Learning soil parameters and updating geotechnical reliability estimates under spatial variability – theory and application to shallow foundations

ABSTRACT

Field data is commonly used to determine soil parameters for geotechnical analysis. Bayesian analysis allows combining field data with other information on soil parameters in a consistent manner. We show that the spatial variability of the soil properties and the associated measurements can be captured through two different modelling approaches. In the first approach, a single random variable (RV) represents the soil property within the area of interest, while the second approach models the spatial variability explicitly with a random field (RF). We apply the Bayesian concept exemplarily to the reliability assessment of a shallow foundation in a silty soil with spatially variable data. We show that the simpler RV approach is applicable in cases where the measurements do not influence the correlation structure of the soil property at the vicinity of the foundation. In other cases, it is expected to underestimate the reliability, and a RF model is required to obtain accurate results.     

The use of mycorrhiza for eco‐engineering measures in steep alpine environments: effects on soil aggregate formation and fine‐root development

Steep erosion‐prone and vegetation‐free slopes are widespread in alpine areas and are often discussed since they have a high socio‐economic damage potential. We present an eco‐engineering approach to test whether a mycorrhizal inoculum improves the establishment of hedge brush layers and in turn soil structural stability on a steep, coarse‐grained vegetation‐free slope in the eastern Swiss Alps. We established (i) mycorrhizal and (ii) non‐mycorrhizal treated eco‐engineered research plots on a field experimental scale, covering a total area of approximately 1000 m² on an east‐northeast (ENE) exposed slope, where many environmental parameters can be regarded as homogeneous. After a full vegetation period, we quantified soil aggregate stability, the formation of water stable aggregates and the fine‐root development. Our results illustrate that the establishment of brush layers without mycorrhizal inoculum increased aggregate stability significantly. Against our expectation and glasshouse experiments, the addition of mycorrhizal inoculum did not have a statistically significant effect after one vegetation period although it tended to increase aggregate stability. Analogously, root length density (RLD) tended to be higher at the non‐mycorrhizal treated site. Aggregate stability was significantly correlated with RLD. Studies on a bigger field experimental scale are inevitable, complement glasshouse studies and lead to a better understanding for a successful application of sustainable eco‐engineering measures in alpine environments. Based on our results and considering the fact that the response time in natural ecosystems may be slower than in laboratory approaches, we conclude that long‐term field studies are necessary to validate results gained through laboratory experiments. Copyright © 2014 John Wiley & Sons, Ltd.     

Storm observations by remote sensing and influences of gustiness on ocean waves and on generation of rogue waves

The impact of the gustiness on surface waves under storm conditions is investigated with focus on the appearance of wave groups with extreme high amplitude and wavelength in the North Sea. During many storms characterized by extremely high individual waves measured near the German coast, especially in cold air outbreaks, the moving atmospheric open cells are observed by optical and radar satellites. According to measurements, the footprint of the cell produces a local increase in the wind field at sea surface, moving as a consistent system with a propagation speed near to swell wave-traveling speed. The optical and microwave satellite data are used to connect mesoscale atmospheric turbulences and the extreme waves measured. The parameters of open cells observed are used for numerical spectral wave modeling. The North Sea with horizontal resolution of 2.5?km and with focus on the German Bight was simulated. The wind field “storm in storm,” including moving organized mesoscale eddies with increased wind speed, was generated. To take into account the rapid moving gust structure, the input wind field was updated each 5?min. The test cases idealized with one, two, and four open individual cells and, respectively, with groups of open cells, with and without preexisting sea state, as well the real storm conditions, are simulated. The model results confirm that an individual-moving open cell can cause the local significant wave height increase in order of meters within the cell area and especially in a narrow area of 1–2?km at the footprint center of a cell (the cell's diameter is 40–90?km). In a case of a traveling individual open cell with 15?m·s^?1 over a sea surface with a preexisting wind sea of and swell, a local significant wave height increase of 3.5?m is produced. A group of cells for a real storm condition produces a local increase of significant wave height of more than 6?m during a short time window of 10–20?min (cell passing). The sea surface simulation from modeled wave spectra points out the appearance of wave groups including extreme individual waves with a period of about 25?s and a wavelength of more than 350?m under the cell's footprint. This corresponds well with measurement of a rogue wave group with length of about 400?m and a period of near 25?s. This has been registered at FiNO-1 research platform in the North Sea during Britta storm on November 1, 2006 at 04:00 UTC. The results can explain the appearance of rogue waves in the German Bight and can be used for ship safety and coastal protection. Presently, the considered mesoscale gustiness cannot be incorporated in present operational wave forecasting systems, since it needs an update of the wind field at spatial and temporal scales, which is still not available for such applications. However, the scenario simulations for cell structures with appropriate travel speed, observed by optical and radar satellites, can be done and applied for warning messages.     

