Discussion: Influence of fluoride on the compressibility of ...

Sridharan, A., Rae, S. M. & Gajarajan, V. S. (1988). GCofechnique38, No. 2,319-321

DISCUSSION

Influence of fluoride on the compressibility A. SRIDHARAN,

(1987). Gbotechnique 37, No. 2, 197-206

S. M. RAO and V. S. GAJARAJAN

E. JuPrez-Badillo, National University of Mtxico This Writer would like to suggest to the Authors, the use of the constant parameters y and K to characterize the compressibility and the variation of permeability of the untreated and Flouride-treated bentonite clay and montmorillonitic soil. The coefficient of compressibility y characterizes the compressibility of clays in their virgin curve by the equation (JuBrez-Badillo, 1975) v/v,

= (fJ/Q1)-y

compressibility of bentonite by a factor of 2.8 and for the Chitoor soil by a factor of 1.57. K characterizes The coefficient of permeachange the variation of permeability of soils by the equation (Juitrez-Badillo, 1983)

where k = Darcy’s coefficient of permeability e,, k, is a known point. Applying Eqn (6) to the Authors’ fig. 5 using as known points those for e = 1.0, results shown in Table 2 were obtained. Introducing these values into Eqn (6) the lowing equations for the four mentioned soils obtained

(1)

where V = volume, 0 = effective pressure and crl, V, is a known point. In terms of void ratios Eqn (1) may be written e = (1 + e,)(a/al)-Y

- 1

(2)

- 1,

e = 2.60 (a/4)-0”6

e = 1.57 (a/4)-“**

- 1

e = 1.53 (u/~)-O”~ - 1

(introducing

C, = 2,3y(l + e,)(o/ol)-’

folare

k = 1.00 ((1 + e)/2)4’8 x 10m9 cm/s

(7)

k = 4.00 ((1 + e)/2)4’8 x 10m9 cm/s ! The theoretical curves describe the experimental curves of fig. 5. From the values of the coefficients of for the untreated and permeachange K fluoride-treated soils it is concluded that treatment increased the coefficient K of bentonite by a factor of 2.8 while it remained unchanged for the Chitoor soil. For the case of Chitoor soil, however, increase k, by a factor of 4 and, with K constant, the increased permeability is by a factor of 4 for the whole range of void ratios.

- 1

The theoretical curves describe the experimental virgin curves of fig. 3, i.e. from u = 0.5 kg/cm2 to higher pressures. The tangential compression index C, is given by the equation (JuBrez-Badillo, 1975)

In terms of pressures (4) may be written as

and the

k = 0.14 ((1 + e)/2)8’9 x 10e9 cm/s

(3)

C, = 2,3y(l + e)

and

k = 0.10 ((1 + e)/2)3’2 x 10m9 cm/s

Applying Eqn. (2) to the Authors’ fig. 3 and using as known points those for g1 = 4 kg/cm* the writer obtained the results shown in Table 1. Introducing these values into Eqn (2) the following equations for the four soils are obtained (a in kg/cm’) e = 2.15 (0/4)-“~’

of montmorillonites

(4) Table 1. Physical properties of treated and untreated specimens

Eqn (2)) Eqn

Sample

(5)

Application of Eqn (5) to the virgin part of the Authors’ fig. 4 gives somewhat smaller values than those reported. From the values of the coefficients of compressibility y for the untreated and fluoride-treated soils it is concluded that treatment decreased the

Bentonite Fluoride-treated bentonite Chitoor soil Fluoride-treated Chitoor soil

319

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e,

Compressibility coefficient: y

1.15

0.45

1.60 0.57

0.16 0.22

0.53

0.14

Void ratio:

DISCUSSION

320

Table 2. Further properties of the treated and untreated specimens Sample

Bentonite Fluoridetreated bentonite Chitoor soil Fluoridetreated Chitoor soil

Coefficient Permeability:

of k,

Coefficient of permeability: K

0.10 x 10e9 cm/s

3.2

0.14 x 10W9 cm/s 1.00 x 10m9 cm/s

8.9 4.8

4XMl x 10e9 cm/s

4.8

Mr. S. T. Bloomer and Dr. P. S. Coupe, Sunderland Polytechnic The Authors provide a detailed insight into the effects of fluoride on several of the commonly measured engineering properties of two particular soils. With respect to the liquid limits of the soils, it is felt that the results obtained by the Casagrande cup method may be improved on by use of the method detailed in BS 1377 (1975), test 2A, using the cone penetrometer. Past research has shown that the Casagrande cup method is affected by the weight of the soil, giving less reliable results than the cone penetrometer method at higher moisture contents. Wroth (1979) suggested that the Casagrande cup involved the dynamic failure of an unstable slope and was dependent on the stability number c/yH where c = undrained shear strength, y = unit weight and H = height of the slope. The failure of the slope is dependent on the density of the soil and its moisture content. If the work of Youssef, el Ramli & el Demery (1965) and others (which suggests the existence of a unique value of soil strength when the soil is at the liquid limit) is accepted, then, using the figures of the Authors, it can be shown that the unit weights of the saturated soils tested at the liquid limit are as given in Table 3. For failure of slopes in the given soils with the same stability number and equal geometrical

