Comparative mechanical behaviors of four fiber-reinforced sand cemented by microbially induced carbonate precipitation

Bulletin of Engineering Geology and the Environment - Microbially induced carbonate precipitation (MICP) has been found promisingly to improve soil mass properties with the environmentally friendly...     

胶凝砂砾石全级配试件与湿筛试件力学性能的试验研究

针对胶凝砂砾石全级配试件与湿筛试件力学性能开展试验研究。结果表明,随着试件尺寸的增加,强度随之下降。边长200 mm立方体抗压强度约为150 mm立方体试件的95%;边长300 mm立方体抗压强度约为150 mm立方体试件的88%;边长450 mm立方体抗压强度约为150 mm立方体试件的83%。180 d龄期边长450、300 mm立方体抗劈强度分别为标准试件的90.1%和94.5%。通过试验发现,湿筛胶凝砂砾石工作性和强度与全级配试件有联系、成规律,可以作为胶凝砂砾石配合比设计的主要试验对象,但需进行必要的胶凝砂砾石全级配试件强度复核试验供大坝安全度溯源。     

胶凝砂砾石料静、动力三轴剪切试验研究

通过不同掺量胶凝材料的静动力三轴试验,研究了胶凝砂砾石料的静、动力力学和变形特性,试验结果表明随着胶凝材料掺入量的增加,胶凝砂砾石试样的应力应变曲线逐渐由非线性向线弹性转变,其力学特性宏观上由弹塑性逐渐转变为线弹性,因此采用土工试验测试方法开展其力学特性的研究工作是适宜的。胶凝砂砾料的静、动力力学指标随着胶凝材料掺入量和养护天数的增加产生一定的提高,但增加量随着胶凝材料掺入量的提高而逐渐减缓;胶凝砂砾料的动力残余变形发展过程曲线宏观上与纯砂砾料保持相似,大体上仍符合半对数衰减规律,随围压、固结应力以及动应力的提高,其动永久变形量相应增大,仍可采用沈珠江提出的动力残余变形公式进行描述。     

微生物诱导碳酸镁沉淀试验研究

碳酸镁和碳酸钙一样也具有胶结作用,且镁矿强度远大于钙矿,因此研究碳酸镁固化技术具有重要意义。本文测量了巴氏芽孢杆菌在培养过程中的吸光度和脲酶活性,并计算得到单位脲酶活性;研究了尿素、氯化钠、醋酸根、Ca2+和Mg2+浓度对脲酶活性的影响;控制温度、pH值和离子浓度对比了钙和镁沉淀效率和不同温度不同尿素浓度下碳酸镁沉淀产率;对比研究了碳酸钙和碳酸镁沉淀下砂土固化效果。结果表明,48 h培养过程中,吸光度和脲酶活性的增长都先缓慢后迅速增长再减小最后停止,单位脲酶活性则是先增加后减小。适当增加尿素或Mg2+浓度可增强细菌脲酶活性,氯化钠和醋酸根浓度对酶活性无明显影响,Ca2+浓度对脲酶活性有明显抑制作用。同一温度、pH值和离子浓度条件下,碳酸镁产率明显小于碳酸钙。而在菌液中添加尿素可促进碳酸镁沉淀生成,且温度相同,尿素浓度越高,碳酸镁产率越大。菌液中添加尿素可使得碳酸镁固化砂土成型并具有一定强度,因此,该方法可解决砂土固化碳酸镁沉淀不足的问题,为后续碳酸镁固化试验奠定基础。     

Effect of coral sand powders and seawater salinity on the impact mechanical properties of cemented coral sand

