factors relating to the landslide process in canadian quickclays

EARTH SURFACE PROCESSES, VOL. 1, 163-172 (1976)

FACTORS RELATING TO THE LANDSLIDE PROCESS IN CANADIAN QUICKCLAYS

IAN SMALLEY

Glacial Soils Project, Department of Civil Engineering, University of Leeds, Leeds, England

SUMMARY

The formation of, and landsliding in extremely sensitive postglacial clay soils (quickclays). could be affected by certain factors including mineralogical composition, interparticle bond type, soil structure, leaching, cementation and particle coatings. Factors which should be taken into account during related investigations include nomenclature and size limits, model soils, distribution. rheology, scientific status, local earthquake intensity and frequency. Landsliding in quickclays occurs largely by means of catastrophic flowslides. These are due to the predominance of short range bonds in the soil system. The short range bonds ensure a very low plasticity index (of the order of 10 or below) and may be due to the inherent nature of the material or to post-depositional events. Mineralogical studies suggest that the former is more critical although cementation plays a major role in Canadian quickclays. The Yatsu philosophy of material investigation should be applied to quickclays on a larger scale: landsliding and slope evolution are ultimately controlled by the forces operating (or not operating) between two adjacent soil particles. A particular set of tectonic, glacial and sedimentological circumstances are required for quickclay formation ; the correct combination rarely occurs which accounts for the very limited occurrence of clay soils with sensitivities of greater than 100.

INTRODUCTION

The Canadian quickclays are found in the valley of the St. Lawrence River and its tributaries and thus they coincide with, and threaten, several centres of population including the cities of Quebec, Montreal and Ottawa. Numerous engineering investigations have been carried out but the geomorphological, sedimentological and mineralogical aspects of these materials have been somewhat neglected, which is regrettable since it is becoming apparent that more basic scientific knowledge of these complex sediments is required before realistic engineering solutions can be provided for the problems they set.

The problem of slope evolution in quickclays has been considered in a geomorphological sense by Yatsu (1967) but the great bulk of work on the subject has had a geotechnical bias with an emphasis on very specific events and with no particular consideration given to the basic problem of how quickclay sediments form and how they fail by landsliding. This paper is an attempt to consider some of the factors which affect the properties of these particular Quaternary marine clays (in a Canadian setting) and to relate some of the geotechnical facts to the geomorphological situation, and to look at the quickclay problem in the widest possible sense. The quickclays are so-called because they exhibit a great loss of strength when disturbed and slope evolution is often by catastrophic landslide, which can be a dangerous event. In Pecsi’s (1969) classification of slope sediments the materials considered would appear under the heading of solifluxia (materials involved in mudflows). The mudflow association seems reasonable and in fact Yatsu (1966, p. 33) has claimed that ‘...it seems reasonable to classify the flowslide of post-glacial clays in terms of mudflows.’ The geomorphological philosophy propounded in this paper follows very closely that outlined by Yatsu. with an emphasis placed on the study of the sediment/soil material as a step towards the understanding of slope failure and evolution. Yatsu (1966, p. 36) has written: ‘When it is realized how scientists of other fields have striven to investigate the properties of materials, it becomes evident that this phase of geomorphological studies has been neglected.. .’

Rewived April 1975 Revised 30 Srptemher 1975

@ 1976 by John Wtley & Sons, Ltd.

163

E.S.P. 1/2-E

164 IAN SMALLEY

The problems of quickclay flowslide mechanism and slope formation in post-glacial marine clays are not just academic: over thirty people died at St. Jean Vianney in 1971 as a result of a sudden, totally unpredicted quickclay flowslide. One consequence of this recognized hazard is that a large amount of geotechnical investigation has been carried out into the factors which affect and control these landslides.

This paper is essentially a discussion of some factors which might be taken into account when develop- ing a theory of quickclay behaviour and when considering in particular the development of flowslides. The problem is a complex and difficult one and quickclay is a complex and difficult material with the remarkable property of being able to change state from a fairly hard, strong brittle solid into a liquid of negligible strength. The factors considered all bear in some way on the central dynamic question, namely ‘how does this change in state occur?’ To understand the landslide process in these particular soils it is necessary to understand how the change of state occurs. Because of the complexity of the problem each factor will be presented in a separate, basically self-contained section and some synthesis will be attempted in the final discussion.

The change in strength of the soil (this word is used in the simple engineering sense as described by Hunt (1972) p. 5 ) on being disturbed gives a measure of the ‘sensitivity’ and possible causes have been surveyed by Mitchell and Houston (1969). The present discussion has much the same aim, although it is rather more speculative and discursive, and concentrates on soils with a very high sensitivity: the so-called quickclays, and particularly the Canadian quickdays.

