Abstract:With the advantages of environmentally friendliness, high strength-to-weight ratio and good seismic performance, glulam has been widely applied to modern structures. The glulam space frame structure is a representative large-span spatial structure. In this paper, the main existing forms of glulam space frame structures are summarized and classified based on practical engineering applications. A comprehensive review of the main research advances of the glulam space frame structure is presented and mainly expounded from the aspects of mechanical behaviors of joints and overall structures. Several issues that should be paid attention to the study of overall structures were proposed. Finally, the further researches worth investigating were put forward. This paper has certain reference significance for the research and can also promote the development of relative engineering applications.

Abstract:Through the accelerated carbonization test in laboratory, the effects of curing method and the replacement rate of recycled coarse aggregate (RCA) on the carbonation resistance of concrete were studied. The results show that the carbonization depth of concrete with curing agent is the smallest under other conditions unchanged, followed by water spray curing and film curing. The carbonation depth of concrete increases with the increase of the replacement rate of RCA. Compared with the ordinary aggregate concrete, the size of the partial carbonization zone and the depth of the carbonization front of concrete with 50% replacement rate of RCA are increased by 7.5% and 23.7%, respectively, while the increase rates are 21.3% and 55.3% for the concrete with 100% replacement rate of RCA, respectively. The influence mechanism of the curing method and the replacement rate of RCA on the carbonation resistance of concrete is analyzed through the water absorption characteristics test. The results show that the capillary water absorption coefficient of concrete cured with curing agent is the smallest, which that cured with sprinkler and film increases in turn. The capillary water absorption coefficient of concrete increases approximately linearly with the increase of the replacement rate of RCA.

Abstract:In order to study the anti-explosion performance of prefabricated reinforced concrete beams connected by grouting sleeve, a numerical model of prefabricated reinforced concrete beams was established. The failure modes and dynamic responses of prefabricated reinforced concrete beams under blast loads were analyzed by using multi-material fluid-structure coupling algorithm. The effects of concrete strength, longitudinal reinforcement ratio and explosive position on the anti-explosion performance of prefabricated beams were studied. The results show that for the prefabricated reinforced concrete beam, because there is a weak interface between old and new concrete at the assembled position, the transmission performance of stress wave in the beam is weakened under the explosion load, and the impact force cannot be effectively transmitted to both sides of the beam, leading to serious brittle failure in the local area. Increasing concrete strength, longitudinal reinforcement ratio and changing explosive position can improve the anti-explosion performance of prefabricated reinforced concrete beams.

Abstract:Based on the extended finite element method (XFEM), the fracture failure process of concrete three-point bending beams with different widths was simulated, and the load-crack mouth displacement curve (P-CMOD) was obtained. The fracture process of three-point bending beam with different width, crack height ratio and span height ratio was simulated, and the stress intensity factor under different width, crack height ratio and span ratio was obtained by using the double K fracture criterion. The influence of these three parameters on the stress intensity factor was studied. The results show that the initiation fracture toughness K_Icⁱⁿⁱ and unstable fracture toughness K_Ic^un of three-point bending beam with span-depth ratio and crack-depth ratio equal to 0.4 basically remain unchanged when the beam width changes from 100 mm to 250 mm. When the beam width was constant, the span-depth ratio was 4 and the crack-depth ratio increases from 0.2 to 0.5, the initiation fracture toughness K_Icⁱⁿⁱ and the unstable fracture toughness K_Ic^un increase first and then decrease. When the beam width was constant, the crack height ratio was equal to 0.4, and the span height ratio increases from 2.5 to 3.5, the initial fracture toughness K_Icⁱⁿⁱ remains unchanged, and the unstable fracture toughness K_Ic^un will gradually increase but the amplitude was small. When the span-to-height ratio was constant, the crack-to-height ratio was equal to 0.4, and the beam width changes in the range of 100~250 mm, the initial fracture toughness K_Icⁱⁿⁱ and the unstable fracture toughness K_Ic^un all increase first and then decrease, indicating that K_Icⁱⁿⁱ and K_Ic^un with small span-to-height ratio have a certain size effect in the beam width direction.

Abstract:In order to study the effects of different polystyrene plastic microsphere contents and different rainfall intermittent ratios on unsaturated infiltration of loess soil under intermittent rainfall conditions, an indoor one-dimensional soil column model was used to conduct vertical infiltration experiments. The results show that: Under the condition of intermittent rainfall cycle, the wetting front velocity of soil gradually attenuates with the increase of intermittent rainfall cycle, the wetting front velocity decay curve conforms to the logistic model, and has a good goodness of fit; Under intermittent rainfall conditions, low content of polystyrene microspheres will make the soil more sensitive to rainfall, reduce the lag time of the response to rainfall, increase the depth of the soil wetting front, and weaken the water holding capacity of the soil, while high content of polystyrene microspheres can increase the lag time of soil response to rainfall, reduce the depth of wetting front, and enhance soil water holding capacity; Compared with the 1∶1 experimental group, the wetting front depth increases under short duration, while the wetting front depth decreases under long duration. Compared with the 1∶1 experimental group, the soil had a more obvious response to rainfall, but the depth of the wetting front decreased within the same duration.

