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Study On Centrifugal Modeling Of Vertical Strip Drains In Soft Foundation

Posted on:2017-02-23Degree:MasterType:Thesis
Country:ChinaCandidate:C WangFull Text:PDF
GTID:2272330488491055Subject:Geotechnical engineering
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One of the airports in Dalian is expanded on the basis of the original airport through sea reclamation, and it is composed of soft foundation, revetment, runway area and construction area. According to the geological survey data, it is difficult and almost impossible to excavate all the soft layer under the dike. Therefore the vertical strip drains are designed to accelerate the consolidation process of the soft soil foundation. It is necessary to take scientific and effective approach to predict the final settlement and the dissipation of the excess pore water pressure of the soft clay with vertical strip drains at the stage of construction and operation. This paper presents the centrifuge study of the above considerations on this project.In centrifuge model tests, the dimension of the prototype structure is reduced to the model scale according to the geometric similarity requirement. Because the centrifugal field in the model at Ng acceleration is almost the same as the gravity field in the prototype structure, the stress state of each point in the model corresponds to that of prototype and the similary rules can be easily deducted. Such as for the centrifuge model consolidation process, the time scale of excess pore water pressure dissipation is 1/N2 of the prototype. Therefore, long time consolidation period of the soft foundation can be simulated by using the centrifuge modeling with a short period of time.The centrifugal model tests were carried out by using the geotechnical centrifuge in China Insitute of Water Resources and Hydropower Research (IWHR). The settlement of the dike and the dissipation of pore water pressure in the foundation during the construction and operation period are the main study purpose of this paper, which is useful for the reference of project feasibility study and structure design.The main work in this paper is as follows:(1)In the centrifuge tests, the silt and silty sand of the foundation are taken directly from the construction site. The mountain stones and broken rocks in prototype were simulated with the particle diameter of 1-5mm sand and 5-10mm gravel respectively, based on the particle similarity rules for centrifuge modeling.The vertical strip drains in the construction site are simulated by cylinder vertical drains, based on the principle of similar consolidation process in average for both type of drains. The stage loading construction in the prototype is simulated by loading dike materials layer by layer at each stop interval of centrifuge running.(2) Laser displacement sensors were installed to measure the settlement of the model. However those sensors can only detect several points because of the limit space of the centrifuge basket and the model box. In order to measure more points of model deformation, a G200 topography measurement system was developed in IWHR with the cooperation of German Innowep Company. Images can be acquired by three cameras at the bottom of the bracket, and then be calculated by reconstruction algorithm by the analysis software to get the surface deformation at any time during or after the test. This paper also presents the theoretical background, working principle and operation method of the system. The monitoring results at 1g test are described, and will be implemented into the centrifuge model test later.(3)The pore water pressure transducers (PPT) were installed in the model foundation. The pore water pressures at different points were observed and analyzed.(4)Based on the measuring data of the laser displacement sensors, it shows the displacement of LS1, LS2 and LS3 in the prototype scale is 2.01m,2.76m and 2.38m after the dike running for 5 years. The Asaoka method and the experience hyperbolic method are used to estimate the final settlement of the three positions according to the measured data. And a comparative analysis has been made between the prediction results of the two methods.(5)The consolidation degree of the foundation Up or Us can be calculated either by the pore water pressure or by the settlement from the results of the centrifuge model test. Compared with the theoretical calculation Urz, several conclusions can be made: ①The Us indicates the average consolidation of the silt and the silty sand under the dike, and in the initial of the running period the Us is larger than Up and Urz,because the settlement developed rapidly;②When loading 5 years, the Up of the soft soil foundation calculated by the pore water pressure data of PPT1-2 has reached 95.9%, and the Us calculated by the settlement data of LS1 has reached 98.1%, and the theoretical calculation of the of the soft soil Urz is 99.3%. The results both from centrifuge test and theoretical calculation indicate that the consolidation of the soft foundation is almost completed after 5 years of operation, which also reflects that the simulation method of vertical strip drains presented in this paper is feasible and reasonable, especially for long term consolidation simulation.
Keywords/Search Tags:vertical strip drains, centrifuge model test, soft soil foundation, consolidation, G200 topography measurement system
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