Study On The Mechanism Of Settlement And Cracking And The Technology Of Dynamic Compaction In Old Subgrade Widening And Reconstruction

With the rapid economic development of our country, the traffic volumes of expressway are rapidly increasing. A considerable number of expressways in service can not meet the demand of increasing traffic volumes and economic development. Thus they need to be reconstructed and extended. The widening extension of the existing expressway not only saves the costs, shortens the construction period, but also occupies less land and is favorable for environmental protection. But, there are three critical problems widening project of expressway. The first one is the differential settlement between the new and the former roadbed. The second one is the non-uniform compaction between the new and the former roadbed. The last one is the consistency of integrity and strength for combination of two roadbeds. Aiming at the critical technical problems in reconstructed project of expressway, numerical simulation, theoretical analysis and in situ experiments were adopted to study the interaction of new and existing roadbed and dynamic compaction. The main points and achievements were as follows.(1) On the basis of surveying of substantive expressway widening projects home and abroad, the existing problems of widening roadbed were systematically investigated and the techniques were evaluated. The mechanism and the main influencing factors were analyzed for differential settlement of new and existing roadbed.(2) The construction process of direct use of existing roadbed was simulated by the finite difference method. The fluid-solid coupled analysis was performed for the differential settlement of roadbed. The influence law on foundation, roadbed and differential settlement, caused by widening mode and height of roadbed, was studied mainly. The initial consolidation of the foundation was calculated, and the simulation was performed according to the construction progress; the velocity vector and sedimentation basin of the embankment were analyzed under different new and old embankment; the largest settlement points' position of the roadbed foundation caused by the different eccentricity was probed. The result showed that there would be great differential settlement in junction of roadbed if the existing roadbed was utilized directly without treatment. The value of differential settlement can not meet the demand of specification.(3) On the basis of the centrifugal loading method, the settlement process of roadbed and road embankment, also the longitudinal-cracking process of base course and surface were simulated by using RFPA with the numerical calculation method of material cracking process analysis. The study on the cracking mechanism of roadbed was carried out, under the condition of unilateral and bilateral widening and different fill depth. It was found that the cracking often happened in the junction of new and existing roadbed. The higher embankment was, the worse stability was, the more serious differential settlement was. With the increase of settlement slip of widening roadbed, the value of additional bending tensile stress and shear stress became higher and higher, and it directly resulted to the longitudinal cracking of the base course. Furthermore, longitudinal cracking of asphalt surface happened, infected by cracks of base course.(4) The key parameters of dynamic compaction in using of existing roadbed were studied. From the results of the tests and analysis of the site dynamic water pressure, the excess pore water pressure and the dynamic soil pressure, the key parameters were drawn such as the optional energy of dynamic compaction and the interval distance of two consolidates. The interaction between rammer and embankment were simulated by FLAC3D, and the dynamic response of the soil under the action of rammers was analyzed. Utilizing dimensional analysis method, the determinations of the optional click tamping energy, the optional number and the reinforcement coefficient of effective height were studied. The formula of reinforcement coefficient of effective height was derived considering geological condition and dynamic compaction parameters. The effectiveness of the use of the dynamic compaction in the existing roads was demonstrated through calculation of road face deflection, and it found that 80cm uniform roadbed played a role as support layer and buffer layer.(5) Two options of trial dynamic vibration were conducted in the embankment test section of the Wei-Ru expressway. The first trial was that fill the new road embankment until it was flush with the existing one and perform dynamic compaction for both two embankments. The second trial was that perform stratified rolling compaction for the new road embankment until it was flush with the existing one and carry out dynamic compaction on the junction. It was presented that the quality of dynamic compaction should be controlled by degree of compaction and length of penetration. And the corresponding testing method was proposed. The formula of cease-compaction standard for road embankment was derived, which was different to the former one. According to the compaction degree tests, rebound and deflection test, and settlement observation, both two options can satisfy the design requirements. But the first one was better than the second one if cost and time were considered. The key points of dynamic compaction were obtained on the basis of practical experience.(6) With regard to disputes and delays caused by construction of dynamic compaction to surrounding environment, the author conducted a research about the hazards to the building and the personnel influence caused by the construction of dynamic vibration. The filed tests were performed and the spectral analysis was done for vibration data using Fast Fourier Transform method. It was found that the main frequency caused by dynamic vibration was close to the frequency of majority of buildings. Moreover, the vertical vibration frequency was also located in the people's sensitive frequency band. According to international general vibration limits and the site and vibration tests, the safe distance of the buildings, the human comfortable limit distance, the work efficiency limit distance, and exposure limit distance were gained through the in situ test and vibration tests. The contraction effect of road embankment was found under the vibration of dynamic compaction and was theoretically analyzed through the transmission way of the stress wave. An approximate formula was introduced through engineering analogy method, which is to show the particle velocity at the height of the embankment.