Soil moisture suction: Its importance and measurement

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Proceedings of the American Society for Testing and Materials, 58, pp.

1205-1217, 1959-07-01

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Soil moisture suction: Its importance and measurement

Penner, E.

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Authorized Reprint from the Copyrighted Proceedings of lhe AwucAN SmETY FoR lbsmrc Mereerer,s

Philadelphia 3, Pa. Volme 58. 1958

SOIL MOISTURE SUCTION-ITS IMPORTANCE AND MEASUREMENT*I

Bv E. Prrvxpnr

SvNopsrs

Perhaps the greatest development in soil mechanics has been in under-standing the mechanical behavior of saturated soils. More recently attempts have been made to deal with soils in the unsaturated state. Under these con-ditions the behavior of the soil depends to a large extent on the moisture status, which for many purposes can be usefully expressed in terms of t'suction," com-monly known also as moisture tension or negative hydraulic potential.

The identity of suction with relative vapor pressure and hence free energy is given together with the advantages of measuring the former in the region of high relative vapor pressures. Since the behavior of water in soils and many related porous materials is similar, the suction concept is not limited to soil.

Both the apparatus and technique used for determining soil moisture suction and unsaturated permeability are described together with the use of the meth-ods in soil engineering.

Many of the practical problems that arise with the use of materials of a porous nature result from their interaction with water. In addition to the influence of water on the performance of porous materials, there is the related aspect of understanding the properties of held water, in particular as it affects moisture transmission.

The problems that result from the in-teraction with water vary with the type or porous material and the service ex-pected from it. For example, the prob-Iems may range from those concerned with moisture transmission in building walls to those involved in the

determina-* Presented at the Sixty-first Annual Meet-ing of the Society, June 22-27,1958.

f This is a contribution from the Division of Building Research, National Resea;rch Council, Canada, and is published with the approval of the Director of the Division.

r National Research Council, Divisionof Build-ing Research, Ottawa, Canada.

tion of shear strength in soils. Acknowl-edging these difierences, there is a dis-tinct need for a more adequate way of describing the moisture status in porous materials than by water content on a weight percentage or per cent saturation basis. Such a measure should describe the held moisture in terms of its tendency to move between similar as well as dis-similar materials. Held moisture in terms of moisture content often gives no real indication of the tendency of the mois-ture to move because two different ma-terials may be in equilibrium with each other at widely difierent moisture con-tents. Also similar materials with a dif-ferent moisture history may not be in moisture equilibrium at the same water content. This second phenomenon in-volves hysteresis and will be referred to later.

The concept of suction that has been widely used in the field of soil science for

N . R . C . 4 9 9 4

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1206

PnNunn oN Sorr,

many years (r)2 provides a basis for a rational and useful method of describing the moisture status of porous materials. More recently it has been usefully em-ployed to a limited extent in some as-pects of soil mechanics and in other engineering fields concerned with the be-havior of moisture in other porous ma-terials. Since the suction concept has

Morsrunr SucrroN

Morsrunr Sonprrox nv Ponous Mernnrars

Soils belong to a class of adsorbents which give a typically S-shaped rela-tionship when the moisture content is plotted as a function of the equilibrium relative vapor pressure. These cw_ves are commonly referred to as sorption iso-therms. Several such curves are shown in

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o o-- o r o o 2 0 0 3 0 0 4 0 0 5 0 0 . 6 0 0 . 7 0 0 8 0 0 9 0 r o o R e l o l i v e V o p o r P r e s s u r e

Frc. l.-Sorption _{Isotherms for a Number of Materials Including Natural Soils.}

been shown to be useful and has led to a better understanding of the behavior of moisture in porous material, it deserves much wider attention. This paper is concerned with a simple description of the nature of suction, some methods of measuring it, and finally it gives a brief account of its application to some soil engineering problems.

2 The boldface numbers in parentheses refer to the list of references appended to this paper, s e e p . 1 2 1 6

-Fig. 1, including some for several ma-terials other than soils.

Although a detailed discussion of the various mechanisms of moisture sorption is beyond the scope of this paper, a brief reference to the subject is useful in describing the suction concept.