COMPOSITION

One of the most famous Canadian quickclay landslides occurred on the 7 May 1898 at St. Thuribe: 3,000,000m3 of soil material was involved and one person was killed. This was a classic quickclay slide in the notorious Leda/Champlain clay. It occurred in the valley of the Blanche river and has been stylishly described by Dawson (1899). The soil material was examined by Peck, Ireland and Fry (1951) and they reported a sensitivity of around 150 and a clay-size fraction of 36per cent. They also found the mineralogical composition to be mainly quartz, with some mica and a trace of montmorillonite. A later examination of the St. Thuribe material by Ladanyi, Morin and Pelchat (1968) gave the same clay-size fraction, the same activity (0.33) and a sensitivity of greater than 200.

Two sets of facts need to be established in a consideration of the composition of quickclays: the mineralogy and the size distribution. It might be expected that materials which are so important in the geotechnical sense would have been examined in minute detail. However, this is not so, partly because geotechnical engineers have not appreciated the importance of mineralogical investigations and partly because the actual investigations are very difficult. Fifteen landslips in Leda clay were listed by Crawford in his 1965 review and since then the St. Jean Vianney event has occurred. Despite this succession of dangerous events, no detailed survey of Leda clay mineralogy appeared until Gillott’s paper in 1971.

The particle size distribution of quickclays is interesting both in its own right and because it has excited strangely little comment or discussion. Berry and Jorgensen’s (1971) published size data for the material from the Ullensaker slide in Norway show that while only 1 per cent was larger than 64pm, 58 per cent was larger than the arbitrary clay cut-off size at 2pm. The following questions arise immediately : ‘why does the material have this particular distribution?’; ‘what is the nature of the material between 64 and 2 pm?’; and ‘is the material larger than 2 pm significantly different from the material smaller than 2pm?’ It is interesting to note that Kerr and Drew (1968), in discussing the flowslides associated with the 1964 Alaska earthquake, presented particle size data for the quickclay material associated with the Turnagain heights landslide and that this had a distribution similar to the Ullensaker material with 42 per cent smaller than 2 pm and none larger than 64 pm.

It is now well known that although quickclays are often thought of as clays, the clay-size fraction need not predominate. Moreover, as Gillott (1971) stated with reference to the Leda clays ‘the clay

LANDSLIDE IN CANADIAN QUICKCLAYS 165

size fractions contain a significant proportion of primary minerals’, so that clay minerals do not predominate either. It may properly be asked whether quickclays are really clays. Is failure to understand the behaviour of quickclays in part due to a persistence in trying to think of them as something which they are not? It is certainly apparent that the landslide process in a normal clay mineral soil proceeds by a very different mechanism from that in a quickclay.

BOND TYPE

‘Bond’ is perhaps not the most satisfactory word from the scientific point of view but it is easier to use than ‘attractive interparticle force’ and is preferred for this reason. Here is one of the roots of the problem: in quickclays a bond operates but may suddenly cease to operate. The understanding of the nature of such bonds must be a central problem in the mechanics of quickclay flowslide behaviour. The phenomenon to be explained is the change in state from a relatively strong soil to a free flowing liquid of negligible strength.

The force which holds the original soil together can be dissipated almost totally. This is not the sort of behaviour to be expected from a clay-mineral material in which the system contains mineral particles with negative electrical charges, dissolved ions and polarized water molecules, all of which conspire to produce a complex long-range bonding force. The complexity of this bond can be judged by the complexity of the phenomenon it produces, namely plasticity. Clay mineral materials tend to be plastic, so that they can be shaped and retain their shape. It is not proposed to enter into a discussion of plasticity here (the interested reader is referred to the review by Bloor (1975)) but it is necessary to point out that plasticity is possible because of the long range nature of the interparticle bond. The cohesiveness of the system does not depend on simple contact. The force is more diffuse: it has been compared with the simple Drude-Lorenz concept of metallic bonding which allows metals to bend and plastically deform and yet retain their strength (Cabrera and Smalley (1973)).

Forces between clay mineral particles have been considered by Low (1968) and he has discussed the various bond models. The attractive bond actually exists between the negatively charged clay mineral particles and the positively charged cations in the soil water system. Thus particle is attracted to particle via the intervening cations. This results in a mobile bond, which is capable of retaining its strength even though the system is deformed.

The alternative to the long-range mobile bond is the short-range fixed bond. This type of bond allows no adjustment, so that if the system is deformed the bond breaks. This is essentially a contact type bond although it could be of the Van der Waals type with an extremely rapid fall-off in strength with distance. It could also be the type of bond which takes some time to form such as a simple cementation bond. The chief characteristic is its specific threshold failure: in one state it is undisturbed and totally effective while in its second state it is broken and totally ineffective. It is much simpler in concept than the long-range bond, and it probably predominates in quickclays.

