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86 4 MEASUREMENTS OF SOIL SUCTION

Vacuum h andp u m p

Figure 4.39 Deairing the tensiometer using a hand-held vacuumpump (from Soilmoisture Equipment Corporation).

spheric pressure within 5 min after the immersion in water,as illustrated in Fig. 4.40. A sluggish response may indi-cate a plugged porous cup, the presence o f entrapped air inthe system, o r a faulty gauge that requires rezeroing.The ceramic cup must be soaked in water prior to itsinstallation in order to avoid desaturation d ue to evapora-tion from the cup. The prepared tensiometer can then beinstalled in a predrilled hole in the field or in a soil speci-men (e.g., a compacted specimen) in the laboratory. It isimportant to ensure good contact between the ceram ic cupand the soil in order to establish continuity between thepore-water in the soil and the water in the tensiom eter tube.

0

-20A$1

9h -408E2rT

-60

-80 0 50 100 1 5 0Elapsed time, t (s)Figure 4.40 Response of tensiometers with respect to time dur-ing immersion in water (from Tadepalli, 1990).

Servicing the Tensiometer@er InsraUationAfter installation of the tensiometer, air bubbles may de-velop within the tensiometer due to several possible rea-sons. Dissolved air may come out of the solution as thewater pressure decreases to a neg ative value. A ir in the soilmay diffise through the water in the ceramic cup and comeout of the solution inside the tube. W hen the w ater pressureapproaches the vapor p ressure of water a t the ambient tem-perature, water mo lecules can m ove freely from the liquidto the vapor form (Le,, cavitation occurs). In other words,the measured absolute pressure is equal to the saturatedwater vapor pressure, ii,o.The minimum vacuum gauge pressure that can be theo-retically developed in the tensiometer is (- 101.3 kPa +uv0). In practice, however, the minimum gauge pressurethat can bemeasured in a tensiometer is approxim ately -90kPa due to the rapid accumulation of air bubbles as thesaturated water vapor pressure is approached. In addition,pore-air passes through the high air entry cup if the matricsuction of the soil exceeds the air entry value of the ce-ramic cup. The use of a ceramic cup w ith an air entry valuegreater than 100 kPa will not improve the measuring rangeof tensiometers since water in the tube w ill always cavitatewhen w ater pressure approaches approximately -90 kPa.It is necessary to check the tensiometer regularly in orderto observe the development of air bubbles in the tube. Th isis particularly crucial when performing measurements onsoils with high m atric suctions. If air bubbles are allowedto accumulate in the tube, the pressure being read on thegauge will slowly increase towards zero (Le., atmosphericpressure).Air bubbles can be removed from the tensiometer usingthe vacuum pump (Fig. 4.39) and a procedure similar tothat followed during the servicing of the tensiometer priorto installation. Having remo ved the air bubbles, deaeratedwater is added to refill the tube and the service cap is tight-ened in place. T he tensiometer reading is then allowed toequilibrate.Je t Rill TensiometersA jet fill-type tensiometer is shown in Fig. 4.41. The jetfill type is an improved m odel of the regular tensiometer.A water reservoir is provided at the top of the tensiometertube for the purpose of removing the air bubbles. The jetfill mechanism is similar to the action of a vacuum pum p.The accum ulated air bubbles are removed by p ressing thebutton a t the top to activate the jet fill action. The jet fillaction causes water to be injected from the w ater reservoirto the tube of the tensiometer, and air bubbles move up-ward to the reservoir.Small tip TensiometerA small tip tensiometer with a flexible coaxial tubing isshown in Fig. 4.42. The tensiometer is prepared for in-stallation using a similar procedure to that described for the

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Reservoir cover4.4 MEASUREMENTS OF MATRIC SUCTION 87

Push button forjet fill action

"0" ring seals

Angle molded porton the side wall

Heavy walled plastic tube

High flow 1barhigh air entry ceramic cup

Water reservoir

Zero point adjuster

Vacuum gauge(0to -100kPa)A flexible temperatureadjusting outer jacket

Figure4.41 Jet fill tensiometer from Soilmoisture Equipment Corporation.regular tensiometer tube. A vacuum pump can be initiallyused to remove air bubbles from the top of the tensiometertube. Subsequent removal of air bubbles can be performedby flushing through the co axial tube.

Flushing is conducted by circulating water from the top

of the tube and opening the water vent screw to the atmo-sphere.Water containing air bubbles will be forced into theinner nylon tube and released to the atmosphere throughthe opened water vent. Usually, this p d u r e s requiredon a daily basis. It is sometimes difficult to get an accurate

Service capRelease collet I E

Water vent screw Water ventretaining nut.ncrewV'O**ing

nai ina -retainer

Vent tubeInner nvl on ube

cup tubeassembly Fr"i7'4)Eigure 4.42 Small tip tensiometer with flexible coaxial tubing (from !bilrnoisNIe EquipmentCorporation).


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88 4 MEASUREMENTSOF OIL SUCTION

0 1 2 3 4 5 6Time, t (hours)Figure 4.43 Typical responses of the small tip tensiometer indecomposed volcanics (from Sweeney, 1982).

suction measurement because water moves from the ten-siometer into the soil each time the tensiometer is serviced.The sm all tip w ith the flexibletube allows the installationof the tensiometer into a relatively sm all soil specimen dur-ing laboratory experiments. As an exam ple, the small tiptensiometer has been installed in a consolidation specimento measure changes in matric suction during the collapseof a compacted silt (Tadepalli, 1990).Figure 4.43 illustrates typical responses of the small tiptensiometer inserted into a d ecomposed vo lcanic soil. T heresponse is plotted in te nns of matric suctions by referenc-ing measurements to atmospheric air pressures. Sweeney(1982) found that this type of tensiometer could only main-tain matric suction equilibrium fo r about one or two daysbefore the suction readings began to drop. However, thetime that equilibrium can be maintained is a function of thematric suction value being m easured.

