Experimental study on compaction and bearing capacity of railway track subballast

Abstract

Density is one of the most important material properties affecting the mechanical properties of granular materials. A poorly compacted railway substructure is more likely to suffer from the accumulation of permanent deformation during its service life. Problems can be avoided by using proper materials and methods during construction. The materials used in railway track substructure layers are typically natural sands and gravels, but their availability is increasingly being limited to more and more local pits. That has made the use of crushed rock aggregates more prevalent. Crushed rock aggregates, especially, must be relatively uniformly graded and coarse-grained. This particular grading is necessary to avoid the frost heave action caused by gradual fouling of the material during its planned service life of 100 years. Due to the uniformly graded materials the compaction of subballast layers has been found difficult. That could lead to an unevenly compacted structure causing excessive permanent deformations and local variations in the strength and bearing capacity of the embankment. A joint research project of the Laboratory of Earth and Foundation Structures at Tampere University of Technology and the Finnish Transport Agency studied the above problem by building a series of full-scale test embankments. The subgrade conditions, grading and the quality of material, thickness of the compacted layer, and construction methods were systematically varied in various test sections. The results obtained from the full-scale construction site were compared to those from model scale in laboratory conditions. All results indicate that grading and moisture content of the material determine the amount of compaction needed for sufficient density and bearing capacity. This paper reveals the effect of modification of compaction procedures on density and bearing capacity values based on laboratory and full-scale tests.