Download Liquefaction Behaviour of Sand During Vibrations

13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 1419
LIQUEFACTION BEHAVIOUR OF SAND DURING VIBRATIONS
Ravi Kant MITTAL1, M.K. GUPTA2 and Sarbjeet SINGH 3
SUMMARY
For prediction of liquefaction behavior, test data from dynamic triaxial and simple shear tests on small
samples have been widely used. Castro [1] has concluded that these tests don’t represent the field
conditions. Gupta [2] have also indicated that studies on small sample tests predict liquefaction of dense
sand deposits with relative densities of 70-80% to a depth of more than 30 m which is anomalous to the
known behavior of dense sands. Hence triaxial test data seems to be in error. For study of liquefaction
behavior of dense sand under surcharge pressure large sand samples of 1.05 m x 0.60 m x 0.32 m were
prepared in shake table. Steady state vibrations of desired accelerations and frequency can be imparted to
the sand sample prepared in shake table. To simulate field conditions at depth different dead weight
surcharge was applied. Overall forty tests were conducted under varying accelerations, relative densities
and surcharge pressures. Dense sand samples have shown dynamic stability at low accelerations and
dilation at high accelerations. This study suggests that compact surcharge fill can be used as antiliquefaction measure.
Key Words: Liquefaction, shake table, surcharge pressure, relative density.
INTRODUCTION
Liquefaction of saturated sands has often been one of the causes of earthquake damage to structures
resulting in loss of life and property. The additional safety in the design of superstructure is not of any
help in the event of liquefaction of foundation soil during an earthquake. It is therefore very necessary that
possibility of liquefaction is examined before hand and necessary remedial measures are adopted against
damages due to liquefaction of foundation soil.
___________________________________________________________________________________
__
1
Lecturer, Civil Engineering Group, Birla Institute of Technology & Science, Pilani, India. Email:
[email protected], [email protected]
2
Professor, Department of Earthquake Engineering, Indian Institute of Technology, Roorkee, India.
3
Engineer, Weidlinger Associates, Inc. Consulting Engineers, Cambridge, MA 02142. Email:
[email protected]
For determining the possibility of liquefaction of a site, two type of laboratory tests are available i) small
sample tests under dynamic triaxial or simple shear conditions and ii) Large sample test on vibration table.
It has been well reported that no uniform agreement to date has been achieved from different tests
performed by different investigators. The tests results both quantitatively as well as qualitatively are
affected by the method of tests and test equipment used ( Peacock [3], Gupta, [4], Castro [1] ). The
method of determining possibility of liquefaction from laboratory triaxial test on small soil sample is not
free from errors (Castro [1], Gupta [2]).
A method for determining the possibility of liquefaction has also been developed which uses the
laboratory test data on large soil samples (Gupta, [4], Gupta [5], Gupta [6]). The approach road
embankment of Tezpur Bridge in Assam was designed using the vibration test results. During Assam
earthquake of August 1988 widespread damage of embankments bridges and foundation soil due to
liquefaction was observed. The high embankment designed using the vibration table test data, performed
satisfactorily (Gupta [7]). This provided confidence in vibration table test on large soil samples.
Liquefaction behavior of saturated sand is largely affected by its relative density and surcharge on it. A
comprehensive vibration table test data on soil sample with surcharge is reported. This paper also,
describes the effect of surcharge on dense sample of sand regarding the liquefaction characteristic.
EXPERIMENTAL SETUP
Tests were performed on horizontal vibration table on a saturated soil sample. A water tight tank one
meter long, 0.6 m wide and 0.4 m high is mounted on a vibration table in which the soil sample is
prepared. A known quantity of water was taken in the tank and a known quantity of air dried sand was
poured in the tank through a constant height to obtain a uniform deposit. The overlying water was
removed and weighed to compute the initial relative density. The desired relative density sand samples
were prepared in test tank and subjected to steady state vibrations of desired accelerations. To simulate
field conditions dead weight surcharge pressures varying from 0.076 kg/cm2 to 0.263 kg/cm2 were applied
on the top of soil by placing concrete blocks of desired weight. All tests were carried out at a frequency of
5cps. Pour pressures were measured by simple glass tube piezometer.
RESULTS AND DISCUSSIONS
Effect of relative density of sample on rise in pore pressure under different accelerations and surcharge
pressure is shown in FIG 1,2,3,4. Pore pressure equal to over burden pressure (u/σ=1) required for
complete liquefaction is indicated by line AB. It was observed that rise in pore pressure decreases with
increase in relative density. In FIG 1 no pore pressure rise is observed beyond 71.5, 76.5 and 79.5%
relative density at 20%, 30%and 40%of g accelerations respectively under zero surcharge indicating that
the dense sand remains stable. However relative density beyond which no pressure rise observed is
different for different acceleration & surcharge pressure.
