人字形切槽巴西圆盘岩石试样复合型断裂渐进过程数值模拟研究

人字形切槽巴西圆盘(CCNBD)岩石试样由于诸多优点,被国际岩石力学学会确定为岩石I型断裂韧度测试建议方法,并被众多学者应用于复合型(包括纯Ⅱ型)断裂试验研究。然而,CCNBD在复合型荷载下的渐进断裂机理尚未完全获悉,复合型断裂韧度测试基于的穿透直裂纹扩展假设并未严格评估,用于复合型断裂研究的合理性还未得到有效验证。采用细观损伤力学软件首次模拟得到CCNBD在复合型荷载下的渐进破坏过程。结果显示:裂纹不仅萌生于人字形韧带尖端,且易产生于切槽边缘,造成真实裂纹前缘为曲线形,与人字形切槽试样断裂韧度测试所基于的穿透直裂纹假设不符;裂纹并非沿着预制人字形韧带平面扩展到其根部,切槽边缘的破裂方向均朝向加载端,形成扭曲的三维翼形裂纹。数值模拟结果同物理实验对比,十分吻合。CCNBD的裂纹形态和渐进扩展规律不符合当前普遍采用的复合型测试原理,因此采用CCNBD试样进行岩石复合型(包括纯Ⅱ型)断裂韧度测试值得商榷。     

Fracture toughness anisotropy in granitic rocks

Four relatively fine grained and homogeneous granitic rocks have been studied to investigate the relationship between their microstructural properties and fracture toughness behaviour. The apparently homogeneous rocks have been found to exhibit strong anisotropy in terms of microcrack distribution and their orientation. The fracture toughness values for these rocks, measured with the cracked chevron notch Brazilian disc method (CCNBD), varied from 0.71 to 1.89 MPa m^0.5, with similar variation observed within the same rock type depending on the specific fracture plane studied. The relationship between these variations and the corresponding microstructural properties along specified planes has been investigated in detail to show a strong correlation between the two. Further confirmation is provided by taking into account the seismic velocity anisotropy observed along the same specified directions. The nature of these variations and their effect on the resulting fracture path and surface are discussed. The results clearly show the difficulty in assigning a unique value of fracture toughness to rocks and rock-like materials without cognizance of their microstructural characteristics.     

Fracture behavior of framing coated glass curtain walls under fire conditions

Breaking glass and its subsequent fallout may markedly affect enclosure fire dynamics. However, little is known about the thermal fracture behavior of framing glass façades in different installation forms. Sixteen 600×600×6 mm³ solar control coated glass panes, installed in exposed, semi-exposed and hidden frames, were heated by a 500×500 mm² n-heptane pool fire to investigate the influence of frame constraints on glass breakage and fallout. Measurements were taken of the time to breakage occurrence, heat release rates, incident heat flux, central gas temperatures and glass surface temperatures. Measurements in relation to crack initiation and propagation as well as loss of integrity of the glazing assembly were also recorded. The experimental results show that all cracks initiated at the edge of covered sections and intersected rapidly, forming islands. The maximum temperature difference and heat flux that the glass can withstand are primarily in the range of 50–90 °C and 9–15 kW/m². Using the finite element method (FEM), the breakage mechanism is demonstrated, the predictions of which are in reasonably good agreement with the experimental results. Among four different installation forms, semi-exposed framing façades showed better fire resistance than exposed framing façades. Meanwhile, the fire resistance of hidden framing façades may depend on the fire location. It is intended that these results will provide practical guidelines for fire safety design.     

Comparative studies on the performance of a roadheader,impact hammer and drilling and blasting method in the excavation of metro station tunnels in Istanbul

Drilling and blasting is the most widely used excavation method in mining and tunnelling especially in hard rock conditions. But in recent years, the application of roadheaders and impact hammers in hard rock, especially in fractured geological formations has increased considerably. However, it is strongly emphasized that the prediction of the machine performance plays an important role in the time scheduling and in the economy of tunnelling projects and accumulated data will serve a sound basis for performance prediction models.This paper presents information on Istanbul Kadikoy–Kartal metro tunnels which are planned to be constructed in two stages, the first one which is in Kozyatagi–Kadikoy direction and the second in Kozyatagi–Kartal direction. The construction method of the Kozyatagi–Kadikoy station tunnels is first summarized and later, the performance of a roadheader, impact hammer and the results of drilling and blasting methods are compared.The results of this study show that machine utilization time is 28.2% for roadheader and 14.2% for impact hammers. Average net cutting rates (NCR) are 32.26 m³/h for roadheader (218.3 m³/day), net breaking rate (NBR) 13.1 m³/h (45 m³/day) for impact hammers and production rate with drill and blast method (D&B) is found to be 187 m³/day.     

