Soil Suction Development under Isotropic Loading and Unloading in a Compacted Residual Soil

Soil Suction Development under Isotropic Loading andUnloading in a Compacted Residual Soil

Desenvolvimento de Sucção em um Solo Residual Compactado sobCarregamento e Descarregamento Isotrópico

Desarrollo de Succión en um Suelo Residual Compactado BajoCarga y Descarga Isotrópica

Fernando A.M. Marinho1, Jamie Standing2, Ricardo M. Kuwagima3

Abstract. Embankment stability reaches a critical point at the end of construction when excess pore pressureshave developed and are only partly dissipated. Slope stability analyses are able to incorporate the development ofpore water pressure during embankment construction, but their accuracy depends on the reliable estimation porepressures. Usually, after the compaction of a layer the soil possesses a negative pore water pressure or suction.That pressure may rise to zero and become positive with the increasing overburden pressure from the subsequentlayers. There are several methods available for estimating the development of pore water pressure generated fromstress increases at the end of construction of an embankment, the one proposed by Hilf (1948) being frequentlyused. These methods do not take into account negative values of pore water pressure that generally exist within un-saturated, compacted soils. Traditionally there have been difficulties measuring suctions greater than 90 kPa, whi-le soils compacted near the optimum water content typically have suctions in a range of 30 to 300 kPa, dependingon factors such as the soil type and compaction energy (Lacerda and Marinho, 1998). The construction of embank-ments using residual soil is common practice in many countries. This paper presents results from tests on compac-ted samples of a residual soil. In some of the tests suction measurements were made with a high capacitytensiometer. Suction changes in response to the application of isotropic confining stresses from zero to 1.7 MPaand unloading back to zero were monitored. The response of the soil tested was found to depend strongly on its ini-tial suction and degree of saturation. The results from these tests have been used to investigate the validity of ex-tending Hilf’s method to take into account initial soil suction.Key words: suction, unsaturated soils, compacted soils, tensiometers, pore water pressure.

Resumo. A estabilidade de aterros compactados atingem um ponto crítico no final da execução do mesmo, quan-do o excesso de pressão na água gerado ainda não foi dissipado. Nas análises de estabilidade de taludes é possí-vel incorporar o desenvolvimento de pressão de água gerado durante a construção, porém sua acurâcia dependede uma estimativa confiável destas pressões. Usualmente, depois da compactação de uma camada o solo apresen-ta uma pressão na água negativa, uma sucção. Esta pressão pode crescer a zero e tonar-se positiva com o aumen-to da sobrecarga das camadas subseqüentes. Existem vários métodos disponíveis para estimar o desenvolvimentoda pressão na água gerada por um acréscimo de tensão no final da construção do aterro. O método proposto porHilf (1948) é o mais freqüentemente usado. Estes métodos não levam em consideração o valor negativo da pres-são da água que existe no solo no seu estado não saturado. Tradicionalmente tem sido difícil medir valores desucção superiores a 90 kPa, sabendo-se que o solo compactado próximo ao teor de umidade ótimo, possui um va-lor de sucção entre 30 kPa e 300 kPa. Estes valores dependem do tipo do solo e da sua energia de compactação(Lacerda e Marinho, 1998). A construção de aterros usando-se solo residual é uma prática comum em vários paí-

Solos e Rochas, São Paulo, 26, (2): 115-128, Maio-Agosto, 2003. 115

1Fernando A. M. Marinho, PhD, University of São Paulo. e-mail: [email protected] Standing, PhD, University of Cambridge. e-mail: [email protected] M. Kuwagima, former MSc Student at University of São Paulo. e-mail: [email protected] em 21/8/2002; Aceitação final em 15/4/2003; Discussões até 30/12/2003.

ses. Este artigo apresenta resultados de ensaios realizados em amostras compactadas de solo residual. Em algunsdestes ensaios a medição da sucção foi feita com o auxílio de um tensiômetro de alta capacidade. A variação dasucção, em resposta a aplicação de uma pressão confinante isotrópica variando de zero a 1,7 MPa e o descarre-gamento até zero, foi monitorado. Observou-se que a resposta do solo ensaiado depende fortemente da sucçãoinicial e do grau de saturação. O resultado destes ensaios foi também utilizado para se avaliar a validade do mé-todo de Hilf quando se leva em consideração a sucção do solo.Palavras-chave: sucção, solos não saturados, solos compactados, tensiômetros, pressão neutra.

