MSc

The soils of highland and midland laterites in Kerala are unique in characteristics
due to the various soil formation processes in the landscape. The midland laterites are often
underlain by plinthite and contain appreciable quantities of gravel, which holds against
supersaturation where as the highland laterites are devoid of plinthite. The hill zones of
Kerala are witnessing frequent landslides during every monsoon. Landslides are induced
by several factors and the soil-rainfall interacting process is one of the important processes,
wherein the soil susceptibility increases landslide hazards by triggering slope failure due
to the draining of water through infiltration, percolation and saturation of soil at an extreme
level. Geotechnical properties / Atterberg limits are the basic measure of soil plasticity as
a function of moisture variation. It includes the Plastic Limit (PL) which is the limit at
which soil changes from semi-solid to plastic and the Liquid Limit (LL) which is the limit
at which soil changes from plastic to liquid state as the water content increases. Atterberg
properties of soil of a region depend on soil physico-chemical properties (soil textural
dynamics, porosity, soil mineral types etc) which strongly influence landslides. To
understand the physical and geotechnical properties of the laterite soils of hill and midland
with respect to their behaviour on progressive saturation with water, the study
“Geotechnical characterisation of lateritic soil profiles in midland and highland plateaus of
Kerala” was carried out in the Department of Soil Science & Agri. Chemistry, College of
Agriculture, Vellanikkara during 2021-22.
The soil profiles for the study were collected from Mulagunnathukavu, Attoor and
Kuranchery (AEU 10) of Thrissur district, Kokkayar, Koottikkal (AEU 14), Koottar and
Pampadumpara (AEU 16) of Idukki district for elucidating morphological, physical,
chemical and geotechnical properties. The results of the physico-chemical analysis showed
that, the soils were acidic in reaction with values ranging from (4.78 to 6.53) in midland
and (4.17 to 6.49) in highland laterites. The Cation Exchange Capacity (CEC) ofthe soils
ranged from 6.2 to 15.3 cmol (+) kg-1
in midlands and 10.40 to 19.60 cmol (+) kg-1
in
highland laterites. Based on the morphological and physico-chemical properties,
the midland and highland were classified under the order Ultisols. The dominant suborder
in all the midland laterites was ustults and the dominant suborder of highland washumults
except for the Kokkayar profile which was under the suborder ustults. Amongthe
midland profiles studied the Mulagunnathkavu soil profile was unique with plinthite
formation in the lower horizon and the soil contained appreciable quantities of gravel and
clay per cent. The Atterberg limits of highland soil was higher compared to midland soil
and it ranged from 31.62 to 54 per cent for liquid limit, 22 to 35 per cent for plastic limit
and 6.79 to 23.61 per cent for the plasticity index. The Atterberg limits of midland soil
ranged from 23.99 to 45 per cent for liquid limit, 18.55 to 29 per cent for plastic limit and
4.71 to 19 per cent for plasticity index. The higher value of Atterberg limits in the highlands
can be attributed to the presence of more fine earth clays in the highlands compared to the
midlands. Comparison of soil properties between midland and highland soil profiles for the
surface and sub soil horizons showed significant differences in the soil properties like
organic carbon, extractable iron oxides, exchangeable sodium, potassium, liquid limit and
plastic limit, coarse sand, gravel per cent and bulk density.The per cent of coarse sand,
gravel per cent and bulk density were higher in midland laterites compared to highland
laterites and the content of organic carbon, extractable ironoxides, sodium, potassium,
liquid limit and the plastic limit was higher for the highland soil compared to midland.
A correlation between plastic limit and liquid limit withdifferent soil properties of
highland profiles showed that the clay has positive correlation and sand has a negative
correlation with Atterberg limits. The random forest classification-regression model ranked
clay as the most important variable affecting the plastic limit. The high clay content
contributed to a directional arrangement of soilparticles leading to the increase in weakly
bound water increasing the plastic limit. The X-ray diffraction analysis results revealed
that kaolin, gibbsite, feldspar, quartz and mica were found in both midland and highland
laterites, which showed the different stages of weathering.
Thus, it can be concluded that, the high rainfall, soft lateritic deposits, highly
plastic clay and intensively weathered rocks, will saturate the entire mass and can cause
loss of shear strength and this can be one of the reasons for landslides in highlands (AEU
14 and 16). The midland laterites (AEU 10) though it contains high illuvial clay the hard
plinthite layer (hardened mix of iron bearing minerals with honeycomb structure and voids
with kaolin) holds against supersaturation and resist clay movement. Therebyensuring
protection against landslides and piping in this AEU 10.