Moisture suction in partially saturated materials (of the class referred to above) at low moisture content arises from the attraction between water molecules and the solid. This type of attraction is

- O o t o _{f r o m P o w e r s o n d B r o w n y o r d} ( 3 ) - - - - _{D o l o fr o m F i l b y o n d M o o s s ( 4 )} - - - _{D o t o fr o m S o i l S o l i d i f i c o t i o n R e p o r t (5 )} . . . Doto Determined b y A u t h o r f o r Ledo Ctoy

---- _{Ooto Delermined by Author f or Whitehorse Silt}

- _{S o t u r o t i o n M o i s f u r e Contenl- Not Given} - - - - _{S o t u r o t i o n M o i s t u r e Conient- 35per cent} - - - _{S o t u r o f i o n M o i s t u r e Gontent- 40 O+} . . . Soturotion Moislure Content- 51.5 per cenf - - - - _{S o t u r o t i o n M o i s f u r e Contenf- 5O,l Der cent}

( o ) Adsorpiion ( b ) oesorption

. t ' /

wood

!P---"

--''

t

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characterized by physical forces rather than chemical and is normally considered to be reversible. In the case of clays, a part of the sorption arises from the hy-dration of the exchangeable ion, but as can be seen from Fig. 1 the general shape of the sorption curve is much the same. The physical arrangement of water molecules in porous materials is not well understood, but it is commonly assumed that the molecules are spread over the solid surface at low relative vapor pres-sures. With increasing vapor pressure, the thickness of the sorbed layer increases until at high vapor pressures the increase in held moisture results in the filling of pores. In this range the forces of sorption can be considered as arising from the surface tension forces operating in the concave water menisci in partially filled pores. In the intermediate vapor pres-sure range both kinds of sorption are probably operative. The forces of attrac-tion acting on the water molecules are reflected in a reduction in vapor pressure, relative to the vapor pressure over a flat water surface at the same temperature. Relative vapor pressure is therefore a direct measure of the forces operating at the surface of the held water.

The relationship between relative vapor pressure and the radius of curva-ture of the water meniscus is given by the Kelvin equation:

t n p / p o : _ r ; #

( 1 )

where:

P : the vapor pressure over the curved surface,

po : the vapor pressure of free water, .y : the surface tension of water, M : the molecular weight of water, d. : the density of the water, R : the gas constant,

? : the absolute temperature, and

0 . 0 8 4 9 . 0 . . 9 3 . 0 . . . 9 9 . 3 9 9 . 9 oo oo 9 9 . 9 9 9 1 0 0 . 0 0. 000147 0. 00147 0. 0147 o . 1 4 7 1 . 4 7 1 4 . 7 r 4 7 . O 0 0 . 0 1 . 4 2 I 1 6 0 t.42 X ro4' 1 . 4 2 X 1 0 3 t.42 X tOz 1 . 4 2 x l 0 | . 4 2 1.42 X 10-r 0

express the forces in another way that is directly related to the negative hydraulic head, or "suctionttof the liquid phase.

The concept of suction is based upon the height, Z, to which water rises in an ideal capillary tube. This height, Z, is dependent on r, the radius of the tube. If the contact angle between the capil-lary tube and the liquid is zero, the radius of curvature of the meniscus and the radius of the capillary tube will be equal. The height of the capillary rise, h, being related to the curvature of the water surface, can therefore be used as a measure of the potential of the water at the surface of the meniscus having

PBNuen oN Sorr, MorsrunB SucrroN

1207

r : the radius of curvature of the meniscus.

This equation shows that the equi-librium vapor pressure, p, over a curved watermeniscus is reduced below thevapor pressure po as a result of surface tension. As will be shown later, relative vapor presssure, though a rational measure of the forces with which water is held, is not always a convenient one, particu-larly at high vapor pressures when many pores are filled and other larger pores contain menisci. Under these conditions, it is often more convenient and useful to

TABLE I.-THE RELATIONSHIPS BE. TWEEN RELATIVE VAPOR PRESSURE. RADIUS OF CURVATURE OF MENISCUS: EQUIVALENT HEIGHT OF COLUMN OF WATER lz, AND pF. Radius of Curvature of Meniscus, r n / P o , per ceDt q a 6 5 4 3

,

I Equiv-alent Heisht of Water, h, cry 107 1 0 6 1 0 6 104 103 1 0 -1 0 0

1208

the same radius, /, even though a com-plete water column does not exist. This is the measure used to express suction. For convenience, Schofield (6) has chosen to express the height of rise in terms of log (- Z), which is now commonly known as pF. Relative vapor pressure, being related to the curvature of the meniscus, is also related to the suction, h.The relationship, which is well known, may be derived from the Kelvin equa-tion.

