A novel horizontal trap-door test system was devised in this study to analyze the face stability of shield tunnels in sands.The test system can be used to investigate both the longitudinal and cross sections of the fa...A novel horizontal trap-door test system was devised in this study to analyze the face stability of shield tunnels in sands.The test system can be used to investigate both the longitudinal and cross sections of the face failure simultaneously at one single apparatus and was employed to perform face stability tests on small-scaled tunnel models at single gravity.The lateral support pressures and failure zones were studied with varying sand materials and earth covers.The results demonstrate that the tunnel face moves back,the lateral active earth pressure on the tunnel face decreases rapidly to a residual value,and the lateral pressure distribution can be categorized into three stages during the failure process:1)initial state;2)pressure dissipation stage;and 3)pressure zone diminution stage.Furthermore,face failure firstly develops from a stable condition to the local failure state,and then continues to develop to the global failure state that can be divided into two sub-zones with different failure mechanisms:rotational failure zone(lower zone)and gravitational failure zone(upper zone).Further discussion shows that under the effects of soil arching,the shape of the gravitational failure zone can adopt arch shaped(most frequent)and column shaped(in shallow tunnels).Limit support pressure for face stability usually appears atδ/D=0.2%−0.5%(ratio of face displacement to tunnel diameter).展开更多
Hot granule medium pressure forming （HGMF） process is a new process in which granule medium replaces the medium in existing flexible-die hot forming process, such as liquids, gases or viscous medium. Hot forming of ...Hot granule medium pressure forming （HGMF） process is a new process in which granule medium replaces the medium in existing flexible-die hot forming process, such as liquids, gases or viscous medium. Hot forming of light alloy sheet parts can be realized based on the properties of granule medium, such as withstanding high temperature and pressure, filling well, sealing and loading easily. In this work, the forming of AA7075 cylindrical parts by HGMF process is taken as an example to establish the constitutive relation and forming limit diagram （FLD） of AA7075 sheet which is related to temperature by hot uniaxial tensile test of sheet metal. Based on the assumption that granule medium is applied to extended Drucker-Prager linear material model, the finite element model of HGMF process is established and the effect of technological parameters, such as forming temperature, blank-holder gap and drawing ratio, on the sheet metal formability, is studied. The limit drawing ratio curve of AA7075 cylindrical parts at forming temperature of 175-300 ℃ is obtained by HGMF process test, and the limit drawing ratio reaches the maximum value of 1.71 at 250 ℃. The results of numerical simulation are consistent with the results of process test, and the forming force, distribution of wall thichness and form of instability are predicted correctly, which provides reference for the application of HGMF process.展开更多
Based on a variety of case histories of site investigations, including extensive bore hole data, laboratory testing and geophysical prospecting at more than 550 construction sites, an empirical formulation is proposed...Based on a variety of case histories of site investigations, including extensive bore hole data, laboratory testing and geophysical prospecting at more than 550 construction sites, an empirical formulation is proposed for the rapid determination of allowable bearing pressure of shallow foundations in soils and rocks. The proposed expression corroborates consistently with the results of the classical theory and is proven to be rapid, and reliable. Plate load tests have been also carried out at three different sites, in order to further confirm the validity of the proposed method. It consists of only two soil parameters, namely, the in situ measured shear wave velocity and the unit weight. The unit weight may be also determined with sufficient accuracy, by means of other empirical expressions proposed, using P or S -- wave velocities. It is indicated that once the shear and P-wave velocities are measured in situ by an appropriate geophysical survey, the allowable bearing pressure as well as the coefficient of subgrade reaction and many other elasticity parameters may be determined rapidly and reliably.展开更多
基金Project(51678037)supported by the National Natural Science Foundation of ChinaProject(2015CB057802)supported by the National Basic Research Program of ChinaProject(BLX2015-20)supported by the Fundamental Research Funds for the Central Universities,China。
文摘A novel horizontal trap-door test system was devised in this study to analyze the face stability of shield tunnels in sands.The test system can be used to investigate both the longitudinal and cross sections of the face failure simultaneously at one single apparatus and was employed to perform face stability tests on small-scaled tunnel models at single gravity.The lateral support pressures and failure zones were studied with varying sand materials and earth covers.The results demonstrate that the tunnel face moves back,the lateral active earth pressure on the tunnel face decreases rapidly to a residual value,and the lateral pressure distribution can be categorized into three stages during the failure process:1)initial state;2)pressure dissipation stage;and 3)pressure zone diminution stage.Furthermore,face failure firstly develops from a stable condition to the local failure state,and then continues to develop to the global failure state that can be divided into two sub-zones with different failure mechanisms:rotational failure zone(lower zone)and gravitational failure zone(upper zone).Further discussion shows that under the effects of soil arching,the shape of the gravitational failure zone can adopt arch shaped(most frequent)and column shaped(in shallow tunnels).Limit support pressure for face stability usually appears atδ/D=0.2%−0.5%(ratio of face displacement to tunnel diameter).
基金Projects(5130538651305385)supported by the National Natural Science Foundation of ChinaProject(E2013203093)supported by the Natural Science Foundation of Hebei Province,China
文摘Hot granule medium pressure forming （HGMF） process is a new process in which granule medium replaces the medium in existing flexible-die hot forming process, such as liquids, gases or viscous medium. Hot forming of light alloy sheet parts can be realized based on the properties of granule medium, such as withstanding high temperature and pressure, filling well, sealing and loading easily. In this work, the forming of AA7075 cylindrical parts by HGMF process is taken as an example to establish the constitutive relation and forming limit diagram （FLD） of AA7075 sheet which is related to temperature by hot uniaxial tensile test of sheet metal. Based on the assumption that granule medium is applied to extended Drucker-Prager linear material model, the finite element model of HGMF process is established and the effect of technological parameters, such as forming temperature, blank-holder gap and drawing ratio, on the sheet metal formability, is studied. The limit drawing ratio curve of AA7075 cylindrical parts at forming temperature of 175-300 ℃ is obtained by HGMF process test, and the limit drawing ratio reaches the maximum value of 1.71 at 250 ℃. The results of numerical simulation are consistent with the results of process test, and the forming force, distribution of wall thichness and form of instability are predicted correctly, which provides reference for the application of HGMF process.
文摘Based on a variety of case histories of site investigations, including extensive bore hole data, laboratory testing and geophysical prospecting at more than 550 construction sites, an empirical formulation is proposed for the rapid determination of allowable bearing pressure of shallow foundations in soils and rocks. The proposed expression corroborates consistently with the results of the classical theory and is proven to be rapid, and reliable. Plate load tests have been also carried out at three different sites, in order to further confirm the validity of the proposed method. It consists of only two soil parameters, namely, the in situ measured shear wave velocity and the unit weight. The unit weight may be also determined with sufficient accuracy, by means of other empirical expressions proposed, using P or S -- wave velocities. It is indicated that once the shear and P-wave velocities are measured in situ by an appropriate geophysical survey, the allowable bearing pressure as well as the coefficient of subgrade reaction and many other elasticity parameters may be determined rapidly and reliably.
