Model Test Apparatus Research Articles

Development of shield model test system for studying the bias load of shield in soil-rock compound strata

The deviation mechanism and control of shield attitude are crucial, especially in soil-rock compound strata. To address this issue, a model test apparatus was established to conduct shield bias-load testing during tunneling. A model test was performed on shield tunneling in upper-soft lower-hard strata to verify the functionality of the developed test system. Subsequently, numerical simulations were conducted to investigate the shield bias moment in such strata and its influencing factors. The results demonstrated that the developed shield bias-load model test apparatus effectively controlled tunneling parameters and monitored the shield's bias moment in real-time. In the upper-soft lower-hard strata, the shield's bias moment increased as the proportion of rock layers in the tunnel face increased in the head-up direction, with the highest increase occurring when the soil and rock ratio in the tunnel face was 1:1. Interaction decoupling analysis in the numerical models revealed that the interaction between the shield shell and the strata was the primary cause of the shield's attitude change in soil-rock compound strata. Additionally, the shield's bias moment increased as the soil-rock modulus ratio decreased, with the bias moment range of the soil modulus being 4.66 times that of the rock modulus. This study provides valuable insights for shield operation in soil-rock compound strata.

Could Hydraulic Fracturing Take Place for Asphalt Core in Embankment Dams through Possible Cracks in the Core?

Hydraulic asphalt concrete is virtually impervious. Hydraulic fracturing due to the generation of pore water pressure can be generally excluded for asphalt facings and asphalt cores in embankment dams. However, when some cracks and/or large voids exist in the asphalt core in dams, hydraulic fracturing could take place during reservoir impounding. Cracks and/or large voids may be caused by either earthquake shaking, large differential settlements during construction and operation, or poor construction quality. Therefore, asphalt specimens with either cracks of different depths or large air porosity were prepared, and a model test apparatus was developed to investigate the possibility of hydraulic fracturing. Model tests were conducted on cylindrical asphalt concrete specimens of 100 mm in diameter and 180 mm in height. The top boundary of the specimens was either prevented (restrained) from moving in the vertical direction or free to move (unrestrained). The model test results for asphalt concrete with cracks under unrestraint conditions were numerically back-analysed. The model test results indicated that the cracks in the asphalt concrete under the restraint conditions could experience an “open–close” progress with increasing water pressure. That suggested that if the asphalt concrete were long, low water pressure would trigger a fracturing through the cracks. Under the unrestraint conditions, low water pressure could cause the asphalt specimens with either cracks or large air porosity to fail. The extensional strains at failure were reduced, and the reductions depended on the magnitude of the crack depth or air porosity. Suggestions are given for designing the asphalt core dams to reduce the possibility of the occurrence of cracks and/or dilations in the core, especially for dams in unfavourable geological and topographical conditions. A specialty contractor is preferable to do the asphalt core job to rule out the possibility of a weak bond between the layers and an air porosity larger than 3.0% in the core.

Experimental and numerical investigation on heat transfer characteristics of vertical ground heat exchangers in karst areas

Heat transfer between the ground heat exchanger and surrounding stratum under the action of groundwater seepage is a potential problem in karst areas, few systematic studies on it have been carried out yet. A model test apparatus was built to experimentally investigate the effects of karst groundwater seepage and ground stratification on the heat transfer characteristics of ground heat exchangers. The heat and seepage transfer coupling model for ground heat exchangers was established, and verified with the measured data from the model test. Base on the model, the influence of seepage velocity and direction on the heat transfer of ground heat exchangers was analyzed and discussed. Test results show that the seepage can promote the temperature field around the heat exchanger to reach the steady state faster, and the influence of seepage on the soil temperature field around the heat exchanger increases with increasing the seepage velocity. The seepage will carry the heat released by the heat exchanger from the upstream side to the downstream, which effectively reduce the accumulation of heat around the heat exchanger. Soil boundary exists the sudden change of temperature when there are great differences in thermal properties between adjacent soil layers, which can be attributed to the influence of ground stratification. The numerical model proposed in this study can well predict the distribution of soil temperature field around heat exchangers in the model test. Simulation results show that the temperature of soil around the heat exchanger will change along the seepage direction.

