Compaction and Soil Mechanics Aspects of Highway Design - Elements of Soil Mechanics - page 446

14.5 Highway design
Highway design in the UK is based on the Highways Agency Design manual for roads and bridges (High-
ways Agency, 2013 ). The main concern in the design of a highway pavement structure is that its overall
thickness from soil to running surface, is such that the soil beneath the road will not be overstressed by
the imposed traffic loads. A useful reference document for readers interested in this subject area is the
ICE Manual of highway design and management (Walsh, 2011) .
The basic components of a road
Both a road and a runway consist of two basic parts, the pavement and the subgrade.
● Pavement : distributes wheel loads over an area so that the bearing capacity of the subgrade is not
exceeded. It usually consists of two or more layers of material: a top layer or wearing surface which is
durable and waterproof, and a base material. For economical reasons the base material is sometimes
split into two layers, a base and a sub-base (Fig. 14.9) .
● Subgrade : the natural soil upon which the pavement is laid. The subgrade is seldom strong enough to
carry a wheel load directly. There are two possibilities:
(a) improve the strength of the subgrade and thereby reduce the required pavement thickness;
(b) design and construct a sufficiently thick pavement to suit the subgrade.
Types of pavement
● Flexible : lean concrete bases, cement-bound granular bases, tar or bitumen-bound macadam, all over-
laid with bituminous surfaces.
● Rigid : reinforced concrete base and surface.
The choice of pavement depends largely upon the local economic considerations. However, rigid pave-
ments are becoming increasingly rare in the UK as the cost and surface running noise associated with
these structures are considered excessive.
14.5.1 Assessment of subgrade strength and stiffness
The strength of the subgrade is the main factor in determining the thickness of the pavement although
its susceptibility to frost must also be considered.
The value of the stiffness modulus of the subgrade is required if the stresses and strains in the pavement
and the subgrade are to be calculated.
Fig. 14.9 Pavement construction.