Effects of thawing-induced softening on fracture behaviors of frozen rock

Due to the presence of ice and unfrozen water in pores of frozen rock,the rock fracture behaviors are susceptible to temperature.In this study,the potential thawing-induced softening effects on the fracture behaviors of frozen rock is evaluated by testing the tension fracture toughness(KIC)of frozen rock at different temperatures(i.e.-20℃,-15℃,-12℃,-10℃,-8℃,-6℃,-4℃,-2℃,and 0℃).Acoustic emission(AE)and digital image correlation(DIC)methods are utilized to analyze the microcrack propagation during fracturing.The melting of pore ice is measured using nuclear magnetic resonance(NMR)method.The results indicate that:(1)The KIC of frozen rock decreases moderately between-20℃ and-4℃,and rapidly between-4℃ and 0℃.(2)At-20℃ to-4℃,the fracturing process,deduced from the DIC results at the notch tip,exhibits three stages:elastic deformation,microcrack propagation and microcrack coalescence.However,at-4℃e0℃,only the latter two stages are observed.(3)At-4℃e0℃,the AE activities during fracturing are less than that at-20℃ to-4℃,while more small events are reported.(4)The NMR results demonstrate a reverse variation trend in pore ice content with increasing temperature,that is,a moderate decrease is followed by a sharp decrease and-4℃ is exactly the critical temperature.Next,we interpret the thawing-induced softening effect by linking the evolution in microscopic structure of frozen rock with its macroscopic fracture behaviors as follow:from-20℃ to-4℃,the thickening of the unfrozen water film diminishes the cementation strength between ice and rock skeleton,leading to the decrease in fracture parameters.From-4℃ to 0℃,the cementation effect of ice almost vanishes,and the filling effect of pore ice is reduced significantly,which facilitates microcrack propagation and thus the easier fracture of frozen rocks.

Numerical study of plasmas start-up by electron cyclotron waves in NCST spherical tokamak and CN-H1 stellarator

According to the physics of tokamak start-up,this study constructs a zero-dimensional(0D)model applicable to electron cyclotron(EC)wave assisted start-up in NCST spherical torus(spherical tokamak)and CN-H1 stellarators.Using the constructed 0D model,the results obtained in this study under the same conditions are compared and validated against reference results for pure hydrogen plasma start-up in tokamak.The results are in good agreement,especially regarding electron temperature,ion temperature and plasma current.In the presence of finite Ohmic electric field in the spherical tokamak,a study on the EC wave assisted start-up of the NCST plasma at frequency of 28 GHz is conducted.The impact of the vertical magnetic field B_(v)on EC wave assisted start-up,the relationship between EC wave injection power P_(inj),Ohmic electric field E,and initial hydrogen atom density n_(H0)are explored separately.It is found that under conditions of Ohmic electric field lower than ITER(~0.3 V m^(-1)),EC wave can expand the operational space to achieve better plasma parameters.Simulating the process of28 GHz EC wave start-up in the CN-H1 stellarator plasma,the plasma current in the zerodimensional model is replaced with the current in the poloidal coil of the stellarator.Plasma startup can be successfully achieved at injection powers in the hundreds of kilowatts range,resulting in electron densities on the order of 10^(17)-10^(18)m^(-3).

Experimental and numerical interpretation on composite foundation consisting of soil-cement column within warm and ice-rich frozen soil

Affected by climate warming and anthropogenic disturbances, the thermo-mechanical stability of warm and ice-rich frozen ground along the Qinghai-Tibet engineering corridor(QTEC) is continuously decreased, which may delay the construction of major projects in the future. In this study, based on chemical stabilization of warm and icerich frozen ground, the soil-cement column(SCC) for ground improvement was recommended to reinforce the foundations in warm and ice-rich permafrost regions. To explore the validity of countermeasures mentioned above, both the original foundation and the composite foundation consisting of SCC with soil temperature of -1.0℃ were prepared in the laboratory, and then the plate loading tests were carried out. The laboratory investigations indicated that the bearing capacity of composite foundation consisting of SCC was higher than that of original foundation, and the total deformation of original foundation was greater than that of composite foundation, meaning that overall stability of foundation with warm and ice-rich frozen soil can be improved by SCC installation. Meanwhile, a numerical model considering the interface interaction between frozen soil and SCC was established for interpretating the bearing mechanism of composite foundation. The numerical investigations revealed that the SCC within composite foundation was responsible for the more applied load, and the applied load can be delivered to deeper zone in depth due to the SCC installation, which was favorable for improving the bearing characteristic of composite foundation. The investigations provide the valuable guideline for the choice of engineering supporting techniques to major projects within the QTEC.

