2553
Lime Remediation of Reactivated Landslides
Traitement à la chaux pour la stabilisation des glissements réactivés
Mesri G.
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, U.S.A.
Moridzadeh M.
Montgomery Watson Harza, Chicago, Illinois, U.S.A
ABSTRACT: Lime improvement of frictional resistance was examined using samples of Brenna Clay Formation from North Dakota.
The montmorillonitic stiff clay samples had a natural water content, plastic limit, liquid limit, clay size fraction, fully softened friction
angle, and residual friction angle, respectively, in the range of 42 to 85%, 20 to 40%, 62 to 154%, 60 to 95%, 14 to 24
, and 7 to 9
.
Immediately after introduction of hydrated lime, pH increased to a range of 12.2 to 12.7; within hours, however, pH began to
decrease. Whereas there was a large increase in plastic limit, the liquid limit response to lime treatment was dependent on the
effective confining pressure. Lime treatment increased fully softened friction angle by 5 to 10
at effective normal stress of 100 kPa
and by 3 to 5
at 300 kPa. Lime treatment increased the residual friction angle by 3 to 6
at both 100 kPa and 300 kPa.
RÉSUMÉ: L’amélioration par addition de chaux de la résistance en frottement est examinée sur des échantillons de la formation
d’argile de Brenna dans le Dakota du Nord. Les échantillons d’argile raide montmorillonitique ont une teneur en eau, une limite
plastique, une limite liquide, une fraction de dimension argileuse, un angle de frottement après remaniement et un angle de frottement
résiduel respectivement de l’ordre de 42 à 85%, de 20 à 40%, de 62 à 154%, de 60 à 95%, de 14 à 24
, et de 7 à 9
. Immédiatement
après l’addition de chaux hydratée, le pH augmente à des valeurs de 12,2 à 12,7 mais commence ensuite à décroître dans les heures
qui suivent. L’augmentation de la limite de plasticité suite au traitement à la chaux est importante, l’augmentation de la limite de
liquidité dépend cependant de la pression de confinement. Le traitement à la chaux augmente l’angle de frottement après
remaniement de 5 à 10
sous une contrainte effective normale de 100 kPa et de 3 à 5
sous 300 kPa. Le traitement à la chaux
augmente l’angle de frottement résiduel de 3 à 6% autant sous une pression de 300 kPa plutôt que de 100 kPa.
KEYWORDS: Brenna clay, frictional resistance, lime treatment, landslides.
1 INTRODUCTION
The effectiveness of lime treatment of soils has been commonly
evaluated in terms of improved workability and increased
undrained unconfined stiffness and compressive strength, in
connection to road and airfield construction (Bell 1996). Soil
improvement is expected to result from the flocculation of clay
minerals and cementing action of lime-soil chemical reactions.
On the other hand if the objective of lime treatment is to
improve long-term stability of first-time or reactivated
landslides in stiff clays and shales, permanent changes in the
size and shape of clay particles must be realized to increase
drained frictional resistance. Lime-soil interactions that may
produce less platey and larger soil particles begin and continue
with time under the highly alkaline pH environment. For
Brenna clay samples treated with lime, measurements of pH as
an indicator of chemical environment, Atterberg plastic limit
and liquid limit as indirect measures of changes in particle size
and shape, and fully softened friction angle and residual friction
angle, were used to examine possible mechanisms of lime-soil
interactions. The main variables, in addition to soil mineralogy,
are soil water content, lime content, and duration of lime-soil
interactions.
2 LIME-SOIL INTERACTION
When dry hydrated lime is thoroughly mixed with a wet soil,
lime is consumed, in the absence of carbonation, through two
mechanisms: (a) part of the lime particles is adsorbed on soil
particles during the mixing process, and (b) part of the
remaining lime is dissolved in the soil porewater. The solubility
of calcium hydroxide in water is rather small (0.75 g/ℓ).
Therefore, the maximum lime content as percent of dry weight
of soil that can dissolve in the porewater during the mixing
process is quite small and a function of soil water content (only
1.5% of lime for 5% lime content at soil water content of
100%). Dissociation of hydrated lime to (OH)
-
and Ca
2+
leads
to a rise in the pH. If enough lime is left, after satisfying the
adsorption, soil porewater becomes saturated and pH increases
to approximately 12.3 to 12.4. Under the strong alkaline
condition, soil mineral particle surfaces become unstable and
begin to dissolve in the porewater. Simultaneously, under the
elevated pH condition, adsorbed lime particles begin to attack
the soil particle surfaces at the points of contact.
Dissolved silica and alumina react with the dissociated
calcium hydroxide and form new compounds. As the dissolved
hydrated lime is used up in the chemical reactions with silica
and alumina, the remaining free lime, if any, dissolves in the
porewater and pH is maintained at 12.3-12.4. The dissolution of
soil particles and local attack of adsorbed lime on the particle
surfaces continue at the initial rate until all free lime is
completely consumed. Thereafter, pH begins to decrease as the
dissociated calcium hydroxide is used up in the chemical
reactions with dissolved silica and alumina. This has been
confirmed by pH measurements and chemical analyses
conducted by Clare and Cruchley (1957) and Diamond et al.
(1964). Dissolution of soil particle surfaces continues at a
decreasing rate, becoming insignificant as pH drops to values
probably less than around 9 (Eades and Grim 1960, Eades et al.
1962, Hunter 1988). The reaction products begin to harden or
crystallize as pH decreases. A calcium hydroxide particle is
attached to more than one soil particle, connecting them
together and producing silt- and sand-sized flocs and
agglomerates (Diamond et al. 1964, Verhasselt 1990). The
Atterberg plastic limit increases, often dramatically, because
large amount of water is enclosed within the flocs and