From the height-of-capillary-rise equa-tion:

c h : r ; . . . . . . ( 2 ) where:

g : gravitational constant, Z : height-of-capillary rise, ? : surface tension of water, d : density of the water, and z : radius of the capillary.

Substitution of. gh in the Kelvin equa-tion leads to the relaequa-tionship:

h : - y h ? / p 0 . . . . . . . . ( 3 ) Mg

Regarding the use of free energy as a measure of the moisture potential, it may be pointed out that the use of the suction concept for a salt-free system does not lose identity with the free energy concept. Although Z measured directly will not include the effect of salt, vapor pressure does. When the held water has reached a state of equilibrium with the vapor in the surrounding environment, the free energy of the vapor and the sorbed moisture will be equal. Conse-quently, the free energy of the vapor may be used to calculate the free energy of the sorbed phase from the expression:

. R T

L F : . t n p / p o . . . . . ( 4 ) M

It is of interes! to note the similaritv

Pprvlcnn

ou Sou- Morsrunn SucrroN

between Eqs 3 and 4, as pointed out by Schofield, and its relationship to the quantity ?/Po, the relative vapor pres-sure. The relationship between relative vapor pressure, radius of curvature of the meniscus, equivalent height of col-umn h, and Schofield's pF have been tabulated (Table I).

There are a number of advantages in using suction instead of relative vapor pressure in many situations. The first

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Frc. 2.-Relation.nip nlt*."r, pF and Rela-tive Humidity at 2O C.

vantage arises from the nature of the relationship between moisture content and relative vapor pressure. Figure 1 shows that at relative vapor pressures below 0.95 the increase in moisture con-tent is relatively small. This is true for soils as well as many other porous ma-terials. Above a relative vapor pressure of 0.95, however, the increase in moisture content is very large for a small change in vapor pressure. UnJortunately, this is the range that is usually of greatest interest in most soils applications in both agriculture and engineering.

There are also great difficulties in the

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accurate measurement and control of relative vapor pressures at the wet end. Since small variations in relative vapor pressure usually correspond to a large change in moisture content, the relia-bility of vapor pressurei measurements above 0.99 is such as to preclude their use. Figure 2, which gives the relation-ship between relative vapor pressure and suction in terms of pF, clearly shows the expanded nature of the pF scale above 4,

'

N-o' Determinotions were Corried O u t i n t h e o o i l e d P o r t i o n

\\

. -,/

\

- \

o r 0 2 0 3 0 4 0 5 0 6 0 7 0 P e r c e n l M o i s t u r e C o n t e n t b y w e i g h t

Frc. 3.-The Hysteresis Loop for a Sample of Leda CIay.

the corresponding range of relative vapor pressure being from 0.993 to 1.

The relative ease of controlling or measuring suction in comparison to rela-tive vapor pressure strongly favors its use. This will be discussed in some detail later.

Moisture movement at near-satura-tion condinear-satura-tions (high relative vapor pres-sures). is probably largely in the liquid phase. Particularly in soils under natural conditions the movement is attributed to stress gradients in the pore water. The suction h in Eq 3 measured directly describes this stress more conveniently and is not complicated by unrealistic parameters.

PeNNBn oN SorL Motsnrnn SucrroN

1209

A note of caution about the compar-ison of Z measured directly with p/ps is in order since the validity of the rela-tionship presupposes a salt-free moisture regime. The quantity Z will be affected by salt in held moisture in a different way than P/Po, b:ut in many cases the difierences may be neglected.

HysrnnBsrs

The equilibrium moisture content of a porous material obtained at a given rela-tive vapor pressure depends on whether equilibrium is approached by wetting or drying. The efiect of tlis phenomenon is shown in Fig. 3 which gives both the wetting and drying curves. At one time the effect was considered to result from nonequilibrium conditions. ft is now known, however, that the sorption proc-ess is in a sense irreversible and gives rise to hysteresis efiects. The theories advanced to explain hysteresis are dis-cussed in detail by Carman (z). The fact that more than one value can be obtained at the same moisture stress condition must be considered whenever the rela-tionships between suction and moisture content are used.

