Electrochemical behavior of pyrite in acidic solution with different concentrations of NaCl

The role of chloride ions in metal sulfides leaching has been well established by researchers, the complexation of chloride ions was considered to be the main effect of chloride ions. In this work, the complexation of chloride ions was neglected by addition of EDTA, a strong complexing agent. In this way, further study was conducted on the effect of chloride ions. The increase of chloride ions concentration from 0 to 2 mol/L, decreased the open circuit potential of pyrite from 264 to 91 mV. Unless otherwise stated, all potentials in this paper are based on the potential of saturated calomel electrode. At 600 mV, the increase of chloride ions concentration acceleraed pyrite oxidation; at 800 and 1000 mV, however, the increase of chloride ions concentration lowered pyrite oxidation rate, and the proportion of elemental sulfur was also decreased with increasing chloride ions concentration; when chloride ions concentration increased from 0 to 2 mol/L, the proportion of elemental sulfur decreased from 32% to 28.4% at 800 mV, and 7.1% to 2.4% at 1000 inV.     

Geochemical characteristics of Holocene sediments from Chuadanga district,Bangladesh: Implications for weathering,climate, redox conditions,provenance and tectonic setting

The present research deals with the geochemical characteristics of the Holocene sediments from Alamdanga area, Chuadanga district, Bangladesh. Main goals of the study are to delineate source rock characteristics, degree of chemical weathering and sorting processes and behavior of redox conditions during deposition of the sediments. Geochemical characteristics of the sediments show comparatively a wide variation in accordance with stratigraphy in their major element contents （e.g. SiO2 69.46-82.13, A1203 2.28-8.88 in wt%）, reflecting the distinctive provenance and in part an unstable period in terms of tectonic activity. Geochemical classification of the sediments shows mostly sub-arkose with few sub-litharenites. Some major and trace elements display comprehensible correlation with A1203 confirming their possible hydraulic fraetionation. The chemical index of alteration （CIA＊）, W＊ index, index of compositional variability （ICV）, plagioclase index of alteration （PIA＊） values and the ratio of SIO2/Al2O3, suggest low degrees of chemical weathering in the source areas as well as immature to moderately mature the sediments. The sediments suggest semi-arid climatic trends within oxic deltaic depositional conditions during the Holocene, at 3-12 ka. Whole rock geochemistry and discrimination diagrams demonstrate the continental signature derivatives, which might have been derived from the felsic to intermediate igneous rocks （granitic plutonic rocks） as well as from quartzose sedimentary/metamorphic provenance. These typical sources are present in a vast region of the Himalayan belt and catchment areas of Ganges. The tectonic setting of the sediments demarcates typically passive margin with slightly continental arc system.     

Occurrence modes of silver in the Ni-Mo-PGE polymetallic layer of the Lower Cambrian black shales in Zunyi,Guizhou Province,South China

The Ni-Mo-PGE polymetallic mineralization of the Lower Cambrian black shales locate in Zunyi, South China and contain abundant noble metals such as Ag, Au and PGE, and especially Ag with average concentration of 64×10-6. The occurrence modes of Ag have been investigated using methods of selective chemical dissolution and transmission electron microscopy. The results demonstrate that the occurrence modes of Ag are complex and diversiform. It might be associated with clay minerals, organic matter, sulfides and also occurred as native silver and sulfides with nanometer in size. Combined with results of previous studies, we suggest that the sulfides, clay minerals and organic matter which hosted in the Ni-Mo-PGE polymetallic ores of black shales can play the roles of important reduction and adsorption geochemical barriers for the enrichment and distribution of silver. This study further implies that the selective chemical dissolution and transmission electron microscopy may pave the way to study the occurrence modes of other noble metals in black shales.     

