Frost heave and heaving pressure measurements in colliery shales

R. J. KETTLE Dqmrtmcvzt of Civil Enginc>~ring, Utliversify ofAston in Birmingham, Birmingham, B4 7ET, Englnnd

A N D

R. I . T. WILLIAMS Dc,partmet7t of Civil Et~gineering. University of S ~ i r r q , Guildford, GU2 5 X H , England

Received October 8, 1975

Accepted January 20, 1976

The paper describes a technique for measuring the pressure generated when heaving is restrained in a frozen soil, freezing being achieved by thermoelectric cooling. Although steps were taken to minimize side wall resistance between the specimen and the test mould significant under-estimation of the pressure was unavoidable and further work is necessary to accurately quantify the resistance.

The tests were performed on specimens of unbound and cement stabilized colliery shale, both unburnt and burnt shales being studied. For the unbound shales, the largest heaving pressures were developed by the finer grained shales, and this supports the theoretical studies that have shown heaving pressure to be inversely proportional to pore size. Cement stabilization did not significantly affect the heaving pressure developed by the coarser grained shales but, with the finer grained shales, it reduced the pressure developed.

Heave and heaving pressure are not uniquely related and, although relationships have been established between these parameters separately for burnt and for unburnt shale, the technique does not at present constitute an alternative to the frost heave test.

The testing programme has shown, however, that thermoelectric devices provide a reliable and efficient means for freezing specimens and an experimental rig is suggested for using them in frost heave testing.

L'article dCcrit une technique de mesure de la pression genCrCe lorsqu'on empcche le soulkve- ment d'un sol gele, le gel Ctant produit par refroidissement thermoelectrique. Bien que des mesures aient ete prises pour minimiser la resistance mobilisee entre I'6chantillon et les parois du moule d'essai, une sous estimation importante de la pression de soulkvement a i t6 inevitable et d'autres travaux sont necessaires pour quantifier cette resistance.

Les essais ont e t t rialisis sur des Cchantillons de schiste houiller, stabilise ou non au ciment, des schistes intacts ou brulCs etant Ctudits. Pour les schistes non cimentes, les pressions de soulkvement maximum ont ete developpCes par les schistes a grains les plus fins, ce qui confirme les etudes theoriques qui ont montre que les pressions de soulkvement Ctaient inversement proportionelles a la dimension des pores. La stabilisation au ciment n'a pas affect6 de fac;on marquie la pression de soul&vement genCrCe dans les schistes i~ grains grossiers, mais elle a provoqui une reduction de pression dans les schistes grains fins.

Le soulkvement et la pression de soulkvement ne sont pas relies de fac;on unique, et, bien que des relations aient Cte etablies entre ces paramktres, sCparCment pour le schiste bruli et le-schiste non brulC, la technique ne constitue pas actuellement une alternative pour I'essai de soulkvement au gel.

Le programme d'essai a toutefois montrC que les dispositifs thermoClectriques constituent un moyen sur et efficace pour congeler des Cchantillons et on suggkre un montage experimental Dermettant des les utiliser dans les essais de soulevement au eel. -

[Traduit par la revue] Can. Geotech. J., 13, 127 (1976)

Introduction In the United Kingdom the frost suscepti-

bility of soils and of highway materials is assessed using the Transport and Road Re- search Laboratory frost heave test, details of which were published some 8 years ago (Croney and Jacobs 1967). The test was de- veloped over a period of 20 years and is

especially valuable because the criteria are based upon a detailed study of the behaviour of soils in relation to their field performance during the severe winters of 1940 and 1947. Although originally introduced for studying the behaviour of soils, it has subsequently been applied to sub-base materials, including cement stabilized materials.

