LABORATORY INVESTIGATION FOR ESTIMATIONTHE SEISMIC RESPONSE OF GROUND
C. Arion, C. Neagu
ABSTRACTLaboratory measurements of soil properties can be used to supplement or confirm the results of fieldmeasurements. They are necessary to establish values of damping and modulus at strains larger thanthose that can be obtained in the field or to measure the properties of materials that do not exist in thenature. The Dynamic Deformation Characteristics of the soil are used in order to calculate seismic
response of ground, earth structures and structure-ground response. To obtain the dynamic proprietiesof the soil we used improved cyclic triaxial equipment that is also used to express phenomenon thatmake soil to fail under seismic loading. The upgrade of the equipment after the training period inJapan at BRI, Tokyo Soil and Tokyo University and during the counterpart JICA program improved
the quality of the obtained results.
INTRODUCTION
The equipments for dynamic laboratory soil testing at National Center for Seismic Risk Reduction
Bucharest Romania (NCSRR) consist of triaxial testing equipment, data acquisition and processing
systems made by Seiken, Japan. Starting to 2003, NCSRR received all those equipments, as a donationfrom Japan International Cooperation Agency (JICA) through the Technical Cooperation Project on
the Reduction of Seismic Risk for Buildings and Structures in Romania.
In designing of engineering structures, when a field seismic investigation is conducted, thedynamic proprieties of the soil must be obtained. A large number of laboratory tests for dynamic
purposes have been developed, and research continues in this area. These tests can generally beclassified into two groups: those that apply dynamic loads and those that apply loads that are cyclic
but slow enough that inertial effects do not occur. The most widely used of the cyclic loading
laboratory tests is the cyclic triaxial test. Cyclic load is usually applied as cyclic axial load bymechanical, hydraulic or pneumatic actuator. In this test a cyclic load is applied to a column of soil
over a number of cycles. The response of load is observed, and the test is repeated at a higher load.
TRIAXIAL EQUIPMENT AT NCSRR
In July 2003 at NCSRR was installed the triaxial equipment. Seiken Inc. Japan made the equipment and thecommissioning. The equipment fulfils all the requirements of The Japanese Geotechnical Society, 2000.
The NCSRR triaxial equipment, Figure 1, can solve the following types of problems:- Static problems with strain level at 10-3 or greater (the main concern regarding the static
problems is used to evaluate the degree of safety of foundations or soil structure against the failure);- Dynamic problems with soils subjected to a strain levels as small as 10-6 (used to evaluate
the soil strength in comparison with stresses induced by external loading and the settlement of groundor structures associated with the deformation of soils.
Specifications of the triaxial equipment
1. Triaxial Cell
1) Air bushed piston of extremely small friction2) Specimen Size: Diameter 50mm × 100mm
Technical University of Civil Engineering, Bucharest (UTCB)

3) Lateral Pressure Capacity: Max. 1Mpa
4) Axial Loading Capacity: 5kN (Equipped with Water-proof and Pressure-proof Load cell 3kN)
2. Vertical Pressure Loading Unit with Air & Water Panel
1) Static Vertical Loading: Electric Strain Control by Mechanical jack, Capacity 5kN, (0.002 – 2
mm/min. Step-less variable type)2) Dynamic Vertical Loading: Stress Control by Pneumatic bellofram cylinder, Capacity 2kN3) Lateral & Back Pressure Loads: Both 0 – 1Mpa by Air regulators, Manual operation
4) Volume Change Apparatus: Double tube burette type 25ml
5) Master Gauge: Dia.200 ×1Mpa, Min. div. 1/500
3. Pneumatic Sine Loader
1) Electric-Pneumatic pressure conversion type & Loading control type2) Range of Vibration: 0.001 – 2 Hz
3) Loading Wave: Sine wave
4) Setting of Load: Both Static Bias & Dynamic loading are 1000 Division, Potentiometer type5) Loading Number of Times: Random Preset type with 6 Figures Digital Counter
6) Pneumatic Pressure Capacity: 1Mpa
7) Electric Power Source: AC Single phase local voltage
4. Transducers & Amplifiers1) Inner Load Cell: 2kN or 500N
2) Large Vertical Displacement Transducer: 25mm3) Pore Water Pressure Transducer: 1Mpa
4) Small Vertical Displacement Transducers (gap sensors) max. +/- 1mm & Amplifiers5) Lateral Pressure Transducer: 1Mpa
6) Volume Change Transducer: 25ml
7) Amplifiers suitable for the above transducers.