碳酸盐风化碳汇与森林碳汇的对比——碳汇研究思路和方法变革的必要性

目前,全球碳循环研究主要集中在海洋碳汇以及陆地土壤和植被碳汇,而对岩石风化碳汇仅考虑地质长时间尺度的硅酸盐风化作用,而认为碳酸盐风化在长时间尺度上对碳汇无贡献。然而,碳酸盐相对于硅酸盐有快得多的溶解速度,且对全球变化(特别是气候和CO2变化)的响应迅速,同时由于生物作用和人为活动的影响,使得碳酸盐风化碳汇的能力需要重新评价。最新的研究发现,由碳酸盐溶解、全球水循环及水生生物光合利用溶解无机碳共同作用,即水-岩-气-生相互作用形成的大气碳汇,远远大于之前只估计了河流输运的无机碳汇,其量级与森林碳汇量相当,因此有必要对传统的碳汇研究思路和方法进行某些变革,这有可能为解决所谓的全球“碳失汇”问题找到一条出路。      

Meteorological causes of Harmattan dust in West Africa

We investigated the temporal dynamics of dust entrainment in the Bodélé Depression, Central Sahara, to better understand the intra-annual variability of aerosol emission in the world's largest dust source. The linkages between dust entrainment and large-scale meteorological factors were examined by correlating several meteorological variables in the Mediterranean and Africa north of the equator with the aerosol concentrations in the Bodélé Depression separately for winter and summer. The methodological tools applied are NCEP/NCAR reanalysis data and the aerosol index of the Total Ozone Mapping Spectrometer (TOMS-AI), available for 15 years from 1978 to 1993. We found that dust mobilisation during the Harmattan season is highly dependent on air pressure variability in the Mediterranean area. High pressure to the north of the Bodélé intensifies the NE trade winds, leading to an increased entrainment of dust in the Bodélé Depression. In summer, dust mobilization cannot be explained by the large scale meteorological conditions. This highlights the importance of local to regional wind systems linked to the northernmost position of the intertropical convection zone (ITCZ) during this time.     

Preparing a seismic hazard model for Switzerland: the view from PEGASOS Expert Group 3 (EG1c)

The seismic hazard model used in the PEGASOS project for assessing earth-quake hazard at four NPP sites was a composite of four sub-models, each produced by a team of three experts. In this paper, one of these models is described in detail by the authors. A criticism sometimes levelled at probabilistic seismic hazard studies is that the process by which seismic source zones are arrived at is obscure, subjective and inconsistent. Here, we attempt to recount the stages by which the model evolved, and the decisions made along the way. In particular, a macro-to-micro approach was used, in which three main stages can be described. The first was the characterisation of the overall kinematic model, the “big picture” of regional seismogenesis. Secondly, this was refined to a more detailed seismotectonic model. Lastly, this was used as the basis of individual sources, for which parameters can be assessed. Some basic questions had also to be answered about aspects of the approach to modelling to be used: for instance, is spatial smoothing an appropriate tool to apply? Should individual fault sources be modelled in an intraplate environment? Also, the extent to which alternative modelling decisions should be expressed in a logic tree structure has to be considered.     

A model for multiphase flow and transport in porous media including a phenomenological approach to account for deformation—a model concept and its validation within a code intercomparison study

Multiphase flow processes in unsaturated cohesive soils are often affected by deformation due to swelling and shrinking as a result of varying water contents. This paper presents a model concept which is denoted ‘phenomenological’ in terms of the processes responsible for soil deformation, since the effects of deformation on flow and transport are only considered by constitutive relations that allow an adaptation of the hydraulic properties. This new model is validated in a detailed intercomparison study with two state-of-the-art models that are capable of explicitly describing the processes relevant for the deformation. A ‘numerical experiment’ with a state-of-the-art reference model is used to produce ‘measurement data’ for an inverse-modelling-based estimation of the model input parameters for the phenomenological concept.     

Estimation of the temporal autocorrelation structure by the collocation technique with an emphasis on soil moisture studies

Abstract

The collocation technique has become a popular tool in oceanography and hydrology for estimating the error variances of different data sources such as in situ sensors, models and remote sensing products. It is also possible to determine calibration constants, for example to account for an off-set between the data sources. So far, the temporal autocorrelation structure of the errors has not been studied, although it is known that it has detrimental effects on the results of the collocation technique, in particular when calibration constants are also determined. This paper shows how the (triple) collocation estimators can be adapted to retrieve the autocovariance functions; the statistical properties as well as the structural deficencies are described. The coupling between the autocorrelation of the error and the estimation of calibration constants is studied in detail, due to its importance for analysing temporal changes. In soil moisture applications, such time variations can be induced, for example, by seasonal changes in the vegetation cover, which affect both models and remote sensing products. The limitations of the proposed technique associated with these considerations are analysed using remote sensing and in situ soil moisture data. The variability of the inter-sensor calibration and the autocovariance are shown to be closely related to temporal patterns of the data.