Resumen. La estabilidad de los aterros compactados alcanza un punto crítico al final de la construcción, cuandoaparece un exceso de presión de poros que no fueron disipados. Durante el análisis de estabilidad de taludes es po-sible incluir el desarrollo de la presión del agua generado durante la construcción del terraplén, pero el nivel deprecisión depende de una estimativa confiable de estas presiones. Generalmente, después de la compactación deuna capa de suelo aparece una succión o presión negativa ocasionada por el agua de los poros, esta presión puedeaumentar hasta cero y llegar a ser positiva con el incremento de sobrecarga de las capas subsecuentes. Existen di-versos métodos que están disponibles para estimar el desarrollo de la presión en el agua generada por el aumentode tensión debido a la construcción de un terraplén; uno de ellos es el método propuesto por Hilf (1948) utilizadocon bastante frecuencia. Estos métodos no consideran los valores negativos de la presión del agua en los poros queexiste generalmente en los suelos no saturados, compactados. Tradicionalmente, fue difícil obtener resultadospara valores de succión mayores a 90 kPa, mientras que para suelos compactados con cifras próximas a la hume-dad óptima, tienen un valor de succión que oscila entre 30 kPa y 300 ka, dependiendo de algunos factores como eltipo de suelo y la energía de compactación (Lacerda y Marinho, 1998). La construcción de terraplenes con sueloresidual es practicada en varios países. Este trabajo presenta resultados de pruebas realizadas en muetras compac-tadas de un suelo residual. En alguna de estas pruebas, las medidas de succión fueron realizadas con tensiómetrode alta capacidad. Las variaciones de la succión fueron acompañadas en respuesta a la aplicación de presión confi-nante isotrópicas que fueron desde cero hasta 1,7 Mpa y durante el descargamiento hasta cero. También, fue ob-servado que la respuesta del suelo ensayado depende directamente de la succión inicial e el grado de saturación.Los resultados de esas pruebas fueron utilizados para verificar la confiabilidad del método de Hilf cuando es con-siderada la succión inicial del suelo.Palabras-clave: succión, suelos no saturados, suelos compactados, tensiómetro, presión neutra.

1. Introduction

It is unusual for a compacted soil to possess apositive pore water immediately after compaction.Usually the pore water pressure is negative. Thedevelopment of pore water pressure followingcompaction and then subsequent loading by furtherlayers of compacted soil is an important aspect toconsider when assessing the stability of an em-bankment. The development of pore water pressu-re during the construction of embankments hasbeen investigated by many researchers (e.g. Wal-ker and Daehn, 1948; Hilf, 1948; Bishop, 1957;Arêas, 1963; Walbancke, 1975; Lins and Sandroni1994).

In embankments analysis, the end of constructi-on pore water pressure can be estimated if the valueof the secant pore pressure parameter, B, is known,where ∆u = B∆σ1. The stability of slopes is directlyassociated with the pore pressure at the end ofconstruction and hence the prediction of the deve-

lopment of the pore water pressure is an importantaspect for design.

The pore water pressure at the end of construc-tion depends on the soil plasticity, placement watercontent, stress level, rate of construction and drai-nage boundary conditions. At low stresses mostcompacted clays possess some suction. Fig. 1 pre-sents data obtained from literature and from the

Figure 1. Suctions measured in compacted soils at their op-timum water content.

116 Solos e Rochas, São Paulo, 26, (2): 115-128, Maio-Agosto, 2003.

Marinho et al.

Authors’ files that illustrate the behaviour of anumber of plastic soils in relation to suction at theoptimum water content after compaction using thestandard Proctor energy. The term water content inthis paper always refers to gravimetric water con-tent.