Complete unit

Coring

@ick h w ensiometersFigure 4.44 shows a Quick D raw tensiometer equipped witha coring tool and a carrying case. The Quick Draw ten-siometer has proven t o be a particularly usefu l portable ten-siometer to rapidly measure negative pore-water pressures.The water in the tensiometer is subjected to tension foronly a short period of time during each measurement.Therefore, air diffusion through the ceramic cup with timeis minimized.The Quick Draw tensiometer can repeatedly measurepore-water pressures approaching - atm when it has beenproperly serviced. When it is not in use, the probe is main-tained saturated in a carrying case which has water-satu-rated cotton surrounding the ceramic cup. The rapid re-sponse of the Quick D raw tensiometer is illustrated in Fig.4.45.Tensiometer Pe fl o m n ce for &ld MeasurementsTensiometers have been used to measure negative pore-water pressures for numerous geo technical engineering ap-plications. O ne example is a cut slope consisting of 5-6 mof colluvium overlying a deep weathered granite (Fig.4.46). Two observation shafts, made of concrete caissonrings, were constructed along the slope (i.e., shaftsA andB in Fig. 4.46). Figure 4.47 illustrates the construction ofthe shaft, along with four circular openings along eachcaisson ring. The op enings functioned as access holes forinstalling tensiometers into the soil at various locationsalong the shaft. The shaft was equipped with wood plat-forms at various elevations, a ladder, and lighting facili-ties, as shown in Fig. 4.47.

Null knob

Figure 4.44 Quick Draw tensiometer with coring tool and cawing case (from SoilmoistureEquipment Corporation).


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4.4 MBASUREMENTS OF MATFUC SUCTION 89

170150-Eg 1 3 0 -.-c1 1 1 0 -w 90-7050

Pr--

--

0 10 20 30 40Time, t (min )Figure 4.45 Typical responses of a Quick Draw tensiometer indecomposed volcanics (from Sween ey, 1982).

Measurements of negative pore-water pressures alongshafts A and B are presented in Figs. 4.48 and 4.49, re-spectively. The results are plotted in terms of matric suc-tion and hydraulic head profiles throughout the depth of theshaft. The results indicate the fluctuation in matric suctionvalues with respect to the time of the year. The largest vari-ations occur near the ground surface (Fig. 4.48). The hy-draulic heads along the depth of the shaft are plotted byadding the negative pore-water pressure head to the ele-vation head. The hydraulic head plots indicate a net down-

' [ 1 High r ise

Completely to hi!

May, 1980

High r ise

Completelyweathered111 graniteF

Figure 4.46 Cut slope of decomposed granite where tensiome-ters were used to measure negative pore-water pressures (fromSweeney, 1982).

Wooden hut- square in planSurface drainaround hut

Wooden platformto carry 3 persons

Figure 4.47 Observation shaft for installing tensiometersin thefield (from Sweeney , 1982).ward water flow towards the water table through the un-saturated zone.

Quick Draw tensiometers have been used in the mea-surement of negative pore-water pressures along the side-walls of two trenches excavated perpendicularto a railwayembankment in western Canada. The soil consisted pre-dominantly of an unsaturated silt. The results are plottedin Fig. 4.50 as contours of matric suction across the em-bankment. It appears that the matric suctions decrease o-

Metr ic suction, (u. - uw ) kPa)0 20 40 80 80 100 0 1 0 20 3 0 40

Hydraulic head (m)

48

1216-E-

o Reading during ceissonexcavation-March to May 19800 13 September 198 0

29 November 198 0A 1 December 1 9 8 036

0-4 m. Sandy clay andsilt (Colluvium )4-6.5 m: Clayey silt(Residual soil)

tL v 9 March 1981'Groundwater tableMay 1980

F@re 4.48 Matric suction profile along shaft A (fromSweeney, 1982).


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90 4 MEASUREMENTS OF SOIL SUCTIONMatric suction, (ua - UW ) (kPa)0

of sandy clay(Colluvium)

Reading during Caissonexcavation24 -March to May 1980FGroundwater tableMay 198032 E4

Hydraulic head (m)0 10 20 30 40

Figure 4.49 Matric suction profile along shaft B (fromSweeney, 1982).

wards the ground surface due to the influenceof he micro-climatic conditions (Le., excessive rainfall).A scanning valve tensiometer system has also been usedfor recording several tensiometer readings through onecentral pressure transducer (Fig. 4.51). The hydraulicscanning valve rotates automatically from one tensiometerto another in order for the transducer to record the read-ings.Osmotic TensiometemAttempts have been made to ovenxme the problem of watercavitation in a conventional tensiometer through the use of

012-8 3678 ---20 - etric suction, (u. - uw) kPa)

62 Suctions i n undisturbed clay

Figure 4.50 Matric suction contours along the s idewalls of twotest trenches in a silt embankment (from Kmhn et al., 1989).

Input fromothertransducer 1Ir

Connections to

Bleed tubedk:gectinCrPotube connections

Figure 4.51 A scanning valve tensiometer system (from An-derson and Burt, 1977).

an osmotic tensiometer (Peck and Rabbidge, 1966). Theosmotic tensiometer uses an aqueous solution that has beeninternally prestmssed to produce a positive gauge pressure.The positive pressure of the aqueous solution is then re-duced by the negative pore-water pressure in an unsatu-rated soil when the osmotic tensiometer comes to equilib-rium. The reduction of the pressure inside the osmotictensiometer is measured using a pressure transducer to givethe measured negative pore-water pressure in the soil.

Using the above concept, highly negative pore-waterpressures can be measured without causing the tensiometersolution to go into tension. However, major difficulties as-sociated with osmotic tensiometers have restricted their use(Peck and Rabbidge, 1%9; Bocking and Fdlund, 1979).The configuration and components of an osmotic ten-siometer are shown in Fig. 4.52. The device consists of aclosed chamber containing an aqueous solution of polyeth-ylene glycol (PEG)with a molecular mass of 20 000. Afilling port is provided on the sidewall for inserting the so-lution into the chamber. The upper end of the chamber issealed with a pressure transducer for measuring the internalpressure of the solution. The lower end of the chamber issealed with a semi-pexmeable membrane attached to a 15bar, high air entry ceramic disk.