Mechanical behaviors of warm and ice-rich frozen soil stabilized with sulphoaluminate cement

The warm and ice-rich frozen soil is characterized by high unfrozen water content, low shear strength and large compressibility, which is unreliable to meet the stability requirements of engineering infrastructures and foundations in permafrost regions. In this study, a novel approach for stabilizing the warm and ice-rich frozen soil with sulphoaluminate cement was proposed based on chemical stabilization. The mechanical behaviors of the stabilized soil, such as strength and stress-strain relationship, were investigated through a series of triaxial compression tests conducted at -1.0℃, and the mechanism of strength variations of the stabilized soil was also explained based on scanning electron microscope test. The investigations indicated that the strength of stabilized soil to resist failure has been improved, and the linear Mohr-Coulomb criteria can accurately reflect the shear strength of stabilized soil under various applied confining pressure. The increase in both curing age and cement mixing ratio were favorable to the growth of cohesion and internal friction angle. More importantly, the strength improvement mechanism of the stabilized soil is attributed to the formation of structural skeleton and the generation of cementitious hydration products within itself. Therefore, the investigations conducted in this study provide valuable references for chemical stabilization of warm and ice-rich frozen ground, thereby providing a basis for in-situ ground improvement for reinforcing warm and ice-rich permafrost foundations by soil-cement column installation.

A dynamic soil freezing characteristic curve model for frozen soil

The soil freezing characteristic curve(SFCC)plays a fundamental role in comprehending thermohydraulic behavior and numerical simulation of frozen soil.This study proposes a dynamic model to uniformly express SFCCs amidst varying total water contents throughout the freezing-thawing process.Firstly,a general model is proposed,wherein the unfrozen water content at arbitrary temperature is determined as the lesser of the current total water content and the reference value derived from saturated SFCC.The dynamic performance of this model is verified through test data.Subsequently,in accordance with electric double layer(EDL)theory,the theoretical residual and minimum temperatures in SFCC are calculated to be-14.5℃to-20℃for clay particles and-260℃,respectively.To ensure that the SFCC curve ends at minimum temperature,a correction function is introduced into the general model.Furthermore,a simplified dynamic model is proposed and investigated,necessitating only three parameters inherited from the general model.Additionally,both general and simplified models are evaluated based on a test database and proven to fit the test data exactly across the entire temperature range.Typical recommended parameter values for various types of soils are summarized.Overall,this study provides not only a theoretical basis for most empirical equations but also proposes a new and more general equation to describe the SFCC.

Dynamic mechanical characteristics of frozen subgrade soil subjected to freeze-thaw cycles

As a widely-applied engineering material in cold regions, the frozen subgrade soils are usually subjected to seismic loading, which are also dramatically influenced by the freeze-thaw(F-T)cycles due to the varying temperature. A series of dynamic cyclic triaxial experiments were conducted through a cryogenic triaxial apparatus for exploring the influences of F-T cycles on the dynamic mechanical properties of frozen subgrade clay.According to the experimental results of frozen clay at the temperature of-10℃, the dynamic responses and microstructure variation at different times of F-T cycles(0, 1, 5, and 20 cycles) were explored in detail.It is experimentally demonstrated that the dynamic stress-strain curves and dynamic volumetric strain curves of frozen clay are significantly sparse after 20F-T cycles. Meanwhile, the cyclic number at failure(Nf) of the frozen specimen reduces by 89% after 20freeze-thaw cycles at a low ratio of the dynamic stress amplitude. In addition, with the increasing F-T cycles,the axial accumulative strain, residual deformation,and the value of damage variable of frozen clay increase, while the dynamic resilient modulus and dynamic strength decrease. Finally, the influence of the F-T cycles on the failure mechanisms of frozen clay was discussed in terms of the microstructure variation. These studies contribute to a better understanding of the fundamental changes in the dynamic mechanical of frozen soils exposed to F-T cycles in cold and seismic regions.