MnesrrnBuBNT ol SucrroN AT Low Rnr.errw Vapon Prussunes While the concept of suction and its use as a measure of potential can be ap-plied over the entire range of moisture from oven dryness to saturation, difier-ent measuring techniques have to be employed for difierent sections of the range. Relative vapor pressure methods are usually best below pF 4. However, from pF 0 to 4 other methods involv-ing the measurement of suction more di-rectly are usually used.

The suction plate technique for the direct measurement of suction involves placing water in a saturated porous plate in tension, by hanging a column of water from the plate. This may also be

accom-t2t0

PnN'rqEn oN Son MorsrunB Sucrrox plished by applying a vacuum to the

water in the plate. If this "negative head" is maintained, a sample of soil placed in contact with this "conditioned" plate will establish moisture equilibrium with it. There will be a transfer of water in the liquid phase between the porous plate and the specimen. The time taken to bring the specimen to the same suction condition as the suction plate depends on the rate of water transmission. The range over which the porous plate is operative depends on the pressure difierence that can be maintained across the porous plate before it desaturates and passes

plate is placed in tension through a vac-uum regulator.

With the types of ceramic porous plates used in this laboratory the suction plate technique is limited to pF's below 3. The apparatus (Fig. a) is similartothat described by Richards (9) and others with some modifications. In the appara-tus shown a rigid suction plate holder made of f-in. brass is used. A rigid plate holder greatly facilitates making an air-tight seal between porous plate and the plate holder and also avoids any volume change between the porous plate and plate holder when the chamber is

pres-surized. The outside diameter of the porous plate is about fi in. smaller than the inside diameter of the plate holder. This groove is filled with a suitable water-resistant hard-setting sealer to provide an airtight seal between the holder and the porous plate. A spiral groove on the side of the plate holder facing the porous plate provides a means of water transfer from the sample to the outside of the ap-paratus. At one end of the spiral, a graduated 1 cu cm pipet is attached and to the other end an ordinary buret is fixed. This provides a means of flushing out any air bubbles when the spiral groove is filled with water. Water is per-mitted to flow through the system until no air bubbles appear. With a 13-qt capacity pressure cooker the porous plate can be about 10 in. in diameter.

S E A L I N G C O M P O U N D . .

-S P I R A L G R O O V E

P O R O U S P L A T E H O L D E R \ P O R O U S P L A T E

Frc. 4.-Suction Plate Apparatus. Regulated Air Pressure Applied Through fnlet b, or Regulated Vacuum at Outlet a.

o

\

P I P E T

air and, therefore, is also a function of the maximum size of the pores. When it passes air it no Ionger operates as a suction plate.

Two methods of maintaining the pres-sure difference across the porous plate in order to condition it to a given suction have been used and both give essentially the same answer (s). By one method a positive pressure is applied through a pressure regulator to the top of a satu-rated porous plate (pressure plate) which is held in a metal plate holder. The bot-tom of the porous plate is in contact with water but open to atmospheric pressure. In the second method the top of the porous plate (suction plate) is kept at atmospheric pressure and the water in contact with the bottom of the

This gives sufficient porous plate area to condition a number of specimens simul-taneously. In practice, for soils, small consolidometer rings with a cutting edge have been found useful as sample holders for undisturbed soils. The calibrated pipet attached to the spiral groove is used to determine when equilibrium is reached.

The moisture content of the specimens at various suction levels may be deter-mined gravimetrically by sampling the specimens or the complete specimen may be removed and weiehed. In the latter

PnrvrBn oN SorL Morsrunp SucrroN

t 2 t r

specimens at intermediate suction levels. The apparatus is shown in Fig. 5. The specimen is placed on the porous plate either in the dry or saturated state de-pending on whether the adsorption or desorption curve is required. Either a pipet or a calibrated capillary tube is used to measure the moisture content change. If large changes in moisture con-tent are envisaged, a long calibrated cap-illary tube is more convenient. This avoids the necessity of resetting the meniscus at frequent intervals.