用SCDC试样测试岩石动态断裂韧度的新方法

 利用大直径(?100 mm)分离式霍普金森压杆对大尺寸(150 mm×80 mm)压缩单裂纹圆孔板(SCDC)试样冲击加载,采用实验–数值–解析法测定了青砂岩的I型动态起裂韧度和动态扩展韧度。试样的起裂时刻和裂纹扩展速度由黏贴在裂尖附近的裂纹扩展计确定,通过对比发现,裂纹扩展计的准确性和灵敏性都比黏贴在同一试样对应位置的普通应变片更好。实验–数值–解析法根据实验数据获取试样两端的加载历程,利用有限元数值计算和普适函数的半解析修正,综合考虑材料惯性效应和裂纹扩展速度对动态应力强度因子的影响,较准静态方法更适于采用大尺寸试样确定岩石动态断裂韧度。实验–数值–解析法所确定的高加载率和高裂纹扩展速度下砂岩的动态断裂韧度值分别随动态加载率和裂纹扩展速度的提高而增加。最后,通过对SCDC试样裂纹扩展路径上应变片的断裂时间分析,确定了利用SCDC试样实现动态止裂的可能性。     

Fracture behavior of a four-point fixed glass curtain wall under fire conditions

The cracking and subsequent fallout of glazing could significantly affect compartment fire dynamics by creating a new opening for air to enter. Twenty-four 1200×1200×6 mm³ soda-lime glass panes in eight different fixing forms were heated by a 500×500 mm² N-heptane pool fire to investigate the influence of fixing conditions on glass breakage and fallout. The time of crack initiation, behavior of crack propagation, heat release rates, central gas temperatures, glass surface temperatures and loss of integrity of the glazing assembly were investigated. The relationship between fixing form and crack behavior is discussed, based on the experimental results. The results show that all the cracks initiated at the supporting point and intersected rapidly, causing glass fallout. Mechanical stress caused by supporting pins and thermal stress caused by glass temperature difference (ranging from 48 °C to 159 °C) are the causes of breaking for this kind of curtain wall. It is concluded that various fixing locations have a significant effect on glass breaking. Among the eight cases, the glass panes whose supporting points were located at 10 cm (Case 1) or 5 cm (Case 8) from the edges performed best: these support locations are recommended in practical engineering because of the good fire resistance and structural beauty of such panes.     

Sulphate resistance of fibre reinforced cement-based foams

This paper describes the results of an investigation on the resistance of plain and fibre reinforced cement-based foams to sulphate exposure. A synthetic foaming agent was used to produce foamed cementitious composites with essentially a closed cellular structure at 1200 kg/m³, 750 kg/m³, and 475 kg/m³. Polymeric microfibres were introduced at 0% and 0.2% volume fraction to result in 6 mixes. Prismatic specimens were immersed in a sodium sulphate solution to be tested in flexure, after specific intervals of exposure, according to ASTM C1609. A comparison with the response of unexposed specimens reveals that the heavier cement-based foams are more susceptible to sulphate attack and perform poorly with an increase in the duration of exposure. On the other hand, the lightest of the mixes—at 475 kg/m³—registered higher flexural strength and toughness factors up to 30 days of exposure before succumbing to sulphate attack. This self-healing response was attributed to the space available in such highly porous composites that allows for the unhindered growth of ettringite without attendant cracking. The presence of microfibres facilitated self-healing, as evident from the flexural toughness factor.     

Study on the influence of joint spacing on rock fragmentation under TBM cutter by linear cutting test