Model experiments to study the hydrothermal variation and resilient modulus of soil subgrade subjected to freeze-thaw conditions

Freeze-thaw (FT) cycles are highly destructive actions on roads in seasonal frozen regions. To examine the mechanical behavior of the subgrade subjected to seasonal FT conditions in close to the actual hydrothermal environment. A hydro-thermal-mechanical (HTM) coupling model test apparatus was developed, FT tests under closed and open boundary conditions were conducted on it. In the tests, hydrothermal variations were investigated and variation behaviors of resilient modulus (MR) were analyzed and compared from the perspective of HTM coupling. The results showed that (i) the models of closed system test (CST) and open system test (OST) experienced similar freezing processes, but the difference in boundary conditions caused a significant freezing rate gap between them; (ii) hydrothermal variations in the two tests were greatly different during the freezing, moisture field variation in CST was relatively typical, while it was more complicated in OST, especially in the unfrozen zone of OST model. In addition, continuous replenishment caused the water content in OST model to be much higher than that in CST model after thawing; (iii) MR was principally related to the state of upper soil layers in both FT tests due to the distribution characteristic of additional stress, and was also affected by the water content of other frozen and unfrozen soil layers. The influence mechanism of water content on OST MR was sophisticated because of the complicated moisture field variation process; (iv) the CST MR tended towards stability after 3 FT cycles. The open system was more adverse to the bearing capacity of the subgrade, with the OST MR after 1 FT cycle even lower than that in CST after 6 cycles.

Development and Application of a Three-Dimensional Geo-Mechanical Model Test System Under Hydro-Mechanical Coupling

To study the surrounding rock stability, water inrush mechanism and support control of the deeply buried tunnel under hydro-mechanical coupling, a hydro-mechanical coupled geo-mechanical model test system is developed, and it is mainly composed of a high-pressure sealed model test chamber, a built-in high hydraulic servo loading system, a high seepage pressure omni-directional loading system, an inclined geologic structure fabrication system and a high precision integrated test system. The internal field of the model test apparatus is 1.0 m in length, 1.0 m in width and 1.0 m in height. It has good sealing and can be tilted at any angle. This model test system can realize the coupling loading of real three-dimensional in-situ stress and seepage pressure, and it adopts the loading mode that the seepage pressure changes with the depth gradient. And it can prepare the inclined geological structure with arbitrary dip angle. A model test of tunnel excavation and support in the Xianglushan diversion tunnel, which is located in Yunnan, China, was carried out by using this model test system. The variations in displacement, stress, seepage pressure and contact pressure are obtained. The results demonstrate that the apparatus is effective, reliable and practical and provide theoretical support for the project.

Model test and discrete element method simulation of shield tunneling face stability in transparent clay

The stability of the shield tunneling face is an extremely important factor affecting the safety of tunnel construction. In this study, a transparent clay with properties similar to those of Tianjin clay is prepared and a new transparent clay model test apparatus is developed to overcome the “black box” problem in the traditional model test. The stability of the shield tunneling face (failure mode, influence range, support force, and surface settlement) is investigated in transparent clay under active failure. A series of transparent clay model tests is performed to investigate the active failure mode, influence range, and support force of the shield tunneling face under different burial depth conditions, whereas particle flow code three-dimensional numerical simulations are conducted to verify the failure mode of the shield tunneling face and surface settlement along the transverse section under different burial depth conditions. The results show that the engineering characteristics of transparent clay are similar to those of soft clay in Binhai, Tianjin and satisfy visibility requirements. Two types of failure modes are obtained: the overall failure mode (cover/diameter: C/D ⩽ 1.0) and local failure mode (C/D ⩾ 2.0). The influence range of the transverse section is wider than that of the longitudinal section when C/D ⩾ 2.0. Additionally, the normalized thresholds of the relative displacement and support force ratio are 3%–6% and 0.2–0.4, respectively. Owing to the cushioning effect of the clay layer, the surface settlement is significantly reduced as the tunnel burial depth increases.

