Measuring Techniques
The method proposed in this article is called the
continuous line source method. With this method, the
thermal conductivity of bentonite-based buffer
material can be measured with different clay dry
density in a single sample. The concept is to put the
thermal probe inside the buffer material sample while
compaction. When a dry density is reached, the
thermal conductivity is measured. With repeating the
process, the relationship between thermal
conductivity and clay dry density can be established.
This method includes two systems – the compaction
mold and the line-source measurement system.
2.1 The compaction mold
The schematic layout of the compaction mold is
in Figure 1. The mold is composed by the following
components.
The inner split mold The four-piece steel split mold
forms a cubic space of 18 cm × 7 cm and 19 cm in
height inside. Thermal hardening process is applied
over the surface of the steel mold to archive a higher
stiffness to resist the scratching from the compaction
process. The outside shape of the split mold when
assembled is a cylindrical cone with transition in
diameter from 25.3 cm at bottom and 24.3 cm at the
top.
The confining ring The ring is made of steel without
thermal process. The outer diameter is 31.5 cm. The
inner shape of the ring is fit with the split inner mold.
The reason for the design is that it will be easier to
remove the inner mold out the ring after the sample is
compacted, and the mold would not slip out when the
compaction force exerts a lateral force through the
sample. The height of the ring is 18 cm, less then that
of the inner split mold. When the ring is put on the
inner split mold assembly, there is a space between
the ring and the base plate. This is to ensure the
confining of the ring on the inner split mold.
The compaction piston The cubic piston is made with
steel and thermal hardening process is also applied on
it. The dimension of the piston is 17.9 cm × 6.9 cm
and 19 cm in height. A threaded rod is installed on
top of it to connect to the loading frame.
The base plate The base plate is also made with
thermal hardening processed steel. The base plate has
a upper deck and a lower deck. Figure 2 shows the
detail of the base plate. A hole is drilled on the upper
deck, while a groove is made at the same place in the
lower deck. The wire of the thermal probe goes
through the hole to connect to the logging system and
the power supply.
The loading system A servo-controlled loading
system was used to apply the compaction loading.
During the compaction procedure, the applied
loading and the displacement of piston were
continuously recorded by a load cell and LVDT. The
capacity of loading system is 1 MN. The maximum
compaction pressure is around 100 MPa. The control
method of loading can be force-controlled or
displacement-controlled. The compaction method
used in this research is uniaxial static compaction By
controlling the distance from the base plate and the
piston, the volume of the sample is determined, and
the clay dry density can be calculated.
Fig. 1 The compaction mold
2.2 The line-source method
The thermal probe or “needle” method is a rapid
and convenient method for measuring thermal
conductivity of soils in the laboratory or in-situ. The
probe is inserted into the soil to be tested and being
thin, should cause little disturbance. It consists of a
heater producing thermal energy at a constant rate
and a temperature sensing element (thermocouple or
thermistor). The rate of rise in the temperature of the
probe depends on the thermal conductivity of the
surrounding medium.
The theory of the line-source method is based on
the theory of the line heat source placed in a
semi-finite, homogeneous and isotropic medium
(Carslaw and Jaeger 1959). The testing equipment
used in this research is based on ASTM D53340 -
Standard test method for determination of thermal
conductivity of soil and soft rock by thermal needle
probe procedure. The apparatus is listed below:
Thermal Needle Probe The thermal probe, which is
15 cm long and 0.3 cm in diameter, consists off a
nichrome heater wire and a T-type thermocouple
(made of copper and constantan wire). The
thermocouple is suitable for its durability and the
temperature range at -200°C ~ 400°C. The heater and
the thermocouple is placed in a stainless tube and
magnesium oxide (MgO) powder is filled in the gaps.
