AN INNOVATIVE DOUBLE CELL TRIAXIAL SYSTEM FOR CONTINUOUS
MEASUREMENT OF THE VOLUME CHANGE OF GASEOUS OR
UNSATURATED MARINE SOILS AND TWO OTHER ADVANCED SOIL
TESTING SYSTEMS
J.-H. YIN
Department of Civil & Structural Engineering, The Hong Kong Polytechnic University
Kowloon, Hong Kong, China
Abstract
This paper introduces three advanced laboratory testing systems for measuring the
stress-strain-strength behavior of soils. The three systems are:
(a) a new Double Cell Triaxial System (DCTS) for continuous measurement of the
volume change of a gaseous (unsaturated or saturated) soil specimen in triaxial
testing (this system is developed by the author).
(b) a Hollow Cylinder Apparatus (HCA) for measuring the behavior of a hollow soil
specimen under conditions of pure shearing, plain strain, rotation of the middle
principle stress, etc., and
(c) a Truly Triaxial System (TTS) for measuring the behavior of a brick-shape soil
specimen under independent control of the three principle stresses.
Calibration results on the DCTS are presented and discussed.
Keywords: Double Cell Triaxial System, Volume Change, Gaseous soils, Unsaturated
soil, Marine Soils
1. Introduction
The measurement of the stress-strain-strength behavior of soils in laboratory is
important for geotechnical applications and research, including the stability and
deformation analysis of submarine mass movements. The measurement can provide soil
parameters, such as strength and deformation parameters, for design analysis and quality
control of new geotechnical structures and performance analysis of existing projects.
The measurement also provides test data for better understanding of the fundamental
behavior of soils and developing improved correlations, models, and theories.
The conditions of a soil in the field are complicated. For example, the soil may be
saturated under water table or unsaturated above water table. Gaseous marine soils in
the seedbed under water table are unsaturated. Generally speaking, the stress state of a
soil under loading is three-dimensional. The measurement of the behavior under the
real field conditions is necessary. This paper introduces three advanced laboratory
testing systems for measuring the stress-strain-strength behavior of soils, that is, (a) a
new Double Cell Triaxial System (DCTS) for continuous measurement of the volume
change of an unsaturated or saturated soil specimen in triaxial testing; (b) a Hollow
Cylinder Apparatus (HCA) for measuring the behavior of a hollow soil specimen under
171
172
Yin
conditions of pure shearing, plan strain, rotation of middle principle stress, etc., and a
Truly Triaxial System (TTS) for measuring the behavior of a brick-shape soil specimen
under independent control of the three principle stresses.
2. A New Double Cell Triaxial System (DCTS)
2.1. EXISTING SYSTEMS
The volume change of a soil specimen is an essential parameter to be measured in
triaxial testing. For a 100% saturated soil triaxial specimen, the volume change of the
specimen (enclosed with rubber membrane and top and bottom caps) during
consolidation or compression is equal to the volume of water coming out of the
specimen (from inside). Therefore, the common measuring method for 100% saturated
specimen is measuring the volume of water coming out of the specimen. However, for
an unsaturated soil specimen, the water volume coming out is no longer equal to the
volume change of the specimen. Alternative methods have been used for unsaturated
soil specimens. But all have significant limitations.
Bishop and Donald (1961) firstly used a modified cell similar to that in Fig.1 for
measuring volume changes of partly saturated soils. An open-top inner cylindrical
container was used inside
a conventional cell. The
Vertical Loading Piston
inner container was filled
with mercury (Bishop and
Outer Water Pressure Cell
Donald 1961). Outside the
Air
inner container was filled
Fill Water up
with water.
Volume
to the Top
Cylindrical Container
Soil Specimen
changes of the partly Inner
with Water
saturated soil specimen
Water
were
measured
by
monitoring the vertical
position of a stainless steel
ball floating on the surface
Outer Cell Air Pressure
Inner Cell Water Pressure
of the mercury using a
Supply Tube (Pressure
Supply Tube (Pressure and
Measurement)
Volume Measurement)
cathetometer.
Following
Bishop and Donald’s work
(1961), Yin (1998) put an
open-top
cylindrical
Soil Specimen Top Water
Soil Specimen Bottom Wate
Drainage Tube (Pressure
Drainage Tube (Pressure or
container
inside
a
Volume Measurement)
or Volume Measurement)
conventional cell. The
inner container was filled Figure1. An existing old cell triaxial system with open inner
cylindrical container for measurement of volume change of a soil
with distilled water to the specimen – a schematic diagram (after Yin 1998).
position as shown in Fig.1.
