Calculation of head difference at two sides of a cut-off barrier during excavation dewatering

This paper proposed a series of simple equations to calculate the head difference at the two sides of waterproof curtain. The barrier effect of waterproof curtain is considered from two situations with respect to without barrier case: (i) groundwater head difference below the barrier and (ii) groundwater head difference by convergence into the opening. The solution for the first situation can be derived from hydraulic analyses and the second situation can be obtained using a numerical analysis. The final groundwater head difference is the sum of these two situation according to the superposition principal. In the proposed equations, the head difference is expressed as a function of the inserted depth of the barrier into confined aquifer, the ratio of the hydraulic conductivity of the aquifer, the thickness of the aquifer, and hydraulic gradient under normal conditions. Finally, the proposed equation was applied to a field case to verify the validity of the proposed approach. Compared with the field data, the results show that the proposed method is reasonable.     

Parametric study of site instability due to presence of a weakening shale layer above sloping bedrock

Several areas of St. Louis County, Missouri have been classified as unstable for any type of construction because of the presence of highly plasticity soil/shale on sloping bedrock. These areas have the potential of becoming unstable due to change in any natural condition that can change its moisture content and/or its steepness. One of the structures constructed in this area showed significant movements several years after its construction. The primary goal of this investigation was to analyze site slope configurations in these problematic areas. A parametric study of two site slope profiles, natural and modified, was performed to investigate the effects of various variables such as slope configuration and shale properties, on the stability of a site where potential of site instability exists due to the presence of a shale/clay layer above sloping bedrock. From the results of the parametric study, design charts were developed that incorporate all variables studied in this investigation. These charts will help engineers make informed decisions and help them recognize conditions, which may cause the site slope to become unstable. This revised version was published online in July 2006 with corrections to the Cover Date.     

Water quality deterioration at a karst spring (Gallusquelle,Germany) due to combined sewer overflow: evidence of bacterial and micro-pollutant contamination

The concurrent use of karst aquifers as drinking water resources and receptors of combined sewer overflow lacking appropriate pre-treatment may cause conflicts between drinking water supply and storm water management. A storm water tank (SWT) for combined wastewater is identified as the source of sporadic contamination of a karst spring (Gallusquelle, “Schwäbische Alb”, SW Germany) used for public water supply. Spring water quality was examined by routine and event sampling and by evaluating physicochemical and microbiological parameters. The total number of microbial colonies growing at 20°C and the number of Escherichia coli colonies rose to values up to four orders of magnitude higher than background, 2–5 days after overflow of the SWT. High concentrations of chloride, sodium, and total organic carbon (TOC) and high values of turbidity coincide with this increase. However, high bacterial contamination is also observed while turbidity and TOC are low. Several wastewater-related organic micro-pollutants such as chlorinated and non-chlorinated organophosphates were detected in the SWT and, depending on their K_ow values and their biodegradability, in lower concentrations at the spring.