The roadbed can be composed of a wide variety of materials. The most predominant materials are local
native soils and soils imported from nearby sources. In the upper layers of the roadbed, imported materials
including cinders, sands, and pit run gravels may be found intermixed with the ballast materials that have
been placed during track surfacing cycles.
f. The composition and thickness of the materials and the drainage conditions existing in the upper two feet of
the roadbed are extremely important because of the high stresses from track loads and exposure to
environmental factors. Roadbed induced track problems such as loss of line, surface, gauge, mud pumping
and ballast fouling in most cases can be traced to one or a combination of deficiencies in the material
properties, thickness, or drainage characteristic within the upper two feet of the roadbed. Therefore, most
roadbed maintenance measures may need to be concentrated at making improvements to the upper two feet
of the roadbed and especially to the interface between the ballast (or sub-ballast) and the roadbed soils in
addition to making improvements to the drainage.
g. Additional details regarding the design and construction of roadbeds are discussed in greater detail in
Section 1.1 through 1.3 of this Manual.