imperial college tensiometer

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Take, W. A. & Bolton, M. D. (2004). Geotechnique 54, No. 3, 229232

229

DISCUSSION

Tensiometer saturation and the reliable measurement of soil suction

W . A . T A K E a n d M . D . B O L T O N ( 2 0 0 3 ) . G eo t e c h n i q u e 5 3 , N o . 2 , 1 5 9 1 7 2

A. Tarantino, Universita degli Studi di Trento, ItalyThe authors address two important issues relating to tensi-ometer measurements: the initial saturation of the high air-entry ceramic and the unreliable response of the tensiometerdue to the inadequate saturation.

Ridley & Burland (1999) suggested a procedure for initialsaturation where the first stage consisted of removal of airfrom the ceramic. This was achieved by applying a vacuumin a chamber partly filled with de-aired water. The authorsassume that the ceramic may adsorb water molecules duringporous ceramic installation and evacuation, and this is likelyto reduce the wettability of the ceramic during the subse-quent inundation. To cope with this problem, the authorspropose a new initial saturation process where air removal iscompleted in two steps: oven-drying and evacuation in theabsence of water.

A very similar procedure has already been suggested byTarantino & Mongiov (2002) using the vacuum chambershown in Fig. 15. The only difference lies in the drying ofthe ceramic, as the tensiometer was placed in a desiccatorcontaining silica gel and not in an oven. The use of thisdouble-compartment vacuum chamber was to overcome an-other supposed limitation of the procedure proposed byRidley & Burland (1999). As free water is present in thevacuum chamber, water vapour continuously develops duringevacuation. This prevents the attainment of very low abso-

lute pressures, and hence reduces the effectiveness of airremoval from the ceramic.

The authors have shown that the initial saturation wassufficient to saturate the tensiometer fitted with the 100 kPaair-entry porous ceramic. However, the initial saturationalone was not sufficient to saturate the ceramic with a higherair-entry value (300 kPa), and a pressurisation was requiredto measure water tension. Similarly, the discusser found thatinitial saturation alone was not sufficient to saturate theTrento tensiometers, which use a ceramic with a very highair-entry value (1500 kPa). After saturation using the satura-tion chamber shown in Fig. 15 and a vacuum pump capable

of a measured minimum absolute pressure of 0.03 kPa, thetensiometers could withstand a minimum gauge pressure ofabout 300 kPa for only 1 or 2 s.

It is then possible that the initial saturation adopted by theauthors and the discusser does not significantly enhance the procedure proposed by Ridley & Burland (1999). The dis-cusser has saturated two Imperial College tensiometers fol-lowing the procedure suggested by Ridley & Burland(1999), and using a large triaxial cell as a saturationchamber and a vacuum pump capable of a minimum abso-lute pressure of about 3 kPa. In spite of the poor vacuumapplied, these two tensiometers later exhibited a satisfactory performance in terms of measurement precision (tensi-ometers pr3 and pr4 in Tarantino & Mongiov , 2000, 2001)that is comparable to that of the best-saturated Trentotensiometers (precision of about 3 kPa relative to theaverage measured value). The experience of the discusser isthat saturation of the ceramic is achieved mainly throughcycles of cavitation and subsequent pressurisation (Tarantino& Mongiov, 2001; Tarantino, 2003). An inadequate initialsaturation simply increases the number of cycles necessaryto obtain a satisfactory performance.

The second issue is the reliable measurement of porewater tension. Tarantino & Mongiov (2001) and Tarantino(2003) showed evidence that inadequate saturation mightresult in readings that appear reliable and stable although

actually incorrect. The authors have demonstrated this in avery rigorous fashion, and have addressed perhaps the main problem in tensiometer measurement, which is how todiscriminate a good measurement from a bad one.

The authors state that tensiometer response times underlow absolute pressures are an excellent indicator of tensi-ometer saturation, implicitly referring to the tests shown inFigs 12 and 14. However, in these tests, water in thetensiometer was subject only to positive absolute pressuresand for a limited period of time ($35 min). The tensiometerresponse may not be as good as supposed when water issubject to tension (gauge pressures less than 100 kPa) and/or when measurement lasts for longer periods (from hours todays). Under these conditions, air cavities may form within

the tensiometer without triggering cavitation, but causing afaulty response of the instrument. Cavity formation in thetensiometer is illustrated by the test shown in Fig. 16, wherethe pressure recorded by an Imperial College tensiometer is plotted against time. The ceramic was wiped after about11 min to let water pressure drop to about 1900 kPa, andthen the tensiometer was immediately replaced in free water.The time required for the tensiometer to return to atmo-spheric pressure was about 0.5 min (Fig. 16(b)).