Coral sand is one kind of the important building materials in coral reef engineering practice. The use of cement as a stabilizing agent can significantly improve the mechanical properties of coral sands and is widely applied in the subbase engineering construction in coral reef islands. Cement-stabilized coral sand structures may contain high contents of fine coral particles and salinity because of the high crushability of coral sands and the existence of seawater surrounding them. In this study, the effects of coral sand powders and seawater salinity on the dynamic mechanical properties of cemented coral sand (CCS) were investigated through the split Hopkinson pressure bar (SHPB) tests and Scanning Electron Microscope (SEM) analysis. It was found that the strength (i.e., the peak stress) of CCS specimens increased firstly and then decreased with the increase of powder content. The specimens reached the maximum peak stress when 3% powder content was included. The initial improvement of CCS strength was attributed to the pore-filling effect of coral powders, namely, the micro pores of the CCS specimens could be more effectively filled with higher percentages of coral powders being used in the experiments. However, excessive coral powders resulted in the reduction of specimen strength because these powders could easily be cemented into agglomerates by absorbing water from the specimens. These agglomerates could reduce the cementation strength between the coarse coral particles and the cement. Meanwhile, the peak stress of CCS specimens was found to be negatively correlated with the average strain rate and the ultimate strain. The degree of specimen fracture was found to be correlated with the amount of specific energy absorption during the tests. Furthermore, the “sulfate attack” caused by the inclusion of salinity of water had different influences on the CCS specimens with different coral powder contents. The ettringite and gypsum produced in “sulfate attack” could fill the pores and lead to cracking of the specimens, significantly affecting the specimen strength.     

Thermal effect on the physical properties of carbonate rocks

The effect of thermal damage on the physical properties of five carbonate rocks has been investigated. The tests were conducted on two marbles and three limestones, mainly composed of calcite but with different grain sizes, porosities, structural and textural characteristics. Cubic samples prepared from these rocks were gradually heated to a specific temperature level of 100, 200, 300, 400 and 500 °C, and gradually cooled down to room temperature without causing thermal shock in order to investigate the effect of heating temperature on physical properties such as microstructure, bulk density, effective porosity and P-wave velocity. Microscopic investigations from thin sections showed that damage in rocks at elevated temperatures was induced in different severity depending on grain size, porosity, structural and textural characteristics. Colour changes were also observed in porous limestones (Lymra and Travertine) due to organic material. In accordance with the degree of calcite dilation depending on heating temperature and in turn new microcrack occurrence, separation along intragrain and/or intergrain boundaries and widening of existing cracks, P-wave velocity decreased to various levels of the initial value, whereas porosity increased. Microscopic analyses and P-wave velocity measurements indicate that compaction of rock structure up to 150 °C occurred and induced calcite dilation had no significant damage effect on the rock material. Compaction of rock structure led to an increase in P-wave velocity and slight decrease in porosity. Most of the damage occurred within 24 h of heating time and further heating treatments brought relatively minor changes in physical properties. Damage intensity was well explained with P-wave velocity and effective porosity values depending on temperature increase.     

富浆胶凝砂砾料筑坝材料配合比试验研究及工程应用

通过对富浆胶凝砂砾料混凝土配合比设计进行试验研究,给出了推荐的配合比。该筑坝材料使工程节约了成本,最大限度的解决骨料紧张的问题,缩短了工期,为其他富浆胶凝砂砾料混凝土筑坝工程提供了设计参数。     

A survey of the physical properties of some carbonate rocks

Samples were taken from the Carboniferous Limestone, the Magnesian Limestone, the Inferior Oolitic Limestone and the Great Oolitic Limestone. Their specific gravities, dry and saturated densities were determined, as were their absolute and effective porosities. The densities decreased and porosities increased with decreasing age of the limestone concerned. The permeability of these samples increased as the porosity increased. Furthermore the strength decreased with increasing porosity, that is, the Carboniferous Limestone was very strong whereas the Great Oolitic Limestone was moderately weak. When saturated the strongest limestone showed the least percentage reduction in strength. The strongest limestone also possessed the highest values of hardness, the weakest having the lowest values. As far as Young's modulus was concerned it tended to increase as density, strength and hardness increased. The Carboniferous Limestone recorded the highest values of Young's modulus, the Great Oolitic Limestone the lowest. Poisson's ratio and related elastic properties were also determined.     

