What is a Lateritic Soil?

Laterite and Lateritic soils – What are they?

In this blog we take a look at a soil type that is common in tropical regions around the globe. We look at the formation, properties and construction uses of lateritic soils. Lateritic soils encompass soils that have undergone some degree of laterisation.

Andrew Lees visits the Cu Chi tunnels in Ho Chi Minh City, Vietnam to tell us about lateritic soils. 

Laterisation - What is it?

Laterisation can occur when parent rock, rich in alumina, iron and silica, is subjected to weathering in regions where the prevailing temperatures are moderate to high and there are distinct wet and dry seasons. There are three stages of laterisation. First the rock weathers and superficially decomposes and subsequently becomes porous. Next, organic material, silica, lime, nitrogen, phosphate, and other mobile elements, are leached out in successive cycles of wet and dry seasons, while sesquioxides (primarily those of Iron, Fe2O3 and Aluminium, AL2O3) are deposited. These accumulated compounds have been referred to as ‘lateritic constituents’. For this to occur the interstices of the rock will be saturated during the wet seasons and dry out completely during the interposing dry seasons. The final stage of laterisation involves dehydration and crystallisation of the resulting products. The laterization process is typically incomplete and the materials can exist at all stages from parent rock to soil. Laterisation generally gives rise to blended horizons in the soil profile.

The degree of laterisation varies considerably with depth:
•    topsoil, 
•    completely weathered - strong laterisation,  
•    highly weathered - moderate laterisation,  
•    moderate to slightly weathered – weak laterisation, 
•    parent rock.

Figure 1 - Degree of laterisation in relation to composition - silica vs the sesquioxides and other compounds (After Netterberg 2014)

What is a lateritic soil?

Lateritic soils are highly weathered residual soils found in tropical and sub-tropical regions, formed as a product of the laterization of rock. The colour of lateritic soils ranges from dark reddish brown, through rusty red, to purple. There are many physical forms of lateritic soil. Particle size can range from clay/silt to sand/gravel. As a result of laterization the particles are typically coated with sesquioxide concretions, giving cementitious characteristics. 

What are the characteristics of lateritic soils?

The characteristics of lateritic soils are dependent upon the synergistic effect of a variety of factors, including the parent rock composition and morphology, rainfall intensity, temperature, hydrology, topography, and depth within the soil profile. Lateritic soils can exhibit the properties of both cohesive and non-cohesive soil. The silt and clay based lateritic soils exhibit cohesive behaviour, whilst those with predominantly sand/gravel size particles exhibit non-cohesive behaviour. Lateritic soils show such a huge variation in geotechnical characteristics that the term can only be used in a general way and not as a soil classification type for engineering purposes.

Figure 2  - Highly variable geotechnical characteristics of lateritic soils (after Kumar et al, 2022)

Laterite vs Lateritic Soil – what is the difference?

This has been the subject of much debate since the term laterite was first introduced by Buchanan in 1807 to describe a material he saw being used for brick making in Malabar, India. He described the material he saw as having soft consistency in-situ but hardening rapidly on exposure. In fact, the name laterite derives from the Latin word for brick – ‘later’. Laterite has been given various definitions over the years. It is probably safe to say that laterite is the result of complete (or mostly complete) laterisation of rock. In a laterised soil profile, the topmost layer of residual material – immediately below any topsoil, may become laterite given time, thought to be a geological epoch of roughly a million years. Fermor, way back in 1911, suggested that a residual material with 90% or more ‘lateritic constituents’ could be considered a true laterite. Residual material that has not completely laterised, but has released silica, has sesquioxides fixated to particles, has no accumulated organic matter and had the reddish brown to rusty red colour, should be characterised as lateritic soil. We could say that a lateritic soil is on the way to becoming laterite, given the right conditions and geological time.
 

Can lateritic soils be used for construction?

Bricks - Lateritic soils are widely used for brick making. This has been done for centuries. In the most basic form, bricks are cut and shaped from the soil profile and left to dry and harden. In some cases, the lateritic soil is reformed in moulds and compressed into hard bricks. The addition of cement or lime greatly improves performance. The strongest, least moisture absorbent bricks are produced by moulding and firing. 

Lateritic soils for road construction – Lateritic soils have been used as a road making material in tropical and sub-tropical countries for centuries. Their performance is greatly influenced by climate, drainage and compaction density achieved. Lateritic soils have unique properties that make them different from other road making materials. This often results in their rejection for use based on conventional testing and specification requirements. Yet despite this, their performance in service has been proven to be good. For this reason, a number of countries have developed specifications suited to lateritic soils. In recent decades medium quality lateritic soils have been used successfully for road base and sub-base layers when stabilised with cement or lime and in some cases the addition of rice husk ash. Mechanical stabilisation of lateritic soils is another option. Parock et al (2013) describe the use of geosynthetics such as Tensar geogrid as an economic alternative to chemical stabilisation. Geogrids are a polymer-based product, used to solve civil and geotechnical engineering problems in or on the ground. Tensar can provide advice on selecting compatible stabilisation geogrids.

Earthworks – Lateritic soils are widely used in embankment construction and the backfill of structures. In addition, these soils have been widely used as structural fill in reinforced soil structures. Erlich and Becker (2010) reporting on their use in reinforced soil structures in Brazil note that the compacted lateritic soil has high strength, low compressibility, and low permeability. Gouw (2016) reports that lateritic soils are often used as fill in reinforced soil structures in Indonesia. He warns that compaction should be undertaken in drained conditions. For advice on the suitability of lateritic soils in Tensartech reinforced soil walls and steep slopes, contact the design team at Tensar at: https://www.tensar.co.uk/solutions/design-support 

References

•    Netterberg, F. 2014. Review of Specifications for the use of laterite in road pavements. ANSARA
•    Kumar, G.S., Saini, P.K., Deoliya, R., Mishra, A.M., Negi,S.K., 2022. Characterization of laterite soil and its use in construction applications: A review. Resources, Conservation & Recycling Advances  Journal. Elsevier
•    Parrock,A., Pleissi, L.L.L., James, G.M., Roe, T., Cook, J., 2013. Designing Cost-effective, Geosynthetic-reinforced Pavements in Equatorial Africa Utilising 2-D Axi-symmetric Finite Element Analyses. GhIGS GeoAfrica 2013 Conference. Accra
•    Gouw,T.L., 2016. Common Mistakes in Designing MSE Wall with Finite Element Method. Geosintetik Conference. Indonesia.
•    Ehrlich,M., Becker,L.D.B., 2010 Reinforced soil wall measurements and predictions. 9th International Conference on Geosynthetics, Brazil