Oxide minerals in the sensitive post-glacial marine clays

Applied Clay Science, 5 (1990) 307-323

307

Elsevier Science Publishers B.V., Amsterdam

Oxide minerals in the sensitive post-glacial marine clays J. Kenneth Torrance Department of Geography and Ottawa-Carleton Centrefor GeoscienceStudies, Carleton University, Ottawa, Ont. KIS 5B6, Canada (Received November 28, 1989; accepted after revision May 15, 1990 )

ABSTRACT Torrance, J.K., 1990. Oxide minerals in the sensitive post-glacial marine clays. Appl. Clay Sci., 5: 307-323. Oxide minerals increase the undisturbed and remoulded strengths of the sensitive marine clays. In Eastern Canada, the oxide mineral contents of sensitive post-glacial marine sediments are greatest in the near-surface, where weathering has occurred, and decrease to an approximately constant value (in the order of 1%) where unweathered. In these marine clays, crystalline hematite or magnetite dominate the iron oxide content, with little, if any, ferrihydrite. The total amorphous mineral content is very low. In Japanese quick clays, in which the minerals are of volcanic origin, ferrihydrite is the dominant iron oxide and total amorphous mineral content is not known. In order for the small quantity of oxide minerals present in the Canadian sensitive marine clays to have measurable effects on undisturbed behaviour, they must be concentrated at points of contact between silicate minerals. Their influence on remoulded behaviour must be the result of their decreasing the electrostatic repulsion between particles, thereby increasing the probability of flocculation. The imperfect mimicking of the natural processes complicates the interpretation of experiments in which oxide minerals are added and problems with the selectivity of chemical extractants complicate the interpretation of experiments in which oxide minerals are removed.

INTRODUCTION

The term "oxide minerals" is commonly used to designate the various crystalline and amorphous materials in soils which consist of iron and/or aluminum in combination with oxygen and hydroxyl. The iron oxide minerals include hematite, goethite, lepidocrocite, maghemite, magnetite (all of which are crystalline) and ferrihydrite (generally described as being amorphous). At least some of these iron oxides can have varying degrees of Al-substitution for Fe in their structure. Gibbsite, the common aluminum oxide, is crystalline. Aluminum and silicon may also occur as allophane, which has shortrange order and a predominance of Si-O-A1 bonds (Wada, 1977 ). Along with these forms, complexes of Fe and AI with organic materials occur.

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The nature and role of the oxide minerals in the sensitive post-glacial marine clays have been relatively little explored. The quantities present are sufficiently low that they are not normally detectable by X-ray diffraction. In general, their effects have been studied by removal rather than by direct observation. This paper will review the characterization of the oxides by X-ray diffraction, M/Sssbauer spectroscopy and selective dissolution analysis, will present new experimental results of selective dissolution analysis, will investigate the rheological influence of their removal and readdition and will assess their role in determining the behaviour of the soil material. OXIDE MINERALS IN SENSITIVE MARINE CLAYS

In a review of the attempts to quantify the mineralogy of the sensitive marine clays using X-ray diffraction, Torrance (1988) found no instances in which oxide minerals were reported. It must be concluded that their concentrations are generally, if not always, below the limits of detection by X-ray diffraction. Torrance et al. (1986) used Mtissbauer spectroscopy to investigate ironbearing minerals in sensitive marine clays. Hematite was present in marine sediments from the Ottawa Valley (South Nation River landslide site) and the Tyrell Sea (Grand Baleine), while magnetite was found in a sediment from the St. Lawrence Lowlands north shore area (St.-Leon-le-Grand). Ferrihydrite was present in a sensitive marine clay derived from volcanic ash in the Ariake Bay area, Japan. The most commonly used methods to investigate the oxides, particularly iron oxides, have been selective dissolution procedures, in which a reagent is assumed to extract selected minerals, yet leave other minerals undamaged. Selective extraction procedures include: dithionite-citrate-bicarbonate (Mehra and Jackson, 1960; Jackson, 1970) and dithionite-citrate (Sheldrick, 1984 ) which yield essentially identical results and are claimed to extract amorphous iron oxides, hematite and goethite, but not to attack magnetite and to have little effect on iron-bearing silicates (Sheldrick, 1984; Borggaard, 1988); acid ammonium oxalate (Chao and Zhou, 1983; Sheldrick, 1984) which is claimed to remove amorphous iron oxides and magnetite and to have little effect on hematite, goethite and iron-bearing silicates; hydroxylamine hydrochloride (Chao and Zhou, 1983 ) which is claimed to selectively extract ferrihydrite; Tiron (Biermans and Baert, 1977 ) which is claimed to extract ferrihydrite; and the Segalen procedure (Segalen, 1968 ) which was designed to extract amorphous oxides from heavily weathered, oxide-dominated tropical soils. None of these procedures, which are based on the dynamics of dissolution and/or affinities of chemicals for various combinations of iron with oxygen, does precisely as claimed and, none is unambiguously selective for ferrihydrite or for a specified crystalline form. Torrance and Percival ( 1988 )