Two general conclusions can be drawn fromthe data shown in Fig. 1. Soils compacted at the op-timum water content always possess suction andthere is no clear trend indicating how the optimumwater content is related to the suction generated.Bishop et al. (1964) also presented data relating thesuction and the compaction water content fromwhich similar observations can be drawn.

Walbancke (1975) analysed data from a num-ber of dams and concluded the following.

• Sandy clays or plastic clays compacted aboveoptimum water content may possess positive waterpressures at the end of construction.

• Plastic clays when compacted at or below theoptimum water content possess significant valuesof suction even when there are significant overbur-den pressures.

Although these statements are well known,most embankments are designed considering onlythe development of positive pore water pressures.

The primary aim of these earlier studies was toevaluate the development of positive pore waterpressure after compaction and as it develops withthe total all-round pressure. The development of ahigh capacity tensiometer (HCT), e.g. that develo-ped by Ridley and Burland (1993), has enabled thedevelopment of the pore water pressures to be fol-low from values as low as -1 MPa to high positivevalues.

Prediction of the development of pore waterpressure is of great importance for the adequate andsafe design of embankments.

Hilf (1948) presented a method for estimatingthe development of pore water pressure as a functi-on of the vertical stress. This method has been usedwith reasonable success in many cases (e.g. Gibbset al., 1960; Arêas, 1963). Although the methoddoes not consider directly any soil suction in itsconcept, it has been suggested that a translation ofthe theoretical curve could take initial suctions intoaccount (e.g. Arêas, 1963).

Site measurements of negative pore water pres-sure are not usually made. Chalmers et al. (1993)

presented predictions and measurements for lowvalues of suction. With the availability of the highcapacity tensiometers it has been possible to mea-sure suction in the laboratory with greater success.However there are still problems with measure-ments in the field.

A laboratory investigation into the develop-ment of pore water pressures in a compacted resi-dual soil has been recently performed (Kuwagima,2000). This paper describes the observed responseof the pore water pressure under different drainageconditions and conditions of isotropic loading andunloading. The results are also compared with theresponse predicted using a modified form of Hilf’smethod.

1.1. Hilf’s method

Hilf’s method of predicting pore water pressuredevelopment is based on oedometer test results anduses Henry’s law and Boyle’s law. Fredlund & Ra-hardjo (1993) give a complete derivation of themethod.

Hilf presented a relation between changes inabsolute pore air pressure and changes in totalstress, i.e. ∆ua and ∆σ:

∆

∆

∆uS hS n

u u m

ao o o

ao a v

=+

− ++

1

11( )

( )

σ (1)

where h is Henry’s coefficient of solubility (~0.02at 20 °C) and S0 and n0 are the initial values of de-gree of saturation and porosity respectively. Thechange in total vertical stress is referred to as ∆σ. InHilf’s analysis, the secant pore air and pore waterpressure parameters are equal, i.e. Bah = Bwh, and sothe equation for the secant pore water pressure pa-rameter can be written as:

B’S hS n

u u m

who o o

ao a v

=+

− ++

1

11( )

( )∆

(2)

These equations were used to obtain relationsbetween increments of pore water pressure andvertical stress (i.e. ∆uw vs. ∆σ and also Bwh vs. ∆σ).It is usual to assume that the suction is equal to zerowhen applying Hilf’s method, this hypothesis re-

Soil Suction Development under Isotropic Loading and Unloading in a Compacted Residual Soil


sulting in a conservative value of pore water pres-sure. The assumption that the pore air pressure isequal to the pore water pressure is made. This maybe acceptable in the case of compressible soils athigher degrees of saturation.

Arêas was probably the first to suggest the in-corporation of the initial suction into Hilf’s met-hod. The idea of translating Hilf’s curve downwardwas based on experimental observation made byArêas (1963). Arêas compared predictions madeusing a modified version of Hilf’s method with agreat number of measurements that he made onTrês Maria Dam, in Brazil. The initial suction usedby Arêas (1963) was calculated indirectly using anestimated pore size diameter. By applying a trans-lation of the Hilf curve, Arêas obtained a muchmore consistent comparison between field measu-rements and predictions.

