the effect of clay minerals on the permeability of sand...

THE EFFECT OF CLAY MINERALSON THE PERMEABILITY OF SAND SOILS

L. LOVAS, D. Se.Design Office for Civil Engineering (Mélyépterv), Budapest, Hungary

ABSTRACT

Four different types of rock forming clay minerals, such as aplite (K.- and Na-feld-spar), kaolin (kaolinitc), illite and bcntonile(montmorillqnite), were mixed with washedsand. They were employed in bot h the flocculated (Ca-ionic) and the dispersed (Na-ionic)state, in amounts of 2, 4 and 6 weight per cent. The mixtures were packedint o testcylinders, and the quantity of the water seeping through the samples was measured atselected time intervals during 100 hours.

The test results obtained are the following : Different clay minerals vary widelywith respect to their effects upon permeability, even when present at 2 per cent quan-tities only. Permeability values exhibit dependence upon the type of the clay minerals;upon the state of the clay minerals — whether it is a state of coagulation or of disper-sion — upon the coagulating or dispersing action of the water penetrating the soil.

RÉSUMÉ

Quatre espèces d'argiles minérales constitutives de roches ont été mêlées à deséchantillons de sables lavés, telles que l'aphte (feldspath potassé ou soudé), le kao-linite), l'illite et le bentonite (montmorillonite). Ces argiles minérales ont été incor-porées sous un état coagulé (ions Ca) et sous un état dispersé (ions Na), en quantitésde 2, 4 ou 6 pour-cent de poids. La quantité d'eau traversant les melanges placés dansdes cylindres d'expérience a été mesurée durant 100 heures.

Les résultats des essais sont les suivants : Lors même que les argiles ne sont pré-sentes dans le sable qu'en doses de 2 pour-cent de poids, leur effet sur la perméabilitvarie beaucoup suivant les différentes espèces. La valeur de la perméabilité dépendnon seulement du type des argiles minérales présentes, mais aussi de leur état de coagu-lation ou de dispersion, ainsi que de l'action soit coagulante soit dispersive de l'eauqui traverse le sol.

1. INTRODUCTION

Securing water supplies from sand soils that contain small percentages of claymineral fractions, and lowering ground water levels in such layers, often present pro-blems to be solved. The permeability of such soils is determinable by means of heldpumping tests or by laboratory measurements. It is also computed by means of for-mulas by HAZKN (2), JAKY (3), and TKRZAGHI ("), on the basis of the particle sizedistribution curves of soil samples obtained from sample borings. The yield of theaquifer is evaluated in consideration of the relationship that exists between particlesize distribution and yielding capacity of the layer, as verified and described by ECKIS (l)KOI:HNE (4), LAMPI. (5) and UBELL (1O). Accordingly, investigations in regard to particlesize distribution play a dominant role in the approach to the suitable verification ofthe permeability of granular soils.

Conventional and standard analyses applied to the establishment of data on par-ticle size distribution only state the weight per cent frequency of the clay fractions.Neither type nor characteristics of the clay minerals are identified by their aid. Investi-gations on the colloidal properties of soil fractions, however, indicate that there is apronounced difference in the water adsorbing capacity of clay minerals, dependenton their type. In view of this, it deemed advisable to undertake studies in order todetect whether these varying adsorbing properties exhibited noticeable effects, even

274

it the clay mineral fractions were occupying the pore spaces of the sand particles binin minor amounts of a few weight percentages.

With the objective to answer above questions, informative investigations on somebasic phenomena were carried out in the Laboratory for Hydrogeology of the Chairfor Mineralogy and Geology of the University of Technical Sciences in Budapest.Data have been collected on four rock forming soils and minerals such as aplite (K-and Na-feldspar), kaolin (kaolinite), illite and bentonite (monlmorillonite), withspecial regard to their role of permeability reducing agents. Clay minerals were employ-ed in both the natural (Ca-, and Mg-ionic) and the sodium-ion adsorption state. Acti-vation was obtained by addition of NaaCO3, by the aid of two methods.

Under natural conditions, clay minerals generally cannot be found occupyingthe limit positions (i.e. complete coagulation, or complete dispersion) that have beenadopted for the experimental studies described in this paper. The tests were carriedout on these bases, because the selected clay minerals are representative bearers ofthe characteristics proper to the various soil materials, and, therefore, it appeared tothe purpose to start studies from these definite, basal limit cases.