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128 CAN. GEOTECH. J. VOL. 13, 1976

Recent research on the frost susceptibility of colliery waste materials at the University of Surrey (Kettle 1973; Kettle and Williams 1969, 1973) has involved extensive use of the frost heave tcst, with both a cold room and a deep freeze cabinct. One of the major disadvantages of the test is the time element involved, a 24 h conditioning period being required prior to the 250 h test run and, in thc case of cement stabilized materials, a maturing period is neces- sary (but not specified) between specimen preparation and initiation of the test itself. The length of the test period has the obvious dis- advantage of directly influencing the time re- quired to reach a decision regarding the suit- ability of a material and, in addition, thc laboratory undertaking the test is 'at risk' over a lengthy period in that mechanical or electrical faults can and do occur whcn operating test facilities of this type on a near-continuous basis.

As a result, further work was directed to- wards examining ways of shortening thc test or obtaining an early indication of the final heave (Kettle 1973). It was found that the heave obtained with most unbound and cement stabilized shales after 100 h of freezing could be uscd to predict the 250 h value with reason- able accuracy but this was not so for the wide range of materials rcported by Croney and Jacobs ( 1967) since thc hcave-time rclation- ships are not of thc same form for all materials.

In the attempt to develop a quick-index test for judging frost susceptibility, work was subse- quently directed towards examining alternative test methods. It had been suggested (Hoekstra et al. 1965; Penner 1968) that measurement of the pressure devcloped when heaving is restrained offered a simplcr test and this was therefore undertaken since, in addition, it was likely to provide fundamental information for understanding the mechanism of frost action in unbound and in cement stabilized materials.

The apparatus used for measuring heaving pressure is based on that used in America by the U.S. Army Corps of Engineers and re- ported by Hoekstra, et al. (1965).

Details of the Apparatus The apparatus, shown in Figs. 1 and 2, con-

sists of a cylindrical steel mould 101.6 mm diameter and 133.3 mm deep, the lower

115.2 mm being tapered on the inside, a re- cessed steel base plate is attached to the lower end of the mould and houses a porous stone sealed to the cylinder by an '0' ring. The therrnoclectric device (TED) rests on an alu- minium cold plate at the top of the mould, with the plate being sealcd against the mould by a 'U' cup seal. Briefly, a thermoelectric device is the reverse of a thermocouple so that, when an electric current is passed through it, one side is cooled whilst heat is given out the opposite side. Mounted above the cold plate, and taking the thrust, is a load cell which bears against a reaction frame. In addition to being tapered, the inside of the mould was coated with PTFE (polytetrafluoroethylene) dry lub- ricant to further reduce frictional forces. The 'U' cup seal is included to minimize heat trans- fer between the cold plate and the cylinder.

The thermoelectric element induces unidirec- tional freezing, the particular element having a rated capacity of 18.7 J/s. It was operated under controlled conditions, with a constant current supply and a regulated flow of watcr through the hot chamber, so that the rate of heat removal was standardized for all tests. The power supply for the thermoelectric ele- ment consisted of a low voltage, direct current unit operated so as to produce a rate of hcat removal of 11.7 J/s. Thc output from the load ccll was plotted on a trace recorder thus giving a permanent record of each test. The load cell had a capacity of 4.5 kN, with an overload capacity of 6.75 kN and, as a check on its reliability, it was calibrated in a standard load- ing frame before and after each test.

Since furring was not expected to present a problem at the heat sink tempcrature of 15 "C, tap water was used for cooling and was run to waste after passing through the element, the measured temperature of the tap water in the header tank remaining within the limits of 13 i- 1 OC.

Test Procedure Both unbound and cement stabilized speci-

mens were made up at the optimum moisture content previously determined by the BS (Bri- tish Standard) compaction test, this being the total amount of water added to the dry shale. They were produced at the same density as that of the spccimens used in the frost heave

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KETTLE AND WILLIAMS

FIG. 1. (a) General layout. (b) Equipment positioned in cabinet. (c) Equipment assembled.

