Agung MULYO

Dicky MUSLIM

Wiyatno HARYANTO

Zufialdi ZAKARIA

HARYANTO, Wiyatno

MUSLIM, Dicky

MULYO, Agung

ZAKARIA, Zufialdi

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Characterization of Potential Slope Failures on Lateritic Soils

Utsunomiya University Academic Information Repository

CHARACTERIZATION OF POTENTIAL SLOPE FAILURES 
ON LATERITIC SOILS 
Wiyatno HARYANTO1∗, Dicky MUSLIM2, Agung MULYO2, Zufialdi ZAKARIA2 
1Post Graduate Student at Universitas Padjadjaran, Faculty of Geological Engineering, 
2Universitas Padjadjaran, Faculty of Geological Engineering,  
Jl. Dipatiukur No. 35 Bandung, 40132, Indonesia.    
*Corresponding author: wiyatno.haryanto@gmail.com
ABSTRACT 
Lateritic soils are commonly consist of unsaturated or partly-saturated soils that have been formed 
by a very intensive weathering process on a parent rock in a tropical climate with high temperature 
and rainfall intensity as well that enabling a leaching of heavy minerals and supergene enrichment. 
The geological structures as indicated by shear zones may form a highly serpentinized rock referred 
to as mylonite layer as a talc-carbonates-altered rock that having a consistency of very soft to soft. 
The slope failures (landslides) are commonly occurred on lithologic contacts (interface of limonite-
saprolite and saprolite-bedrock as well) in which the failure mechanisms are presumed controlled by 
presence of a thin soft soil layer with high water content. The erratic ground profiles on the lateritic 
soils such as undulating, irregular, and anisotropic layers, presence of water table, existence of soft 
soils on a shear zone, and insufficient of geotechnical investigation program lead some difficulties to 
determine a potential slope failure in the slope stability analysis. A geological modelling that 
introducing specific valuable information regarding the position of the shear zones, phreatic lines, 
ground profiles, and lithologic contacts should be taken into account to obtain a more reliable slope 
stability analysis result. By a proper geological modeling approach, the potential slope failures and 
its mechanism can be mitigated prior to the landslide occurs.          
Keywords: Lateritic soils, lithologic contact, shear zone, slope failure, geotechnical investigation. 
1. INTRODUCTION
There are a lot of number of landslide events occurred in lateritic soils that commonly caused by
high rainfall intensity within a long term period. Several failures on a slope excavation had involved
a change of slope geometry (height of bench, width of safety berm, angle of individual slope, inter-
ramp slope, and overall slope), lithologic contacts (interface of limonite-saprolite and saprolite-
bedrock), uncertainty of erratic ground profiles, and presence of a water table on site. Some author’s
personal notes concluded the slope failures of the lateritic soils are due to either any uncertainty of
geological model, presence of an unrecognized soft soil layer, high rainfall intensity within a long
term period, and improper geotechnical analysis approach.
To mitigate recurrence of the landslide disaster incidents mainly in lateritic soils, characterization of
the potential slope failures should be well organized by means of a systematic geotechnical
investigation program to obtain a sufficient data for geotechnical slope stability analysis purpose.
2. LITERATURE REVIEW
Lateritic soils is a residual soil rich in sesquioxides of iron with some mineral enrichment that have
been developed in the process of chemical weathering and supergene enrichment under a tropical
climatic conditions (Ahmad, 2006). It is commonly consist of unsaturated or partly-saturated soils
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that have been formed by intensively weathering process on a parent rock with high temperature and 
rainfall intensity as well that enabling a leaching of some heavy minerals.  
Tuncer (1976) has empirically explained a relationship between shear strength parameters of lateritic 
soils as represented by mechanical properties (cohesion and angle of internal friction) with index 
properties such as specific gravity, void ratio, and moisture content shown in the Figure 1. Referring 
to the data, maximum cohesion about 30 psi (or equal with 207 kPa) were achieved in a specific 
gravity value about 3.0, meanwhile angles of internal frictions increased slightly corresponding with 
the increasing of the specific gravity, and cohesion values archly decreased as void ratio and 
moisture content increased.     
(a) (b) 
(c) (d) 
Figure 1. Relationship between: (a) cohesion vs specific gravity, (b) internal friction angle vs specific gravity, 