Abstract:In order to solve the technical problems of gushing and collapse of excavation surface when the earth pressure balance shield passes through the silty fine sand stratum with high water pressure, this paper studies the gushing prevention and control technology of earth pressure balance shield under the silty fine sand stratum with high water pressure by means of screening test, modifier performance test, sand layer gushing simulation device design and field engineering verification. Test and research results: 1) according to the geological characteristics of silty fine sand of the project, determine the reasonable variety and proportion of modifier, and optimize the bentonite swelling water ratio, expansion time and mixing ratio; 2) Based on the similarity theory, a set of device for simulating the occurrence and evaluation of sand gushing is designed to intuitively reflect the effect of modifier on sand samples under different working pressures, so as to provide technical support for residue soil improvement test; 3) By evaluating the velocity v and horizontal distance of sand sample ejected from the gushing device ΔL and other parameters, the relevant proportion and mixing amount of modifier required for specific graded sand samples are determined, and the implementation scheme of anti gushing residue improvement is formed. Combined with the actual application effect of shield tunneling through Luntou sea sand layer, guide the on-site bentonite improvement parameter control, verify the reliability of bentonite related parameters and gushing simulation device, avoid the gushing risk of shield tunneling through water rich sand layer.

Abstract:The unconfined uniaxial compression test of the improved sand under different curing agent contents was carried out in laboratory. The corresponding numerical model was established by using a particle flow numerical simulation software and the numerical simulation test was carried out. In this paper, the microscopic failure mechanism of specimens under different curing agent contents in different loading stages is studied, and it is found that the content of curing agent significantly affects the compressive strength, deformation resistance and macroscopic failure mode of specimens. With the increase of the curing agent content, the peak strength and the elastic modulus increase, while the peak strain decreases. According to the development of the micro cracks, the contact force chain and the particles displacement field evolution process, it can be seen that as the curing agent content increases, specimens are prone to exhibit stress concentration and local failure and gradually develop from the overall failure into local crushing damage. Meanwhile, the bending degree of force chain decreases gradually after fracture and the displacement direction by disorder gradually disperses evenly and orderly, while the failure mode of sample changes from shear failure to tension failure.

Abstract:In order to explore the modification effect of different modifiers on various performance indexes of matrix asphalt, three different modified asphalts, SBS modified asphalt, TPE modified asphalt and high viscosity and high elasticity modified asphalt were selected for indoor test and comparison. Through the cone penetration test of modified asphalt mortar, PG high temperature classification test, temperature scanning test and MSCR test, the pavement effects of the three modified asphalts are systematically evaluated from the aspects of shear strength, medium and high temperature performance, permanent deformation resistance and deformation recovery. The results show that the preparation of modified asphalt and mortar needs to strictly control the preparation time, shear rate, temperature and other parameters within a reasonable range to ensure that the material meets the requirements. The shear strength of various asphalt mortars has a good correlation with temperature, and the correlation coefficient R² of each regression equation is very high, and the TPE modified asphalt mortar has a high shear strength. The rheological tests of three modified asphalts show that TPE modified asphalt has the best medium-high temperature anti-denatured ability and high temperature creep recovery ability, followed by high viscosity and high elasticity modified asphalt, and SBS modified asphalt is lower than the first two asphalts.

Abstract:In order to more intuitively observe the process of grouting diffusion and mud film formation in the shield mud film formation test, this paper developed a set of visual grouting test system for the excavation surface of the mud-water shield. The visualization test of the mud infiltration process of the mud-water shield under variable conditions, recording the infiltration images of each transverse and longitudinal section, and binarizing the images, exploring the influence of formation particle size and slurry viscosity on the mud diffusion pattern, and evaluating the formed mud The research results show that the permeability of the slurry in the formation is an important criterion for evaluating the quality of mud film formation, and the pore size of the formation and the viscosity of the mud are important factors affecting the penetration distance of the slurry. Under the condition of high permeability soil layer, reducing the opening ratio of the shield machine cutter head and reducing the grouting pressure can help to form a mud-skin type mud film. The test results have important guiding significance for the mud engineering of shield tunneling.

Abstract:In order to reveal the influence of the pulse effect of ground motion on the dynamic response of the immersed tunnel, a finite element model for the dynamic analysis of a bi-directional four-lane immersed tunnel was established based on the Foshan immersed tunnel structure in Guangzhou. The nonlinear dynamic constitutive model of soil was introduced into the dynamic finite element model, and the seismic responses of 40 groups of forward directional pulse ground motions (FD) as well as the corresponding non pulse ground motions (NP) with velocity pulse excluded were compared and analyzed by inlaying ground motions horizontally at the bottom of the model. The results show that: (1) the response of FD ground motion of immersed tunnel is larger than NP ground motion (displacement, acceleration and stress); (2) with the increase of PGV of ground motion, the structural responses also show an increasing trend. At the same time, the corresponding influence coefficient K of structural responses pulse is large when the influence coefficient R of peak velocity is large, indicating that the ground motion with velocity pulse has a greater response to the structure; (3) with the increase of pulse cycle T_p,structural responses of pulse influence coefficient K increased at first and then decreased, Moreover the structural responses are larger when T_p performed in the interval near the first natural vibration period of the structure, The damage performed similarly showed that there was significant influence of pulse period of pulsed ground motion on the seismic response of the immersed tunnel.