Outside the inner container was filled with air. When the specimen volume is changed,
for example, decreased under axial loading, the water table in the inner container will
come down. At this time, water was supplied into the inner container through the “Inner
Cell Water Pressure Supply Tune” as shown in Fig.1 to maintain the water table in the
An innovative double cell system
173
inner container at the previous position. In this way, the water volume (measured by
burette) entering the inner container is equal to the compression volume change of the
specimen. This process is tedious and cannot be done automatically. The accuracy is
questionable since the water table position was judged by naked eye readings.
Wheeler (1988) extended the idea of Bishop and Donald (1961) and developed a double
cell triaxial system for testing soils with large gas bubbles. The schematic diagram of
Wheeler’s double cell system (Wheeler 1988) is shown in Fig.2(a).
In particular, the volume
of the inner cell was
Vertical Loading Piston
2
Shaft
measured by using water
O-ring Seals
Rolling Diaphragn
burette. The vertical axial
Outer Water Pressure Cell
load
was
measured
Water
σ
outside
the
cell
using a
1
Inner Water Pressure Cell
Soil Specimen
Water σ
local proving ring. As
shown in Fig.2(a), the
shaft with the loading
piston extends up and
comes out of the outer
Outer Cell Water Pressure
Inner Cell Water Pressure
cell. Leaking could be a
Supply Tube (Pressure and
Supply Tube (Pressure and
Volume Measurement)
Volume Measurement)
problem and this was
minimized by using a
rolling
diaphragm
Soil Specimen Top Water
Soil Specimen Bottom Water
between
the
loading
Drainage Tube (Pressure or
Drainage Tube (Pressure or
Volume Measurement)
Volume Measurement)
piston and inner cell top
shaft (Fig.2a). Wheeler
Vertical Loading Piston
2
(b)
(1988) reported limited
Shaft
Leaking
calibration results and
Outer Water Pressure Cell
found
that
the
1
σ
relationship of volume
Water Water σ
changes of the inner cell
Inner Water Pressure Cell
Soil Specimen
with cell pressure up to
400 kPa was non-linear
and the largest inner cell
volume change was 0.7
Outer Cell Water Pressure
cm3 at cell pressure of
Inner Cell Water Pressure
Supply Tube (Pressure and
Supply Tube (Pressure and
Volume Measurement)
400kPa.
Chen et al.
Volume Measurement)
(2001) showed a double
wall triaxial system in
Soil Specimen Top Water
Soil Specimen Bottom Water
Fig.2(b).
A
few
Drainage Tube (Pressure or
Drainage Tube (Pressure or
Volume Measurement)
Volume Measurement)
limitations
of
the
Figure 2. Two existing double cell triaxial systems for measurement of arrangement in Fig.2(a)
volume change of a soil specimen – (a) after Wheeler (1988) and (b) and (b) are :
(a)
o_cell
i_cell
o_cell
i_cell
after Chen, et. al. (2001).
(a) Leaking is a potential
problem even though a rolling diaphragm or the shaft is used. This is because the
rolling diaphragm or the shaft is subject to differential pressure of σi_cell (i.e. the
174
(b)
(c)
(d)
(e)
Yin
inner cell pressure σi_cell at point 1 minus the outside atmospheric pressure of zero
at point 2).
The rolling diaphragm in Fig.2(a) or the top cap in Fig.2(b) will deform under the
differential pressure of σi_cell. This will affect the volume changes of the inner
cell.
The shaft is subject to an extension under the differential pressure of σi_cell. This
will cause volume changes of the inner cell and errors in measuring volume
changes of the soil specimen.
The vertical load was measured externally and the frictional force between the
piston and the shaft is included. This will cause an error in the measurement of the
actual vertical load on the soil specimen.
The water volume changes were all measured using water burette. This is not
convenient for computer control and automatic data acquisition.
2.2 AN IMPROVED DOUBLE CELL TRIAXIAL SYSTEM
Based the pioneering work above, the author has developed an improved Double Cell
Triaxial System (DCTS) for continuous measurement of the volume change of an
unsaturated or saturated soil specimen in triaxial testing as shown in Fig.3. The main
new features of the DCTS, which are different from the modified cells proposed by
Bishop and Donald (1961), Yin (1998), Wheeler (1988), and Chen et al, (2001) are, as
shown in Fig.3 :
(a) The inner cell is totally
enclosed within the outer cell.