The initial internal chamber pressure of the solution isdeveloped through an osmotic process. This chamber pres-sure is highly positive, and can be in the range of 1400-2000 P a . This pressure is generated when the sensor isplaced in water, and is regarded as the reference pressure


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4.4 MEASUREMENTS OF MATRIC SUCf'ION 91r4 Electrical connector

Plastic O-ringClamping ring(2-1nillrpOre pelticon)

. ressure transdu cer(Dynisco APT 310B )

Chamber containing2.83 cmaof 3q%Aqueous solutionof PEG 2oooO

15 bar high airentry ceramic diskFigure 4.52 Osmotic tensiometer constructed at the Universityof Saskatchewan (from Bocking and Fredlund, 1979).

of the osmotic tensiometer. The reference pressure co m -sponds to equilibrium conditions at a standard temperatureof 2OoCwith the device imm ersed in distilled water at zerogauge pressure. Therefore, the osmotic process is gener-ated by immersing the osmotic tensiom eter into water.Water flows into the cham ber as a resu lt of the large dif-ference between the solution concentration across the semi-permeable membrane. O n the o ther hand, the solution mol-ecules cannot pass hrough the semi-permeable memb rane.How ever, water flows into the closed chamber, causing thesolution pressure to increase as recoded by the pressuretransducer. The flow of w ater will diminish as the internalpressure in the chamber increases. Eventually, equilibriumis reached between the internal pressure of the solution andthe osmotic suction across the semi-permeable membrane.The reference pressure under equilibrium conditions is afunction of the concentration and the molecular mass of thesolution, as illustrated in Fig. 4.53 for the PEG polyeth-ylene glycol) solution.The osmotic tensiometer can now be placed in contactwith an unsaturated soil where the pore-water pressures arenegative. During the equilibration process, a small amountof water will flow out of the chamber through the semi-permeable membrane. The solution pressure is reduced byan amount equal to the negative pore-water pressure in thesoil. The solution pressure reduction is measured on thepressure transducer. Pore-air cannot enter the chamb er be-cause of the high air entry ceramic disk and the high pos-itive pressure within the chamber.The first major difficulty associated with the osm otic ten-siometer is its inability to maintain a constant referencepressure with time. Figure 4.54 demonstrates the decreas-ing reference pressures of two osmotic tensiometers inequilibrium with distilled water at 20C over a period of

0 0.004 0.008 0.012 0.016 0.020Concentration (molar)Figure4.53 Osmotic pm sure of polyethylene glycol(PEG)o-lutions (from Bocking and F d u n d , 1979).260 days. There are seveml possible causes for the reduc-tion in the reference pressure with time (Peck and Rab-bidge, 1969). How ever, the m ost likely explanation for thepressure reduction is the minute leakage of the confinedsolute through the semi-permeable membrane. It was alsofound that the mem brane is susceptible to physical deteri-oration with time (&king and Fredlun d, 1979).The second major difficulty associated with the osmotictensiometer concerns changes in the internal referencepressure as a result of changes in the am bient temperature.Figure 4.55 shows the decreasing reference pressureof tw oosmotic tensiometers as th e tem perature i n c w s . T h epressures shown in Fig. 4.55 have been corrected for thepressure drift with time (Fig. 4.54). The effect of temper-ature on the reference pressure makes it extreme ly difficultto measure negative pore-water pressure s othe r than in acontrolled temperature environment. Even under con-trolled temperatures, there is significant drift of the refer-ence pressure with time. To date, a solution has not beenfound to overnome these problems.Axis-TrensWn TechniqueMeasurem ents of negative pore-water pressure can be madeusing the axis-translation technique. T he measurement is

Unit number 06777 (dual memb rane, 30% PEG)Drift =0.78 kPa/day, 1.22% (of 190 8 kPa)/month4 Unit num ber 6726 6 (Dual membrane, 30% PEG)Drift =3.36 kPa/day, 6.61% (o f 152 3 kPa)/month. " ' , ' ' ' ' '

L

1 - - -I5 0 0 1 ' ' ' ' ' ' ' ' ' ' ' ' ' ' 10 10 0 200 300

Time since filling, t (days)Figure 4.54 Reference pressure drifts of tw o osmotic tensiom-eters with time (from Bocking and Fredlund, 1979).


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92 4 MEASUREMENTSOF SOIL SUCTIONI2 2300$

2100

190055.-.- 1700--

1500Ut 1300k 6 10 14 18 22 260 Tem per atur e, t (OC)

Figure 4.55 Decreasing reference pressures of two osmotic ten-siometers with increasing temperatures (from Bocking and Fred-lund, 1979).

performed on either undisturbed o r compacted specimens.This technique was originally proposecl by Hilf (1956), asillustrated in F ig. 4.56.An unsaturated soil specimen was placed in a closedpressure chamber. The pore-water pressure measuringprobe consisted of a needle w ith a saturated high air entryceramic tip. T he probe w as connected to a null-type pres-sure measuring system through a tube filled with deairedwater, with a mercury plug in the middle. As soon as theprobe was inserted into the specimen, the water in the tubetended to go into tension and the Bourdon gauge beganregistering a neg ative pressure. T he tendency o f the w aterin the measuring system to go further into tension wascountered by increasing the air pressure in the chamber.Eventually, an equilibrium condition was achieved whenthe mercury plug (Le., the null indicator) remained sta-tionary. The difference between the air pressure in thechamber and the measured negative w ater pressure at equi-librium was taken to be the matric suction of the soil,(u, - U d .

When the a ir pressure is atmospheric (Le., u, =0), thematric suction value is numerically equal to the negativepore-water pressure. T he axis-translation technique simplytranslates the origin of reference for the pore-water pres-sure from standard atmospheric conditions to the final airpressure in the cham ber (Le., axis translation; H ilf, 1956).As a result, the water pressure in the measuring systemdoes not become highly negative, and the problem of cav-itation is prevented.Hilf (1956) demonstrated that the pore-water pressureincreased by a n amount equal to the increase in the ambientchamber air pressure. In o ther words, the soil matric suc-tion remained constant when measured at various ambientair pressures. T he condition of no flow maintained duringthe measurement of matric suction is the justification forthe axis-translation technique.The axis-translation technique has also been used byother researchers. Olson and Langfelder (1965), used themodified pressure plate apparatus shown in F ig. 4.57. Theprocedure used was as follows. A soil specimen is placedon top of a saturated high a ir entry disk in an air pressurechamber. The air entry value of the disk must be higherthan the matric suction to be measured. A 1 kg mass isplaced on top of the specimen to ensure a good contactbetween the soil specimen and the high air entry disk. Theplacement of the specimen onto the ceramic disk and theassemblage of the cell are performed as rapidly as possible(Le., within approximately 30 s) .The water pressure in the compartment below the highair entry disk is maintained as close as possible to zeropressure by increasing the air pressure in the chamber. T hepressure transducer connected to the water compam nent isused as a null indicator.

Compound Bourdon gauge(-101.3 kPa to 202 .6 kPa)\Pressure gauge

.-L H i g h air entryceramic tipUnsaturated soi lspecimen \ ressure chamberompressed air

Figure 4.56 Original setup for the null-type, axis-translationdevice for measuring negative pore-water pressures (fromHilf,1956).

Pressure transdu cerFigure 4.57 Pressure plate apparatus for measuring negativepore-water pressures using the axis-translation technique (fromOlson and Langfelder, 1965).


