8/11/2019 Function and Composition of a Surface Layer

1/2

unction and composition of a surface layer

ARNING ELEMENT OBJECTIVES

fter you have learned this element you should be able to:

explain the function of a surface layer;

describe when a soil is suitable to be used as surfacing material;

describe the functions and desirable quantities of the various fractions in a surface layer soil.

road can be built up of a number of different layers: and weakening the base and subgrade, subgrade, sub-base, base and s

yer. Roads which are built for traffic by a few light vehicles or on very good soils do not need all these different layers; it is u

fficient to put on a good surface layer. Sometimes even the natural soil is strong enough to carry the projected number of

ehicles. Surface layers serve to spread the traffic load (usually through a base), so that the subgrade (natural) soil is protecte

om overloading and deformation. Bituminous surface layers also seal the surface and prevent surface water (rain) from

enetrating and weakening the base and subgrade.

the case of a gravelled road this function is carried out partly by the gravel layer and partly by the camber of the road.

g 6

he type (bitumen, gravel, stabilised soil or natural soil) and thickness of the surface layer should ideally be determined by thxpected traffic density and the type of the natural soil. However, usually other factors, such as available funds, the location o

ad and the availability of suitable material in the area are more important when surface layers are designed.

he suitability of a soil for a surface layer depends on its behaviour in dry and wet weather. In dry weather a fairly high propo

clay is desirable to bind the particles together and to prevent corrugation. However, in wet weather the presence of a lot o

the mixture is disadvantageous, because it makes the surface slippery and ruts are easily formed. Therefore, the specificati

r a soil to be used as a surface layer have to be a compromise between the ideal requirements for wet and dry weather.

s a rough guide, a mixture of 35-65 per cent stones, 20-40 per cent sand and 10-25 per cent clay will produce an optimum so

surface layer. A higher proportion (up to 65 per cent) of suitably sized stones is preferable, as the strength and density of th

ones will provide a stronger surface. The gravel will behave better if the stones in the mixture are of various sizes well grad

8/11/2019 Function and Composition of a Surface Layer

2/2

that the voids in between the particles, are filled. Angular-shaped particles are preferable, because they will lock togethe

etter than round particles.

ure clay and silts, mixtures of silt and clay, and organic soils are generally unsuitable for surface layers.

he stony particles to be used in a surface layer should not be bigger than 3 cm in order to obtain a reasonably smooth finish

gger particles should be crushed by hand or by roller.

g 7

good surface layer should spread the traffic load evenly to the underlying base and prevent water from penetrating. It shou

e resistant to corrugation and provide a non-slippery and dust-free surface.

though such surfaces can hardly be made with earth or gravel, fair results can be achieved by avoiding pure clays and silts,

ixtures of clays and silt, and organic soils.

mixture of some 10-25 per cent clay and a good gravel (i.e. small stones of different sizes + sand) can provide a very good su

aterial.

nd/clay mixtures also provide reasonable surfaces, although the strength and durability are less than a mixture in which sto

ravel) are present. Here again, not more than 25 per cent clay should be present in the mixture