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130 CAN. GEOTECH. J. VOL. 13. 1976

Reaction frame

~hermoelectricl I device

$tde

Sample mou Id t

FIG. 2. Schematic drawing of apparatus (sample size 115 m m X 102 m m diameter).

test, so that the results of the two tests could be directly compared.

Tests on Unbound Material Sufficient water to produce the optimum

value was added to the shale and the two were mixed together for 13 min. The cylindrical mould was bolted to a steel base plate and the mixed material compacted into the mould in three layers, each layer being given 25 blows with the standard compaction hammer. The inside of the tapered section of the mould had the same dimensions as a standard compaction mould and this procedure produced specimens with dry density values within -tl% of the maximum value determined in the standard test.

After campaction, the upper surface of the specimen was carefully smoothed with a 100 mm diameter steel block, excess material being trimmed off so that the height of the specimen was 115 mm, corresponding to the trimmed height of a standard compaction speci- men. The mould was removed from the base plate and bolted to the recessed plate which,

having been fitted with a porous stone, was then connected to a water supply. The upper cold plate was positioned, together with the load cell, and the whole assembly placed in the reaction frame inside the deep freeze cabinet, as shown in Fig. 1, the reaction frame being constructed from 20 mm X 200 mm steel plate. The specimen was allowed to saturate for 24 h, as in the case of specimens in the frost heave test, the cabinet being operated in such a way that the air temperature gradually reduced within the initial 16 h to +4 OC. At the start of the test, the trace recorder, load cell and thermoelectric device were switched on and the whole assembly rigidly secured in the reaction frame with a pre-load of 2 kg. The subsequent increase in pressure was registered on the trace recorder and the test continued until a maximum pressure was reached, the period depending on the particular shale under test. The results of the tests are given in Table 1 together with the amount of material finer than 75 pm. In order that they may be discussed in relation to frost susceptibility, values of frost heave and absorption are also tabulated, the latter having been shown elsewhere (Kettle 1973) to be an important property in predict- ing the freezing behaviour of colliery shales.

Tests on Cement Stabilized Shale The test procedure was slightly different for

these materials due to the need for a maturing period before testing. To avoid the standard mould being used for a single specimen throughout this period, frost heave specimens ( 1 52.4 mm x 101.6 mm, diameter) at the BS standard level of compaction were used. The specimens were wrapped in polythene and dipped in wax to prevent change in moisture content. After 7 days they were stripped and weighed, and then slid into a tapered cylindrical steel mould. The mould was 168 mm long and the inside diameters were 1.25 mm larger than those of the frost heave mould, thus allowing the tapered specimens to be slid into position. Some specimens had high spots, caused by slight expansion on extrusion from the frost heave mould, and these were filed down.

The procedure was then the same as that followed for the unbound shales and the results of the tests are given in Table 2.

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TABLE 2. Effect of cement content on heaving pressure

Sample

Unbrirtzt Betteshanger Chislet Peckfieid Snowdown Tilmanstone 1

KETTLE A N D WILLIAMS 131

TABLE 1. Results of heaving pressure tests on unbound shale

Heaving pressure* Amount finer Average than 75 pm Absorption Individual results Mean heave

Sample (%) (%I (MN/m2) (MN/m2) (mm)

Unburnt Peckfield 3 8 8 .0 0.62,0.66,0.55,0.58 0.60 6.4

0.59 Aberpergwm 9 5.2 0.19, 0.23 0.21 12.9 Bedlay 2 1 3.2 0.32, 0.39 0.36 7.9 Bentley 16 6.7 0.27, 0.29 0.28 10.2 Betteshanger 6 2.9 0.15,0.09,0.12 0.12 5.0 Brodsworth 11 4.5 0.12, 0.14 0.13 6.4 Bullcroft 43 7.0 0.76,0.81 0.79 7.3 Chislet 10 3.5 0.15, 0.19 0.17 10.7 Cortonwood 10 5.5 0.15,0.21 0.18 7 .5 Rothwell 31 7.8 0.48, 0.52 0.50 12.7 Snowdown 3 3.1 0.07, 0.08 0.08 4.1 Thorne 28 7.8 0.35, 0.42 0.39 15.9 Tilmanstone 1 19 8.9 0.56, 0.55 0.56 50.8