There is evidence for the lack of long-range bonding in the very low plasticity indices (PI) of classic quickclays. The St. Thuribe material mentioned above has a PI of 12. If a predominance of short range bonds is responsible for the observed solid-liquid transformations in quickclays the question arises as to the origin of these bonds. In particular, are they simp1y.a consequence of the special nature of the quickclay material, or are they the result of specific post-depositional events?

SOIL STRUCTURE

Do quickclays have a characteristic structure with a high collapse potential which facilitates the change from solid to liquid? Early investigations of soil structure suggested that this might be the case and structure is undoubtedly a factor to be taken into consideration. An early postulated structure was the so-called ‘cardhouse’ type (Goldschmidt (1926)). Recently some doubt has been cast on the existence of this structure in clay mineral soils (Barden (1972), Moon (1972)) because evidence is accruing that clay mineral particles tend to exist in soil systems as packets or domains and not as single particles.

166 IAN SMALLEY

However as the possibility of structures constructed from single clay mineral particles appears to recede another possibility emerges. Electron microscope studies of sensitive soils have suggested that the quartz particles present tend to have a very platy habit (Hammond et a1 (1973), Krinsley and Smalley (1973)) and it is possible that these contribute to very open structures of the cardhouse type. This platy habit is rather a mystery since it is well known that quartz does not have a well marked cleavage plane. Terzaghi (1955) certainly believed that as particle size decreased in a soil the proportion of plate shaped particles increased, but he did not suggest any particle forming mechanism. It is possible that the platy habit is the result of processes occurring within the original quartz containing igneous rock and that the shape of the small sedimentary particle is directly influenced by cooling stresses in granitic masses (Smalley (1974), Smalley and Krinsley (1974)).

NOMENCLATURE AND SIZE LIMITS

In the terminology of engineering geology any soil particles which have nominal diameters of less than a certain arbitrary value (usually 2pm) are defined as clay particles. This practice tends to foster the assumption that particles smaller than 2 pm are necessarily in some way fundamentally different from particles larger than 2pm. This feeling is reinforced by the division of soils into ‘cohesive’ and ‘cohesionless’ materials because cohesive soils tend to be ‘clay’ soils and cohesionless soils tend to be composed of larger particles. The arbitrary nature of the 2 pm cutoff is sometimes forgotten.

Clay mineral particles are undoubtedly small and it seems reasonable to expect the fraction smaller than 2pm to contain them. However, it is known to contain small primary mineral particles as well. In fact. it appears that, in the case of quickclays, the content of clay mineral particles may be relatively small, and certainly not large enough to predominate.

While, given long usage, it is impractical to change the name of this distinctive soil, it is important to appreciate that quickclays are not clays in the clay mineral sense and that only about 40per cent of the particles may actually meet the clay size requirements. Mankind has been aware of the importance of such material certainly since Medieval times. For example, landslides in quickclay are mentioned in the Icelandic Skalholt Annaler of the 14th century (Rosenqvist (1966)). Soderblom (1969) has suggested that the Swedish term ‘Kvicklera’ was probably first used in 1767. The English term is of later origin and it may not be too late to suggest that the preferred term should be quickclay rather than quick clay or quick-clay. At least ‘quickclay’ suggests a special material rather than a clay which happens to be quick.

LEACHING

The most elegant and scientifically satisfying theory advanced to explain quickclay properties is that advanced and developed by Rosenqvist (1953, 1966). The original formulation was fairly simple and envisaged sedimentation of clay mineral particles in a salt sea environment leading to an open structure rich in cations. The cations provided bond strength and the initial soil system was fairly strong. Post- glacial movements raised the soil and exposed it to percolating fresh water which tended to leach out the cations. This weakened the soil until only its structural integrity kept it intact; so that a meta- stable structure was produced.

Kazi and Moum (1973) have observed that leaching actually produces a reorientation of particles within the clay structure. It appears that the increased parallelism of the particles could correspond to a more collapsed particle arrangement which should lead to a lower sensitivity. It may be that increased repulsive forces mask this and produce a net increase in sensitivity.

The persistent problem with reference to the leaching explanation of very high sensitivities would appear to lie in the observation of quickclays which do not have sufficient clay mineral material prcscnt for leaching to be the predominant cause of quickness. This might apply more to the Canadian quickclays than to the Scandinavian material. Another possible problem lies in the existence of rigid quickclays such as the material at Toulnustouc River investigated by Conlon (1966). This had a very high sensitivity


but required a large energy input to bring about the characteristic change of state. Conlon interpreted his observations in terms of an open structure made rigid by cementation bonds. When the cementation bonds were broken the structure collapsed.