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4.4 MEASUREMENTS OF MATRIC SUCTION 93A similar pressure plate design was used in several re-

search studies at the University of Saskatchewan. The de-vice is shown in Fig. 4.58. It is important for the apparatusto have a system to flush air bubbles from below the highair entry disk in order to keep the compartment above thetransducer saturated with water.

Pressure values m shown in Fig. 4.59 to illustrate howa highly negative pore-water pressure can be measuredusing this apparatus. Let us suppose that a soil specimenhas an initial pore-water pressure of -250 kPa when placedonto the saturated, high air entry disk. The specimen willimmediately tend to draw water up through the ceramicdisk, causing the pressure transducer to commence regis-tering a negative value. The cover must be quickly placedon top of the device, and the air pressure in the chamber isincreased until there is no futther tendency for the move-ment of water through the high air entry disk. At equilib-rium, the chamber air pressure could be 255 kPa, while thewater compartment would register 5 kPa. Therefore, thematric suction of the soil is 250 kPa.Typical response time curves for the measurement ofmatric suction, using the axis-translation technique, are il-lustrated in Figs. 4.60 and 4.61 (Widger, 1976; Filson,1980). The response curves generally exhibit an Sshape, with a relatively fast equilibration time. Pressureresponse versus time is a function of the permeability char-acteristic of the high air entry disk and the soil. The resultsshown in Figs. 4.60 and 4.61 were obtained during matricsuction measurements on a highly plastic clay (Le., Reginaclay).Figures 4.62 and 4.63 present water content versus ma-tric suction relationships for compacted specimens of Re-gina clay and glacial till, respectively. The matric suctionswere measured using the axis-translation technique. Rea-sonably good agreement in the data has been obtained by

Stainless,steelchamber

Line to air pressure supply

Pressure transducersupply

Figure 4.58 Pressure plate apparatus for measuring negativepore-water pressures using theaxis-translation technique (the de-sign at the University of Saskatchewan).

-9ir pressurePreeaure transducer

WiresinitialStTBSBBsa = o a =U.U =0 (atmospheric)U,=-250 kPazimwum% smSUmQ(a u =0(u. - u =250 kPa

Rgure 4.59 Schematicshowing he p~ ssu rehanges associatedwith the measurementofmatric suction using a null-type pressureplate apparatus (from P d lu n d , 1989).

ua=256 kPauw =5kPa(a u.) =0(0. - u,) =260 kPa

several resemhers, indicating the reliability of this tech-nique.Similar water content versus matric suction relationshipswere obtained for several compacted soil types by Olsonand hngfelder (1%5); and Mou and Chu (1981). The re-sultsm shown in Figs. 4.64 and 4.65. Again, there is adistinct relationship between decreasing water contents andincreasing matric suctions. There is, however, a differencein the water content versus matxic suction Elationships ob-tained from static and kneading compaction. This differ-ence appears to be caused by different soil structures re-sulting from the different methods of compaction (Mou andChu, 1981).In general, the null-type axis-translation technique canbe used to measure negative pore-water pressures in thelaboratory with reasonable success and accuracy. High airentry disks with a maximum air entry value of up to 1500kpa are commercially available (Table 4.6). Theoreticalstudies on the axis-translation technique suggested that thetechnique is best suited to soils with a continuous air phase(Bocking and Fredlund, 1980). The presence of occludedair bubbles in the soil specimen can result in an overesti-mation of the measured matric suction, In addition, air dif-fusing through the high air entry disk can cause an under-estimating of the measured matric suction.4.4.3 Indirect MeasurementsThe indirect measurement of soil matric suction can bemade using a standard porous block as a measuring sensor.


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94 4 MEASUREMENTSOF SOIL SUCTIONI I+a400 -

a5300 w=35.1%

A w=36.8%0 w =33.2%0 w=28.8%

Undisturbedspecimensof Regina clay--U.-..)3 /#-o-o-o

0.1 1 10 1 0 0 loo0

1 00 -

Elapsed time, t (min)Figure 4.60 Response versus time for matric suction measurements on Regina clay using theaxis-translation technique (from Widger, 1976).

A wide range of porous materials has been examined fortheir soil-water characteristic relationship in order to selectthe most appropriate material for making the sensor. Thesematerials include nylon, fiberglass, gypsum plaster, clayceramics, sintered glass, and metal.The porous block sensor must be brought into equilib-rium with the matric suction in the soil. At equilibrium,the matric suction in the porous block and the soil are equal.

The matric suction is inferred from the water content of theporous block. The water content of the porous block canbe determined by m easuring the electrical or thermal prop-erties of the porous block. These properties are a functionof the w ater content, and can be established through Cali-brations. In the calibration process, the porous block issubjected to various matric suctions, and its electrical orthermal properties are m easured. As a result, the measured

Figure 4.61 Response versus time for matric suction measumments on Regina clay using theaxis-translation technique (from Filson, 1980).


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4.4 MEASUREMENTS OF MATRlC SUCTION1300r

95

"22 24 26 20 30 32 34 36Water content,w (%)

Figure 4.62 Water content versus suction for specimens ofcompacted Regina clay.

electrical or thermal property of the porous block in equi-librium with a soil can be used to determine the matricsuction in the soil through use of the calibration curve.

The indirect measurements based on the electrical prop-erties of a porous block have been found to be sensitive tothe presence of dissolved salts in the pore-water (Richards,1974). On the other hand, the indirect measurements basedon the thermal properties of the porous block show littleeffect from the dissolved salts in the pore-water or varia-tions in the ambient temperature. As a result, the thermalsensor is the most promising device for the indirect mea-surements of matric suction (Richards, 1974), Its workingprinciple and application are explained in the followingsections.Thennal Conductivity SensorsThermal properties of a soil have been found to be indic-ative of the water content of a soil. Water is a better ther-mal conductor than air. The thermal conductivity of a soilincreases with an incneasing water content. This is partic-ularly true where the change in water content is associatedwith a change in the degree of saturation of the soil.Shaw and Baver (1939) developed a device consisting ofa temperature sensor and heater which could be installeddirectly into the soil for thermal conductivity measure-ments. It was found that the presence of salts did not sig-nificantly affect the thermal conductivity of the soil. How-

Water content, w (%)Figure 4.63 Water content versus matric suction for specimensof compacted glacial till.

ever, different soils required differentcalibrations in orderto relate the thermal conductivity measurements to the watercontents of the soil. Johnston (1942) suggested that thethermal conductivity sensor be enclosed in a porous me-dium that had a calibration curve. The porous cover couldthen be brought into equilibrium with the soil under con-sideration. Johnston (1942) used plaster of pans to encasethe heating element.