The studies of particle size distribution data and the cation exchange i.e. cationadsorption procedures were carried out by the aid of the new pepti/.ation methodinvented by SztPESi (6-8) (Hungary). The theoretic grounds for this method can bedescribed as follows:

Pcptization serves the purpose of obtaining the maximum dispcrsity of aggregatedsoil particles. In general, this highest degree of dispersion is attainable, if suitable Naelectrolytes are added in quantities adequate to the given state of the system, and areadsorbed in the suitable surface positions. When the electrolytes are added in lower orhigher quantities, or if their cation adsorption is not adequate as to degree and posi-tion, only a partial peptization i.e. a partial dispersion will occur, or complete coagu-lation will set in. No predetermined reliable data can, therefore, be given on the amountof the peptizer required by whatever kind of soil material. This quantity, as well asthe velocity of electrolyte reaction to adsorption arc functions of a series of factors,such as the morphologic state (structure), the quantity and the quality of the clayminerals; the exchangeable cation sort and its surface position; the quantity and thequality of the so-called free electrolytes in the system; the amount of organic matter;the amorphous components; etc.

SzEPi.sr's practical procedure elaborated on above theories, presents a solutionfor the determination of the optimum quantity of the peptizer. It is a simple procedure,and requires no special equipment. By virtue of its effectiveness, it may lay claim towidespread application.

In the following, description and evaluation are given of the experimental studieson the reducing effects of the clay minerals on infiltration rate.

2. DESCRIPTION OF THE BASAL MATERIALS UTILIZED FOR THE PERMEABILITY TESTS

2.1. Characteristic Data on the Sand Samples

2.1.1. Limiting boundaries of the particle size distribution of the employed sandsare represented by two limit curves in figure 1. The particle size frequency curves ofall sand samples are falling in between.

The effective grain size diameters Oio% of the sands are varying between 0.10 and0.11 mm, their uniformity coefficients

uOio%

arc ranging between 1.70 and 2.18.275

Particle Size Distribution

Clay Silt Fine sand (Mo) Sand100'/-

TO —

SO —

X

X

id

0.0002

2.1.2. The chemical compositions of above sand samples are varying betweenthe following limit data:

SiO2 93-95 per centAI2O3 2-3 per centFeO.3 1-2 per centCaO 0.5-1.0 per centMgO 0.5-0.8 per centTotal of alkalis (Na2O - K2O) 0.5-1.0 per cent

Above analysis shows that the sand samples employed for the experiments werecontaining 92 to 94 per cent quartz beside 3 to 4 per cent feldspar.

2.2. Characteristic Data on Rocks Containing Individual Clay Minerals

The rocks and clay minerals that were mixed to the sand samples, both in a naturalstate (i.e. in a form of Ca-cation adsorption) and in an artificially obtained state ofsodium-cation adsorption were the following:

The minerals bearing Ca — and partly Mg — as initially dominant exchangeablecation (the Ca-minerals) have been activated to so-called sodium-minerals, bymeans of SZEPESI'S patented method (6).

2.2.1. The colloid-chemical and rheological properties of the rocks and mineralscited are listed in Table 1.

276

Rock Type Employed

Aplite(deposit from Székcsfchérvâr

Kaolin(deposit from Sârisâp)

Illite(deposit from Fiizérradvàny, namedSârospatakit by Magdefrau)

Ca-bentonite(deposit from Istenmezeje)

Per Cent Minera

QuartzNa-FeldsparK-Feldspar

KaoliniteQuartz

IlliteQuartzHeldspar

MontmorilloniteVolcanic glassQuartz

Composition

422830

55 to 6040 to 45

75205

82144

2.2.2. Soil-mechanical Properlies of the Clay Minerals

2.2.2.1. Partile Size DistributionThe standard wet mechanical analyses were performed by mixing the soil samples

both with distilled and with tap water. This parallel use was motivated because tapwater had been applied to the permeability tests, hence it was advisable to examineits coagulating effect.

The particle size distributions of the clay minerals in the Na- and the Ca-cationadsorption state, observed in distilled and tap water systems, are plotted in figures2 to*5.


Clay Silt Fine sand (Mo) Sand

Q0002

so

kO

30

20

10

— • - , " - - . - .

. _ . , .

, - ' _

—

j.

- / / /•"": ni"/r

—

- - - - - - - • — - -

- - - • - • • - •

Kaolin

. _ . . . _ . .

- — - - •

• - - - - • — • — - - •_._. —.. .. _ —i

—

—

—

- - — - - — —

— — .