Joint spacing is one of the most important geological factors influencing rock fragmentation by TBM cutters and TBM performance. In order to study the influence of joint spacing, full-scale linear cutting tests have been conducted for the Beishan granite samples with different joint spacing (i.e. one intact sample, two jointed samples with joint spacing of 100 mm and 400 mm). For different joint spacing, the influence of penetration depth on rock fragmentation was also explored by varying the penetration depth with an interval of 0.5 mm. During the test process, the three directional forces acting on the TBM cutter were recorded, and the rock chips formed by each cutting pass were weighed, respectively. By analysing the cutting force, crack initiation/propagation and rock chips, the influences of joint spacing on rock fragmentation process by TBM cutter were investigated. The test results showed that the increase of penetration depth cannot improve the TBM breakage efficiency after reaching a certain value for the intact rock sample, and the normal force for intruding the intact rock is larger than that for intruding the rock jointed samples. It is also found that the sample part below the joint plane is intact, thus joint can restrain the crack propagating cross the joint plane and facilitates the chips formation on the cutting surface. For the rock sample with joint spacing of 100 mm, two rock fragmentation modes were found during the cutting process. One mode is that the cracks initiate from the crushed zone under TBM cutter, and the cracks propagate to the joint plane, consequently form large rock chips. The other one is that the cracks initiate from the joint plane and then propagate to the rock cutting surface, and the cracks initiate before the formation of the crushed zone under the cutter. For the rock sample with joint spacing of 400 mm, there are two rock fragmentation stages, i.e., the normal rock fragmentation stage and the joint-controlled rock fragmentation stage. There is a transitional process between these two stages, and also the median crack can be promoted to propagate vertically to joint plane due to the joint existence. This study can provide useful guidance for operation optimization and performance prediction for TBM operating in jointed rock masses.     

Effects of steel fiber addition on mechanical properties of concrete and RC beams

C20 and C30 classes of concrete are produced each with addition of Dramix RC-80/0.60-BN type of steel fibers (SFs) at dosages of 0, 30, 60 kg/m³, and their compressive strengths, split tensile strength, moduli of elasticity and toughnesses are measured. Nine reinforced concrete (RC) beams of 300 × 300 × 2000 mm outer dimensions, designed as tension failure and all having the same steel reinforcement, having SFs at dosages of 0, 30, 60 kg/m³ with C20 class concrete, and nine other RC beams of the same peculiarities with C30 class concrete again designed as tension failure and all having the same reinforcement are produced and tested under simple bending. The load versus mid-span deflection relationships of all these RC and steel-fiber-added RC (SFARC) beams under simple bending are recorded. First, the mechanical properties of C20 and C30 classes of concrete with no SFs and with SFs at dosages of 30 and 60 kg/m³ are determined in a comparative way. The flexural behaviours and toughnesses of RC and SFARC beams for C20 and C30 classes of concrete are also determined in a comparative way. The experimentally determined (mid-section load)–(SFs dosage) and (toughness)–(SFs dosage) relationships are given to reveal the quantitative effects of concrete class and SFs dosage on these crucial properties.     

Monitoring the energy-use effects of cool roofs on California commercial buildings

Solar-reflective roofs stay cooler in the sun than solar-absorptive roofs. Such “cool” roofs achieve lower surface temperatures that reduce heat conduction into the building and the building's cooling load. We monitored the effects of cool roofs on energy use and environmental parameters in six California buildings at three different sites: a retail store in Sacramento; an elementary school in San Marcos (near San Diego); and a four-building cold storage facility in Reedley (near Fresno). The latter included a cold storage building, a conditioning and fruit-palletizing area, a conditioned packing area, and two unconditioned packing areas.Results showed that installing a cool roof reduced the daily peak roof surface temperature of each building by 33–42 K. In the retail store building in Sacramento, for the monitored period of 8 August–30 September 2002, the estimated savings in average air conditioning energy use was about 72 Wh/m²/day (52%). On hot days when the afternoon temperature exceeded 38 °C, the measured savings in average peak demand for peak hours (noon–5 p.m.) was about 10 W/m² of conditioned area. In the school building in San Marcos, for the monitored period of 8 July–20 August 2002, the estimated savings in average air conditioning energy use was about 42–48 Wh/m²/day (17–18%). On hot days, when the afternoon temperature exceeded 32 °C, the measured savings in average peak demand for hours 10 a.m.–4 p.m. was about 5 W/m² of conditioned area. In the cold storage facility in Reedley, for the monitored period of 11 July–14 September 2002, and 11 July–18 August 2003, the estimated savings in average chiller energy use was about 57–81 Wh/m²/day (3–4%). On hot days when the afternoon temperature exceeded 38 °C, the measured savings in average peak-period demand (average cooling-power demand during peak demand hours, typically noon–6 p.m.) was about 5–6 W/m² of conditioned area.Using the measured data and calibrated simulations, we estimated savings for similar buildings installing cool roofs in retrofit applications for all 16 California climate zones. For similar retail stores in climate zones 2 and 4–16, installing a cool roof can save about 6–15 kWh/m²/year of conditioned area. In climate zones 2–16, estimates of average peak demand savings for hours noon–5 p.m. range from 2.9 to 5.8 W/m². For similar school buildings in climate zones 2–16, installing a cool roof can save from 3 to 6 kWh/m²/year of conditioned roof area. For all 16 climate zones estimates of average peak demand savings for hours noon–5 p.m. range from 2.6 to 3.8 W/m². In similar cold storage buildings in all 16 climate zones, installing a cool roof can save about 4.5–7.4 kWh/m²/year of conditioned roof area. In all 16 climate zones, estimates of average peak demand savings for hours noon–5 p.m. range from 3.9 to 6.6 W/m².     

