2006 Vol. 17, No. 4
The changes in the thermohaline circulation(THC) because of the increased CO 2 in the atmosphere play an important role in future climate regimes. In this article, a new climate model developed at the Max-Planck Institute for Meteorology is used to study the variation in THC strength, the changes of North Atlantic deep-water (NADW) formation, and the regional responses of the THCin the North Atlantic to increasing atmospheric CO 2 . From 2000 to2100, under increased CO 2 scenarios(B1,A1B, and A2), the strength of THC decreases by 4 Sv(106 m 3 /s), 5. 1 Sv, and 5. 2 Sv, respectively,equivalent to a reduction of 20%, 25%, and 25. 1% of the present THC strength. The analyses show that the oceanic deep convective activity significantly strengthens in the Greenland-Iceland-Norway(GIN) Seas owing to saltier (denser) upper oceans, whereas weakens in the Labrador Sea and in the south of the Denmark Strait region (SDSR) because of surface warming and freshening due to global warming. The saltiness of the GIN Seas is mainly caused by the increase of the saline North Atlantic in-flow through the Faro-Bank(FB) Channel. Under the scenario A1B, the deep-water formation rate in the North Atlantic decreases from 16. 2 Sv to12. 9 Sv with increasing CO 2 .
To study the variation pattern of aero conductivity of different porous media under low pressure conditions, three kinds of media are selected. These include sandy clay loam, fine sand, and medium sand, and air as fluid to conduct soil column ventilation tests. Pressure at both ends of the columns is measured under different ventilation flow ratesduring testing. The test results show that the aero conductivity, solved by Darcy’ s law, is not a constant. It is a variable, which increases first when air flow velocity is less than 0. 258 7 cm/s for sandy clay loam, 0. 637 3 cm/s for fine sand and then decreases when air flow velocity is bigger than that with the increase of the ventilation flow rate when the medium is determined. By analyzing various factors that influence the flow resistance, the reasons for variation in aeroconductivityare found asfollows:first, the change of pore structure results in better ventilation;second, the relationship between pressure head loss and air flow velocity is nonlinear, and it is beyond the condition of the laminar flow domain to which Darcy’ s law can be applied, when the air flow rate increases to a certain value and the flow velocity is in the transition range to turbulent flow.
To ensure the stability of a tunnel during construction, rock bolts are usually installed,which affects the stress distribution around the tunnel. Therefore, it is necessary to study the effects of rock bolting on the stress distribution around the tunnel. In this article, the effects of rock bolting on the stress distribution around the tunnel, including the position and orientation of bolts, the overburden depths, and the bolt lengths, are simulated using the ANSYS software with an elastoplastic model. The effect of multiple bolts of 2 m and 1 m lengths on the stress distribution in the roof and on the lateral sides of a tunnel and at different overburden depths is considered. An important finding is that the tensile stress region that is very dangerous for rock in the bottom of the tunnel growsrapidly with increasing overburden depths when rock bolts are installed only in the roof or on the lateral sides of a tunnel. The determination of the length of the rock bolt used around a tunnel is dependent on the loads and the integrity of the rock mass around the tunnel. In addition, rock bolting around the tunnel can obviously reduce the coefficients and the size of the region of stress concentration, especially when installed in high-stress areas. This fact is very important and essential for the design of tunnels and ensuresengineeringsafety in tunnel engineering.
Several cracks were found on the surface of the concrete foundations that support the steel towers of the Luohe(漯河)-Huaiyang (淮阳) high-voltage electricity transmission line that is 20 years old and situated in an inland salt lake environment in North China. To analyze the deterioration mechanism that led to this condition, field investigations were carried out and several tests were conducted on the soil and the affected concrete, including XRD (X-ray diffraction), electric probe analysis, and chemical analysis. The results show that the concentration of sulfates is very high (0. 39% wt. - 0. 67% wt.) in both the surrounding soil and the material around the coarse aggregates inside the concrete.Hence, sulfate attack could be one of the main causes of concrete deterioration. The percentage of sulfates in the surface layer of the concrete was higher than that in the inner layers of the concrete. The sulfates penetrate into the concrete and react with the hydrates of cement to form ettringite(AFt), which leads to increase in the volume and cracking of the concrete.