Fig. 1 Triaxial Testing Apparatus Model DTC-367 by Seiken.Inc. Japan (Photo at NCSRR)
The automatic stress strain path control and monitoring for triaxial test is as follows (Fig. 2):

Fig. 2. Example of monitoring of axial pressure and axial cyclic deformation

1. For lateral loading is use a pneumatic pressuring system. An air pump and a servo EP
transducer make possible the accurate control of the air-pressure.
2. For axial loading it is important to have a high resolution as well as a large stroke of axial
displacement. We use a servo-pneumatic loading system.
3. For very small vertical displacements a pair of gap sensors are used
Loading pattern of the NCSRR laboratory
The loading pattern used at NCSRR laboratory might be:
1. Monotonic loading tests can be conducted with varying speed of loading. The
conventional static loading tests employ a rate of loading to failure on the order of few minutes. The
monotonic loading test conducted in less than a few seconds to failure are classified as rapid loading
tests. Example – evaluate the strength of soil under exhibited blast loading (detonation, bombs).
2. Cyclic load application following the static monotonic loading (example – evaluate thestrength of soils during the earthquakes). The initial phase of the static monotonic shear stressapplication representing a sustained pre-earthquake state of stress which exists in a soil element. The
phase of loading is considered as simulating the cyclic shear stress application during the earthquakes.
3. Cyclic monotonic loading is performed firstly to investigate the effects on strength andstiffness deterioration of soils due to seismic shaking. Static properties are changed from the initialvalues (example – post-stability analysis of dams and embankments).
4. Monotonically increasing cyclic loading – to study the static strength of soil while is being
subjected to vibrations.
The difference between static and dynamic loading conditions is in the term of time ofloading and is expressed in terms of speed of loading or rate of straining (speed effect or rate effect).
The “Time of loading” is defined as ¼ of the period at which the load is reciprocated. If load
application last more than 0.1 sec then we have “static problems” and if load application have a shortertime of application we have “dynamic problems”. In Table 1 are presented the description of realloading and consequently the simulation in the laboratory.
Table 1. Simulation of natural or artificial phenomenon in the laboratory tests
Number of cycles
1

Phenomenon
Dropping bombs or blasting

Duration of loading
10-3 – 10-2 seconds

10 – 20 withdifferent
amplitudes
100 – 1000

Earthquakes(the period of each impulse is
between 0.1 to 3 seconds)
Pile driving, vibro-compaction

0.02 – 1 seconds

104 - 105

Machine foundation(for compressors, electric generator)
Parking, water waves, pavements ofrailroads,

Very large

Frequency of the loads
10 – 60 Hz
Frequency of the loads
10 – 60 Hz
0.1 – few seconds

EffectImpulse or
shock load

Fatigue
Repetition effectCYCLIC UNDRAINED TRIAXIAL TEST ON SOILS
In the engineering approach the earthquake wave travels from the fault to the ground surface though theseismic bedrock where earthquake motion can be defined to be a function with respect to the distancefrom the fault, and the engineering seismic base layer at which earthquake motion is not affected by theexistence of the surface ground, i.e., the subsoil above the engineering seismic base layer. The schematicwave propagation from the earthquake source to the ground surface is drawn in Figure 3. The earthquake
wave propagates vertically in the surface ground because the ground becomes softer to the groundsurface. In addition, since the soil is soft, it may exhibit nonlinear behavior under the large earthquake.
The nonlinear behavior of the surface ground is caused by the vertically propagating of S-wave. The
amplification of the surface ground has two characteristics that work in opposite way. The first one isamplification. Since S-wave velocity becomes smaller to the ground surface, energy accumulates
resulting in amplification. Therefore as the ground becomes softer, amplification of the wave becomeslarger. The other is deamplification. Shear strength decreases as the ground becomes softer, whichindicates peak acceleration becomes smaller at softer ground. These two characteristics indicate that,
amplification occurs under the small ground shaking, but amplification becomes small as the groundshaking becomes large.

Fig. 3. Schematic wave propagation from the earthquake source to the ground surface (Yoshida. & Iai, 1998)

Shear stress

The 2nd type of CNRRS loading pattern is commonly used to determine the strength of soils
under seismic loading conditions. A specimen is consolidated first and subjected to the initial stress σS.
A sequence of prescribed shear stress cycles is then applied to the test specimen with relatively small
amplitude. When the points reached at the end of each sequences are connected in the plot of thestress-strain diagram, Figure 2. Axial stress is the stress applied in the axial direction of the specimen,
while lateral stress is the stress applied in the radial direction of the specimen. The difference in thesestresses during the cyclic undrained loading process is defined as cyclic deviator stress. All the stressvalues are computed at the mid-point of the height of the specimen.
This test method covers the determination of the relationships between the single amplitude ofcyclic deviator stress or the cyclic stress ratio, which is applied under undrained condition with cyclic
triaxial apparatus, and the number of cycles required to cause a specified value of double amplitude ofaxial strain or to cause a specified value of excess pore water pressure ratio.