Burnt Granville 11 7.5 0.26, 0.22 0.24 19.2 Newdigate 6 6.2 0.19,0.15 0.17 16.0 Silverdale 12 8.1 0.31, 0.32 0.32 46.5 Thornely 9 8.3 0.26 0.26 30.9 Tilmanstone 3 1 I 11.0 0.28, 0.23 0.26 42.5 Wheatley Hill 13 7.8 0.30 0.30 27.5

Part Burnt Tilmanstone 2 12 8.0 0.26, 0.25 0.25 9.9

*All these tests were carried out in the short mould and no allowance has been made for the effects of sidewall resistance. ?There was a suggestion that this material had been subjected to some abnormal treatment on the tip and it is not representa-

tive of the material in colliery tips.

Mean value of heaving pressure (MN/m2)

Unbound Cement stabilized

Amount finer 10% cement than 75 pm Short Long 5% cement

(%I mould mould 7 days 7 days 3 months

Burnt Tilrnanstone 3 11 0.26 0.36 0.36 0.40 * Part Burnt Tilmanstone 2 12 0.25 0.35 0.34 0.41 * NOTES: NO allowance has been made for the effects of sidewall resistance. All cement stabilized specimens tested

in the long mould. *Indicates that no test was carried out.

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CAN. GEOTECH. J. VOL. 13, 1976

TABLE 3. Measurement of sidewall resistance (comparison of the use of short and long mould on unbound material)

Mould used Force to overcome Equivalent Sample for test resistance (kN) pressure (MN/m2)

-- -

Chislet Short 0.35 0.043 Long 0.19 0.023

Tilrnanstone I Short 0.41 0.051 Long 0.22 0.027

Silverdale Short 0.60 0.073 Long 0.23 0.028

TABLE 4. Measurement of sidewall resistance (additional tests)

Mould used Force to overcome Equivalent Sample for test resistance (kN) pressure (MN/mZ)

Brodsworth Short 0.42 Snowdown Short 0.30 Snowdown* Long 0.15 Tilmanstone 2 Short 0.63 l'ilmanstone 2* Long 0.17 Peckfield* Long 0.31

*Indicates test o n stabilized material.

Discussion of the Test Method Repeatability of the Test

Although the test procedure followed was based directly on the work of Hoekstra et al. (1965), preliminary tests were carried out to ensure that the equipment gave repeatable re- sults. Five tests were therefore carried out on nominally similar specimens of an unburnt shale from Peckfield colliery and the results are included in Table 1. These were considered to be satisfactory, the coefficient of variation being 5 % , and it was decided that tests on two nominally similar specimens would give an acceptable measure of heaving pressure for the testing programme.

Effects of Sidewall Resistance Sidewall resistance is due in part to the

formation of ice between the specimen and the mould and in part to the frictional resistance of particle contact at this interface. Hoekstra et al. (1965) reported that the resistance was minimized by tapering the mould and by apply- ing PTFE lubricant on the inside of the mould, but they did not give values for the resistance developed. Since two types of mould were used in the colliery shale study, it was necessary to

obtain data on the sidewall resistance devel- oped.

Measurements were therefore made of the force required to displace a number of speci- mens on completion of the restrained heave test. An increasing force was applied to the base of the specimen whilst the upper rim of the mould was held, and sidewall resistance was defined as the maximum force necessary to cause a relative movement of 1 mm as mea- sured by a dial gauge. This procedure was not entirely satisfactory because it involved re- moving the mould from the cabinet and partly dismantling the mould so that the full effects of adfreeze may not have been measured and, furthermore, water liberated by local thawing may have reduced the frictional part of the resistance.

