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CHAPTER 4

Measurements of Soil Suction

The role of matric suction as one of the stress state vari-ables for an unsaturated soil was illustrated in Chapter 3.The theory and components of soil suction will be pre-sented first in this chapter, followed by a discussion of thecapillary phenomena. Various devices and techniques formeasuring soil suction and its com ponents a~ described indetail in this chapter. Each device or technique is intro-duced with a history of its development, followed by itsworking principle, calibration technique, and performance.4.1 THEORY OF SOIL SUCTIONThe theoretical concept of soil suction was developed insoil physics in the early 1900s (Buckingham, 1907; G a d -ner and Widtsoe, 1921; Richards, 1928; Schofield, 1935;Edlefsen and Anderson, 1943; Childs and Collis-George,1948; Bolt and Miller, 1958; Corey and Kemper, 1961;Corey et al., 1967). The soil suction theory was mainlydeveloped in relation to the soil-water-plant system. T heimportance of soil suction in explaining the mechanical be-havior of unsaturated soils relative t o engineering problem swas introduced at the Road Research Laboratory in En-gland (Croney and Coleman, 1948; Croney et al., 1950).In 1965, the review panel for the soil mechanics sympo-sium, Moisture Equilibria and Moisture Changes inSoils (Aitchison, 1965a), provided quantitative defini-tions of soil suction and its components from a thermody-namic context. These definitions have become acceptedconcepts in geo technical engineering (Krahn and Fredlund,1972; Wray, 1984; Fredlund and Rahardjo, 1988).Soil suction is commonly referred to as the free energystate of soil water (Ed lefsen and And erson, 1943). The freeenergy of the soil water can be measured in terms of thepartial vapor pressure of the soil water (Richards, 1965).The thermodynamic relationship between soil suction (orthe free energy of the soil water) and the partial pressureof the pore-water vapor can be written as follows:

J . = -- RT In (s)V W O @ lJ 4 0

whereJ. =soil suction o r total suction (kPa)R =universal (molar) gas constant [Le., 8.31432T =absolute temperature [Le., T = (273.16 + t o )O = temperature (C)

J/ (mol K)1(K)1

v w 0= specific volume of water or the inverse of thep w =density of water (Le., 998 kg/m3 at t o =20C)o,=molecular mass of water vapor (Le., 18.016u, =partial pressure of pore-water vapor (H a )

density of water [Le., l / p w ) (m3/kg)]

- kg/kmol)-uu0 = saturation pressure of w ater vapor o ver a flat sur-face of pure water at the sam e temperature (kPa).Equation (4.1) shows that the reference state for quan-tifying the components of suction is the vapor pressureabove a flat surface of pure w ater (i.e., water with no saltsor impu rities). The term i i v / i i vo s called relative humidity,RH 96). If we select a reference temperature of 2 0 C , theconstants in Eq. 4.1) give a value of 135 022 kPa. Equa-tion (4.1) can now be written to give a fixed relationshipbetween total suction in kilopascals and relative vaporpressure:

(4.2)Figure 4.1 shows a plot of Eq. (4.1) for three differenttemperatures. Th e soil suction, $, is equal to 0.0 when therelative humidity, RH Le., i i v / i i v o ) ,s equal to 100% [E@(4.1)]. A relative humidity value less than 100% in a soilwould indicate the presence of suction in the soil. Figure4.1 also shows that suction can be extremely high. For ex-ample, a relative humidity of 94.24% at a temperature of20C corresponds to a soil suction of 8000 kPa. The rangeof suctions of interest in geotechnical engineering will cor-respond to h igh relative humidities.

4.1.1 Componentsof Soil SuctionThe soil suction as quantified in terms of the relative hu-midity [Eq. 4.1)] is commonly called total suction. It

J. = -135 022 In ( iZv / i ivo) .

64

Copyright 1993 John Wiley & Sons Retrieved from: www.knovel.com

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4.1 THEORY OF SOIL SUCTION 651 0 0

80

Bo

40

20

01o2 1 0 3 ta4 IO? 1 0 107

Total suction,tb (kPe)Figure4.1 Relative humidity Venus total suction relationship.

has two components, namely, matric and osmotic suction.The total, matric, and osmotic suctions can be defined asfollows (Aitchison, 1965a):Matric or capillary component of free energy-In suctionterms, it is the equivalent suction derived from the measum-ment of the partial pressure of the water vapor in equilibriumwith the soil water, relative to the partial pressure of the watervapor in equilibrium with a solution identical in compositionwith the soil water.Osmotic (or solute) component of free energy-In suctionterms, it is the equivalent suction derived from the measum-ment of the partial pressure of the water vapor in equilibriumwith a solution identical in composition with the soil water,relative to the partial pressure of water vapor in equilibriumwith free pure water.Total suction or free energy of the soil water-In suctionterms, it is the equivalent suction derived from the measure-ment of the partial pressure of the water vapor in equilibriumwith a solution identical in composition with the soil water,relative to the partial pmssure of water vapor in equilibriumwith free pure water.The above definitions clearly state that the total suctioncorresponds to the free energy of the soil water, while thematric and osmotic suctions are the components of the freeenergy. In an equation form, this can be written as follows:

(4.3)= (u , - U J + a(u , - u,) =matric suction

u, =pore-air pressureu , =pore-water pressure

7r =osmotic suction.The spelling of the erm matric is in accordance with the recommen-dation of the Committee on Terminology of the Society of Soil Scienceof America. The definitionis from their Glossary of Soil Science Termi-nology (1963, 1970. and 1979).

Figure 4.2 illustrates the concept of total suction and itscomponent as dated to the free energy of the soil water.The matric suction component is commonly associated withthe capillary phenomenon arising from the surface tensionof water. Surface tension has been described in Chapter 2,and is the result of the intermolecular forces acting on mol-ecules in the contractile skin. The capillary phenomenon isusually illustrated by the rise of a water surface in a cap-illary tube (Fig. 4.2).In soils, the pores with s m a l l radii act as capillary tubesthat cause the soil water to rise above the water table (Fig.4.3). The capillary water has a negative pressure with re-

Measuredsystem

O v l < ovoEoil water

Reference Suctionmedium

Metric,(U. - uw)

osmot ic,Ea 7rlpure water1

Figure 4.2 Total suction and its components: matric and os-motic suction.


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66 4 MEASUREMENTSOF SOIL SUCTIONMaonified

J. .soil particles

Radius of meniscus0.0002 cm------ ---------.

. . .... . . ,.,' ,':'. ..: . : : .. . .. .,. , :. , .. . . , . . ...,... , . . . , . . . . . . .Flgure 4.3 Capillary tubes showing the air-water interfaces atdifferent radii of cuwatuve (from Janssen and Dempsey, 1980).spect to the air pressure, which is generally atmospheric(i.e., u, =0) in the field. At low degms of saturation, thepore-water pressures can be highly negative, with valuesas low as minus 7000 kPa (Olson and Langfelder, 1965).In this case, he adsorptive forces between soil particles arebelieved to play an important role in sustaining the highlynegative pore-water pressures in soils.

Consider a capillary tube filled with a soil water. Thesurface of the water in the capillary tube is curved and iscalled a meniscus. On the other hand, the same soil waterwill have a flat surface when placed in a large container.The partial pressure of the water vapor above the curvedsurface of soil water, U,, is less than the partial pressure ofthe water vapor above a$& surface of the same soil water,u, , , (i.e., E,

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4.2 CAPILLARITY 67the water content versus matric suction relationship in soils(Le., the soil-water characteristic curve). This relationshipisdifferent or the wetting and drying portions of the curve,and these differences can also be explained in terms of thecapillary model.4.2.1 Capillary HeightConsider a small glass tube that is inserted into water underatmospheric conditions (Fig. 4.5). The water rises up inthe tube as a result of the surface tension in the contractileskin and the tendency of water to wet the surface of theglass tube (i.e., hygroscopic properties). This capillary be-havior can be analyzed by considering the surface tension,T,, cting around the circumference of the meniscus. Thesurface tension, T,, cts at an angle, a, rom the vertical.The angle is known as the contact angle, and its magnitudedepends on the adhesion between the molecules in the con-tractile skin and the material comprising the tube (i.e.,glass).Let us consider the vertical force equilibrium of the cap-illary water in the tube shown in Fig. 4.5. The verticalresultant of the surface tension (i.e., 2.rr r T,cos a) s re-sponsible for holdingtheweight of the water column, whichhas a height of h, (i.e., r r 2 h, pw g ):

(4.4)where

2 r r T,cos a = .rrr2h,p,gt = radius of the capillary tubeT,= surface tension of watera =contact angleh, =capillary heightg =gravitational acceleration.