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report that the extractant which is most selective for ferrihydrite depends on the soil material being investigated. They also concluded that even the more gentle extraction procedures (dithionite-citrate, acid ammonium oxalate, hydroxylamine hydrochloride, and Tiron) overestimate the amount of amorphous materials present. The Segalen procedure, the least selective of the above list, has been the one most used to assess the role of amorphous materials in the sensitive marine clays. This procedure has been variously claimed to extract: amorphous materials, including amorphous iron oxides, alumina and silica (McKyes et al., 1974; Yong et al., 1979 ); amorphous materials and some hornblende (Bentley and Smalley, 1978 ); amorphous oxides, with qualifications (Locat et al., 1984 ); and "amorphous" materials (Quigley et al., 1985 ). Estimates of up to 24% amorphous material have been reported for some Leda clay materials. The seriousness with which the Segalen procedure overestimates the amorphous content in the sensitive clays has gradually been recognized. Bentley et al. (1980) found the procedure to dissolve hornblende, mica, chlorite and amorphous iron oxide from Norwegian soil materials. Quigley ( 1980 ) criticized all the extraction procedures which involve strong acids, bases and chelating agents as being too aggressive and requiring correction for dissolution of chlorite and biotite. The consistent use of "amorphous" by Quigley et al. ( 1985 ) also reflects the belief that the procedure was extracting both oxides and "amorphous" rinds from glacially ground rock flour. Torrance ( 1988 ) has been even more critical of the Segalen procedure since X-ray diffraction of three Canadian sensitive marine clays (Torrance et al., 1986) revealed complete destruction of chlorite by the fourth (of the standard eight) alternating acid-base extractions and severe damage to micaceous minerals. Surprisingly, hornblende appeared to be much less affected than the phyllosilicates. The extremely aggressive character of the Segalen extraction, when applied to the sensitive post-glacial marine clays, places the interpretations of all experiments using this procedure in doubt. The correlations between physical properties and Segalen-extracted materials (Yong et al., 1979) can no longer be attributed to the presence of amorphous materials, and because many minerals may be attacked, it is not possible to differentiate their relative importance in determining the material properties. The dithionite-citrate-bicarbonate (DCB) procedure, which is recognized to extract some iron from iron-bearing silicates (Borggaard, 1988 ), extracted only 0.53% Fe203, in a single extraction, from the South Nation soil (Torrance, 1984 ). Indeed, the total Fe203, A120 3 and SiO2 extracted was less than 1%. Since DCB is known to attack some aluminosilicate minerals (Borggaard, 1988 ), the amount of the alumina and silica that is amorphous, while unknown, is small. The M6ssbauer spectroscopy investigations of Torrance et al. (1986) revealed that most of the iron oxide removed was hematite of very fine particle size, although the possibility that a small proportion might