Quality assessment and hydrogeochemical characteristics of groundwater in Agastheeswaram taluk,Kanyakumari district,Tamil Nadu,India

The present study investigates the hydrogeochemical characteristics of groundwater quality in Agas- theeswaram taluk of Kanyakumari district, Tamil Nadu, India. A total of 69 groundwater samples were collected during pre- and post-monsoon periods of 2011-2012. The groundwater quality assessment has been carried out by evaluating the physicochemical parameters such as pH, EC, TDS, HCO3, Cl, SO42-, Ca2＋, Mg2＋, Na＋ and K＋ for both the seasons. Based on these parameters, groundwater has been assessed in favor of its suitability for drinking and irrigation purpose. Dominant cations for both the seasons are in the order of Na＋〉 Ca2＋〉 Mg2＋ 〉 K＋ while the dominant anions for post monsoon and pre monsoon have the trends of CI 〉 HCO3 〉 SO42- and HCO3- 〉 CI 〉 SO42-, respectively. Analytical results observed from various indices reveal that the groundwater quality is fairly good in some places. Analytical results of few samples show that they are severely polluted and incidentally found to be near the coasts, estuaries and salt pans in the study area. The Gibbs plot indicates that the majority of groundwater samples fall in rock dominant region, which indicates rock water interaction in the study area. The United States salinity （USSL） diagram shows that the groundwater is free from sodium hazards but the salinity hazard varies from low to very high throughout the study area. This reveals that the groundwater is moderately suitable for agricultural activities. The observed chemical variations in pre-monsoon and post-monsoon seasons may be the effect to rock-water interactions, ion-exchange reactions, and runoff of fertilizers from the surrounding agricultural lands.     

LA-ICP-MS U-Pb zircon geochronology and Hf isotope,geochemistry and kinetics of the Daxigou anorthosite from Kuruqtagh block,NW China

Kuruqtagh block is the best area for Precambrian geology in Xinjiang Autonomous Region, NW China, since it exposed complete Precambrian lithology units. The study of this ancient base will deepen the understanding of the Precambrian evolution of the Tarim Basin. In this paper, we studied the petrology, geochemistry, zircon LA-ICPMS U-Pb chronology and zircon Hf isotope of Daxigou anorthosite （DA） which is located at the northem margin of Tarim craton and discussed the rock formation, tectonic and geological significance. Zircons from the intrusions display oscillatory zoning and high Th/U ratios （0.39-1.35）, implying their magmatic origin. Zircon LA-ICP-MS U-Pb dating results indicate that they formed during the Paleoproterozoic age with the weighted 2~6pb/238U average age of 1818~9 Ma, which is significantly different from former＇s Neoproterozoic age, and is co- incidentally identical with its associated syenite granite age within the error range. Studies on petrogeochemistry suggest that DA belongs to medium-sodium peraluminous alkaline type, rich in Pb, La, Th and LILE, and poor in HFSE （Gd, Nd, and Ta）. The chondrite-normalized REE pattern is slightly to the right form. The average Y~REE is 317.2~ 10-6; HREE show moderate fractionation [average LREE/HREE is 14.71, average （La/Yb）N is 24.77; average （LaJSm）N is 3.85, and average （Gd/Yb）y is 3.46]; and the 6Eu and 8Ce are not obvious. Their initial Hf isotope ratios and Hf two-stage model ages range from -6.6 to -4.43 and 2.63 to 2.74 Ga, respectively. Taken together, it is sug- gested that Daxigou anorthosite is a typical volcanic anorthosite and its primary magma could be contaminated by the partial melt Neoarchaean crust and mainly formed in the arc environment, which recoded the tectonic-magma activities response of the Tarim refers to the amalgamation of the supercontinent Columbia.     