0 Total suction(psychrometer) --Matric suction(pressure plate)A Osmotic suction(squeezing techn ique)- smotic plus

I0 ' ! ' ' I ' ' ' ' ' ' 1I9 1 1 13 15 17 19Water content, w (%)Figure 4.4 Total, matric, and osmotic suctions for glacial till(fromKrahn and Fredlund, 1972).

planation of each device is given later. The measurementrange and comments related to each device are shown inTable 4.2.

4.2 CAPILLARITYThe capillary phenomenon is associated with the matricsuction component of total suction. The height of waterrise and the radius of curvature have direct implications on

Table 4.2 Devices for Measuring Soil Suction and Its ComponentsName of Device Suction Component Measured Range (P a ) Comments

Psychrometers Total 100"- - OOO Constant temperature environmentFilter paper Total (Entire range) May measure matric suction whenTensiometers Negative pore-water pressures or 0--90 Difficulties with cavitation and air

requiredin good contact with moist soildiffusion through ceramic cupatric suction when pore-airpressure is atmospheric

Null-type pressure plate Matric 0- 500 Range of measurement is a function(axis translation) of the air entry value of theceramic diskThermal conductivity Matric 0--400+ Indirect measurement using avariable pore size ceramic sensorPore fluid squeezer Osmotic (Entire range) Used in conjuction with apsychrometer or electricalconductivity measurement

sensors

"Controlled temperature environment to f 0.001"C.


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68 4 MEASUREMENTSOF SOIL SUCTIONCapillary tube

DatumVi. 'Water

-

Pressure distribu tion

Atmospheric airpressure(u. =0)

u w =-P w hc Qc- one of negativewater pressures

- o f -Water pressures

' 0 v

2r -r =Radius ofFigure 4.5 Physical model and phenomenon related to capillarity.

Equation (4.4) can be rearranged to give the maximumheight of water in the capillary tube, h,:(4.5)TSP w gR sh, =-whereR, = radius of curvature of the meniscus (Le., r/cos

The contact angle between the contractile skin for purewater and clean glass is zero (Le., a = 0). When the CYangle is zero, the radius of curvature, Rs, s equal to theradius of the tube, r (Fig. 4.5). Therefote, the capillaryheight of pure water in a clean glass is

0).

(4.6)The radius of the tube is analogous to the pore radius in

soils. Equation (4.6) shows that the smaller the pore radiusin the soil, the higher willbe the capillary height. The cap-illary height can be plotted against the pore radius usingJ3q. (4.6) where the contact angle is assumed to be zero(Fig. 4.6).4.2.2 Capillary PressurePoints A, B, and C in the capillary system shown in Fig.4.5 in hydrostatic equilibrium. The water pressures atpoints A and B are atmospheric (Le., uw at A =u, at B,

2TSPwgr'h, =-

which is equal to 0.0). The elevationof points A and B onthe water surface is considered as the datum for the system(i.e., zero elevation). As a result, the hydraulic heads (Le.,elevation head plus pressure head) at points A and B areequal to zero.Point C is located at a height of h, from the datum (i.e.,elevation head is equal to h,). The hydrostatic equilibriumamong points C, B, and A requies that the hydraulic headsat all three points be equal. In other words, the hydraulichead at point C is also equal to zero. This means that the

Surface tension . T. =72.76 m N/m10'

-E-l o o 5

.P-10-1=B

rrn

10-210-3 1 0 - 2 10-1 l o 0Pore radius, r (mm)Figure 4.6 Relationshipamong p o ~ eadius, matric suction, andcapillary height.


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4.2 CAPILwRlTY 69pressure head at point C is equal to the negative value ofthe elevation head at point C. The water pressure at pointC an be calculated as

u w = -Pwgh, (4.7)where

u, =water pressure.The water pressures above point A in the capillary tubeare negative, as shown in Fig. 4.5. The water in the cap-illary tube is said to be under tension, On the other hand,the water pressures below point A (Le., water table) arepositive due to hydrostatic pressure conditions. At point C,the air pressure is atmospheric (Le., u, =0) and the waterpressure is negative (i.e., u, = -p,gh,) . The matric suc-tion, (u , - u,), at point C can then be expressed as fol-lows:SubstitutingEq. 4.5) into Eq. (4.8) gives rise to matricsuction being written in terms of surface tension:

(4.9)Equation (4.9) is the same as the equation for the pres-sure difference across a contractile skin as presented inChapter 2 . The radius of curvature, R,, can be considered

analogous to the pore radius, r , in a soil by assuming azero contact angle (Le., a =0). As a result, the smallerthe pore radius of a soil, the higher the soil matric suctioncan be, as shown in Fig. 4.6.The above explanation has demonstrated the ability ofthe surface tension to support a column of water, h,, in acapillary tube. The surface tension associated with the con-tractile skin results in a reaction force on the wall of thecapillary tube (Fig. 4.7). The vertical component of thisreaction force produces compressive stresses on the wall of

2TS(u , - u,) =-.RS

Fagure 4.7 Foxces acting on a capillary tube..

the tube. In other words, the weight of the water columnis transferred to the tube thmugh the contractile skin. Inthe case of a soil having a capillary zone, the contractileskin results in an increased compmssion of the soil struc-ture. As a result, the presence of matric suction in an un-saturated soil increasesthe shear strength of the soil.4.2.3 Height of the Capillary Rise and Radius EffectsThe effects of the height of capillary rise and the radius ofcurvature on capillarity are illustrated in Fig. 4.8, as pre-sented by Taylor (1948).A clean capillary tube of radius,r, allows pure water to rise to a maximum capillary height,h,, as shown in Fig. 4.8(a). However, the water rise in acapillary tube may be restricted by the limited length of thetube [Fig. 4,8(b)]. Adecrease n the capillary height resultsin an increase in the radius of curvature, R,, as indicatedby Eq. (4.5) (Le., h, =2Ts/(p,gRs)) . For a constant ra-dius of the tube, the increase in R, causes an increase inthe contact angle since R, is equal to (r lcos a).The radius or opening of the tube is a significant factorin the development of capillary rise, as illustrated in Fig.4.8(c) and (d). In both caws, the tube has a bulb with aradius of r I,which is larger than the radius of the tube, r .The presence of the bulb at the midheight of the capillaryheight, h,, prevents the water from rising up beyond thebase of the bulb [Fig. 4.8(c)J. In other words, the non-uniform opening along the capillary tube can prevent thefull development of capillary height. On the other hand,the capillary height, h,, can be fully developed if the bulbis filled by submerging it below the water surface and thenraising it above the surface [Fig. 4.8(d)].The development of capillary rise in a soil isalso affectedby the pore size distribution in the soil, as shown in Fig.4.8(e). The water surface in the soil can rise to the capillaryheight, h,, through continuous soil pores with radii that amsmaller than or equal to r . Capillary heights greater than h,may also develop if the height of the soil 'column is ex-tended. The higher capillary rise corresponds to the poreradii that are smaller than r . However, the water surfacecannot rise within the large openings at the center of thesoil column [Fig. 4.8(e)].

