Evaluating the feasibility of using guanidinium salts as a means of in-situ soil strengthening

Adam Leik, Ian Fleming, Chi Pu

In the proceedings of: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Soft and sensitive clays

ABSTRACT: There are a variety of engineering problems for which conventional geotechnical solutions are either impractical or ineffective; thus, the need for a multi-disciplinary approach has become apparent. When it comes to slope stability, challenging sites are found worldwide in clayey soils and weak rocks. In Saskatchewan and eastern Alberta, particular problems are associated with shallow soil deposits in highly plastic smectitic clays, clayey tills, and in weak shales with lenses of highly plastic smectitic clay. Given the extremely low residual shear strengths of these materials, shallow creeping failures are common and threaten infrastructure such as roads, bridges, and pipelines. The current research is intended to evaluate the feasibility of modifying the porewater chemistry of such soil deposits using guanidinium chloride (C(NH2)3Cl) to effect mineralogical alteration through ion exchange, as a means of insitu soil strengthening. The effect of guanidinium on geotechnical properties (relevant to slope stability) for various local soils has been evaluated through batch tests and lab-scale treatment trials. Using four local prairie soils, the effect that guanidinium salts have on soil behaviour was evaluated through testing of index properties, x-ray diffraction, and residual shear strength.

RÉSUMÉ: Il existe une variété de problèmes d'ingénierie pour lesquels les solutions géotechniques classiques sont peu pratiques ou inefficaces; ainsi, la nécessité d'une approche multidisciplinaire est devenue évidente. En ce qui concerne la stabilité des pentes, les sols argileux et les roches fragiles se rencontrent partout dans le monde. En Saskatchewan et dans l'est de l'Alberta, des problèmes particuliers sont associés aux dépôts de sol peu profonds dans des argiles smectitiques hautement plastiques, des tills argileux et dans des schistes faibles à lentilles en argile smectitique hautement plastique. Compte tenu de la résistance extrêmement faible au cisaillement de ces matériaux, les défauts de fluage superficiels sont fréquents et menacent des infrastructures telles que les routes, les ponts et les pipelines. Les recherches actuelles ont pour objectif dvaluer la faisabilité de modifier la chimie de leau interstitielle de tels dépôts en utilisant du chlorure de guanidinium (C (NH2)3Cl) pour effectuer une altération minéralogique par échange dions, comme moyen de renforcer les sols in situ. L'effet du guanidinium sur les propriétés géotechniques (relatives à la stabilité des pentes) de divers sols locaux a été évalué à l'aide de tests par lots et d'essais de traitement en laboratoire. En utilisant quatre sols locaux des Prairies, leffet des sels de guanidinium sur le comportement du sol a été évalué en testant les propriétés de lindice, la diffraction des rayons X et la résistance au cisaillement résiduelle. Shallow slope failures are prevalent throughout Saskatchewan and eastern Alberta due to the presence of highly plastic smectitic clay lenses within clayey till formations. These clay lenses tend to be rich in montmorillonite and, with the exception of the sediments deposited from Lake Agassiz, are likely derived from short-lived pro-glacial lakes that formed during the retreat of ice sheets (Quigley, 1980). Due to the frequency of this sort of failure and their often highly publicized nature (i.e. the 2016 oil spill in the North Saskatchewan River that resulted from a pipeline rupture), an alternative approach to typical geotechnical solutions is being investigated (CBC Saskatoon, 2019). This altered approach is one based upon geochemistry, specifically the mineralogical alteration of smectitic soils via the use of guanidinium salts. Preliminary results of batch style laboratory testing have indicated that guanidinium salts lead to lower liquid limits and an increase in the residual angle of shearing resistance, ´RES. The latter of these two properties is of particular interest for the aforementioned shallow slope failure problems. Guanidinium salts are readily dissolved and biodegraded in naturally occurring surface water (Mitchell, 1987). This property of organic salt makes it an excellent candidate for potentially being used as an insitu treatment method for slow moving landslides. 1.1 Literature Review The nature and composition of the minerals within a soil are key factors governing the mechanism of shear (Lupini, et al., 1981; Skempton, 1985). The three clay minerals that traditionally receive the most attention, in order of increasing activity, are: kaolinite, illite, and montmorillonite. The focus of the work presented here is on montmorillonite soils, and therefore much of the discussion will be made regarding this specific type of clay mineral. Montmorillonite is a 2:1 phyllosilicate comprised of successive links of aluminum octahedrons (Gibbsite Sheets) and silicon tetrahedrons. Figure 1 provides a schematic of a montmorillonite mineral. Weak Van der Waal bonds hold each mineralogical unit together, and therefore each unit is easily separated and susceptible to

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