In 1955, L . A . Richards patented an electrothermal ele-ment for measuring moisture in porous media (U.S. atent2 718 141). The element consisted of a resistance ther-mometer which was wrapped with a small heating coil. Theelectrothermal element was then mounted in a porous cupand sealed with ceramic cement. Richardsproposed the useof a sandy silt material for the porous block, It was sug-gested that the porous cup should have an air entry valueless than 10 kPa.Bloodworth and Page (1957) studied three materials foruse as a porous cup for the thermal conductivity sensors.Plaster of pans, fired clay, or ceramic and castone (Le., acommercially available dental stode powder) were used inthe study. The castone was found to be the best materialfor the porous cup.

Phene et al. (1971) developed a thermal conductivitysensor using a Germaniump n diode as a temperature sen-


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96 4 MEASUREMENTSOF SOIL SUCTION1650

1600nE\ 550-.-tg 1500tmn 1450

1400

-20iii4,2 --I 60i809 - 1 0 0a

10 12 14 16 18 20 22 24Weter content,w (94)

4 6 8 10 12 14 16Water content, w (%J

Figure 4.64 Negative pore-water pressure measurements on compacted specimens using theaxis-translation technique (from Olson and Langfelder, 1965).

400 - 1 I 1 I IStatic compactionKneading compaction\

0a-g 2 0 0 -g 100-.--3)0.-E

0 I t f i * I18 20 22 24 26 28 30 32

n 1.7Ez 1.6\D*CI.-

1.56Water content, w (%J

Figure 4.65 Compaction curves and corresponding water con-tent versus matric suction for an expansive clay in Texas (fromMou an d Chu, 1981).

sor. T he sensor was w rapped with 40-gauge Teflon-coatedcopper wire that served as the heating coil. The sensingunit was embedded in a porous block. The optimum di-mensions of the porous block were calculated based on atheoretical analysis. The block must be large enough tocontain the heat pulse (particularly for the saturated sensor)without interference from the surrounding soil. Also, it wasfound that the higher the ratio of the thermal conductivityto the dif isiv ity , the higher the precision with which thewater content could be measured. The distribution of thepore sizes was also important.Gypsum, ceramics, and mixtures of ceramics and ca-stone were examined as potential porous block materialsby Phen e et al., (1971). It was found that the ceramic blockexhibited a linear response and provided a stable solid ma-trix.In the mid-I970s, Moisture Control System Inc. ofFindlay, OH,manufactured the MCS 6OOO hermal con-ductivity sensor. The sensor was built using the same de-sign and construction procedures used by Phene et al.,(1971). Th e manufactured sensors were subjected to a two-point calibration. The suggested calibration curves wereassumed to be inear from zero suction to a suction of 300kPa. Above 300 kPa, the calibration curves were empiri-cally extrapolated. In the region above 300 kPa, the cali-bration curves became highly nonlinear and less accurate.


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4.4 MEASUREMENTS OF MATRIC SUCTION 97The MCS 6OOO sensors have been used for matric suc-

tion measurements in the laboratory and in the field (Pic-ornell et al., 1983; Lee and Fredlund, 1984). The sensorsappeared to be quite suitable for field usage, being insen-sitive to temperature and salinity changes. Relatively ac-curate measurements of matric suction were obtained in therange of 0-300 kPa. Curtis and Johnston (1987) used theMCS 6OOO sensors in a groundwater recharge study. Thesensors were found to be quite responsive and sensitive.The results were in good agreement with piezometer andneutron probe data. However, Moisture Control SystemInc. discontinued production in early 1980, and the MCS6OOO sensor is no longer commercially available.

In 1981, Agwatronics Inc. in Merced, CA, commencedproductionof the AGWA thermal conductivity sensors. Thedesign of the sensor was changed from previous designs,but was based on the research by Phene et al., (1971).There were several difficulties associated with the AGWAsensor that resulted in their replacement by a new design,the AGWA-I1 sensor in 1984.A thorough calibration study on the AGWA-I1 sensorswas undertaken at the University of Saskatchewan, Canada(Wong et al., 1989; Fredlund and Wong, 1989). Severalother difficulties were reported with the use of the AGWA-I1 sensors. These include the deterioration of the electron-ics and the porous block with time. The AGWA-I1 sensorshave been ;sed for laboratory andmatric suctions on several researchet al., 1987; Sattler and Fredlund,1989).

field measurements ofstudies (van der Raadt1989; Rahardjo et al.,

Theory of OpemtionA thermal conductivity sensor consistsof a porous ceramicblock containing a temperature sensing element and a min-iature heater (Fig. 4.66). The thermal conductivity of theporous block varies in accordance with the water contentof the block. The water content of the porous block is de-

Epoxyseal

Temperaturesensingintegratedcircui t

Epoxy

Epoxy backing

.Plastic jacket- eaterresistor- eramicporousmed ia

Figure4.66 A cross-sectionaldiagram of the AGWA-I1 thermalconductivity sensor(fromPhew et al. , 1971).

pendent upon the matric suctions applied to the block bythe surrounding soil. Therefore, the thermal conductivityof the porous block can be calibrated with respect to anapplied matric suction.

A calibrated sensor can then be used to measure the ma-tric suction by placing the sensor in the soil and allowingit to come to equilibrium with the state of stress in the pore-water (Le., the matric suction of the soil). Thermal con-ductivity measurements at equilibrium am related to thematric suction of the soil.

Thermal conductivity measurements are performed bymeasuring heat dissipation within the porous block. A con-trolled amount of heat is generated by the heater at the cen-ter of the block. A portion of the generated heat will bedissipated throughout the block. The amount of heat dis-sipation is controlled by the presence of water within theporous block. The change in the thermal conductivity ofthe sensor is directly related to the change in water contentof the block. In other words, more heat will be dissipatedas the water content in the block increases.

The undissipated heat will result in a temperature rise atthe center of the block. The temperature rise is measuredby the sensing element after a specified time interval, andits magnitude is inversely proportionalto the water contentof the porous block. The measured temperature rise is ex-pressed in terms of a voltage output.Calibmtion of SensorsAGWA-I1 sensors are usually subjected to a two-point cal-ibration prior to shipment from the factory.One calibrationreading is taken with the Sensors placed in water (Le., zeromatric suction). A second calibration reading is taken withthe sensors subjected to a suction of approximately 1 atm.This calibration procedure may be adequate for some ap-plications. However, it has been suggested that a more rig-orous calibration pmedure is necessary when the sensorsare used for geotechnical engineering applications (Fred-lund and Wong, 1989).

