Dynamic characteristics of coal specimens with varying static preloading levels under low-frequency disturbance load

The mechanical properties of residual coal pillars under the influence of upward mining disturbances significantly affect the safety of residual mining activities on working faces.This study conducted low-frequency disturbance dynamic uniaxial compression tests on coal specimens using a self-developed dynamic-static load coupling electro-hydraulic servo system,and studied the strength evolutions,surface deformations,acoustic emission(AE)characteristic parameters,and the failure modes of coal specimens with different static preloading levels were studied.The disturbance damage is positively correlated with the coal specimen static preload level.Specifically,the cumulative AE count rates of the initial accelerated damage stage for the coal specimens with static preloading level of 60%and 70%of the uniaxial compressive strength(UCS)were 2.66 and 3.19 times that of the 50%UCS specimens,respectively.Macroscopically,this behaviour manifested as a decrease in the compressive strength,and the mean strengths of the disturbance-damaged coal specimens with 60%and 70%of UCS static preloading decreased by 8.53%and 9.32%,respectively,compared to those of the specimens under pure static loading.The crack sources,such as the primary fissures,strongly control the dynamic response of the coal specimen.The difference between the dynamic responses of the coal specimens and that of dense rocks is significant.

Numerical Simulation of Vacuum Preloading for Chemically Conditioned Municipal Sludge

Municipal sludge is a sedimentation waste produced during the wastewater process in sewage treatment plants.Among recent studies,pilot and field tests showed that chemical conditioning combined with vacuum preloading can effectively treat municipal sludge.To further understand the drainage and consolidation characteristics of the conditioning sludge during vacuum preloading,a large deformation nonlinear numerical simulation model based on the equal strain condition was developed to simulate and analyze the pilot and field tests,whereas the simulation results were not satisfactory.The results of the numerical analysis of the pilot test showed that the predicted consolidation degree was greater than that measured by the field tests,which is attributed to the relatively low permeability layer formed during the preloading process of the prefabricated vertical drain.To better reflect the consolidation process of the conditioned sludge,a simplified analysis method considering the low permeability layer around the prefabricated vertical drain was proposed.The initial permeability coefficient of the low permeability layer is determined via numerical simulations using finite difference method.The predicted settlement curve was in good agreement with the measured results,which indicated that the numerical simulation based on the equal strain condition considering the relatively low permeability layer can better analyze the consolidation process of ferric chloride-conditioning sludge with vacuum preloading.

A New Method of Combination of Electroosmosis, Vacuum and Surcharge Preloading for Soft Ground Improvement

As a rapid and effective ground improvement method is urgently required for the booming land reclamation in China＇s coastal area, this study proposes a new combined method of electroosmosis, vacuum preloading and surcharge preloading. A new type of electrical prefabricated vertical drain （ePVD） and a new electroosmotic drainage system are suggested to allow the application of the new method. This combined method is then field-tested and compared with the conventional vacuum combined with surcharge preloading method. The monitoring and foundation test results show that the new method induces a settlement 20% larger than that of the conventional vacuum combined with surcharge preloading method in the same treatment period, and saves approximately half of the treatment time compared with the vacuum combined with surcharge preloading method according to the finite element prediction of the settlement. The proposed method also increases the vane shear strength of the soil significantly. The bearing capacity of the ground improved by use of the new proposed method raises 118%. In comparison, there is only a 75% rise when using the vacuum combined with surcharge preloading method during the same reinforcement period. All results indicate that the proposed combined method is effective and suitable for reinforcing the soft clay ground. Besides, the voltage applied between the anode and cathode increases exponentially versus treatment time when the output current of power supplies is kept constant. Most of the voltage potential in electroosmosis is lost at electrodes, leaving smaller than 50% of the voltage to be effectively transmitted into the soil.

Laboratory study for effects of vacuum preloading on physical and mechanical properties of soft clayey soils

A new triaxial apparatus was designed and manufactured. It is able to applysurcharge and combined vacuum-surcharge pressures on soil samples, and allows for monitoring ofexcess pore-water pressure, axial strain or settlement, and volumetric strain during the process ofconsolidation. Tests were performed using the apparatus on undisturbed soft clayey soil samples,which were collected from Wenzhou, Zhejiang Province, China, at average natural water content 72. 5%. The consolidation behavior of theclay has no rigorous difference, whether it is consolidatedunder the vacuum, surcharge, or combined vacuum-surcharge preloading. The study shows that somephysical properties of the soft clayey soils are changed and mechanical properties are improved tosupport excessive loads transferred to the soil foundation due to construction.

