaction test results of CL remolded samples (γd=1.60g/cm3)
with comparison of the results of frozen samples.
Fig. 7 Liquefaction test results of SM remolded samples (γd=1.40g/cm3)
with comparison of the results of frozen sample.
RESONANT COLUMN TEST
RESULTS AND ANALYSES



Physical Properties of Frozen
Samples
Ten frozen samples were trimmed for
resonant column testing, two from
borehole S-2 and eight from borehole
S-1.
The dry densities are in the range of
1.19 to 1.46 g/cm3.
Physical Properties of
Frozen Samples


Three out of ten samples have fines
contents less than 50%, and are
classified as SM or SC. The other seven
samples have fines content 60~90%.
According to the fines content, the
samples can be grouped in to three
categories as 20%, 40~60%, and
80~90% of fines content.
Table 3 Physical properties of frozen samples for resonant column tests
No.
FS1
FS2
FS3
FS4
FS5
FS6
FS7
FS8
FS9
FS10
Depth (m)
14.3
14.1
12.2
11.2
13.2
12.5
12.7
13.8
13.0
13.2
Gs
2.72
2.72
2.71
2.72
2.71
2.71
2.71
2.71
2.71
2.71
γd (g/cm3)
1.26
1.46
1.19
1.20
1.28
1.22
1.25
1.25
1.41
1.21
FC(%)
79.9
86.5
47.8
91.8
89.7
61.9
83.7
19.3
85.7
41.7
LL
37.3
39.9
23.4
42.1
36.3
24.0
25.2
*
23.5
29.9
PL
19.3
20.6
19.8
19.9
16.8
18.9
17.2
*
18.5
21.8
PI
18.0
19.3
3.6
22.2
19.5
5.1
8.0
*
5.0
8.1
Type
CL
CL
SM
CL
CL
CL-ML
CL
SM
ML
SC
Percent finer by weight(%)
100
90
FS1
FS2
FS3
FS4
FS5
FS6
FS7
FS8
FS9
FS10
80
70
60
50
40
30
20
10
0
10
1
0.1
0.01
0.001
Particle size(mm)
Fig. 8 Particle size distribution curves of frozen samples for resonant column tests
Maximum Shear Modulus
of Frozen Samples

The frozen samples were first
consolidated at 100kPa effective
confining pressures and their resonant
frequencies were measured, then the
confining pressures were increased at
50kPa increment up to 300kPa and
resonant frequencies were measured at
each stress interval.
180
FS1
FS3
FS5
FS7
FS9
Gmax (MPa)
160
140
FS2
FS4
FS6
FS8
FS10
120
100
80
60
0
50
100
150
200
250
Effective confining pressure (kPa)
Fig. 9 Maximum shear modulus of frozen samples
300
350
COMPARISONS WITH IN-SITU
TEST RESULTS



In-situ SPT Test Results
Three boreholes were drilled and
standard penetration tests (SPT) were
performed at each 1.5m interval.
Three types of hammers (safety,
automatic, and donut) were used and
the driving energies were measured.
In-situ SPT Test Results

The subsoil conditions for Boreholes No.
1 and No. 2 are alternate layers of CL
and SM. The subsoil conditions for
Borehole No. 3 are mostly SM. The
energy ratio is the highest for safety
hammer (about 70~85), the second for
automatic hammer (about 60~80), and
the lowest for donut hammer (about
55~75).
Liquefaction Resistance
Estimated by SPT-N


The simplified procedure based on SPTN1 as suggested by 1996 NCEER
workshop was used to evaluate the
seismic liquefaction potential of the insitu soils.
The cyclic stress ratio (CSR) and the
cyclic resistance ratio (CRR) were
calculated.
calculation of CSR
Seed and Idriss formulated the following
equation for calculation of CSR
   0.65amax / g  vo /  vo
 rd
CSR   av /  vo
Sandy soils containing some
fines

Studies in China suggest that the
potential for cyclic liquefaction in silts
and clays is controlled by grain size,
liquid limit, and water content. When a
soil satisfies the following three criteria,
the soil might experience liquefaction
problem.
Three criteria



Percent finer than 0.005mm
≤15%
(4a)
Liquid limit
≤35%
(4b)
Water content
＞0.9 × liquid limit
(4c)
Liquefaction of CL

According to the results of Table 1 and
Fig. 3, the CL frozen samples have clay
content (＜0.005mm) in the range of
17~40%, liquid limit 27%, and nature
water content in the range of 30~40%.
The CL samples satisfy only the criteria
of Eqs. 4b and 4c, however, the high
content of clay particles might prohibit
the soil to liquefy.
Comparison Between
Laboratory and In-Situ Tests


According to the tendency curves in Fig.
4, the cyclic stress ratios of the frozen
samples at 15 cycles (correspondent to
M=7.5 earthquake) can be estimated.
To be consistent with in-situ anisotropic
stress conditions, the CRR obtained
from laboratory tests on the frozen
samples should be converted .
converted equation:
 )  [(1  2K o ) / 3]( /  o )
( /  vo
σ´o : the isotropic effective confining stress
used in the laboratory tests.
Ko : the at rest lateral stress coefficient.
Fig. 10 Comparison of the CRR obtained from laboratory tests on CL frozen samples
with in-situ SPT tests of Borehole No.1.
Fig. 11 Comparison of the CRR obtained from laboratory tests on CL frozen samples
with in-situ SPT tests of Borehole No.2.
Fig. 12 Comparison of the CRR obtained from laboratory tests on SM frozen sample
with in-situ SPT tests of Borehole No.3.
Comparisons of Maximum Shear
Modulus with Shear Wave
Velocities


Three CPT tests (boreholes YLA, YLB,
and YLC) were performed with
measurements of shear wave velocities
at the test site.
The maximum shear modulus of the
frozen samples was converted to shear
wave velocity.
Converted equation:
Vs 
Gmax

0
50
100
Vs (m/s)
150
200
250
300
0
2
4
YLA
YLB
YLC
FS
6
Depth (m)
8
10
12
14
16
18
20
Fig. 13 Comparisons of the laboratory results with in-situ shear wave velocities
CONCLUSIONS


Fines content influences a lot the
ground freezing efficiency. The ground
freezing efficiency is very poor for
clayey soils and very good for sandy
soils.
In-situ frozen samples have much
higher liquefaction resistance than
remolded samples.
CONCLUSIONS (Cont.)

The liquefaction resistance of CL frozen
samples is smaller than the one
predicted from SPT-N1. This indicates
that clayey soils experienced certain
degree of disturbance during freezing
process.
CONCLUSIONS (Cont.)

The liquefaction resistance of SM frozen
sample is about the same as the one
predicted from SPT-N1. This indicates
that the disturbance was quite small
during freezing and sampling process.
CONCLUSIONS (Cont.)

According to the laboratory testing on
frozen sample and the in-situ SPT tests,
the sandy soils (SM) of the test site
would liquefy during the Chi-Chi
Earthquake which is consistent with the
phenomena of sand boils observed at
the test site.
CONCLUSIONS (Cont.)

The maximum shear modulus of frozen
samples compares quite well with insitu measured shear wave velocities. It
indicates that the disturbance due to
freezing and sampling process is
acceptable.