Research On Mechanical Properties And Engineering Application Of Supporting System For Dry Dock Foundation Pit Of Immersed Tunnel

In recent years,the immersed tunnel has been used more and more widely in the passage across rivers and straits in our country due to its many advantages.The problems existing in its application process are also increasing gradually.As a site for the fabrication of immersed tunnel,the dry dock engineering needs to meet the working conditions of large area pouring for a long time.Therefore,the dry dock foundation pit is faced with long-term exposure without supporting system and must go through the process of excavation and periodic storage and drainage.Under such complicated conditions,it is very significant to determine the supporting system of the foundation pit reasonably.With the background of dry dock engineering of Ruyifang Radiological System Project(Phase I Project),the thesis carried out the comparison and selection of the supporting schemes of the dry dock foundation pit under the restricted conditions.The supporting system scheme of common diaphragm wall + recyclable anchor cable and cellular diaphragm wall was adopted initially.On this basis,the bearing mechanism,mechanical characteristics,reliability and influencing factors of the supporting scheme of recyclable anchor cable and cellular diaphragm wall in different stages of dry dock foundation pit were explored.The optimization idea of the initial scheme was put forward.The main research work and results are as follows:(1)Based on the characteristics of the dry dock of immersed tunnel,combined with the construction conditions and the surrounding environment feature,the comparison and selection of the dry dock foundation pit supporting scheme were carried out.Considering the standards of "large cantilever pit depth,long-term safety and reliability,sufficient working space,no underground waste",the supporting form of common diaphragm wall+ recyclable anchor cable and cellular diaphragm wall was put forward.The key technical problems existing in the initial scheme were identified based on the summary and analysis experience of previous projects.(2)The internal action mechanism of the recyclable anchor cable was analyzed and a fine model of the recyclable anchor cable was established by numerical method.The construction conditions of the whole process of dry dock excavation,tunnel section prefabrication,floating transportation and docking were fully considered.The mechanical properties of the recyclable anchor cable under the action of dry dock excavation and water level change were revealed and verified by comparison with the field test results.(3)The common tension anchor cable model was established and the results of the recyclable anchor cable were compared to clarify the differences between the design ideas.The results showed that the axial force of the recyclable anchor cable in the free section was the same as that of the common anchor cable,which presented a "rectangular" distribution.But it was different in the anchor section.Common anchor cable axial force distribution was "ladder",however the recyclable anchor cable was still "rectangle".The maximum difference between the recyclable anchor cable and the common was 11%,which shows that there were obvious deficiencies in the current design of recyclable anchor cable by using design software and ideas for common anchor cable.In order to meet the requirement of recyclable anchor cable,the recyclable anchor cable axial force was be revised by the maximum ratio of recyclable anchor cable and common anchor cable.(4)A fine model analysis of cross perforated steel plate joint used in cellular diaphragm wall was carried out.The mechanical properties of rigid joints in common dry dock foundation pit was evaluated.The rigid joints of cross perforated steel plates were in the overall safe and reliable state within the limit range of the displacement of dry dock foundation pit.The rigid joints of cross perforated steel plates used by cellular diaphragm wall need no additional stiffness reduction in practical engineering.It can be regarded as the joint action of diaphragm wall and reinforced concrete wall.(5)A three-dimensional calculation model of the cellular diaphragm wall was established to discuss the displacement of the diaphragm wall in the process of dry dock foundation pit excavation.The comparison and analysis were made with the field measured values.On this basis,the mechanical response characteristics of cellular diaphragm wall and its influencing factors were analyzed.The sensitivity of wall deformation to cover plate size,wall size,joint form was studied detailedly and the optimization idea in the later stage of the design scheme was proposed.