Vacuum preloading combined electroosmotic strengthening of ultra-soft soil

To assess the effectiveness of vacuum preloading combined electroosmotic strengthening of ultra-soft soil and study the mechanism of the process, a comprehensive experimental investigation was performed. A laboratory test cell was designed and applied to evaluate the vacuum preloading combined electroosmosis. Several factors were taken into consideration, including the directions of the electroosmotic current and water induced by vacuum preloading and the replenishment of groundwater from the surrounding area. The results indicate that electroosmosis together with vacuum preloading improve the soil strength greatly, with an increase of approximately 60%, and reduce the water content of the soil on the basis of consolidation of vacuum preloading, howeve~ further settlement is not obvious with only 1.7 mm. The reinforcement effect of vacuum preloading combined electroosmosis is better than that of electroosmosis after vacuum preloading. Elemental analysis using X-ray fluorescence proves that the soil strengthening during electroosmotic period in this work is mainly caused by electroosmosis-induced electrochemical reactions, the concentrations of Al2O3 in the VPCEO region increase by 2.2%, 1.5%, and 0.9% at the anode, the midpoint between the electrodes, and the cathode, respectively.

Laboratory model study of newly deposited dredger fills using improved multiple-vacuum preloading technique

Problems continue to be encountered concerning the traditional vacuum preloading method in field during the treatment of newly deposited dredger fills.In this paper,an improved multiple-vacuum preloading method was developed to consolidate newly dredger fills that are hydraulically placed in seawater for land reclamation in Lingang Industrial Zone of Tianjin City,China.With this multiplevacuum preloading method,the newly deposited dredger fills could be treated effectively by adopting a novel moisture separator and a rapid improvement technique without sand cushion.A series of model tests was conducted in the laboratory for comparing the results from the multiple-vacuum preloading method and the traditional one.Ten piezometers and settlement plates were installed to measure the variations in excess pore water pressures and moisture content,and vane shear strength was measured at different positions.The testing results indicate that water dischargeetime curves obtained by the traditional vacuum preloading method can be divided into three phases:rapid growth phase,slow growth phase,and steady phase.According to the process of fluid flow concentrated along tiny ripples and building of larger channels inside soils during the whole vacuum loading process,the fluctuations of pore water pressure during each loading step are divided into three phases:steady phase,rapid dissipation phase,and slow dissipation phase.An optimal loading pattern which could have a best treatment effect was proposed for calculating the water discharge and pore water pressure of soil using the improved multiple-vacuum preloading method.For the newly deposited dredger fills at Lingang Industrial Zone of Tianjin City,the best loading step was 20 kPa and the loading of 40-50 k Pa produced the highest drainage consolidation.The measured moisture content and vane shear strength were discussed in terms of the effect of reinforcement,both of which indicate that the multiple-vacuum preloading method has a better treatment effect not only in decreasing the moisture content and increasing the bearing capacity,but also in increasing the process uniformity at different depths of foundation.

Improvement of silty clay by vacuum preloading incorporated with electroosmotic method

A laboratory test was performed to assess the effectiveness of vacuum preloading incorporated with electroosmotic (EOM) treatment on silty clay (combined method) for reclamation projects like new disposal ponds, where the horizontal electrode configurations beneath the soil layer were possible and the drainage pipes and the prefabricated vertical drains (PVDs) system could be easily installed in advance before the sludge dragged from sea bed or river bed was filled into the site. Three groups of tests were conducted on the silty clay from Qinhuai River in Nanjing, China. The model is able to apply vacuum pressure at the bottom of the soil layer and a direct current electric field simultaneously. It is also possible to measure the pore pressures at different depths of soil column, and the changes in settlement and volume with the elapsed time. In this study, the vacuum preloading method, vacuum preloading applied at the bottom (VAB method), was applied and the cathodes were installed beneath the soil layer. The results obtained indicate substantial reduction in water content, and increases in dry density and undrained shear strength in comparison with those obtained by the vacuum preloading only, particularly at the positions close to the anode. The combined method utilizes the vertical drainage flow created by the electroosmosis integrating the horizontal drainage flow created mostly by the vacuum pressure. The total drainage flow can be calculated as a result of the vertical drainage flow by electroosmosis only and the horizontal drainage flow by the vacuum preloading only. The way of placement of the cathode and the anode in the combined method also overcomes the disadvantage of EOM method itself, i.e. the appearance of cracks between the anode and the surrounding soil. Moreover, it is observed that the vacuum preloading plays a primary role in earlier stage in deduction of free pore water; meanwhile, the electroosmotic method is more efficient in later stage for absorbing water in the diffused double layers of soil.

