Materials for capping layer

The materials used for the construction of the capping layer or the formation of subgrade may be (a) collective granular soil material (b) unbound mixtures of natural, artificial and recycled aggregates and (c) soil material treated (stabilised) with hydraulically bound binders. 

The collective granular material could be type GW, GP or GM according to the Unified Soil Glassification System type A-l-a or A-l-b according to the ASTM classification system or type 6F1, 6F2 or 6F3 according to Highways Agency classification. 

As a rule of thumb, cement binder is preferred to be used on soils with low plasticity index (PI lower than 10% to 12%). Higher plasticity soils are normally stabilised with lime. 

In general, the capping material should conform to the national specification followed. Table 10.2 gives the characteristic property requirements of the capping materials recommended to be used in the United Kingdom. 

Sieve size (mm) and other properties Selected granular material Granular material complying with CEN EN 13285 (2010) Cement-stabilised material Lime- or lime and cement-stabiSsed material  

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The materials for capping layer may be selected granular material or cement- and lime-stabilised material, as those described in Section 10.3.1. Soil stabilisation is also recommended to be considered, when highly cohesive soil material is encountered. For details, see Section 10.3.2. 

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