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be ferrihydrite could not be ruled out. These M6ssbauer spectroscopy results, raise the question as to whether it is appropriate to use the term "amorphous minerals" when referring to the unweathered sensitive marine clays of Eastern Canada. Admittedly, the iron oxide minerals in the sensitive marine clays are not detectable by X-ray diffraction, but this would appear to result from the small amount present rather than their lack of crystallinity. Because the term "amorphous" is commonly used in the Leda clay literature, it might be useful to attempt to clarify what "amorphous" implies. The Penguin "Dictionary of Geology" (Whitten and Brooks, 1972 ) defines amorphous as a "term applied to material having no regular arrangement of atoms" and the Anchor Press/Doubleday "Dictionary of Geological Terms" (Bates and Jackson, 1984) defines amorphous as being "literally, without form; applied to rocks and minerals having no definite crystalline structure". In c o m m o n practice, amorphous minerals are considered as those which do not yield well-defined X-ray diffraction peaks. Eggleton and Fitzpatrick ( 1988 ) presented examples of the broad peaks characteristic of 2-line and 6line ferrihydrite and Wada ( 1977 ) presented a typical pattern for allophane. X-ray diffraction of the post-glacial marine clays yields well-defined peaks for the primary silicates and the phyllosilicates, but no X-ray diffraction peaks are observed for the oxide minerals. Occasionally a small peak occurs at 0.269 nm, the strongest reflection for hematite, but this cannot be unambiguously interpreted, because the amphiboles can exhibit reflections in the same area (Brindley and Brown, 1980). The lack of definitive peaks for the iron oxides is undoubtedly because the amount present is below the limit of detection; the crystallinity of that present has been demonstrated by M6ssbauer spectroscopy (Torrance et al., 1986). The situation of the post-glacial marine clays, in which quartz and feldspars commonly dominate in all size fractions, poses a series of further questions regarding amorphous silica. These include: What is meant by amorphous silica? Is the surface layer of quartz particles (regardless of their size), where the silica tetrahedra are distorted, a zone of amorphous quartz? The answers are not always clear. Silica which has entered solution by weathering and then accumulated on the surface of another mineral in a quasicrystalline or noncrystalline form would qualify, without question, as being amorphous. The distorted surface layer of quartz and feldspars, in which the degree of distortion decreases with depth from the surface, presents a less clear case. This zone originates because of the existence of a discontinuity (the surface) and, if this layer is removed, internal structural adjustment should be essentially instantaneous, and a new distorted zone would be created almost as rapidly as the exterior is removed. If this argument is valid, no extraction process can be designed to extract only the distorted material that was originally present, since new distorted material is constantly being created. If this distorted surface zone is considered to be amorphous, we have an unsolvable dilemma in

OXIDE MINERALS IN THE SENSITIVE POST-GLACIAL MARINE CLAYS

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clearly delineating the limits of amorphous material. This author prefers to exclude the distorted surface zone of silicate particles from being considered as amorphous and prefers to restrict the term to particles which lack a crystalline core. Some very small particles which had originally been quartz and feldspars would meet this criterion. Clelland et al. (1952) reported that grinding can produce a vitreous layer on quartz that is 30 to 150 m m thick, while Gordon and Harris (1955) reported that silica particles smaller than 0.5/tm are essentially X-ray amorphous with virtually no crystalline core. Some of the quartz and feldspars in the post-glacial marine clays is in this size range. The clay minerals, with their layered structure, and the oxide minerals, with their strips of octahedra, do not have this transition zone at their surface. In summary, for the oxide minerals, previous work has found that hematite is the dominant iron oxide in sensitive marine clays from the Ottawa Valley and the Tyrell Sea, that magnetite is the dominant iron oxide in an example from the St. Lawrence Lowlands and that ferrihydrite is the dominant iron oxide in a sensitive marine clay derived from volcanic materials in Japan. The nature and amounts of crystalline or amorphous alumina and of amorphous silica remain matters of speculation, although a convincing argument can be made that the amount is probably small. At this time, it can be conclusively stated that the amount of amorphous material in the sensitive postglacial marine clays has been greatly overestimated by investigators, particularly by those using the Segalen procedure. Therefore, the use of the Segalen procedure to estimate the amount of amorphous material should be immediately discontinued. EXTRACTABLE IRON OXIDES IN THE SENSITIVE MARINE CLAYS

"Selective" chemical extractions have been performed on several sensitive marine clay cores. These results will be reviewed. DCB- or DC-extractable iron versus depth (Fig. 1 ) is reported for Drammen, Norway (Moum et al., 1971 ), for Mud Creek, and Touraine in the Ottawa Valley, Canada (Torrance, 1975 ), and for the near-surface zone at Gatineau, Quebec (in the Ottawa Valley) and at St. Barnab6 and Henryville, Quebec (in the St. Lawrence Lowlands). Acid ammonium oxalate (AAO) extractions were also performed at Gatineau, St. Barnab6 and Henryville. In all cases more iron oxides were extracted from the surface zone than from deeper in the soil. The only plausible explanation for this increased extractable iron is weathering of iron-bearing primary minerals. The soil colour suggests that hematite is the oxide produced. Below the zone of weathering, the depth of which varies with location, the iron oxide extracted probably represents the hematite a n d / o r magnetite originally in the sediment, along with a small amount of less crystalline material. In the surface zones, the AAO extraction always extracted less iron than

312

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1. Extractable iron oxides in s o m e sensitive m a r i n e clays: • = extracted by D C B o r DC; o = extracted by AAO.