The above capillary tube analogy also appliesto soil con-ditions in nature. The nonuniform pore size distribution ina soil can result in hysteresis in the soil-water character-istics curve. At a given matric suction, the soil water con-tent during the wetting and drying pmesses am different,as illustrated by the examples shown in Fig. 4.8(c) and (d),respectively. In addition, the contact angle at an advancinginterface during the wetting process is diffemnt from thatat a d i n g nterface during the drying prucess (Bear,1979). The above factors, as well as the presence of en-trapped air in the soil, are considered to be themain causesfor hysteresis in the soil-water characteristic curve.In spite of its simplicity, the capillary model has somelimitations in its application to describing the mechanical


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70 4 MEASUREMENTS OF SOIL SUCTION

Figure 4.8 Height and radius effects on capillarity (fromTaylor, 1948).

behavior of unsaturated soils. An apparent anomaly willoccur when the capillary model is incorporated into the for-mulation of pore fluid compressibility, as later, explainedin Chapter 8. The use of pore radius in the capillary equa-tion [Le., Eq. 4.9)] causes the model to be impractical forengineering practice. In addition, there are other factorsthat contribute to being able to sustain highly negative pore-water pressures in soils, such as the adsorptive forces be-tween clay particles.4.3 MEASUREMENTS OF TOTAL SUCTIONEnvironmental changes and changes in applied loads pro-duce a change in the water content of the soil. The initialwater content of compacted soils appears to have a directrelationship with the matric suction component (Fig. 4.4).On the other hand, the osmotic suction does not seem tobe sensitive to the changes in the soil water content. As aresult, a change in the total suction is quite representativeof a change in the matric suction. Therefore, total suctionmeasurements are of importance, particularly in the highsuction ranges where the matric suction measurements aredifficult to obtain.The following sections discuss the direct and indirectmeasurements of total suction. The free energy of the soilwater (Le., total suction) can be determined by measuringthe vapor pressure of the soil water or the relative humidityin the soil. The direct measurement of relative humidity ina soil can be conducted using a device called a psychrom-eter. The relative humidity in a soil can be indirectly mea-sured using a filter paper as a measuring sensor. The filterpaper is equilibrated with the suction in the soil.4.3.1 PsychrometersThermocouple psychrometers can be used to measure thetotal suction of a soil by measuring the relative humidityin the air phase of the soil pores or the region near the soil.The relative humidity is related to the total suction in ac-cordance with Eq. 4.1) where &&,) is equal to the rel-ative humidity, RH.There are two basic types of thermocouple psychrome-

ters, namely, the wet-loop type (Richards and Ogata, 1958)and the Peltier type (Spanner, 1951). Both types of psy-chrometers operate on the basis of temperature differencemeasurements between a nonevapomting surface (Le., dIybulb) and an evaporating surface (Le., wet bulb). The dif-ference in the temperatures between these surfaces is re-lated to the relative humidity.The wet-loop and the Peltier-type psychrometers differin the manner by which the evaporating junction is wettedin order to induce evaporation. In the wet-loop psychrom-eter, the evaporating junction is wetted by placing a dropof water into a small silver ring. In the Peltier psychrom-eter, evaporation is induced by passing a Peltier currentthrough the evaporating unction. The Peltier current causesthe junction to cool below the dewpoint, resulting in thecondensation of a minute quantity of water vapor on thejunction. The Peltier psychrometer is most commonly usedin geotechnical engineering and is described in the follow-ing sections. The Seebeck and the Peltier effects are themain principles behind the operation of the Peltier psy-chrometer.Seebeck W ect sSeebeck (1821) discovered that an electromotive force (i.e.,emf) was generated in a closed circuit of two dissimilarmetals when the two junctions of the circuit have differenttemperatures [Le., T nd (T+AT)], s illustrated in Fig.4.9. This phenomenon is referred to as the Seebeck effect,which allows the use of two dissimilar wires (Le., a ther-mocouple) to measure temperature. One junction of the cir-cuit is maintained at a constant temperature for a reference,while the other junction is used for sensing a difference intemperature. A microvoltmeter can be installed in the cir-cuit to measure the Seebeck electromotive force, which isa function of the temperature difference between the twojunctions.Peliier EffectsPeltier (1834) discovered that when a current is passedthrough a circuit of two dissimilar metals, one of the junc-tions becomes warmer, while the other junction becomes


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4.3 MEASUREMENTSOF TOTALSUCTION 71

MetalA

Micrwo l tmeter

Metal,r:-Meta l BT T+ATp V= microvoltageT =temperatureFigure 4.9 Electrical circuit to illustrate the Seebeck effect.cooler, as illustrated in Fig. 4.10. Passing the current inthe opposite direction, as shown in Fig. 4.10, will producea reverse thermal condition at the two junctions. This phe-nomenon is referred to as the Peltier effect, and it allowsthe use of thermocouples for the measurement of relativehumidity.

The Peltier effect can be used to cool a thermocouplejunction to reach the dewpoint temperature comspondingto the surrounding atmosphere. As a result, water vaporcondenses on the junction. Upon terminationof passing thecurrent, the condensed water tends to evaporate to the sur-rounding atmosphere, causing a further reduction in thetemperature at the junction. The temperature reduction is afunction of the evaporation rate, which is in turn affectedby the water vapor pressure in the atmosphere. If the am-bient temperature and the temperature reduction due toevaporation are measured using the Seebeck effects, therelative humidity of the atmosphere can be computed.

There is a maximum degree of junction cooling that canbe achieved using the Peltier current (Spanner, 1951). Theelectrical current that produces the Peltier cooling also pro-duces the Joule heating effect. The Joule heating effect isthe heat produced by the work done against friction alongthe thermocouple wires. Spanner (1951) showed that thenet cooling effect is a quadratic function of the current, andthat there is a maximum value beyond which the Jouleheating will dominate.Different types of thennocouples have different degrees

Metal Metal

A ,Til-Meta l Bwarmer cooler

Figure 4.10 Electrical circuit to illustrate the Peltier effect.

of maximum cooling. The maximum cooling results in aminimum dewpoint temperature that can be reached by thethermocouple. This, in turn, imposes a restriction on thelowest relative humidity (or the highest soil suction) thatcan be measured using a thermocouple psychrometer. Thelower the relative humidity, the lower is the dewpoint tem-perature associated with its water vapor pressure.Peuier PsychmmeterA typical Peltier psychrometer, often called a Spanner psy-chrometer, is shown in Fig. 4.11. The thermocouple con-sists of 0.025 mm diameter wires of constantan (i.e., cop-per-nickel) and chrome1 (Le., chromium-nickel). Thewires are welded together to forman evaporatingor a mea-suring junction. The other ends of the wires am connectedto 26 American Wire Gauge (AWG) copper lead wires toform a reference junction. The highly conductive copperwires have a large diameter (Le., large thennal mass) inorder to serve as heat sinks that can maintain a constanttemperature at the reference junction. The heat sinks arerequired to adsorb the Joule heat generated near the refer-ence junction as the measuring junction is being cooled.

The maximum degree of cooling generated by the chro-mel-constantan thermocouple is about 0.6"C below theambient temperature (Brown and Ba~tos, 982). This max-imum cooling represents the lowest relative humidity (Le.,94%)or the upper limit of the total suction (Le., 8oookPa)which can be measured using the thermocouple psychrom-eter. The lowest suction which can be measured using athermocouple psychrometer is approximately 100 kPa un-der a controlled temperature environment of f0.001"C(Krahn and Fredlund, 1972). This lower limit correspondsto a relative humidity approaching 10096. A slight lower-ing of the temperature as the 10096 relative humidity isapproached will immediately produce condensation on thethermocouple.

W C nsu lated cableMeltable shrinktubing l inerPolypropyleneshrinkMyl ar shield tubing linerWhi teEpoxy resin

Teflon plug

Color-coded insulation

Copper-constantanjunctio n (soldered)(Reference unction)Chrome1(0.026 m m )Welded junctio n

Constantan400Mesh inner liner 4(0.026 mm ) I200 Mesh outer liner Y &Teflon disk

Stainless s teel screen capFigure 4.11 Scmn-caged single-junction Peltier thermocouplepsychrometer (from Brown and Collins, 1980).


