General report session I: Phase transformations

Engineering Geology,

13 (1979) 1--6 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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G E N E R A L R E P O R T SESSION I: PHASE T R A N S F O R M A T I O N S

General reporters: D.M. A N D E R S O N

and S. K I N O S I T A

INTRODUCTION

The contributions to this symposium consist o f a wide variety o f observations and experiences on the processes, nature and behavior characteristic o f freezing and thawing of soils, earth and rock on the one hand and the utilization of this knowledge in the exploitation of the favorable properties o f frozen ground in engineering applications on the other. Consequently, it has been found convenient to arrange the papers contributed b y the participants into t w o sessions reflecting this fundamental division. Session I is devoted to the first o f these two categories and Session II is devoted to the second. In all, eleven papers have been assigned to the first session. They deal for the most part with the basic processes and effects of freezing both from the point of view o f microscopic as well as macroscopic phenomena. In this report we call attention to what seem to us the points of major importance of each and pose certain questions that seem w o r t h y of attention in the discussion period provided at the end o f Session I. REVIEW

OF SUBMITTED

PAPERS

Penner and Walton report in their paper "Effects of temperature and pressure on frost heaving" the effect o f the boundary condition (overburden pressure) on the moisture transport. They find that the moisture suction from the unfrozen part to the freezing interface decreases with the increasing overburden pressure and the suction changes to the expulsion when it goes b e y o n d a certain value of overburden pressure (shut-off pressure): the magnitude o f this shut-off pressure is more strongly d e p e n d e n t on the particulate nature o f the soil than the freezing temperature or freezing technique used. The paper by Kinosita, "Effects of initial soil-water conditions on frost heaving characteristics", is a discussion o f experiments carried o u t at a field test site in Hokkaldo, Japan. The water table was set initially at 4 different levels in identical test basins. Soils tested were silt and sand, respectively. Soil-water m o v e m e n t during frost heaving was observed and measured under ambient conditions during the winter of 1976--77 and related to air temperatures, frost penetration and heaving rate. It was observed that the higher the initial water content, the greater was the heave. Even in the test basins with no free water table, heave was observed due to migration of water to

the freezing front, drying the soil below. Water movement and frost heave was greatest during the coldest periods of the winter and were greater in the frost-susceptible silt than the non-frost-susceptible sand. In the paper "The research of the frost heave of non-water-saturated loamy soil in field conditions" by Karlov the heave behavior of two kinds of softs (disturbed and undisturbed loamy soils) under various overburden pressures in the field conditions is studied. He finds that the heave amount of the ground surface decreases in the freezing period next year; it is due to the change of soil structure; especially the change is intensive when many distinct ice lenses exist in the frozen state. The paper by Horiguchi, " E f f e c t of the rate of heat removal on the rate of frost heaving", describes a series of experiments using powdered alumina carborundum and zeolites in size ranging from 0.5/~ to 80 ~, equivalent spherical diameter. The general conclusion was that for most o f these materials, as presumably for most if not all earth materials that they simulate, the rate of frost heaving increased with the rate of heat removal to a maximum rate and then decreased. The maximum rate of heave observed increased as the size of the particles decreased. This is quite understandable under the conditions of the experiments in which water m o v e m e n t was upward to the freezing plane from an infinite water supply below; the maxim u m rate at which capillary water could be drawn up into the powder to freeze is finite in every case and the o p t i m u m rate for maximum frost heaving is greatest for the finer grained powders. These experiments are another demonstration that ice segregation must be attributed principally to the properties of water and not to soil or other earth materials. Frost heaving p h e n o m e n a can be induced in most powdered substances if a suitable liquid is employed. Chamberlain and Gow report in their paper "Effect of freezing and thawing on the permeability and structure of soils" the change of soil behavior (permeability and structure) by the repeating of freezing and thawing. They find that lesser effects are given on silt than clay. The paper by Takagi "Segregation freezing as the cause of suction force for ice lens formation" claims to distinguish "a new freezing mechanism" that is responsible for the development of the negative pressure gradient ("suction force") that is set up ahead of the freezing front. A series of derivations involving this idea are then developed to predict frost heaving pressures. There is much in this paper with which one can agree b u t there are a number of difficulties that make complete agreement difficult. It is d o u b t f u l in the first place that a new mechanism has in fact been discovered. Further discussion of this claim may prove enlightening and it would seem that additional discussion of the stresses identified on p.97 in terms of the relevant experimental data available could be useful. The paper by Menot, "Equations of frost propagation in unsaturated porous media", is a welcome contribution. In a clear, concise manner the essential physical processes and suitable mathematical expressions are identified and chosen, respectively, and the simplifying assumptions em-