DC. At Gatineau, the amounts extracted by AAO and DC converged at 4 m depth. At St. Barnab6 the quantity extracted by AAO became greater at approximately 2.2 m and this continued throughout the depth investigated. At Henryville, the amounts converged at approximately 2.5 m and were compa-

OXIDE MINERALS IN THE SENSITIVE POST-GLACIAL MARINE CLAYS

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rable below that depth. The following interpretation is placed on these comparative extraction results. St. BarnabL At St.-L6on-le-Grand, 10 km from St. Barnab6, magnetite was the only crystalline iron oxide detected by MiSssbauer spectroscopy (Torrance et al., 1986). This should also be the case at St. Barnab6. The greater effectiveness of AAO over DC reflects this dominance of magnetite. The DC is believed to have extracted some iron from the finely divided magnetite and both DC and AAO would extract the less crystalline forms. Gatineau. At Gatineau, which is 45 km from the South Nation River site and believed to have hematite as its dominant iron oxide, DC is presumed to have extracted both hematite and amorphous iron oxides, but AAO also dissolved some hematite along with less crystalline forms. Henryville. The Henryville soil has not been investigated by M~ssbauer spectroscopy, so one can only speculate as to whether magnetite and hematite are present in roughly equal amounts in the unweathered soil or if the oxides are sufficiently poorly crystalline that both extractants are equally effective. The pattern for AAO-extractable iron with depth suggests that magnetite is probably present and, considering the location of the site near the Appalachian Mountains, hematite eroded from the mountains would also be expected to be inherited in the sediment. Unfortunately the selective chemical extraction procedures do not allow reliable discrimination of oxide forms in cases such as this. Regardless of the uncertainties, it can be definitively concluded that the extractable iron content increases upon weathering of these marine clays. This depth to which the increase is apparent coincides roughly with the depth to which oxidizing conditions have penetrated. GEOTECHNICAL ROLE OF THE OXIDES

Undisturbed behaviour The geotechnical role of the oxides has generally been considered to involve their probable role as cementing agents. The high undisturbed strengths of the soil material at the Toulnustouc River landslide site in Quebec, and its failure at very small strains, led Conlon (1966) to propose that "at the contacts between the soil particles there are strong bonds. These bonds act as if they were brittle 'cementing' material, perhaps crystalline in nature.'" He further suggested that cementation was important in all the Canadian marine clays. Kenney et al. (1967) found that the apparent preconsolidation of a marine clay from Labrador was drastically reduced by leaching with EDTA, which extracts carbonates and "weakly crystalline iron and aluminum compounds". Quigley ( 1968 ) believed that aluminum and iron hydroxide precipitates were responsible for the strong bonding exhibited by the clay from the

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Toulnustouc River landslide site. Loiselle et al. ( 1971 ) leached brackish-water clay sediments, which they described as "highly sensitive, stiff, and very brittle", from Outardes, Quebec, with EDTA, and found lesser preconsolidation pressures than in unextracted specimens. Sangrey ( 1972 ), from indirect evidence, postulated that the hydrous and anhydrous oxides of iron and aluminum, which may act as cements, derive from weathering of "fresh" minerals, particularly amphiboles and pyroxenes. Bentley and Smalley (1979) and Quigley (1980) emphasized that cementation bonds, which may originate from precipitation of carbonates and/or various oxides, would increase the undisturbed strength of the marine clays and contribute to the development of high sensitivities. Unfortunately during the 1970s, so-called "amorphous" minerals, extracted by the Segalen procedure drew a lot of attention. As already discussed, this extraction, while undoubtedly removing amorphous materials, also attacks crystalline oxides, clay minerals and other silicates. As a consequence, the selected geotechnical properties which correlate with the amount of material removed by the Segalen procedure (Yong et al., 1979 ) cannot be attributed solely to amorphous oxides or total oxides. That iron and/or aluminum oxide accumulation can lead to cementation is abundantly clear from the existence oflaterite (Hamilton, 1964 ), plinthite (Buol et al., 1980), and ortstein (Wang et al., 1978). These develop in soils which range from sand-dominated to clay-dominated. Such cementation should increase the undisturbed strength of the soil and cause a more brittle behaviour to be exhibited. The evidence already presented, strongly suggests that cementation by oxides occurs in at least some of the post-glacial marine clays. The way in which the cementing agent accomplishes its task, is not well understood. No electron micrographs of the sensitive marine clays show the presence of an obvious cementing agent. During the period when amorphous materials were believed to be present in large amounts, Bentley and Smalley ( 1978 ) proposed that an amorphous coating enveloped the crystalline particles and had continuity throughout the structure. With the amount of amorphous material now being recognized as low, this last model is unrealistic and, with the microcrystalline character of at least the iron oxides (Torrance et al., 1986 ), the "perhaps crystalline" bonds envisioned by Conlon (1966) seem more probable. Some speculative calculations are useful. If a single extraction, by whichever of the DCB and AAO procedures is more aggressive for the particular oxide (Fe203, A1203, SiO2), is assumed to extract all the possible cementitious material, then the upper limit for Leda clay from the South Nation River landslide site is 0.54% Fe203, 0.07% A1203 and 0.27% SIO2, or 0.88% of the total mass (Torrance, 1988 ). The total volume of this material, based on the specific gravities of hematite, gibbsite and quartz, would be 0.234 c m 3 per