A hydrostatic pressure at 30000 PSI is applied on the
probe to compress the tube and the fill to form the
probe. Therefore, the strength of the thermal probe is
suitable for the experiment configuration in this
research. To protect the wires during compaction, a
metal mesh tube with TEFLON liner covers the
connecting wire all the way through the hole on the
base plate. The position of the probe in the sample is
kept in the central by first putting half amount of the
sample in the mold, and bending the flexible wire to
lay the probe on the surface, and then cover with the
other half of the sample. The resistance of the
nichrome heater wire is checked with Ohm meter
before and after each experiment.
Fig. 3 The thermal probe
Constant Current Source A power supply is used to
produce a constant current.
Thermal Readout Unit Agilent 34970A data
acquisition/switch unit with HP 34901A 20-channel
armature multiplexer is used for temperature logging
with time. While using the T-type thermocouple with
this system, an ice-bath should be used to create a
known reference temperature (0°C) in order to
prevent the internal junction error. The resistance of
the nichrome heater and the voltage of the power
supply is also calibrated and recorded with this
system to ensure the quality of the experiment.
The calculation of thermal conductivity is
described in the standard suggested by ASTM D5334.
2.3 The procedure
This method consist the following assumptions.
1. The water inside the sample is
incompressible.
2. The sand (crushed granite) particles in
the sample are incompressible for the
sake of its high stiffness comparing to
clay.
3. The volume change of the sample is
contributed by the compaction on clay
only.
Tien and Wu (2003) described the relationship
between clay dry density, volumetric ratio and weight
ratio in the compaction process of the bentonite-sand
(or crushed granite) buffer material.
With these assumptions, the clay dry density can
be determined by the distance from the piston to the
base plate.
The procedure for the continuous line-source
method is described below.
Determine the clay dry density at final stage The
clay dry density is determined by the dry weight of
the clay proportion in the sample and the volume
formed by the inner split mold and the piston. The
final stage of experiment means that the volume of
the sample equals to 18cm × 7cm × 7cm. This prism
shape of sample is valid for the geometry
specification by ASTM D5334. The gross weight
remains constant all over the experiment. Once the
clay density at the final stage is determined, the gross
weight of bentonite-sand (or crushed granite) powder
that will spur into the mold is determined.
Determine the Levels of clay dry density that the
measurement will be executed After the final
(maximum) clay dry density is determined, the other
clay dry density, at which the thermal conductivity
measurement will be conducted, can be decided.
Then the positions of the piston can be also decided.
Pour the mixtures into the mold Half of the powdery
mixture of bentonite and sand (crushed granite) is put
into the mold and tamped to form a flat surface. The
flexible wire of the thermal probe is immersed in the
sample near the mold (as shown in Figure 3). After
tamping the first layer, the thermal probe is bended to
lay on the surface and temporarily fixed on it. The
other half of the powder is then poured onto it and
temped.
Metal m esh tube
with TEFLON liner
Th e r m o c o u p l e
Fig. 3 The position of the thermocouple inside the sample
Apply the initial contact load The piston is lowered
to the surface of the sample, and then a contact load
of 0.5kN is applied. The settlement of the sample at
this sequence is large due to the loose powdery state
of sample. The force applying rate is slow to allow
the particles to be rearranged. The readings of load
cell will be stable when the settlement of the sample
stops.
The follow sequence can be repeated to perform
thermal conductivity measurement at different clay
dry density that is determined previously.
Compact to the designated clay dry density
Displacement-controlled loading is applied at a
displacement rate of 0.1mm/min. After the piston
reaches the designated position, the position is fixed
for 30 minutes for the sample to be stabilized.
Thermal conductivity measurement The
measurement for thermal conductivity is now
performed. First the data logging system start to
record the temperature by the thermocouple at a
period of 0.5 sec. Then the power supply is turn on to
input the heat and the time is recorded as the start of
measurement. The heating time for the measurement
is 1200 sec. After the test, the sample needs to be
cool down to room temperature for the next stage of
measurement. The cooling time for the materials we
used is 40 minutes, and this time interval can overlap
with the compacting and stabilizing time at next stage.
Therefore, once the heating is stopped, the piston
starts to compact to the next stage of dry density, and
followed by stabilization time.
