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Quickclays as products of glacial action: a new approach to their nature, geology, distribution and geotechnical properties—a short comment
Engineering Geology, 7 (1973) 359--363 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Discussion
QUICKCLAYS AS PRODUCTS OF GLACIAL ACTION: A NEW APPROACH TO T H E I R N A T U R E , GEOLOGY, D I S T R I B U T I O N AND GEOTECHNICAL P R O P E R T I E S - - A S H O R T COMMENT
C.F. MOON Department of Civil Engineering, Sheffield Polytechnic (Great Britain) ( A c c e p t e d for p u b l i c a t i o n May 2, 1974)
There appears to be a certain b o d y of evidence in support of the approach to the quickclay failure mechanism postulated b y Cabrera and Smalley (1973). Experimental work performed in the past shows that aqueous mixtures of very fine quartz particles of the order of 2pm diameter exhibit unusual rheological properties (e.g. Freundlich and Juliusburger, 1935). Simple tests carried o u t by myself show that such a material having a water content of a b o u t 35% exhibits a p h e n o m e n o n termed shear-rate blockage (Boswell, 1961) which is probably best described as a t y p e of "super-dilatancy". The shear strength of this material is totally dependent u p o n the rate of shear to which it is subjected. In the undisturbed state, the material has a reasonable stability, however, a gentle deformation (especially gentle vibration) causes it to take up a fluid consistency which flows quite easily. A rapid deformation on the other hand, causes the strength to drastically increase and in fact the material behaves as a brittle solid under these conditions. It appears that the fluid--solid transition is abrupt and occurs at a well-defined threshold level of shear rate. This effect is shown purely diagrammatically in Fig.1. If the shearing is stopped the material immediately reverts to its initial condition. It is of interest to note that Pryce-Jones (1948) observed a similar effect with a starch--water mixture. This liquefaction of an otherwise solid material at a low shear rate compares favourably with the observations of quickclay phenomena cited b y Cabrera and SmaUey. If this then is correct, it seems that many workers (e.g. Ackermann, 1948) have focussed their attention on the wrong rheologicat property, i.e., t h i x o t r o p y which can be regarded as the antithesis of dilatancy. However, it should be borne in mind that as certain authorities have observed, the addition of a small a m o u n t of true clay material to a dilatant soil may induce thixotropic properties (Boswell, 1961). So here, the matter becomes somewhat: enigmatic. An interesting fact arises concerning the shape of the quartz particles which the authors suggest are plate-shaped. Freundlich and Juliusburger (1935) in experiments with fine quartz p o w d e r (1--15~m diameter) also suggest this: "Concentrated suspensions of this fine quartz powder in water or aqueous solutions show, when stirred, streaks of a silky lustre, a sign that the particles
360 "1"
f brittle solid
Z UJLu Lf)
semi-solid
0
Lfluid
I
JP
SHEAR RATE threshold level
Fig.l, Diagrammatic explanation of shear rate blockage. are distinctly non-spherical." Although I have n o t observed this personally, it seems feasible if a cleavage control does operate in the fracturing of small quartz particles as the authors suggest. From the evidence above and that presented by Cabrera and Smalley there emerges a strong suggestion that quickclays could be composed of a certain percentage of inactive particles of a super-dilatant nature. There are t w o minor points contained in the paper with which I do n o t agree. The authors state (p.126) that a 2-/~m diameter non-clay particle will settle in water at about 2/~m/sec. This assumes an equivalent spherical diameter of 2/~m. If the particles are tabular as postulated, taking the frictional drag on the particle into account, the settling velocity will surely be much lower than the quoted value. Secondly, Cabrera and Smalley quote from a review of mine ( p . l 1 9 ) in which I have said that single particle clay mineral structures do n o t exist (Moon, 1972). I did in fact, state that these structures are almost certainly non-existent in a sedimented clay b u t on the available evidence it seems likely that they are present in a clay which is sedimenting.