In Table 3, the values given for tests on unbound shales relate to material compacted in situ in the short mould but to preformed specimens slid into the long mould. In all three cases, greater sidewall resistance is mobilized in the short mould and this is consistent with the intimate contact likely to have been created at the mould to material interface. These re- sults suggest that sidewall resistance amounts

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KETTLE AND WILLIAMS 133

to an equivalent pressure of 0.02 to 0.07 MN/m2, and this is endorsed by the additional tcsts reported in Table 4 but the total resis- tance, including the full effects of adfreeze, is likely to be higher.

In view of the difference in resistance be- tween the two moulds, some unbound frost heave specimens were tested in the longer mould and the results given in Table 5 indicate that a higher pressure is obtained when the unbound material is tested in this manner. This is due, at least partly, to the difference in side- wall resistance, although differences in the specimen hcight and method of compaction could also ir 'luence the results. It is interesting to note that the mould effect is more pro- nounced with the coarser grained burnt shales than with the finer grained unburnt shales, and this is in agreement with the findings reported in the U.S.A. (Kaplar 1968). With the finer grained unburnt shales, the effect of sidewall resistance is probably modified by the lubricat- ing action of the clay colloids and this action would not apply to the burnt shales.

Thc effects of sidewall resistance have not therefore been accuratcly quantified but the values obtained give some indication of the probable magnitude of the restraint involved.

Discussion of Heaving Pressure Hoekstra et al. (1965) plotted the logarithm

of heaving pressure against timc, thereby sug-

TABLE 5. Comparison between tests carried out in the two moulds on unbound specimens

Heaving pressure (MN/m2)

Sample Short mould Long mould

Ut~burtzt Bedley 0.39 Betteshanger 0.12 Chislet 0.17 Snowdown 0.08 Tilmanstone 1 0 .56

Burnt Silverdale 0.32 Tilmanstone 3 0.26

Part B~tr t~t Tilmanstone 2 0.25

NOTE: Specimens tested in the long mould were prepared in the standard frost heave mould and then slid into the long mould.

gesting early equilibrium, whereas typical plots using linear scales in Fig. 3 show that the heaving pressure approaches a limiting value at an ever-decreasing rate. Indeed Sutherland and Gaskin (1970), using a different experi- mental technique, reported that slight increases could be detected after freezing for ovcr 150 h. To limit the period and to provide a means of quantifying the data, heaving pressure is defined in this paper as that pressure at which the rate of increase is less than 0.001 MN/mVh. On this basis, values of heaving pressure for the unbound shales are given in Table 1 and range from 0.08 to 0.79 MN/n12.

The maximum heaving pressure for a givcn porous material is developed in an open system with free exccss of water (Everett 1961; Pen- ner 1967), such that the unfrozen soil is sat- urated because under these conditions the suc- tion force in the unfrozen soil is zero. In this work the specimens were tested in an opcn system with free access of water so that the soil was in a state of capillary saturation at the commenccment of the test. In this regard, even if the soil at the start of cooling is unsaturated, the degree of saturation increases towards the cold end bcfore ice forms. When icc forms the soil immediately below the ice will definitcly

. Bullcroft . Chiskt

I I 0 Snowdown

FIG. 3. Graph of heaving pressure against duration of test (typical results for unbound, unburnt shales).

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134 CAN. GEOTECH. J . VOL. 13, 1976

saturate if water is available (Wissa and Martin 1968).

At the completion of the restrained heave test, moisture content determinations on the unfrozen shale from some of the unbound specimens indicated that the degree of satura- tion exceeded 95%. In such an open system, the amount of energy expended in lifting the water will be small in comparison with the heaving pressure generated (Kaplar 1970) but this could lead to some under-estimation of the heaving pressure, particularly with the less per- meable finer grained shales because, with these, considerable energy could be expended in water transport. However, the initial saturation, flow paths, soil compaction, and soil characteristics are the same for a given material in both the heaving pressure and the frost heave tests. It is interesting to note that other workers have measured the heaving pressures in open sys- tems either with free access to water (Taber 1930; Sutherland and Gaskin 1970), or with free excess of water (Hoekstra et al. 1965; Penner 1967).