The predicted and observed pore water pressu-res at the central zone of the dam obtained by Arêas(1963) are presented in Fig. 2. Although there is re-asonably good agreement between the data and thepredictions, the introduction of the newly develo-ped high capacity tensiometers, capable of measu-ring soils suctions, offers an opportunity toinvestigate Hilf’s method in greater depth and withgreater confidence.

1.2. Testing equipment

The tensiometer used in the study performed(Kuwagima, 2000) is similar to the one describedby Ridley and Burland (1993). The key aspect ofthe tensiometer is its ability to measure suctionshigher than 100 kPa. This potential is largely due tothe saturation procedure. Details concerning thesaturation procedure used can be found in Marinhoand Chandler (1994), Marinho and Pinto (1997)

and Tarantino and Mongioví (2001). The tensio-meter is mounted within the base of a triaxial cellwith readings taken using a data logger.

The porous ceramic fitted to the tensiometerused in this work has an air-entry value of pressureof 500 kPa. A paste of the same soil is used to im-prove the contact between the soil water and thewater inside the tensiometer. The response time ofthe tensiometer is strongly related to the water con-tent of the material used between the soil and theporous ceramic (Marinho 2000).

1.3. Soil characteristics

The soil used in the study was sampled from theupper layer of a residual soil profile, weatheredfrom gneiss, located at an experimental site of SãoPaulo University. The fundamental characteristicsof the soil in its compacted state have been studiedby Stuermer (1998). Following this study, Marinhoand Stuermer (1998 and 2000) presented results re-lating to the retention capacity of the soil.

Since residual soil is intrinsically heterogene-ous, all the material used for the studies was collec-ted at the same time from one location. The soilwas mixed in the laboratory to form one homoge-neous unit, which was divided up and placed in fivecontainers. The five samples were individuallycharacterised in order to confirm their homogene-ity. Figure 3 illustrates the particle size distributioncurves for the five samples.

Table 1 presents the percentage smaller than2 µm and the specific gravity for each sample.

The Atterberg limits determined for each sam-ple are presented in Table 2, i.e. the results for the

Figure 2. Comparison between predicted and observed porewater pressures (after Arêas, 1963).

Figure 3. Particle size distribution of the soil (Stuermer,1998).

Marinho et al.


liquid limit (wl), plastic limit (wp) and plasticity in-dex (Ip). It should be pointed out that the liquid li-mit was obtained using the Casagrande apparatus.

A series of compaction tests were performed onthe homogenised soil using different compactionenergies. The results of these tests are presented inFig. 4 along with the values of suction after com-paction and lines of iso-suction. These lines wereestimated by interpolating between the results fromsuction measurement made on each sample, aftercompaction. Suctions in this earlier work were me-asured using the filter paper method (Stuermer,1998). For the standard Proctor, the suction at theoptimum water content can be seen to lie between200 kPa and 50 kPa.

The dry unit weight and the optimum watercontent, for the sample tested at each compactionenergy are presented in Table 3.

The Soil-Water Characteristic Curve (SWCC)is another means of characterising a soil in terms ofits suction-void ratio (or suction-gravimetric watercontent) relationship as it dries out and desaturates.The effect of the energy of compaction on theSWCC has been found not to be significant (e.g.Marinho and Stuermer, 2000; Leong and Rahardjo,

2002). The results presented in this paper refer tosamples that were allowed to dry just after compac-tion. No wetting path was induced. This systematicprocedure was followed as the process of wettingand drying may affect the relation between watercontent and suction.

1.4. Suction measurements

Two techniques were used to determine the soilsuction: the filter paper method (Chandler et al.1992) and the high capacity tensiometer developedby Ridley and Burland (1993). The filter paperused was the Whatman n. 42, quantitative filter pa-per. The procedure used for taking tensiometer me-asurements followed that described by Marinhoand Chandler (1994) and Marinho and Pinto(1997).

1.5. Initial characteristics of the samples

After the dynamic compaction of the soil theinitial suctions were measured using the filter pa-per method. Figure 5 presents the results obtainedfor the samples compacted using the standard Proc-tor. It is important to emphasise that the data pre-sented in this way do not represent the Soil WaterCharacteristic Curves for the soil.