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100 g of soil. The surface area of the South Nation soil was determined to be approximately 60 m2/g, by the ethylene glycol monoethyl ether method of Mortland and Kemper (1965). If it is assumed that these extracted oxides were uniformly spread over the surface, the thickness of the layer formed would be 0.039 nm, which is much less than the 0.28 nm diameter of the oxygen atom. One must conclude that the extractable oxides cannot be uniformly distributed. Reasonable assumptions for the characteristics of the oxides must be made. Mtissbauer spectroscopy has shown that the iron oxide (44% of the extracted material, by volume) is dominantly microcrystalline hematite (Torrance et al., 1986), which has a c-axis dimension of 1.37 nm (Eggleton et al., 1988 ). It seems conservative to assume that a minimum of 3 unit cell thicknesses, or approximately 4 nm thickness, would be needed for crystalline response during Mrssbauer spectroscopy investigations. This is not greatly out of line with the estimate of less than 20 nm for the hematite particles in the South Nation soil material (Torrance et al., 1986). With these assumptions, the 0.54% iron oxide could cover a maximum of 0.43% of the surface. With similar assumptions for the A1203 and SIO2, about 1% of the total surface area could be coated by the three oxides. If the assumptions are reasonable and the material that was extracted by DCB and AAO is the cementing agent, it must be localized at the points of contact between particles if it is to have a substantial effect. With the iron oxides shown to be microcrystalline, Conlon's (1966) proposal of a crystalline bonding agent would appear to be at least partially substantiated. The alumina and silica might indeed be present as an amorphous form, probably similar to allophane (Wada, 1977). In summary, it is concluded that the iron oxides, and other oxides extractable by DCB and AAO, have a cementing action which accounts for some of the brittleness and undisturbed strength exhibited by marine clays. To have these effects, the oxides must be concentrated at points of contact between particles. Undoubtedly, the oxides also contribute to the strength which develops in the surface crust upon weathering.

Remouided behaviour and oxide addition experiments The influence of oxide minerals on the remoulded behaviour of clays, including Leda clays, has been investigated by the influence of its removal and by the influence of its addition. Hendershot and Carson (1978) observed substantial decreases of the liquid limit, plastic limit and plasticity index of a Leda clay from Gatineau, Quebec upon removal of DCB- and AAO-extractable Fe, A1 and Si. Torrance (1984) reported similar results for the DCB extraction of the South Nation River landslide soil and a quick clay from Ariake Bay, Japan. M o u m ( 1967 ) reported an increase in liquid limit and plasticity when iron compounds were added and when a natural clay was stored in an