Each material has a characteristic heaving pressure and it has been shown (Miller et al. 1960; Everett 196 1 ; Penner 1967) from funda- mental thermodynamic concepts that this maxi- mum heaving pressure is inversely proportional to the pore radius, this being the case for both unbound and cemented materials (Everett and Haynes 1965). In the course of the work, measurements were not made of the pore size or of pore size distribution, but the particle size distribution was determined for all the shales and this is considered (Yong and Osler 1967; Penner 1968) to be qualitatively related to pore size and pore size distribution. It has been suggested (Penner 1966, 1967) that the maximum heaving pressure is particularly de- pendent on the smallest pores and, in this work, the fraction of soil finer than 75 pm is likely to be especially influential in determining the size of the smallest pores since it forms the matrix around the coarser particles in an engi- neering soil.

In Fig. 4 the values of maximum heaving pressure are plotted against the percentage of fine material for the various shales and it can be seen that a reasonably well-defined relation- ship is obtained. This is particularly the case for unburnt materials, with only one result not

x Unburnt shales

1 . Burnt shales

(Tilrnanstone, unburnt)

x /

I - Fraction of soil finer than 7 5 , ~ m ( % )

FIG. 4. Graph of heaving pressures against the proportion of fine material for the unbound shales.

fitting into the pattern. Although the results from the burnt shales are more limited in num- ber, they show that higher pressures are de- veloped for a given amount of fine material.

In Fig. 5, the possibility of using the rela- tively straightforward heaving pressure deter- mination as a means of predicting frost suscep- tibility is examined by plotting heaving pressure against heave, using the values given in Table 1. It is immediately apparent that there is no unique relationship between these properties, and the results are therefore discussed sep- arately for the two types of shale.

Unburnt Material Of the 13 samples tested, only the materia1

from Tilmanstone was classified as highly frost susceptible and only three as marginally frost susceptible. It fo1Iows that the majority of the unburnt shales were non-frost susceptible and this limits the comments that can be made on the results.

The general pattern, however, is of interest. In Fig. 4, shales containing only a Iow propor- tion of fine material are shown to develop low values of heaving pressure. In addition, the voids in these soils are large enough for the water in them to behave as bulk water and so heave is limited (Chalmers and Jackson 1970).

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KETTLE AND WILLIAMS 135

I x Unburnt shales

I L R 9 0 Criteria 201 IEimrn 1 Highly frost susceptlble

50-

40 -

Marginally frost susceptible

10-

Burnt shales x (Tilinanstone

I unburnt)

7 I

/ Heave (mm) 1

Marginally frost susceptible

10- X x

I Heaving prossuie (MN l m 2 ) I

0.20 0 .40 0.60 0 8 0

FIG. 5. Graph of heave against heaving pressure for various unbound shales.

These comments apply to the shales, in Fig. 5, that develop pressures less than some 0.20 MN/m2.

At the other extreme, Fig. 4 shows that shales containing a high proportion of fine material develop high values of heaving pres- sure but the heave is low due to the restricted flow of water through the small pores to the freezing front. This applies particularly to those shales in Fig. 5 developing pressures in excess of some 0.50 MN/m2.

It might therefore be expected that the greatest tendency to heave would be detected in shales developing heaving pressures within the range of 0.20 to 0.50 MN/m2, but the limited number of results within this range does not allow conclusions to be drawn. It is inter- esting to note, however, that Hoekstra et al. (1965) identified the range of 0.07 to 0.55 MN/ma as being critical in terms of frost susceptibility based on American criteria using the test developed by the U.S. Army Cold Regions Research and Engineering Laboratory (Haley and Kaplar 1952; Kaplar 1965).