ployed are clearly stated. The system o f differential equations that results reduces to an elliptic hydraulic problem for saturated soils whereas it is parabolic under unsaturated conditions. The paper by Outcalt, " T h e effect o f iteration frequency on a numerical model o f near surface ice segregation" is a continuation of his effort to improve the mathematical modeling of ice segregation during frost penetration. He describes the non-linear model he has developed and concludes that an iteration frequency of 10 sec yields realistic results. His analysis reveals that vapor diffusion is a significant process in the vicinity of the freezing front and it should not be neglected. This point Could profitably be pursued in the discussion period. The manner in which the latent heat of condensation and solidification and microscopic heat transfer is accounted for is of interest as well as other physical processes that are either explicitly or implicitly taken into account. The paper by Jumikis, "Cryogenic texture and strength aspects of artificially frozen softs", is a discussion o f some o f the principal factors that determine cryogenic textures in artificially frozen soil from the point o f view o f an application of artificial soil freezing to the construction of a coffer dam. Various freezing regimes lead to a variety of soil-ice textures that in turn affect the deformation and strength properties of the frozen soil. These are discussed in terms o f symmetrical arrangements of brine lines in a coffer dam that is to be strengthened and rendered impermeable. The paper by Yong, Cheung and Sheeran, "Prediction of salt influence on unfrozen water content in frozen softs", is a report of the comparison of experimental data giving the dependence of the unfrozen water content of three, salty, frozen clays on temperature with predictions obtained by means of an equation derived earlier by Banin and Anderson from classical thermodynamical arguments. The paper is difficult to follow in detail because of the a t t e m p t by the authors to construct a more comprehensive theory from that already at hand by combining it additively with another contribution from double layer theory. Although the result is a phase composition curve (i.e., unfrozen water content as a function of temperature) that is qualitatively realistic, the m e t h o d does not appear to be rigorously justifiable. Early in the paper the authors point out that the results o f unfrozen water content determinations of montmoriUonite in distilled water yielded higher values at a given temperature than the same clay in 10-3 M NaC1, contrary to the theoretical prediction. What seems to have been overlooked, however, is the fact that clays are soluble in distilled water and undergo rapid disintegration yielding aluminum and silicon ions which then hydrate. This produces an electrolytic solution o f uncharacterized ionic strength which could easily account for the discrepancy. Thus the limitations o f the existing theory are n o t clearly demonstrated and, at the end, it appears that the improvement being proposed is n o t fully adequate. The paper by Pusch, "Unfrozen water as a function of clay microstructure", describes the differences in two soft ftlitic clays, one laid d o w n in fresh water, the other in salt water. Two easily distinguished microstructures

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resulted. It was f o u n d that microstructure geometry had an observable effect on the unfrozen water content as a function of temperature. The effect of differing states of agglomeration and consolidation on t h e extent of the ice--water interface thus has a significant effect on the phase composi tion curve. The methods of microstructure characterization and ways of taking it into account in predicting or allowing for the different behavior that results from differing microstructures could profitably be explored in the discussion o f this paper. PROPOSALS F O R DISCUSSION

Penner and Walton Concerning their study, it may be argued that the shut-off pressure may also be d e p e n d e n t on the creep property of the unfrozen and the frozen part within the specimen box, and also on the constraining condition, for example, the dimension o f the specimen box. We would appreciate their c o m m e n t on them.

Kinosita Questions that might be raised during the discussion of this paper might explore the plans for using this field test facility and the types o f experiments of the highest priority that should be conducted there in the future.

Karlov (1) We would like to find o u t if there is any difference in the height distance o f suction zone in unfrozen part between disturbed and undisturbed soils. (2) Also we are interested in finding o u t if there is any structural change in soil after freezing and thawing. (3) He states that overburden pressure gives no effect on the heaving character. We would like to know his view a b o u t any possible effect if the pressure changes in a more wide range. (4) We appreciate his explanation a b o u t the main reason of nonuniformity o f heaving.

Horiguchi A question that might be pursued in discussion of this paper is h o w to combine the equations if heat and moisture fluxes in a suitable moving coordinate system to predict the observed behaviour. Another question would be: What additional experiments might be done with the apparatus e m p l o y e d in this investigation?

Chamberlain/Gow (1) We want to find out the particle size distribution of the soils used. (2) We want to confirm whether they are suggesting that the change of permeability corresponds to the amount of distinct ice lenses formed in the frozen state.

Takag/ (1) His study is based on a proposal that there exists a heterogeneous layer of water absorbed between the particle and the ice, its thickness being maintained at a constant value during the freezing process. We appreciate his explanation about why the thickness remains constant, and how related this layer is to the unfrozen water. (2) As to segregation freezing and in-situ freezing, an explanation is needed about how these freezing types are related to the thermal equilibrium or mechanical equilibrium. Menot Questions that might be explored during the discussion of this paper should examine the appropriateness of the assumptions and the consequences of other possibilities of interest. The phenomenological description of the hydraulic gradient proposed in the concluding paragraph merits some explanation and justification. Outcalt (1) We hope to have clearer explanations of the simple model and vapor model he uses. (2) We would like to find out if it is possible to simulate when the initial soil temperature is nonuniform within the ground, which is what we find in nature. (3) We appreciate his explanation abqut the relation between an iteration frequency and the length of freezing period. Jumikis (1) We hope to confirm that the main difference between artificial and natural freezing is the orientation of freezing direction. (2) In his study in passive freezing there exists stationary heat flow. We want to find out if there is any possibility for a stationary growth of an ice layer in the freezing interface. (3) We should like to know about the creep property of the soil-ice wall. (4) Which textures are to be preferred and how can they be achieved?

Young/Cheung/Sheeran Some questions that might be addressed in the discussion of this paper are: (1) Is the assumption that the classical thermodynamic theory derived for solutions can be combined additively with a result obtained from a concomitant application of double layer theory justifiable? (2) To what extent is the earlier theory inadequate at low electrolyte concentration (assuming that the actual concentrations after equilibrating with the partially soluble clay can be determined)? (3) Is the area CDE in fig.5 which is equivalent to AHf clearly enough defined in riga to permit accurate measurements of phase composition data by this method?

Pusch (1) We would like to see if he thinks of any quantitative expression for the degree of particle aggregation or the density of particle aggregates. (2) We want to know the method he used for measuring the unfrozen water content. (3) We appreciate his comment about whether the behaviour of a remolded sample comes closer to that of undisturbed samples when freezing and thawing are repeated.