Unsaturated Stratum Research Articles

Analysis of pipeline leakage in unsaturated stratum: A new seepage-diffusion model

Accurate prediction of the spread of fluids leaking from underground pipelines is crucial for risk assessment. In this study, we developed a method for predicting the diffusion range of pipeline seepage fluids within an unsaturated stratum. First, we derived a seepage–diffusion equation based on the generalized Darcy’s law and subsequently analyzed the mechanism of fluid diffusion in the unsaturated stratum. We then constructed a model for the seepage–diffusion of a pipeline leakage fluid, incorporating variables such as the saturation, permeability coefficient, diffusion pressure, and diffusion distance of soil microelements across various time intervals, using a stepwise algorithm in tandem with the Green–Ampt model and VG–Mualen permeability coefficient function. We investigated the influence of fluid self-gravity, saturated permeability coefficient, initial saturation, and intra-pipe pressure on the diffusion distance of the pipeline leakage fluid in an unsaturated stratum, focusing on specific cases. The results indicate that an increase in the saturated permeability coefficient, initial saturation, and intra-pipe pressure leads to an increase in the fluid diffusion distance. A simulation test was conducted to validate the proposed seepage–diffusion model. The findings of this study can be employed to predict the diffusion range of pipeline leakage fluids in various formation types, providing a vital foundation for pipeline leakage accident management.

A simple numerical simulation method for unsaturated stratum under the water environmental load

A simple numerical simulation method is proposed to investigate the mechanical response of the unsaturated stratum under the water environmental load. The proposed numerical simulation method applies the water environmental load to the unsaturated stratum as the load condition, and adopts the developed non-orthogonal elastoplastic constitutive model for unsaturated soil to describe the mechanical behaviors at the material scale. The mechanical response of unsaturated stratum is obtained by solving the discretized governing equation of the mechanical equilibrium. The proposed numerical simulation method is implemented based on the commercial finite element software ABAQUS. The effectiveness of the proposed method is verified through the element tests on the Botkin silt, the element tests on the Pearl clay, and a model test of the rising water level on a sand column. The numerical simulations of the rising and decreasing groundwater level in the unsaturated stratum are conducted. During the rising groundwater level, the non-collapsible stratum produces rebound deformation, the collapsible stratum below the groundwater level occurs rebound deformation, and the collapsible stratum above the groundwater level mainly produces settlement deformation. During the decreasing groundwater level, both the collapsible stratum and the non-collapsible stratum produce the settlement deformation.

Study on the Influence of Back-fill Grouting Parameters in Unsaturated Strata on the Seismic Performance of Shield Tunnels

In this study, based on unsaturated soil theory, the diffusion behavior of shield tunnel back-fill grouting in unsaturated strata was considered, and Van Genuchten equation was introduced for deriving calculation formula for grouting diffusion radius. Then, an analysis flow for the influence of back-fill grouting parameters on seismic performance of shield tunnel was proposed, and the analysis was carried out with an actual project example. The results show that, compared to the saturated state, owing to the existence of matrix suction in the unsaturated soil, the permeability of the stratum is reduced, resulting in a slower increase in the grouting diffusion radius and smaller ultimate grouting distance. In this project, after using asphalt-cement grouting material, increasing the grouting time and grouting pressure, as well as reducing the elastic modulus of the material can decrease the seismic response of the tunnel. When the grouting time t = 60 min, grouting pressure P0 = 0.5 MPa, elastic modulus E = 49.1 MPa, the seismic-reduction rate of the structure can reach 25.65%. Finally, the fitted equations for the grouting time, pressure, and elastic modulus of the grouting material with the relative displacement and seismic-reduction rate of the tunnel were obtained.

Analysis of One-Dimensional Consolidation for Unsaturated Soils under Piecewise Cyclic Loading

This paper studies the one-dimensional (1D) consolidation behavior for unsaturated stratum subjected to piecewise cyclic loading. Combined with the widely accepted consolidation theory of unsaturated soils, a semianalytical method was employed to investigate the consolidation of unsaturated foundation considering piecewise cyclic loading in the Laplace domain. Furthermore, the reduced solutions were produced to perform the verification work accompanied by the results in the existing literature. Finally, a case study was conducted to explore the consolidation characteristics under piecewise cyclic loading (i.e., triangular and trapezoidal cyclic loadings). Parametric studies were carried out by variations of excess pore pressures and settlement against the ratio of air-water permeability coefficients, depth, and loading parameters. The research proposed in this paper can provide theoretical basis for the ground treatment of unsaturated soils, especially for rationally accelerating consolidation or avoiding sudden settlement.

Consolidation of Unsaturated Drainage Well Foundation with Smear Effect under Time-Dependent Loading

In real practice, the construction of drainage wells is always accompanied by smear effect, whereas it is scarcely discussed in the field of unsaturated soils. In this case, on the basis of the widely-accepted consolidation theory for unsaturated stratum by Fredlund, this paper studies the equal strain consolidation characteristics of unsaturated foundation with drain wells affected by smear effect under time-varying loading (exponential loading as a case). Firstly, with the consideration of corresponding boundary conditions, the governing equations under the equal strain hypothesis are obtained. Afterwards, decoupling process, constant variation method and Fourier series expansion theory are utilized to get the analytical solutions, which are then verified to be credible by means of the finite difference method. Finally, the consolidation patterns of the drainage well foundation modeling in unsaturated soils are studied against hydraulic coefficients, modeling sizes and loading parameter. It reveals that the smear effect can significantly restrain the consolidation process of the foundation by drain wells, while different parameters will lead to various results.

Water services with independent providers in peri-urban Maputo: Challenges and opportunities for long-term development

Water service delivery to most residents of peri-urban areas of greater Maputo depends largely on alternative service providers, mostly in the form of small-scale independent providers (SSIPs). This paper discusses the present and long-term challenges facing SSIPs in supplying quality water of sufficient quantity in peri-urban Maputo and possible human health risks associated with the consumption of water provided by SSIPs. Extensive water sampling and analyses were conducted to evaluate the physicochemical and bacteriological quality of water provided by independent providers and the associated human health risks. Borehole pumping tests, the results of which were interpreted using the graphical method of Jacob, were used to evaluate the regional aquifer potential, the long-term impacts of its exploitation and the aquifer vulnerability to external contamination. From the results of borehole pumping tests it was concluded that the present yields are in average 33% lower than estimated safe yields and that larger than present yields therefore can be exploited. The aquifer vulnerability to external contamination (e.g. by E. coli and nitrates) is low, mainly because of low hydraulic loads and the existence of a rather thick (10 to 30 m) sandy unsaturated stratum where bacteria die-off and biological denitrification probably occurs. However, the aquifer vulnerability to sea sea-water intrusion is high. Currently, the health risks posed to consumers relying on services provided by SSIPs are small; even so, 13 out of 35 controlled boreholes had either total coliform or faecal coliform levels higher than the WHO standard. In the long run SSIPs may face more serious water quality problems due either to over-exploitation of the aquifer system or increased hydraulic loads resulting from increased population density.

Factors Involved in Segregation of Oil and Gas from Subterranean Water

The process of oil and gas accumulation is mainly a segregation of these substances from the water circulating in the earth's crust. The segregation is caused by intermolecular forces and differences of pore size. The hypothesis that synclinal oil has accumulated in dry or unsaturated strata is rejected. Tardiness of edge-water encroachment and irregularities in the shape of oil and gas accumulations deserve attention.