Table 1. Results from gradings: percentage smaller than2 µm and specific gravity.

Sample <0.002 mm (%) Specific gravity Gs

1 43.8 2.73

2 44.0 2.74

3 44.7 2.75

4 43.2 2.73

5 40.8 2.74

Table 2. Atterberg limits for the residual soil (Stuermer,1998).

Sample wl (%) wp (%) Ip (%)

1 48 28 20

2 48 29 19

3 49 29 20

4 49 31 18

5 48 28 20

Average values 48 29 19

Figure 4. Compaction curves and curves of iso-suctions(Marinho and Stuermer, 2000).

Table 3. Compaction parameters (Stuermer, 1998).

Compaction Test γdmax (kN/m3) woptimum

Standard Proctor (SP) 15.0 25

Modified Proctor (MP) 16.7 19



Figure 5 also presents the initial condition ofthe specimens used for the oedometer tests and forthe triaxial tests that are described later. It can beobserved that there is reasonably good agreementbetween the initial states of all samples when com-pared with the data presented by Marinho and Stu-ermer (2000) for the same soil. The suctionsindicated for the oedometer specimens were infer-

red from the soil retention curve. It is also evidentthat the oedometer specimens possessed lower va-lues of voids ratio.

2. The Soil-Water CharacteristicCurves

Figure 6 shows the Soil-Water CharacteristicCurves obtained from a gradual drying process for

Figure 5. Initial suction for standard Proctor energy.

Marinho et al.


the samples compacted using the standard Proctorenergy.

Small but discernible differences in the behavi-our of the specimens at the beginning of the curves

are clearly visible in the relation between watercontent and suction. Since the specimens were notwetted after compaction, this behaviour probablyreflects the initial structure of the soil set up during

Figure 6. Soil-Water Characteristic Curves for the samples compacted using the standard Proctor (Marinho and Stuermer,

2000).



compaction, which is dependent on its water con-tent. At higher suctions the SWCC only reflectschanges in the microstructure.

2.1. Test procedures

The specimens used for measurement of thesuction with the tensiometer were statically com-pacted to achieve the same values of water contentand dry unit weight obtained when the standardProctor procedure was carried out. The specimensused for the oedometer tests were trimmed from thesamples dynamically compacted using the stan-dard Proctor procedure. The difference in theSWCCs between samples dynamically and stati-cally compacted is small, although there is a ten-dency for higher suctions in the latter (e.g. Olson.and Langfelder, 1965).

2.2. Oedometer tests

Figure 7 presents results from oedometer testsperformed on four of the dynamically compactedsamples. The initial characteristics of each sampleare presented in Table 4 and also in Fig. 5. The va-lues of initial suction presented in Table 4 havebeen obtained from the respective SWCC (Mari-nho and Stuermer, 2000).

3. Application of Hilf’s Method

Hilf’s method has been applied, using the re-sults of the oedometer tests (e.g. to obtain mv, over arange from 1 to 500 kPa), to provide a relationshipbetween pore water pressure and change in totalstress ∆σ for these samples. Figure 8 presents theresults obtained for the samples tested (note that inall cases the initial pore water pressure is taken tobe zero).

Also plotted in Fig. 8 are proposed curves fortaking initial suctions into account. These havebeen constructed by translating the curves obtainedusing Hilf’s method, by an amount equivalent tothe initial suction.

4. Triaxial Tests

A triaxial cell was adapted to incorporate a ten-siometer in its base. The tensiometer was positio-ned such that a gap of 0.5 mm was left between thebase and the porous ceramic. This gap was filledwith slurry, prepared using the same soil, but closeto its liquid limit, to enhance the contact betweenthe tensiometer and the soil water.

After the specimen was assembled in the triaxi-al cell, with its base in contact with the tensiometer,a period was set to allow the suction to reach equili-brium. The equilibrium time was usually about30 min.

The isotropic pressure was applied using anelectronically-controlled hydraulic constant pres-sure system and registered using a conventionalpressure transducer.