A

d
s

P

A

B

B’

C

C’

Shear strain
Fig. 4. Construction of a shear stress versus residual curve from multistage loading test results

Low amplitude moduli obtained from cyclic triaxial test

Though soil deformation under seismic loading is relatively small, its modulus is dependent on dynamic
stress or strain level. Soil moduli such as Young’s modulus and shear modulus decrease as the level ofstress or strain increases. Therefore nonlinearity of dynamic deformation characteristics is significant inseismic response analysis. All the soil moduli E, ν, G, h depends on strain range but the dependency of νis considered rather small. In Figure 5 is shown the methods of measurement and response analysis ofsoil moduli. The evaluation of shear modulus of soils at very small levels of strains was a main concernof researchers. This modulus is called maximum shear modulus, initial shear modulus, or low amplitude
shear modulus and is noted by Gmax or G0. It is the custom to take the strain amplitude of γa=10-5 fordefining the low-amplitude shear modulus G0. To obtain a 10-4 – 10-5 axial strain we must use a verysmall deviator stress.

Fig. 5. Strain dependent soil properties, measurement, and analysis (Tatsuoka & Shibuya 1991)

Emax

d
1
Eeq 

 a SA 1011
q = a - r

Gmax





W

 a SA  L  100



2H n

Gsec

Esec

q = a - r

Procedure to calculate the dynamic deformation characteristics
The G- γ (E - γ) and h - γ relationships obtained in DDC (dynamic deformation characteristics tests) are
generally recognized to express the deformation characteristics of soil. The definitions of Young’s
modulus, shear modulus and damping are showed in Fig.6.
Eeq
1

a

1

1
a

a



W



xa)SA

Fig. 6 Definitions of Young’s modulus, shear modulus and damping

The Young’s modulus and the hysteretic damping ratio, h (%) are obtained using the following relations:
1 W
d
1
Eeq 

 100
 and h  a SA 10
2 W
where
σd is cyclic stress amplitude (cyclic deviator stress)(a)SA is single amplitude cyclic axial strain
W: damping energy in a single loading cycle, which is defined as the area of the hysteretic loop on thedeviator load, versus axial displacement curve
W: equivalent elastic energy input in a single cyclic loading
Upgrading of the equipments

Problems related with damping measurements at very small strains
Conducting cyclic triaxial deformation property test with our equipment, unfortunately, sometimes weobserved some differences between measured value of the damping and the real ones. The errors of
damping calculation at very small strain, 10-4 – 10-5 become for us the most important issue. In Figure 7
is presented an example of an unusual damping value of 5,63%, resulted from calculations, bigger
damping than a usual value of 2% corresponding to that very small strain (Arion, 2004).

Example:damping h= 5.63%;
axial strain 0.00027%,
dev. stress 2.3kN/m2

Fig. 7 Example of monitoring of deviator stress and axial strain (left) and verification of the damping
and modulus test results using the additional computation (right)
During the training period in Japan, Prof Towhata and Prof Tatsuoka from Tokyo University suggestions
about our problems with damping calculation at very small strain might have the following causes:
1. Effects of bedding error at the top and bottom ends of specimen (the deformation of theloading piston and specimen cap);
2. Frequency of loading;
3. Noise of the power supply;
4. Sensitivity of the pickup (output of the pickup) (A/D –converter); 16 bit (216) or 12bit (212);

5. Time gap: for example 1/100 sec between the force and displacement measurement canincrease the damping;
6. Signal/noise ratio – S/N ratio;
7. Cell pressure capacity (bigger – small resolution; small capacity- fine resolution);
8. Type of soil specimen (stiff clay, soft rock – more problems in measurements).
Also during the discussions they recommend the use of LDT type sensors instead of measurement
system made with gap sensors. This comment might be questionable.
Upgrading the equipments
From the eight points list we focus especially on a 4th and a 7th. Together with people from SEIKEN
INC.(1-28-5, Higashi-Ueno, Taito-ku, Tokyo, 110-0015), Mr. Itoh and Mr. Soyama, and with thefinancial help from JICA we did the following to the equipment:- Changed AD board of the triaxial apparatus. A new 16Ch/16Bit High-speed Analog
Multi-Function I/O Board was installed to convert analog input signals to digital
equivalents (performing analog-to-digital conversion). This card is the high-speed, highprecision type that performs A-D conversion at a conversion speed of 1 microsecond perchannel and a resolution of 16-bit. The board has buffer memory available as a FIFO orring buffer to hold 16 megabytes of data. This enables sampling to be executed in the
background independently of the processing power of the PC;- Changed the load cell. The old model of load cell, LP-200 with 2000N capacity was
replaced with the new one LP-50, with 500N capacity;- The new version of the acquisition and processing software and also 2 types of steel springs(for evaluation of the time gap) were forwarded to Romania.
The chosen upgraded solutions for triaxial equipment solve the problems with damping calculation andimprove accuracy of data recorded. The results might be observed in Figures 8.
135h (%) upper boundh (%) average