A kaolin paste was then applied to the porous ceramic,and the paste was allowed to dry out slowly. After reachinga pressure of about 2600 kPa (595 min) the tensiometerwas again replaced in free water (Fig. 16(c)). The equalisa-tion time was much larger in this case, being about 7 min.

The reduced permeability is probably due to the expansionof the air cavities in the porous ceramic. Here, it wasnecessary to reach a very high tension to expand thecavities, but lower tensions might be sufficient to enlarge

Vacuum Vacuum

De-aired water

Screw valve (closed position)O-ringsTensiometer

Fig. 15. Schematic layout of the Trento double-compartmentvacuum chamber


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cavities if the initial degree of saturation of the ceramic islow. These cavities may then cause false tensiometer read-ings.

Air cavities may expand not only under tension but alsounder high positive pressures. Tarantino & Mongiov (2001)reported the case of a tensiometer stored for 1 year in asaturation chamber under a pressure of 4 MPa. After re-moval from the chamber, the tensiometer could sustain atension of 650 kPa for some seconds only. However, thesame tensiometer could sustain very high tensions (greaterthan 2000 kPa) after just one cycle of cavitation and subse-quent pressurisation at 4 MPa. Despite the high positive

pressures, air cavities had expanded in the tensiometer whenstored for 1 year because of air diffusion.

These two examples suggest that the degree of saturationof the ceramic must be regarded as a dynamic variable, inthe sense that it might change with time. As a result, thetypes of test shown in Figs 12 and 14 may or may not beindicators of adequate saturation for the case where longermeasurements are carried out and/or a tension is applied towater.

To verify the reliability of suction measurement, Tarantino& Mongiov (2001) and Tarantino (2003) have suggestedindicators of adequate saturation to be checked before andafter measurement. These indicators were defined on the basis of long-term measurement using more tensiometers

placed on the same sample. Nonetheless, we consider thatthe straightforward way to verify measurement reliability isto measure suction using at least two tensiometers simulta-neously and checking that they give the same readings.

Authors replyThe authors would like to thank Dr Tarantino for his

interest in the paper. The discusser has provided usefuladditional information regarding the independent develop-ment of a similar technique for the initial saturation oftensiometers. Although motivated by different intentionsavery dry filter element in the authors case and the mini-misation of the absolute pressure at saturation by the dis-

cusserthese two techniques share the key features of adried filter element and the evacuation of the tensiometer inthe absence of water. Despite providing a higher degree ofinitial saturation, the discusser has debated the merits ofthese techniques as inadequate initial saturation simplyappears to increase the number of cycles necessary to obtaina satisfactory performance of the instrument.

This difference in opinion perhaps arises from the differ-ences in the magnitude of the positive pressures used in thesecond stage of saturation. One objective of the paper wasto investigate the process of saturation to permit the meas-urement of soil suction with more sensitive, low-pressure-range devices that cannot be subjected to large saturation pressures. For example, one version of the tensiometerdesigned specifically for the accurate measurement of lowsuctions has a full-scale range of 100 kPa. Even if a portionof the safe over-pressure range is used to apply a saturation pressure of 200 kPa, a consideration of the physics ofsaturation (equation (2)) indicates that the initial degree ofsaturation, Si, must be greater than 0.96 to saturate theceramic filter sufficiently . In contrast, saturation using posi-tive pressures of the order of 4 MPa (e.g. Tarantino &Mongiov , 2001) is likely to be capable of saturating all butthe most poorly saturated of devices, given time. Thus itfollows from equation (2) that the significance of the highdegree of initial saturation will be diminished if very largepositive pressures are applied to saturate the device.

The typical pressure range of the base device upon whichthe new tensiometer is built is 700 kPa. At the higher

saturation pressure of 1 MPa, the initial degree of saturationwill be less important, but it is the experience of the authorsthat the higher degree of saturation ensures the saturation ofa device fitted with a 3 bar air-entry ceramic on the firstshort pressurisation. When saturating up to a dozen of thesedevices for inclusion into model tests, the authors are of theopinion that this benefit alone is worth the extra effort ofensuring a high initial saturation.