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72 4 MEASUREMENTS OF SOIL UCTION

14B0 12E,3 10

864 0 2 0 40 60 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0Time (min )

Figure 4.12 Equilibration times for a thermocouple psychrom-eter with various protective coverings, placed over a 0.3molalKC1 solution at 25C (fromBrown, 1970).A protective housing is usually p rovided around the ther-

mocouple wires. Protective covers take the form of a ce-ramic cup, a stainless steel screen, o r a solid (stainless steelor teflon) tubing with a screen end w indow. The selectionof the type of the protective cover depends on the appli-cation of the psychrometer. Th e time required for the watervapor equilibration is affected by the type of the protectivecover, as dem onstrated in Fig. 4.12. The ceramic cup ap-pears to have a long equilibration time, which m ay not bepractical in many situations.The psychrometer device is connected to a control unitfor applying the Peltier cooling current. The psychrometeris also connected to a microvoltmeter for measuring thegenerated electromotive force during the evaporation pro-cess.Measurements of total suction are conducted by sus-pending a psychrometer in a closed system containing a

Read : null emf Cool

I 1

a b

Cool

C

soil specimen. The relative humidity is measured afterequilibrium is attained between the air near the thermocou-ple and the pore-air in the soil specim en. Isotherm al con-ditions among the temperature of the soil, the air, and thepsychrometer must be achieved prior to conducting themeasurements. A controlled temperature environment off 0.001"C is required in order to measure total suctionsto an accuracy of f10 Pa (Krahn and Fredlund, 1972).Thermal equilibrium within the psychrom eter is assured byobtaining a zero reading on the microvoltmeter.The processes associated with the relative humidity mea-surement when using a Peltier psychrometer are best illus-trated using Fig. 4 .1 3 and the following explanation:

a) Isothermal equilibrium between the psychrometer andthe surrounding atmosphere must be achieved priorto a measurem ent being taken. This is indicated by azero voltage reading.b) At an elapsed time of 15 s, a small electrical current(Le., 5 mA) is passed through the psychrometer cir-cuit from the constantan w ire to the chrome1 wire fora period of 15 s. The passage of an electrical currentin this direction causes the m easuring junction to cooldue to the Peltier effect. As the temperature at themeasuring junction drop s below the dewpoint corre-sponding to the surrounding atmosphere, water vaporcondenses on the measuring junction. During thecondensation process, the temperature at the measur-ing junction remains at the corresponding dewpointtemperature.c) At the end of the 15 s period of cooling, the Peltiercurrent is then terminated.d) As soon as the cooling process is stopped, the con-densed water on the measuring junction starts toRead

d

Read

e

Read :null em f

fFigure 4.13 The operational principle of the Peltier thermocouple psychrometer suspended in asealed chamber over a soil specimen (from Van Havered and Brown, 1972).


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4.3 MEASUREMENTS OF TOTAL SUCTION 73evaporate to the surrounding atmosphere. The tem-perature at the measuring junction starts to drop be-low the dewpoint temperature as evaporation begins.As a result, the microvoltmeter m o d s the electro-motive force on a strip chart recorder. The generatedelectromotive force is a function of the temperaturedifference between the measuring junction and thereference junction (Le., the Seebeck effect). The mi-crovolt reading increases rapidly to a maximum valuewhich is a function of the relative humidity in thesurrounding atmosphere. The drier the atmosphere,the higher will be the microvolt output during theevaporation process.e) Having reached the maximum output correspondingto the maximum evaporative cooling, the microvoltoutput decreases rapidly to a zero reading. The de-creasing output indicates that the temperature at themeasuring junction is increasing towards the ambientor the reference junction temperature.

f ) The microvoltmeter gives a zero reading when thetemperature at the measuring junction becomes equalto that of the reference junction.

Psychrometer CalibmtjonThe calibration of a psychrometer consists of determiningthe relationship between microvolt outputs from the ther-mocouple and a known total suction value. The calibrationis conducted by suspending the psychrometer over a saltsolution with a known osmotic suction under isothermalconditions. The calibration is performed by mounting thepsychrometer in a sealed chamber.Filter papers are placed at the base of the chamber andgenerally saturated with a solution of NaCl or KCl. The

osmotic suctions for NaCl and KC1 solutions at differentmolalities and temperatures are summarized in Tables 4.3and 4.4, respectively.

Under isothermal equilibrium conditions, the water va-por pressure or the relative humidity in the calibrationchamber corresponds to the osmotic suction of the salt so-lution. Therefore, the psychrometer can be calibrated atvarious suction values by using different molalities (or os-motic suctions) for the salt solution. Isothermal conditionsare maintained by placing the chamber in a constant tem-perature bath, as illustrated in Fig. 4.14.The calibration process results in a set of calibrationcurves corresponding to various temperatures (Fig. 4.15).Each curve relates the psychrometer reading to a corre-sponding total suction. The maximum output from the mi-crovoltmeter is taken as the psychrometer reading. Thepsychrometer can then be used to measure the total suctionin a soil specimen by using the established calibrationcurves.

The calibration curves shown in Fig. 4.15 appear to in-crease from zero to a maximum microvolt value, and thendecrease sharply to lower values. The maximum microvoltvalue indicates the maximum total suction that can be mea-sured using psychrometers. Psychrometer readings beyondthis point are highly variable, with the largest variabilityoccumng at high temperatures. This characteristic occursbecause there is a maximum degnx of cooling that can beachieved. The curves in Fig. 4.15 indicate a range for themaximum measurable total suction from 7000 to 8000 kPa,comsponding to a temperature range between 0 and 35"C,respectively.The response time of a psychrometer depends on its pro-tective cover (Fig. 4.12) and the magnitude of total suctionbeing measured (Fig. 4.16). The response time varies from

Table 4.3 OsmoticSuctionsof NaCI solutions (fmm Lange, 1967)Temperature

NaClMolality 0C 7.5" C 15C 25 "C 35"COsmotic Suction (kPa)

00.20.50.71 o1.51.71.81.92.0

0.083620702901416963597260773081908670

0.086021362998431866067550803585309025

0.0 0.0 0.0884 915 9462200 2281 23623091 32 10 33284459 4640 48156837 7134 74117820 8170 84908330 8700 90408840 9240 96009360 9780 10 160


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74 4 MEASUREMENTS OF SOIL SUCTIONTable 4.4 Osmotic Suctions for KC l Solutions(&om Campbell and Gardner, 1971)

Molality 0C 10C 15C 20C 25C 30C 35C00.100.200.300.400.500.600.700.800.901.oo

0.04282712291628202524202814320836013993

0.043685912771693210825232938335337694185

0.04448741300172421482572299634238464272

0.045289013241757219026233057349239284366

0.045990513471788223026723116356140074455

0.046792013701819226827193171362540804538

0.047493513921849230627653226368841534620

a few hours at several thousands kilopascals suction toabout two weeks at 100 kPa suction (Richards, 1974). Itappears that the psychrometer requires a considerably longtime for equalization when used to measure low suctionvalues.Hamilton et al. (1981) epoxted serious problems asso-ciated with corrosion on the thermocouples. The responsecharacteristicsof a failing or dirty psychrometer is difficultto interpret, as illustrated in Fig. 4.17.The corrosion prob-lem can be attributed to the acidic environment in the soil.It is important to clean the psychrometer thoroughly aftereach calibration or usage, in accordance with the instruc-tions given by the manufacturer.Psychrometer Pe@imnance

temperature environment using undisturbed soil specimensfrom the field.A small soil specimen is placed into a stainless steel orLucite chamber, together with the thermocouple psy-chrometer, as illustrated in Fig. 4.18. he entire assemblyis then placed in a constant temperature bath, as shown inFig. 4.14.The temperature of the bath should be main-tained at a constant temperature, within fO.OO1 C (Krahnand Fredlund, 1972). In other words, the thermoregulatormust be able to respond to a fluctuation in temperature off0.001"C.The soil temperature is expected to be main-tained within the same degree of accuracy or greater dueto the buffering effect of the glass beaker.Figures 4.19 and 4.20present the relationships betweentotal suction and initial water content forcomDacted glacialIPsychrometers are useful far measuring high suctions insoils. In situ measurements of total suctions using psy-chrometers are generally not mommended because signif-icant temperature fluctuations occur in the field. However,laboratory measurements can be conducted in a controlledtill and compacted Regina clay, respectively. h e otal suc-tion measurements were conducted using thermocouplepsychrometers. It should be noted that these relationshipsare different from the soil-water characteristic curves forthe soils since the results were obtained from various soil

ThermoregulatorTo relay Stirrer

Psychrometer

-19 m mStyrofoam

- 6 m mLuc i te

Figure4.14 Schematic diagram of a constant temperaturebath (fmm Krahn and Fredlund, 1972).


