Design of Conventional Rail Track Foundations.

Abstract

The background, philosophy and approach to the problem are discussed and previous work is summarized. Measurements of the distribution of vertical stress in the subgrade for given depths of ballast, track structural configurations and loading conditions are described. It is shown that the mean maximum vertical stress in the subgrade can be predicted with reasonable accuracy by simple elastic theory. The results of laboratory work in which samples of soil were subjected to repeated triaxial loading are presented. For the cohesive soils tested a level of stress was found above which repeated applications of load caused large permanent deformations and below which permanent deformations were small and terminating. Based on these findings a method of track foundation design is developed in which the depth of ballast required to prevent excessive deformation of the subgrade can be predicted from the results of a simple laboratory repeated load test and a knowledge of the traffic loading to be carried. The Paper concludes with the results of field observations in which the actual behaviour of sites in terms of movement of the subgrade was measured for depths of construction both greater and less than those predicted by the design method. It is shown that small decreases in construction depth from the design depth produce large increases in deformation rate whilst large increases in construction depth over the design depth produce little return in terms of reduced rates of deformation. It is concluded that whilst further research is required the proposed design procedure already produces construction depths that are apparently close to the optimum for the fine grained soils so far investigated.

Test study on the influences of dynamic stress and load history to the dynamic properties of the remolded red clay

“…Moreover, as previously outlined, a decreased dynamic stress is shown to be associated with and increased vibration time. Consequently, below a specified level of dynamic stress, soil deformation does not occur, with this specified level of stress defined as the critical strength (σcr) (Heath, Waters, Shenton & Sparrow, 1972), as previously demonstrated (Liu, 2010;Li, 2010;Zhou, 1998). Based upon results and figures about dynamic stress obtained within the current study, characteristic σcr for test samples are presented in Table 3.…”

Cyclic Triaxial Tests on Clay Subgrades for Analytical Pavement Design

“…Such a design then only needs to consider permanent deformation as a function of strength and material stiffness based on limiting applied stresses through the composite foundation structure (assuming the capping and sub-base have sufficient strength to avoid internal permanent deformations). Such an approach forms the basis for the method of determining the design thickness of the unbound layers for UK railway track (Heath et al, 1972), in which RLTT data were used to define a limiting stress (i.e. 0.5q max ) that was then related to empirically measured/observed field performance.…”

Measurements of transient ground movements below a ballasted railway line

“…The depth of ballast and sub-ballast (sometimes termed the granular layer) are specified so that stresses applied to the subgrade, from the superstructure and train loads, are reduced (through spreading) sufficiently to prevent failure of the subgrade by excessive deformation (plastic strain) or progressive shearing of the soil (Selig & Waters, 1994). Historically, the depth of the granular layer needed to prevent failure was estimated on the basis of experience (Heath et al, 1972), with standards generally using equations developed empiricallyfor example, the American Railway Engineering Association manual (AREA, 1996) and the Union Internationale des Chemins de fer Code 719R (UIC, 1994). Raymond (1985) modified the AREA design method using the Casagrande soil classification system (Casagrande, 1948) to relate safe bearing pressure to subgrade types.…”