Ground freezing ′85 — A summary

Ground freezing ′85 — A summary

ColdRegionsScienceand Technology, 13 (1987) 301-306 Elsevier SciencePublishers B.V., Amsterdam-- Printed in the Netherlands 301 Meeting Report G R ...

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ColdRegionsScienceand Technology, 13 (1987) 301-306 Elsevier SciencePublishers B.V., Amsterdam-- Printed in the Netherlands

301

Meeting Report

G R O U N D F R E E Z I N G "85 - - A S U M M A R Y T . H . W . Baker 1, H.L. Jessb er g er 2, B.D. Kay 3 and N. M a e n o 4 1Institute for Research in Construction, National Research Council of Canada, Otttawa, Ontario, K 1A OR6 (Canada) 2Department of Civil Engineering, Ruhr University, Bochum (West Germany) ZDepartment of Land Resource Science, University of Guelph, Guelph, Ontario (Canada) 4Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido (Japan)

(ReceivedJuly 14, 1986; accepted in revised form September 15, 1986)

ABSTRACT

INTRODUCTION

The Fourth International Symposium on Ground Freezing was held at the Keio Plaza Hotel Sapporo, Japan from August 5-7, 1985. The Symposium was organized by a National Committee under the chairmanship of Dr. S. Kinosita, Director of the Institute of Low Temperature Science, Hokkaido University. Two hundred and nine participants from 13 countries presented 107 papers that were reviewed and prepared into two proceedings volumes. Volume 1 was published by A.A. Balkema Publishers and Volume 2 was published by the Hokkaido University Press. The Symposium was divided into four main sessions: I ThermalProperties andProcesses (20papers); H Frost Action (24 papers); III Mechanical Properties and Processes (31 papers); IV Engineering Design and Case Histories (32 papers). Achievements in the understanding and use of artificial ground freezing can be seen in the increasing number of projects utilizing this technique to manage difficult problems in ground engineering. These symposia permit the sharing of experience, leading toward innovative and economical designs. The proceedings provide an excellent record of advances in basic knowledge to improve the state-of the-art in artificial ground freezing.

Artificial ground freezing has been used in construction projects in the last few decades, to stabilize earth materials and to control groundwater seepage. During this period many significant advances have been made in ground freezing technology, primarily associated with predicting the effects of freezing on soil properties, predicting the refrigeration requirements and accurate placement of freeze pipes. Three previous international symposia were held to bring together specialists to share and discuss their experiences with the use of artificial ground freezing in construction. The first symposium was held in Bochum, West Germany in 1978, the second in Trondheim, Norway in 1980 and the third in Hanover, New Hampshire, U.S.A., in 1982. At these symposia the participants presented excellent papers on the results of theoretical and experimental studies and case histories of successful construction experience. Although many advances have been made in each of these areas since the first symposium, it was felt by the International Organizing Committee that a gap still existed between theory and practice. Therefore, the objective of the fourth symposium was to emphasize the relation between theory and design and application in construction practice.

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IN M E M O R Y OF DR. TSUTOMU TAKASHI Dr. Tsutomu Takashi, who passed away on August 6, 1983, has left us a rich legacy of theory and experimental work describing the flow of water during the freezing of weak, highly compressible soils. He was involved with the use of artificial ground freezing in Japan, particularly related to the large sewers and subways constructed in Tokyo and Osaka. Most of this work was carried out between 1958 and 1983, a period he spent almost entirely as Director of the Technical Research Division of Seiken Company Limited. Dr. Takashi's papers at the first three International Symposia on Ground Freezing spanned the spectrum from theoretical studies to applied engineering studies. His contributions at the Fourth International Symposium on Ground Freezing, in his home country, were sadly missed.

SESSION 1. THERMAL PROPERTIES A N D PROCESSES

heave rate, attained when ground freezing produces soil-free ice lenses. They found that the critical freezing rate is a linear function of the temperature gradient in the soil.

