Overconsolidation effects of ground freezing

Engineering Geology, 18 (1981) 97--110

97

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

OVERCONSOLIDATION EFFECTS OF GROUND FREEZING

EDWIN J. CHAMBERLAIN

U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, N.H. (U.S.A.) (Accepted for publication February 4, 1981) ABSTRACT

Chamberlain, E.J., 1981. Overconsolidation effects of ground freezing. Eng. Geol., 18: 97--110. Temporary ground freezing is a valuable technique for stabilizing soft soils during construction. It imparts large increases in strength and bearing capacity to most soils. However, freezing can cause significant changes in soil structure and density which can lead to adverse settlement during thaw. Settlement of clay soils after freezing and thawing is the result of the suction forces that draw pore water to the freezing front. These suction forces cause an increase in the effective stress on the clay beneath the freezing front, and thus cause an overconsolidation of the clay. As these suction forces often exceed 1 atm, their direct measurement is not easy. A technique for indirectly determining the maximum suction occurring during freezing is presented which utilizes the apparent memory that clay soils have for maximum past (preconsolidation) pressures. Suctions as large as 532 kN m -2 were observed after freezing and thawing a clay soil which was initially consolidated to 128 kN m -2. The volume changes resulting from the freezing and thawing of clays were related to the plastic limit and were observed in the laboratory to be as high as 25%. If provisions are n o t made to account for these volume changes in a ground freezing project, considerable damage to structures can occur from settlement and the resulting stresses. INTRODUCTION

Ground freezing is now well established as a valuable technique for temporarily stabilizing soils during construction. Freezing of soils can be used to impart large increases in strength and bearing capacity and to cut off groundwater flow. Ground freezing also occurs as a consequence of liquid gas s~torage and as a natural occurrence in seasonal frost and permafrost regions. At the First International Symposium on Ground Freezing in Bochum, West G ermany, we learned of the problems of artificial ground freezing for construction. During this conference, the basic principles of ground freezing and the processes o f ice segregation and frost heaving were explored, and attempts were made to determine the freezing performance of full-scale structures. The potential consequences of thawing frozen ground, however, were explored very little. Thawing of frozen ground can be an important aspect in

98

the evaluation of ground freezing projects since freezing can cause significant changes in soil structure and density which can lead to adverse settlements during thaw. This report will discuss the consolidation of soil during thaw and its possible effects on ground freezing projects. FREEZE-THAW CONSOLIDATION THEORY

In order to understand the freeze-thaw consolidation process, it is important to f i ~ t review the physical processes involved. Nixon and Morgenstern (1973) have shown that this process is best presented in terms of effective stress. T h e effective stress p is equal to the total or applied stress p minus the excess pore water pressure u: p=p--u

Figure 1 illustrates the process. A clay soil is fully consolidated (u = 0) to point a on the virgin compression curve where the effective stress is equal to the applied stress. The sample is then frozen unidirectionally with free access to water and, in terms of total stress, undergoes a net increase in void ratio to point b due to the expansion of water to ice and the intake of water from the reservoir to form segregated ice. During freezing, however, the large negative pore-water pressures that develop cause an increase in the effective stress immediately below or within the region of freezing. Discrete bands of

\Virgin Compression Curve \

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Total stress curve during freezing and thawing for the bulk sample \ a ~ E f f e c t i v e stress curve during freezing "~ i/h/~wingwithin discrete clay layers

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> CP

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b and b' Log Total Stress O-or Effective Stress 0-' Fig.1. Theorized thaw consolidation process. (After Chamberlain and Gow, 1978.)