Determination of physical properties of carbonate rocks from P-wave velocity

Velocity, density, porosity, void ratio, water absorption by weight and P-wave velocity tests were conducted on 14 different carbonate rocks. Strong correlations between P-wave velocity and all the physical properties of the rock were found. The relations follow a linear function. The equations developed were compared with others in the literature. Although it appears that the physical properties of the tested carbonate rocks can be estimated from P-wave velocity, the validity of the derived equations must be checked for other carbonate rocks as the equations given are likely to be specific to the test method and vary with rock type, degree of saturation and presence of bedding planes.      

Unconfined compressive strength and ductility of fiber-reinforced cemented sand

A series of unconfined compression tests were performed on specimens of fiber-reinforced cemented sand (FRCS) to evaluate how fiber inclusion affects the measured strength and ductility characteristics of cemented sand. Lightly cemented sand with three different cement ratios (2, 4, and 6% by weight of soil) was mixed with four different fiber ratios (0, 0.3, 0.6, and 1% by weight of soil) and then compacted into a cylindrical specimen. Polyvinyl alcohol (PVA) fiber, which adheres well to cement, was randomly distributed throughout the cemented sand. The test results indicate that the inclusion of PVA fiber has a significant effect on both the unconfined compressive strength (UCS) and the axial strain at peak strength. The increase in the UCS was most apparent in the 2% cemented specimen wherein the UCS increased more than three times as the fiber ratio increased up to 1%. The ductile behavior of the FRCS is quantified by the deformability index, D, which is a ratio of the axial strain at peak strength of fiber-reinforced specimen to that of non-fiber-reinforced specimen. In the cases of 1% fiber ratio, the values of D were greater than four, regardless of cement ratios.     

Strength enhancement of geotextile-reinforced carbonate sand

The mechanical behavior of carbonate sand reinforced with horizontal layers of geotextile is invetigated using a series of drained compression triaxial tests on unreinforced and reinforced samples. The main factors affecting the mechanical behavior such as the number of geotextile layers, their arrangement in specimens, confining pressure, particle size distribution, geotextile type and relative density of samples were examined and discussed in this research. To make a precise comparison between the behavior of reinforced siliceous and carbonate sand, triaxial tests were performed on both types of sands. Results indicate that geotextile inclusion increases the peak strength and strain at failure, and significantly reduces the post-peak strength loss of carbonate specimens. The amount of strength enhancement rises as the number of geotextile layers increases while two other parameters including confining pressure and particle size affect adversely. The strength enhancement of reinforced carbonate sand is greater than the corresponding siliceous sample at high axial strains. Reinforced and unreinforced carbonate specimens exhibit more contractive behavior than their corresponding siliceous samples and tend to dilate at higher axial strains. By increasing the relative density of the samples, the peak strength of reinforced specimens rises due to enhanced interlocking between geotextile layers and sand particles. This process continues as long as the geotextile is not ruptured. The utilization of geotextiles with high mass per unit areas was found to be uneconomical due to slight differences between the strength augmentation of geotextiles with high and low mass per unit areas. It should be noted that geotextile layers limit the lateral expansion of specimens which leads to changing the failure pattern from a shear plane to bulging between the adjacent layers of geotextile.     

机制砂物理特性对水泥胶砂强度的影响研究

通过对筛选的19家甘肃机制砂生产厂家的生产情况进行调研,基于灰色关联分析理论,研究不同厂家机制砂的水泥胶砂强度,分析机制砂相关指标与水泥胶砂强度的内在规律,间接得到控制水泥胶砂强度的关键参数,对机制砂的生产和使用起到控制作用.结果表明,影响水泥胶砂抗压、抗折强度的主要因素是机制砂的石粉含量、亚甲蓝值和粉料质量指数.     

