Note: Descriptions are shown in the official language in which they were submitted.
1070517
This invention concerns a method and equipment for
determining the pore water pressure in a soil, especially clay.
At present there are a large number of methods of
determining pore water pressure. A distinction is made
between "closed" and "open" measuring systems. In a closed
measuring system only a relatively small change in volume is
needed for ~he registration of the pressure, whereas in an
open system a large change in volume is necessary. This
invention is primarily intended for the measurement of the
pore water pressure in clays, and consists of a closed
measuring system.
The biggest disadvantage of the closed measuring
systems available today is that the measuring unit is fitted
to the pressure sounding tip. This means that the tips are
expensive and that it is difficult to make a functional check
of the pressure sensing element.
The purpose of the invention in question is to offer a
method in which the above disadvantages can be avoided. This
is done using the following procedure for determining the
pore water pressure in a soil:
1. A water-filled tube bearing a pore pressure probe
at its lower end is installed at the required
level. The pore pressure probe bears a filter
through which the pore pressure in the soil is
conveyed.
2. A measuring device is lowered down the tube and
fitted on to the pore pressure probe.
3. The pore pressure is then allowed to stabilize
after which a reading is taken which represents
the pore pressure in the soil.
4. The measuring device is disconnected from the pore
pressure probe and a reading is taken which
1070517
represents the water pressure in the tube.
5. The pore pressure is calculated by
comparison of the reading for pore water
pressure and the water pressure in the tube,
using a calibration factor for the measuring
device.
In accordance with another aspect, the invention
relates to a device for determination of pore water pressures
which comprises a water-filled tube, a pore pressure probe
with a filter at the lower end of the tube, a measuring device
with a pressure transducer, means for detachably connecting
the measuring device to the pore pressure probe from within the
water filled tube, and an electric cable which connects the
measuring device to a read-out unit.
Below a description will be given of an
embodiment of the invention with reference to the attached
drawings, wherein:
Fig. 1 shows an embodiment of a device according
to the invention;
Fig. 2 shows at a larger scale a cross section
through an embodiment of the pore pressure probe; and
Fig. 3 shows at the same scale as Fig. 2 a section
through an embodiment of the measuring device.
The device shown in Fig. 1 includes a water-filled
tube 1 which has been installed in the ground. At the lower
; end of the tube 1 a pore pressure probe 2 has been attached.
An embodiment of a pore pressure probe will be shown in greater
- detail in Fig. 2. The device shown ~n Fig. 1 also includes a
measuring de~ice 3, which has been lowered down the tube 1, and
3Q fitted tightly on to a nipple on the pore pressure probe 2.
Fig. 3 shows a preferred embodiment of a measuring device at a
-2-
~0705~7
larger scale. As shown in Fig. 1, the measuring device 3 is
connected to an electronic read-out unit 4 via an electric
cable 5.
Fig. 2 shows a pore pressure probe 2 at a larger
scale. This is screwed onto the threaded tube 1 at its lower
end. The pore pressure probe shown here is very narrow and
has a conical form. At the lower end of the probe 2 a filter
6 has been a~tached. This filter is connected via a duct 7
to a nipple 8. The measuring device 3 shown in Fig. 3 is
lowered onto this nipple. At its lower end, this measuring
device has a sleeve 9 which fits onto the nipple 8 when the
measuring device 3 is lowered. Above the sleeve 9 in the
measuring device 3 there is a pressure transducer 10, which is
connected to the read-out unit 4 via an electric cable 5.
Pore pressure measurements with the device
described above are taken by installing a water-filled tube 1
with a pore pressure probe 2 to the required level. After the
disturbance in the soil caused by the penetration of the probe 2
has dissipated, the measuring device 3 is lowered onto the
nipple 8 on the pore pressure probe 2. After a short pause
while the pore pressure stabilizes, a reading is taken by the
read-out unit 4. This reading applies to the pore pressure at
the level of the filter 6. After this, the measuring device 2
is disconnected from the nipple 8. A reading which represents
the water pressure in the tube 1 is then taken. As this water
pressure is known, due to the fact that the length of the tube
1 is known, the pore pressure can be calculated with the help
of the following formula:
u = pO + k ~ml m2)
where u = pore water pressure
O = water pressure in tube
.~-~,
'~,~1'
1070517
k = calibration coefficient for the measurement
system
ml = reading for pore water pressure
m2 = reading for water pressure in tube
Qh = distance between pressure transducer and
centre of filter.
The readings mentioned above are those supplied by
the measuring device 3. These values are then converted by
multiplying by the calibration coefficient to give a pressure
expressed in a suitable unit, for example in cm water column.
The procedure described above is thus very simple
for the people doing the fieldwork. Only two readings are
taken: that of the pore water pressure, and that of the water
pressure in the tube. For example one is thus not dependent
on being able to check possible zero shift of the measurement
system.
The equipment is also very simple, and has distinct
advantages over equipment previously known. If, for example,
the method described above is used for long-time measurements,
it is a great advantage that the measuring device is not built
into the pore pressure probe, which would make the probes
expensive, as well as making it difficult to subject the
pressure sensing element to functional testing.
Another factor which can cause interruption in pore
pressure measurements, especially in clay, is corrosion of the
pore pressure probe 2. To avoid interruptions of this kind,
the pore pressure probe can be made of nylon with a sintered
ceramic filter.
The invention is naturally not limited to the
method or devices described above. For example, the pore
pressure probe can have a cylindrical shape with mainly even
1070517
thickness instead of the shape shown in Fig. 2. This
cylindrical type of pore pressure probe is especially suited
to long-time measurements. Also, the measuring device 3 can
have a different construction without departure from the
scope of the invention. The invention can thus be varied
freely within the scope of protection according to the
following patent claims.
_5_