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4.3 MBASUREMENTS OF TOTAL SUCTION 75

Total suction, J / (kPa)Figure 4.15 Psychrometer calibretion curves at various temper-atures (from Brown and Caztos, 1982).

1210

420

k otal suction = 2820 kPa f 15 hrsl+

0 6 10 15 20 25 30 35Time (days)Figure4.16 Response tim es for laboratory psychrometers (fromRichards, 1974).

1 Normal

TimeFigure4.17 Comparison of responses from a nonna l and a dirtyor failing psychrometer, as obtained from the same suction (fromHamilton, 1979).

Psychrometer cablenReducer orifice

Screen cage psychrometer

1oi l chamber4%Figure 4.18 Stainless steel sample chamber with a seal psy-chrometer in place (from Brown and Collins, 1980).specimens compacted to different densities and at differentwater contents. A soil-water characteristic curve describesthe water content versus suction relationship for a singlesoil specimen. Figures 4.19 and 4.20 clearly indicate aunique relationship Between soil suction and initial watercontent for a particular compacted soil, regardless its drydensities. The in s i t u suction of the same compacted soilin the field can then be inferred from this type of relation-ship (Figs. 4.19 and 4.20) by measuring its water content.This applies only when the soil has ust been compacted inthe field.Comparisons between suction measurements using ther-

3200

2800

- 24005

4008 10 12 14 16 18Initial w ater Content, w o (%)Figure 4.19 Total suction versus initial water content relation-ship for glacial till (from Krahn and Fredlund, 1972).


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76 4 MEASUREMENTS OF SOIL SUCTION' i ' i 1 1800 , 1

1 ,'LO' !2 ' !4 ' !6 ' 28 30

Init ial water content, w o (%)Figure 4.20 Total suction versus initial water content relation-ship for Regina clay (from Krahn and Fredlund, 1972).

mocouple psychrometers and suction measurements usingthe filter paper technique are shown in Figs. 4.21 and 4.22.Figure 4.21 illustrates laboratory measurements of totalsuctions on soil samples from various depths at a locationnear Regina, Sask., Canada. The results indicate reasona-bly close agreement between both methods of total suctionmeasurement as long as the filter paper is not in contactwith the soil.Suction (kPa)

10 10 0 lo00 loo00

E-

1

rj.;il ttil lFigure 4.21 Suction profile versus depth obtained using ther-mocouple psychrometers and the filter paper method (from vander Raadt et al. , 1987).

n 0 5 1 0 15 2 0 2 5 30Compaction water content (%)Figure 4.22 Comparison of independent measurementsof totalsuction on a compacted silty sand (fromDaniel et al., 1981).

The basic Peltier psychrometer (Fig. 4.11) with a singlemeasuring junction has been found to be extremely sensi-tive to slight temperature gradients. In addition, the single-junction psychrometer cannot be used to measure the am-bient temperature around the measuring junction. There-fore, a double-junction Peltier psychrometer (Fig. 4.23) hasbeen developed in order to eliminate the drawbacks asso-ciated with a single-junction psychrometer.The double-junction psychrometer has two chromel-constantan thermocouples. The two constantan wires areattached to a constantan lead, while the two chrome1 wiresare attached to different copper leads. The Peltier currentis applied to the circuit TN Fig. 4.23), causing a coolingat the left measuring junction. The psychrometer output ismeasured between the two copper leads, N and P, s adifference between the tw o junctions. The psychrometeroutput is relatively free from any extraneous electrical out-puts associated with temperature fluctuations within thePsychrometer chamber. Any electromotive force generatedat one measuring junction due to a thermal gradient is com-pensated by an opposing electromotive force at the other

Teflon rodConstantan (26 AWG)Copper (26 A WG)Reference junc tion

Copper (26 A WG)

Reference unc tionChromel (0.025 mm )Measuring unct ionMeasuring unct ionwhromel (0.025 mm )

Constantan(0.025 mm)Figwe 4.23 Double-junction temperature-mmpensated Peltierthermocouple psychrometer (fmm Van Haveren and Brown, 1972and Meeuwig, 1972).


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measuring junction. Therefore, the double-junction psy-chrometer is called a temperature-compensated psychro-meter. In addition, the output measured between the con-stantan lead wire, T, nd the copper lead wire, P, ives theambient temperature within the psychrometer chamber.

4.3.2 Filter PaperThe filter paper method for measuring soil suction was de-veloped in the soil science discipline, and has since beenused primarily in soil science and agronomy (Gardner,1937; Fawcett and Collis-George, 1967; McQueen andMiller, 1968;Al-Khafaf and Hanks, 1974).Attempts havealso been made to use the filter paper method in geotech-nical engineering (Ho, 1979;Tang, 1979;McKeen, 1981;Khan, 1981;Ching and Fredlund, 1984;Gallen, 1985;McKeen, 1985;Chandler and Gutierrez, 1986).Recent ex-perience with the use of the filter paper method on the stud-ies of airport pavement subgrades and swelling potentialprofile of expansive soils by McKeen (1985), as indicatedthat this method deserves further consideration. At present,the filter paper method has not gained general acceptancein geotechnical engineering. There is a need for further re-search relative to the use of this technique in engineering.Principle of Measumment ( m r aper Method)From a theoretical standpoint, it is possible to use the filterpaper method to measure either the total or the matric suc-tion of a soil. The filter paper is used as a sensor. The filterpaper method is classified as an indirect method of mea-suring soil suction.

The filter paper method is based on the assumption thata filter paper will come to equilibrium (Le., with respectto moisture flow) with a soil having a specific suction.Equilibrium can be reached by either liquid or vapor mois-ture exchange between the soil and the filter paper. Whena dry filter paper is placed in direct contact with a soil spec-imen, it is assumed that water flows from the soil to thepaper until equilibrium is achieved (Fig. 4.24).When a dryfilter paper is suspended above a soil specimen (Le., nodirect contact with the soil), vapor flow of water will occurfrom the soil to the filter paper until equilibrium is achieved(Fig.4.24). Having established equilibrium conditions, thewater content of the filter paper is measured.

The water content of the tilter paper corresponds to asuction value, as illustrated by the filter paper calibrationcurve shown in Fig. 4.25.Theoretically, the equilibriumwater content of the filter paper corresponds to the matr icsuction of the soil when the paper is placed in contact withthe water in the soil. On the other hand, the equilibriumwater content of the filter paper corresponds to the totalsuction of the soil if the paper is not in contact with thesoil. Therefore, the same calibration curve is used for boththe matric and total suction measurements.The tilter paper method can be used to measure soil suc-

4.3 MEASUREMENTS OF TOTAL SUCTION 77Tw o filter

. . . .. . . ( .