Numerical Simulation of a Two-Phase Flow with Low Permeability anda Start-Up Pressure Gradient

A new numerical model for low-permeability reservoirs is developed.The model incorporates the nonlinear characteristics of oil-water two-phase flows while taking into account the initiation pressure gradient.Related numerical solutions are obtained using a finite difference method.The correctness of the method is demonstrated using a two-dimensional inhomogeneous low permeability example.Then,the differences in the cumulative oil and water production are investigated for different starting water saturations.It is shown that when the initial water saturation grows,the water content of the block continues to rise and the cumulative oil production gradually decreases.

A rate-dependent constitutive model for saturated frozen soil considering local breakage mechanism

A rate-dependent constitutive model for saturated frozen soil is vital in frozen soil mechanics,especially when simultaneously describing the nonlinearity,dilatancy and strain-softening characteristics.The distribution of the non-uniform strain rate of saturated frozen soil at the meso-scale due to the local icecementation breakage is described by a newly binary-medium-based homogenization equation.Based on the field-equation-based approach of the meso-mechanics theory,the interaction expression of the strain rate at macro-and meso-scale is derived,which can give the strain rate concentration tensor at different crushed degrees.With the thermodynamics and empirical assumption,a breakage ratio in the rate-dependent form is determined.This overcomes the limitations of the existing binary-medium-based models that are difficult to simulate rate-dependent mechanical response.Based on these assumptions,a newly binary-medium-based rate-dependent model is proposed considering both the ice bond breakage and material composition characteristics of saturated frozen soil.The proposed constitutive model has been validated by the test results on frozen soils with different temperatures and strain rates.

Stabilized effects of L-S cement-mixed batter pile composite foundation for existed warm frozen soil subgrade

In permafrost regions with warm frozen soil,subgrade thaw-collapse phenomenon commonly occurs,facing thaw collapse problems of the existed frozen soil subgrade,thus it is difficult to use traditional methods such as active cooling and passive protection technology to stabilize the existed warm frozen soil subgrade.This study derives a novel stabilizer method,a long-short(L-S)cement-mixed batter pile composite foundation to stabilize the existed warm frozen soil subgrade.To solve the thawcollapse problems in warm frozen soil subgrade,high water content and large compressibility characteristics were compared between soft soil and warm frozen soils.Theoretical analysis of heat conduction and numerical simulation of finite element model were used to study the freeze–thaw process and evaluate the stabilized effects of the L-S cement-mixed batter piles on the warm frozen soil foundation of the Qinghai-Tibet Highway.Furthermore,the thaw process and mechanical properties of foundation and piles were analyzed by introducing the hydration heat factor in the thermodynamic control equation.The results indicate that the thawing displacement of the existed warm frozen soil subgrade was reduced owing to the“support”and“grasp”effects of the L-S cement-mixed batter piles on the surrounding soil.The composite ground formed by strengthening the warm frozen ground with batter piles could considerably improve the bearing capacity of the existed warm frozen ground,effectively restrain the deformation of the upper embankment,and improve the strength of the ground.The analysis can provide method for the construction design of cement mixing batter pile foundation in cold regions.

Dynamic evaluation of a scaled-down heat pipe-cooled system during start-up/shut-down processes using a hardware-in-the-loop test approach

Micro-mobile heat pipe-cooled nuclear power plants are promising candidates for distributed energy resource power genera-tors and can be flexibly deployed in remote places to meet increasing electric power demands.However,previous steady-state simulations and experiments have deviated significantly from actual micronuclear system operations.Hence,a transient analysis is required for performance optimization and safety assessment.In this study,a hardware-in-the-loop(HIL)approach was used to investigate the dynamic behavior of scaled-down heat pipe-cooled systems.The real-time features of the HIL architecture were interpreted and validated,and an optimal time step of 500 ms was selected for the thermal transient.The power transient was modeled using point kinetic equations,and a scaled-down thermal prototype was set up to avoid mod-eling unpredictable heat transfer behaviors and feeding temperature samples into the main program running on a desktop PC.A series of dynamic test results showed significant power and temperature oscillations during the transient process,owing to the inconsistency of the rapid nuclear reaction rate and large thermal inertia.The proposed HIL approach is stable and effective for further studying of the dynamic characteristics and control optimization of solid-state small nuclear-powered systems at an early prototyping stage.