Burnt Material All six samples were frost susceptible and

yield a reasonable relationship in which in- crease in heaving pressure is associated with increased heave.

For a given heaving pressure, the burnt shales heave considerably more than do the unburnt shales and this is attributed to the higher absorption and greater permeability of the burnt material (Kettle 1973). In this con- text, it is interesting to note from Table 1 that the unburnt shale sample from Tilmanstone, regarded as a stray result in the discussion of the unburnt shales, has an absorption value comparable with the values measured on burnt shales.

Stabilized Material The results given in Table 2 show that the

heaving pressure generated by a shale when stabilized with cement is of the same order of magnitude as that of the unbound shale and especially so when the comparison is based on unbound material tested in the long mould since the mould restraint conditions are then comparable.

The agreement is particularly good for the shales from Betteshangcr, Chislet, Tilmanstone 3, and Tilmanstone 2. The amount of material finer than 75 pm for these shales is within the range of 6 to 12% so that they are relatively coarsely graded. The material from Snowdown, however, has only 3% finer than 75pm, but no explanation can be offered for the relatively poor agreement in this instance other than to observe that the pressure developed by this material is in any case very low.

The shales from Peckfield and from Tilman- stonc 1 contain respectively 38 and 19% of material finer than 75 pm and it is apparent that treatment with cement has caused a lower pressure to be developed in both cases. The change in pressure is indicative of a change in pore size and is attributed to the finding (Chadda 1970) that the addition of cement to fine grained soils causes aggregation of the clay colloids to produce fewer but larger sized pores in the material, and heaving pressure is inversely proportional to pore size (Everett 1961 ). Evidence in support of this observation was found in the main investigation (Kettle 1973) when the addition of cement to fine

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136 CAN. GEOTECH. J. VOL. 13, 1976

grained shales increased both permeability and heave.

Since the pores in cement stabilized material are progressively filled with the products of hydration, additional tests were carried out on materials after a curing period of three months, and the heaving pressures were then greater. The results in Table 2 also show that the finer shales stabilized with 10% cement developed marginally lower pressures than when they were stabilized with only 5 % cement and this may also be due to the increased aggregation.

Frost Weave Tests in the Restrained Weave Apparatus

As a separate study, tests were carried out on some unbound s~ecimens but without re- straint so as to allow heave to occur. These were performed on frost heave specimens placed in the longer mould. The heave value was determined using a brass rod and dial gauge in the same way as for the standard frost heave test, and the results are given in Table 6. It was not convenient to place thermocouples in these specimens as the connections would have prevented the specimen being readily slid into the mould. but thern~ocouples were used in some specimens in the shorter mould. The temperature gradient through these specimens is shown in Fig. 6, with the zero isotherm being about one-third from the base of the svecimen as with the frost heave specimens (Croney and Jacobs 1967; Kettle and Williams 1973). Un- der the fixed operating conditions a similar location can be expected for the longer speci- mens.

However, the sidewall resistance will affect the modified heave tests. In the standard frost heave test (Croney and Jacobs 1967) the

TABLE 6. Conlparison of frost heave results

Frost heave (mm)

Cold room Pressure mould Sample and trolley without restraint

Urzbwtzt Brodsworth 6 .4 1 . 1 Chislet 10.7 2 .7

Bum t Binley 18.9 7 .6 Tiimanstone 3 41.6 12.9

TOP

\

Depth

BASE

FIG. 6. Temperature distribution through a speci- men in the short mould. Equilibrium distribution was achieved within the initial 6 h of the test, but obviously this was influenced by the material under test.

specimens are loosely wrapped in waterproof paper and surrounded by coarse, dry sand, so that resistance due to particle friction at the boundary is virtually eliminated. In addition, as the inside of the paper is not in intimate contact with the specimens, the amount of ad- freeze resistance is likely to be low. Earlier, it was shown that significant sidewall resistance is mobilized when specimens are frozen in the restrained heave moulds so that it is unlikely that there will be direct relationship between the two test methods. Indeed, whilst the trend in the few results given in Table 6 is the same in both tests, there is no simple relationship between the results.