含不同胶结充填物的碳酸盐岩层面剪切力学特性试验研究

 层面是层状岩体稳定性的主控结构面,层面的剪切力学行为及抗剪强度对建造于层状岩体中的地下工程具有重要影响。由于成岩时期物化作用的差异,同一工程场址常发育不同地质特征(胶结充填物、粗糙度等)的层面。为了深入了解不同类型层面的剪切力学特性,利用乌东德水电站地下厂房区域取得的胶结层面试样开展常法向应力及峰后降法向应力直剪试验。试验结果表明,不同地质特征的层面表现出不同的剪切力学行为,按有无明显峰值可将剪切变形–剪应力曲线分为2类,无明显峰值层面的粗糙度和表面附着物均有别于有明显峰值层面。法向变形与法向应力直接相关,但与层面类别之间的关系不明显。不同类型层面还表现出不同的剪胀行为及相应的剪胀角差异。胶结层面抗剪强度低于完整岩石但高于已分离层面,对层状岩体起弱化作用。按层面地质特征对其进行分类有助于进一步理解层状岩体的复杂力学行为,而选择不同层面力学模型可使计算更贴近实际工程岩体特征。     

Experimental study on mitigating wind erosion of calcareous desert sand using spray method for microbially induced calcium carbonate precipitation

Wind erosion is one of the significant natural calamities worldwide, which degrades around one-third of global land. The eroded and suspended soil particles in the environment may cause health hazards, i.e.allergies and respiratory diseases, due to the presence of harmful contaminants, bacteria, and pollens.The present study evaluates the feasibility of microbially induced calcium carbonate precipitation(MICP)technique to mitigate wind-induced erosion of calcareous desert sand(Thar desert of Raj...     

Shear properties of cemented rockfills

Application of cemented rockfilling to underground mining could not be separated from the corresponding backfill's shear strength properties. The shear of cemented rockfill (CRF)-rock wall and the shear interaction occurring within CRFs both have some disadvantageous failure chances. In this study, we tried to investigate the complete shear properties of CRFs using direct shear and triaxial tests of cemented granite rockfill. Large-scale triaxial testing was held to accommodate the large CRF sample. Direct shear testing on the prepared flat and smooth surfaces was assessed with brief conversions and their corrections were used to approximate the shear strength envelopes of CRF joint interfaces. Two types of CRFs with the same aggregate size and distribution but different unconfined compressive strengths (UCSs) due to different mixture designs indicated insignificant differences between their basic friction angles, and also their asperity inclination angles. Nevertheless, investigation between direct shear test and triaxial test showed that the specimen with higher UCS tended to have a slightly lower friction angle but a higher cohesion than the other one.     

Constitutive model for monotonic and cyclic responses of loosely cemented sand formations

This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands. An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock. To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.     

Modelling tensile/compressive strength ratio of artificially cemented clean sand

The present work proposes a new theoretical model for predicting both the splitting tensile strength (q_t) and the compressive strength (q_u) of artificially cemented sand and assesses their ratio for a given material. The proposed model is based on the concept of the superposition of the failure strength contributions of the sand and cement phases. The sand matrix obeys the concept of critical state soil mechanics, while the strength of the cemented phase can be described using the Drucker-Prager failure criterion. The analytical solutions are compared against the results of tests on three different types of cemented clean sand cured for different time periods. While the analytical relation fits the experimental data well, it also provides a theoretical basis for the explanation of some features related to the experimentally derived strength relationships for cemented clean sand. The value of the power relationship between the strength and the porosity/cement ratio index seems to be governed by the soil matrix properties, while the interdependency of the strength and the curing time can also be captured. For a given cemented sand, the model equally confirms the existence of a unique tensile/compressive strength ratio (q_t/q_u), independent of the curing time and primarily governed by the compressive to tensile strength ratio (or the friction properties) of the cement. It is also confirmed that the q_t/q_u ratio changes within a narrow range for different frictional properties of the cementing phase.     

A constitutive model for evaluation of mechanical behavior of fiber-reinforced cemented sand

The aim of this study is to present a constitutive model for prediction of the mechanical behavior of fiber-reinforced cemented sand. For this purpose, a generalized plasticity constitutive model of sandy soil is selected and the parameters of the model are determined for three types of sandy soils using the results of triaxial tests. Next, the proposed model is developed using the existing models based on the physico-mechanical characteristics of fiber-reinforced cemented sand. The elastic parameters, flow rule and hardening law of the base model are modified for fiber-reinforced cemented sand. To verify the proposed model, the predicted results are compared with those of triaxial tests performed on fiber-reinforced cemented sand. Finally, the efficiency of the proposed model is studied at different confining pressures, and cement and fiber contents.