Geochemistry of mafic rocks and melt inclusions and their implications for the heat source of the 14.0°S hydrothermal field,South Mid-Atlantic Ridge

Fresh rocks sampled from the 14.0°S hydrothermal field of the South Atlantic Ridge can be divided into two categories： olivine-gabbro and basalt. The olivine-gabbro is composed mainly of three types of minerals： olivine, clinopyroxene and plagioclase, while a multitude of melt inclusions occur in the plagioclase phenocrysts of the basalts. We analyzed the whole-rock, major and trace elements contents of the basaks, the mineral chemistry of phenocrysts and melt inclusions in the basalts, and the mineral chemistry of olivine-clinopyroxene-plagioclase in the olivine-gabbro, then simulated magma evolution within the crust using the COMAGMAT program. The whole-rock geochemistry shows that all the basalts exhibit typical N-MORB characteristics. In addition, the mineral chemistry characteristics of the olivine-gabbro （low-Fo olivine, low-Mg# clinopyroxene, high-TiO2 clinopyroxene, low-An plagioclase）, show that strong magma differentiation occurred within the crust. Nevertheless, significant discrepancies between those minerals and phenocrysts in the basalts （high-Fo olivine, high-An plagioclase） reflect the heterogeneity of magma differentiation. High Mg# （-~0.72） melt inclusions isobaric partial crystallization simulations suggest that the magma differentiation occurred at the depth shallower than 13.03 km below the seafloor, and both the vertical differentiation column shows distinct discrepancies from that of a steady-state magma chamber. Instead, a series of independent magma intrusions probably occurred within the crust, and their corresponding crystallized bodies, as the primary high-temperature thermal anomalies within the off-axis crust, probably act as the heat source for the development of the 14.0°S hydrothermal system.     

Geological Relationships Between the Intrusions,Country Rocks,and Ni-Cu-PGE Sulfides of the Kharaelakh Intrusion,Noril′sk Region:Implications for the Roles of Sulfide Differentiation and Metasomatism in Their Genesis

The Ni-Cu-platinum group element sulfide ore deposits of the Kharaelakh Intrusion,Noril′sk Region,Siberia,represent a large concentration of sulfides associated with a small differentiated intrusion formed at the edge of the Siberian Craton in the roots of the Siberian Trap flood basalt.The deposit is associated with an intrusion that occupies a flanking periclinal structure adjacent to the Noril′sk-Kharaelakh Fault.The intrusion is strongly differentiated and comprises taxitic gabbrodolerites,picritic gabbrodolerites,and gabbrodolerites within the main body which in turn forms a chonolith within a sheet-like intrusion that extends laterally to form extensive undifferentiated sills of gabbrodolerite.The intrusion substantially replaces the stratigraphy of the country rocks,and although it appears to have exploited the axis of structures developed in response to transtension,the intrusion has created space by both mechanical dilation of stratigraphy and magmatic replacement of pre-existing sedimentary rocks.The frontal lobes of the main intrusion have complex apophyses of gabrodolerite on a range of scales that demonstrate replacement of the sedimentary rocks and link to the development of an extensive metamorphic halo in the country rocks.This halo is much narrower over the main body of the intrusion,and these observations have implications for the thermal history of the intrusion.Mg-skarns and breccias are developed in the roof of the main body of the intrusion.Within the intrusion,the taxitic rocks contain vesicles and the blebby sulfides developed in the picritic and taxitic gabbrodolerites appear to have a linkage to volatile phases.Cuprous sulfide mineralization developed at the roof of the Kharaelakh Intrusion is associated with metamorphosed and skarn-bearing country rocks,and appears to have been generated by a combination of sulfide fractionation and associated metasomatism.The geological relationships appear consistent with a chonolith model for the development of the differentiated intrusion and mineralization,but the extent of metasmorphism of the country rocks appears to be related to the unusual thickness of gabbrodolerite apophyses at the flanks of the intrusion rather than metamorphism produced by the passage of mafic magma through the intrusion.Variations in disseminated sulfide compositions and metasomatic textures in the skarns are described,and a model is proposed which balances traditional views on the evolution of the magma conduits with the impact of magmatic fluids transported through the magma column(i.e.transmagmatic fluids).The importance of structures in controlling the nature of the conduit,and the resultant small intrusions with excess sulfide is a feature of many other Ni-Cu sulfide deposits including Voisey′s Bay,and it is suggested that the sulfides are more likely to have beentransported from depth into their final resting place rather than developed by in-situ equilibration of sulfide with fresh magma in the chonolith.     