99

soil and ice form as freezing progresses and the soil bands are overconsolidated to point b'. Upon thawing, the effective stress path within the discrete bands of soft is depicted along line b--c to point c where the pore pressures are in equilibrium with the applied load, and the material has undergone a net decrease in void ratio from point a to point c. The superficial manifestation of this process is that the soil consolidates during thaw. However, the consolidation of the soil occurs during freezing and the observations made during thaw are of the extrusion of the excess melt water from the thawing segregated ice. LABORATORY OBSERVATIONS Thaw strain

Chamberlain and Blouin (1978) showed that large decreases in the void ratio of fine-grained dredged material slurries could be caused by freezing and thawing. For instance, Fig.2 shows that the void ratio of a dredged clay material with a liquid limit of 71% and a plasticity index of 41% was significantly reduced by freezing and thawing. In terms of volume change, Fig.3 shows that this material and several others were reduced in volume by as much as 25% by freezing and thawing. There appears to be a practical limit to which soils can be overconsolidated by freezing and thawing. (The theoretical limit is, of course, the shrinkage limit.) Fig.4 shows a plot of volumetric strain versus the ratio of the initial water content w o to the plastic limit wp for a number of materials studied by Nixon and Morgenstern (1973), Chamberlain and Blouin (1978) and Chamberlain and Gow (1978). The zero thaw strain intercept of the best linear fit line falls very close to a wo/wp ratio of 1. The plastic limit, thus, 3.5

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Fig.4. Freeze--thaw strain as a function of the ratio of initial water content to plastic limit.

101 appears to be the minimum water content that can be obtained by freezing and thawing. This concurs with Tsytovich's (1975) observation that "the density of soil aggregates may become quite high (during freezing), equaling the density of clays at the plastic limit". The best-fit line shown in Fig.4 was obtained by the method of leastsquares linear regression. The standard deviation is 4% strain and the correlation coefficient is 0.80. This plot allows the determination of the potential thaw strain from knowledge of the initial water content and the plastic limit. For a particular ground freezing project, the potential thaw strain may be estimated by knowing wo and Wp. If wo is equal to Wp, then little or no thaw strain would be expected. If, however, wo is equal to 2 or 3 times wp, then 13 or 25% -+ 14% thaw strain may be expected for most conditions. It should be noted that this is the potential thaw strain. For this to occur, free drainage of the melt water must be possible. Preconsolidation pressure

As previously discussed, overconsolidation of soils during freezing is caused by the increase in effective stress in the material beneath the freezing front. The negative pore-water pressures that cause the effective stress increase can be measured with tensiometers if the tension does not exceed I arm (actual experience at C R R E L indicates that 0.8 atm is the practical limit).Above 1 arm, water cavitates in a tensiometer. For tensions exceeding Iatm, Martin and Wissa (1973) found that by back-pressuring the pore water, measurements of the reduction in positive pore-water pressure during freezing could be made and the moisture tension calculated. Another, simpler method is proposed by the present author. This method relies on the apparent m e m o r y that fine-grained soilshave for m a x i m u m past (preconsolidation) pressures. Fig.5 illustratesthe method as applied in the laboratory. A clay slurry was consolidated in increments to a normal pressure Pn = 16 kPa and subjected to freeze--thaw cycling. O n completion of three freeze--thaw cycles, the pressure was again increased in increments until the loading curve approached the virgin consolidation curve. The preconsolidation pressure Pc was then constructed using the empirical method of Casagrande (1936). The difference between pn and Pc is the effective negative pore-water pressure resulting from freezing (uf). In this case uf = --240 kPa or approximately --2.5 atmospheres, a much larger value than can be measured with simple tensiometers. Figure 6 shows that for a higher initial value ofpn (128 kPa), u~ is even larger (--532 kPa or approximately --5 atmospheres). Rate o f thaw consolidation and thaw permeability

Chamberlain and Blouin (1978) noted that consolidation always occurred m u c h more rapidly after a thaw than after the application of individual load increments. For instance, Fig.7 shows that the coefficient of