1.2 Unfrozen water in frozen earth materials Five papers were presented in relation to the behaviour of unfrozen water in earth materials. Iwata's and Kuroda's papers were based on theoretical treatments and papers by Horiguchi, Xu and Chen and Wong included mainly laboratory results. Theoretical treatments of unfrozen water of necessity make many important assumptions of the physical states and conditions. It was suggested that more accurate experimental information should be gathered before theoretical analyses can be reliably applied to real soil systems. Laboratory studies of unfrozen water were based on the DSC (differential scanning calorimeter), thermometry and NMR (nuclear magnetic resonance) techniques.

1.3 Other thermal properties 1.1 Freezing behaviour of earth materials Ten papers in total were concerned with the freezing behaviour of earth materials. Smith and Riseborough discussed possible errors that may arise in thermal predictions by common assumptions of thermal properties of frozen soils, and McCabe and Kettle pointed out the effect of the addition of coarse particles in transforming a highly frost susceptible matrix to a non-frost susceptible mixture. Studies of ground freezing with liquid nitrogen and underground cryogenic cavities were reported by Jessberger, Cames-Pintaux and others. Mizoguchi and Nakano described observations of vertical profiles of water content, electrical conductivity and temperature during freezing of unsaturated soils. Seasonal ground freezing and thawing studies were reported by Tsuchiya and others for specific locations in Japan. They showed the important effect of snow cover on the maximum frost depth. Kubo and others discussed the accuracy of the freezing index estimated from general daily meteorological data. Takeda et al. presented studies of the maximum

Xia presented an analysis of the mechanics of thermal crack formation in frozen ground and showed the large influence of soil type, water content, vegetation and snow cover on the spacing of cracks. Aguirre-Puente et al. presented a study of the sublimation rate of ice in porous media and developed a theoretical computation model. Observations of thermal conductivity were presented by Sawada and Ohno for clay, silt and sand in both the frozen and unfrozen states. Horiguchi and Maeno discussed pressures developed during the freezing of water drops and rigidly confined water. The breakdown of supercooling was noted to cause pressure increases as large as 30 MPa.

1.4 Future needs The chemical composition and physical structure of earth materials are so complex that their mechanical, thermal, electrical and other physical properties cannot be described uniquely by only giving bulk densities, particle sizes, temperature, and total water content. It is postulated that the

303 included water may exist in various physical states and that the freezing of water in earth materials causes far more complicated processes. Two points should be mentioned here: one is the characterization of materials and the other is the definition of terminologies. As mentioned above, earth materials are quite complicated mixtures; the normal characterization method for soils is not concise enough for one earth material to be adequately compared with another. The characterization of materials, especially in relation to the physical states of in situ water is one of the most important subjects that needs further investigation. The physical meaning of terminologies used in the presented paperg often seems to be different; for example, activation energy and unfrozen water. There is no doubt that physical quantities should be clearly defined and be understandable to other researchers.

SESSION III. FROST ACTION 2.1 Segregation potential Papers presented in the session on frost action clearly indicated improved communication between the engineers who need to predict the response of soil materials to ground freezing using a minimum number of field measurements and the scientists who are pursuing that elusive but comprehensive theory of frost heaving. O f particular significance is the development of frost susceptibility tests which had utilized concepts evolving through studies of water flow in freezing soils. This type of interaction was illustrated, for example, in several papers which assessed the utility of "segregation potential" (Konrad and Morgenstern, 1981) expressed in terms of readily measured parameters such as liquid limit and clay content. Several papers presented in the frost action session utilized the concept of "segregation potential"; however, this concept may be applicable only during the transient stage of freezing. Data presented by Ishizaki and Nishio showed heave rate could either increase or decrease with an increase in temperature gradient depending on how the gradient

was established and at what stage in the heaving process the analysis is done. This paper raised serious questions about the generality of the "segregation potential" concept. These authors introduced an alternative approach to predicting frost heave which is relatively simple and should be adaptable to field conditions. In this case, the rate of frost heave under a wide variety of thermal conditions was found to be linearly related to the temperature at the base of the last ice lens to form.

