Unit ix. Pavement structural layers

Pre-reading tasks

1. Translate the following words and memorize them:

stress roadbed to induce to attenuate surfacing abrasion to employ wear

binder smoothness roughness porous subbase subgrade to saturate to distribute

2. Match the English words with their Russian equivalents:

1. pavement structural layers а) водонепроницаемый

2. semi-rigid structure b) илистый грунт

3. base course c) несущая способность

4. wearing course d) конструкционные слои дорожной одежды

5. impervious to water e) глинистый грунт

6. protective (sealing) coat f) полужесткая конструкция

7. polished surfacing g) морозозащитный слой

8. pavement base h) несущий слой

9. weak bed soil i) суглинистый грунт

10. silty soil j) слабый грунт земляного полотна (основание)

11. winter moisture accumulation k) скользкое покрытие

12. frost resistance l) многослойная конструкция

13. loamy soil m) защитный слой

14. bearing strength n) основание дорожной одежды

15. clayey soil o) слой износа

16. anti-frost heave course p) морозостойкость

17. multilayer structure q) зимнее влагонакопление

Reading task

3. Read the text carefully and do the tasks that follow:

To ensure all-year-round operation of traffic on a road independent of weather conditions, the carriageway is covered with a pavement which is a rigid or semi-rigid structure laid on the surface of the roadbed and resisting traffic stresses and climatic factors.

Traffic stress diagram for a multilayer pavement:

a) diagram of vertical stresses; b) diagram of horizontal stresses;

1 – wearing course; 2 – base course; 3 – subbase; 4 – subgrade;

5 – homogeneous soil; 6 - pavement

The stresses induced in the pavement by motor vehicle wheels attenuate with the depth. This enables the pavement to be designed in the form of a multilayer structure, employing materials whose strengths vary for each layer and are determined in accordance with the magnitude of the acting forces. The pavement consists of following layers:

1. Surfacing is the upper, and most rigid, layer of the pavement. It is comparatively thin, but resists well the abrasion and the impacts caused by the wheels, and also the effect of weather conditions. Usually, the surfacing is the most expensive part of the pavement and, therefore, is laid to the minimum admissible thickness. The surfacing provides the required road service qualities (surface smoothness, high coefficient of adhesion). Surfacing usually comprises two coats or courses - the base course, on which depend the basic qualities of the surfacing, and a wearing course, which is not regarded in calculations and which is periodically renewed as it wears out. When the surfacings are made of weak materials, which are subject to appreciable wear, a special wearing course made of strong stone material treated with organic binders is necessary, which may be periodically renewed in the course of road operation.

If the surfacing is not sufficiently impervious to water and may be destroyed during freezing or drying out in hot arid weather conditions, it is covered with a thin protective or sealing coat by surface treatment with a binder and a filling of fine sand. Surface treatment is also used for increasing the roughness of polished surfacings.

2. Below the surfacing base coat is the pavement base, a strong bearing layer of stony material or stone with a binder. This layer is designed to distribute the individual wheel-loads over the roadbed or sub-base.

The pavement base is not subject to the direct action of automobile wheels. Therefore, materials of less strength than those used for the surfacing or the wearing course can be employed in its construction.

When the base is protected from the action of surface water - in the case of an impervious surfacing - it may become saturated by water drawn upwards from the roadbed during winter frost penetration. For this reason, in the northern regions materials used for base construction have to satisfy certain requirements concerning frost resistance.

3. The sub-base is a layer of earth or stone materials, resistant to moisture, inserted when necessary between the pavement base and the roadbed to reduce the required thickness of the pavement base. The sub-base is made of gravel, slag, soil treated with binding agents, sand, etc.

On sections where the roadbed comprises silty, loamy and clayey soils, inside which winter moisture accumulation may occur, a sub-base of porous materials is introduced. This consists of a sand or gravel layer which drains away excess water from the upper layers of the roadbed, drains the pavement structure and increases the bearing strength of the roadbed. It is termed a drainage or anti-frost heave course.

If the roadbed is composed of stable, impervious sand, sandy loam or gravel soils, a sub-base is not necessary.

4. The subgrade comprises the thoroughly compacted upper layers of the roadbed, upon which are laid the layers of the pavement. The subgrade receives all the distributed pressure of traffic loads and, therefore, is a very important element of the pavement structure. The stability of road pavements can be ensured only on a heterogeneous, well compacted roadbed with adequate drainage. The increase of roadbed soil resistance to external loads, its drainage and the uniformity of water conditions are the best means for ensuring pavement stability and reducing its cost. No increase in the thickness of the pavement base can guarantee the strength of a pavement laid on a weak bed soil.

Comprehension check

4. Answer the following questions:

1. Is the pavement a multilayer structure?

2. What do you know about stresses in each layer?

3. Do the strengths of materials for each layer of the pavement vary?

4. What are the main layers of the pavement?

5. How many courses does the surfacing have? Characterize them.

6. What course of the surfacing is often renewed?

7. When is the surfacing covered by a sealing coat?

8. What is the function of the pavement base?

9. Why is the pavement base of less strength than surfacing?

10. How can the pavement base be saturated by water?