361 REFERENCES Ackermann, E., 1948. Thixotropie und Fliesseigenschaften feink6rniger BSden. Geol. Rundsch., 36: 126--134. Boswell, P.G.H., 1961. Muddy Sediments. Heifer, Cambridge, 140 pp. Cabrera, J.G. and Smalley, I.J., 1973. Quickclays as products of glacial action: a n e w approach to their nature, geology, distribution and geotechnical properties. Eng. Geol., 7: 115--133. Freundlich, H. and Juliusburger, F., 1935. The plasticity of powdered slate from Solnhofen and the thixotropic behaviour of its suspensions. Trans. Farad. Soc., 30: 333--338. Moon, C.F., 1972. The microstructure of clay sediments. Earth-Sci. Rev., 8: 303--321. Pryce-Jones, J., !948. The flow of suspensions. Thixotropy and dilatancy. Proc. Durham Univ. Phil. Soc., 10: 427--467.
QUICKCLAYS AS PRODUCTS OF GLACIAL ACTION: A NEW APPROACH TO THEIR NATURE, GEOLOGY, DISTRIBUTION AND GEOTECHNICAL PROPERTIES - - A REPLY
I.J. SMALLEY and J.G. CABRERA
Department of Civil Engineering, University of Leeds, (Great Britain) Department of Civil Engineering, Federal University of Paraiba (Brazil) (Accepted for publication May 2, 1974)
Several of the points raised by Moon (1974) merit further discussion, in particular the observation about particle shape. At about the same time as the original paper (Cabrera and Smalley, 1973) was published a paper by Moss et al. (1973) appeared which casts a new light on the quartz particle platelet problem. Moss et al. pointed out that quartz in granitic rocks (the source of most sedimentary clastic quartz) contained many defects and that these could influence subsequent particle production. If, as has been suggested (Carter, 1965), plastic deformation in quartz occurs essentially by slip along an (0001) plane and this deformation can occur in granitic quartz before it is released as sedimentary particles (Smalley, 1974b) then the shape observations made on small quartz particles become more comprehensible. Another point concerns the nature of the inactive particles in quickclays. The original formulation of the short-range bond theory was based on a concept of quickclays composed essentially of quartz and feldspar particles, which could be compared to a soil which was dominated by active clay mineral particles. The terminology has been criticised by Fairbridge (private communication) and the concept by Rosenqvist (private communication). Fairbridge suggests using the term lithic to describe the non-clay mineral particles, and the suggestion may be useful if some modifications suggested by Rosenqvist are adopted. In particular the absolute division between clay minerals and non-clay minerals may be an unacceptable generalization; it may be more
QUICKCLAYS AS PRODUCTS OF GLACIAL ACTION: A NEW APPROACH TO T H E I R N A T U R E , GEOLOGY, D I S T R I B U T I O N AND GEOTECHNICAL P R O P E R T I E S - - A S H O R T COMMENT
C.F. MOON Department of Civil Engineering, Sheffield Polytechnic (Great Britain) ( A c c e p t e d for p u b l i c a t i o n May 2, 1974)
There appears to be a certain b o d y of evidence in support of the approach to the quickclay failure mechanism postulated b y Cabrera and Smalley (1973). Experimental work performed in the past shows that aqueous mixtures of very fine quartz particles of the order of 2pm diameter exhibit unusual rheological properties (e.g. Freundlich and Juliusburger, 1935). Simple tests carried o u t by myself show that such a material having a water content of a b o u t 35% exhibits a p h e n o m e n o n termed shear-rate blockage (Boswell, 1961) which is probably best described as a t y p e of "super-dilatancy". The shear strength of this material is totally dependent u p o n the rate of shear to which it is subjected. In the undisturbed state, the material has a reasonable stability, however, a gentle deformation (especially gentle vibration) causes it to take up a fluid consistency which flows quite easily. A rapid deformation on the other hand, causes the strength to drastically increase and in fact the material behaves as a brittle solid under these conditions. It appears that the fluid--solid transition is abrupt and occurs at a well-defined threshold level of shear rate. This effect is shown purely diagrammatically in Fig.1. If the shearing is stopped the material immediately reverts to its initial condition. It is of interest to note that Pryce-Jones (1948) observed a similar effect with a starch--water mixture. This liquefaction of an otherwise solid material at a low shear rate compares favourably with the observations of quickclay phenomena cited b y Cabrera and SmaUey. If this then is correct, it seems that many workers (e.g. Ackermann, 1948) have focussed their attention on the wrong rheologicat property, i.e., t