| The high filled subgrade stability and post-construction settlement, differential settlement in excavation-backfilling boundaries and bump at bridge-head are three difficult issues in high filled subgrade foundation engineering. In soft subgrade, these problems are especially significant, due to the low bearing capacity while high compressibility of the soil. Under both the self weight of the embankment and the cyclic traffic loading, large settlement deformation and differential settlements are produced, which in turn resulting crack related damages to roads. These crack-related problems not only impact the appearance of the roads, but more significantly, quicken the deterioration of the road surface and even the road foundation, thus impact the life of the roads, increase the maintenance costs and cause economical losses.The study described in this thesis is based on a first grade road linking Heshan and Chaolian Bridge (called Riverside Road hereafter in this thesis), which belongs to the G325 line project between Jiujiang Bridge and Jiangmen. Along this road, soft soil is widely distributed, and the depth is generally big. The embankment height of most sections of the road ranges from four to eight metres, which is classified as High filled or Very High filled. The Riverside Road strikes parallel to the Xi River and an approximate 10 km long part of the road levels with the river bank. Between the road and river bank, plantation belt is designed. Therefore, the Riverside Road is featured high filled, deep soft soil distribution and paralleling to river bank.Firstly, intensive investigation of the cracks appeared on the eight sections of the road (K2+180～K23+555) is carried out. For prioritized sections of monitoring, the width, length and depth of the longitudinal cracks are surveyed, and their development with time was observed by further repeated surveys. The methods used for survey of the cracks utilized mainly include: making marks on both ends of cracks to investigate the development of crack length with time; gluing glass pieces across the crack on selected locations to observe if the glass was torn by the fast expansion of crack width; half inserting pairs of steel nails into both sides of the crack at specific spacing between two neighboring pairs along the strike of the crack and then measuring the distance between the nails on the crack's both side and logging the change with time to monitor the development of crack width; taking samples of the cracks in the structural layer of the road surface and observing the characteristics of the crack face and its change rules with depth.Secondly, Through the intensive field investigation and making use of the corresponding design, construction, and monitoring documents of the sections where cracks appeared, the basic information regarding the crack developments in the whole road is obtained. In order to analyze and clarify the causes of the cracks and also to identify the relation and differences between the cracks in different sections, statistics classification of cracks by the embankment height, soft subgrade treatment method, depth of soft soil is carried out for every section. That is considered of statistics of the distribution of length of cracks in the whole road and each section by this condition or in the case of two or even three conditions.Thirdly, based on the obtained monitoring data and the corresponding design, construction, monitoring documents of the Riverside Road, calculation of differential settlements on typical high filled sections is performed, utilizing the differential settlement model. The settlement characteristic and strain-stress development in the soil of these sections are also disclosed and compared with practical monitoring data. In order to explore the relation of differential settlement with embankment height and also with soft soil depth, numerical simulation of differential settlements under varied combinations of embankment height and soft soil depth, based on the nonlinear finite element theory and numerical model constituted accordingly, are carried out. Through the numerical analysis, the relation among the three is acquired, which enabled further exploration of the crack formation mechanisms, causes and the crack development rules for high filled road. According to the analyses results, reference can be made for optimization of design of soft ground treatment and embankment projects, which would help to effectively control of road foundation settlement, therefore protect the road from the damage of longitudinal cracks.Lastly, solutions are made on preventing the development of longitudinal cracks based the latest researches in and out of China, which are expected to provide guidance for practical engineering activities. The conclusions are drawn. Reasonable and feasible recommendations are made. This study has provided guidance for industry, supplying a theoretical reference for construction of soft and high filled subgrade, structural design of road surface, crack initiation and development prediction and sensible selection of road surface construction material.
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