Leaching would certainly have the effect of shortening the range of the interparticle bonds, or at least of causing a reduction in strength. If the Norrish (1954) equation is accepted as giving a reasonable indication of attractive energy in a clay mineral system, then for the situation when the cations are far apart (i.e. few cations left after leaching):

E A = (aue)/(2&)

where E , is the attractive energy, (T is the surface charge density, u is the ionic valence, d is the distance between the ions and the surface and E is the dielectric constant (see Low (1968), p. 6). Essentially, this applies for one cation in the system: the fewer the cations the lower the overall attractive energy and the more easily the structure is disrupted. Thus it can be seen that leaching is probably a significant factor in causing quickness in some soils. However, it is appropriate to search for other possible factors in any attempt to establish an acceptable general explanation of quickclay behaviour.

MODEL SOILS

A fairly obvious way of testing a scientific theory is to construct a model of the system which fulfils all the known conditions of the theory and test its behaviour in comparison with the natural phenomenon. Pusch and Arnold (1969) attempted this with a clay mineral based model quickclay. They wanted to investigate the leaching theory of quickness production and set out to reproduce the events leading to the formation of a quickclay. The material used was almost pure illite, containing relatively few primary mineral particles. Pusch and Arnold failed to leach it into a sensitive state. Their method of producing the clay sediment (by centrifuge) was subsequently criticized and the experiments were repeated using unforced sedimentation. This allowed a sensitivity of around 16-20 to be developed (Pusch. private communication). However. the high sensitivities characteristic of quickclays ( > 100) were not produced. These experiments appear to cast doubt on the pre-eminence of the leaching theory as an explanation of quickclay formation.

If the balance between clay mineral particles and primary mineral particles in quickclays is critical and the problem is being tackled via model experiments. the appropriate initial step is to examine the two extreme conditions, Pusch and Arnold examined the all-clay-mineral soil and Moon (1973) has carried out some initial experiments on all primary-mineral-particle soils. It is very difficult to produce artificially quartz particles with diameters of the order of 2 pm. In fact, the comminution limit for quartz is around 1 pm, but a series of experiments has been carried out which indicates that the critical change in state from solid to liquid can be observed in these model soils.

A third type of model has been used by Tavenas, Roy and La Rochelle (1973). The material consisted essentially of a mixture of kaolinite, montmorillonite and Portland cement. It was designed to reproduce the mechanical properties of Canadian quickclays and appears to have been fairly successful although it does not approximate very closely to the true constitution of quickclays found in nature.

DISTRIBUTION

Quickclays occur in Scandinavia, eastern Canada and Alaska. They occur in regions associated with Quaternary glaciations and they are properly described as post-glacial soils. How does their association with the Quaternary ice sheets affect their properties and is there a direct link between their glacial origins and their observed properties'?

If a large proportion of finely ground primary mineral particles is required to give quickclays their high sensitivities, then glacial grinding of rock provides an obvious source.

A large supply of such material (rock flour) is found only in the high energy environments associated with glaciated regions. However. in addition to a particle formation mechanism, a suitable sedimentation

168 IAN SMALLEY

system is also required, and this dual requirement ensures that quickclays are fairly rare. A further requirement to account for the limited distribution of quickclays in subaerial location is post-glacial uplift, another concomitant of continental-scale glaciation. In sum, the continental ice-sheets provided the soil particles and post-glacial uplift exposed the marine deposits formed from them.

RHEOLOGY

In displaying a rapid change of state from a hard, brittle, relatively strong solid into a free flowing liquid, quickclays present a unique and unsolved rheological problem. It is rather surprising that rheolo- gists have not given more thought to this challenge, especially as the problem of quicksands was tackled forty years ago (Freundlich and Juliusberger (1 935)).

Kerr and Drew (1968) have suggested that the Anchorage (Alaska) quickclay was thixotropic. Acker- mann (1948) proposed that Norwegian quickclays were not thixotropic. This apparent conflict is resolved to some extent when the problem of definition is investigated. The classic definition of thixotropy (see Van Olphen (1963), p. 132) requires that strength diminish on disturbance and be regained when disturbance ceases. The Kerr and Drew approach emphasises the loss of strength on disturbance but is less rigorous about strength regain. Ackermann was particularly concerned about strength regain.

Eden and Mitchell (1970) have observed dilatant behaviour in Leda clay. This involves an expansion on initial deformation. This might reasonably be expected in a system which possesses a fairly rigid structure, that is a system largely composed of primary mineral particles and held together by short range bonds could be dilatant. The initial failure stresses would be tensile, pulling the particles apart and causing the structure to collapse. Conlon (1966) has made some very penetrating observations on the tensile strength of quickclays and it has been suggested that interparticle tensile stresses are generally important in the initiation of flowslide type slope failures (Smalley (1972)). In general, however, consideration of tensile stresses has not made much impact on practical soil mechanics, although Kezdi and Horvath (1973) have reported some interesting results.

Perhaps the most important and interesting rheological observations have been made by Bentley (1974). He was able to show, using a coaxial type viscometer and a Moon-type model quickclay, that changes in chemical environment affected a primary mineral particle soil in much the same way as they affected a clay mineral particle soil. Changing the cation concentration had a marked effect on a suspension of quartz particles in water. In fact, the effect was such as to suggest that leaching pheno- mena, formerly thought to relate more or less exclusively to clay mineral soils, might also reasonably be expected in primary mineral particle soils.