Atypical progress of frozen shoulder after COVID-19 vaccination:A case report

BACKGROUND After vaccination was mandated worldwide,various adverse effects associated with the coronavirus disease 2019(COVID-19)vaccination,including shoulder pain,have been reported.Here,we report a case of new-onset shoulder pain after BNT162b2(Comirnaty,Pfizer-BioNTech)mRNA vaccination.CASE SUMMARY A 50-year-old man visited our rehabilitation center with left shoulder range of motion(ROM)limitation that had persisted for more than 5 mo.The history included no specific noteworthy events,except vaccination.The pain in the patient’s left deltoid muscle appeared 1 day after the second BNT162b2 vaccination and intensified to severe pain.The patient self-administered aspirin,with which the pain subsided immediately,whereas ROM limitation persisted.At the first visit,the patient complained of dull pain and ROM restriction of the left shoulder(flexion 130°,abduction 110°,and external rotation 40°).Among the diagnostic studies conducted for the evaluation of the shoulder,magnetic resonance imaging showed a thickened coracohumeral ligament.Nerve conduction studies and needle electromyography showed no electrodiagnostic abnormalities.The patient received comprehensive rehabilitation for 7 mo and had an overall improvement in pain and ROM of the left shoulder.CONCLUSION In this case of severe shoulder pain after COVID-19 vaccination that subsided immediately with aspirin treatment,the exact cause and mechanism of pain are unclear.However,the clinical symptoms and diagnostic workups in our report suggest the possibility that the COVID-19 vaccination triggered an immunochemical response that resulted in shoulder pathology.

On-Site Assessment of a Cryogenic Disinfectant for the Alpine Environment and Outer Packaging of Frozen Items

Objective To study the effectiveness and feasibility of cryogenic disinfectants in different cold scenarios and analyze the key points of on-site cryogenic disinfection.Methods Qingdao and Suifenhe were selected as application sites for the manual or mechanical spraying of cryogenic disinfectants.The same amount of disinfectant(3,000 mg/L)was applied on cold chain food packaging,cold chain containers,transport vehicles,alpine environments,and article surfaces.The killing log value of the cryogenic disinfectant against the indicator microorganisms(Staphylococcus aureus and Escherichia coli)was used to evaluate the on-site disinfection effect.Results When using 3,000 mg/L with an action time of 10 min on the ground in alpine regions,the surface of frozen items,cold-chain containers,and cold chain food packaging in supermarkets,all external surfaces were successfully disinfected,with a pass rate of 100%.The disinfection pass rates for cold chain food packaging and cold chain transport vehicles of centralized supervised warehouses and food processing enterprises were 12.5%(15/120),81.67%(49/60),and 93.33%(14/15),respectively;yet,the surfaces were not fully sprayed.Conclusion Cryogenic disinfectants are effective in disinfecting alpine environments and the outer packaging of frozen items.The application of cryogenic disinfectants should be regulated to ensure that they cover all surfaces of the disinfected object,thus ensuring effective cryogenic disinfection.

Effects of confining pressure and temperature on strength and deformation behavior of frozen saline silty clay

Buildings are always affected by frost heave and thaw settlement in cold regions,even where saline soil is present.This paper describes the triaxial testing results of frozen silty clay with high salt content and examines the in-fluence of confining pressure and temperature on its mechanical characteristics.Conventional triaxial compression tests were conducted under different confining pressures(0.5–7.0 MPa)and temperatures(-6℃,-8℃,-10℃,and-12℃).The test results show that when the confining pressure is less than 1 MPa,the frozen saline silty clay is dominated by brittle behavior with the X-shaped dilatancy failure mode.As the confining pressure increases,the sample gradually transitions from brittle to plastic behavior.The strength of frozen saline silty clay rises first and then decreases with increasing confining pressure.The improved Duncan-Chang hyperbolic model can describe the stress-strain relationship of frozen saline silty clay.And the parabolic strength criterion can be used to describe the strength evolution of frozen saline silty clay.The function relation of strength parameters with temperature is obtained by fitting,and the results of the parabolic strength criterion are in good agreement with the experimental results,especially when confining pressure is less than 5 MPa.Therefore,the study has important guiding significance for design and construction when considering high salinity soil as an engineering material in cold regions.