The responses of the pavement in service are the basis for the design of the semi-rigid base course asphalt pavement. Due to the dynamic characteristics of wheel loads and the temperature loads,the dynamic response analysis is very significant. In this article, the dynamic analysis of asphalt pavement under moving wheel loads is carried out using finite element method coupled with non-reflective boundary method. The influences of the base modulus, thickness, the vehicle velocity, the tire pressure,and the contact condition at the interface are studied usingparametric analysis. The results of numerical analysis show that it is not appropriate tosimply increase the base modulus or thickness in the design. It would be beneficial if the base design is optimized synthetically. The increase of damping is also beneficial to the pavements because of the surface deflection and the stresses declination. Furthermore, the good contact condition at the interface results in good performance because it combines every layer of the pavement to work together. As overload aggravates the working condition of the pavement, it is not allowed.
In order to study the method of numerical modeling for constitutive relations of clay, on the basis of the principle of interaction between plastic volumetric strain and plastic generalized shear strain, the two constitutive functionals that include the function of stress path were used as the basic framework of the constitutive model, which are able to demonstrate the dependence of stress path. The two partialdifferential cross terms appear in the expression of stress-strain increment relation, which are used to demonstrate the interaction between plastic volumetric strain and plastic generalized shear strain.The elasoplastic constitutive models of clay under twokinds of stresspaths, CTCand TC, have been constructed using the triaxial test results. The three basic characteristics of deformation of soils, pressure sensitivity, dilatancy, and dependence of stress path, are well explained using these two models. Using visualization, the three-dimensional surfaces of shear and volume strains in the whole stress field under stress paths of CTC and TC are given. In addition, the two families of shear and volumetric yield loci under CTC and TC paths are plotted respectively. By comparing the results of deformation under these two stress paths, it has been found that, there are obviousdifferences in the strain peaks, the shapes of strain surfaces, and the trends of variation of volumetric yield loci, however both families of shear yield loci are similar. These results demonstrate that the influences of stress path on the constitutive relations of clay are considerably large and not negligible. The numerical modelingmethod that can sufficiently reflect the dependence of stress path is superior to the traditional one.
The surface subsidence induced by mining is a complex problem, which is related with many complex and uncertain factors. Genetic programming(GP) has agood ability todeal with complex and nonlinear problems, therefore genetic programming approach is proposed to predict mining induced surface subsidence in this article. First genetic programming technique is introduced, second, surface subsidence genetic programming model is set up by selecting its main affective factors and training relatingto practical engineeringdata, and finally, predictionsare made by the testing of data, whose results show that the relative error is approximately less than 10%, which can meet the engineering needs, and therefore, this proposed approach is valid and applicable in predicting mining induced surface subsidence. The model offers a novel method to predict surface subsidence in mining.
The Caocun (曹村) loess profile of Zhangbianyuan (张汴塬) is located at the southeast margin of the Loess Plateau, which is a typical representative of loess stratum in the Sanmenxia(三门峡) area. In the 145 m thick profile, aloess-paleosol sequence occurs fromL 1 to L 33 , underlainby ared clay bed. The B/M boundary was recorded at the bottom of L 7 , the Jaramillo event at S 11 to S 12 , the Olduval event at L 23 toL 26 , and the M/G boundaryat the bottom of L 33 near the red clay. The magnetic susceptibility of the loess deposits, as well as the carbonate and granularity components, indicates that there have been many periodic paleoclimate changes, including a total of 13 cold events that each lasted more than 40 000 a. A high-resolution chronostratigraphic profile has been established using an age model according to grain characteristics. A clear correlation existsbetween records of the paleomonsoon in the Caocun loess succession and SPECMAP from 3×10 5 a B. P. to the present. The Caocun profile can act as a control site in the southeastern Loess Plateau. Its paleoenvironmental records relate to eastern Asian paleomonsoon theory.