A more thorough calibration of thermal conductivity sen-sors can be performed by applying a range of matric suc-tion values to the sensors which are mounted in a soil.Readings of the change in voltage output is a measure ofthe thermal conductivity (or the water content) of the po-rous block under the applied mattic suction. The matricsuction can be applied to the sensor using a modified pres-sure plate apparatus (Wong et al., 1989; Fredlund andWong, 1989).

The sensor is embedded in a soil which is placed on thepressure plate (Fig. 4.67). The soil on the pressure plateprovides continuity between the water phase in the porousblock and in the high air entry plate. In addition, the soilused in the calibration must be able to change its watercontent at a low matric suction (i.e., low air entry value),as shown in Fig. 4.68. The matric suction is applied by


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98 4 MEASUREMENTSOF SOIL SUCTION

30Calibration soil:9Wosilt and 10%Ottawa sand25 -

-2 0 -P -

1 5 -8 -c :g 10-

E -c-

-

5 - -

\Modified pressureplateFigure 4.67 Pressure plate calibration setup fo r thermal conductivity senson (from Fredlund andWong, 1989).

0

increasing the air pressure in the pressure plate apparatus,but maintaining the w ater pressure below the pressure plateat atmospheric conditions.The change in voltage output from the sensor can bemonitored periodically until matric suction equilibrium isachieved. The above procedure is repeated for various ap-plied matric suctions in ord er to obtain a calibration curve.A number of thermal cond uctivity sensors can be calibratedsimultaneously on the pressure plate. During calibration,

1 1 ' 1 i l l l I I " " I l l '

the pressure plate setup should be contained within a tem-perature-controlledbox .Figure 4.69 shows a typical response curve for theAGWA-I1 sensor resulting from the ap plication of differentair pressures during the calibration process. T he curve in-dicates an increasing equalization time as the applied ma-tric suctions increase. F or the calibration soil indicated inFig. 4.68, the sensor has an eq ualization time in the orderof 50 h for an applied m atric suction below 150 kPa. The


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4.4 MEASUREMENTS OF MATRIC SUCTION 99

I I I50 100 150 200 250 300 3 50Elapsed tim e, t (hours)

suction measurements above 175 kPa cornspond to thesteeper portion of the calibration curve, which has a lowersensitivity to changes in thermal conductivity.

AGWA-11 sensors have shown consistent, reproducible,and stable output readings with time (Fredlund and Wong,1989). The sensors have been found to be responsive toboth the wetting and drying processes. However, somefailures have been experienced with the sensors, particu-larly when subjected to a positive water pressure. The fail-ures are attributed to moisture coming into contact with theelectronicssealed within the porous ceramic (Wong et al.,1989). Also, there have been continual problems with theporous blocks being too fragile. Therefore, the sensor mustbehandled with great am. ven so, here is a percentageof the sensors which crack or cnrmble during calibrationorinstallation.lLpicalResults ofMahic Suction Measunm entsLaboratory and field measurements of matric suctions usingthe MCS 6OOOand the AGWA-I1 thermal conductivity sen-sors have been made involving several types of soils. Thesoils have ranged from highly plastic clays to essentiallynonplastic sands. The sensors have been installed either inan initially wet or an initially dry state. The results fromthe MCS 6OOO sensorsare presented first, followed by theresults from the AGWA-I1 sensors.

Figure 4.69 Time response curves for a thennalchanges in applied air pmssum (or matric suction).

equalization time for a sensor is affected by the permeabil-ity and thickness of the calibration soil. In addition, thepermeability and the thickness of the high ai r entry disksalso affect the equalization times.More than 100 AGWA-I1 sensors have been calibratedand used at the University of Saskatchewan, Canada. Typical nonlinear calibration curves for the AGWA-I1 sensorsare shown in Fig. 4.70. The nonlinear response of the sen-sors is likely related to the pore size distribution of the ce-ramic porous block. Similar nonlinearities were also ob-served on the calibration curves for the MCS 6OOO sensor.

The nonlinear behavior of the AGWA-11 sensors may beapproximated by a bilinear curve, as illustrated in Fig.4.70. The bt.eaking points on the calibration curves aregenerally around 175 kPa. Relatively accurate measure-ments of matric suction dan be made using the AGWA-I1sensors, particularly within the range of 0-175 kPa.Matric

- 400-soil:10% Ottawa sand- 9o%sik3 300. I 7 I

Sensor reading (mV)Figure 4.70 Calibration curves for tw o AGWA-Il thermal con-ductivity sensors.

conductivity sensor (AGWA-II) subjected to

The MCS 6OOOSensorsLee (1983) studied the performance of the MCS 6OOO her-mal conductivity sensor. The laboratoryand field measure-ments of matric suctions in glacial till =hown in Figs.4.71 and 4.72, respectively. The laboratory measurements


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100 4 MEASUREMENTS OF SOIL SUCTIONDesorption cycle saturatedsensor &dry hole600 standard compaction

1400 - bsorption cycle dry- sensor &dry holeg 1200 standard compaction.* Dry sensor &wet hole7 l o 00 standard compactionDry sensor &dry ho!emodif ied compaction:6 800

u) 600

-3-'g3 4000.-L

200010 14 18 22 26

3t3

2 8 -cf 2 0 -8z 1 2 -Pe

e

4

44 r

Initially

Glacial til lI I 1 1 1 1 1 1 I I i I I I I I I 1 1

.P 36>t3

2 8 -cf 2 0 -

z 1 2 -Pe

e

8

4

83 hours 413 hours

Initially

Initiall y dry sensors

Glacial til lI I 1 1 1 1 1 1 I I i I I I I I I 1 1

1 1

Water content, w (%) Time, t (min )Figure 4.71 Laboratory measurements of matric suction in gla-cia1 till using thermal conductivity senson (MCS OOO).