L. . .

my.

90

so

70

so

30

«o

x

x

to

0.0002 0.00! 0.02 O.I

Legend

Ca m'/h tsp Hater Ca Mttr distilled

Na • •Ha . .

Fig. 3.


Clay Silt Fine sand (Mo) Sand

so

so

JO

so

so

L

-•'

^/ i i

71117. --

y— —

—- H— -

i

Wife

• • - -

. . .

0,0002 0,02

Fig. 4.

mi.

so

so

- — X

20

2mm

278

my* Wi-

Legend^

Ca with tap water Ca with distilled water

Ha • • • Ha •

Fig. 5.

2.2.2.2. Plastic Limits of the Clay Minerals

Name ofClay Mineral

Ca-apliteNa-aplitcCa-kaolinNa-kaolinCa-illiteNa-illiteCa-bentoniteNa-bentonite

Liquid LimitU

sandlike22.048.045.4

149.888.4

131.5151.6

Plasticity Indexlw

indeterminable6.1

22.521.8

117.058.479.2

104.2

Above limit values were measured at intervals of 18 hours of wetting. Plasticityvalues of Na-bentonite have also been registered after 60 hours, with results as follows:

Lw = 602.0 per cent, I» — 544.7 per cent

279

2.2.2.3. Linear Shrinkage Values of the Clay Minerals

Ca-apliteNa-apliteCa-kaolinNa-kaolinCa-illiteNa-illiteCa-bentoniteNa-bentonite

Clay Minerals•

(U%pcr 18 hr)

(Lai% per 60 hr)

ShrinkageLimit

Sw

8.06,55.9

13.611.610,525.0

2.2.3. The permeability tests were carried out with tap water, the total hardnessof which ranged 12 to 13 German hardness.

3. PERMEABILITY TESTS

3.1. Equipment and Method applied

The scheme of the equipment used for the experimental procedures is shown infigures 6 and 7. By aid of the laboratory instruments represented there, a continuousfiltration through the soil samples was successfully effectuated, with water temperaturesranging between 23 and 26°C, kept 3 to 5°C higher than has been the surroundinglaboratory air temperature.

Permeability Test Equipment

[Ground Scheme)Electric keyboard

rhermoregylator / / A Thermometer

Fig. 6.

280

I Procedure Scheme)

Thermormlßtor.\Tap

Measuringcylinder

3.2. The Medium Cylinder

Fig. 7.

Plastic cylinders of 55 cm height and 10 cm inside diameter were employed (Fig. 8).At the bottom of the cylinder a screen (mesh size 0.58 mm) was fixed to a wire netthat was braced at a height of 1 cm. At distances of 5 centimeters upward, a total of8 piezometric tubes (i.d. 0.5 cm) was set in spiral arrangement along the inside of thecylinder wall. A millimeter scale division was fixed at the outside of the cylinder wall.

Four centimeters above the highest placed piezometric tube, an overflow pipewas adjoined (i.d. 1.5 cm). Approximately 5 centimeters under their upper flange,the cylinders were holed at three points and hanged onto a wooden framework bymeans of soft metallic cords fixed into these holes.

3.3. Emplacement of the Soil Samples

The emplacement of the soil samples to be examined was carried out in the follow-ing manner (Fig. 8 b-b).

A 2 cm thick mat of 2 to 5 mm size gravel, and then another 2 cm thick mat of0.32 to 1.0 mm size filter sand were spread over the screen. It was onto this doublefilter bed that the agglomerated soil sample was carefully packed in layers of 5 cm.The sample column was of a total height of 35 cm.

A screen was placed on top of the column in order to protect the sample fromdamages that might eventually be caused by the water flowing from the rubber pipe.

3.4. The Course of the Procedures

Employing above method, permeability tests were carried out parallely on 5 soilsamples, one of which was pure sand, the other four were mixes of aplite, kaolin,illite and bentonite respectively.

281

For these six experimental runs, the mixtures were prepared on a weight per centbasis, such as follows:

Soil Sample Cylinder

a-a b-b

Piezometnctube

Overflowpipe

(

Piezometnc tubes_ ] /

Runs

1.2.3.4.5.6.

Weight per cent

246242

IILU

O

Soilsample

-v i-i SanchGravei

IJ \urer-z /

.Overflow pipe

i Screen

ç

Screen

' /

1 / Scale:^ N , 0 5 10 cm

[ ^ f/^yr^ 8.

