Hydrological responses to early-peak rainfall in unsaturated rooted soils.

Vegetation has been commonly used in sponge city to remediate problems related to rainstorm events. Unlike uniform rainfall which has been widely studied, effects of early-peak rainfall on hydrological responses in vegetated soils are unclear. Besides, there is a lack of quantitative method of accurately measuring wetting front (WF). This study aims to propose a new WF tracing method, and explore the hydrological responses to early-peak rainfall in unsaturated soils vegetated with dwarf mondo grass. During soil column tests, WF position, matric suction, volumetric water content, surface ponding and overflow drainage were measured. The new WF tracing method works reasonably well for all cases. As compared to uniform rainfalls, early-peak rainfalls caused (1) earlier onsets of ponding (by 20 minutes for vegetation case and by 5 minutes for bare soil) and overflow (by 52 minutes for vegetation case and by 37 minutes for bare soil), (2) greater overflow velocity (by 28% for vegetation case and by 41% for bare soil), and (3) slightly more total overflow amount. Vegetation delayed the ponding/overflow generations, and decreased total overflow drainage, due to enhanced infiltration of surface soil. At 5 cm depth, high-density mixture of fine and coarse roots caused an increase in the saturated water content (θs) and a reduction in the residual water content (θr), because of root-induced changes in soil structure. At 10 cm depth, low-density fine roots caused reductions in both θs and θr, and increased air-entry value, as roots occupy the pores.

Mesoscopic and macroscopic investigation of a dolomitic marble subjected to thermal damage.

Thermal loading is an important factor that could lead to the weakening and deterioration of rock materials. Understanding the thermal properties of rocks and their evolution under different high temperatures is important in the post-fire-hazard evaluation and cultural heritage conservation. Yet it is challenging to understand the evolution of thermally-induced changes in rock properties and to quantitatively study degrees of thermal damage when samples are limited. This study investigates the effects of high temperatures (i.e., 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C) on a dolomitic marble using combined mesoscopic and macroscopic testing techniques. The test results show that increasing marble temperature led to a deterioration of physical properties (i.e., increasing open porosity and weight loss; but decreasing P-wave velocity) and mechanical properties (i.e., increasing axial strain corresponding with the peak stress; but decreasing uniaxial compressive strength, Young's modulus, and brittleness). There existed a threshold temperature of 600 °C, which marks different thermal damage mechanisms. Below the threshold, the rock deterioration was mainly caused by physical changes such as crack propagation and grain breakage, which can be characterized by mesoscopic parameters (i.e., linear crack density and mineral grain size distribution). On the contrary, when the temperature was higher than the threshold, the deterioration was caused by chemical changes, including mineral decomposition and re-crystallization, which was indicated by the changes in mineral compositions and relative atomic mass calculation. Based on the experimental results (e.g., mineralogical and physico-mechanical changes) and obtained relationships between the parameters in mesoscale and macroscale, a novel scheme for thermal damage evaluation is proposed to estimate thermally-induced changes in macroscopic parameters (e.g., Young's modulus) based on the corresponding mesoscopic parameters (e.g., particle size distribution and linear crack density).

Seepage characteristics of three-layered landfill cover system constituting fly-ash under extreme ponding condition.

A multi-layered final cover system is constructed over the landfill after it reaches its full capacity to minimize water ingress into the underlying hazardous waste. Three layered landfill cover are designed for areas experiencing very humid climatic conditions. Under the effects of climate change, the occurrences of extreme rainfall events become more frequent and this has resulted in catastrophic floods and hence extreme ponding. This study investigates the seepage characteristics of three-layered capillary barrier cover systems under an extreme ponding condition of 1.5 m water head, through detailed laboratory column tests and finite-element seepage analysis. Four 1.2 m-tall columns having different configurations (C1-C4) were studied. Fly ash (FA) was used to amend the surface and barrier layers in columns C2 and C4, in line with the novel concept of "waste protect waste". Spatiotemporal variations of volumetric water content of the four columns were monitored for three years continuously. With FA amendment in the surface layer and an inclusion of a 0.01 m thick geosynthetic clay liner between the drainage and barrier layers, the onset of basal percolation was significantly delayed until 700 days of ponding, compared to 115 days without FA amendment. Capillary flow dominated the gravitational flow and perched water table was formed as waterfront advanced from the drainage to barrier layers. Further seepage analysis considering a realistic humid climate boundary condition showed that all four configurations were successful in preventing basal percolation for 800 days.

Mechanisms of hydrogen sulfide removal by ground granulated blast furnace slag amended soil.

Ground granulated blast furnace slag (GGBS) amended soil has been found able to remove gaseous hydrogen sulfide (H2S). However, how H2S is removed by GGBS amended soil and why GGBS amended soil can be regenerated to remove H2S are not fully understood. In this study, laboratory column tests together with chemical analysis were conducted to investigate and reveal the mechanisms of H2S removal process in GGBS amended soil. Sulfur products formed on the surface of soil particle and in pore water were quantified. The test results reveal that the reaction between H2S and GGBS amended soil was a combined process of oxidation and acid-base reaction. The principal mechanism to remove H2S in GGBS amended soil was through the formation of acid volatile sulfide (AVS), elemental sulfur and thiosulfate. Soil pH value decreased gradually during regeneration and reuse cycles. It is found that the AVS plays a significant role in H2S removal during regeneration and reuse cycles. Adding GGBS increased the production of AVS and at the same time suppressed the formation of elemental sulfur. This mechanism is found to be more prominent when the soil water content is higher, leading to increased removal capacity.