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oxidizing environment. Above certain minimum pH values, Yong and Ohtsubo ( 1987 ), observed an increase in the Bingham yield stress of kaolinite after the addition of 5% ferrihydrite. The yield stress increase occurred above pH 4.3 when the addition was done at pH 9.5 and above pH 5.3 when the addition was done at pH 3.0. Ferrihydrite apparently increases the attractive interactions within these systems. Quigley ( 1980 ) and Torrance ( 1983 ) have recognized that the oxide mineral content must be below certain undetermined thresholds, if the low remoulded strengths required for designation as quick clays are to be attained upon leaching of the post-glacial marine clays. Extraction by DCB decreased the yield stress at constant water content of Ariake Bay quick clay and of Leda clay from the South Nation River landslide (Torrance, 1984). Marine clay sediments from St.-L6on-le-Grand, Quebec and Grand Baleine, Quebec responded in the same manner. In further investigations with the South Nation River soil, a large batch of soil was extracted by DCB to remove oxides (0.48% Fe203 was removed), then washed thoroughly with 1 M NaC1 (to remove the extractant and to attain Na-saturation). The yield stress-water content relationships, at 20 and 2%o salinity, were determined for this extracted material and for the material after readdition of 0.5, 2.0 and 8.0% iron oxide using two methods of readdition. In the first method, goethite, which had been prepared by bringing a 0.17 M FeCI3 solution to pH 12 by addition of 2 M NaOH and aging overnight (Ainsworth et al., 1985 ), was added. This addition of crystalline iron oxide was intended to mimic the situation of crystalline iron oxides co-sedimenting with the other minerals during sediment accumulation. In the second method, 20 ml of 0.17 M FeC13 and 5 ml of 2 M NaOH were alternately added to a soil suspension (which was continuously stirred) until enough chemicals had been added to produce the desired amount of iron oxide. This procedure was intended to mimic, undoubtedly imperfectly, the formation of iron oxides by weathering of iron-bearing minerals after sedimentation. The alternating addition of the chemicals was done to avoid severe pH changes. The yield stress-water content relationships for these materials and for the original natural material are compared in Fig. 2. At both 20 and 2%0 salinity, oxide extraction shifted the response towards lower yield stresses at constant water content, with the shift being more pronounced at low salinity. Oxide readdition shifted the response back towards higher yield stresses. In all cases, the oxides which were produced with the soil present had a greater effect than the addition of an equal amount of crystalline oxide. At 20%0 salinity (Fig. 2b), addition of 0.5 and 2% crystalline oxide had no apparent influence, whereas the formation of 0.5 to 2% oxide in the presence of soil returned the response to approximately that of the natural soil before extraction. The addition of 8% oxide, by either method, caused a major increase in the yield stresses. At 2%0 salinity (Fig. 2a), 2% crystalline oxide returned the reponse to approximately that of the natural soil, whereas

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interpolation suggests that 1% had a comparable effect, if the oxide was formed in the presence of soil. The effect of oxide addition and the difference between the methods of addition was always more pronounced at low salinity. These results reinforce the conclusion that relatively small quantities of oxide in the sensitive marine clays can have a measurable influence on the remoulded behaviour of the soil and that the effect increases with increasing oxide content. The results also suggest that oxides produced during weathering have a greater influence than crystalline oxides which co-sediment with the other minerals. Addition of crystalline aluminum hydroxide, prepared by the neutralization of A1C13by NaOH, to DCB-extracted South Nation marine clay had similar effects to the iron oxide additions (Fig. 3 ). At 20O/oosalinity, 0.5 and 2% additions had little influence but 8% dramatically shifted the response to higher yield stress values. At 2%o salinity, 0.5% aluminum hydroxide may have had a slight dispersing effect but 2 and 8% both shifted the response to higher yield stress values. Indeed for the 8% additions, the yield stresses at 2%o were slightly greater than those at 20%o. This latter observation was unexpected and suggests that the mode of action of the iron oxides and alumin u m hydroxides may differ. To have an influence on the remoulded behaviour of soils, the oxides must change the interactions between particles. This would be expected, because the oxides are either positively charged or neutral at the pH of the sensitive marine clays and would, as a result, locally neutralize or screen the negative charge of the clay particles. This neutralizing or screening action should encourage attractive interaction between particles, which would lead to the higher yield stresses observed. SYNTHESIS A N D C O N C L U S I O N S

Oxide minerals influence both the undisturbed and remoulded properties of the sensitive marine clays. Their presence increases the bearing capacity and shear strength. The liquid and plastic limits and the plasticity are reduced when oxides are extracted (Hendershot and Carson, 1978 ) and the yield stress of remoulded soil, determined by viscometry and by fall cone, is also reduced. Because oxide presence affects both undisturbed and remoulded behaviour, the amount of oxide present and its nature should affect both slope stability and post-failure behaviour. Of particular interest with quick clays is that the characteristics of landslides - size, mode of failure and runout distance of the debris - may also be influenced. The oxides must act by influencing particle interaction. In the case of the undisturbed behaviour, the small quantity present must be preferentially concentrated at points of contact in order to have a significant effect, and for the remoulded behaviour the possibility of flocculation must be enhanced by ox-