The suction or pressure plate technique

Fro. S.-Small Suction Plate or Pressure Membrane Apparatus. Regulated Air Pressure Applied Through Inlet b or Regulated Vacuum at Outlet o.

To Operate as a Pressure Membrane Apparatus Place Membrane from c to c'. method, the relationship between suction

and moisture content is then deduced by finding the weight of oven-dried soil after the final suction determination. This method is subject to a number of errors in practice: the specimen cannot always be removed intact from the porous plate and, once removed, good contact is some-times not easily achieved between the plate and the specimen.

In a second t'suction" apparatus used at this laboratory, which is in many ways similar to apparatus described by Rich-ards (8) and Croney and Coleman (10), the moisture content gains and losses are measured volumetrically. This tech-nique is limited to one specimen at a time but does away with weighing

is used to determine suction - moisture-content relationships in the range from pF 0 to 3. However, as pointed out, suc-tion determinasuc-tions by the vapor pres-sure technique are not reliable below pF 4. In the range pF 3 to 4, the "pressure membrane" technique described by Richards (r2) and others may be used. The same apparatus shown in Fig. 5 may be used with one important addi-tion. To withstand the large pressure dif-ference required to induce pF's between 3 and 4 without passing air, a membrane with very fine pores must be used, such as Visking sausage skin. In the apparatus shown, the membrane is placed over the porous plate with the sample in contact with the membrane. The walls of the

t212

apparatus are constructed of |-in. brass sufficiently strong to withstand air pres-sures equivalent to pF 4. The change in water content of the specimen may again be measured volumetrically or gravi metrically. In practice, the latter has been found more reliable because small air leaks in the membrane frequently in-validate volumetric measurements of moisture content change. The apparatus shown has been found particularly useful since by merely adding the membrane, one apparatus can be used over the entire range from pF 0 to 4.

There are, of course, many other techniques described in the literature for the determination of suction in the low pF range (r0). In the author's experience, the techniques described in this paper have proved satisfactory from the point of view of simplicity of apparatus and technique both for undisturbed and pre-pared specimens. The suction technique may be used with equal facility for rigid and compressible materials. For rigid porous materials such as brick, special care must be taken to provide a good contact between the suction plate and specimen in order to obtain satisfactory rates of moisture transmission.

The apparatus described are for inde-pendent methods of measuring relation-ships between suction and moisture content The suction technique is also used frequently as a means of controlling the moisture status while other proper-ties are being measured. To do this, the suction technique is incorporated into the design of larger apparatus where the parameters to be studied are a function of the moisture status. This can be an invaluable asset in many types of ex-periments with porous materials. Two general approaches are possible: one may either measure the variation in suction as the experiment progresses or impose a definite suction throughout the experi-ment. The exact methods involved are a

a

Ppnron oN Sorr, Morsrunp Sucrrox

matter of adapting the suction technique to the apparatus being used.

Elrocr oF TEMPERATTTRE oN SucrroN MBesrrnrunrqts fn the low suction range, the effect of temperature change on suction is due mainly to surface tension variation with temperature. It is assumed that the es-tablishment of the equilibrium is through the liquid phase. Under these conditions, variation is approximately 0.1 pF units for a 10 C change in temperature, ac-cording to Croney and Coleman (tt), pro-vided the proper precautions are taken. A further difficulty, owing to t€mpera-ture fluctuation, arises from condensation on the inside surfaces of the apparatus. At this laboratory, suction plate, pres-sure membrane, and unsaturated perme-ability determinations are carried out in a small temperature-controlled room where temperature variations do not exceed +0.1 C of control point.

Sour ApprrcATroNs oF TrrE Sucrrolq Coxcapr rN Sors

Shear Strength oJ Unsaturated. Soils.-The well-established concept of effective stress for saturated soil

o ' : c - 1 . ! , . . . . . . . . . ( 5 )

where:

o' : efiective normal stress, d : applied normal stress, and 27 : pr€ssure in the pore fluid,

has reCently been extended to unsatu-rated soil by Australian workers (13-15). This equation has been rewritten in the form

o ' : o * P o . . . . . ( 6 )

where the quantity p" is the suction (that is, negative pressure) of the soil moisture and is substituted as a positive quantity. According to Terzaghi, the ef-fective stress is that part of the total stress which produces measurable effects

PBNrvrn oN Sorr such as consolidation or an increase in shearing resistance. This means that these "efiects" can be attributed in part to suctionl this has already been demon-strated with reasonable certainty by the Australian workers.