A true triaxial geomechanical model test apparatus for studying the precursory information of water inrush from impermeable rock mass failure

To study the precursory information of water inrush from impermeable rock mass failure, a true triaxial geomechanical model test apparatus was developed. The test apparatus designed is 5.5 m in length, 4.0 m in width and 2.5 m in height. It is mainly composed of a steel frame system, a hydraulic control system, a water pressure loading system and a data monitoring system. The main feature of the true triaxial model test apparatus is its ability to apply high, uniform compressive loads in all three principal directions to perform tests under true triaxial static loading conditions. The apparatus can also be used to study the solid-fluid interaction of the surrounding rock mass and water. A test sample can be placed into and removed from the apparatus automatically and conveniently by hydraulic control software. A model test of the water inrush in the Xiema Tunnel, which is located in Chongqing, China, was carried out by using this apparatus. The size of the experimental sample was 1500 × 1000 × 1000 mm. The variations in displacement, stress and seepage pressure were investigated. The test results indicate that the apparatus is stable and reliable.

Experimental Study on Contact Erosion Failure in Pavement Embankment with Dispersive Clay

AbstractSoft subgrades are not strong enough to provide the required degree of compaction for a pavement embankment’s foundation. Therefore, it is required to treat the subgrade soil or construct a working platform on top of the soft subgrade to ensure proper compaction of the subsequent layers. The constriction sizes of the granular materials used for a working platform are comparatively higher to restrict the erosion of overlying embankment materials. This condition would lead to severe material loss in the embankment layer in cases where the embankment is constructed with dispersive soil, which becomes structurally unstable in wet conditions and appears to disperse in water. This paper investigates the contact erosion failure at the foundation of a pavement embankment constructed with dispersive soil as a result of groundwater fluctuation. Experimental studies were conducted on a laboratory model test apparatus designed to simulate the conditions in the pavement embankment’s foundation. The experimenta...

모형토조실험을 이용한 강우시 토층의 포화속도 산정

본 연구에서는 강우강도에 따른 토층내 포화속도를 산정하기 위하여 모형실험장치를 개발하고 일련의 모형실험을 수행하였다. 모형실험장치는 모형토조, 강우재현장치 및 계측장치로 구성되어 있다. 모형지반(60 cm × 50 cm × 15 cm)은 상대밀도 75%의 주문진 표준사로 조성하였으며, 강우재현장치는 강우강도의 조절이 가능하도록 하였다. 그리고 토층내 강우침투시 깊이별 체적함수비 및 모관흡수력의 변화를 측정하기 위하여 TDR과 Tensiometer를 설치하였다. 모형실험결과 토층의 입도가 균등하고 투수계수가 상대적으로 크므로 강우시 지표면에서 습윤전선이 형성되어 하강하는 것이 아니라 하부 바닥면에서부터 지하수위가 형성되어 상승하면서 포화가 진행되었다. 강우시 토층내 흡입응력의 변화를 살펴본 결과 토층 내에서 체적함수비가 증가함에 따라 흡입응력은 감소하며, 체적함수비가 20-30% 사이에서 흡입응력은 비교적 빠르게 감소하였다. 강우강도와 토층의 평균포화속도를 회귀분석한 결과 강우강도에 따른 평균포화속도는 V_savg (cm/sec) = 0.068I_R (mm/hr)와 같이 제안할 수 있다. We constructed a model test apparatus to evaluate the dependence of the saturation velocity (V_s) in soils on rainfall intensity (I_R). The apparatus comprises a soil box, a rainfall simulator, and measuring sensors. The model grounds (60 cm × 50 cm × 15 cm) were formed by Joomunjin standard sand with a relative density of 75%. The rainfall simulator can control the rainfall intensity to reenact the actual rainfall in a soil box. Time Domain Reflectometer (TDR) sensors and tensiometers were installed in the soils to measure changes in the volumetric water content and matric suction due to rainfall infiltration. During the tests, the soil saturation was determined by raising the groundwater table, which was formed at the bottom of the soil box. [Please check that the correct meaning has been maintained.] The wetting front did not form at the ground surface during rainfall because the soil particles were uniform and the coefficient of permeability was relatively high. Our results show that the suction stress of the soils decreased with increasing volumetric water content, and this effect was most pronounced for volumetric water contents of 20%-30%. Based on a regression analysis of the relationship between rainfall intensity and the average saturation velocity, we suggest the following equation for estimating the saturation velocity in soils: V_savg (cm/sec) = 0.068I_R (mm/hr).