Editor D. Koutsoyiannis

Citation Zwieback, S., Dorigo, W., and Wagner, W., 2013. Estimation of the temporal autocorrelation structure by the collocation technique with an emphasis on soil moisture studies. Hydrological Sciences Journal, 58 (8), 1729–1747.     

In situ N and C labelling of indigenous and plantation tree species in a tropical mountain forest (Munessa, Ethiopia) for subsequent litter and soil organic matter turnover studies

We propose a novel pragmatic approach of in situ ¹⁵N and ¹³C isotope labelling of trees for subsequent litter decomposition and turnover studies under field conditions. Using this method the labelling of even large trees under natural conditions is possible and compared to tree labelling under artificial conditions in greenhouses the in situ approach is less expensive. ¹³C and ¹⁵N labelling were carried out simultaneously via photosynthesis by tree gassing with ¹³CO₂ and by stem injection of ¹⁵NH₄¹⁵NO₃. The aims of this study were: (i) to produce a sufficient quantity of labelled plant material for subsequent field incubation studies and (ii) to investigate the effectiveness and distribution of in situ ¹⁵N (¹⁵NH₄¹⁵NO₃) and ¹³C (¹³CO₂) labelling of Podocarpus falcatus, Croton macrostachys, Prunus africana and Cupressus lusitanica. The following targets need to be achieved: (i) Assuming almost natural litter fall conditions, enough labelled plant material must be produced in situ for the turnover experiment; (ii) intra-plant tracer enrichment shall be homogeneous; (iii) tracer enrichment should be comparable for different tree species; and (iv) tracer enrichment must be sufficient for subsequent litter turnover studies using the stable isotope approach. Our results clearly demonstrated that several kilograms of labelled plant material can be produced in situ. For many ecosystems, this amount is sufficient for a long term litter turnover experiment on a field scale under almost natural litter fall conditions. However, intra-plant label uptake of ¹³C and ¹⁵N was heterogeneous so that only leaves (litter) should be used for the turnover study. It could be shown that only a part of the labile C and N fraction in the leaves was labelled. Nevertheless, label uptake was sufficient for subsequent litter turnover studies.     

Geochemically induced shifts in catabolic energy yields explain past ecological changes of diffuse vents in the East Pacific Rise 9°50'N area

The East Pacific Rise (EPR) at 9°50'N hosts a hydrothermal vent field (Bio9) where the change in fluid chemistry is believed to have caused the demise of a tubeworm colony. We test this hypothesis and expand on it by providing a thermodynamic perspective in calculating free energies for a range of catabolic reactions from published compositional data. The energy calculations show that there was excess H₂S in the fluids and that oxygen was the limiting reactant from 1991 to 1997. Energy levels are generally high, although they declined in that time span. In 1997, sulfide availability decreased substantially and H₂S was the limiting reactant. Energy availability dropped by a factor of 10 to 20 from what it had been between 1991 and 1995. The perishing of the tubeworm colonies began in 1995 and coincided with the timing of energy decrease for sulfide oxidizers. In the same time interval, energy availability for iron oxidizers increased by a factor of 6 to 8, and, in 1997, there was 25 times more energy per transferred electron in iron oxidation than in sulfide oxidation. This change coincides with a massive spread of red staining (putative colonization by Fe-oxidizing bacteria) between 1995 and 1997. For a different cluster of vents from the EPR 9°50'N area (Tube Worm Pillar), thermodynamic modeling is used to examine changes in subseafloor catabolic metabolism between 1992 and 2000. These reactions are deduced from deviations in diffuse fluid compositions from conservative behavior of redox-sensitive species. We show that hydrogen is significantly reduced relative to values expected from conservative mixing. While H₂ concentrations of the hydrothermal endmember fluids were constant between 1992 and 1995, the affinities for hydrogenotrophic reactions in the diffuse fluids decreased by a factor of 15 and then remained constant between 1995 and 2000. Previously, these fluids have been shown to support subseafloor methanogenesis. Our calculation results corroborate these findings and indicate that the 1992-1995 period was one of active growth of hydrogenotrophic communities, while the system was more or less at steady state between 1995 and 2000.