2.2 Thermal regime The experimental conditions used in an increasing number of laboratory studies are approximating temperature gradients observed under field conditions and studies are being carried out over extended periods of time thus making extrapolation to field conditions more credible. For instance, studies reported by Xu et al. used temperature gradients as low as 0.02 ° C/cm and studies reported by Ohrai and Yamamoto extended for more than 6000 hr. Akagawa et al. defined 3 stages of frost heaving: transient, steady state and long-term. Although it is doubtful if different mechanisms are operating in these different stages, the relative significance of different mechanisms may change. It is clearly important that laboratory oriented research focus on all three stages using thermal regimes appropriate to field conditions and measurement times which are much longer than previously used, if laboratory measurements are to be effectively extrapolated to field conditions.

2.3 Secondary frost heaving The concept of secondary frost heaving and the formation of an ice lens at the point where the effective stress is zero (Miller, 1978) has provided an insight into the formation of banded ice lenses and water flow as frost heaving progresses from a transient to a steady state condition. However, the study of Ohrai and Yamamoto provided convincing evidence ofregelation and the simultaneous growth and disappearance of ice lenses in the frozen zone behind the last ice lens. Unless this can be attributed to minor changes in temperature gradients it is not

304 immediately obvious how this observation can be explained by the model of frost heaving proposed by Miller (1978).

2.4 Variability in field conditions Spatial variability in soil conditions, and temporal variability in thermal conditions in the field, continue to present a major barrier in the application of models generated under laboratory conditions to the conditions existing in the field. Considerable effort is being devoted in other disciplines concerned with water flow, in overcoming this barrier. However, there was little evidence, on the basis of papers presented at this symposium, that researchers associated with ground freezing are really addressing this problem yet.

2.5 Computer models of frost heaving Considerable effort has been expended in the last decade on developing computer models which describe the frost heaving process. A relatively small number of papers were presented on this topic suggesting that this area of activity may have achieved a degree of maturity. Efforts are currently being redirected into two areas: (a) simplifying the models so that microcomputers can be used, thereby diminishing the considerable cost associated with running many of the earlier models which describe coupled heat and water flow, and (b) trying to understand more completely the processes associated with frost heaving before expending further effort on computer modelling. Both approaches are particularly appropriate, given current needs in the field and the gaps in our existing knowledge of the processes of frost heaving.

SESSION III. MECHANICAL PROPERTIES AND PROCESSES 3.1 Physics and mechanics of frozen, freezing and thawing soils This session included 35 papers on the physics and mechanics of frozen, freezing and thawing soils. The physics papers were concerned with the relation-

ship between the physical and chemical properties and the processes of freezing and thawing. The mechanical studies examined the laws governing the behaviour of the frozen, freezing and thawing soils under load and established relationships between stress, strain, temperature and time. These relationships were reported for use to assist in the design of artificially frozen earth structures and to predict their performance. Several papers were presented on the mechanical properties of frozen soils under static and dynamic loading in tension, compression and shear. Four papers by Baker and Konrad, Ebel, Kinosita and Ryokai and Vyalov et al. discussed procedures for laboratory strength testing in support of artificial ground freezing. Of the four papers on modelling the creep behaviour of frozen earth structures, two (Berggren and Furuberg, Fish) present phenomenological creep models. The effect of freeze-thaw cycling was discussed in relation to the physical and mechanical properties and to frost action. One paper by Blanchard and Fremond proposed a model to determine the deformations associated with frost heave and subsequent thaw settlement. Physical properties were presented by Bourbonnais and Ladanyi for frozen sand and clay at low temperatures down to - 160°C.

3.2 Standardization of testing procedures Several groups are presently looking at the development of guidelines for determining the physical, thermal and mechanical properties of frozen soils using existing laboratory equipment and technology. These groups include: (a) ISGF Working Group on Testing of Frozen Soils under the chairmanship of Mr. F.H. Sayles, USA-CRREL, Hanover, N.H., U.S.A. The first report of this Committee was published in Volume 2 of the Proceedings of the Third International Symposium on Ground Freezing. (b) Japanese Committee on Mechanical Properties of Frozen Soils under the chairmanship of Dr. S. Kinosita, Director of the Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. This Committee is affiliated with the Japanese Society of Soil Mechanics and Foundation Engineering. The first report of this Committee is

305 published in Volume 2 of the Proceedings of the Fourth International Symposium on Ground Freezing. (c) Permafrost Subcommittee of the Associate Committee on Geotechnical Research, National Research Council of Canada under the chairmanship of Mr. G.H. Johnston, Institute for Research in Construction, National Research Council of Canada. This Committee is sponsoring a laboratory testing manual on permafrost. The Editorial Committee is chaired by Mr. J.A. Heginbottom, Geological Survey of Canada, Ottawa, Canada. Standardization of field and laboratory testing procedures is required to provide a basis for comparison, statistical treatment, and correlation of data from different sources. Of particular need are procedures related to the sampling, handling and laboratory freezing of test specimens used to determine the mechanical properties required for the design of artificially frozen earth structures.