Vertical Loading Piston
O-ring Seals
De-aired water is used to fill
Outer Water Pressure Cell
both the inner cell and the
Water
σ
outer cell.
Load Transducer Cell
2
o_cell
Inner Water Pressure Cell
1
Water σi_cell
A
A
Outer Cell Water Pressure
Supply Tube (Pressure and
Volume Measurement)
Soil Specimen Top Water
Drainage Tube (Pressure or
Volume Measurement)
Outer Perspex Cell
Wall
Inner Perspex Cell
Wall
Soil Specimen
Inner Cell Water Pressure
Supply Tube (Pressure an
Volume Measurement)
Water
Soil Specimen Bottom Wa
Drainage Tube (Pressure o
Volume Measurement)
Soil Specimen
Water
(b) Both outside and inside
the inner cell are subject to
the same magnitude of the
cell pressure σi_cell (inside)=
σo_cell (outside).
(c) Because of the same
water pressure σi_cell, both the
wall and top cap of the inner
cell will have negligible
deformation. This will avoid
errors caused by the inner cell
deformations.
(d) Since the inner cell water
pressure σi_cell is equal to
outside cell water pressure
A-A Section
σo_cell, the hydraulic gradient
Figure 3. A new double cell triaxial system for measurement of along the piston at the inner
volume change of a soil specimen – a schematic diagram.
cell top cap (from Pint 1 to
An innovative double cell system
175
Point 2 as shown Fig.3) is zero. Thus no water flow will occur along the gap
between the piston and the inner cell top cap. This will avoid errors due to water
flow/leaking at the gap. In fact, an O-ring is used at the gap.
(e) A submersible electric load cell is placed inside the inner cell and used to measure
the vertical load on the soil specimen directly. This will avoid the error due to the
friction between the piston and the cell caps (inner and outer cell caps).
(f) All water volumes are measured by electric volumemeter. All data, such as vertical
load, pore water pressure, and volume changes are collected automatically by a PC
computer.
The DCTS has been made and set-up at the soil laboratory as shown in Fig.4. The inner
cell with a soil specimen and the submersible load cell is shown in Fig.5a. The outer cell
with the inner cell inside is shown in Fig.5b. For the DCTS in Fig.5, the outer cell has
an internal diameter D of 230mm, height H of 425mm and the wall thickness T of 8mm.
The inner cell has an internal diameter d of 90mm, height h of 235mm and the wall
thickness t of 6mm. The load cell has dimensions of thickness of 30mm and diameter of
65mm. The axial load piston has a diameter of 20mm. The standard size for a soil
specimen is diameter of 50mm and height of 100mm.
2.3 CALIBRATION OF THE DCTS
A solid copper specimen with diameter of 50mm and height of 100mm was first used to
calibrate the Double Cell Triaxial System (DCTS). Since the solid copper specimen is
considered incompressible
under a pressure, say, up
to 600kPa, therefore, the
Loading Frame
copper specimen can be
used to assess the volume
changes of the inner cell
Vertical Loading
Piston
and the outer cell under
Outer Water
increasing pressure.
Pressure Cell
(Perspex Wall)
Soil Specimen Bottom
Water Drainage Tube
(Pressure or Volume
Measurement)
All tubes and holes were
de-aired.
The
copper
Automatic
specimen was installed in
Volumemeter
the inner cell. A standard
Data-Logger
rubber membrane was put
on the copper specimen.
Soil Specimen Top
The vertical filter stone
Water Drainage Tube
and nylon cap were put on
(Pressure or Volume
Measurement)
the top of the specimen.
The specimen was then
Figure 4. The real set-up of the Double Cell Triaxial System (DCTS) in
sealed by using double Othe Soil Mechanics Laboratory.
rings on the rubber
membrane. In order to make the water inside the rubber membrane fully saturated, a
back-pressure of 100 kPa was applied with the cell pressure in the inner cell and the
outer cell increased to 105 kPa accordingly. The B-value was checked and found to be
0.99.
Inner Water
Pressure Cell
(Perspex Wall inside)
176
Yin
Load Cell
Soil
Specimen
(a)
(b)
After the back-pressure
saturation, the top drainage
tube valve of the specimen
was closed to make the
copper
specimen
undrained.
