English in railway construction.

UNIT 1. DEGRADATION OF RAILWAY TRACK

Vocabulary

congestion – затор (в движении), задержка

to degrade – разрушаться, приходить в негодность

wheel loads – нагрузки на колеса

rail pads – нашпальная прокладка

coarse – крупнозернистый

an embankment – насыпь

rail deflections – скручиваемость рельса

to facilitate – способствовать

alignment – выверка прямолинейности

lateral spreading – боковое уширение

subsidence – оседание грунта

liable – подверженный

Railway networks are developing very quickly all over the world for a variety of reasons, including reducing congestion and as an environmentally friendly alternative to flights. Large expanding networks already exist in Europe and Asia, with proposals for projects in the US and Brazil. This text concentrates upon the mechanics of track infrastructure. In order to design tracks optimally, a balance must be found between initial costs and track performance. To do this it necessary to understand how various components of the track behave under the applied wheel loads.

A typical track layout consists of the following components; steel rails and absorbing rail pads are fixed to a series of pre-stressed concrete sleepers. The sleepers lie on the ballast, a coarse granular material consisting of 30 to 60mm diameter pieces of crushed rock. Below this is the sub-ballast, a sandy material which also helps to spread the wheel loads and facilitate drainage. All this lies on top of the natural soil or an embankment referred to as the subgrade. It is the role of the foundation components; the sleepers and ballast layers to ensure that the wheel loads are spread such that the subgrade is undamaged. Some subgrades have been especially problematic, for example, soft clays, causing large rail deflections which the trains have difficulty operating on and creating ground vibration waves.

There are a number of ways by which track may degrade and lose strength and stiffness, resulting in poorer performance. It happens above layers which cannot spread the load sufficiently and plastic movements occur in the subgrade. There may be a progressive failure where the subgrade near the edges of the track become remoulded and is no longer able to support the overlying track. The other possible occurrence is that of ballast pockets which cause track subsidence. Subgrade problems are the most difficult track problem to deal with, often requiring a total reconstruction of the track.

The ballast under repeated loading is liable chip and crack, causing a loss of stiffness, a decrease in its ability to drain the track and putting the rails out of alignment. Ballast may also deform by lateral spreading, when the particles migrate outwards making the formation wider and thinner.

At all times the main stone ballast should be 300mm, in order to allow for maintenance and good load spreading. The sub-ballast should be at the very least 600mm, although increasing this would improve performance and design life.

Answer the questions:

1. What is the typical layout of a track?

2. What helps to spread the wheel loads?

3. Why does the track lose strength and stiffness?

4. Where does a progressive failure occur?

5. Why may ballast deform?

Exercises: