Stiffness and strength of structural layers from cohesionless material

Ulf Gerber,Olga Nabochenko,Mykola Sysyn,Jandab Zarour

doi:10.5604/01.3001.0013.2776

Abstract

The deformation modulus and permissible stress are two independent parameters that depict the carrying capacity of foundations, including earthworks and ballast layer. Nevertheless, while designing the track superstructure or controlling its state, they are considered separate to each other, even though they are terms of the same measure. The scientific problem is due to the practical necessity of unified building rules and standards. The carrying capacity of earthworks and foundations is regulated with standards based both on deformation and on stress criteria, which are not related to each other. This plays particularly important role for railway ballast layer, as an intermediate between the solids and soil. The objective of the present research is to estimate the relationship between deformation modulus and the strength of ballast layer. An overview of modern approaches according to the relation between the stiffness, deformation modulus, elasticity and strength of soils and crushed stone is done. The strength of ballast layer is considered depending on the experimental test: the direct shear test, compressive strength in the uniaxial or biaxial stress state. Load transfer model in crushed stone is proposed. The load transfer angle and cone of loading distribution are determined based on the load transfer and compressive strength models. The relation between deformation modulus and strength is derived from two simple laboratory experiments with cohesionless ballast material. The experiment tests have shown that the ballast stiffness as well as its strength are influenced with the support stress. The measurement of elastic and residual settlements for the different support stress values enables to determine the relation. It can be potentially used for the development of methods for the ballast compaction control, unification of construction norms. The research result should be considered as an approach for unification of two different ways to reflect the carrying capacity of ballast layer.

Highlights

The ballast bed is, mechanically considered, cohesionless crushed stone
The ground mechanical state is classified as permissible if the existing stress does not exceed some permissible stress value, that depends on the corresponding deformation modulus (Fendrich and Fengler, 2013)
This paper demonstrates the fundamental relationship between deformation modulus and strength as two alternative ways to quantify the carrying capacity of layers from crushed stone using two simple experiments

Summary

Introduction

The ballast bed is, mechanically considered, cohesionless crushed stone. it is the most difficult element of the railway superstructure to understand and predict. The load can be expressed as the mean contact stress between sleeper and ballast, and the load capacity as the corresponding permissible contact stress (Esveld, 2001) This approach derives from the mechanics of materials, which classifies a structure element as stable if the existing stress does not exceed the permissible stress (strength). The ground mechanical state is classified as permissible if the existing stress (load) does not exceed some permissible stress value, that depends on the corresponding deformation modulus (Fendrich and Fengler, 2013) This approach, at first sight, seems strange, because the permissible stress and the elasticity properties of materials are interrelated. The soil mechanics describes the strength behavior of the crushed rock in a very clear way on the basis of the direct shear test (Fig. 1).

Compressive strength in the uniaxial stress state

Experimental determination of deformation modulus and strength

Conclusions and subsequent studies