CEMENTATION

Cementation at particle contacts can give rise to a rigid material with a relatively high tensile strength. The time since the last glacial maximum is available for the cement to be deposited and, in the early stages at least it appears that the cement can be deposited quite rapidly (see Smalley (1967)). The topological genus-time relationship is such that a rigid network can be formed fairly quickly in a cementing system. Various workers have drawn attention to the importance of cementing agents (e.g. Conlon (1966), Kenney et al (1967), Eden and Mitchell (1970), Sangrey (1972)). They seem to be of greater significance in the Canadian quickclays than in the Scandinavian materials.

A cementation bond between two primary mineral particles could be considered the archetypal short range bond. It is a strong bond located exactly at the contact between the particles. Its lithological nature makes it a strong bond (the cementing material is usually silica or calcium carbonate or an iron salt) and it produces a strong soil. When it is disturbed with sufficient force, however, it breaks and the strength drops to zero (or virtually zero), i.e. a sudden change in property can be observed.

McKyes et al (1974) have studied amorphous coatings on soil particles from Champlain Sea sediments and have detected relatively large amounts of amorphous material which presumably has a cementing function. Solution treatments removed 11-12 per cent of amorphous material which was essentially silica


and iron oxide. It was also found that the amorphous material appeared to affect the results of quantitative X-ray diffraction experiments and that removing it allowed much more primary mineral material to be detected.

EARTHQUAKE FREQUENCY AND INTENSITY

The St. Lawrence River Valley has the highest incidence of earthquakes of any part of eastern North America. The average cumulative seismic hazard index ( J ) is 8.53 f 0.92 (Howell (1973)). J is defined in relation to I , the earthquake intensity at any location, and not to I,,,, the intensity at the epicentre. Every earthquake, no matter how distant, contributes to the Cumulative Hazard Index of every place. It takes ten earthquakes of intensity 7 (Mercalli scale) to equal one of intensity 8. As a result, only the largest earthquakes observed at any location make a significant contribution to the Cumulative Hazard Index.

Howell (1973) has listed 23 large earthquakes, of intensity 8 or over, which have occurred in eastern North America between 1622 and 1971. Eight of these major earthquakes occurred in Quebec, and the St. Lawrence River Valley as a whole has experienced one or more earthquakes of intensity 7 or greater in each 35 year period since 1622 except 1692-1726. Howell presents his data in 35 year periods, and he classifies the St. Lawrence River Valley as a region of continuous hazard. This high level of earthquake activity must contribute to the landslide risk, and it provides a remarkable geological coincidence. It is strange that in one of the few areas where glacial action has provided the right material and postglacial uplift has provided the right setting, tectonic events deep within the earth should provide the seismic energy to trigger quickclay landslides. These factors combine to provide real hazards in the St. Lawrence valley and Parkes and Day (1975) do well to draw attention to the dangers.

There does not appear to be a direct relationship between a large earthquake event and a subsequent major landslide but more sophisticated statistical analysis will be needed before the nature of the relation- ships can be established satisfactorily.

THE SCIENCE OF QUICKCLAY INVESTIGATION

It is appropriate to consider briefly to what extent quickclay science has progressed in terms of the currently popular concept of ‘scientific revolutions’ as developed by Kuhn (1970). Table I shows the suggested stages in scientific development, applied to a particular field of interest. Walker (1973) has already applied them to the ‘Turbidite’ concept in geology and the following application of them to the ‘Quickclay’ concept seems fitting.

Stage 1 represents the pre-scientific stage when crusts over liquid clay were widely accepted and there was little direct investigation or communication. Stage 2, that of the emergence of the first paradigm, may be identified with the publication in English in 1953 of Rosenqvist’s leaching theory. It is suggested that the state of the science now approximates to Stage 3 and that enough shortcomings of the leaching theory are visible to suggest the need for a revised paradigm. There is no reason,

Table I. Stages in scientific development (after Kuhn (1970), via Walker (1973))

1. Early random observations. No guidance from pre-existing theory: each worker develops his own hypotheses. 2. Emergence ofJirst paradigm. One of the hypotheses proves successful and is adopted by a group of scientistsit

then guides their research activities. 3. Crisis. Facts or experimental results are found to be at variance with the paradigm. As more and more discrepancies

are found, a state of professional crisis may develop. 4. Revolution. A new theory, capable of explaining the discrepancies, emerges. During a scientific revolution, the

old paradigm is rejected and replaced by a new one. 5 . Mopping up. The new paradigm is elaborated during a period of ‘normal science’ (or ‘mopping-up operations’).