Table 3. Unit weights of the specimens tested at liquid limit Specimen

Bentonite Fluoride-treated bentonite Chitoor soil Fluoride-treated Chitoor soil

Specific gravity

Liquid limit:

%

Unit weight: kN/m3

2.81 2.64

495 256

ll@O 11.88

2.72 2.58

124 81

13.66 14.82

properties (as is the case with the Casagrande cup) then the ratios of strengths in the extreme case is as high as 14~82/11GO = 1.35. This shows that a variation in strength (and hence liquid limit) is caused by the weight of the soil, using the Casagrande method. However, the effects of soil weight using the cone penetrometer are practically negligible (Houlsby, 1982). In the case of the fluoride-treated Chitoor soil (the most dense of the four soils) the weight displaced by a British Standard 8Og, 30” cone falling into the soil at the liquid limit (i.e., a penetration of 20mm) is O+g. The displaced weight of soil is in the region of 1o/o of the weight of the cone and therefore may reasonably be neglected and the test regarded as being independent of the weight of the soil. The Authors detail their experimental procedure, but fail to mention whether the different points on the flow curve for the determination of liquid limit in treated specimens were achieved by the addition of distilled water or by that of 0.5N sodium fluoride solution, with which they were originally equilibrated. If the former was the case, would this not effectively leach out the fluoride and dilute its effect? In practical cases of pollutant migration, such as the instances mentioned in the Paper, the pore fluid within the soil would be a mixture of fluoride and water. It is therefore thought to be more representative if the soil has the particular pollutant as its pore fluid. Authors’ reply The Authors appreciate the comments of Juarez-Badillo and hope to make use of his constant parameters in future studies. Bloomer and Coupe suggest that the variations in liquid limits of the untreated and treated clays, determined by Casagrande’s apparatus, possibly arise from variations in the weight of the specimens and recommend verification by the cone penetrometer method. Determining the liquid limits of bentonite and fluoride-treated Chitoor soil by the latter procedure indicated slight variations from the Casagrande results (Table 4). The results therefore resolve the apprehension of the Writers. The variations in liquid limits of the clay Table 4. Comparison etrometer test results

of Casagrande

Specimen

Liquid limit: % Casagrande’s test

Bentonite Fluoride-treated Chitoor soil


and cone pen-

495 81

Cone penetrometer method 510 70

DISCUSSION

specimens on fluoride treatment essentially arise from changes in interparticle electrical forces. The different points in the flow curve for the treated specimens were achieved by addition of distilled water which is incapable of leaching mineral-bonded fluoride (Huang & Jackson, 1965). It is agreed that in practical situations, the pore fluids within soils will contain some amount of dissolved fluoride ions. The presence of excess dissolved salts in the pore water was deliberately avoided so as to eliminate their effect on the plasticity and compressibility characteristics of bentonite, and highlight solely the influence of bonded fluoride.

REFERENCES British Standards Institution (1975). Methods of test for soils for civil engineering purposes, BS 1377, pp. 17-19. London:

Her Majesty’s

Stationery

Offtce.

321

Houlsby, G. T. (1982). Theoretical analysis of the fall cone test. GCotechnique 32, No. 2, 11 l-l 18. Huang, P. M. & Jackson, M. L. (1965). Mechanism of reaction of neutral fluoride solution with layer silicates and oxides of soils. Proc. Am. Soil Sci. 29, 661665. Juarez-Badillo, E. (1975). Constitutive relationships for in A&y. Soil soils. Symp. Recent Developments Behaviour and Appln to Geotech. Structures, pp. 231257. Kensington: University of New South Wales. Juarez-Badillo, E. (1983). General permeability change equation for soils. Int. Conf: Constitutive Laws for Engng Materials, pp. 205-209 Tucson: University of Arizona. Wroth, C. P. (1979). Correlations of some engineering properties of soils. Proc. 2nd. Int. Conf: Behaviour of Of-Shore Structures, London, 121-132. Youssef, M. S., el Ramli, A. H. & el Demery, M. (1965). Relationships between shear strength, consolidation, liquid limit, and plastic limit for remoulded clays. Proc. 6th Int. ConJ: Soil Mech. and Fdn Engng, Montreal 1, 126-129.
