Nitrogen Transformation in Waterlogged Soil in Indian Rice Fields: A Review

No other element can match the essentiality of nitrogen in soil for growth and development of plants and its transformations among various forms viz., nitrite, nitrate, ammonium are mostly mediated by microbes. As its transformation depends on major microbial activity and thus, there is a huge difference between the transformation of it in aerobic soil and anaerobic soil. This difference mainly arises due to presence and absence of oxygen. The absence of oxygen in soil creates anaerobic condition and thus promotes the growth of anaerobic microbes. In submerged soil, applied nitrogen is lost in various forms such as volatilization, denitrification, leaching and runoff out of which ammonium volatilization causes the maximum loss. The recovery of applied nitrogen, as recorded from various filed experiments in India, has been found to vary from 28 to 34% for submerged rice. The chemistry of nitrogen in submerged soil is quite interesting for this review.

A Probabilistic Approach to the Spatial Variability of Ground Properties in the Design of Urban Deep Excavation

Uncertainty in ground datasets often stems from spatial variability of soil parameters and changing groundwater regimes. In urban settings and where engineering ground interventions need to have minimum and well-anticipated ground movements, uncertainty in ground data leads to uncertain analysis, with substantial unwelcomed economical and safety implications. A probabilistic random set finite element modelling (RSFEM) approach is used to revisit the stability and serviceability of a 27 m deep submerged soil nailed excavation built into a cemented soil profile, using a variable water level and soil shear strength. Variation of a suite of index parameters, including mobilized working loads and moments in facing and soil inclusion elements, as well as stability and serviceability of facing and the integrated support system, are derived and contrasted with field monitoring data and deterministic FE modelling outputs. The validated model is then deployed to test the viability of using independent hydraulic actions as stochastic variables as an alternative to dependent hydraulic actions and soil shear strength. The achieved results suggest that utilizing cohesion as a stochastic variable alongside the water level predicts system uncertainty reasonably well for both actions and material response; substituting the hydraulic gradient produces a conservative probability range for the action response only.

HYDRAULIC HEAVE – DESIGN CHARTS AND DESIGN FORMULA FOR THE REQUIRED EMBEDDED LENGTH

For the determination of the required embedded length for the safety against hydraulic heave several approximate solutions exist. However, most of these solutions do not take into account the geometrical boundary conditions such as width B and length L of the excavation as well as the thickness of the aquifer S. Thus, values obtained by such simplified approximate solutions can easily lead to either uneconomical or unsafe design. For this reason investigations on the safety against hydraulic heave have been carried out at the Chair of Geotechnical Engineering at RWTH Aachen University. Based on the results of numerous calculations dimensionless design charts have been generated. With the help of these design charts the required embedded length T can be determined quite easily taking into account the difference of the ground water level H, the Thickness of the aquifer S, the geometrical dimensions B and L of the excavation and the unit weight of submerged soil γ′. In addition to these design charts a formula has been developed. By use of this design formula the required embedded length can directly be determined taking into account the before mentioned boundary conditions.