(c) cohesion vs void ratio, and (d) cohesion vs moisture content.
It is well recognized that the soils behavior as aforementioned is attributed to the variation in soil 
structure with varying moisture content, such that, as the soil dries out part of the hydrated colloidal 
iron and aluminum oxides dehydrates and forms strong bonds among certain soil grains which, in 
turn, causes an increase in strength (Tuncer, 1976). A weakest thin layer (very soft to soft clayey 
materials) is commonly encountered in a transitional zone close to the lithologic contact between 
limonite and saprolite in which the water table fluctuated along the years. The situation becomes 
more complicated when the problematic soils present in the cut of slope in which topographic 
depression (commonly a swampy area, sag ponds, and small basins) constitutes a tectonic product of 
pull-apart structure occasionally appears on the top hill. In this zone, runoff water have been trapped 
in the depression within a hydrate environment, therefore weathering becomes very intensively 
formed a montmorilonite clay soil or called as black cotton soils (Wesley, 2010). 
Haryanto, et al. (2010) explained the lateritic soils profile is commonly divided into some layers 
referring to the exploration data and former investigation, as follows: (1) overburden layer that 
occupies the upper layer, soft and reddish brown to dark, comprises association of goethite and 
limonite mineral, and occasionally hematite, (2) limonite layer that occupies below overburden layer, 
fine grained, reddish brown to yellow, slightly soft, rich of iron oxides from limonite cover up the 
whole area with the thickness until tens meter. Some manganese oxide mineral and occasional talc, 
tremolite, chromiferous, silica, and gibbsite mineral are encountered in this layer, (3) saprolite layer 
which has brownish yellow and slightly reddish color, brittle, medium stiff to stiff, occupies the 
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underneath of limonite layer with thickness averages about 7 m, formed by leaching of fine grained 
limonite and garnierite, silica, manganese oxides deposit of transitional zone from limonite to 
bedrock, occasionally quartz mineral encountered fill crack & fissures, weathering of major mineral, 
chlorite, and garnierite; structure and texture of origin rock is commonly can be distinguished 
obviously, and (4) bedrock layer, constitutes a bottom of the laterite profile as fresh peridotite 
bedrock with no weathering process occurred, yellow pale until greenish grey, very dense to massive. 
In this bedrock, strongly fracturing affect opening structure that filled by garnierite and silica 
minerals.  
Typical of laterite profile is generalized into a red limonite soils (hematite) and a yellow limonite 
soils (goethite) as ferruginous zone, saprolite as supergene enrichment zone, and blue zone as a 
bedrock layer shown in the following Figure 2 below (Haryanto, et al., 2010; 2016). 
(a) (b) 
Figure 2. Typical of laterite profiles: (a) at SRK site, (b) at PTA site. 
Haryanto, et al. (2016) explained some difficulties to retrieve a very thin soft soil layers in lateritic 
soils as a potential slip surface by means of the standard penetration test (SPT) in a geotechnical 
exploratory drilling program. The carefulness of a drilling operator to operate the whole field tests 
and engineering geologist during undertaking description of the core samples to collect a valid and 
reliable data is absolutely required. For the first 10 m depth, the N-SPT are commonly ranging from 
0 – 3 blows/foot (soil consistency very soft) then relatively increased with depth until 5 – 8 
blows/foot (soil consistency soft to medium stiff) meanwhile the shear strength parameters in 
effective stress such as cohesion about 5 – 15 kPa and angle of internal friction about 15 – 20°. For 
the next deeper position after penetrating a capillary fringe zone that commonly as lower limonite up 
to saprolite layer in which the water table fluctuated (partly saturated), the presence of a thin soft 
layer should be identified by recognizing appearance of the shear zone (highly serpentinized) that 
characterized by colors of greenish grey mylonite clays, brecciated, and high water content.  
3. METHODOLOGY
To characterize a potential slope failure on a cut of slopes of lateritic soils, an advanced geotechnical
investigation as should be optimized by extending scope of current geotechnical investigation
method by means of: (1) increasing number of effective shear strength parameters (since undrained
shear strength parameters are not applicable), (2) identifying shear zone, (3) discovering lithologic