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Fig.5. Determination of the preconsolidation pressure Pc and the effective pore-water tension uf after freezing for Ellsworth clay at an initial consolidation pressure Pn = 16 kPa. consolidation for the thaw-consolidated case can be as m u c h as two orders o f magnitude greater than t h a t for the normally consohdated, never frozen conditions. T he consolidation coefficient is increased by freezing and thawing because the permeability is increased. F o r instance, Fig.8 shows the effect of freezing and thawing on the vertical permeability (permeability in the direction o f freezing) o f r e m o l d e d Ellsworth clay soil which has a liquid limit of 45% and a plasticity index of 20~o. It can be seen t h a t even t hough decreases in void ratio occur as a result o f freezing and thawing, very large increases in vertical permeability occur. A t low applied effective stresses (P = 1.71 kPa, f o r instance) the permeability is increased by a fact or of 100 while the void ratio is decreased by 29% after freeze--thaw cychng. At higher applied effec-

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tive stress ~ = 140 kPa, for instance) both changes were smaller, the permeability increasing by a factor of 24 and the void ratio decreasing by 26%. Similar results (Fig.9) were obtained more recently for permeability measurements made in the horizontal plane normal to the direction of freezing. Thin sections and thaw permeability Thin sections of frozen samples made in the horizontal and vertical planes reveal features that explain the increase in permeability after thawing. Fig.10, for instance, shows the thin sections made for the clay material described above. The vertical thin section (photo A) reveals intersecting vertical and horizontal ice features, the vertical ice-filled cracks being 2--5 mm apart and the horizontal ice lenses being only a few tenths of a millimeter apart. The horizontal thin sections taken at elevations marked by arrows at b and c are also shown (photos B and C) in Fig.10. It can be seen that the vertical cracks are actually closed polygonal features. These vertical ice-filled cracks account for the increase in vertical permeability that occurs in the thawed state. In Fig.ll, it can be seen that these cracks remain as discontinuities, even in the thawed soil. The vertical permeability increases in the thawed soft because flow occurs through the shorter, less resistant crack paths rather than through the more tortuous interstitial

104

paths as occurs in the never frozen soil. The same argument m a y be made for the increase in horizontal permeability; the discontinuities left b y the melted ice lenses provide paths of reduced flow resistance. The vertical icefilled cracks and the horizontal ice lenses need n o t be continuous to cause an increase in the permeability as these features frequently intersect (Fig.10). Soils do n o t have to develop either the vertical cracks or the horizontal ice lenses for the increase in the permeabilities to occur. For instance, the vertical permeability of a non-plastic silt soil was observed to be increased significantly by freezing and thawing (Fig.12), but magnified thin sections of the frozen material revealed no visible ice features. The increased permeability in this case is probably a result of a rearrangement of clay particles in the void space formed b y the coarser particles, and thus, a decreased flow resistance through the voids. SETTLEMENT OF GROUND FREEZING PROJECTS

A survey of the literature has revealed little discussion of settlement problems during the thawing o f ground freezing projects. Jones and Brown (1978) briefly discussed thaw settlement in relationship to ground freezing projects. According to these authors, Endo (1969) observed thaw settlement for a subway construction project to be a b o u t 20% greater than the a m o u n t of heave occurring during the freezing period. 1

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105

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Fig.8. Vertical permeability for Ellsworth clay. (After Chamberlainand Gow, 1978.) The fact that settlement associated with thawing at ground freezing projects has not been widely reported does not necessarily mean that it has not occurred. In many cases, the builtAn factor of safety in the design of structures placed in excavations made during ground freezing projects may have precluded thaw settlement. Other factors, such as the absence of free drainage for melt water and arching of the thawed soil adjacent to structures may also influence the potential thaw settlement. Nonetheless, thaw settlement should be considered when designing ground freezing projects. Two examples of the types of problems that may occur if precautions are not taken are illustrated in Fig.13, Figure 13a shows that thaw settlement may occur adjacent to a vertical shaft and that this settle-

106

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Fig.11. Polygonal cracks in thawed Toledo Island dredged material. (After Chamberlain and Blouin, 1978.)