Examining the Pathological Diagnostic Impact of Frozen Sections in Breast Cancer

Objective:To analyze the diagnostic value of frozen section pathology in the diagnosis of breast cancer.Methods:A total of 50 patients with breast tumors treated between July 2021 and February 2023 were randomly selected as samples.Both paraffin section and frozen section diagnoses were conducted.The paraffin section results served as the gold standard for evaluating the value of frozen section examination.Results:Among the frozen section diagnoses,48 cases(96.00%)were confirmed,1 case was misdiagnosed(2.00%),and 1 case was delayed(2.00%).Among the confirmed patients,45 cases(90.00%)were entirely consistent,and 3 cases(6.00%)were basically consistent.The diagnostic rate of the frozen section was 96.00%,compared with 100.00%for the paraffin section(P>0.05).The diagnostic time of the frozen section(35.25±2.11 min)was significantly shorter than that for the paraffin section(6911.36±58.36 min;P<0.05).Conclusion:Frozen section diagnosis is rapid and demonstrates relatively high diagnostic accuracy.It can guide doctors in determining whether to pursue breast-conserving treatment and aid in selecting appropriate surgical methods.This is beneficial for preventing unnecessary medical interventions and reducing the need for secondary surgeries in breast cancer patients.

基于含冰量的冻结砂土-混凝土接触面蠕变特性

保证高含冰量冻土区桩基础的长期稳定性是多年冻土区桥梁桩基础安全服役中的关键问题,为研究含冰量对冻土-混凝土接触面蠕变特性的影响,采用自行研制的大型蠕变剪切仪,在−2℃条件下开展含冰量为6%、12%、16%、23%、36%、60%、80%的冻结砂土与混凝土接触面蠕变试验.试验结果表明:在恒定的剪应力作用下,除含冰量为6%试样出现加速蠕变外,其他试样仅出现衰减蠕变及稳定蠕变2个阶段;随含冰量的增大,试样黏性变形占比增大,含冰量为80%试样的黏性变形超过总变形量的80%;稳定蠕变速率受到干密度及含冰量的综合影响,含冰量为16%时稳定蠕变速率最小;Burgers黏弹性模型能较好地模拟高含冰量冻结砂土-混凝土接触面蠕变曲线;随着含冰量的增大,初始剪切模量和稳定蠕变阶段黏滞系数先增大后减小,初始蠕变阶段的渐进剪切模量呈幂函数减小,初始蠕变阶段黏滞系数呈幂函数增大.

冻结重塑黏土分数阶蠕变本构模型分析

人工冻土的蠕变特性受温度场、水分场、应力场的相互影响,而传统的岩土本构模型未能考虑温度、含水率等因素。为了表达冻土介于理想固体和理想流体之间的某种“勾兑效应”,基于岩土的经验模型,将分数阶导数理论引入其中,建立受水、热、力三场耦合影响的人工冻土分数阶蠕变模型。将西安某煤矿黏土重塑并进行不同温度、含水率的单轴抗压和单轴蠕变试验,得到温度和含水率对冻结重塑黏土强度特性和蠕变特性的影响规律。分析冻结重塑黏土在不同负温、含水率及两种应力等级下的蠕变曲线,得到lg t-lgε曲线的线性关系,进而得出冻结重塑黏土分数阶蠕变模型相关参数与温度、含水率的函数关系。对比分数阶冻土蠕变模型试验值与计算值,发现该模型能够很好地反映冻土在不同应力等级下其蠕变随温度和含水率的变化规律,对指导寒区冻土工程设计与施工具有重要意义。

补体C3水平对冻融胚胎移植妊娠结局的早期预测价值

目的探讨补体C3对冻融胚胎移植(F-ET)妊娠结局的早期预测价值。方法前瞻性收集378个F-ET周期相关资料,依据补体C3预测F-ET妊娠结局的最佳截断值分为A组(补体C3≤1.05)120个周期;B组(补体C3>1.05)258个周期,比较两组结局。分析B组补体C3预测F-ET自然流产的最佳截断值。结果年龄是F-ET妊娠成功的危险因素(P<0.05);补体C3和胚胎类型是F-ET妊娠成功的保护因素(P<0.05)。补体C3对F-ET妊娠结局的受试者工作特征曲线(ROC)曲线下面积为0.702,最佳截断值为1.05 g/L,其预测临床妊娠灵敏度为87.60%、特异度为52.00%。B组临床妊娠率(67.05%)和胚胎着床率(52.75%)明显高于A组,差异有统计学意义(P<0.05)。补体C3早期预测F-ET后自然流产最佳截断值为1.32 g/L,ROC曲线下面积为0.760,灵敏度为69.00%、特异度为81.20%。结论补体C3对早期预测F-ET妊娠结局有一定的临床意义,当补体C3超过1.32 g/L可能会导致自然流产率升高。