加载速率对岩石动态断裂韧度影响的实验研究

为了获得岩石在高加载速率作用下的动态断裂韧度值并分析加载速率的影响,由分离式霍普金森压杆入射杆杆端附加劈尖及其基座对边切槽圆盘试样施加动态劈裂载荷。把应变片粘贴在裂纹尖端附近获得裂纹扩展时间;将劈裂载荷时间历程及裂纹扩展时间输入有限元计算模型,获得试样的起裂动态断裂韧度值。结果表明,在加载速率18.85×104MPa.m1/2s-1以下,大理岩的动态断裂韧度值随着加载速率的增大而上升,但上升趋势逐渐减弱。断裂韧度数值在高加载速率下呈现出明显的离散性。     

Automatic fire detection in road traffic tunnels

Fire detection experiments in a road traffic tunnel were performed in the Runehamar test tunnel 5th–8th March 2007. The Runehamar test tunnel is a full profile road traffic tunnel, 1.65 km long, located outside Åndalsnes, Norway. The goal was to examinate smoke and heat detection systems to determinate what kind of principle best suited for detecting a fire in an early stage. The systems were tested during small Heptane pool fires, varying between 0.16 m² and 1 m², giving heat release rates from 0.2 MW to 2.4 MW accordingly, and one car fire of about 3–5 MW, and with wind conditions varying from 1.1 m s^?1 to 1.6 m s^?1. The size of the fires, were designed to be in the range from impossible to difficult to detect. The results were conclusive. Earliest detection of a car fire, fire starts inside, was by smoke detection given fixed limits (3000 μg m^?3). With open pool fires, or immediate flames, continues fibre optical heat detection systems was faster given the limits 3 °C/4 min.     

A comparative experimental investigation of concrete,reinforced-concrete and steel-fibre concrete pipes under three-edge-bearing test

Three-edge-bearing and crack size measurement tests were carried out on plain concrete, reinforced-concrete, and steel-fibre concrete pipes of 500 mm diameters. The average three-edge-bearing strength and crack size of steel-fibre concrete pipes having steel fibres of RC80/60-BN type at a dosage of 25 kg/m³ turned out to be 82% greater and 47% smaller than those of plain concrete pipes, and 6% greater and 15% smaller than those of reinforced-concrete pipes, respectively. Tests on those steel-fibre concrete pipes having a steel fibres dosage of 40 kg/m³ revealed that a steel fibres dosage of 25 kg/m³ seems to be close to optimum because a 60% increase in the amount of steel fibres engenders only minor improvements. By these findings, steel-fibre concrete pipes are more economical than and mechanically and physically superior to reinforced-concrete pipes.     

Drip sealing of tunnels in hard rock: A new concept for the design and evaluation of permeation grouting

This paper presents a new pre-excavation grouting concept to prevent dripping and reduce the inflow into a railway tunnel. For this purpose, the tunnel’s roof was drip-sealed using colloidal silica and the walls and invert of the tunnel were grouted with cement. The grouting design process followed a structured approach with pre-investigations of core-drilled boreholes providing parameters for the layout. Water pressure tests and pressure volume time recordings were used for the evaluation. Results showed that the design was successful: the total transmissivity was reduced from 4.9 × 10^?08 m²/s to the measurement limit (1.6 × 10^?08 m²/s), and the dripping was reduced to eight spots from the roof. Improved rock characterisation showed that the grout hole separation was within the transmissivity correlation length and that grouting efficiency depends to a large extent on the dimensionality of the flow system of the rock mass.     

Strength and toughness improvement of cement binders using expandable thermoplastic microspheres

The effect of Expandable Thermoplastic Microspheres (ETM) loading on the fracture resistance and indirect tensile strength of cement binders is studied. Portland white cement (PWC) was used as the matrix in the current study. Loadings of 0.1%, 0.35%, 0.5%, 0.75% and 1%, by weight, of ETM were added to the dry cement. Semi-circular bend specimens, 152 mm in diameter and 27 mm thickness with different notch depths were fabricated to study the crack resistance of the compounds, J_c. For the indirect tensile tests, circular specimens, 50 mm in diameter and 12.7 mm thickness were used. All specimens were left to cure under water for 7 days. A 2.5-fold increase in the indirect tensile strength was achieved at an ETM loading of 0.35% by weight. A nearly threefold increase in the fracture resistance occurred at the 0.1% ETM loading. The thermal resistivity of the compounds increased by 30% for a 1% Expancel loading. Fracture surface examination revealed that the ETM facilitated the permeation of water by creating pores. Thus, an optimum strength and fracture resistance was achieved between 0.1% and 0.4%.     