The 2. 34 cm-wide garnet porphyroblast in the Paleoproterozoic felsic granulite from the Huangtuling area, North Dabieshan, has been reinvestigated for compositional variation in light of Cacomposition X-ray mapping to obtain peak P-T conditions of granulite-facies metamorphism. A new core-rim traverse was conducted through where there is little influence on Ca-profile and slight modification in Mn-, Mg-and Fe-profiles with the highest Mg/(Mg+Fe) value of 0. 467. Reasonable peak P-T conditions were estimated to be 1. 50- 1. 70 GPa and 1 100- 1 150 ℃ according to TWQ-based garnet-Al -orthopyroxene thermobarometry. These estimations suggest that the Huangtuling granulite once was subjected to ultrahigh-temperature (UHT) granulite-facies metamorphism following a high-pressure granulite-faices metamorphic stage, implying that a deep subduction and collision process relevant to the Yangtze block occurred in the Paleoproterozoic time, probably as aresponse tothe global assembly event of the Columbia supercontinent.
Apatite fission track (AFT) thermochronology of seven samples from the Xiaonanchuan (小南川) pluton in the Kunlun (昆仑) pass area was carried out, for the purpose of determining the timing of cooling and the relation between the exhumation and the morphotectonic processes. The AFT ages yield low denudation rates of 0. 020- 0. 035 mm/a during the late Miocene, which correspond to a stable geomorphic and weak tectonic uplifting environment. The low denudation rates can be considered as the approximate tectonic uplifting rates. The AFT geochronology shows paroxysmally rapid cooling since the Pliocene and an apparent material unroofing of more than 3 km in the Xiaonanchuan area. This was not the result of simple denudation. The rapid cooling was coupled with the intensive orogeny since the Pliocene, which was driven by tectonic uplifting. The accelerated relief buildingwasaccompanied by a series of faulting, which caused the basin and the valley formation and sinking. The space pattern of the AFT ages also shows differential uplifting, which decreases northwardly. This trend is supported by the regional AFT data, which indicate that the exhumation decreases northwardly in eastern Kunlun.This trend also exists in east -west orientation from the western Kunlun range to the eastern. The uplifting trend is also supported by geomorphic characteristics including the elevation and the relief differences as well as the distribution of the Late Cenozoic volcanism.
Toexamine the degree of contamination in the DongtingLake(洞庭湖) area, surface sediments samples were collected and investigated in thisstudy. This lake liesin the south of the middle Yan-gtze River, and it is the second largest fresh-water lake in China. These samples were analyzed for the concentrations of the following 6 elements(Cd, Cr, Cu, Ni, Pb and Zn) collected from 46 locations.The index of geo-accumulation was used to assess the degree of contamination of the total trace elements and the assessments showed that the contamination of Cd reached strong to very strong pollution. The speciation of metal in sediments was analyzed usingthe modified BCR(Community Bureau of Reference) sequential extraction technology and the water column-contamination risk was evaluated by the calculated contamination factors. The results of the speciation of metal indicated that more easily mobilized forms (exchangeable, water soluble, and acid soluble) were predominant for Cd, especially in the samples from East Dongting Lake. In contrast, the largest amount of lead was associated with the iron and manganese oxide fractions, and Cu, Zn, Cr, and Ni analyzed were mainlydistributed in residual phase at an average percentage higher than 60% of the total metals. The potential risk to the lake’ s water contami-nation was the highest in East Dongting Lake based on the calculated contamination factors. The assessment results using geo-accumulation index were compared with the information obtained by trace metal speciation and both results were consistent.
On the basis of the relationship between the carbonate content and the stratal velocity and density, an exercise has been attempted using an artificial neural network on high-resolution seismic data for inversion of carbonate content with limited well measurements as a control. The method was applied to the slope areaof the northern South ChinaSea near ODP Sites1146 and 1148, and the resultsare sat-isfactory. Before inversion calculation, a stepwise regression method was applied toobtain six properties related most closely to the carbonate content variations among the various properties on the seismic pro-files across or near the wells. These include the average frequency, the integrated absolute amplitude, the dominant frequency, the reflection time, the derivative instantaneous amplitude, and the instantaneous frequency. The results, with carbonate content errors of mostly ±5% relative to those measured from sediment samples, show a relatively accurate picture of carbonate distribution along the slope profile.This method pioneers a new quantitative model to acquire carbonate content variations directly from high-resolution seismic data. It will provide a new approach toward obtaining substitutive high-resolution sediment data for earth systemstudies related to basin evolution, especially in discussing the couplingbe-tween regional sedimentation and climate change.