Croney and Coleman (t0) suggest that for saturated compressible clays, the pressure void ratio curve determined by the consolidation technique is in fact a suction moisture content drying curve. At any particular applied pressure in the consolidometer the pressure, 1, is equal to the effective stress, o', at equilibrium since the pore pressure, u, is 0. A simi-larity between consolidation curves and suction - moisture-content curves de-termined by the suction plate and pres-sure membrane techniques was the basis for the Australian workers concluding that, for compressible clays, the concept of effective stresses extended to pore pressures which are truly negative with respect to free water at atmospheric pressure. Noncohesive soils did not re-spond in a similar way beyond the range where the soil pores emptied under ten-sion forces.

The shear strengths of unsatwated soils is obviously of great importance for Australian conditions. In a recent paper, Aitchison (r5) concludes that the varia-tion of sucvaria-tion in the pore water for the Australian environment appears to be a major determinant of soil strength in cohesive soils.

Eqai,hbri,um Moi.sture Distribution in Soil Above a Shallow Water Table:

The moisture changes that occur in the soil underneath airfield pavements, etc., subsequent to construction has been ex-tensively studied by the British Road Research Laboratory (16-19). The prem-ise is that subsequent moisture changes may invalidate the design based on soil strengths determined before construc-tion. ft appears that the main

determi-MorsrrrnB SucrroN

t213

nants of the equilibrium moisture condi-tions above a constant water table are embodied in the following relationship:

a P : s * u . . . ( 7 ) where:

P : normal pressure,

s : the moisture suction of the soil, u : the pore water pressure, and a : the fraction of the normal pressure

that is efiective in changing the suction.

The relationship apparently is most easily determined for the case of an im-permeable surface cover. To determine the equilibrium moisture-content dis-tribution, a is either measured with tensiometers or calculated from the lo-cation of the ground water table. P is calculated from the total weight of the wet soil above the point in question, and a can be evaluated from loading and shrinkage tests. Thus s can be calcu-Iated and, from suction - moisture-con-tent curves, the estimated moisture content is obtained. Research workers who developed this method report good success with it; however, to what extent the method has been generally accepted by others is not known. There is no question of the usefulness of predicting, in advance of construction, the moisture content changes that will occur under structures.

U nsaturated M oisture Flow :

Moisture flow in the liquid phase for unsaturated soils follows Darcy's law, s : ki, under isothermal conditions. In contrast to saturated flow, & is not a constant but is a function of the average moisture suction. For example, if the Ievels of suction are 10 and 2O cm sepa-rated by a distance l, the velocity of flow will be difierent than when the levels of suction are 190 and 200 cm, again sep-arated by distance l, although the gradi-ents are the same. To a large extent the

t 2 t 4

_{Pnnurn oN Sorr Morsruna Sucrror}

variation of fr depends on the state of saturation. The greatest variation occurs in uniform sands or silts which have a critical suction where almost complete desaturation occurs. At suctions greater than this, the liquid phase through the pore system is discontinuous, and the greatly redu.ced flow is through the ad-sorbed phase.

determined in the Iaboratory (21,22). Nevertheless, the concept of suction potential as a driving force has contrib-uted to the understanding of unsaturated flow in nature. This type of flow is thought to account for the distribution of water at equilibrium above the shallow water table as discussed in the previous section. b 3 . 5

€

( E

'

-o L ? 2o0 300 400 500 600 700 800 S , , T e n s i o n c m W o t e r

Frc. 6.-Flow Rate as a Function of Sr with & Held at Zero.