This limited study, and the work on heaving pressures, has shown that thermoelectric de- vices are extremely efficient and reliable for freezing soil. It may be that effort should be directed towards examining the possibility of employing these devices for frost heave tests, an approach that may be particularly suitable for organizations wishing to carry out only a limited number of tests.

A cooling chamber runnhg at +4 OC can probably be achieved by using a normal domes- tic cabinet and a suggested arrangement is

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KETTLE AND WILLIAMS 137

- Heave

Cabinet ( 1 ternperat~re+4~~ \ 1

52.4 mrn x 101.6 cimen 8 r n 3

shown in Fig. 7. As in the standard test, the specimen is wrapped in waterproof paper and stood on a porous disc in a copper specimen holder. The specimen is placed in an environ- ment maintained at +4 "C, so that the sides of the specimen need to be well insulated to minimize the lateral heat gradient. To this end it is suggested that the coarse, dry sand should be replaced by a hollow, cylindrical sleeve of polystyrene. The specimen should be cooled with a standardized, thermoelectric device lo- cated on a cold plate, at the upper end of the specimen.

With such an arrangement the sidewall re- sistance would be similar to that in the standard frost heave test, whilst the total weight of the aluminium cold plate and the thermoelectric device would not need to exceed 500 g. Ini- tially, tests in such an arrangement would require correlation with those in the standard equipment so as to establish a correction factor. Three similar rigs would be sufficient for most testing requirements and they would probably be constructed more cheaply than setting up equipment for the existing frost heave test.

-

Conclusion The measurement of heaving pressurc gave

repeatable values which have been discussed in relation to the properties of the materials tested. Although further work is required to allow for the sidewall resistance developed by

~ v e r t ~ o w bowl

friction and adfreezing, the investigation allows the following conclusions to be drawn.

( 1 ) Each material has a characteristic hcav- ing pressure, higher values being developcd with shales containing appreciable amounts of fine material. For a given amount of fine matc- rial, higher pressures arc developed by burnt than by unburnt shales.

(2) Heave and heaving pressure are not uniquely related, separate relationships being obtained for unburnt and for burnt shales. With the unburnt shales, heaving pressures within the range of 0.20 to 0.50 MN/m2 may be indicative of frost susceptiblc material whereas all the burnt shales tested were frost susceptible and developed pressures greater than 0.15 MN/m2.

(3) The heaving pressure generated by a cement stabilized shale is of the same order of magnitude as that generated by the same shale when unbound except that the finer grained shales develop a marginally lower heaving pressure, possibly due to aggregation of the clay colloids by the cement. Similarly an in- crease in cement content leads to further aggrc- gation and would be likely to lead to lower heaving prcssures being developed at the higher cement contents. However, prolonged curing is associated with a reduction in porc size and this leads to an increase in heaving pressure.

(4 ) The use of thermoelectric devices is shown to be a convenient and reliable method for freezing soils and further work is recom- mended to develop their usc in frost heave testing.

Acknowledgments This study forms part of a research financed

by the National Coal Board and the authors wish to express their thanks for the support given and for permission to publish the paper. The views expressed are those of the authors and not necessarily those of the National Coal Board.

They also wish to express their thanks to the Department of thc Environment, Engineering Intelligence Division, who financed the pur- chase of the equipmcnt used for measurement of the restrained heaving pressures. Finally, they wish to thank their colleague Dr. D. J. Hannant for his many useful comments during the course of the investigation.

FIG. 7. Suggested arrangement for heave test using a thermoelectric device.

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138 CAN. GEOTECH. J. VOL. 13, 1976

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CRONEY. D. and JACOBS. J . C. 1967. The frost susceptibil- ity of soils and road materials. Road Res. Lab.. Rep. LR90.

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