Numerical model of soft ground improvement by vertical drain combined with vacuum preloading

The rapid development of high-speed transportation infrastructure such as highway and high-speed railway has resulted in the advancement of soft soil improvement techniques. Vacuum preloading combined with vertical drains has been proved to be an effective method in the treatment of soft foundation. A three-dimensional numerical analysis of the coupled methods was presented, in which the smear zone and the well resistance were taken into account. The variations of the basic soil parameters including the permeability coefficient and the coefficient of volume compressibility were considered in the numerical model. The result of the numerical model was then compared to the measured value. The results indicate that the decrease of coefficient of volume compressibility accelerates the consolidation of the soil while the influence of hydraulic conductivity is insignificant. A cube drain presents the closest result to the real situation compared to the other equivalent methods of prefabricated vertical drain (PVD). The case study indicates that the numerical model with variation of soil parameters is closer to the measured value than the numerical model without variation of soil parameters.

Analysis of ground deformation development and settlement prediction by air-boosted vacuum preloading

Vacuum preloading has been widely used to improve soft soils in coastal areas of China.An increasing amount of evidence from field operations has shown that conventional vacuum preloading is prone to clogging in prefabricated vertical drains(PVDs)and demands a large volume of sand fills.In recent years,air-boosted vacuum preloading has been developed to overcome these limitations;however,this method still requires more data to verify its performance.In this study,a field test for air-boosted vacuum preloading was conducted,and a large-strain two-dimensional(2D)finite element(FE)model was developed and validated against the field test data.Then,a series of FE parametric analyses was performed to assess key factors,i.e.the air injection pressure,the injection spacing,and the characteristics of cyclic injection,which affect the performance of the air-boosted vacuum preloading.The results showed that the ground settlement and lateral displacement of the soils increased due to an increase in the injection pressure,a decrease in the injection spacing,or increases in the number and duration of the injection cycles.Based on the parametric analyses,an empirical formula for ground settlement prediction was proposed and compared with a case history reported in the literature,showing good agreement.

Rigorous Axially Symmetric Consolidation Solution of Vacuum Combined Linear Loading Surcharge Preloading

Vacuum combined surcharge preloading is an effective method which has been used in ground treatment. This study presents the analytical solution of vertical drains with vacuum combined surcharge preloading. The surcharge was considered to be time-dependent preloading due to its reflection of the real situation.Moreover,the free strain,instead of equal strain hypothesis of the surface,was adopted for the mathematical rigor. Both vertical and horizontal drainages,along with the well resistance were considered.Besides,the time-dependent surcharge preloading was resolved with the impulse theorem which has been presented in the study,and the solutions for pore water pressure and degree of consolidation were derived by applying the approach of separation of variables.Furthermore,the solution is compared with previous solutions and numerical solutions,and the results verify the correctness of the proposed model.

Simulation Vacuum Preloading Method by Tri-Axial Apparatus

It is very important to control any risk of instability of embankment during vacuum construction, the simulation vacuum preloading method using tri-axial apparatus is proposed to predict the behavior of soft soil improvement in the laboratory, as well as to make this method become familiar and easier in the future. The tri-axial apparatus is used instead of the large-scale one, which has been performed by Bergado (1998) and Indaratna (2008). The tri-axial test on small size specimen can be carried out in one week compared to the large-scale apparatus takes one month for big specimen. In addition, the lateral deformation as well as the shear strength increase with time can determine accurately.

