Measuring and testing – Testing of material
Reexamination Certificate
2002-09-23
2004-04-13
Williams, Hezron (Department: 2856)
Measuring and testing
Testing of material
C073S038000, C073S076000, C073S073000, C073S866000
Reexamination Certificate
active
06718835
ABSTRACT:
FIELD OF THE INVENTION
This disclosure concerns an invention relating generally to pressure plate extractors for soil testing, and more specifically to pressure plate extractors intended for leak-free operation.
BACKGROUND OF THE INVENTION
The soil water characteristic curve (SWCC), a parameter which relates suction (matric, total, or both) to water content or saturation, is essential for characterizing the hydraulic and mechanical behavior of unsaturated soils. The method used to measure the SWCC depends on the texture of the soil (coarse vs. fine) and the magnitude of the suctions that must be established. For finer textured soils (silts, clays, and silty or clayey sands), a pressure plate extractor is normally used. A pressure plate extractor generally includes two key components, a pressure chamber (also referred to as a pressure cell) which allows pressurization of its interior, and a porous drain plate which rests within the pressure chamber in communication with soil to be tested, and which receives water or other liquids from the soil during pressurization. The drain plate is usually a ceramic disk, although polymeric membranes are used when very high suctions (>1500 kPa or 150 m of water) are being applied. The structure and operation of pressure plate extractors is better understood with review of common configurations of prior extractors.
FIG. 1
illustrates an exemplary pressure plate extractor
100
(commonly referred to as a “Tempe cell”) used for applications where lower suctions (<100 kPa or 10 m of water) are to be applied. The pressure chamber is defined by a lid
102
, a base
104
, and a cylindrical sidewall
106
(wherein the lid
102
and base
104
are also provided in cylindrical forms between which the sidewall
106
may be fit). A porous drain plate
108
is provided on the base
104
to receive water or other liquid from a soil sample provided atop the drain plate
108
in a retaining ring
110
. The base
104
has a recess
112
wherein the liquid may be received. A pressure inlet
114
is provided in the lid
102
for connection to a compressed air cylinder or other pressure source, and a drain outlet
116
is provided in the base
104
to receive water or other liquid expelled from the soil sample into the drain plate
108
during pressurization. O-ring seals
118
are provided between the pressure chamber sidewall
106
and the lid
102
and base
104
, and also between the drain plate
108
and base
104
. A nut-screw arrangement
120
is provided whereby the lid
102
may be urged against the sidewall
106
, which in turn urges against the drain plate
108
and base
104
, to close the pressure chamber for pressurization.
When testing at higher pressures is desired, a pressure plate extractor having a more robust pressure chamber is generally used, with an exemplary arrangement being illustrated in FIG.
2
. Here, the pressure plate extractor
200
has a pressure chamber defined by a lid
202
and a combined base and cylindrical sidewall
204
. A porous drain plate
206
receives water or other liquid from a soil sample provided in a retaining ring
208
. A metal screen
210
is situated at the bottom of the drain plate
206
, and the screen
210
and the bottom of the drain plate
206
are then enclosed (with the screen
210
held to the bottom of the drain plate
206
) by a rubber membrane
212
which is clamped about the edges of the drain plate
206
by a wire wrapping
214
. A drain outlet tube
216
then extends from the exterior of the sidewall
204
to the space between the bottom of the drain plate
206
and the rubber membrane
212
. A pressure inlet
218
extends through the sidewall
204
, and O-ring seals
220
are provided between the lid
202
and sidewall
204
to deter depressurization of the pressure chamber. A nut-screw arrangement
222
is provided to urge the lid
202
against the sidewall
204
to close the pressure chamber for pressurization.
When using the foregoing extractors
100
and
200
, the air pressure inside the pressure chamber is elevated via pressure inlets
114
and
218
, and atmospheric pressure is generally maintained at the drain outlets
116
and
216
(and thus on the sides of the drain plates
108
and
206
in fluid communication with the drain outlets
116
and
216
). Drying SWCC can be measured by first saturating the soil sample, and then applying a series of different pressure differentials (often referred to as “suctions,” since water is pulled from the soil sample owing to lower pressure at the drain outlets
116
/
216
) between pressure inlets
114
/
218
and drain outlet
116
/
216
. Different amounts of water are expelled at different pressure differentials, and the expelled water is measured (gravimetrically or volumetrically) at each suction to define the SWCC.
Although the operating principles of the pressure plate extractors
100
and
200
are conceptually simple, mechanical problems are common, with air leakage being a particular problem. Leakage is highly undesirable because it can invalidate the test results, and since a test to determine SWCC of a sample can take from two weeks to several months to run, an invalid test run can result in significant loss of time and money (and can significantly delay projects wherein the SWCC is needed to proceed). In extractors such as extractor
100
, leakage is most prevalent at the outer edge or the bottom of the drain plate
108
from air bypassing the adjacent O-ring seal
118
. A common solution is to glue the drain plate
108
in place on the base
104
using epoxy or another adhesive applied around the edge of the drain plate
108
, but because the adhesive bond is permanent, the drain plate
108
usually cannot be removed for later cleaning, test preparation, etc. without damage. Also, the rigid connection caused by the epoxy between the drain plate
108
and the base
104
can lead to cracking of the drain plate
108
owing to the pressure differential between the recess
112
and the interior of the pressure chamber, and owing to loading of the drain plate
108
by the sidewall
106
when the sidewall
106
is urged towards the base
104
to seal the pressure chamber. These problems lead to an unfortunate tradeoff: the lid
102
must be tightly clamped to the base
104
to deter leaks, but this is more likely to crack the drain plate
108
(and conversely, air leaks may result if stress on the drain plate
108
is relieved in order to avoid damage). As a result, some degree of leakage always occurs and must be tolerated, though it degrades the quality of the SWCC test results.
The extractor
200
encounters similar problems in that air leakage occurs between the drain plate
206
and the rubber membrane
212
owing to poor sealing by the wire wrapping
214
or other sealing arrangement. Decreases in test accuracy from leakage of the extractor
200
are particularly unfortunate since test data from the extractor
200
are inherently not as precise as for the extractor
100
, owing to the relatively small size of the soil sample used in the extractor
200
, and also owing to inefficiencies in collecting expelled water in the extractor
200
. These collection inefficiencies primarily arise from difficulties in collecting all water from the screen
201
and membrane
212
, and air diffusion through the drain plate
206
interfering with measurements.
Additionally, both of the extractors
100
and
200
depicted in
FIGS. 1 and 2
have limited sealing capacity between their lids, sidewalls, bases, and drain plates, since their seals
118
/
220
are set within recesses and can only be compressed to a limited extent. If the seals
118
/
220
grow less flexible over time (as is common), they may fail to provide the necessary degree of sealing regardless of how far their lids and sidewalls are urged towards their bases.
Owing to the importance of accurate SWCC measurements to civil and environmental engineering projects, and the cost and time involved in obtaining accurate SWCC measurements, there is a substantial need for improvements in
Benson Craig Herbert
Wang Xiaodong
DeWitt Ross & Stevens S.C.
Fieschko, Esq. Craig A.
Jackson André K.
Williams Hezron
Wisconsin Alumni Research Foundation
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