Description
The influence of changes in pore water chemistry on the residual strength of clays has been widely reported in the technical literature. Ramiah (1970) reported variations in the residual angle of friction j’r of about 4°. Similar conclusions were reached by Kenney (1977), Moore (1991), Di Maio (1996a, 1996b), Maggiò et al. (2002) for various pure and natural clays and by Steward and Cripps (1983) for pyritic shale.
Moore (1991) carried out a systematic laboratory investigation of this issue and reported that clays saturated with monovalent sodium cations consistently resulted in lower residual strengths than clays saturated with calcium divalent cations. The type of cation can account for changes in residual strength of up to approximately 40% for montmorillonite and 15% for kaolinite clay minerals.
The concentration of salts in in the pore water was found to result in furhter differences in residual strength. Moore (1991) also showed that increasing concentrations of seawater result in increasing residual strength in natural clays too, suggesting that seasonal fluctuations in the concentration of salts in pore water can modify the residual strength of natural calys. This observation, which can be particularly significant for coastal landslides, is corroborated by field observations (Moore, 1988; Moore and Brunsden, 1996).
Mesri and Olson (1971) showed that the void ratio of clay samples decreased when subjected to a long term increase in the concentration of NaCl, thus increasing consolidation and stability. NaCl are especially known for long term stability of sensitive clays as the presence of cations change the surface tension on the clay minerals. Long term leaching of NaCl destabilizes clays and when the content of NaCl becomes too low the clay becomes quick (NGU, 2002). In spite of this it is not found that NaCl is used for increasing stability of clays by groundwater exchange.
Instead, the most common technique for lowering landslide susceptibility by modification of groundwater chemistry is to add lime to the soil, often creating lime columns in the ground. The methods for creating lime columns in the ground are the same as described for mechanical deep mixing, permeation grouting and jet grouting.
Lime-stabilization has been applied especially to soft and sensitive clays (Rogers and Glendinning, 1997).
It is widely reported that lime migrates from the columns, stabilizing also the surrounding clay. Stabilization is achieved due to formation of calcium silicate hydrate and calcium aluminate hydrate; both gels crystallize in the pores of the clay (Rogers and Glendinning, 1996). The migration has been reported over great distances, probably due to hydraulic gradients. Bell (1996) investigated the effect of lime stabilization on both clay and till and found that till did not show any significant increase in stability to tratment with lime. Migration of ground water into lime columns has also been observed.
The effects of lime columns in clay may be summarized as follows (Rogers and Glendinning, 1997):
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Increased strength of an annular zone of clay surrounding the columns, caused by lime-clay reaction;
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Clay dehydration;
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Generation of negative pore-water pressure;
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Over-consolidation of the soil in the shear plane;
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Columns strength.
The stabilization of an embankment of loose clay shale fill was attempted in Thailand in 1977. Lime piles were installed in a regular grid with a spacing of 3 m (Figure 1). Holes 15 cm in diameter were augered by hand down to natural hard ground, and lime and water were poured into the holes and topped up daily for two months.
Based on measurements at four locations, Ruenkrairergsa and Pimsarn (1982) report a significant change in soil properties two years after installing the lime piles: the water content of the clay decreased by up to 6.0 %, the cohesion increased by up to 15.7 kN/m² and the friction angle increased by up to 8.1°.
Design methods
Although some experimental case histories are reported in the literature, some of which characterized by a reasonable degree of success, there is no consolidated and reliable design approach at this stage for landslide stabilization besed on modifications of groundwater chemistry, which at present remains wholly empirical.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Applicability to spreads and flows to be carefully evaluated on a case by case basis, bearing in mind the risk that installation iteself could trigger movement |
| Topple | 0 | |
| Slide | 6 | |
| Spread | 4 | |
| Flow | 4 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 6 | Only applicable in clays, but stabilizing effects depend on continued treatment |
| Debris | 0 | |
| Rock | 0 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 0 | Groundwater chemistry conditioned through relatively small boreholes, can be used in medium to very deep landslides. |
| Shallow (0.5 to 3 m) | 4 | |
| Medium (3 to 8 m) | 8 | |
| Deep (8 to 15 m) | 8 | |
| Very deep (> 15 m) | 8 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Long term operation of injection boreholes make this technique applicable only when movement is extremely slow or very slow (maximum 1.5 m/year or 5 mm/day) |
| Slow | 0 | |
| Very slow | 6 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 0 | Uses groundwater for diffusion from injection hole to soil mass; best suited to sites with high groundwater levels and a moderate groundwater flow |
| High | 8 | |
| Low | 4 | |
| Absent | 0 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 6 | Not applicable close to water courses. Potential pollution of watercourses during construction or from subsequent diffusion of salts.. |
| Snowmelt | 6 | |
| Localized | 0 | |
| Stream | 0 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 4 | Case histories indicate contrasting results. Extend and effectiveness of diffusion unpredicatble. Needs continuous maintenance to remain effective. |
| Feasibility and Manageability | 4 | Mostly experiemntal at this stage. Some succesful case histories exist, but no established design practice |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Relatively simple to implement |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 6 | Installation cost is moderate, but requires maintenance |
References
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Bell F.G. (1886) ”Lime stabilization of clay minerals and soils” Engineering Geology, 42, 223-237.
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Di Maio C. (1996a). “The influence of pore fluid composition on the residual shear strength of some natural clayey soils”. Proceedings of 7th International Symposium on Landslides. Trondheim, Vol. 2, Balkema, 1189-1194.
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Di Maio C. (1996b). “Exposure of bentonite to salt solution: osmotic and mechanical effects”. Géotechnique 46(4), 695-707.
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Kenney T.C. (1977). “Residual strength of mineral mixtures”. Proceedings 10th International conference on Soil Mechanics and Foundation Engineering, 1, 155-160.
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Maggiò F., Pellegrino A., Urciuoli G. (2002). “Influence of pore fluid composition on the mechanical behaviour of a marine plastic clay”. Environmental Geomechanics – Monte Verità,
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Mesri G., Olson R.E. (1971) ”Consolidation characteristics of montmorillonite” Géotechnique, 21, 341-352.
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Moore R. (1988) “The clay mineralogy, weathering and mudslide behaviour of coastal cliffs”. PhD thesis, University of London.
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Moore R. (1991). “The chemical and mineralogical controls upon the residual strength of pure and natural clays”, Géotechnique 41(1), 35-47.
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Moore R., Brunsden D: (1996). “A physico-chemical mechanism of seasonal mudsliding”, Géotechnique, 46(2), 259-278.
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NGU (2002) http://www.ngu.no/upload/Geofarer/Skred/Leirskred/Om%2Oleirskred.pdf
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Ramiah B.K. (1970). “Influence of chemicals on the residual strength of silty clay”. Soils and Foundations, 10, 25-36.
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Rogers C., Glendinning S. (1996) ”The role of lime mitigation in lime pile stabilization of slopes” Quarterly Journal of Engineering Geology & Hydrogeology, 29(4), 273-284.
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Rogers C., Glendinning S. (1997) ” Improvement of clay soils in situ using lime piles in the UK” Engineering Geology, 47, 243-257.
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Ruenkrairergsa T., Pimsarn T. (1982). “Deep hole lime stabilisation of active landslide for unstable clay shale embankment”. Proceedings 7th S E Asia Geotechnics Conference, 22-26 November 1982, Hong Kong, 631-645.
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Steward H.E., Cripps J.C. (1983). “Some engineering implications of chemical weathering of pyritic shale”. Quarterly Journal of Engineering Geology, 16, 281-289.