Health risk assessment for typical and abandoned chromium-contaminated sites

Chromium pollution were investigated by sampling the abandoned chromium contaminated sites in Chongqing. Its health risk was assessed following the procedures made by USEPA. The results showed that the most seriously polluted land was found at the former production area （G4） with average Cr（VI） concentration of 3369.2 mg kg-1 on the surface soil. The assessment indicated that there was high risk of non-carcinogens for children. The assessed risk of the first layer of office area （G1） for children, chromium slag transition （G2）, drainage pipeline chromium slag transition （G3） and production area （G4） for both children and adults were not acceptable （〉1.00× 106）. It was strongly suggested that the accessible measures of remediation should be taken for a portion of contaminated sites before the reuse of abandoned lands.     

Fluid inclusion,siliceous rock geochemistry of Shewushan lateritic gold deposit,Hubei Province,eastern China: Implication for the genesis of primary orebody

The Shewushan gold deposit is located 16 km southwest of Jiayu County, Hubei Province, eastern China, which is the largest lateritic gold deposit in Asia, consisting of a series of mineralized faults containing gold grades of 1.0–19.5 g/t set within a larger, lower-grade(0.2–1.0 g/t) zone. According to the fluid inclusions study, the homogenization temperature ranges from 70–350℃, and concentrates between 140–220℃. The laser Rama results show that the CO2 and CH4 exist in mineralized fluid. In addition, the major and trace element and REE geochemical data show that the genesis of the siliceous rocks is hydrothermal genesis, formed by mineralized fluid bearing SiO2 transmitted along faults to the surface, and replace the carbonate rocks to begin deposition. Primary gold mineralization is attributed to hydrothermal activity that followed the main period of tectonic deformation of the Indosinian orogeny, which caused the regional detachment regime in southeast Hubei. The reversed fold and the fault system formed the fluid migration channel in the Shewushan area. EPMA results show that Au exists in arsenopyrite(850×10-6–1550×10-6 Au) and pyrite(470×10-6–1340×10-6 Au). Therefore, based on the above results and combined with the field observation, we suggest that the genesis of primary orebody in Shewushan deposit is carlin type, while not weathered residual type.     

Zircon U-Pb age and Hf isotopic characteristics of the Huangtuliang monzonitic granite,North Hebei Province,China

The Huangtuliang monzonitic granite outcrops on the northern side of the Huangtuliang gold mining district, Chicheng, North Hebei Province. Our predecessors only made isotopic age determination using the K-Ar method. Through LA-MC-ICP-MS zircon U-Pb dating and zircon Hf isotopic composition determination, this study acquired the age of 244.8±2.0 Ma(MSWD=0.57) on the basis of the weighed mean 206Pb/238U ratio, indicating that the Huangtuliang monzonitic granite was formed during the Middle Triassic period, which is the product of Early Indosinian magmatic activities in the region of North Hebei. εHf(t) values vary relatively evenly, within the range of-10.65--14.03, with an average of-12.14. The two-stage evolution model ages, tDM2, vary between 1943 and 2144 Ma, implying that the rock-forming materials of the Huangtiliang monzonitic granite mainly came from the Paleoproterozoic recirculated crustal materials, though a small quantity of enriched-mantle materials would have been involved.     