170 IAN SMALLEY

of course, why the new paradigm should not complement and expand the original one, as Einstein’s view of the Universe took over from that of Newton.

Communication within the field of quickclay research tends to be good and. to a degree. this may be because a high proportion of results eventually become available in widely understood languages; little significant material is published only in minority languages. Research is centred in the critically affected regions and tends to have a local rather than a universal application. Moreover, it tends to have an overwhelmingly geotechnical emphasis. This is understandable because of the urgently practical aspects of the problem but it is possible that some interesting general aspects are being consequently obscured or are at least underdeveloped. This is where the geomorphological view is needed and where geomorphologists can make a specific contribution. The study of slope stability and evolution in the Canadian (and Scandinavian) quickclays demands the expertise of both civil engineers and geomorphologists. To quote Yatsu again: ‘Geomorphologists have at long last had to come, hat in hand, before the civil engineers to beg instruc- tion in the matter of slope stability. a matter long understood by the engineers. Howcver, don’t take this too sorely to heart. With a little effort we can not only catch up but also surpass them. Soil mechanics has come to a standstill in a certain sense.’ (Yatsu (1966), p. 102).

In the ten years since Yatsu wrote that many views on slope stability and evolution have changed and in some areas a modest revolution has occurred but the two groups involved can still learn a lot from each other.

DISCUSSION AND CONCLUSIONS

In which directions can the study of quickclays and flowslides and related problems in slope stability and slope evolution most profitably develop? As far as quickclays are concerned, more information is needed on the mineralogy of the materials and on the nature of the constituent particles. For example, it is important to determine whether the Canadian quickclays are essentially the same as the Scandinavian quickclays or whether, in investigating the change of state phenomenon, quite different criteria should be considered. Clearly, a model which would predict quickclay landslides would be useful and a series of techniques to prevent them altogether would be quite invaluable. If the short range interparticle bond is critical for quickclay formation. the obvious approach would seem to be to find some way of ‘lengthening’ the bond. Adding cations strengthens the clay mineral structure (and possibly the primary mineral structure as well) and could raise the PI by allowing bonds to move without breaking. Anything which increases the PI is a step towards preventing a quickclay failure. The Norwegians have used salt wells in a practical attempt to stabilize quickclays (see Moum, Sopp and Loken (1968)) and it seems that if quickclays could be converted from brittle materials to plastic materials, the chances of the solid-liquid transformation occurring would be reduced, thus reducing the geotechnical hazards.

The discussion of mechanism serves to lend emphasis to the obvious but central point that the critical relationship is the nature of the interparticle bond. In other words, it is essential to determine what holds the quickclay system together so effectively and then allows it to collapse so totally. It has been argued that some sort of short range bond is involved. It is extremely unlikely that it is the sort of bond associated with the phenomenon of plasticity for, if it is, it must be drastically altered in nature. Conlon (1966) measured PIS in the Toulnustouc quickclay of less than 5, a value which could probably be used to fix one approximate limit of the PI scale for soils. the other being perhaps a sodium montmorillonite with a PI of over 500 and wry long range bonds.

Farth flows which occur in the Canadian Leda clay have a characteristic mode of failure and final shape. Crawford (1965) has stated that ‘they appear to be initiated by a rather small bank failure which then retrogresses in a series of slides in which part of the soil is completely remoulded and flows out of the small opening in the bank carrying large chunks of relatively undisturbed soil with it. The resulting crater is usually pear shaped.’ Some authorities prefer to call the Leda clay the Cham- plain clay (Tavenas et a1 (1973)); it was this material which was involved in the St. Jean Vianney


slide of 1971. St. Jean Vianney is situated 2.4km north of the Saguenay River and approximately 10 km west of Chicoutimi in Quebec. The slide involved more than 8 x lo6 m3 of soil from an area of about 300,000m2, the material moving a t a speed of approximately 25 km/hr. The village had been constructed on quickclay which formed in a branch of the Champlain Sea and it was badly damaged by the sudden slide. The landslide occurred in the middle of a crater left by an ancient landslide and it was later concluded that the slide may have involved soils that had been remoulded by the ancient slide 500 years before. The fact that it took place in remoulded material suggests very strongly that the critical factors lie in the intrinsic nature of the soil material rather than in the mode of deposition of the initial sediment.

Some thermogravimetric investigations have been carried out on the St. Jean Vianney material and these suggest that it contains a relatively small proportion of clay mineral material, but that it does contain a modest amount of calcium carbonate. which could act as a cementing agent. More detailed results have been published elsewhere (Smalley et al (1975)). Accurate quantitative determination of clay mineral contents of soils is difficult and this accounts for the relative lack of analytical data. Determinations have tended to be concentrated on X-ray diffraction methods and it now appears that the amorphous coatings present in the quickclays could significantly affect the accuracy of these analyses (McKyes et al (1974)). The most significant fact of quickclay composition appears to be the lack of clay mineral material; this ensures a predominance of short range bonds. The lack of clay minerals in the Canadian quickclays has been observed before; Beland (1956) noted the very small quantity of clay minerals in the material which was involved in the Nicolet slide of 1955. but the significance of these observations does not seem to have been appreciated.