Figure 4.7 3 Equalization times for the MCS OOO sensors forglacial till and Regina clay compacted at various water contents.

were performed on com pacted specimens. Figure 4.71 in-dicates that the initially wet sensor gives a lower matricsuction than the initially dry s ensor for the same w ater con-tent in the soil.The equ alization times required for the MCS ensor areshown in Figure 4.73 for measurements in glacial till andRegina clay. The initially wet sensors have longer equal-ization times (Le., maximum 413 h) than the initially drysensors (i.e., 83 h). This pattern was consistent in bothsoils.Unreliable suction measurem ents using thermal conduc-tivity sensors have been attributed to poor contact betweenthe porous block and the soil, the entrapment of air duringinstallation (Nagpal and Boersma, 1973), and temperatureand hysteretic effects. Poor contact between the porous

200 Shower SnowingV W V l

I No reading whensoil temperaturebelow zero O Cepth=0.15 m-g 120- b.- Sensor location changed,V3 100-L..-3 8 0 -

-"O 5 10 15 20 25 30 35 40 45

block and the soil will cause the sensor to read a high suc-tion value (Richard, 1974). Th e temperature effects on theMCS OOO ensor readings in R egina clay are illustrated inFig. 4.74.Tire AG WA-ZZ SensorsResults of laboratory measurements using the AGWA-I1sensors on highly plastic clays from Sceptre and Regina,Saskatchewan are shown in Figs. 4.75, 4.76, and 4.77.The soils were sampled in the field using Shelby tubes.Matric suction measumments on compacted so ils have alsobeen performed on a silt from Brazil (Fig. 4.78). The re-sults indicate that a considerably longer equ alization timewas required for the sensor to equilibrate when the watercontent of the specimen was low (Fig. 4.78) than when thewater content of the specimen was high (Figs. 4.75, 4.76,and 4.77). Th e longe r equalization time is attributed to the

-5 0 0

2 1 0 0

aO"2O 22 24 26 28 30 32

Elapsed time, t (days) Temperature ("C)-re 4.72 Field measurementsof matric suction in glacial tillusing thermal conductivity sensors (MCS-aooO). Figure4.74 Temperature effect on the MCS 6ooo sensor read-ings in Regina clay.


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4.4 MEASUREMENTSOF MATRIC SUCTION 101

Elapsed time, t (hours)Figure 4.75 Laboratory measurements of matric suction on a highly plastic clay from Sceptre,Sask., Canada (w =39.3%).

lower coefficient of permeability of the soil specimen as itswater content decreases.Several laboratory measuremen ts were conducted usingtwo senson inserted into each soil specimen. One sensorwas initiallyairdried, nd he other was initially saturated.The initially saturated sensor was submerged in water forabout two days prior to being installed in the soil. The sen-

sors were inserted into predrilled holes in eith er end of thesoil specimen. The specimen with the installed sensors waswrapped in aluminum foil to prevent moisture loss duringthe measurement. The responses of both sensors weremonitored immediately and at various elapsed times aftertheir installation. The results indicate that the time requiredfor the initially dry sensor to come to equilibrium with the

Elapsed time. t (hours)Figure4.76 Laboratory measurements of matric suctions on a highly plastic clay from Sceptre,Sask., Canada (w=34.1%).


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102 4 MEASUREMENTS OF SOIL SUCTION7 0 0

600

;a 50 0n6

- I IHi@hly laStiC clayWater content:35.1%

Elapsed time, t (hours)Figure 4.77 Laboratory measurements of matric suction on a highly plastic clay from DarkeHall, Regina, Sask., Canada (w =35.1%).

soil specimen is less than the equilibrium time required forthe initially saturated sensor to com e to equilibrium.On the basis of numerous laboratory experiments, itwould appear that the AGW A-I1 sensors that were initiallydry yielded a matric suction value w hich was close to thec o m t value. In general, the initially dry sensor shouldyield a value which was slightly high. On the other hand,the initially wet sensor yields a value which was too low.

Table 4.8 gives the interpretation of the results presentedin Figs. 4.75-4.78 inclusive.On the basis of many laboratory tests, it is recommendedthat if only one sensor is installed in an undisturbed s am -ple, the sensor should be initially dry. If the sensors havebeen calibrated using at lest seven data points, the readingsobtained in the la b ra to q should be accurate to within atleast 15 kPa of the correct value, provided the matric suc-700 Compacted si l t f rom Brazi l

Water content :15.2%600 -

in i t ial ly dry sensor1\ II ln i t ial lv saturated I

Elapsed time, t (hours) D

F igure 4.78 Laboratory measurementsof matric suctions on a compacted silt from Brazil (w =15.2%).


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4.4 MEASUREMENTS OF MATRIC SUCT~O N 103Table 4.8 Interpretationof Laboratory Matric Suction Measurements

Initially InitiallyWater Dry Wet Best

Soil Type No. (4%) Wa) orpa) ( e a )Figure Content Sensor Sensor Estimate

Sceptre clay 4.75 39.3 120 100 114Sceptre clay 4.16 34.1 136 108 126Regina clay 4.77 35.1 i6o 150 157Brazil silt 4.78 15.2 100 68 90

tion reading is in the range of 0-300 kPa. It may take four-seven days before equilibrium is achieved. If the sensorsare left in situ for a long period of time, the measurementsshould be even more accurate.Results from laboratory measurements of matric suctionhave been used to establish the negative pore-water pres-sures in un distuw samples of Winnipeg clay taken fromvarious depths within a railway embankment (Sattler et al.,1990). The samples were brought to the laboratory for ma-tric suction measurements using the AGWA-II sensors. Themeasured matric suctions were comted for the removalof the overburden stress, and plotted as a negative pore-water pressure profile (Fig. 4.79). The results indicated thatthe negative pore-water pressures approached zero at theaverage water table, and were, in gene&, more negativethan the hydrostatic line above the water table.

Field measurementsof matric suction under a controlledenvironment have been conducted in the subgrade soils ofa Department of Highways indoor test track at Regina, Sas-katchewan ( h i et al,, 1989). The temperature and the rel-ative humidity within the est track facility were controlled.Twenty-two AGWA-I1 sensors were installed in thesubgrade of the test track. The subgrade consisted of ahighly plastic clay and a glacial till. The sensorswere ini-tially airdried and installed into predrilled holes at variousdepths in the subgrade. The sensor outputs were recordedtwice a day.Typical matric suction measurements on the compactedRegina clay and glacial till subgrade are presented in Figs.4.80 and 4.81. Consistent readings of matric suction rang-ing from 50 to 400 kPa were monitored over a period ofmore than five months prior to flooding the test track. The

Pore-water pressure,u (kPa)Figure 4.79 Negative pore-water pressures measured using the AGWA-II thennal conductivitysensorson undisturbed samples.