A full-scale physical model test apparatus for investigating the dynamic performance of the slab track system of a high-speed railway

This study involves the development of a full-scale physical model test apparatus to investigate the dynamic performance of the slab track system of a high-speed railway. A portion of a railway ballastless track with embankment and underlying soil strata was built inside a steel box (15 m long, 5 m wide and 6 m high). A sequential-load-generating system consisting of eight high-performance actuators was developed to apply a load that was equivalent to that generated by a train moving at speeds up to 360 km/h. Two loading conditions can be implemented in the tests, a stationary cyclic loading (using one actuator) or a simulated moving train loading (using all eight actuators). Three test cases were performed to demonstrate the capacity of the proposed test facility. The first case was to test the load-sharing ratio of fasteners beneath the rails to determine the load distribution on the track slab. A formula, based on the test results, was proposed to determine the transfer process of the loads between the train wheel and the rail fasteners. The second case was a stationary cyclic loading test, which applied loads with varying frequencies to the rails using a single actuator. The resonance frequency of the track/embankment/soil system was determined in these tests to be about 16 Hz, which coincided with theoretically computed results and field measurements. The final case was to simulate the dynamic excitation resulting from the passage of a train on the track at various speeds (up to 360 km/h). The test results on track vibrations in the physical model test showed reasonably good agreement with field measurements. These three test cases fully demonstrate the ability of the newly developed full-scale test facility to simulate dynamic excitation from trains and its ability to explore the dynamic performance of railway structures under train loadings.

주문진 모래를 이용한 선반식 옹벽의 수평토압 측정 모형시험

최근 국내에서도 구조물의 안정성을 향상시키고 수평토압의 크기를 줄여주는 선반식 옹벽에 대한 관심이 높아지고 있다. 이에 따라 모형시험 등 연구도 수행되기 시작했는데 기존 모형시험은 시험조건도 다양하지 않으며, 시험방법 등에서 개선해야 할 점이 있다. 이 연구에서는 개선된 모형시험 방법으로 여러 가지 다양한 조건에 대하여 모형시험을 수행하여 선반식 옹벽의 수평토압 감소효과를 확인하고자 하였다. 모형지반으로는 옹벽모형지반으로 일반적으로 많이 사용되는 주문진 모래를 이용하였으며, 1.2m의 모형옹벽을 15개 층으로 나누어 층마다 2개씩 총 30개의 로드셀을 사용하여 옹벽 높이별 수평토압을 측정하였다. 연구결과 선반식 옹벽은 캔틸레버식 옹벽에 비해 수평토압이 작게 발생하였으며, 선반 길이, 선반위치 등에 따라 수평토압의 크기가 영향을 받는 것으로 나타났다. Recently, the retaining wall with the relieving platform has received increasing interests also in Korea in that it can reduce the lateral earth pressure and provide more stability than conventional retaining wall. Previous studies with model tests studies covered only a limited test conditions. In this study, total 15 model tests were performed for various conditions with improved model test apparatus to confirm the effect of decreasing the lateral earth pressure on the retaining wall with the relieving platform. Jumoonjin sand was used for model soil and 2 load cells were used for each 15 layers to measure the lateral earth pressure. Based on the experimental results, the lateral earth pressure of the retaining wall with the relieving platform is less than the that of cantilever wall. The length of the platform and the location of the platform are the key factors influencing the lateral earth pressure.