h i x o t r o p y which can be regarded as the antithesis of dilatancy. However, it should be borne in mind that as certain authorities have observed, the addition of a small a m o u n t of true clay material to a dilatant soil may induce thixotropic properties (Boswell, 1961). So here, the matter becomes somewhat: enigmatic. An interesting fact arises concerning the shape of the quartz particles which the authors suggest are plate-shaped. Freundlich and Juliusburger (1935) in experiments with fine quartz p o w d e r (1--15~m diameter) also suggest this: "Concentrated suspensions of this fine quartz powder in water or aqueous solutions show, when stirred, streaks of a silky lustre, a sign that the particles
360 "1"
f brittle solid
Z UJLu Lf)
semi-solid
0
Lfluid
I
JP
SHEAR RATE threshold level
Fig.l, Diagrammatic explanation of shear rate blockage. are distinctly non-spherical." Although I have n o t observed this personally, it seems feasible if a cleavage control does operate in the fracturing of small quartz particles as the authors suggest. From the evidence above and that presented by Cabrera and Smalley there emerges a strong suggestion that quickclays could be composed of a certain percentage of inactive particles of a super-dilatant nature. There are t w o minor points contained in the paper with which I do n o t agree. The authors state (p.126) that a 2-/~m diameter non-clay particle will settle in water at about 2/~m/sec. This assumes an equivalent spherical diameter of 2/~m. If the particles are tabular as postulated, taking the frictional drag on the particle into account, the settling velocity will surely be much lower than the quoted value. Secondly, Cabrera and Smalley quote from a review of mine ( p . l 1 9 ) in which I have said that single particle clay mineral structures do n o t exist (Moon, 1972). I did in fact, state that these structures are almost certainly non-existent in a sedimented clay b u t on the available evidence it seems likely that they are present in a clay which is sedimenting.
361 REFERENCES Ackermann, E., 1948. Thixotropie und Fliesseigenschaften feink6rniger BSden. Geol. Rundsch., 36: 126--134. Boswell, P.G.H., 1961. Muddy Sediments. Heifer, Cambridge, 140 pp. Cabrera, J.G. and Smalley, I.J., 1973. Quickclays as products of glacial action: a n e w approach to their nature, geology, distribution and geotechnical properties. Eng. Geol., 7: 115--133. Freundlich, H. and Juliusburger, F., 1935. The plasticity of powdered slate from Solnhofen and the thixotropic behaviour of its suspensions. Trans. Farad. Soc., 30: 333--338. Moon, C.F., 1972. The microstructure of clay sediments. Earth-Sci. Rev., 8: 303--321. Pryce-Jones, J., !948. The flow of suspensions. Thixotropy and dilatancy. Proc. Durham Univ. Phil. Soc., 10: 427--467.
QUICKCLAYS AS PRODUCTS OF GLACIAL ACTION: A NEW APPROACH TO THEIR NATURE, GEOLOGY, DISTRIBUTION AND GEOTECHNICAL PROPERTIES - - A REPLY
I.J. SMALLEY and J.G. CABRERA
Department of Civil Engineering, University of Leeds, (Great Britain) Department of Civil Engineering, Federal University of Paraiba (Brazil) (Accepted for publication May 2, 1974)
Several of the points raised by Moon (1974) merit further discussion, in particular the observation about particle shape. At about the same time as the original paper (Cabrera and Smalley, 1973) was published a paper by Moss et al. (1973) appeared which casts a new light on the quartz particle platelet problem. Moss et al. pointed out that quartz in granitic rocks (the source of most sedimentary clastic quartz) contained many defects and that these could influence subsequent particle production. If, as has been suggested (Carter, 1965), plastic deformation in quartz occurs essentially by slip along an (0001) plane and this deformation can occur in granitic quartz before it is released as sedimentary particles (Smalley, 1974b) then the shape observations made on small quartz particles become more comprehensible. Another point concerns the nature of the inactive particles in quickclays. The original formulation of the short-range bond theory was based on a concept of quickclays composed essentially of quartz and feldspar particles, which could be compared to a soil which was dominated by active clay mineral particles. The terminology has been criticised by Fairbridge (private communication) and the concept by Rosenqvist (private communication). Fairbridge suggests using the term lithic to describe the non-clay mineral particles, and the suggestion may be useful if some modifications suggested by Rosenqvist are adopted. In particular the absolute division between clay minerals and non-clay minerals may be an unacceptable generalization; it may be more