Numerical study of rheologic and soil damage for ground improvement by vacuum preloading

Based on the characteristics and reinforcement mechanism of vacuum preloading applied in high way soft foundation, the theoretical calculation method of vacuum preloading was presented. According to the equivalent method derived from sand drain foundation subjected to plane and axisymmetric conditions, the three dimensional problem was transformed into plane strain problem. The modified Komala-Huang model was used to model the rheological behavior and the damage theory was also introduced to consider the effect of damage caused by construction disturbance. Numerical simulations were carried out for a high way test section. The results show that simulations of the viscoelastic-plastic-damage model are in good agreement with the field measurements. It suggests that the calculation method, simplified load and boundary condition used in this study are reasonable.

Characterization of Strength-Shift Behavior of Bolts Connected with and without Preloading

Bolted joins allow for more efficient joining of parts(mechanical and structural components)when assembly and disassembly,called interchangeability,are concerned.Also and for this reason,they are the most widely used in the final assembly,already in construction,in a state of final consolidation of the structure.This work began in a pedagogical action to compare the experimental results with those obtained by Eurocode 3(EN 1993-1-8)in simple joints with screws subject to cutting,the study evolved the analysis of the behavior of the screws themselves in the joint.Tests were carried out with great dimensional accuracy,for new bolted connections,with different bolt’s number,according to different arrangements and repeated in cases of simple and preloaded tightening.The experimental work,carried out on a uniaxial tensile testing machine,consisted in subjecting the connections to a growing effort so that the bolts,being subject to the cut,would break through the thread-free zone.The analysis of results and their comparison(in force—displacement diagrams)allow drawing conclusions regarding the true behavior of the bolted connections.The characterization of the thresholds of the initial zone of the force—displacement curves,in the cases of connections with preloading—was the objective of study.The transfer of forces and the influence of friction on the bonds without and with preloading were then characterized.The main objective of this project is to know the meaning of the calculated values in the preloaded and non-preloaded links by Eurocode 3.

3D Nonlinear Analysis of the Preloading Filling Spiral Case of the Jinping-I Hydropower Station

In consideration of the contact between the steel spiral casing and the peripheral reinforced concrete, a nonlinear analysis for the combined bearing structure of the JinPing-I preloading filling spiral case has been made with ANSYS. Contrasts of the stress of the reinforcing bars, the biggest width of the crack and the outspread section of the crack has been made with the different parameters of preloading pressure and reinforcement scheme, resulting in a reasonable preloading pressure and reinforcement scheme. This conclusion has been applied to the spiral casing of JinPing-I hydropower station.

Consecutive Impact Loading and Preloading Effect on Stiffness of Woven Synthetic-Fiber Rope

We studied consecutive impact loading on woven high-modulus polyethylene rope, which is used in robotics fields. An impact tester was developed to conduct the experiments. Five consecutive impact loads (five drops) were applied to the rope and the stiffness of the loading part that corresponds to each drop was evaluated. The stiffness of the woven ropes was affected strongly by consecutive impact loading. The change in stiffness is undesirable in some applications such as in robotic fields. Therefore, we have proposed a method that can optimize changes in stiffness by applying a preload before impact testing (preload treatment). The experimental results show that preload is an efficient way to reduce changing rope stiffness. We have also proposed an empirical equation that can estimate the rope stiffness after arbitrary preload treatment, and this equation is a function of the number of drops and the static preload level. The equation can be used to determine the preload treatment conditions to stabilize the stiffness of the woven ropes before they are used in engineering fields.

Consolidation behavior of Tianjin dredged clay using two air-booster vacuum preloading methods

This paper presents model tests(macro aspect)and microstructure tests(micro aspect)for investigating the consolidation behavior of Tianjin dredged clay using the prefabricated vertical drain air-booster vacuum preloading(PAVP)and tube air-booster vacuum preloading(TAVP)methods.The mechanism of air-booster vacuum preloading(AVP)using a spring-like system is explained.The main difference between these two methods is the air-boosting equipment.A new anticlogging air-booster prefabricated vertical drain(PVD)is used in the PAVP technique and a self-designed air-booster tube is used in the TAVP technique.In the model tests,a comparison of the variables that are monitored during reinforcement(vacuum pressure,surface settlement,water discharge,and pore-water pressure)and after reinforcement(water content,dry density,and vane shear strength)is conducted.The results indicate that the consolidation behavior of Tianjin dredged clay using the PAVP method is better than that using the TAVP method.PAVP more efficiently mitigates the issue of water-draining PVD clogging and significantly accelerates drainage consolidation.In addition,in the microstructure tests,a comparison of the variables that are monitored after reinforcement(via scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP))is conducted,and the results further explain the model test results.

