School of Built Environment & Development Studies, University of KwaZulu-Natal
Abstract
To reduce railway track maintenance costs and meet the growing demand for rail travel
the railway industry needs to significantly increase the performance of old existing
tracks and design any new tracks accordingly. In this thesis, a new full-scale laboratory
Geopavement & Railway Accelerated Fatigue Testing (GRAFT) facility at Heriot-Watt
University is developed to study the performance of both unreinforced and reinforced
railway track substructure systems. The new GRAFT facility enables accelerated testing
of full-scale railway tracks and innovative railway products under realistic railway
loading conditions. The unreinforced track systems represent typical railway tracks in
the UK while the reinforced track systems represent sections of track implemented with
various geosynthetic products. GRAFT consists of a track constructed within a steel
tank. The track comprises a 750mm clay subgrade layer overlain by a clay formation
layer overlain by a 300mm ballast layer. The track includes three hardwood sleeper
sections overlain by an I-section steel beam which has similar stiffness properties to a
BS 113 A rail section. Cyclic loading is applied to the track from a hydraulic testing
machine with the centre sleeper directly under the loading actuator. The loading
mechanism replicates a repeated quasi static single wheel load on the central sleeper of
one half of a 3m long section of railway track.
Based on the results found from the testing programme in GRAFT empirical
relationships are developed between the unreinforced track performance in terms of
track settlement and stiffness and the subgrade modulus, applied load and number of
applied cycles. These relationships fit the GRAFT data presented in this thesis well and
it is thought that they could be used (tentatively) to estimate track settlement on track
after tamping/ballast renewal/new track. These relationships are shown to be consistent
with other well known track settlement models and they indicate that subgrade stiffness
and applied vertical load are two of the most significant parameters that influence track
substructure deterioration.
The results found from the reinforced track tests quantify the improvement in track
performance available with each product under various track conditions. Two ballast
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reinforcement products have been tested; XiTRACK reinforcement and geocell
reinforcement, along with a reinforced geocomposite used primarily for separation at
the ballast/subgrade interface. In addition, a geocomposite product designed to replace a
traditional sand blanket, used on the tracks where severe subgrade erosion conditions
prevail, has been tested in GRAFT under flooding conditions. The most significant
results show that XiTRACK reinforcement can considerably improve the performance
of railway tracks while the performance of the track implemented with the sand blanket
replacement product indicates that currently a traditional sand blanket with a geotextile
separator is the recommended option for tracks with subgrade wet spots. From all the
data recorded empirical settlement models are proposed for each of the geosynthetics
compared for reinforcement purposes. These models form the basis for reinforced track
design graphs that could potentially be used to form part of an initial cost-benefit
analysis of different track reinforcement techniques considered for improving track
performance and reducing maintenance.
In order to use the track settlement design graphs developed within this thesis (in the
field) a reliable measure of subgrade stiffness needs to be made on track. A reliable insitu
measuring device could enhance railway site investigations. Several in-situ
measuring devices that could potentially be used to measures subgrade stiffness and
strength in the field have been tested within GRAFT. The devices studied include the
Dynamic Cone Penetrometer (DCP), Light Falling Weight Deflectometer (LFWD),
Pocket Penetrometer and Proving Ring Penetrometer. The accuracy of these devices is
compared to Plate Load Tests (PLT) and unconfined compression strength tests and
suggestions towards the use of such devices on track made. The results indicate that the
DCP has the potential to be a quick and accurate in-situ measuring device for railway
track site investigations.
The GRAFT facility and the results found in GRAFT have been validated using a basic
static 3D FE computer model termed SART3D (Static Analysis of Railway Track 3D).
The program has been calibrated to GRAFT by modifying the FE mesh for the
dimensions of GRAFT and inputting the GRAFT track properties. The validated results
from this thesis have direct practical implications to the railway industry in terms of
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design recommendations on how best to investigate and improve key geotechnical
parameters that influence railway track performance and hence reduce maintenance
costs and extend asset life. A review of current design procedures used in the railway
industry is given and a new design procedure is suggested to reduce track maintenance
and offer an optimised design and maintenance strategy
