12. Railway track ballast
Published:January 01, 2001
Railway track formations generally consist essentially of a layer of coarse aggregate, or ballast, in which the sleepers are embedded (see Fig. 12.1). The ballast may rest directly on the subgrade or, depending on the bearing capacity, on a layer of blanketing sand. The layer of ballast is intended to provide a free draining base which is stable enough to maintain the track aUgnment with the minimum of maintenance. The function of the blanketing sand is primarily to provide a filter to prevent contamination of the ballast by fine particles derived from ascending waters (see Fig. 12.2).
The fundamental engineering is reasonably well understood. It concerns the transmission of the load from the base of the sleeper on to an area of closely packed ballast beneath. The sleeper bears on to the points of the angular ballast which are, in consequence, liable to fracture or abrade under load, a problem more severe with concrete than with wooden sleepers. If the layer of ballast is not close-packed and of sufficient depth there is a tendency for the sleepers to move up and down as the wheels of the traffic pass in succession, eventually causing the sub-grade to be disturbed.
Consequently the ballast layer must be thick enough to hold the track in position and prevent traffic loads from distorting the subgrade, while the aggregate under the sleepers must be tough enough to resist the abrasion and degradation caused by the intermittent traffic loads. The main requirement, confirmed
Figures & Tables
Aggregates: Sand, gravel and crushed rock aggregates for construction purposes
In 1985, the Geological Society published Aggregates as the first volume in its Engineering Geology Special Publication series. It met with immediate acclaim, being awarded the Brewis Trophy by SAGA in 1986.
“If your work involves the use of aggregates, buy this book and read no further; this volume will be an essential and valuable reference that you will use for many years.” (Canadian Geotechnical Journal 1988)
In 1989, the working party whose work had resulted in the publication of Aggregates was reconvened to revise, update and extend their report. Each chapter was reviewed by independent referees. The second and greatly improved edition, published in 1993 and reprinted in 1998, represented the distillation of a vast body of knowledge and experience held not only by the members of the working party, but also by many international experts, scientists and engineers who contributed as reviewers, referees and corresponding authors.
Owing to continued demand for this unique reference book, a group of aggregate specialists was convened in 1999 in order to review thoroughly and update Aggregates for this third edition.
Outline of contents: Introduction; Occurrences; Field investigations; Extraction; Processing; Classification; Testing; Aggregates for concrete; Aggregates for mortar; Unbound aggregates; Bituminous bound aggregates; Rail ballast; Filter media; Appendix: Aggregate properties; Glossary; Index.
Working Party Members and/or third edition Reviewers: Mr L. Collis (formerly Sandberg); Professor P. G. Fookes (Chairman; consulting engineering geologist), Mr R. A. Fox (formerly RMC Aggregates (UK) Ltd), Professor G. P. Hammersley (formerly Laing Technology Group, now BRE), Mr P. M. Harris (formerly BGS), Dr I. E. Higginbottom (formerly Wimpey Environmental Ltd), Mr J. Lay (RMC Aggregates (UK) Ltd), Dr G. Lees (formerly University of Birmingham), Mr D. I. Roberts (Land and Mineral Resource consultants), Mr A. R. Roeder (formerly British Cement Association), Dr I. Sims (Secretary; formerly Sandberg, now STATS Limited), Dr M. R. Smith (formerly Imperial College, now the Institute of Quarrying), Dr R. G. Thurrell (formerly BGS), Dr G. West (formerly TRL).