Roca Idriss
G/GmaxP5

G/GmaxG/Gmax upper boundP0

G/Gmax averageh (%) lower bound

P5

P1

P4

20

Roca - h

P3

0.4

25

P2

Roca - G/Gmax

0.6

15

P2
P1

Gravel Seed
G/GmaxG/Gmax lower bound10

Hysteretic damping ratio h (%) .

30

P3

0.8

P0

0.2

gravel Seed

00.0001

5

0
0.001

0.01

0.1

1

10

0.0001

0.001

Single amplitude shear strain, ()sa(%)

0.01

0.1

1

10

Single amplitude shear strain, ()sa(%)

Fig. 8a Test results for sandy soils from Bucharest and comparison with analytical model curves
1

30
Vutecic IP=50 - h

0.9

P0

0.7

P1
P2

Vutecic IP=200
G/Gmax

Vutecic IP=0 - G/Gmax

P2

G/Gmax

0.6

P7
P8

P5
P6

0.4
0.3

P8

0.2

Vutecic IP=50 - G/Gmax

0.1

Vutecic IP=200 - G/Gmax

15

Vutecic IP=0 - h
Vutecic IP=15 - h

P7Vutecic IP=15 - G/Gmax

00.0001

20

P5
P6

P3
P4

0.5

25

P3
P4

P1

Vutecic IP=30 - h

10Vutecic IP=100 - h

Vutecic IP=30 - G/Gmax

Vutecic IP=200 - h

Hysteretic damping ratio h (%)

0.8

P0

5

Vutecic IP=100 - G/Gmax

0

0.001

0.01

0.1

Single amplitude shear strain, ()sa(%)

1

100.0001

0.001

0.01

0.1

1

10

Single amplitude shear strain, ()sa(%)

Fig. 8b Test results for cohesive soils from Bucharest and comparison with analytical model curves

Split barrel sampler for clay soils

Trimming and preparation of clay sample

Shear failure of tested sample

Trimming and preparation of clay sample
Shear failure of tested sample
Fig. 9. Sampling, preparation and testing of the soil samples

Preservation of soil samples

Results from the tests

During last two years we conduct series of dynamic triaxial tests especially on the clay soils from
Bucharest. In the Figures 8a and 8b are represented relations of shear modulus ratio G/GO versus shear
strain and the strain dependent damping for the laboratory samples. Also we represented thestrain-dependent modulus and damping curves quoted in the literature (Vucetic & Dobry, 1986). In
Figure 9 are presented the steps of preparation of the soils in our laboratory.
ACKNOWLEDGEMENT

The authors would like to acknowledge the funding provided by Japan International Cooperation Agency,
JICA and the technical support from the Japanese counterpart (late dr. Masahito Tamura) in the TechnicalCooperation Project on the Reduction of Seismic Risk for Buildings and Structures in Romania.
REFERENCES

Arion, C., 2004: „Report on soil tests and investigations (laboratory and field experiments)”,
JICA/CNRRS/BRI.
SEIKEN, Inc., 2003. Technical documentation of triaxial testing apparatus. Model No. DTC – 367
Tatsuoka, F. & Shibuya, S. 1991. Deformation characteristics of soils and rocks from field andlaboratory tests, Proc., 9th Asian Reg. Conf. on SMFE, Bangkok
Tatsuoka F., Lo Presti D.C.F. and Kohata Y., 1995. Deformation characteristics of soils and soft rocks
under monotonic and cyclic loads and their relationships, SOA Report, Proc. of the Third Int. Conf. on
Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St Louis 2, 851-879.
The Japanese Geotechnical Society, 2000. Standards of Japanese Geotechnical Society for Laboratory
Shear Test (English Version).
Yoshida, N. & Iai S., 1998. Nonlinear site response and its evaluation and prediction. Proc. 2nd
International Symposium on the Effect of Surface Geology on Seismic Motion, Yokosuka, Japan, pp.
71-90, 1998.
Vutecic M. and Dobry R. “Pore pressure build-up and liquefaction at level sandy sites during
earthquakes”,1986. Research report No. CE-86-3 Rensselaer Polytechnic Institute, 616p.