What is Laterite?

Laterite (pavement material)

Definition

Laterite is a tropical soil rich in iron and aluminium oxides that road builders use as gravel in pavement layers.

The AFCAP review (Netterberg, 2014) describes it as a tropical soil that advanced weathering produced. The weathering removes bases and silica, and the iron and aluminium oxides build up. This page covers laterite as a pavement material. The base course page covers the layer in general. The wearing course page covers the top layer.

Laterite, lateritic soil and plinthite

The word has many meanings. The AFCAP review says that there is no universally accepted definition. Geology, pedology and engineering each use their own. Pedology dropped the word and uses plinthite instead.

The review uses these terms:

• Laterite. A strongly cemented material with at least 50 percent iron and aluminium oxides as the cement.
• Lateritic soil. A material with about 20 to 50 percent of the cement. It may be less developed, or the host soil may still dominate.
• Lateritic material. The general term for both. The review proposes it for road engineers.

Other local names include murram in East Africa, cabook in Sri Lanka and ferricrete in southern Africa. The review proposes that road engineers use lateritic material as the general term and do not use ferricrete.

Where it occurs

Laterite forms mostly in the humid tropical and sub-tropical zones. These include Africa, India, South-East Asia, Australia, and Central and South America. Climate shifts in the past mean that some laterite lies outside today’s tropics (2.3.4).

Buchanan first described it in India in 1807. It was soft in the ground and hardened fast in the air, so builders cut it into bricks. The name comes from the Latin word for brick.

Why it fails standard tests

Laterite often fails the limits that road authorities apply to gravel, and it still performs well in service (4.1). The AFCAP review gives these reasons:

• Grading. It often has gaps in the sand fraction. A mass-based sieve test exaggerates this, because the iron-rich coarse part is heavier than the fine part. The coarse part has a specific gravity of 3.0 to 3.5 and the fine part about 2.7. A volume-based grading can look better (3.2.1).
• Porous particles. They raise every moisture content measurement, including the Atterberg limits.
• Clay type. Kaolinite is the main clay. It has a non-expansive lattice, so the material swells less than with other clays.
• Plasticity and strength. Laterites often have a plasticity index of 15 to 20. Their soaked CBR is often below the normal minimum of 80 percent (1.1).
• Preparation of samples. Normal laboratory work breaks aggregations of silt and clay that stay whole during construction (4.1).

The properties that matter most are the grading, the Atterberg limits, the strength of the coarse particles, and the compaction and bearing strength (3.2).

Criteria for a laterite base

Charman (1988) proposed criteria for a lateritic gravel base under a thin bituminous surface in the tropics. The AFCAP review lists them in Table 4-2.

Charman gives a Los Angeles abrasion value of 65 percent or less for a minor road base. For the other base rows it is 50 percent or less. The CBR is for samples that soaked for 4 days. The base samples for traffic above 0.3 million are at 100 percent of modified compaction density. The others are at 95 percent.

The review says that Charman suggested these bases for low volume roads only. Brazil uses fine-grained lateritic soils as base for up to 1,500 vehicles a day and 5 million equivalent standard axles (5.2). Main Roads Western Australia (MRWA, 2002) sets criteria for a base under a thin bituminous surface. The soaked CBR is 80 or more and the liquid limit is 25 or less. The dryback is 85 percent of optimum moisture or less (Table 4-10).

Thickness of a laterite layer

The AFCAP review gives material criteria. It gives no layer thickness. We found no thickness standard that is specific to a laterite wearing course.

The FHWA gravel roads manual gives a general rule for gravel. The thickness depends on the number of heavy trucks and the strength of the roadbed (Table 6). The table is for gravel in general and not for laterite.

The manual says that a gravel road loses surface gravel to traffic, erosion and rain. The design has to allow for the loss (A2). It also gives 25 to 50 mm (1 to 2 in) as a typical allowable rut depth for an aggregate surfaced road. The subgrade page covers the layer that sets the CBR class.

A gravel surface needs different material than a base. The manual’s sample state specification allows 4 to 15 percent passing the No. 200 sieve in surface gravel. The plasticity index is 4 to 12. For base gravel it allows 3 to 12 percent and a plasticity index of 0 to 6. The plastic fines bind the surface.

Construction

AFCAP section 5.2 reports these practices for laterite bases:

• Compact at close to optimum moisture content. A laterite that is compacted dry of optimum often gives a poor surface and does not reach its strength.
• Compact to a high degree. A high degree of compaction keeps the strength when the laterite is wet.
• Cure low plasticity gravel for about one day after the water goes in. Lateritic sands may need 4 or 5 days.
• Break large particles of concretionary laterite with a grid roller. Paving machines work poorly because they segregate coarse and fine particles.
• Shape the top for drainage. Delay the seal until the base is dry and the surface is firm. This can take 1 to 2 months.

Laterite on an airfield

No text from ICAO, EASA or the FAA that we read names laterite. We found no airfield specification for it. The FAA crushed aggregate base, Item P-209, has these limits (AC 150/5370-10H, 209-2.1):

• A Los Angeles loss of 45 percent or less.
• At least 90 percent of particles with two fractured faces, and 98 percent with one.
• A fine aggregate liquid limit of 25 or less and a plasticity index of 5 or less.

These limits are tighter than the road criteria above. A laterite layer in an airfield pavement therefore needs its own engineering justification. It also needs a strength report. For aircraft of 5 700 kg or less, ICAO Annex 14 (2.6.8) asks for the maximum allowable aircraft mass and tire pressure. An example is 4 800 kg/0.60 MPa. Above 5 700 kg it uses the ACR-PCR method (2.6.2).

The pavement page covers the layers of an airfield pavement.