Preliminary experiments with an un-saturated permeability apparatus re-ported previously by the author (20) com-pare flow rates below pF 3. The flow rates shown in Fig. 6 were obtained by hold-ing the suction Sz at one end of the specimen at zero and increasing the suc-tion at the other end 51 in small incre-ments. These results show familiar char-acteristics of unsaturated flow. At low tensions the flow rates are higher in silt than in clay, but this reverses at higher tensions. At present no satisfactory method for predicting unsaturated flow with any degree of certainty is avail-able. As a consequence, it is usually

Suction in Relation to Frost Heaaing: The phenomenon of frost heaving as related to the porformance of shallow foundations or structures, roads, and airports is still not completely under-stood. Much of the present knowledge stems from the early work of Taber (23) and Beskow (24). More recently Haley et aI (25), Ruckli (26), and Jumikis (27) have made notable contributions. The author QO, 2849) has recently studied the mechanism of ice lensing in the laboratory using artificial and natural soils. The concept of suction has been useful in studying the equilibrium

pres-Cloy

-\

\

---_

_{--_}

---.---\ _ - :

lir-PrNwen oN SorL Morsrrrnr SucrroN

1 2 t 5

sure relationships in the ice-water-soil

system and has led to the development of a tentative theory of ice lensing in soils.

The driving force for moisture move-ment is the suction developed at the ice-water interface. This force is the most important feature of frost heaving since it must exist for ice lenses to form. Ex-periments show that the suction has in-creased to a maximum in a closed system when the ice lens growth stops, that is, at equilibrium, but it is greatly influenced by the dimensions of the pores and the overburden pressure. The thermody-namic equilibrium conditions have been described based on a simple model of the ice-water interface in a small element of the granular system.

Electri,cal Resistance Block Method' of Measuring Moisture i'n Soil:

A reliable nondestructive method of measuring soil moisture content in the field has still not been perfected, but recent developments with the neutron meter show promise. The electrical re-sistance block technique of measuring moisture is discussed here partly be-cause it is still used in practice and also because this technique demonstrates rather an important principle as follows. The equilibrium moisture condition be-tween dissimilar porous materials exists when the suctions are similar although the moisture cbntents rlaay vary widely. The electrical resistance meters made of porous materials are therefore more appropriately referred to as suction meters.

Earlier workers noted that the elec-trical resistance of soil was a function of its moisture content, Investigations re-vealed that soil density, particle-size distribution, mineral type, and salinity of the soil solution influence resistance

sufficiently to limit its usefulness as a measure of moisture content. To over-come these difficulties the electrodes were buried in a porous block which was then placed in contact with the soil. In this way it was hoped to limit the varia-tion of resistance to moisture content only.

In one method of calibration the resistance meter is buried in a block of saturated soil selected from the site where the meter is to be installed. The soil is then dried in stages and allowed to come to equilibrium. The electrical resistance is measured at each equilib-rium state, and the soil is sampled for moisture content. In this way a complete calibration for the drying cycle can be obtained. This calibration, however, ap-plies to only the one soil used in the calibration.

In this laboratory a calibration was carried out to establish the direct rela-tionship between suction and electrical resistance of the moisture blocks. The suction of the meters was controlled using suction plate and pressure mem-brane techniques, and electrical resist-ance readings were taken when suction equilibrium was established. If the fur-ther relationship between suction and moisture content for the soil is deter-mined, all the information is available to convert electrical resistance readings of the meters to moisture content of the soil with which it is in suction equilib-rium. The commercial moisture meters that did not have a flat contact surface were embedded in plaster of Paris. In this way, the same method of calibration could be used for all the meters irrespec-tive of type.

In practice, the electrical resistance method of measuring the moisture con-tent of soils is beset with many dif-ficulties. These are outlined in some detail by Penner et al (so;.

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The concept of suction has been usefully employed both in practice and as a research tool. The suction char-acteristics of a porous material are in essence sorption isotherms, but the interpretation of the phenomena has led to new methods of determining and expressing the affinity of a porous ma-terial for water and is particularly use-ful in the high relative vapor pressure range. In this region the water is held largely in the macropore system. The concept of suction and its method of determination provides a realistic

ap-proach to many problems in soil en-gineering.

Acknowled,gments:

The author wishes to express his appreciation to Mr. N. B. Hutcheon for his guidance in preparing tJris paper and to Mr. R. F. Legget for his continuing interest in moisture problems, and to Mr. J. M. Kuzmak, who designed the apparatus shown in Fig. 5 of this paper. This is a contribution from the Divi-sion of Building Research, National Research Council, Canada, and is pub-Iished with the approval of the Director of the Division.

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