Sealing of fractures in claystone

The sealing behavior of fractures in clay rocks for deep disposal of radioactive waste has been comprehensively investigated at the GRS laboratory. Various sealing experiments were performed on strongly cracked samples of different sizes from the Callovo-Oxfordian argillite and the Opalinus clay under relevant repository conditions. The fractured samples were compacted and flowed through with gas or synthetic pore-water under confining stresses up to 18 MPa and elevated temperatures from 20 °C to 90 °C. Sealing of fractures was quantified by measurements of their closure and permeability. Under the applied thermo-hydro-mechanical (THM) conditions, significant fracture closure and permeability decrease to very low levels of 10^?19 to 10^?21 m² were observed within time periods of months to years. The properties of the resealed claystones are comparable with those of the intact rock mass. All test results suggest high sealing potentials of the studied claystones.     

Investigation of failure behavior of continuous rock mass around cavern under high internal pressure

A series of physical and numerical model tests were performed to investigate the failure behavior of a continuous rock mass surrounding a silo-shaped cavern under high internal pressure. This research aims to provide information on fracture initiation and propagation in the rock mass around an underground gas storage cavern occurring as a result of applying high internal pressure under different controlled conditions. By scaling down the prototype 200 times, synthetic rock specimens containing silo-shaped hole were confined to vertical pressures of 12.5 and 25 kPa, with a K_o of 0.5, 1 and 3 in the two horizontal directions. The pressure was gradually applied in the silo until fracture initiated and propagated. The photogrammetric analysis provides insights into the response of the rock mass during the application of internal pressure and assists in identifying the failure path that occurred. The resulting fracture patterns indicate that the lateral earth pressure coefficient at rest, K_o, has a strong influence on the position of crack initiation and the propagation direction of the failure path. Supplemental numerical analyses were carried out to evaluate the failure mode, which cannot be addressed by the model tests. The numerical method, based on finite element method, considers stress analysis with localization and is developed to capture the formation of discrete fractures in a continuum. It is found that the method is able to accurately represent the factors that affect the fracture pattern and that a qualitative agreement between the experimental and numerical results can be established. A comparison between experimental and numerical results indicates that the fracturing process was caused by tensile cracking.     

Estimating geometrical and mechanical REV based on synthetic rock mass models at Brunswick Mine

This paper uses a case study from Brunswick Mine in Canada to determine a representative elementary volume (REV) of a jointed rock mass in the vicinity of important underground infrastructure. The equivalent geometrical and mechanical property REV sizes were determined based on fracture systems modeling and numerical experiments on a synthetic rock mass. Structural data collected in massive sulphides were used to generate a large fracture system model (FSM), 40 m×40 m×40 m. This FSM was validated and subsequently sampled to procure 40 cubic specimens with a height to width ratio of 2 based on sample width from 0.05 to 10 m. The specimens were introduced into a 3D particle flow code (PFC3D) model to create synthetic rock mass (SRM) samples. The geometrical REV of the rock mass was determined based on the number of fractures in each sampled volume (P₃₀) and the volumetric fracture intensity (P₃₂) of the samples. The mechanical REV was estimated based on the uniaxial compressive strength (UCS) and elastic modulus (E) of the synthetic rock mass samples.The REV size of the rock mass was determined based on a series of statistical tests. The T-test was used to assess whether the means of the samples were statistically different from each other and the F-test to compare the calculated variance. Finally, the coefficient of variation, for the synthetic rock mass geometrical and mechanical properties, was plotted against sample size. For this particular site the estimated geometrical REV size of the rock mass was 3.5 m×3.5 m×7 m, while the mechanical property REV size was 7 m×7 m×14 m. Consequently, for engineering purposes the largest volume (7 m×7 m×14 m) can be considered as the REV size for this rock mass.     

MODE II AND MIXED MODE I-II ROCK FRACTURE RESEARCH

MODE II AND MIXED MODE I-II ROCK FRACTURE RESEARCH@于海勇$黑龙江科技学院!教授