Figure 4.24 Contact and noncontact filter paper methods formeasuring matric and total suction, respectively (from AI-khafafand Hanks, 974).

tion over a wide mnge of values. The measunements aregenerally performed in the laboratory by equilibrating a fil-ter paper with an undistuhxi or disturbed soil specimenobtained from the field.Measurement and Calibration Techniques ( m e r PaperMethod)The following technique of measurement and calibration iswritten in accordance with a tentative ASTM standard onthe filter paper method (ASTM CommitteeD18 on Soil andRock). The filter paper sensor must be of the ash-free,quantitative Type II as specified by ASTM standard spec-ification E832. Whatman No. 42 and Schleicher andSchuell No. 589 White Ribbon are two commonly usedbrands of filter paper. A typical filter paper has a disk sizewith a diameter of 55 mm. Filter papers from the samebrand a= considered to be identical in the sense hat allfilter paper disks have the same calibration curve.The equipment associated with the filter paper methodconsists of large and small metal containers, an insulatedbox, a balance, and a drying oven. The arge container mustbe able to contain a soil specimen of approximately 200 g.The container should be treated to prevent lusting. The

0.11. - . . 10 60 100 150 200Water content of fil ter paper, wt (%)

Flgure 4.25 A typical calibration curve showing measured filterpaper water contents for applied suctions (fromMcQueen andMiller, 1%8).


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78 4 MEASUREMENTSOF SOIL SUCTIONlarge container has an air-tight lid, and is used to equili-brate the soil specimen and the filter paper for a period ofseveral days. The small container, with a volume of ap-proximately60cm3, is used to contain the filter paper dur-ing oven drying for its water content measurement. Thesmall container should be as light as possible, consideringthe small mass of the filter paper.An insulatedbox canbe used to store the large containerswith the soil specimens and filter papers during the equil-ibration period. The box should be maintained at a constanttemperature within floc.An accurate balance with aminimum capacity of 20 g and a readability of O.OOO1 gshould be used when weighing the filter paper during itswater content measurement.Some researchers pretreat the filter papers prior to its usein order to prevent fungal and bacterial growth during theequilibration period (Fawcett and Collis-George, 1967; andMcQueen and Miller, 1968). Solutions of 396 Pentachlor-ophenol, C6C150H, or 0.005% HgCl have been used topretreat the filter papers. However, recent studies do notindicate any difference in the results obtained from the pre-treated and untreated filter papers (Hamblin, 1984; Chan-dler and Gutierrez, 1986).

The most common practice is to have the filter paper ini-tially dry, and then allow it to adsorb water from the soilspecimen during equilibration. All calibration curves ap-pears to have been established using initially dry filter pa-pers. Therefore, if initially wet filter papers are to be usedin the suction measurements, it may be necessary to estab-lish new calibration curves using initially wet filter papers.There appears to be some hysteresis in the water contentversus suction relationship for filter paper upon wetting anddrying (Lykov, 1961).The filter paper is initially oven dried for several hours.The dry filter paper is then cooled and stored in a desiccantcontainer. Meanwhile, a soil specimen is placed in a largecontainer. The soil specimen should almost fill the con-tainer (Fig. 4.24) in order to reduce the equilibration time.The noncontact procedure can be used by placing twodr y filter papers on a perforated brass disk that is seated ontop of the soil specimen, as shown in Fig. 4.24. The con-tact procedure can be used by placing three stacked filterpapers in contact with the soil specimen, as illustrated inFig. 4.24. For the contact procedure, the center filter paperis generally used for the suction measurement, while theouter filter papers are primarily used to protect the centerpaper from soil contamination.Once the filter paper and the soil specimen are in thelarge container, the container is sealed with plastic electri-cal tape. The sealed container is then stored in the insulatedbox for equilibration. It appears that the ambient temper-ature does not affect the filter paper results provided thetemperature variations during equilibration are minimized(Al-Khafaf and Hanks, 1974). Suctions in the filter papershould be allowed to equilibrate for a minimum period of

0 2 4 6 8 1 0 1 2 1 4Equilibration ime ( day s )Figure 4.26 Increasing water content of the initially dry filterpaper during the equilibration period (fromTang, 1978).

seven days. Figure 4.26 illustrates the increasing watercontent of initially dry filter paper as equilibration occurswith the soil specimen. The results indicate that seven daysis sufficient for the equilibration pupose.At the end of the equilibration period, the filter papersare removed from the large container using a pair of tweez-ers, and their water contents are determined using the smallmetal containers. The water contents of both filter papersused in the noncontact measurement can be measuredindependently. In the contact measurement, the watercontent of the center filter paper is of primary importance.The other filter papers are primarily for protective pur-poses.Extreme care must be exercised when measuring thesmall masses associated with the filter papers. The filterpaper should be transferred from the large containerto thesmall container within a short period of time (e.g., 3-5 s).This short period of transfemng time will minimize waterloss or gain between the filter paper and the surroundingatmosphere. The small container containing the filter papermust be closed and weighed immediately in order to deter-mine th e mass of the filter paper and the adsorbed water.

The container along with the filter paper is then placedin an oven at a temperatuxe of 110 f 5C. In the oven,the lid of the container should be removed to allow waterto escape from the filter paper. Having removed all of thewater from the filter paper, the container, along with thedry filter paper, m weighed with the lid in place in orderto determine the dry mass of the filter paper. The differencebetween the dry mass and the wet mass of the filter paperis used to compute the equilibrium water content of thefilter paper.The equilibrium suction is obtained from the calibrationcurve (Fig. 4.27) by using the measured equilibrium watercontent of the filter paper. The equilibrium suction is as-sumed to be equal to he suction in the soil specimen. Usingthe noncontact pmedure, the suction values deter-


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4.3 MEASUREMENTS OF TOTAL SUCTION 79

0 10 20 30 40 SO 60 70 80 90Filter paper water content, WI (%)Figure 4.27 Calibration curves for two types of filter paper.

mined from the two filter papers can be averaged whendetermining the soil suction, provided the two filter papersgive similar water contents.The calibration curve for a specific filter paper can beestablished by measuring the water content of filter paperin equilibrium with a salt solution having a known osmoticsuction. In principle, the filter paper calibration is similarto the calibration of a psychrometer. The filter paper shouldbe suspended above at least 50 cm3 of a salt solution. Theprocedure for ensuring equilibration and measuring thewater content is the same as those used during the mea-surements of soil suction. Various filter paper water con-tents can then be plotted against the differing osmotic suc-tions to give the calibration curve.The calibration curve for filter papers always exhibits bi-linearity, as shown in Fig. 4.27. The lowerpart of the curverepresents the high range of filter paper water contentswhere the water is believed to be held by the influence ofcapillary forces. On the other hand, the upper part of thecalibration curve represents lower water contents where thewater is believed to be held in an adsorbed water film withinth e filter paper (Miller and McQueen, 1978).

It should be emphasized that the filter paper technique ishighly user-dependent, and great cam must be taken whenmeasuring the water content of the filter paper. The balancemust be able to weigh to the nearest O.OOO1 g. Each dryfilter paper has a mass of about 0.52 g, and at a watercontent of 3096, the mass of water in the filter paper isabout 0.16 g.The Use of the Filter Paper Method iu h t k eA question of immediate concern to the practicing engineeris, What is the accuracyof the suction measurement when

using the filter paper method? The question is still beinganswered, but the following typical resultsrn presented inorder to provide some indication of the accuracy of the fil-ter paper method. A comparison between the results of suction measure-ments using the filter paper method and psychrometers isshown in Fig. 4.21. The results from the noncontact filterpaper agree fairly closely with the psychrometer results,indicating that total suction was being measured. However,the contact filter papers did not exhibit as consistent resultswith respect to depth. This is believed to be due to poorcontact between the filter paper and the soil specimen,which resulted in the total suction being measured in manyinstances, insteadof the matric suction over the depth rangeof 0-5 m (Fig. 4.21).

It appears to be difficult to ensure good contact betweenthe soil specimen and the filter paper. For t h i s reason, totalsuction will generally be measured when using the filterpaper method. Figure 4.28 demonstmtes a close agreementbetween total suction measurements obtained when usingthe filter paper method and thermocouple psychrometers.Total suction profiles in a montmorillonite clay in Texas(Fig. 4.29) have been pdicted using the filter papermethod (McKeen, 1981). The results appear to agreeclosely with the psychrometer measumments.

