Earth Bed

a structure that serves as the foundation for the superstructure of a railroad track or the pavement of a road. The earth bed absorbs the load from the rails and crossties (sleepers), ballast, and rolling stock on a railroad and from the pavement and motor-vehicle traffic on a highway by distributing it uniformly over the underlying natural ground. An earth bed is one of the main elements of a road; its condition determines the good working condition of rail-road track and road pavement.

Railroad earth beds consist of embankments and cuts, as well as borrow pits, spoil banks, drainage devices (side ditches, troughs, catch drains, banquette cuts, and drains), and reinforcing and protective structures. The width of the main roadbed adopted in the USSR for single-track lines (on straight sections of track) is 7 m for lines of the first and second categories, 5.8 m for lines of the third category, and 5.5-5.8 m for lines of the fourth category. The width is correspondingly reduced to 6 and 5.2 m in rocky, largefragment, and sandy soils (except fine and silty sands). If a second track is planned, the distance between its axis and the edge of the bed is set at 3.5 m. On curved sections the width of the bed is increased by 0.1-0.5 m on the outer side, depending on the radius of the curve.

A highway roadbed is composed of the roadway, two shoulders, and a dividing strip or railings. The width of the roadbed is 27.5 m or more for roads of the first category, 15 m for roads of the second category, 12 m for roads of the third category, 10 m for roads of the fourth category, and 8 m for roads of the fifth category. The steepness of the embankment and excavation (cut) slopes depends on the soil characteristics; the geological, hydrographic, and climatic conditions of the locality; construction methods; embankment height; and excavation depth. Methods used to reinforce the banks of earthen roadbeds include the planting of perennial grasses, the laying of sod or flagstones, the planting of bushes, the use of fascine, and covering with built-up, cast, or reinforced concrete.

Earth beds are usually constructed by special mechanized groups or teams, which have such excavating and earthmoving machines as excavators, dump trucks, pull-type and motor scrapers, grader elevators, motor graders, screw-type planers, and machines for packing the soil of embankments and for finishing work.