Figure 7. Results from consolidation tests on four of thecompacted samples.

Table 4. Initial characteristics of the samples.

S-1 S-2 S-3 S-4

w (%) 21.9 22.0 25.1 25.0

γ (kN/m3) 14.8 15.4 14.9 15.5

e 0.82 0.74 0.80 0.74

S (%) 73 82 86 92

mv (m2/kN) 1.47*10-4 1.12*10-4 1.90*10-4 3.69*10-4

Initial suction (kPa) 500 400 150 170

Marinho et al.


Results from initial measurements on the triaxi-al specimens are presented in Table 5 and also inFig. 5.

The results from the triaxial tests are presentedin Fig. 9. It should be noted that the maximum timeof testing was about 7 h. Under undrained conditi-ons, as the total stress on the sample is increased,the water pressures measured by the HCT increase(i.e. suctions reduce). There is a rapid response ini-tially as the air and water pressures increase withconsequent particle rearrangement (rememberingthat if the sample were fully saturated there would

be no change in effective stress and no particle re-arrangement would occur). As the total stress in-creases there is a lesser propensity for particlerearrangement because of a combined effect ofsample densification and reduction in the volumeof air. Thus a stage can be reached where only thepresence of air will affect the response of the porewater pressure. This is evident from the shape ofthe triaxial test results in Fig. 9, where the rate ofchange of pore water pressure reduces after the ini-tial increments of total stress.

In Hilf’s method the coefficient of compressi-bility (mv) influences the shape of the curves pre-sented in Figs. 8 and 9. The values of mv used wereobtained from oedometer tests, which implies a Ko

condition. The suction was measured under isotro-pic conditions, which may in part justify the diffe-rences obtained.

The shape of the curves obtained from the ex-perimental results differs significantly from thoseobtained using Hilf’s method. However, curveswith similar characteristics to those shown havebeen observed in other studies (e.g. Bishop andHenkel, 1962, p. 187).

The difference arises from the initial degree ofsaturation, S, of the samples. Of particular signifi-cance are cases where values of S are very low,where the air present within the soils is in continu-ous phases rather than in the form of occluded bub-bles or in a dissolved state. In these cases the waterpresent is mostly in the form of menisci between

Figure 9. Comparison between Hilf’s method and the actualpore water pressure measurements (Marinho et al. 2002).

Table 5. Characteristics of the triaxial samples at the start oftesting.

Sample w(%)

S(%)

Initial suctionfrom HCT (kPa)

SD3 25.1 86 143

SD4 25.1 87 134

SD5 25.1 87 186

SD6 22.5 71 332

SD7 22.5 71 375

SD9 22.5 72 320

SD12 24.5 81 117

SD13 24.5 81 115

SD14 24.5 79 120

SD18 24.5 81 115

Figure 8. Curves generated using Hilf’s method and theproposed correction for the soil suction (Marinho et al.,2002).



soil particles. Water pressures are consequentlymuch lower than the air pressures, with high sucti-ons being generated from the tension forces actingacross these menisci. The response of soils at hig-her degrees of saturation, where air is in the form ofoccluded bubbles or is dissolved, follow more clo-sely the trend of behaviour predicted using Hilf’smethod (Marinho et al., 2002).

4.1. Pore water pressure response times

The response time of pore water pressure in thespecimens to change in confining pressure is plot-ted against time in Figs. 10 to 12. These figures re-present typical behaviours of the specimens nearthe optimum water content and the dry side of thecompaction curve. Samples SD4 and SD9 (Figs. 10and 11) were at and below the optimum water con-tent respectively.

The following aspects of sample behaviourhave been investigated:

• sensitivity of the measuring system to the con-tact between the specimen and the tensiometer;

• speed of response of the pore water pressure toloading and unloading;

• the increase of suction at constant pressure;• the initial increase and subsequent decrease of

suction after unloading increment;• the slower response of specimens compacted

at the dry side of the compaction curve.Frequently there was some scatter of data du-

ring the initial stages of the tests (denoted by regionA in the figures). This is caused by the initial move-ment of the sample during setting up (e.g. in pla-cing the membrane). The measuring system ishighly sensitive to small disturbances and theseresponses probably represent real local changes insuction in the vicinity of the tensiometer.