Fig. 8.

Clay Minerals

Ca clay mineralCa clay mineralCa clay mineralNa clay mineralNa clay mineralCa clay mineralmixed with Na2CO3

282

TABLE l

Colloid-chemical and rheological properties of clay mineralsin a stale of 6 per cent dispersion, investigated by A. P. I. Norms.

Colloid-chemical andRheological Properties

APLITE

(Székesfehérvar)KAOLIN

(Sârisâp)ILLITE

(Fuzérradvâny)BF.NTONITE

(Istenmezeje) t

Ca-minerals(natural, coagulated)

oo

Viscosity (Stormer, 600 r.p.min)

Thixotropy (Stormer)

Filtration loss (Free water at 7 atm,30 min, from 500 ml dispersion)

Na-minerals(state of complete dispersion byNa2 CO3, SZEPESI method)

Viscosity

Thixotrophy

Filtration loss

Addition of Soda : usual method,mixing with Na2 CO3

Viscosity

Thixotropy

Filtration loss

1.5 cPgel strength :

immed. : 0 g10 min. : 0 g

500 ml

(1% Na2CO3)2.5 cP

gel strength :immed. : 0 g10 min. : 0g

245 ml

(l%NaoCO3)1.5 cP

gel strength :immed. : 0 g10 min. : 0 g

385 ml



362 ml

(l%Na2CO3)3.5 cP


165 ml

(l%Na^CO3)1.5 cP

gel strength :immed. ' 0 g10 min. : 0 g

240 ml



94 ml

(l%NaBCO3)3.5 cP


12 ml

(l%Na2CO3)3.0 cP


45 ml



250 ml

(4,5%Na2CO3)60.0 cP


7 ml

(4,5%Na2CO;))8.0 cP


32 ml

Natural minerals and rocks, such as aplitc, kaolin and illitc were made containingI per cent Na2COn; a 4.5 per cent quantity of Na2CC>3 was admixed to Ca-bentonites.Per cent data were related to the respective clay mineral amounts.

According to SZEPESI'S experimental studies, these data proved to be the repre-sentative values for the optimum ratio of sodium-carbonate. The sodium-carbonatewas admixed to dry clay minerals. The exchange of Ca-cations to Na-cations throughsodium-carbonate has taken place by the action of the test water filtrating throughthe clay-sand mixtures. The objective of this method was to obtain informative dataon natural phenomena that are often representing states intermediate between completefiocculation and complete dispersion.

The experimental runs were all continuous operations lasting 100 hours. Afterexpiration of this time interval, the medium cylinders were emptied, cleaned andrepacked with sand or with clay-mineral sand mixtures.

In the course of above procedures, data have been collected on infiltration rate,on water temperatures, on laboratory air temperatures; the heights of the watercolumns in the piezometric tubes were observed and registered.

Runs 1 to 5 have been repeated in order to gain reassurance as to that the testresults were reliable.

4. DISCUSSION OF TEST RESULTS

4.1. Experimental Data

As can be seen above, permeability of soil samples was studied at eleven experi-mental runs.

The discharge rates of the water transmitted by the sand samples at equal inter-vals of time, have shown deviations from the mean values, ranging from 4 to 37 percent in limit cases.

The data collected at the twice repeated measurements of the discharge rates ofthe water seeping through sand samples with clay mineral contents, showed deviationsfrom the mean values varying between 12 and 86 per cent.

One reason for these deviations is the difference in the particle size distributionof the samples employed. Determined on the basis of the limit curves of the particlesize distributions and by the HAZEN formula, the permeability coefficient "k" of thesands was found varying between 1.16 x 10~2 and 1.41 x 10~2cm/sec. With thesedata, the calculated limit values of water discharge through the samples ranged 62.3to 75.7 cubic centimeters per minute (mean value 69.0 cu cm/min; deviation from themean values approximately ±9%). Deviations may also be caused by errors committedat preparing the soil samples.

As can be seen from the results, in the case of sand, discharge rates stabilizeddisplaying but little fluctuations within 10 hours. As to clay-mineral-sand mixtures,the discharge rates could be considered as practically constant or, on certain cases,as constantly decreasing after a period of 50 hours.

Piezometric examinations and measurements on changes in water and air tempera-tures had to be passed over. The corrections that had been available by means of datagained by such procedures (e.g. effects of changes in temperatures upon water dis-charge) had been of minor importance than had been the errors caused by them.