Facies analysis and depositional environments of the Oligocene–Miocene Asmari Formation, Zagros Basin, Iran

The Asmari Formation(a giant hydrocarbon reservoir)is a thick carbonate sequence of the Oligocenee Miocene in the Zagros Basin,southwest of Iran.This formation is exposed at Tang-e-Lendeh in the Fars interior zone with a thickness of 190 m comprising medium and thick to massive bedded carbonates.The age of the Asmari Formation in the study area is the late Oligocene(Chattian)eearly Miocene(Burdigalian).Ten microfacies are defned,characterizing a gradual shallowing upward trend;the related environments are as follows:open marine(MF 8e10),restricted lagoon(MF 6e7),shoal(MF 3e5),lagoon(MF 2),and tidal fat(MF 1).Based on the environmental interpretations,a homoclinal ramp consisting of inner and middle parts prevails.MF 3e7 are characterized by the occurrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp,infuenced by wave and tidal processes.MF 8e10,with large particles of coral and algae,represent a deeper fair weather wave base of a middle ramp setting.     

Late Tremadoc and Early Floian Graptolites from Central Hunan, China

<正>Fourteen graptolite species of the Late Tremadoc and Early Floian are described from the Nanba section in the Yiyang area,Hunan Province.Among them,Paradelograptus onubensis, Kiaerograptus bulmani,K.kutchini and Paratemnograptus isolatus of the Late Tremadoc are first found in China.Based on the current graptolite materials,the correspondingly complete graptolite zones of Upper Tremadocian in China are recognized as follows(in ascending order):the Adelograptus tenellus Zone,the Aorograptus victoriae Zone,the Araneograptus murrayi Zone and the Hunnegraptus copiosus Zone.In the Nanba section,the Hunnegraptus copiosus Zone is overlain directly by the Tetragraptus approximatus Zone,there is no hiatus below the base of Floian.     

LA-ICP-MS Zircon U-Pb Geochronology and Petrology of the Muchang Alkali Granite, Zhenkang County, Western Yunnan Province, China

<正>The Muchang composite intrusion is located about 14 km southeast of the Fengwei town and south of the Baoshan-Zhenkang block.The rift-related intrusion consists of the early-stage riebeckite nordmarkite in the east and west sides and the discontinuous marginal zone,the late-stage main body of dominant riebeckite granite,and minor aegirine granite.Studies on petrological geochemistry and LA-ICP-MS zircon U-Pb dating of the late-stage riebeckite granite,origin and evolution of the Muchang alkali granite and the relationship between the granite and the associated skarn-type Luziyuan Pb-Zn deposit are discussed in this paper.The results show that the Muchang alkali granites belong to A-type granites,which are characterized by enrichment in Al_2O_3,SiO_2,total alkali and Fe, depletion in MgO and CaO contents with high FeO_T/MgO ratios.The REE concentrations are relatively high,exhibiting highly fractionated LREE patterns with significantly Eu negative anomaly. The Muchang granites are obviously enriched in lithophile elements(e.g.,Rb,K,U and Th) and high field strength elements(e.g.,Zr,Hf,Nb,Y and Ga) with high 10000×Ga/Al ratios and depleted in Sr, Ba,Ti,Cr and Ni,which are similar to those of the A-type granites and quite different from those of S-type andⅠ-type granites.The LA-ICP-MS zircon dating results of the Muchang granite gave a weighted mean age of 266.2±5.4 Ma(2σ),suggesting that they were formed in the stage of extension at the end of post-collision at Middle Permian and the consumption of Paleo-Tethys ocean took place before 266 Ma. It is suggested that the unexposed intermediate-acid intrusive rocks in the Luziyuan ore district,which is the "sister" rocks material of the Muchang granites and related closely to Luziyuan Pb-Zn mineralization,were the product of Middle Permian.     

Weiwoboidae fam.nov.of 'Higher' Fulgoroidea (Hemiptera:Fulgoromorpha) from the Eocene Deposits of Yunnan,China

<正>A new family of so-called‘higher' planthoppers,Weiwoboidae fam.nov.,from the Lower Eocene of Yunnan is described.A new monotypic genus,Weiwoboa gen.nov.,with Weiwoboa meridiana sp.nov.is also described and illustrated.The characters of tegmen venation of the new family,its evolution,and supposed relationships are briefly discussed.     