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104 4 MEASUREMENTS OF SOIL SUCTION$7001 : ; I I , ! ,i -;00 content =7.4%-400.- 30 0In 200,- 100I

I I I I b I I I

0; ldo 260 300 4 0 5;)o & 760 8 0 40Elapsed t ime, t (hours)

Figure 4.80 Measurements of matric suction using the AGWA-I1 thermal conductivity sensors under a controlled environment inthe test track facility (Department of Highways, Regina, Can-ada).

sensor responded quickly upon flooding (Fig. 4.82). Theresults demonstrated that the AGWA-II sensors can pro-vide stable measurements of matric suction over a rela-tively long period of time.Matric suction variations in the field can be related toenvironmental changes. Several AGWA-I1 sensors havebeen installed at various depths in the subgrade below arailroad. Th e soil was a highly plastic Regina clay that ex-hibited high swelling potentials. Matric suctions in the soilwere monitored at various times of the year. The resultsclearly indicate seasonal v ariations of matric suctions in thefield, with the greatest variation occurring near g round sur-face (Fig. 4.83).

Thermal conductivity sensors appear to be a promisingdevice for m easuring matric suction either in the laboratoryor in the field. However, proper calibration should be per-formed on each sensor prior to its use. The calibration studyon the AGWA -I1 sensors revealed that the sensors are quitesensitive for measuring m atric suctions up to 175 kPa.It is possible that further improvem ents on thermal con-ductivity type sensors will further enhance their perfor-mance. For example, a better seal around the electronicswithin the sensor could reduce the influence of soil water.Also, a stronger, more durable porous block would pro-duce a better sensor for geotechnical engineering applica-tions. These improvemen ts would reduce the mortality rateof the sensor.

4.5 MEASUREMENTS OF OSMOTIC SUCTIONSeveral procedures can be used to m easure the osmotic suc-tion of a soil. For example, it is possible to add distilledwater to a soil until the so il is in a near fluid condition, andthen drain off some effluent and m easure its electrical con-ductivity. The conductivity measurement can then be lin-early extrapolated to the osmotic suction corresponding tothe natural water content. T his is kno wn a s the saturationextract procedure. Although the p rocedure is simple, itdoesnot yield an accurate measurement of the i n situ osmoticsuction (Krahn and Fredlund, 1972).A p sychrometer can also be placed ove r the fluid extractto measure the osmotic suction, but this procedure, like-

1000 2000 3000 4Elapsed time, t (hours)

Figure 4.81 Measurements of matric suction using th e AGWA-I1 thermal conductivity sensorsunder a controlled environment (Test track facility, Department of Highways, Regina, Canada).


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4.5 MBASUREMENTS OFOSMOTIC SUCTION 105

- 1500.E 1000-_ .

'Slrid' : :.:, : ',., ,: :;:.,.::.i-Regina clay 86501 .o

5 500 Reg insclay Sandzij ,Sand-', 6 I , Ial 0 '

0 3000 8ooo Irkpermeable 'O0ODistance (mm) membraneL*Oo0 50000 1 0 0 0 0 0 150000 200000

Tim, t (min)(a) (b)

Figure 4.82 Cross-section and location of measurements of matric suction using the AGWA-I Ithermal conductivity sensors under a controlled environment (a) sensor locations; (b) sensor re-sponses, (Test track facility, Departmentof Highways, Regina, Canada).

wise, gives poor results. It is the use of the pore fluidsqueezer technique that has proven to give the most rea-sonable measurements of osmotic suction.4.5.1 SqueezingTechniqueThe osmotic suction of a soil can be indirectly estimatedby measuring the electrical conductivity of the pore-waterfrom the soil. Pure water has a low electrical conductivityin comparison to porn-water which contain s dissolved salts.The electrical conductivityof the pore-water from the soilcan be used to indicate the total concentration of dissolvedsalts which is related to the osmotic suction of the soil.The pore-water in the soil can be extracted using a porefluid squeezer which consists of a heavy-walled cylinderand piston squeezer (Fig. 4.84). The electrical resistivity(or electrical conductivity) of the pore-water is then mea-sured. A calibration cu w e (Fig. 4.85) can be used to relatethe electrical conductivity to the osmotic pressure of the

700

800

5 002E400537 300-

200100

8

ONov Jan Mar May July Sept1984 1985

Figure4.83 Summary plot of matric suction measurements ver-sus time of year for various depths in Regina clay in Saskatche-wan (from van der Raadt, 1988).

soil. The results of squeezing technique measurements ap-pear to be affected by the magnitude of the extraction pres-sure applied. Krahn and Fredlund (1972) used an extrac-tion pressure of 34.5 MPa in the osmotic suctionmeasurements on the glacial till and Regina clay.Figures 4.86 an d 4.87 present the results of osmotic suc-tion measurements on glacial till and Regina clay, respec-tively. The measurements were conducted using thesqueezing technique. The measured osmotic suctions areshown to agree closely with the total minus the m atric suc-tion measurements. In this case, he total and the matricsuctions were measumd independently. The discrepancies

-L6. 4T

L URubber (neoprene d isk)

Perforated plate, 1.6 mm thick(filter paper su pport)tainless steel wir e-screenisk, 1.6 mm thick

Rubber (neop rene) wash er4.8 mm thickEffluent passage reamedto fit nose of syringepassage

k 5 4 . 0 4Figure 4.84 The design of the pore fluid squeezer (from Man-heim, 1966).


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106 4 MEASUREMENTS OF SOIL SUCTION

Figure 4.85 Osmotic pressure versus electrical conductivity re-lationship for pore-water containing mixtures of dissolved salts(from U.S.D.A.Agricultural Handbook No. 60, 950).

shown at low water contents for the glacial till (Fig. 4.85)are believed to be attributable to inaccurate measurementsof matric suction (Krahn and Fredlund, 1972).The close agreement exhibited in Figs. 4.86 and 4.87indicates the reliability of the squeezing technique for os-motic suction measurements. The results also support the

2 600k - otal minus matric suction---- By squeezing technique0 9 1 1 13 16 17 19 21Water content,w (%)

Figure 4.86 Osmotic suction versus water content forglacial till(fromKrahn and Fredlund, 1972).

m

- otal minus matric suction60 0kz 200.-0 20 22 24 26 20 30 32I Water content,w (96)

Figure 4.87 Osmotic suction versus water content for Reginaclay (from Kmhn and Fredlund, 1972).

validity of the matric and osmotic suctions being compo-nents of the total suction [Le., Eq. 4.3)].It appears that the osmotic suction is relatively constantat various water contents (Figs. 4.86 and 4.87). Therefore,it is possible to use the osmotic suction as a relatively fixedvalue that can be subtmcted from the total suction mea-surements in order to give the matric suction values.