In view of the foregoing, the above test results and the relationships observedshall now be examined with respect to discharge rates of water filtering through thevarious soil samples.

284

Discharge Curves of 4plife-Sand Mixtures

rime

Send

'tie

. J Sand'tvi

\

V api,

Sand,

le

fica apile

Fig. 9. (Time in hours)

Discharge Curves of Kaolin-Sand Mixtures

g i - - ^ - - • - . ^ :.—J—SfTtfr-i


4.2. Discharge Rales of Water Filtering through Various Soil Samples

Discharge rates measured during the test procedures are found plotted on graphs9 to 12. For the investigations on the reducing action of the various clay mineralsupon permeability, reference data utilized were the mean discharge values that havebeen observed at the end of 100 hours' testing time. These values are also expressedby percentage data resulting from relating above values to the mean water dischargefrom pure sand (Table 2).

285

TABLE 2Discharge Rales of Waier Filtering through Various Soil Samples

Measured in the 100th hour of testing time (cu cm/min)

Clay-minerals

2% Ca

4% Ca

6% Ca

2%Na

4% Na

Ca-clay mine-ral mixedsimply with2 per centNa2CO.i

Sand

73.0- loo %

73.0-= 100 %

73.0- 100 %

73.0

-- wo %73.0

• -• 1 0 0 %

73.0=• 100 %

Sand-aplite

46.2--• 63 %

12.0- 1 6 %

7.5- 10%

28.3- 39 %

10.6- 1 5 %

36.5- 50 %

Sand-kaolin

33.5-"46 %

3.2

• - • 4 %

2.0- 3 %

4.6— 6 %

1.2- 2 %

15.4- 21 %

Sand-illite

22.0=-- 30 %

1.4- 2 %

0.20- 0.3 %

0.56-• 0-8 "A,

1.10- 0.1 %

1.7- 2%

Sand-bentonite

42.0"- 57 %

5.7= 8 %

0.70•^ 1 %

0.003•=• 0.004 %

0.000-- 0.000 %

2.2- 3 %

There are two points of view from which data listed in Table 2 may be compared:a) The effect of different clay minerals on discharge, when they are added at

equal quantities;b) The effect of different clay minerals on discharge, when they are added at

different quantities.

5. EVALUATION OF TEST RESULT'S

Statements made on the basis of above test results are the following:5.1. The difference as to the extent in which individual clay minerals display

their particular effect upon the discharge of water from sand soils is even noticeable,when the sand soils are containing clay minerals at amounts of but 2 per cent.

5.2. Diminution of the discharge rates is not only dependent on the clay type,it also depends on the degree of dispersion and of hydration of the clay minerals.

5.3. Clay minerals display considerable variations as to their particle size distri-bution, the degrees of their hydration effects, and, consequently, as to their capacityof inhibiting the passage of water through soils, depending on whether they are eithercoagulated by Ca-cation adsorption or dispersed by Na-cation adsorption processes.

In the case of sand mixtures containing clay minerals at 2 per cent amounts, thesedifferences appear as follows:

aplitekaolinill j tcbentonite

(montmorillonite)

less than half an order of magnitudenear to one order of magnitudemore than two orders of magnitudenear to four orders of magnitude

286

Discharge Curves of lllite-Sand Mixtures

Time

a c x x t o s o e o m s o s o m

— A 1— \ e

no •

so •

I "j ,.

Dis

char

ge (

cubi

c c

s

«5

0,1

^.K,_ J

.._

.._

-h-

_._

1-

• " • • • » 4 !

1^3. _|_..i3

-

Sa

id*/,;

id'2'i

•»«

id

|_. _

Cs ill

o o

te

le

—

z

~~.1

- 4 -


5.4. Described conditions are well determined by the characteristics of the 6 percent hydrous dispersions of Ca- and Na-elcctrolytic adsorption systems: first of allby viscosity data, and by the water rate expelled on 7 atm pressure within half anhours' time, i.e. the so-called filtration loss value (Table 1).

Above evidences have already been confirmed by earlier interpretations relativeto clay minerals of similar characters and compositions (7).

5.5. The characterization of the properties of the various clay minerals describedin point 5.3 may also be suitably represented by semi-logarithmic grain-size curves(Fig. 2 to 5), plotted on the basis of the new analytic method of sedimentation (pepti-zation) by SZEPKSI. Standard hydration techniques only refer to undefined intermediatepositions, without finding the possible limits, that are precisely established by meansof the SzEi'h.si method.