Since
the
purpose of the calibration
was to assess the volume
changes of the inner cell
and the outer cell. Keeping
the specimen undrained
(or all drainage tube valves
closed) would avoid the
volume changes (errors)
due to the water squeezed
out of the gag between the
rubber membrane and the
copper
specimen
and
between the cap/filter stone
and the specimen.
The measured volume
changes of the inner cell
and the outer cell with
effective cell pressure (cell
pressure minus the initial
cell pressure of 105 kPa)
are shown in Fig.6. It is
seen that the relationship of
the outer cell volume
change and the effective
cell pressure is non-linear;
while the relationship of
the inner cell volume
change and the effective
cell pressure is linear.
The volume changes of the
outer cell is 3 to 4 times of
the volume changes of the
inner cell. For example, the
accumulated
volume
change under the effective
pressure of 400 kPa is
1.523 cm3 for the outer cell
Figure 5 A close-up view of the inner cell (top) and a close-up view of
and 0.40 cm3 for the inner
the outer cell (bottom).
cell.
An innovative double cell system
177
Compared to the results
reported
by
Wheeler
0
100
200
300
400
500
(1988), the volume change
0.0
of the inner cell was 0.7
cm3
under
effective
pressure 400 kPa. Using the
0.5
DCTS, the inner cell
volume change is only 0.40
cm3, that is, 0.57% of 0.7
y = 0.0010x
1.0
cm3 using the design by
R2 = 1.0000
Wheeler (1988). Furthermore, the relationship of
1.5
the inner cell volume
Inner cell volume change
change and the effective
Outer cell volume change
cell pressure using the
2.0
DCTS is linear; while that
Figure 6. Volume changes of a copper specimen under isotropic reported by Wheeler (1988)
compression measured by the changes of (a) water volume of the inner was non-linear. A straight
cell and (b) the water volume of the outer cell with back pressure equal line has been used to fit the
to 100kPa and the specimen drainage valve closed.
relationship of the inner
cell volume change and the effective cell pressure as shown in Fig.6. The fitting
equation is:
∆Vi _ cell = 0.0010σ e _ cell
(1)
Inner/outer cell volume change (cm3)
Effective inner/outer cell presure increase (kPa)
where ∆Vi _ cell is the accumulated volume change of the inner cell and σ e _ cell is the
effective cell pressure. The volume of the copper specimen is 196.25 cm3. The inner cell
volume change of 0.4 cm3 is only 0.20% of the copper specimen volume and negligible
for the volume strain calculation of a soil specimen.
Time (min)
The marine clay used in
the testing was taken from
depth 1m to 2m at a
Water volume change coming out of (or into) soil specimen
marine site in Hong
5
Water volume change of inner cell
Kong’s waters. The marine
clay was in dark grey color
10
and were a mixture of clay,
silt and fine sand with
15
occasionally shells and
coarse particles. In order to
obtain
uniform
and
20
consistent soil samples, the
marine
deposits
were
25
sieved in wet condition
Figure 7. Volume changes of a saturated marine clay during through a sieve with an
consolidation measured by the changes of (a) the water coming out of opening size of 150 mm.
(or into) the clay specimen and (b) water volume of the inner cell.
The marine clay after wet
sieving had a composition
of silt and clay with some fine sand. The composition is 27.5% of clay, 58.4% of silt
0
Volume change (cm3)
0
200
400
600
800
1000
1200
1400
1600
1800
178
Yin
and 14.1% of fine sand. The marine clay was re-consolidated in cylindrical mould with
300mm in diameter and 450mm high. The basic properties of the marine clay are
specific gravity Gs=2.664, liquid limit wL =60.0%, plastic limit wP =28.5%, plasticity
index IP =31.5%, and initial water content w=57.4% (after re-consolidation but before
odometer testing).
A thin-wall plastic tube with 50mm internal diameter was pushed into the reconsolidated marine clay in the mould. The clay sample in the tube was then extruded
out and trimmed to form a specimen of 50mm in diameter and 100m in height. The clay
specimen was installed in the inner cell following the code of BS 1377 (1990). A backpressure of 200 kPa with a cell pressure of 205 kPa was applied to ensure near 100%
saturation. B-value measured was 0.99.