After the filling the cell the suction was allowedto equilibrate before applying increments of cellpressure. The immediate response of the tensiome-ter to changes in cell pressure can clearly be seen.

The general trend observed during the tests wasthat an increase or decrease in cell pressure led tocorresponding increase or decrease in pore waterpressure. Specimens SD4 and SD9 were compac-ted at and close to the optimum water content res-pectively. Comparing the general behaviour ofthese two specimens it can be observed that thesample compacted at a water content below opti-

mum (SD9, Fig. 11) had a reduced response to in-creases in cell pressure compared with the speci-men at optimum (SD4, Fig. 10). Once appliedpressures reach a certain value (about 500 kPa inthis case) there are only negligible changes in porewater pressure at and above this level. This behavi-our is probably associated with the compressibilityof the specimen. Figure 13 presents the results obtai-ned with specimens SD4 and SD9, plotted at thesame scale. It can be seen from Fig. 13 that the res-ponse of the pore water pressure to the increments ofisotropic pressure is similar in both specimens up toabout 600 kPa. At higher values of cell pressure thepore water pressure response of specimen SD4 wasgreater than the response of the specimen SD9.

The points denoted B in Figs. 10, 11 and 12 in-dicate specific points where a loading pressure in-crement was applied. In each case the response tothe increment is an immediate positive develop-ment of pore water pressure followed by a reducti-on towards an equilibrium value. This behaviourwas observed in all specimens, regardless of theirinitial state. It may be associated with the soil pastebetween the specimen and the tensiometer, or itmay be due to the compressibility of the air presentin the specimen. After the initial few increments ofpressure, the pore water responses to equal incre-ments gradually reduce. In the cases of the samplesat and below the optimum water content, there is agradual reduction in the response of the pore waterpressure to applied increments of cell pressure. Asthe suctions diminish, the initial peak observed atthe start of each increment also disappears with amuch more rounded profile when pore water pres-sures are positive (see Fig. 11).

Figure 10. Response of sample SD4 to loading and unloa-ding.

Marinho et al.


A similar behaviour during the unloading pathcan be observed in Figs. 10 and 12. Upon the releaseof pressure the suction initially increases and thenimmediately starts to reduce (denoted by point C).

In Fig. 12, after a number of different loadingand unloading steps a high cell pressure (1.2 MPa)was applied and maintained for about 70 min. Itcan be observed that the increase in pore waterpressure was not followed by an immediate reduc-tion, but the pore water pressure gradually reducedtowards an equilibrium value, starting at point D.This may be associated with the low compressibi-lity of the paste at that time.

4.2. Drainage conditions during isotropic triaxi-al tests

In order to investigate the effects of drainageduring a test a series of tests were performed underdrained and undrained conditions. Figure 14 pre-

sents the results obtained for specimens compactedclose to the optimum water content. In this state theair is likely to be occluded and so little flow of freeair from the sample boundaries would be expected.In the test on specimen SD-C, an impermeable topcap was used while for specimen SD-14 there wasa porous ceramic at the top with the connecting val-ve closed. Differences in the responses observedunder these two conditions were not significant,although a tendency for lower pressure develop-ment was observed for the air-undrained tests. Nodrainage of water was observed during the drainedtests.

4.3. Loading and unloading responses

The effects of loading and unloading and thepotential for hysteresis were investigated in six ofthe specimens tested. The results from two tests aregiven in Fig. 15. Specimen SD-4 was compacted ata water content of 25.1% (S = 87%) and the com-paction water content of the specimen SD-7 was



Figure 13. Response of specimens SD4 and SD9 to pore

water pressure during loading.

Figure 14. Loading paths with and without drainage.