Figure 4.30 shows the results of filter paper measure-ments of total suction on a highly plastic clay from Eston,Canada (Ching and Fredlund, 1984).While augering sev-eral boreholes, water content samples were taken at about0.3 m intervals. Three filter papers were included with eachwater content samples. These were allowed to equilibratefor one weak. The measured suction on the highly swellingclay ranged from 2000 to 6OOO Wa. Although there is no


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80 4 MEASUREMENTS OF SOIL SUCTIONI2

-2 i o i i o * 103 104 106to Total suction as m easured by filter paper (kPa)n =Number of pointsR2 =Coefficient of determinationo =Standard error

Figure 4.28 Comparisons of total suction measurements ob-tained using the filter paper method and psychrometers (fromMcKeen, 1981).direct Confirmation of these measurements, the results ap-pear to be reasonable for this deposit.

Another total suction profile (Fig. 4.31) was obtainedfrom an excavation in shale using the filter paper method(McKeen, 1985). The drying effects near the surface of theexcavation (Le., down to a depth of 0.6 m) result in highsoil suctions [Fig. 4.31(a)]. Correspondingly, this portionof the profile also has a low water content [Fig. 4.31(b)]in the soil. The filter paper method has also been used toestimate the in s i tu stress state of London clay with rea-sonable success (Chandler and Gutierrez, 1986).

It may also be possible to use the filter paper techniquefor in situ measurements of (total) suction (Fredlund,1989). A proposed scheme for measuring suctions insubgrade soils is shown in Fig. 4.32. The filter papers

Suction (kPa)i o i i o 2 i o 3 10 io6

2

56

I i IFlgure4.29 Total suction profiles as determined using the filterpaper method and thermocouple psychrometers (From McKeen,1981).

Total suction (kPa)100 500 lo00 5 0 0 0 loo000123- 45

O 678

A

g 5

91 0 1 I I I I I . I t L

Atterberg imits of Eston clayLiquid l imit 94%Plastic l im it 31%Plasticity index 63%Figure4.30 Total suction profiles forEston clay using the filterpaper method.

would be left in place for about one week, and then re-moved for measurement of their water contents. New filterpapers could then be installed and allowed to equalize foranother week. Although this scheme has not been used todate, it appears to have possibilities as a low-cost, approx-imate technique to estimate total suction.

The filter paper method appears to have a wide range ofmeasuring capability corresponding to soil suctions from afew kilopascals to several hundred thousand kilopascals(Fawcett and Collis-George, 1967; McQueen and Miller,1968). However, the measurements must be performedwith great care. In addition, only the noncontact filterpaper procedure can be assured of measuring total suction.The contact filter paper procedure may measure eitherthe total or the matric suction, depending on the degree ofcontact between the soil and the filter paper.

4.4 MEASUREMENTS OF MATRIC SUCTIONMatric suction can be measured either in a direct or indirectmanner. The negative pore-water pressure is measuredusing direct methods. The pore-air pressure, which is gen-erally atmospheric in the field, minus the negative pore-water pressure gives the matric suction.High air entry ceramic disks are used for direct mea-surements of negative pore-water pressures. Therefore, theproperties of high air entry ceramic disks are presented priorto describing different ways to perform a direct measure-ment.

Several types of porous sensors are used for performingindirect measurements of matric suction. The electrical andthermal properties of a standard ceramic are a function of


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4.4 MEASUREMENTS OF MATRIC SUCTION 81

Figure4.31Suction (kPa) Water content, w (%)

Total suction and water content profiles in a shale excavation (from McKeen,

its water content, which in turn is a function of the matricsuction. A measurement of the electrical or thermal prop-erties of the sensor indicates the matric suction both in thesensor and in the surrounding soil. Indirect measurementsof matric suction based on the thermal properties of thesensor are described in this chapter.4.4.1 High Air Entry DisksA high air entry disk has small pores of relatively uniformsize. The disk acts as a membrane between air and water(Fig. 4.33). The disk is genemlly ceramic, being made ofsintered kaolin. Once the disk is saturated with water, aircannot pass through the disk due to the ability of the con-tractile skin to resist the flow of air.The ability of the ceramic disk to withstand the flow ofair results from the surface tension, T, , developed by thecontractile skin. The contractile skin acts like a thin mem-brane joining the small pores of radius, R,, on the surfaceof the ceramic disk. The differencebetween the air pressureManufactured by Soilmoisturc Equipment Corporation, Santa Barbara,CA I

Casing

Figure 4.32 Scheme for using filter papers to measurn total suc-tion.

1985).

above the contractile skinand he water pressure below thecontractile skin is defined as matric suction. The maximummatric suction that can be maintained across the surfaceofthe disk is called its air entry value, (u , - u,,,),,. The airentry value of the disk can be illustrated using Kelvinsequation:

(4.10)TS(43 - U w l d =-RS

where(u , - uJd 3: air entry value of the high air entry diskTs=surface tension of the contractile skin or

the air-water interface (e.g., Ts=72.75mN/m at 20C)R, = radius of curvature of the contractile skinor the radius of the maximumpore size.

Surface tension, T,, changes slightly with temperature.The air entry value of a disk is largely controlled by the

\ Water, uw

Water compartment

To measurin g systemFigure4.33 Opemting principle of a high air entry disk as de-scribed by Kelvins capillary model.


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82 4 MEASUREMENTSOF OIL SUCTIONTable 4.5 High Air Entry DisksUsed at Imperial College (From Blight, 1966)

Air EntryPorosity, with Respect to W ater, kd (u , - u,),,Coefficient of Permeability Value,

Type of Disks n (% I (m/s) (kPa)Doulton Grade P6A 23 2.1 x 10-9 152Aerox "Celloton" Grade VI 46 2.9 X 214Kaolin-consolidated from a 45 6.2 X lo-" 317Kaolindust pressed and fired 39 4.5 x 10-'O 524slurry and fired

radius of curvature, R,, f the largest pore in the disk. Th esize of the pores is controlled by the preparation and sin-tering process used to manufacture the cemmic disk. Thesmaller the pore size in a disk, the greater will be its airentry value [Eq. (4.o)].Th e properties of several types ofhigh air entry disks used for unsaturated soils research atImperial College, London, are listed in Tab le4.5.The ability of he high air entry disk to withstand a dif-ference between air and water pressures makes the disksuitable for the direct measurem ent of negative pore-waterpressures in an unsaturated soil. The disk is used as aninterface between the un saturated soil and the pore-waterpressure measuring system. W ater in the disk acts as a finkbetween the pore-water in the soil and the water in themeasuring system. At the same time, air cannot passthrough the high air entry disk into the measuring system.The separation of the air and water across a high air entrydisk can be achieved only as long a s the matric suction ofthe soil does not exceed the air entry value of the disk.Once the air entry value of the disk is exceeded, air willpass through the disk and enter the measuring system. Th epresence of air in the measuring system causes em ne ou smeasurements of the pore-water pressure in a closed sys-tem,Figure4.34 hows the air passage characteristics of threedisks mentioned in Table 4.5. he plots indicate the airentry value or the maximum matric suction sustainableacross the disk.The properties of several high air entry disks manufac-tured by Soilmoisture Equipment C orporation are tabulatedin Table 4.6. he disks are identified according to their airentry values, which are expressed in bars (Le., one bar isequal to 100 kPa). Th e water coefficientof permeability ofa disk was measured by mo unting the disk in a triaxial ap-paratus and placing water above the disk. An air pressurecan then be applied to the water, producing a gradientacross the high air entry disk. T he volume of water flowingthrough the disk is measured using a water volume changeindicator. Details on the equipment are presented in Chap-ter 10.