3.3 Physical and mechanical properties of saline soils The physical and mechanical properties of frozen saline soils is of interest to many researchers, particularly those working in marine environments. The mechanical properties of frozen saline soils are greatly affected by the presence of even small amounts of salt in the range from 0 to 5 ppt. Of particular interest in understanding the behaviour of frozen saline soils is the effect of salt concentration and temperature on the unfrozen (salt) water content and the resultant changes in cohesion between the interstitial ice and the soil matrix. The influence of the soil type on the ice--salt water equilibrium is of great importance.

3.4 Numerical modelling Numerical models to simulate and predict the ground movements associated with freezing and thawing should be pursued, particularly in regard to phenomenological relationships. Performance monitoring of artificially frozen earth structures is required to check the model predictions.

SESSION IV. ENGINEERING DESIGN AND CASE HISTORIES 4.1 Tunnels and ground freezing The papers on tunnelling are related to model tests in the laboratory, description of case histories, and summarizing the ground freezing work performed in Japan. One paper by Huang and Speck is related to a tunnel in permafrost at Fairbanks, Alaska. The application of ground freezing in tunnelling is used increasingly worldwide. Special methods for drilling, freezing, excavating and lining of tunnels have been developed, but extra precautions have to be taken in cases where buildings, structures or even rivers are to be underpassed.

4.2 Pipelines The investigation of the influence of heated or chilled pipelines on the surrounding ground and vice versa is an extremely important field of research including practical application. This was discussed not only at the ground freezing symposia but also at other conferences. Despite laboratory investigations and numerical calculations it seems to be necessary to work with well-instrumented experimental pipelines in order to evaluate various influences and to adjust physical and mathematical models.

4.3 LNG storage tanks The technique of storing liquified natural gas (LNG), in tanks in the ground, was developed and used to a large extent in Japan. In this long-term experience it was found that there is a need for largescale in situ tests to obtain the frost heave pressures. Horizontal pressures are included by thermal stresses and frost heave. In order to reduce this pressure, heating systems are installed surrounding the tanks.

4.4 Mine shafts Three papers dealt with the design of frozen shafts, trying to find or to reconfirm an applicable design method for this very significant and complicated problem. Obviously there are successfully

306 applied design methods available. In the reporter's opinion it is not desirable to have just one single design method. Due to the complexity of the problem, each method will have short-comings. For large frozen shaft projects a comparison could be made between the results of several design methods.

4.5 Future design needs The number of practical applications of artificial ground freezing has increased. Therefore, it seems to be valuable to continue the exchange of knowledge and experience in this field. An increasing cooperation between scientists and practically oriented engineers can be recognized. The scientists contribute to the solution of important problems and the engineers appreciate the advantage of basic research. The benefit of this positive development is that in the future more precise design methods not only will be published but also will meet the needs of the designing engineer.

It is expected that new numerical models will be developed for application in design. If new material parameters are required by the models, methods for establishing these parameters will be required also. For the upcoming ISGF '88 (July 26-28, 1988) in Nottingham, England, it is desirable to strengthen the cooperation between scientists and engineers. Furthermore, the effort should be concentrated to meet the requirements of the end users who are contractors and clients. The end users should be encouraged to document and publish their results and case histories.

REFERENCES Konrad, J.M. and Morgenstern, N.W. (1981). The segregation potential of a freezing soil. Can. Geotech. J., 18: 484-491. Miller, R.D. (1978). Lens initiation in secondary heaving. Proc. Int. Symposium Frost Action in Soils, Lulea, Sweden, Vol. 2, pp. 68-74.