Retaining Structures - Elements of Soil Mechanics - page 250

If N is greater than 4, the distribution in Fig. 8.20e is used, provided that K a is greater than 0.4. If N is
less than 4, or if 0.2 < K a < 0.4, the distribution in Fig. 8.20f is used. With respect to Fig. 8.20e , m is
generally taken as 1.0. For soft clays, however, m can reduce to ≈ 0.4.
8.8 Reinforced soil
The principle of reinforced soil is that a mass of soil can be given tensile strength in a specific direction,
if lengths of a material capable of carrying tension are embedded within it in the required direction.
This idea has been known for centuries. The Bible quotes the use of straw to strengthen unburnt clay
bricks, and, from ancient times, fascine mattresses have been used to strengthen soft soil deposits prior
to road construction. Ziggurats, built in Iraq, consisted of dried earth blocks, reinforced across the width
of the structure with tarred ropes. However the full potential of reinforced soil was never realised until
Vidal, who coined the term 'reinforced earth', demonstrated its wide potential and produced a rational
design approach in his paper of 1966. There is no doubt that the present day use of reinforced soil struc-
tures stems directly from the pioneering work of Vidal.
Reinforced soil can be used in many geotechnical applications but, in this chapter, we are only concerned
with earth retaining structures.
A reinforced soil retaining wall is a gravity structure and a simple form of such a wall is illustrated in
Fig. 8.21. Brief descriptions of the components listed in the figure are set out below.
Soil ill
The soil should be granular and free draining with not more than 10% passing the 63 μ m sieve.
Reinforcing elements
(1) Metals
Originally many reinforced soil structures used thin metallic strips usually 50-100 mm wide and some
3-5 mm thick. Metals used were aluminium alloy, copper, stainless steel and galvanised steel, the latter
being the most common. The common property of these materials is that they all have high moduli
of elasticity so that negligible strains are created within the soil mass.
(2) Plastics
Since the mid-1970s there has been an increasing use of geosynthetics as reinforcement in reinforced
soil, either in strip form or in grid form, such as Tensar geogrid. Geosynthetics have the advantage of
greater durability than metal in corrosive soil, and their tensile strength can approach that of steel. In
grid form, plastic reinforcement can achieve high frictional properties between itself and the surround-
ing soil. The main disadvantage of plastic reinforcement is that it experiences plastic deformation when
subjected to tensile forces, which can lead to relatively large strains within the soil mass.
Reinforcing strips
Facing units
Concrete footing
Fig. 8.21 Typical reinforced soil retaining wall.