contacts, and (4) recognizing the extrinsic contributing factors on the slope failures.
However, the geotechnical exploratory drilling program is still the most reliable method to observe 
soil/rock engineering properties and identifying some critical information to be used in a slope 
stability analysis as part of landslide hazard mitigation. In addition, a geophysical method e.g. 
electrical resistivity tomography (ERT) is introduced to retrieve subsurface condition accurately.  
4. RESULT AND DISCUSSION
There are several approaches to characterize the potential slope failures on lateritic soils; one of the
common methods is geotechnical exploratory drilling to sink core barrel and retrieving undisturbed
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soil samples (UDS) to obtain index and mechanical properties of soil and/or rock mass for 
engineering purposes. For cut of slope stability design, the effective stresses parameter (effective 
cohesion, c’ and effective angle of internal friction ϕ’) are compulsory to be explored extensively 
since the slope geometries would be designed for long term utilization due to the water table allowed 
to percolate and dissipated gravitationally. However, the undrained shear strength parameters (Su) 
from standard penetration test (SPT) and triaxial-UU (undrained unconsolidated) tests are not 
allowed because the analysis result would be much different with the actual slope condition.  
In an effective stress condition that represented by some triaxial CD (consolidated drained) tests, the 
existence of water content is playing a significant role that commonly influencing shear strength 
parameters. At least, the 60% of water content is considered as optimum moisture content to 
contribute the peak strength as shown in the following Figure 3 below.  
(a) (b) 
Figure 3. Relationship between: (a) effective cohesion and water content and (b) effective angle of 
internal friction and water content. 
Refer to the chart as abovementioned, after exceeding 60 – 80% of water content, the effective 
stresses parameter decrease significantly. A value of the water content more than 100% in the 
lateritic soils becomes a serious concern as an indicative data for assuming further geotechnical 
models. The statement is emphasized by a discovery of carbonaceous clay layer and plenty of 
organic material with high water content even more than 120% until tens meters thickness. For the 
slope stability concern, this problematic soil should be carefully assessed in effective condition. 
Furthermore, the high water contents of lateritic soils are commonly encountered in 7 – 35 m depth 
following the water table as shown in the following Figure 4.    
Figure 4. Relationship between water content and depth. 
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Relationship between water content and unit weight of moisture soils is inversely correlated in 
which the higher water content the lower unit weight, meanwhile the unit weight of moist soils are 
relatively not influenced by difference of specific gravity value as shown in Figure 5 below.  
(a) (b) 
Figure 5. Relationship between: (a) moist unit weight and water content, and (b) moist of unit weight 
and specific gravity. 
The presence of a very moist clayey layer with soft consistency and appearance of shear zones in 
lateritic soils should be taken into account and well recognized by geotechnical engineers prior to do 
geotechnical modeling. Based on author’s empirical experiences at some slope failures on lateritic 
soils, a very moist soft clay layer with low to medium plasticity were a slip surface with presence of 
groundwater seepage on the scarps. In the same way, the shear zones as characterized by a greenish 
grey mylonite clayey layer were a slip surface as well in which the material mostly wet, friable, and 
partly altered.  
(a)  (b) 
Figure 6. Core samples: (a) reddish brown clayey silt (MH) at 3.50 – 4.00 m depth in limonite layer, soft, 
medium plasticity, very moist, and (b) greenish grey mylonite clay (CH) at 20.00 – 20.50 m depth              
in saprolite layer, very soft consistency, very moist. 
The intrinsic factor that contributing the slope failures on the lateritic soils is presence of lithologic 
contacts or geological structures (i.e. crack, fault, folding, discontinuities, and others) referred 
to as interface of limonite-saprolite and saprolite bedrock layers. The contacts can be obtained by 
distinguishing homogeneity of the core samples taken from geotechnical exploratory drilling that 
characterized by similarity of physical and chemical properties for example color, texture, structure, 