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ide presence. The explanations, for both cases, must be related to the characteristics of the oxides themselves, and most probably to their charge characteristics. The surface charge of the oxide minerals is pH-dependent, and at all times both positive and negative charge sites are present (Herbillon, 1988). The isoelectric points of hematite and goethite (the pH at which the number of positive and negative charge sites are equal) lie in the pH 7.5-9.3 range (Schwertmann, 1988 ). For the Canadian and Scandinavian marine clays (pH approximately 8 ) this implies a very low net surface charge on the oxide particles, with some uncertainty as to its sign. If the in-situ pH is below the isoelectric point, an attractive force should exist between oxides and clays. In the least favourable probable situation, with the pH slightly above the isoelectric point, repulsive forces between the clay surfaces and the oxide particles should be low. Yong and Ohtsubo ( 1987 ) demonstrated that the interaction between oxides and kaolinite is more intimate if the oxide and clay first become associated at a pH at which the oxide bears a substantial net positive charge (i.e. the pH is well below the isoelectric point). Schwertmann's ( 1988 ) observation that aggregation in surface soils increased with increasing surface area of iron oxide and with decreasing pH supports these arguments. The available evidence for the sensitive marine clays of Eastern Canada (Torrance et al., 1986 ) indicates that the iron oxides present in unweathered material are microcrystaUine and have co-sedimented with the other minerals. The freshwater transporting environment was probably mildly acidic and the sedimentation environment (estuarine to marine conditions ) would be in the near-neutral to alkaline range. Attractive associations between oxides and silicates would be expected under these conditions, but the links should be of the weaker type hypothesized by Yong and Ohtsubo (1987). Presumably the oxides would retain some possibility of independent mobility, and would be able to move, at least short distances, from their points of initial association. It follows that they should be able to migrate towards points of contact between silicate particles, where the intensity of the negative electric field is the greatest. The provision of additional strength to the contact would constitute a cementing action. The remoulded behaviour would be influenced in a similar manner. The association of the oxide minerals with the silicate surfaces screens or neutralizes the negative charge on the silicate, decreases the repulsive force between the particles and increases the probability of flocculation. Increased attractive interaction between mineral particles in a suspension or slurry will lead to increased resistance to flow. In the surface zone where oxidizing conditions occur, at least periodically, more iron oxide is present - presumably as a weathering product. This oxide undoubtedly contributes to the strength of the peds developed in the surface crust during weathering (Eden and Mitchell, 1970 ) but, because of the close-

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spaced joint system in the weathered zone, probably contributes only indirectly - through the influence o f the ped structure on failure m e c h a n i s m - to the strength o f the soil at low overburden pressures. In unweathered soil materials, the small quantity o f oxide minerals present, less than 1%, must be concentrated in the area o f interparticle contacts for t h e m to exert the influences on undisturbed properties that have been observed. Our knowledge o f the nature, abundance and geotechnical role o f the oxide minerals in silicate-dominated soils remains inadequate. The small amounts present in the sensitive marine clays complicate the investigation of their role and how they act. To date, m o r e has been learned through investigations o f the influence o f their removal, than o f their addition. Both approaches have limitations. In the case o f extraction, the procedures are not as specific as would be desirable and other minerals are also damaged, to greater or lesser extents, by the various extraction procedures. In the case o f addition, it is difficult to adequately m i m i c the natural processes and to obtain the appropriate oxide form or mix o f oxide forms. Regardless, it is apparent that the oxides exert an influence on soil behaviour, even when present in small amounts and undetectable by X-ray diffraction. Much remains to be learned about the effect o f oxide presence and the mechanisms by which the oxides exert their influence, not only in the sensitive marine clays but also in other silicate-dominated soils and in oxide-dominated soils. ACKNOWLEDGEMENTS The assistance o f A. Brereton, J. Cliff, J. Hanright, A. Kroeker, D. Onysko and N. Tomas in various stages o f the laboratory investigations, and comments on the manuscript by J.B. Percival are gratefully acknowledged. The research was supported by the Natural Sciences and Engineering Research Council, Grant A8503.

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