Second, the discusser raises the important issue of stable bubble growth in the ceramic filter element under highabsolute tensions, and questions whether tensiometer re-sponse times under low absolute pressures are thereforeuseful indicators of tensiometer saturation. This scenario has been investigated in a further set of experiments using a

tensiometer fitted with a nominal 3 bar air-entry ceramicfilter element. This tensiometer was initially saturated in thetwo-chamber saturation apparatus before being subjected toa saturation pressure of 1 MPa.

Following the experimental methodology described in thepaper, the response time of the instrument was then assessed by subjecting the device to 10 kPa pressure increments of100 s duration between 0 and 100 kPa, and between 100and 0 kPa. The device was then removed from the chamberand allowed to generate a pressure of 250 kPa under theaction of evaporation (Fig. 17(a)). At this point, the devicewas submerged in free water and allowed to equilibrate toatmospheric pressure before the response time was againassessed.

The lower half of this stepwise calibration exercise isshown in Fig. 17( b). The response of the device was ob-served to be indistinguishable from that observed before thegeneration of the absolute tension, andas shown in the

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Fig. 16. (a) Response of the tensiometer associated with slowdrying of the paste applied to the porous ceramic; (b)equalisation in free water before measurement; (c) equalisationin free water after measurement

230 DISCUSSION


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figureindicates little lag-time observed between the appliedand measured pressures. The device was then subjected to afurther pressure of 500 kPa by evaporation (Fig. 17(c)), before being plunged into free water. Upon pore pressureequalisation, the device was again assessed for its responsetime under low absolute pressures. These results, presentedin Fig. 17(d), indicate that even the short-term generation ofthis higher tension has reduced the permeability of the filterelement, despite not having initiated nucleation.

Rather than the absolute magnitude of the applied tension,the authors would suggest that it is the fact that the air entryvalue has been exceeded that gives rise to this response, andthat reported by the discusser (Fig. 16). Although the air-entry

value of a ceramic is often quoted as a single value, the poresize distribution will ensure that air entry is not an instanta-neous process. Rather, as the tension is increased past thenominal air-entry value, the menisci at the exposed surface of

the tensiometer will start retreating into the ceramic, beginningwith the largest of exposed pores. The thicker the filterelement, the longer this process can continue before nucleationoccurs, the more unsaturated the surface of the ceramic canbecome, and the larger the tension that can be recorded in thereservoir of the device. The difference in response time ob-served by the discusser is a result of a longer period ofdesaturation. This observation, in combination with the experi-ment described herein, indicates that largest measurable ten-sion in the reservoir should not be the defined as the largestmeasurable suction, as applied suctions greater than theair-entry value of the ceramic will initiate the process ofdesaturation. As the process of desaturation will always be

accompanied by a reduction in permeability, any techniqueevaluating this property either quantitatively or qualitatively(as in the technique proposed by the authors) will provide agood indication of tensiometer saturation.

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Fig. 17. Reduction in tensiometer response times with stable air entry

DISCUSSION 231


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REFERENCESRidley, A. M. & Burland, J. B. (1999). Discussion: Use of tensile

strength of water for the direct measurement of high soilsuction. Canadian Geotechnical Journal, 36, 178180.

Tarantino, A. (2003). Panel report: Direct measurement of soilwater tension. Proc. 3rd Int. Conf. on Unsaturated Soils, Recife3 (in press).

Tarantino, A. & Mongiov, L. (2000). Experimental investigationson the stress variables governing unsaturated soil behaviour at

medium to high degrees of saturation. In Experimental evidenceand theoretical approaches in unsaturated soils (eds A. Taranti-no and C. Mancuso), pp. 319. Rotterdam: Balkema.

Tarantino, A. & Mongiov, L. (2001). Experimental procedures andcavitation mechanisms in tensiometer measurements. Geotechni-cal and Geological Engineering 19, Nos 34, 189210.

Tarantino, A. & Mongiov, L. (2002). Design and construction of atensiometer for direct measurement of matric suction. Proc. 3rd

Int. Conf. on Unsaturated Soils, Recife 1, 319324.

232 DISCUSSION

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