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Nature, 212, 5069, pp. 1463-1464, 1967-05-01
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Movement of air through water in partly saturated soils
Williams, P. J.
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A H A ~ Y Z E D
(Rep1 itue~l f r orjr Noture, Vol. 212, N o . 5069, pp. 1463- 1464, Decerrtbo 24, 1966)
Movement
ofA i r through W a t e r in Partly
Saturated Soils
WHEN tho water content of a soil is reduced belo\\. satura- tion, the interfaces bctwceil air ancl water lie within pores, and are c~wvecl bccause of surfaco tension, G. Tho pressure
of tho nrntcr
P,,
is lower tlxm that of the surronnding airP a
b y.ivhoro r , is the raclius of tho intcrfaces. As the wator contont is reclnced, drainago occurs from progressively smaller openings, a n d the interface radius decreases. Tho pressure of tho water also falls, in accordance wit11 equation 1, producing tllc woll known "suction-moisturo contont" rclationship.
I n many situations of geotechnical and agricultural interest tho soil is partly saturated. I n a sample of the soil tho interfaces betwcen tho external air a n d the pore
vato or are curved but localized near t h e snrfacc. I n addi- tion, there are usunlly bubbles of air within tho soil, totally entmppccl by urater-filled porcs. They m a y be sphorical, or sufficiently large t o occupy a number of inter- connected pores. If tho lsatcr pressure IS uniform, then
such a bubble can only have thc same pressure a s tho cstornal air if its irltcrfi~ce (or intorfaccs) with the wator also has thc radi~ls T S . Thcso intcrfaces nsnally have somc other radins, uucl accordingly thc air pressui9e within the bnbblcs is i ~ o t nsnally the same as t h a t of the atmosphere. Becausc the solubility of air in water depencls on t h e air pressure (Henry's law), it folloxvs t h a t there mill often b e a gradient in t h c concentration of clissolvcd air in the water. Whon the interface (or intcrfaces) oEa bubble is of greater mdins than t h a t a t thc snrfarc of' t h c soil sample, tho gradient will occur from thc viciility of the samplc surfncc towards t l ~ c vicinity of thc hnbblc. The cxiste~lcc of a concciltration gradicrlt rcsnlts in a d~fCt~s~on of air in solu- tion ton-ards the rcgion of IOU er coilcciltmtion. Wllen tho saturation concentration is osccedecl. howovcr. air will come out of solutioil. An entrapped bnbblo can thus be onlargccl. Tho process will bc a continning one. As tho raclius of t l ~ o I)ul)blc li~crcascs. t l ~ c air pressure within it falls furtilor so tlltxt t,llc satui~ation concurltration in its vicirlity is f u ~ thcr rccluc~ctl. I t s g r o l r t l ~ inay ovontually
bo stopped by its cxtci~sio~l into small porcs, whore the illtorfaces w ~ l l again hnvc a. mlall radius.
'I'liis procoss ol' air ino\ cinoilt b y tlirS~~sioil througli \valor can,
or
course, also occur in tho oppositc dircctioil :ii~d causo cntrappod air to dccrcasc in oxtont or to dis- nppoar. This will bo tho caso whon tlie intorfaccs of tho ontmppecl air havo tho smaller radius.The proccss is relevant to studies of iifiltratioil a d of drying of soil llorizons ncar the soil surfacc, to the intor- pretation of laboratory suction-moisture content tests, and t o geotechnical considerations of the mechanics of partly saturate j soils. Few articles in the wide literature in these fiolds, however, give even scant attention t o the significaricc of air movement in this may.
A simple esperiinent has been deviscd to obscrvo thc process. A soil ranging from fine silt to medium silt was prcparcd by crushing chiolite (NaI'.iUF,). This ilaturally occurring mineral has a refractive index closc t o that of water (in which it is practically insoluble), and when tho pores of the soil are filled mith water small samplcs are cluito trailsparcnt. Air within the porcs is fully visible. Such a sample was placcd in a specially constructed pressure membrane apparatus. consisting basically of a small 'Perspes' cell, part of which could bc placod under a microscope. The sample mas placcd i n t.hc cell, above a drdnago opcning covorcd by a membrane
Htor. Thc air pressuro in t h e cell could bc raiscd, and drainago of water from the sample t o tho outside could occur through the membrane. Air lealrage does not occur because of the smallness of the pores of the mcmbrarie which remain totally filled with water a t the air prcssurcs in question.
After the air pressuro is raised, clrainagc occurs fiunl thc sample b u t soon appcars to stop. Tho pressure 01
the water in tho pores IS then unif'onnly atmospheric,
becauso the water phase is in continuous connexion wit11 water in tho drainago tub0 undcr tho csternal (room) pressure. The intorfaces between the air and the water are then curved mith a radius which depends o n the pressure applied t o the air. These interfaces, depending
on the poro size, will to some extent lic within the "soil". The air pressure can he seloctcd with rcgard t o tho porc size. snch that the air only extends into a few largor pores near tho sample surfaco. I n o11c such tcst, a n air prcssilro of 0.22 kg/cm2 \\-as applicd.
Fig. l a shows ontrappod air bubblos, sit,uatc\cl :1- 4 m111 nrithin an otherwise saturated part of tlic samplc. T l ~ c photograph was taken about 20 min after t,hc air pressurc in the ccll was raiscd. As tlic willcr prcssuro had by
this timc again become ~uliformly atmospheric, volume changes of the bubbles due t o temporary changcs of watcr pressuro wero complotccl. St~bscc~iicnt,lj: t h c bubbles eiilargcd slo~vly and 26 h latci. had tlic forms sllowll ill Fig. l b . By this timc enlargeincnt had slowed down, ancl it is a ~ p a r e n t that tho intorfacos (now l y ~ n g within porcs
a 6
Fig. 1. Tlucc bubbles entrapped i n s p:~rt,lg snturaletl soil sil~llplc. a,
Shortly aflcr the cst,ablishment of an est,ernal air w;lter-prcssurc tlifkr- cncc, Pn - Ptu = 0.2'7 lig/cm2; 6 , the sn~xlc bubblcs 2G 11 Inter. surrounding tho cavities ill \\.liicll tho bubbles originally occurred) again arc of o size (givcn by ccluat,ioil 1) conlpnr- able uvith t h a t 01 tllc i~ltcrfnccs to thc cshcrnal air.
I n the prrseul csainplc tho gcomotry is complicatetl nncl allows only a, rougll estimate of t,k~c rate ; ~ t which t h c b~ibblcs should grow. Thc obsel>\wl rates arc compatiblt: with this ostimatc. The csperimctntal observations reported here ore also sufEcicnt t o lcavc littlc clo~tbt as to the causc of the well line\\-n ancl problelnzitical entry of air illto cloviccs for measuring porc 1va.tcr prcTssurc which arc used in field ineasuremeilts in partly saturated soils. Even though such devices are isolated from soil air b y filtcrs with fine pores totally filled wit11 n-atcr, air occurs within thc device aftcr a pcriocl of time and p r c ~ c n t ~ s its correct operation. It moves by cliff~filsion i'l,oill tl~c: sinall and rostrictccl bul.~bles in t1lc:soil pores. to t11(~ la~,gcr
bubblc wlfilich can grow in thc chamber of thc: 1)rcssuro m c ; ~ a ~ ~ r i n g tle~icc.
So11 Mccha~l~cs Sect~oil, Dix-~sion of Building Rescarch, S a t ~ o n a l Iicsearch (Jouncil, Ottan-a, C,uiada.