22.5% (S = 71%). The results suggest that the spe-cimen with a higher initial degree of saturation didnot exhibit significant hysteresis during the loadingand unloading. However, a decrease in suction oc-curred in the specimen with a lower degree of satu-ration as it was unloaded to zero confining pressurefollowing the initial loading. This behaviour wasobserved in all specimens tested, i.e. samples withlower initial degrees of saturation ended up with lo-wer suctions after unloading. Making an analogybetween the isotropic loading, under constant wa-ter content, and the results presented in Fig. 4 onecan observe a decrease in suction with an increasein dry unit weight and vice versa, since the curvesare slightly inclined. The behaviour is associatedwith the compressibility of the specimen during theunloading process. A lower degree of saturation isprobably reached as a consequence.

In considering the results shown in Fig. 15, itshould be noted that the initial suction of sampleSD4 was lower than the general air-entry value ofthe soil, while specimen SD7 possessed a suctionhigher than the general air-entry value. The defini-tion of the general air entry is presented by Mari-nho and Chandler (1993). It refers to the point werethere is a change in the gradient between the degreeof saturation S and suction for samples drying froman unsaturated state.

It can be observed in Figs. 15 and 14 that theincrease in pore water pressure at high levels ofconfining pressure did not show a direct correspon-dence as would be expected if the soil were satura-ted. In Fig. 15 for the specimen with a lower degreeof saturation (SD7 with S = 71%) the relation bet-ween pore water pressure and applied pressure was

about 0.01, and for the specimen SD4 (S = 87%)the relation was 0.45. This suggests that the sampledid not reach saturation under the pressure appliedwithin the test duration.

5. Conclusions

The use of high capacity tensiometer (HCT)instruments for investigating unsaturated soil be-haviour is becoming increasingly important. Itwould not have been possible to make the observa-tions or comparisons given in this paper withoutthe use of such a device. Problems associated withthe contact between the HCT and the sample havenot been addressed in this paper, but are consideredto be of fundamental importance regarding the reli-ability of the results. There is a great necessity toimprove tensiometer design to allow long-termmeasurements to be made.

Several characteristics of the partly saturatedresidual soil tested have been investigated in thisstudy. Because of the heterogeneous nature of resi-dual soils, the field samples were mixed together toform a uniform homogeneous mix. The response ofthe soil to loading and unloading is strongly associ-ated with the presence of air as can be seen from theresults presented. Tests performed on specimenscompacted statically and dynamically indicated thefollowing behaviour.

• Samples tested under isotropic conditionswith open access to air exhibited smaller degrees ofpore water pressure generation under isotropic loa-ding than those tested under undrained conditions.However, differences were very small. Those sam-ples tested to observe the effects of the drainageconditions were close to the optimum water con-tent and air within them was mostly occluded. Furt-her research is required to investigate the effects ofdrainage more thoroughly.

• The samples compacted at about 3% belowthe optimum water content did not generate signifi-cant positive pore water pressures for the level ofcell pressures used. This behaviour suggests thatthe compressibility of these samples is such thatthe degree of saturation does not increase duringthe loading.

• During the unloading of the samples a smalldecrease of suction developed compared with valu-es during loading for samples compacted at watercontents below optimum. No apparent change was

Figure 15. The effects of loading and unloading on sucti-

ons.

Marinho et al.


observed for the samples compacted near the opti-mum water content. The increase in suction forsamples compacted below optimum is probably as-sociated with changes in volume of the soil skele-ton during unloading.

• The behaviour found during the tests perfor-med is strongly related to the characteristics of thesoil, particularly in terms of water storage charac-teristics. For this soil the dry unit weight has astrong influence on the suction. This indicates thatany change in volume due to loading or unloadingmay significantly affect the suction generated.

The use of Hilf’s method to evaluate the deve-lopment of pore water pressure leads to an over-prediction of the pore water pressure. The correcti-on of Hilf’s curves taking into account the suctionmay under-predict the pore water pressure at lowtotal stresses and over-predict at higher stresses.

Acknowledgments

The work presented in this paper is part of a re-search project sponsored by the São Paulo StateAgency for Research (FAPESP). The third authoris grateful for the sponsorship by the Brazilian Go-vernment Agency CAPES.

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Soil Suction Development under Isotropic Loading and Unloading in a Compacted Residual Soil

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