5.6. Plastic limit values, that are resulting from the standard plastic limit test,do not give quite satisfactory data on clay mineral character. Above studies refer tothis fact, in revealing that Na-clay minerals of much finer size range than the naturalCa-clay minerals do not display higher water adsorbing capacities proportionate totheir larger specific surface, and to their higher degree of dispersity. That is becauseNa-clay minerals, consequently their adsorption equilibrium sets in later too. (Seerise of liquid and plastic limits of bentonite, after 60 hrs. Point 2.2.2.2.).

287

Discharge Curm of Bentonite-Sand Mixtures

o l o x x t o s o e o i o s o s o

8 . os - •

o t i x x f o s o e o i o l ß s o t o o


288

5.7. The linear shrinkage values being computed on the basis of liquid limitmeasures are also concerned with above statement.

5.8. Calculated with the HAZKN, JÂKY and TERZAGIII formula, the reductiveeffect of 4 weight per cent clay mineral contents on the permeability coefficient "A",and thus on the yielding capacity, appears most insignificant. This statement is result-ing from a slight diversion of the size frequency curve of clay-sand mixtures in relationto the grain size distributions of pure sand. Whereas, investigations described abovehave verified, that 4 weight per cent illite reduce permeability by almost 3 orders ofmagnitude, while 4 weight per cent montmorillonite (bentonite) effectuate completeimpermeability or cause complete water retention.

CONCLUSION

Projected on natural phenomena, the results of"above investigations reveal them-selves as described, for example, in the following: With feldspar present, — the pre-dominant rock of which is aplite — -, there is no significant difference as to permeability,whether the water transmitted is hard, soft, or alkaline. With montmorillonite (bento-nite) present, the coagulating or dispersing effect of the water and of the substancesdissolved therein (electrolytes), cause essential differences in the permeability valuesof the soil.

The primary objective of the above informative investigations was to study cer-tain problems from new aspects. Character and number of the data obtained do notauthorize to form any mathematical formulas as yet. The subject requires further effi-cient elaborations. On the qualitative basis, however, the tests have verified relation-ships, that, up to the present, were excluded from consideration by the qualitativesoil tests applied in hydrology and soil mechanics.

REFERENCES

(') ECRIS, R. (1934) : South Coastal Basin investigation, geology and ground-waterstorage capacity of valley fill. Bull. 45. Calif. l)iv. Water Resource Sacramentop. 279.

(2) HAZKN, A. (1893) : Some physical properties of sands and gravel with specialreference to their use in filtration. 24th Annual Report, Mass. State Board ofHealth, 1962. Pub. Doc. 34, pp. 539-556.

(:i, JAKY, J. (1944) : Talajmechanika (Sail mechanics). Budapest, Magyar EgyetemiNyomda.

(4) KOKHNE, W. (1948) : Grundwasserkunde. Slutgart, E. Schweizerbart'seheVerlagsbuchhandlung.

(5) LAMPL, H. (1955) : A talaj szemcseösszetetcle es fizikai tulajdonsàgai közöttiösszefüggesek. (Re lationship between particle size distribution and physical propertiessoils). Budapest, Vizügyi Közlemenyck, No 3,4.

(6) SZEPESI, K.. (1958) : Eljàràs natrium bentonit cloàllitàsàra földalkali bentonitokböl.Producing Na-bentonite from the bentonite of alkaline soils). Patented in Hungary,No. 1767; India No. 59775; Italy, No. 577109; German Federal Republic, No.1062683; Yugoslavia, No. 21780; France, 1192942.

(7) SZEI'IISI, K. and LOVAS, L. (1962) : Abdichtungsarbeiten mit Bentonil. Wasser-wirtschaft-Wassertechnik, 12. Jahrgang, H. 5, pp. 209-215.

(8) SZEPESI, K. and MAJER, J. (1963) : Neuartige Sedimentationsanalyse. TonindustrieZeitung. At press.

(9) TERZAGHI, K. (1955) : Influence of geological factors on the engineering propertiesot sediments. Economic Geology, Fiftieth Anniversary, Volume, pp. 557-618.

(10) UHELI, K.. (1958) : Az elméleti kuthidraulika môdszereinek gyakorlati alkal-mazâsa. ( Practical application of theoretical methods in well hydraulics). Budapest,Vizügyi Kozlcmények, No. 2.

289

the effect of clay minerals on the permeability of sand...

Documents