These two sequences are repeated till the final
stage of dry density is reached and the thermal
conductivity measurement is conducted. After the test,
the compaction force is released at a slow rate of
1kN/s. The sample is then taken out cut into slices to
measure the water content for comparison
Measuring Techniques The method proposed in this article is called the continuous line source method. With this method, the thermal conductivity of bentonite-based buffer material can be measured with different clay dry density in a single sample. The concept is to put the thermal probe inside the buffer material sample while compaction. When a dry density is reached, the thermal conductivity is measured. With repeating the process, the relationship between thermal conductivity and clay dry density can be established. This method includes two systems – the compaction mold and the line-source measurement system. 2.1 The compaction mold The schematic layout of the compaction mold is in Figure 1. The mold is composed by the following components. The inner split mold The four-piece steel split mold forms a cubic space of 18 cm × 7 cm and 19 cm in height inside. Thermal hardening process is applied over the surface of the steel mold to archive a higher stiffness to resist the scratching from the compaction process. The outside shape of the split mold when assembled is a cylindrical cone with transition in diameter from 25.3 cm at bottom and 24.3 cm at the top. The confining ring The ring is made of steel without thermal process. The outer diameter is 31.5 cm. The inner shape of the ring is fit with the split inner mold. The reason for the design is that it will be easier to remove the inner mold out the ring after the sample is compacted, and the mold would not slip out when the compaction force exerts a lateral force through the sample. The height of the ring is 18 cm, less then that of the inner split mold. When the ring is put on the inner split mold assembly, there is a space between the ring and the base plate. This is to ensure the confining of the ring on the inner split mold. The compaction piston The cubic piston is made with steel and thermal hardening process is also applied on it. The dimension of the piston is 17.9 cm × 6.9 cm and 19 cm in height. A threaded rod is installed on top of it to connect to the loading frame. The base plate The base plate is also made with thermal hardening processed steel. The base plate has a upper deck and a lower deck. Figure 2 shows the detail of the base plate. A hole is drilled on the upper deck, while a groove is made at the same place in the lower deck. The wire of the thermal probe goes through the hole to connect to the logging system and the power supply. The loading system A servo-controlled loading system was used to apply the compaction loading. During the compaction procedure, the applied loading and the displacement of piston were continuously recorded by a load cell and LVDT. The capacity of loading system is 1 MN. The maximum compaction pressure is around 100 MPa. The control method of loading can be force-controlled or displacement-controlled. The compaction method used in this research is uniaxial static compaction By controlling the distance from the base plate and the piston, the volume of the sample is determined, and the clay dry density can be calculated. Fig. 1 The compaction mold 2.2 The line-source method The thermal probe or “needle” method is a rapid and convenient method for measuring thermal conductivity of soils in the laboratory or in-situ. The probe is inserted into the soil to be tested and being thin, should cause little disturbance. It consists of a heater producing thermal energy at a constant rate and a temperature sensing element (thermocouple or thermistor). The rate of rise in the temperature of the probe depends on the thermal conductivity of the surrounding medium. The theory of the line-source method is based on the theory of the line heat source placed in a semi-finite, homogeneous and isotropic medium (Carslaw and Jaeger 1959). The testing equipment used in this research is based on ASTM D53340 - Standard test method for determination of thermal conductivity of soil and soft rock by thermal needle probe procedure. The apparatus is listed below: Thermal Needle Probe The thermal probe, which is 15 cm long and 0.3 cm in diameter, consists off a nichrome heater wire and a T-type thermocouple (made of copper and constantan wire). The thermocouple is suitable for its durability and the temperature range at -200°C ~ 400°C. The heater and the thermocouple is placed in a stainless tube and magnesium oxide (MgO) powder is filled in the gaps. A hydrostatic pressure at 30000 PSI is applied on the probe to compress the tube and the fill to form the probe. Therefore, the strength of the thermal probe is suitable for the experiment configuration in this research. To protect the wires during compaction, a metal mesh tube with TEFLON liner covers the connecting wire all the way through the hole on