Fig.7 shows the results of volume changes with time under effective cell pressure of
50kPa. Two methods of volume measurement were used: (a) Method A by measuring
the volume of water coming out of the specimen and (b) Method B by measuring the
water volume change of the inner cell. Fig.7 shows that the curves of accumulated
volume change with time using the two methods are very close. The volume measured
using the inner cell (Method B) is slightly larger than that measured using Method A. In
particular, at the end of the consolidation i.e. time of 1560 mins, the ∆Vi _ cell using
Method B is 21.1 cm3; while ∆Vw _ cell using Method A is 20.7 cm3. The relative
difference is
∆Vi _ cell − ∆Vw _ cell / ∆Vw _ cell =
21.1 − 20.7 / 20.7 = 1.93%. The total
volume of the clay specimen is Vo=196.25 cm3. The volume strain error is
∆Vi _ cell − ∆Vw _ cell / Vo = 21.1 − 20.7 / 196.25 = 0.20%. This error may be negligible.
3. A Hollow Cylinder Apparatus (HCA)
A fully computer controlled Hollow Cylinder Apparatus (HCA) has been installed in
Soil Mechanics Laboratory for soil testing. The soil specimen is hollow with an internal
diameter of 50mm, outer diameter of 100mm and height of 100mm. Cyclic loading can
be applied at a frequency of up to 20Hz. Unsaturated soil testing functions have also
been incorporated with the HCA. The HCA is being used to measure the timedependent stress-strain behavior of Hong Kong Marine Clays. The stress states in HCA
are shown in Fig.10.
An innovative double cell system
179
4. A Truly Triaxial System (TTS)
A new Truly Triaxial System has been installed in the Soil Mechanics Laboratory for
soil testing as shown in Fig.9. The soil specimen has a brick-shape with a height of
150mm and a cross-section of 70mm by 70mm. The vertical stress and lateral stress are
applied using oil jacks and are the
major principle stress and the
middle principle stress respective.
The minor principle stress is
applied by oil pressure in the
chamber when the door is closed
(see the outside view in Fig.9).
The stress states in the TTS are
shown in Fig.10.
5. Remarks
From the work presented above,
the following remarks are
presented:
(a) The new Double Cell
Triaxial System (DCTS)
has advantages over the
existing modified cells.
The DCTS is more
accurate and reliable and
has
continuous
measurement of the volume Figure 8. A new Hollow Cylinder System for soil testing (static
changes of unsaturated and and cyclic loading up to 20Hz).
saturated
soils
in
consolidation and compression.
(b) The calibration data using a solid copper specimen show that the outer cell
volume change is 3 to 4 times of that of the inner cell volume change. For
effective pressure of 400 kPa, the inner cell volume change is only 0.4 cm3 and
0.20% of the copper specimen volume. The error of 0.2% may be negligible for
the volume strain calculation of a soil specimen.
(c) In drained compression, the relative difference of volume changes measured
using Method A and Method B is 5.62%. The volume strain error in terms of
∆Vi _ cell − ∆Vw _ cell / Vo is only 0.21%. This error is small and considered
acceptable for most soils.
180
Yin
(a)
(b
Figure 9. A Truly Triaxial System for soil testing – (a) inside view and
(b) outside view.
(d) The new HCA and TTS are needed to study the stress-strain-strength behavior
of soils in more complicated stress states, which are closer to those in the field.
Figure 10. Stress states in HCA and TTS.
An innovative double cell system
181
6. Acknowledgements
Financial supports from a RGC grant (PolyU 5041/01E and PolyU Account No.Q414)
of the University Grant Council of the Government of Hong Kong Special
Administrative Region and from the Hong Kong Polytechnic University are
acknowledged.
7. References
BS 1377, (1990). Methods of Test for Soils for Civil Engineering Purposes, British Standards Institute,
London, 1990.
Bishop, A.W. and Donald, I.B. (1961). The experimental study of partly saturated soils in the triaxial
apparatus. Procc. 5th Int. Conf. Soil Mech. and Found. Engineering, Paris, Vol1, pp.13-21.
Chen, Z-H, Lu, Z-H., PU, Y-B. (2001). “Unsaturated soil triaxial apparatus CT system and application”,
Chinese Journal of Geotechnical Engineering Vol.23, No.4, pp.387-392.
Wheeler, S.J. (1988). The undrained shear strength of soils containing large gas bubbles. Geotechnique,
Vol.28, No.3, pp.399-413.
Yin, Z.Z. (Editor) (1998). “Settlement and Consolidation of Soil Mass”, China Electric Publication House (in
Chinese).