Effect of surcharge on rise in pore pressure for varying relative density of sample under different
accelerations 20%g and 40%g are shown in FIG 5 & 6 respectively. From FIG 5 & 6, it is observed that
with increase in overburden pressure, initially pore pressure increases, after a certain threshold value, it
started decreasing. Hence for a sand deposit of particular relative density & expected maxi ground
acceleration there is a value of surcharge pressure beyond which rise in pore pressure decreases sharply.
However it may be noted that difference between pore pressure rise and line of complete liquefaction keep
on increasing as surcharge pressure increases which shows increase in liquefaction resistance of soil with
every increase of surcharge pressure. Which suggest compacted surcharge fill can be used as antiliquefaction measure.
FIG 1 PORE PRESSURE Vs RELATIVE DENSITY FOR ZERO
SURCHARGE PRESSURE
POR
E
PRE
SSU
RE
(cm)
30
25
20
15
10
5
0
30
40
50
70
60
80
RELATIVE DENSITY (%)
20% g
30% g
40% g
LINE AB
FIG 2 PORE PRESSURE Vs RELATIVE DENSITY
FOR SURCHARGE 0.076 Kg/sq. cm
POR
E
PRE
SSU
RE
(cm)
50
40
30
20
10
0
30
40
50
60
70
80
90
100
RELATIVE DENSITY %
20 %g
40 % g
110
FIG 3 PORE PRESSURE Vs RELATIVE DENSITY
FOR SURCHARGE .170 Kg/sq cm
POR
E
PRE 60
SSU 40
RE 20
0
(cm)
30
40
50
60
70
80
90
100
RELATIVE DENSITY (%)
20 %g
40 % g
FIG 4 PORE PRESSURE Vs RELATIVE DENSITY
FOR SURCHARGE .263 Kg/sq cm
POR
E
PRE
SSU
RE
(cm)
30
20
10
0
30
40
50
60
70
80
90
RELATIVE DENSITY (%)
40 %g
70 % g
100
FIG 5 PORE PRESSURE Vs OVER BURDEN PRESSURE
160
120
100
80
60
40
20
0
0
50
100
150
200
250
300
350
OVER BURDEN PRESSURE (gm/cm2)
80%
60%
Dri = 40%
LINE AB
FIG 6 PORE PRESSURE Vs OVER BURDEN PRESSURE
FOR 20% g
160
140
PORE PRESSURE (cm)
PORE PRESSURE (cm)
140
120
100
80
60
40
20
0
0
50
100
150
200
250
300
OVER BURDEN PRESSURE
80%
60%
RD = 40 %
LINE AB
350
Figure 7 shows pore pressure v/s acceleration at a relative density of 80% under different surcharge
pressure. In there dense sample it is observed that negative pore pressure developed at high acceleration of
70%or more. Dilation of samples was observed visually in test tank. These results indicate dense sands are
not prone to liquefaction and show greater stability under vibration. Higher surcharge pressure doesn’t
allow dilation of sample and negative pore pressure decreases. Thus under surcharge pressure conditions
also dense sands do not liquefy.
FIG 7 PORE PRESSURE VS ACCELER ATION AT RELATIVE
DENSIT Y OF 80%
ACCELERATION % g
PORE PRESSURE (cm)
60
70
80
90
100
0
-1
-2
-3
0.263 kg/cm2
0.123 kg/cm2
0.029 kg/cm2
ZERO
CONCLUSIONS
Possibility of liquefaction decreases with increase in relative density of sand. However, desired relative
density of sand deposit for no liquefaction depends on surcharge pressure and expected ground
acceleration. Dense sand shows dynamic stability at low acceleration and at high acceleration dilation i.e.
negative pore observed under different surcharge pressure. Thus dense sand may not liquefy under both
with and without surcharge condition. Vibration table tests appears to be more close to field conditions
compare to small sample tests under triaxial test. Surcharge pressure effects the liquefaction behavior of
sand in a characteristics manner and suggests that compacted surcharge can be used as anti-liquefaction
measure.
ACKNOWLEDGEMENTS
Author’s thankfully acknowledge, experimental facilities provided by Department of Earthquake
Engineering, I.I.T., Roorkee.
REFERENCES
1.
Castro G, Poulos SJ. ”Factors affecting liquefaction and cyclic mobility” Journal of Geotechnical Engineering Division, ASCE, 1977;Vol.103,No.GT6: 501-506.
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Gupta MK, Sharma HM. “Possibility of liquefaction during an earthquake” Bulletin Indian
Society of Earthquake Technology, 1977; vol. 14, no. 3: 101-109.
Peacock WH, Seed HB. ”Sand Liquefaction under cyclic loading simple shear conditions” Journal
of Soil Mechanic and Foundation Engineering Division, ASCE, 1968; Vol.94, No.SM3: 689-708.
Gupta MK. ”Liquefaction of sands during earthquakes” Ph.D. Thesis, University Of Roorkee,
Roorkee, 1977.
Gupta, M.K, Prakash S. “A new realistic approach for liquefaction analysis, “Bulletin Indian
Society of Earthquake Technology 1986; Vol. 23, No. 3.
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