108 I

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ment may affect nearby structures as well as the shaft liner itself. Down-drag on slender well castings in thawing permafrost has been a major consideration in the design of casings in Prudhoe Bay (Goodwin, 1978). Tunnels excavated using the ground freezing technique for stabilization may also be affected by settlement during thaw (Fig.13b). Problems may occur both from a loss of support for the tunnel liner and settlement. Differential settlement may be a particularly severe problem in transition regions at the boundaries of a ground freezing project or where marked differences in soil type occur. Other ground freezing projects, such as retaining walls and foundations, may also be affected by thaw settlement. Methods of determining the amount and rate of thaw settlement have been reported by Morgenstem and Nixon (1971), Tsytovich (1975), and several others. Particular care must be taken in selecting the consolidation coefficients, because, as previously noted, these factors can be greatly affected by freezing.

109 CONCLUSION

Freezing of soft clayey soils can cause a significant change in soil properties. Plastic soils may be overconsolidated by freezing. Thawing of these softs may result in large settlements. The amount of settlement appears to be linearly related to the ratio of the initial water content to the plastic limit, the maximum amount of thaw consolidation being determined by the plastic limit. Preconsolidation pressures can greatly exceed in-situ prefrozen pressures because of large increases in pore-water tension during freezing. The preconsolidation pressure and the pore-water tension due to freezing can be indirectly determined from the void-ratio/effective-stress plot by applying the method of Casagrande (1936) to the loading curve of the thawed material. Both vertical and horizontal permeabilities of soft plastic softs can be greatly increased by freezing. This results in much higher consolidation coefficients during thaw.

After F

froze1 Soil

After

Thowing

t Bedrock

(a)

~'//.~,~. ,~

Tunnel

Frozen Soil

Thaw Settlement (b)

Fig.13. Possible effects of thaw settlement on ground freezing projects.

110

Few cases of adverse thaw settlement at ground freezing projects have been reported. However, the potentially large settlements and rapid consolidation times should be considered in ground freezing projects to preclude structural damage not previously noted, particularly when working in soft clayey soils. ACKNOWLEDGEMENTS

I wish to acknowledge the contributions of S.E. Blouin and A.J. Gow to this work and the financial support of the U.S. Army Cold Regions Research and Engineering Laboratory and the U.S. Army Corps of Engineers Dredged Material Research Program. F.H. Sayles and B.D. Alkire provided valuable reviews of the manuscript. REFERENCES Casagrande, A., 1936. The determination of the preconsolidation load and its practical significance. Proc. 1st Int. Conf. Soil Mech. Found. Eng., Cambridge, Mass., 60 pp. Chamberlain, E.J. and Blouin, S.E., 1978. Densification by freezing and thawing of fine material dredged from waterways. Proc. 3rd Int. Conf. Permafrost, Edmonton, Alta., pp.623--628. Chamberlain, E.J. and Gow, A.J., 1978. Effect of freezing and thawing on the permeability and structure of soils. Proc. Int. Syrup. Ground Freezing, Ruhr-Univ. Bochum, pp.31--44. Endo, K., 1969. Artificial Soil Freezing Method for Subway Construction, Jpn. Soc. Cir. Eng. Goodwin, M.A., 1978. Handbook of Arctic Well Completion. World Oil. Jones, J.S. and Brown, R.E., 1978. Design of tunnel support system using ground freezing. Proc. Int. Syrup. Ground Freezing, Ruhr-University, Bochum, pp.235--253. Martin, R.T. and Wissa, A.E.Z., 1973. Frost susceptibility of Massachusetts soils -- evaluation of rapid frost susceptibility tests. MIT. Soils Publ., 320, R73-60:256 pp. Morgenstern, N.R. and Nixon, J.F., 1971. One-dimensional consolidation of thawing soils, Can. Geotech. J., 8: 558--565. Nixon, J.F. and Morgenstern, N.R., 1973. The residual stress in thawing soils. Can. Geotech. J., 10(4): 571--580. Tsytovich, N.A., 1975. The Mechanics of Frozen Ground (English translation edited by G.K. Swinzow.) McGraw-Hill, New York, N.Y., 426 pp.