CVC(強制圧密脱水)工法の室内モデル装置の改良とその適用

CVC(強制圧密脱水)工法の室内モデル装置の改良とその適用

A401 QUASI-STATIC ANALYSIS OF THE WHEEL LOAD VARIATION OF PNEUMATIC SUSPESION RAILWAY VEHICLES ON TWISTED CURVES

Railway vehicles of pneumatic suspension have a tendency to produce large wheel road variation on a twisted curve due to operation of the height control valve of air springs installed between both side air springs of the truck. In this paper, equations of moment equilibrium of the pneumatic suspension vehicle on the twisted curves are introduced quasi-statically, and effects of nonlinear characteristics of the height control system on the wheel load variation are analyzed. Analytical results are also examined by using the full size model test apparatus. The result shows preset pressure value of the air pressure balance valve plays an important role on the wheel load.

A comparison of X-ray fluorescence spectrometric (XRF) techniques for the determination of metal traces, especially in plastics

A comparative study of energy-dispersive X-ray fluorescence techniques (without and with polarized radiation) and a wavelength-dispersive technique was carried out in order to investigate their sensitivity for the quantitative determination of 3d transition metal traces in plastic materials. For this purpose, liquid calibration samples in paraffin oil for the elements Cr, Mn, Co, Ni and Cu were prepared. Calibration lines in the concentration range of 1–100 μg/g were evaluated for commercially available XRF-instruments and the results were critically compared. Limits of detection (LOD) ranged from 0.1 to 0.7 μg/g for wavelength-dispersive XRF and energy-dispersive XRF with polarized radiation. The LOD's for energy-dispersive XRF without polarized radiation ranged from 0.9 to 8.6 μg/g for elements in the same atomic number range between 24 and 29. The application of these optimized methods to the quantitation of wear and corrosion phenomena in plastic extruders and in a relevant model test apparatus, the annular slit tribometer, is elucidated.

Research Concerning Feedforward Water Level Control Taking Self-evaporation of Hot Water in Piping into Consideration

The two-phase flow state that originates with the transient self-evaporation of hot water on the upstream side of a water level regulating valve, decreases the flow rate capacity of the valve, and accordingly causes the water level in equipment to rise. The purposee of our research was to establish an advanced control technique to prevent an excessive rise in the water level in equipment. To do this, we proposed a feedforward water level control technique using dynamic analyses with a computer model and confirm the applicability of this technique in experiments with a model test apparatus. One feature of the new technique is that valve openings are decided according to feedforward signals that indicate the degree of self-evaporation that has occurred, and then, valve openings are corrected by feedback signals that indicate a deviation in the water level in equipment. Test results of the new feedforward control system showed that rises in the water level could be reduced to thirty percent of those seen when conventional feedback control systems are used.

Study on the Heat Indication Test of Steam Turbine Rotor Forgings : 4th Report, On Further Factors Affecting the Magnitude of Type A, C' and D Deflections

In the heat indication tests, high pressure and medium pressure steam turbine rotor forgings have shown more frequently type A, C' and D deflections compared with low pressure ones. This phenomenon has hitherto been ascribable to the metallurgical properties as these forgings contain some special and carbide-forming alloy elements. In the present experiment, however, it has been clarified by using the heat indication model test apparatus that the one of the causes of the aforesaid phenomenon is the diversity of geometrical shape among the forgings. Additional experiments have shown the effect of : 1. The rates of heating and cooling at the test thermal cycles, 2. Some characteristics of furnace for heat indication test. Thus type A, C' and D deflections can disappear in a certain manner without re-heat-treatment. This is an important fact which suggests that these three deflections do not occure due to the metallurgical properties of forgings.