Correlative Studies of FWD, LDW and DCP Testing of Railway- Track Sub-Ballast Structures in Israel

Abstract

The performance of railway tracks depends to a large extent on the stiffness and
strength of their subgrades and sub-ballast layers. Among the various methods of in-situ evaluation
of stiffness, the Falling Weight Deflectometer (FWD) and some additional small-scale dynamic
devices, such as the Light Drop Weight (LDW), have been gaining popularity in recent
years. The LDW device substitutes in Israel for the German plate-bearing device by assuming
that its surface modulus is equal to the static deformation modulus obtained from the regular
plate-bearing test. The use of these various devices creates the need to study the correlation between
results obtained with the LDW device and those obtained by using traditional approaches,
such as FWD and the Dynamic Cone Penetrometer (DCP). The present work focuses on the correlation
between these devices for local silty and clayey soils and local granular sub-base material.
Studies have indicated that different types of equipment can produce different values for
surface stiffness; furthermore, the experimental relationships between the different tests appear
to be variable and perhaps site dependent. This paper also describes the DCP tests that were
conducted together with the FWD and LDW tests in order to correlate stiffness results with
CBR values. As a continuation of this line of investigation, the sensitivity of the depth of influence
of the FWD device was then studied through (a) a theoretical analysis of a two-layered
elastic model and (b) a correlative study of the FWD surface modulus results and the variation
in CBR values with depth through DCP testing. The study concluded that although it is difficult
(a) to determine a specific depth of influence and (b) to develop a unique relationship between
stiffness and CBR values, approximated answers may be suggested.