Mineralogy and radioactivity of pegmatites from South Wadi Khuda area,Eastern Desert,Egypt

Stohl F.V.and Smith (1981) The Crystal Chemistry of the Uranyl Silicate Minerals[M].Department of Geosciences,the Pennsylvania State University,University Park.Pennsylvania.

Stuckless J.S.,Nkomo I.T.,Wenmer D.B.,and Van Trump G.(1984) Geochemistry and uranium favourability of the postorogenic granites of the north-eastern Arabian Shield,Kingdom of Saudi Arabia[J].Bull Fac.Earth Sci.King Abdel Aziz Univ.6,195-209.

William S.B.,Hanson S.L.,and Falster A.U.(2006) Samarskite-Yb:A new species of the samarskite group from the Little Patsy pegmatites,Jefferson County,Colorado[J].Canadian Mineralogist.44,1119-1125.

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新疆哈密金窝子矿田逆冲推覆构造及其控矿作用

金窝子矿田控矿构造复杂,成矿条件独特,金矿化类型多样。经研究,金窝子矿田的形成与中天山地块向北山裂谷带逆冲推覆作用有关。逆冲推覆造就了金窝子矿田北、中、南3个不同性质的断裂带及不同的金矿化物理、化学环境,并分别控制了金矿田与其相关的金矿化类型及其成矿作用和金矿成矿系列。     

“Giant pressure shadow” structure and ore-finding method of tectonic stress field in the Tongchang Cu-Au polymetallic orefield,Shaanxi, China: II. Dynamics of tectonic ore-forming processes and prognosis of concealed ores

John Ridley (1993) The relations between mean rock stress and fluid flow in the crust:With reference to vein-and lode-style gold deposit[J].Ore Deposit Reviews.8,23-37.

Kapper P.and Zeeh S.(2000) Relationship between fluid flow and faulting in the Alpine realm (Austria,Germany,Italy)[J].Sedimentary Geology.131,147-162.

Li Siguang (1976) Research Method of Geomechanics[M].Science Press,Beijing (in Chinese).

Liu Zhaochang,Li Fanyou,Zhong Kanghui,and Li Wei (1996) Tectonic Evolution and Metallogenesis on the Western Margin of the Yangtze Platform[M].pp.17-67.University of Electronics Science Press,Chengdu (in Chinese with English abstract).

Lu Guxian and Li Xiaobo (1996) Study on Tectonophysics and Tectonochemistry in Metallogeny[J].Journal of Geomechanics.2,38-40 (in Chinese).

Marc Polliand and Robert Moritz (1999) Basement-hosted quartz-barite sulfide veins in the French Alps:a record of Alpine tectonic fluid expulsion in external crystalline massifs-structural,fluid inclusion and isotopes (S and Sr) evidences[J].Economic Geology.94,37-56.

Minissale G.,Magro G.,Martinelli O.Vaselli,and Tassi G.F.(2000) fluid geochemical transect in the northern Apennines (central-northern Italy):Fluid genesis and migration and tectonic implications[J].Tectonophysics.319,199-222.

Shen Shumin,Zheng Fangfang,and Liu Wenying (1994) Three Dimensional Migration Potential Fields of the Tectonic Stress Driving and the Fluid Within Rocks[M].pp.37-42.Press of Earthquake,Beijing (in Chinese).

The institute of Chinese Irrigation and Electricity (1991) Notebook of Rock Mechanics[M].pp.25-29.Publishing House of Irrigation Works and Electric Power,Beijing (in Chinese).

Wang Dongbo (1991) Geochemistry of the South Qinling Au-Ag Ore Belt[M].pp.1-60.Central South University of Industry Press,Changsha (in Chinese with English abstract).

Wang Feng (1998) Concise Experimental Mechanics[M].Science Press,Beijing (in Chinese).

Wang Jiayin (1976) Discussion of Stress Minerals[M].Geological Publishing House,Beijing (in Chinese).

Wang Xiaofeng (1994) Flow in Rock and the Concentration and Dispersition of Elements[M].pp.119-131.Seismological Press,Beijing (in Chinese).