Abstract:The main safety problems and reinforcement design schemes of the substandard dike of a dike project, focuses on the comparative analysis of grouting seepage prevention schemes of the dike, and designs the optimized cone probe grouting in detail. On this basis, the saturated-unsaturated, stable-unstable seepage finite element calculation theory is adopted to calculate the stable seepage field and the unstable seepage field under the condition of sudden drop of water level of the dike before and after seepage control and reinforcement by cone probe grouting, and the seepage behavior such as the variation law of pore water pressure of the dike body is studied, and the corresponding stability calculation of the dike slope is carried out. The results show that, after the reinforcement by cone probe grouting, the seepage flow and escape slope of the embankment are obviously reduced, the infiltration line is lowered, and the anti-seepage effect is remarkable, and the safety factor of anti-sliding stability of the embankment slope is obviously improved under various working conditions.

Abstract:In order to study the influence of plants on soil evaporation rate and erosion resistance characteristics and the changing rules, two types of plants commonly used for slope protection, ryegrass and alfalfa, were selected for evaporation test and erosion resistance test. The evaporation rate of samples during plant growth period was analyzed under different plant seed proportion conditions, and the vegetation coverage after 14 days of plant growth is obtained by combining image binarization method. The anti erosion characteristics of soil under different plant seed proportion were analyzed. The mixed plant seeds used in the plant growth sample were set with five ratios, the ratios of ryegrass and alfalfa were 1∶0, 0.7∶0.3, 0.5∶0.5, 0.3∶0.7 and 0∶1 respectively, and the plant growth cycle was set at 14 days. The results shows that alfalfa has a more significant effect on soil evaporation rate in the early stage of vegetation growth and development, and the evaporation rate of all proportions of samples tend to be consistent in the stable growth period; Using the method of image binarization, it is concluded that the highest vegetation coverage is 80.1% when the ratio of ryegrass to alfalfa is 3∶7; With the increase of the vegetation coverage of the mixed proportion sample, the developed plant stems and leaves reduce the impact of rainwater on the soil and weaken the runoff erosion. Different types of roots combine with the soil to increase the depth of root influence and enhance the reinforcement effect, and enhance the anti scouring performance of the soil.

Abstract:In order to study the deformation and failure of the slope under rainfall conditions, a typical slope model was established, and the seepage analysis was carried out through the finite element software Geostudio. The water content of the slope soil under different rainfall intensities was imported into the particle flow method. The weight and contact parameters of the particles are modified according to the position of the particles, and seepage force is applied to the seepage area. When the rainfall intensity is greater than the maximum infiltration capacity of the slope, the dragging effect of the slope runoff on the particles is added. The deformation of the slope and the failure process of the landslide under different rainfall intensities are analyzed. The results show that when rainfall occurs, the slope deformation is mainly caused by settlement, and the maximum deformation occurs at the top of the slope; When the rainfall intensity is large, the particles at the toe of the slope will slide due to the scouring effect, and the soil mass of the rear slope will be induced to produce a progressive traction landslide.

Abstract:In order to study the fracture initiation behavior in the near-wellbore area, hydraulic fracturing tests were carried out by uniaxial hydraulic fracturing equipment. The results show that the injection rate has a significant influence on the initiation time of fractures in the near-wellbore range. And the initiation time decreases with the increase of injection rate. The influence of injection rate on fracture initiation pressure can be ignored. Fracture initiation has strong correlation with the total amount of injection fluid. Besides, the roughness of fracture surface was analyzed through image comparison. The results show that the roughness of fracture surface is different along different positions. The roughness of fracture surface is larger along bedding plane.

Abstract:To study the detection effect of the high-density electrical resistivity method on chromium-contaminated sites, the resistivity model of chromium pollution was established. The finite difference method and the least square method were used to perform forward and inversion to analyze the resistivity characteristics of chromium pollution. Then, the detection effect of different pollution concentrations under different device types is obtained. Subsequently, the high-density electrical resistivity method was carried out on a chromium contaminated site in Hebei, and the distribution of the underground pollution plume was obtained. The detection results were verified by drilling. Theoretical analysis and actual detection results show that the resistivity of underground soil polluted by chromium is relatively low, which is significantly different from that of uncontaminated soil. With the weakening of pollution degree, the identification ability of high-density electrical resistivity also decreases and the detection effects of different device types are also different. The above research results provide an important theoretical and practical basis for the site investigation of the same type of heavy metal pollution in the future.