consistency, weathering degree, presence of certain minerals, and others. Accordingly, the drill-
holes data and lithologic contacts shall be used to calibrate an advanced geophysical electrical 
resistivity tomography (ERT) survey to obtain a more accurate subsurface condition for geotechnical 
modeling purposes and slope stability analysis as shown in the following Figure 7 below.  
20.00 – 20.50 m depth 
3.50 – 4.00 m depth 
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Figure 7. The lateritic soils profile based on ERT and drill-holes 
(LIM = Limonite, SAP = Saprolite, BRK = Bedrock) 
The subsurface condition including lithologic contacts and geological structures that resulted from 
reconciliation of the geotechnical exploratory drilling and ERT survey are considered as a reference 
to presume the probable slip surfaces and potential slope failures in the slope stability analysis.  
Several extrinsic factors that can be contributing the slope failures on the lateritic soils such as high 
rainfall intensity within a long term period, poor drainage system on the crest and toe of slopes, 
presence of earthquake, and changes of land use and tributary system. The extrinsic factors should 
be considered in geotechnical modeling and mitigation programs.  
5. CONCLUSION
Some critical information that should be taken into account in a geotechnical investigation program 
such as lithologic contacts (interface of limonite-saprolite and saprolite-bedrock), presence of water 
table, problematic soils, shear zone (serpentinization), slicken sides, and others that should be 
recorded in the drill-logs. The problematic soils should be explored more extensively for example: 
presence of swelling clays, clayey shales (expansive soils), scaly clays, carbonaceous clays with 
high organic content, and others that occasionally encountered in the lateritic soils.  
Carefulness in geotechnical modeling should be a concern when water content of the soils exceeding 
100% due to the shear strength parameters drops significantly. For the slope stability concerns on 
the problematic soils should be assessed in effective condition and cut slopes should be conducted 
by staging in a long time frame.  
As it is known, a geotechnical investigation program has limitation to deliver more accurate data; 
therefore one of geophysical survey methods becomes another option to overcome the information 
gaps in between of the geotechnical drill-holes by means of electrical resistivity tomography (ERT) 
to scanning subsurface condition and geological structures by reading out the electrical resistivity of 
the ground profile.  
ACKNOWLEDGEMENT 
This paper is dedicated to Prof. Dr. Ir. Adjat Sudradjat, M.Sc., Dr. Ir. Vijaya Isnaniawardhani, MT., 
Dr. Eng. Boy Yoseph CSSSA, ST., MT., Dr. Muhamad Sapari Dwi Hadian, ST., MT., as our 
respectful counterpart in scientific discussion, and all academic communities in Universitas 
Padjadjaran (UNPAD), Bandung. 
REFERENCES 
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Ahmad, W., 2008. Laterite Geology and Mineralization Processes. Technical Presentation, Vale 
Inco Exploration, Unpublished.  
Haryanto, W., Hariyadi, H., Hendrartijanto, R., Ardhiwishesa, P., 2010. Influence of Matric Suction 
on Unsaturated Soil Shear Strength Behavior and Slope Stability: Case Study on Anoa 
South MHR Slope in Mine Operation Area of PT INCO Tbk Sorowako. 19th Annual 
Professional Meeting at Indonesian Mining Association, Balikpapan, Indonesia.   
Haryanto, W., Kumarawarman, B., Wanni., 2016. Geophysical Approach to Resolve Geotechnical 
Challenges on Slope Failures Modeling in Residual Soil. International Conference on 
Problematic Soils (SOFT SOILS 2016), Bandung, Indonesia.  
Ng, C. W. W., Leung, A. K., Xu, J., 2011. The Theory and Application of Unsaturated Soil 
Mechanics in Slope Engineering. Proceedings of 9th Indonesian Geotechnical Conference 
and 15th Annual Scientific Meeting, Indonesian Society for Geotechnical Engineering 
(ISGE), ISBN No: 978-979-96668-9-5, Jakarta, Indonesia.    
Tuncer, E. R., 1976. Engineering Behavior and Classification of Lateritic Soils in Relation to Soil 
Genesis. Iowa State University, United States of America.  
Wesley, L. D., 2010. Fundamentals of Soil Mechanics for Sedimentary and Residual Soils. John 
Wiley & Sons, Inc., ISBN No: 978-979-29-2633-0, Hoboken, New Jersey.  
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CHARACTERIZATION OF POTENTIAL SLOPE FAILURES ON LATERITIC SOILS

CHARACTERIZATION OF POTENTIAL SLOPE FAILURES ON LATERITIC SOILS

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