Th e flow of water through a high air entry disk is plottedagainst elapsed time, a s shown in Fig. 4.35. he plot showsa straight line indicating steady-state seepage through thedisk. The volume of water divided by the cross-sectionalarea of the disk an d the elapsed time gives the coefficientof permeability of the disk. In general, the coefficientofpermeability of the disk decreases with an increasing airentry value.The air entry value and the permeability of a high entrydisk should be measured prior to its use in unsaturated soiltestings. Figure 4.36 resents the air p assage characteris-tics of high air entry disks from Soilmoisture EquipmentCorporation. The measured air entry values appear to behigher than the nominal values specified by the manufac-turer. The results of measurements of the air entry valuesand the coefficientsof permeability for various high air en-try disks are sum marized in Table 4.7.4.4.2 Direct MeasurementsThere are two devices commonly used for the direct mea-surement of negative pore-water pressures. These are thetensiometer and the axis-translation apparatus. Tensiome-ters utilize a high a ir entry ceramic cup a s an interface be-tween the measuring system and the negative pore-waterpressure in the soil. Tensiometers can be used in the lab-

5" -0 50 100 150 200 250 300 350- Appl ied rnatr ic suct ion, ( ua - u (kPa)8Figure 4.34 Air passage characteristicsof three high air entrydisks (fromBishop and Henkel, 1962).


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4.4 MEASUREMENTS OF MATRlC SUCTION 83

4 bar h igh air ent ry disk-Pressure =3q 5 kP a/

?/

//r' ~///&/

1 15 ar high air entry d iskr Pre:sure =55 kPa,/

Table 4.6 Propertiesof High Air Entry DisksManufactured by SoilmoistureEquipment Corporation (Manufacturer's Results)Air Entry

Approximate Coefficient of Permeability Value,Type of Disks ( X io-' mm) ( 4 s ) W a )Pore Diameter with Respect to Water, kd (ua - U d d

1/2 bar high flow 6.0 3.11 x 1 0 - ~ 48-621 bar 2.1 3.46 x 10-~ 138-2071 bar high flow 2.5 8.60 x lo-' 131-1932 bar 1.2 1.73 1 0 - ~ 241-3103 bar 0.8 1.73 x 317-4835 bar 0.5 1.21 x 1 0 - ~ >55015 bar 0.16 2.59 x lo-'' >1520

oratory and in the field. On the other hand, the axis-trans-lation apparatus can be used only in the laboratory. Theuse of the axis-translation concept was described in Chap-ter 3.TensiometersA tensiometer measures the negative pore-water pressurein a soil. The tensiometer consists of a high air entry, po-rous ceramic cup connected to a pressure measuring devicethrough a small bore tube. The tube is usually made fromplastic due to its low heat conduction and noncorrosive na-ture. The tube and the cup are filled with deaired water.The cup can be inserted into a precored hole until there isgood contact with the soil.Once equilibrium is achieved between the soil and themeasuring system, the water in the tensiometer will havethe same negative pressure as the pore-water in the soil.

"0 2 4 6 8 1 0Elapsed tim e, t (rnin )

Figure 4.35 Steady-state seepage of water through a high entrydisk (fromFredlund, 1973).

The pore-water pressure that can be measured in a ten-siometer is limited to approximately negative 90kPa dueto the possibility of cavitation of the water in the tensiom-eter. The measured negative pore-water pressure is nu-merically equal to the matric suction when the pore-airpressure is atmospheric (Le., u, =zero gauge pressure).When the pore-air pressure is greater than atmosphericpressure (i.e., during axis translation), the tensiometerreading can be added to the ambient pore-air pressure read-ing to give the matric suction of the soil. The measuredmatric suction must not exceed the air entry value of theceramic cup. The osmotic componentof soil suction is notmeasured with tensiometers since soluble salts are free tomove through the poms cup.There are several types of tensiometers available fromSoilmoisture Equipment Corporation. Figure4.37 shows aregular tensiometer with a Bourdon-vacuum gauge to mea-sure the negative pore-water pressure. The negative pore-water pressure in the tensiometer tube canalsobe measuredusing a water-mercury manometer or an electrical pressuretransducer, as indicated in Fig. 4.38.Cassel and Mute(1986) iscussed the sensitivity of various measuring de-vices on the response time of a tensiometer. In general, anincrease in the gauge sensitivity will decrease the responsetime of the tensiometer. The increased gauge sensitivityalso results in less water movement between the soil andthe tensiometer, and subsequently a more accurate mea-surement of suction. A higher permeability of the ceramiccup will also result in a lower response time for the ten-siometer.The tensiometer tube in Fig. 4.37 has a diameter of ap-proximately 20 mm and various lengths up to 1.5 m. Inother words, tensiometer cups can be installed in the fieldto a depth of 1.5 m below the ground surface. However,the negative water pressure recorded at the ground surfacemust be corrected for the elevation head corresponding tothe water column in the tensiometer. The longer the ten-


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84 4 MEASUREMENTSOF SOIL SUCTION4

l 3

- 2u)E2PkCI5 1

01 0 0 1 1 0 1 2 0 130 1 4 0 150 160Matric suct ion, (ua - UW ) (kPa)Figure 4.36 Air passage characteristics of one bar, high air entry disks (fromRahardjo, 1990).

siometer tube, the greater will be the correction. This cor-rection results in a more negative water pressure beingmeasured than that recorded by the measuring device. Alength of 1 .5 m corresponds to a pressure correction of 15.2kPa .Servicing the TensiometerPrior to InstauationThe service cap at the top of the tensiometer tube (Fig.4.37) is used to facilitate the filling of the tube with deaer-ated water, the sealing of the tube during measurements,and the servicing of the tensiometer. The tensiometer must

be serviced properly prior to its use in order to obtain re-liable results. Details on the preparation, installation, andusage of a tensiometer are presented by Cassel and Klute(1986).During prepamtion for installation, the ceramic cupshould be checked for signs of plugging; air bubbles shouldbe removed from the tensiometer, and the response time ofthe tensiometer should be checked. The ceramic cup canbe checked by placing the empty tensiometer upright in apail of water and allowing the cup to soak in the waterovernight. An unplugged cup will allow water to fill thetensiometer tube.The removal of air bubbles is performed by applying a

Table 4.7 Permeability and Air Entry Value Measurements on High Air Entry Disksfrom Sotlmoisture Equipment Corporation (from Fredlund, 1973; Rahardjo, 1990)Diameter Thickness Air Entry Valueof the of the of the Disk, Coefficient of

Disk disk (% - U d d Permeability of theType of Disks (mm ) (mm) Disk, kd (m/S)1 bar 19.0high flow 19.019.0

19.019.0

101.65 bar 56.815 bar 56.8

57.0

6.46.46.46.46.410.06.23.13 .1

115 5.12 X lo-'13 0 3.92 x lo-'110 3.98 x lo-'130 5.09 x lo-'150 5.60 x lo-'

1.30 x 10-9>200 4.20 x lo-'6:t2 x lo-''8. 1 X


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4.4 MEASUREMENTS OF MATRlC SUCTION 85Service cap

" 0" in g cap seal

Port, molded intobody tube

Heavy clear plastic

Bourdon vacuum

"0" r ing s tem sea lI V

High air entryceramic cupI- -

Figure 4.37 Conventional tensiometer from Soilmoisture Equipment Corporation.

Mercury Bourdon Pressuremanometer vacuum transducergauge

Manualobservation-Po

F i o nanual

IS cup/

To chart recorderfo r continuousobservation

Groundsurface

(1 (2 ) (3)Figure4.38 Several measuring systems for a tensiometer(fromMomson, 1983).

vacuum of approximately80 Wa to the top of the tensiom-eter tube for a period of 30-60 s. The vacuum can be ap-plied using a hand-held vacuum pump, as shown in Fig.4.39. This process will remove air bubbles from the ce-ramic cup, the Bourdon gauge, and from the imperfectionson the wall of the tube. Having released the applied vac-uum, deaerated water is added to refill the tube and theservice cap is tightened in place. The water in the tube isthen subjected to a negative pressure of approximately 80kPa by allowing water to evaporate from the ceramic cup.Under this negative pressure, air bubbles may reappear inthe tube, and the above procedure shouldbe repeated untilthe tube is essentially fme of air bubbles. It is important tohave an air-free tensiometer tube in order to ensure correctreadings and rapid responses.The response time of a tensiometer can be checked bydeveloping a negative water pressure of approximately 80kPa by evaporation from the ceramic cup and then im-mersing the ceramic cup into water. The negative waterpressure in the tensiometer should increase towards atmo-

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