胚胎冷冻保存时间对首次冻融胚胎移植临床结局和出生子代的影响

目的探讨胚胎冷冻保存时间对首次冻融胚胎移植(FET)妊娠结局和出生子代的影响。方法回顾性分析2014年1月至2022年6月期间在江西省妇幼保健院辅助生殖中心行首次FET的4075例妇女的临床资料。根据胚胎冻存时间分为4组:A组(胚胎冻存≤3个月,n=993)、B组(胚胎冻存4~6个月,n=1757)、C组(胚胎冻存7~12个月,n=880)和D组(胚胎冻存13~24个月,n=445),比较各组患者的一般资料、胚胎移植情况、妊娠结局和新生儿结局,采用多元Logistic回归分析胚胎冻存时间对妊娠结局和新生儿结局的影响。结果4组患者的胚胎冻存时间、女方取卵年龄、体质量指数(BMI)、原发不孕占比等一般资料有显著性差异(P<0.05)。4组患者间获卵数、可利用胚胎数、冷冻年份、内膜准备方案和单胚胎移植率比较有显著性差异(P<0.01),囊胚移植率无显著性差异(P>0.05)。4组患者FET后的临床妊娠率、多胎率、流产率等均无显著性差异(P>0.05);A~D组的活产率分别为50.86%、53.27%、49.55%和51.01%,组间无显著性差异(P>0.05)。单胎活产婴儿的出生结局比较,4组新生儿的早产率、极早产率、低出生体重率、极低出生体重率、巨大儿率、男女性别比和出生缺陷发生率等均无显著性差异(P>0.05)。多元Logistic回归分析结果显示,胚胎冻存时间对活产率、β-HCG阳性率、临床妊娠率、种植率、流产率和早产率等均无显著影响(P>0.05)。结论胚胎玻璃化冻存2年内,胚胎冻存时间不影响首次FET的活产率,也不增加早产、低出生体重和出生缺陷发生风险。

温度对季冻区粉质黏土强度及变形特性的影响

为研究温度变化对天然状态及饱和状态下粉质黏土强度以及变形的影响,以典型季冻区广泛分布的粉质黏土为研究对象,通过全球数字系统(global digital system,GDS)非饱和土三轴测试系统对天然状态下非饱和土以及GDS温控式静/动三轴测试系统对饱和粉质黏土,在不同温度条件下进行三轴试验。对试验结果进行研究分析可得:非饱和以及饱和粉质黏土的应力-应变曲线均呈现出应变硬化的特性。非饱和粉质黏土的黏聚力随温度降低表现出不断增加的趋势,而内摩擦角随温度降低逐渐减小,但整体变化趋势较小。饱和土的黏聚力与非饱和土变化趋势相同,随温度的降低其黏聚力逐渐增加,但相同温度时其黏聚力小于非饱和土,其内摩擦角则呈先减小后增加的转变趋势。无竖向压力作用时,相同围压条件下,非饱和与饱和粉质黏土轴向变形量随温度降低而增加,相同温度条件下,两者的轴向变形量随围压的增加会有所减小。试验成果可为季冻区粉质黏土地层工程的设计施工提供理论依据。

农业冻土冰含量测量方法试验研究

农业冻土中的冰含量作为冻土冻融过程的关键变量之一,影响着土壤水分动态变化、土壤养分流失和农田生态环境维稳等。根据质量守恒原理和体积变换关系,自制一种农业冻土冰含量测量的试验装置,进行-20、-25、-30℃和土壤含水率为15.7%、23.5%、31.4%的冰含量测量试验。结果表明,农业冻土样品中冰含量随冻结温度降低冰质量含量增加。冻结温度达到-30℃时,冰质量含量达到一个恒定的峰值。相同冻结温度不同含水率状态下,冰质量含量百分比趋于一个平均值,且最大值和最小值之差约为2%。