the base plate. The position of the probe in the sample is kept in the central by first putting half amount of the sample in the mold, and bending the flexible wire to lay the probe on the surface, and then cover with the other half of the sample. The resistance of the nichrome heater wire is checked with Ohm meter before and after each experiment. Fig. 3 The thermal probe Constant Current Source A power supply is used to produce a constant current. Thermal Readout Unit Agilent 34970A data acquisition/switch unit with HP 34901A 20-channel armature multiplexer is used for temperature logging with time. While using the T-type thermocouple with this system, an ice-bath should be used to create a known reference temperature (0°C) in order to prevent the internal junction error. The resistance of the nichrome heater and the voltage of the power supply is also calibrated and recorded with this system to ensure the quality of the experiment. The calculation of thermal conductivity is described in the standard suggested by ASTM D5334. 2.3 The procedure This method consist the following assumptions. 1. The water inside the sample is incompressible. 2. The sand (crushed granite) particles in the sample are incompressible for the sake of its high stiffness comparing to clay. 3. The volume change of the sample is contributed by the compaction on clay only. Tien and Wu (2003) described the relationship between clay dry density, volumetric ratio and weight ratio in the compaction process of the bentonite-sand (or crushed granite) buffer material. With these assumptions, the clay dry density can be determined by the distance from the piston to the base plate. The procedure for the continuous line-source method is described below. Determine the clay dry density at final stage The clay dry density is determined by the dry weight of the clay proportion in the sample and the volume formed by the inner split mold and the piston. The final stage of experiment means that the volume of the sample equals to 18cm × 7cm × 7cm. This prism shape of sample is valid for the geometry specification by ASTM D5334. The gross weight remains constant all over the experiment. Once the clay density at the final stage is determined, the gross weight of bentonite-sand (or crushed granite) powder that will spur into the mold is determined. Determine the Levels of clay dry density that the measurement will be executed After the final
(maximum) clay dry density is determined, the other
clay dry density, at which the thermal conductivity
measurement will be conducted, can be decided.
Then the positions of the piston can be also decided.
Pour the mixtures into the mold Half of the powdery
mixture of bentonite and sand (crushed granite) is put
into the mold and tamped to form a flat surface. The
flexible wire of the thermal probe is immersed in the
sample near the mold (as shown in Figure 3). After
tamping the first layer, the thermal probe is bended to
lay on the surface and temporarily fixed on it. The
other half of the powder is then poured onto it and
temped.
Metal m esh tube
with TEFLON liner
Th e r m o c o u p l e
Fig. 3 The position of the thermocouple inside the sample
Apply the initial contact load The piston is lowered
to the surface of the sample, and then a contact load
of 0.5kN is applied. The settlement of the sample at
this sequence is large due to the loose powdery state
of sample. The force applying rate is slow to allow
the particles to be rearranged. The readings of load
cell will be stable when the settlement of the sample
stops.
The follow sequence can be repeated to perform
thermal conductivity measurement at different clay
dry density that is determined previously.
Compact to the designated clay dry density
Displacement-controlled loading is applied at a
displacement rate of 0.1mm/min. After the piston
reaches the designated position, the position is fixed
for 30 minutes for the sample to be stabilized.
Thermal conductivity measurement The
measurement for thermal conductivity is now
performed. First the data logging system start to
record the temperature by the thermocouple at a
period of 0.5 sec. Then the power supply is turn on to
input the heat and the time is recorded as the start of
measurement. The heating time for the measurement
is 1200 sec. After the test, the sample needs to be
cool down to room temperature for the next stage of
measurement. The cooling time for the materials we
used is 40 minutes, and this time interval can overlap
with the compacting and stabilizing time at next stage.
Therefore, once the heating is stopped, the piston
starts to compact to the next stage of dry density, and
followed by stabilization time.
These two sequences are repeated till the final
stage of dry density is reached and the thermal
conductivity measurement is conducted. After the test,
the compaction force is released at a slow rate of
1kN/s. The sample is then taken out cut into slices to
measure the water content for comparison
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