Liebling and Kerr (1965) proposed that the ma.jor critical conditions which contribute to quickclay movement appear to be: substantial quantities of layer lattice silicates of colloidal size (about 40per cent or more by dry weight), substantial pore water (44 per cent). a reduction in electrolyte concentration below 5 g of salt per litre, and addition of a dispersant. These all apply to a clay mineral based concept of a quickclay and appear to have very little relevance to the real Canadian quickclays. The layer lattice silicates of colloidal size (i.e. the clay minerals) do appear to be critical hut in exactly the opposite sense to that stated by Liebling and Kerr: quickclay materials involved in the characteristic flowslides have a very small clay mineral content. The critical condition which contributes to quickclay movement is a predominance of short range bonds in the soil system. In the Canadian quickclays this is brought about by the mineral composition of the soil which consists essentially of primary minerals, and by cementation at particle contacts. The slide process is facilitated by a high pore water content and slide initiation probably owes something to the high local level of earthquake activity. The major factor affecting the formation of the quickclays was the action of Quaternary glaciers-they are truly glacial soils.

REFERENCES

Ackermann. F. (1948). ‘Thixotropie und Fleisseigenschaften feinkorniger Boden’. Grol. Rundschau, 36. I 2 6 134. Barden. L. (1972). ‘The relation of soil structure to the engineering geology of clay soils’, Quart. J . Eng. Geol. 5 . 85-102. Beland, J. (1956). ‘Nicolet landslide-November 1955’, Proc. Geol. Assoc. Canada, 8, 143-156. Bentley. S. P. (1974). ‘Inactive particle model quickclay: Studies on cation exchange capacity and rheological behaviour’. MSc

Berry. R. W. and Jorgensen, P. (1971). ‘Grain size, mineralogy and chemistry of a quick-clay sample from Ullensaker slide,

Bloor, E. C. (1957). ‘Plasticity: A critical survey’, Trans. Brit. C m ~ n i . Soc.. 56. 423-481. Cabrera, J. G. and Smalley. I. J. (1973). ‘Quickclays as products of glacial action: a new approach to their nature, geology,

Conlon, R. J. (1966). ‘Landslide on the Toulnustouc River, Quebec’, Can. Geotech. J . 3 113-144. Crawford. C. B. (1965). ‘Engineering studies of Leda clay’, in Soils iti Canada. ed. R. F. Legget. Roy. SOC. Canada Spec. Pub.

Dawson. G. M. (1899). ‘Remarkable Iiindslip in Portneuf County, Quebec’, Geol. Soc. Amer. Bull., 10. 484-490. Eden, W. J. and Mitchell. R. J. (lY7Oj. ‘The mechanics of landslides in Leda clay’, Can. Grotcch. J.. 7 285-296. Freundlich. H. and Juliusberger, F. (1935). ‘Quicksand as a thixotropic system’, Trans. Faraday Soc.. 31. 769-774. Gillott, J. E. (1971). ‘Mineralogy of Leda clay’, Canadian Mineralogist, 10. 797-811. Goldschmidt, V. M. (1926). ‘Undersokelser over lersedimenter’, Nordisk Jordhrugs Forskning 6 7 434445.

thesis, University of Leeds.

Norway’. Eng. Geol., 5 73-84.

distribution and geotechnical properties’. Eng. Grol.. 7 115- 133; and 7. 361-363.

3. Univ. Toronto Press 2nd. ed 1965. pp. 200-217.

172 IAN SMALLEY

Hammond, C., Moon, C. F. and Smalley. I. J. (1973). ‘High voltage electron microscopy of quartz particles from post-glacial

Howell, B. F. Jr. (1973). ‘Earthquake hazard in the Eastern United States’, Earth and Mineral Sci., 42. 41-45. Hunt, C. B. (1972). Geology of Soils, W. H. Freeman, San Francisco, 344 p. Kazi, A. and Moum, J. (1973). ‘Effect of leaching on the fabric of normally consolidated marine clays’, Proc. Int . Symp. Soil

Kezdi, A. and Horvath, Gy. (1973). Tensile and flexural strength on cohesive soils’, Acta Tech. Acad. Sci. Hungar., 74, 43-61, Kerr, P. F. and Drew, I. M. (1968). ‘Quick clay slides in the USA.’ Eng. Geol., 2, 215-238. Krinsley, D. H. and Smalley, I. J. (1973). ‘Shape and nature of small sedimentary quartz particles’, Science, 180, 1277-1279;

Kuhn, T. S . (1970). The Structure of Scientific Revolutions, (2nd. ed) Univ. Chicago Press 210 p. Ladanyi, B., Morin, J. P. and Pelchat, C. (1968). ‘Post-peak behaviour of sensitive clays in undrained shear’, Can. Geotech.