Yang Kaiqing (1986) Research subjects and orientation on the theory of tectono-genesis and tectono-metallogenesis[J] Bullentin of the Institute of Geomechanics.Geological Publishing House,Beijing.(7),1-10 (in Chinese with English abstract).

Yu Chongwen,Chen Kuang,Bao Zhengyu,and Chen Yaoting (1998) Dynamics of Hydrothermal Ore-forming Process[M].pp.1-6.Press of China University of Geoscience,Wuhan (in Chinese with English abstract).

Zhai Yusheng (1996) Questions of tectonics and fluid and ore-forming processes[J].Earth Science Frontes.23,230-236 (in Chinese with English abstract).

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Assessment of the industrial utilization of scoria materials in central Harrat Rahat, Saudi Arabia

Although basaltic volcanic scoria deposits are widespread in and around the scoria cones present in the Harrats, very few occurrences have been evaluated and still fewer have been exploited. The scoria from a quarry in central Harrat Rahat was investigated and assessed for its industrial utilization. Cubes were prepared from several concrete mixes using scoria as lightweight coarse and fine aggregates in different percentages. The compressive strength values of the cubes were found to be acceptable and satisfy the ASTM (1995) requirement for structural concrete. The scoria was also assessed for its utilization as a cement additive. Pozzolanic activity was tested according to the Italian standards and found to be acceptable. The strength activity index with Portland cement and the effectiveness of scoria admixture in controlling alkali-silica reactions were tested according to ASTM (1995) standards. Mortar cubes were specially prepared for these studies using different mixes and different storage procedures. The results satisfied the ASTM (1995) requirements as cement additive. The utilization of scoria as a heat-insulating material was tested and the results were found to satisfy the ASTM (1995) requirements. This fact suggests it could be utilized in the manufacture of the building blocks. It is recommended to evaluate the other scoria deposits, exploit the economically feasible ones and utilize them for different industrial applications. The manufacturing of heat-insulating concrete or building blocks using scoria is of prime importance as an energy saver.     

Flow rule effects in the Tresca model

The Tresca failure criterion is used regularly in geotechnical engineering to compute the failure loads of clay soils deforming under undrained conditions. When this criterion is used together with the finite element method a plastic flow rule must also be incorporated in the elasto-plastic soil model.

The effects of the flow rule on the performance of a non-linear analysis using an elastic perfectly plastic soil model obeying the Tresca failure criteria are discussed in this note. Application of this model in a three-dimensional analysis causes computational difficulties, due to the gradient discontinuities that exist at the corner of the Tresca yield surface. Such discontinuities can be removed from the yield (or failure) surface using different methods. Two of the most widely used methods in removing singularities from the yield surface and their overall performances in a three-dimensional finite element analysis are discussed.

The motivation for this study comes from a concern raised by Randolph and Puzrin [Randolph MF and Puzrin AM Upper bound limit analysis of circular foundations on clay under general loading. Geotechnique, (2003);53(9):785–796, [5]] about reported instances of under predictions of the collapse loads by finite element analysis [Taiebat HA and Carter JP Numerical studies of the bearing capacity of shallow foundations on cohesive soil subjected to combined loading. Geotechnique, (2000);50(4):409–418, [7]] and [Taiebat HA and Carter JP Bearing capacity of strip and circular foundations on undrained clay subjected to eccentric loads. Geotechnique, (2002);52(1):61–64, [8]], [Gourvenec S and Randolph M Effect of strength non-homogeneity on the shape of failure envelopes for combined loading of strip and circular foundations on clay, Geotechnique, (2003);53(6):575–586, [4]], when it is usually expected that finite element results should overestimate the true collapse loads. The intent of this study is to demonstrate and reiterate that although the finite element method is an extremely powerful analytical tool for solution of engineering problems, it is nevertheless subjected to approximation errors due to simplifications that are necessarily made to prevent other numerical difficulties.