基于实际预压载作业的自升式平台环境载荷图谱绘制新思路

The environmental load chart is an important technical support required for the jack-up drilling platform to facilitate its adaptation to different operating waters and ensure the safety of operation.This chart is a crucial part of the platform operation manual.The chart data are closely related to external factors such as water depth,wind,wave,and current conditions of the working water,as well as to the structural characteristics of the platform itself and the number of variable loads.This study examines the platform state under extreme wind,wave,and current conditions during preloading.In addition,this study focuses on the difference between the ultimate reaction force of the pile leg during preloading and the reaction force of the pile leg without considering any environmental load before preloading.Furthermore,the relationship between the difference and the new reaction force of the pile leg caused by the combination of different environmental conditions is established to facilitate the construction of a new form of environmental load chart.The newly formed chart is flexible and simple;thus,it can be used to evaluate the environmental adaptability of the platform in the target well location and provides the preloading target demand or variable load limit according to the given environmental constraints.Moreover,the platform can perform personalized preloading operations,thereby improving its capability to cope with complex geological conditions,such as reducing punch-through risks.This condition reduces the load on jacking system devices and increases its service life.

Effect of preload forces on multidimensional signal dynamic behaviours for battery early safety warning

Providing early safety warning for batteries in real-world applications is challenging.In this study,comprehensive thermal abuse experiments are conducted to clarify the multidimensional signal evolution of battery failure under various preload forces.The time-sequence relationship among expansion force,voltage,and temperature during thermal abuse under five categorised stages is revealed.Three characteristic peaks are identified for the expansion force,which correspond to venting,internal short-circuiting,and thermal runaway.In particular,an abnormal expansion force signal can be detected at temperatures as low as 42.4°C,followed by battery thermal runaway in approximately 6.5 min.Moreover,reducing the preload force can improve the effectiveness of the early-warning method via the expansion force.Specifically,reducing the preload force from 6000 to 1000 N prolongs the warning time(i.e.,227 to 398 s)before thermal runaway is triggered.Based on the results,a notable expansion force early-warning method is proposed that can successfully enable early safety warning approximately 375 s ahead of battery thermal runaway and effectively prevent failure propagation with module validation.This study provides a practical reference for the development of timely and accurate early-warning strategies as well as guidance for the design of safer battery systems.

Research on Optimal Preload Method of Controllable Rolling Bearing Based on Multisensor Fusion

Angular contact ball bearings have been widely used in machine tool spindles,and the bearing preload plays an important role in the performance of the spindle.In order to solve the problems of the traditional optimal preload prediction method limited by actual conditions and uncertainties,a roller bearing preload test method based on the improved D-S evidence theorymulti-sensor fusion method was proposed.First,a novel controllable preload system is proposed and evaluated.Subsequently,multiple sensors are employed to collect data on the bearing parameters during preload application.Finally,a multisensor fusion algorithm is used to make predictions,and a neural network is used to optimize the fitting of the preload data.The limitations of conventional preload testing methods are identified,and the integration of complementary information frommultiple sensors is used to achieve accurate predictions,offering valuable insights into the optimal preload force.Experimental results demonstrate that the multi-sensor fusion approach outperforms traditional methods in accurately measuring the optimal preload for rolling bearings.

Near resonance vibration isolation on a levered-dual response(LEDAR)Coulomb-damped system by differential preloads/offsets in linear springs

The levered-dual response(LEDAR)Coulomb-damped system attains near resonant vibration isolation by differential preloads/offsets in linear springs.It takes the advantages of both the preloads/offsets in linear springs and the guiderail friction for realizing different levels of vibration isolation.The isolation capacities are investigated on the strategies with both the horizontal and vertical guiderails,with the horizontal rail only,and without guiderails.The compressive preloads generally result in the consumption of most of the initial excitation energy so as to overcome the potential threshold.The isolation onsets at the frequency ratio of 1∓0.095 on the left-hand side(LHS)and the right-hand side(RHS)of the lever are relative to the load plate connector.The observed near resonant isolation thus makes the LEDAR system a candidate for the isolation of the mechanical systems about resonance while opening a path for simultaneous harvesterisolation functions and passive functions at extreme frequencies.