Liebling, R. and Kerr, P. F. (1965). ‘Observations on quick-clay’, Bull. Geol. Soc. Amer., 76. 853-878. Low, P. F. (1968). ‘Mineralogical data requirements in soil physical investigations: in Mineralogy in Soil Science and Engineering’,

McKyes, E., Sethi. A. and Yong, R. N. (1974). ‘Amorphous coatings on particles in sensitive clay soils’, Clays and Clay Min.,

Mitchell. J. K. and Houston, W. N. (1969). ‘Causes of clay sensitivity’, J. Soil Mech. & Foundations Dio. Amer. Soc. Ciu. Enq.,

clay soils’. J . Materials Science, 8. 509-513.

Structure, Gothenburg 1973, pp. 137-152.

and 184, 183-184.

J. , 5. 59-68.

Soil Sci. Soc. Amer., Spec. Pub. 3, pp. 1-34.

22. 427433.

95. 845-871. Moon, C. F. (1972). ‘Microstructure of clay sediments’, Earth Sci. Reo., 8. 303-317. Moon, C . F. (1973): ‘Quickclays as products of glacial action: A new approach to their nature, geology, distribution and geotechnical

Mourn, J., Sopp, 0. 1. and Loken. T. (1968). ’Stabilization of undisturbed quick clay by salt wells’, Vag-Ock uatten bygarren,

Norrish, K. (1954). ‘The swelling of montmorillonite’, Disc. Faraday Soc., 18. 12CL-134. Parkes. J. G. M. and Day, J. C. (1975). ‘The hazard of sensitive clays: a case study of the Ottawa-Hull area’, Geog. Rev.,

Peck, R. B., Ireland, H. 0. and Fry, T. S. (1951). ‘Studies of soil characteristics: the earth flows of St. Thuribe, Quebec’, Dept.

Pecsi, M. (1969). ‘Genetic classification of slope sediments’, Biul. Peryglac., No. 18, 15-27. Pusch, R. and Arnold, M. (1969). ‘The sensitivity of artificially sedimented organic-free illitic clay’, Eng. Geol., 3. 135-148. Rosenqvist, I. Th. (1953). ‘Consideration of the sensitivity of Norwegian quick clays’. Gcwtechnique, 3. 195-200. Rosenqvist. I. Th. (1966). ‘Norwegian research into the properties of quick clay-a review’. Eng. Geol., 1. 445450. Sangrey, D. A. (1972). ‘Naturally cemented sensitive soils’, Geotechnique, 22. 139-152. Smalley. I. J. (1967). ‘A simple model of a diagenetic system’, Sedimentology, 8. 27-33. Smalley, I. J. (1972). ‘Boundary conditions for flowslides in fine particle mine waste tips’, Trans. Inst . Min. Metall., sect. A.

Smalley, I. J. (1974). ‘Fragmentation of granitic quartz in water’, Sedimentology, 21. 633-635. Smalley, I. J. and Krinsley, D. H. (1974). ‘Quartz cleavage and quick clays’, Science, 184. 183-184. Smalley, I . J., Bentley, S. P. and Moon, C. F. (1975). ‘The St. Jean Vianney quickclay’, Can. Mineral., 13. 247-254. Soderblom, R. (1969). ‘Salt in Swedish clays and its importance for Quick clay formation’, Proc. Swedish Grotech. Inst. no.

Tavenas, F., Roy, M. and La Rochelle, P. (1973). ‘An artificial material for simulating Champlain clays’, Can. Geotech. J . , 10.

Terzaghi, K. (1955). ‘Influence of geological factors on the engineering properties of sediments’, Econ. Grol., 50th Anniv. vol.

Van Olphen, H. (1963). An Introduction to Clay Colloid Chemistry, Interscience 301 p. Walker, R. G. (1973). ‘Mopping up the turbidite mess’: in Evolving Concepts in Sedimentology, ed.. R. N. Ginsburg, Johns Hopkins

Yatsu, E. (1966). ‘Rock Control in Geomorphology’, Sozosha. 8, 3-chome. Kandanishikicho. Cliiyodaku, Tokyo. Yatsu, E. (1967). ‘Some problems on mass movements’, Grog. Annaler, 49. ser. A, 396401.

properties-A short comment’. Enq. Geol.. 7. 359-361.

14. no. 8, 23-29; also published in Norwegian Geotechnical Inst. publ. 81.

65. 198-213.

Civil Eng., Univ. Illinois Studies, Soil Mech. Series no. 1.

81. A31-A37.

22.

489-503.

557-61 8.

Univ. Press, pp. 1-37.

factors relating to the landslide process in canadian quickclays

Documents