It is widely recognized that clays and shales can demonstrate membrane properties. When a hydraulic head differential exists across a membrane-functioning clay-rich barrier, some of the solute is rejected by the membrane. This process is known as hyperfiltration. Some shallow geologic environments, including aquitards bounding shallow perched aquifers and unconfined aquifers, some river and stream beds, and some lake bottoms contain clay-soil mixes. Many engineering structures such as landfill liners, mixed soil augered barriers, and retention pond liners also consist of soil-clay mixes. No previous testing has been performed to investigate the likelihood that hyperfiltration may occur in such mixed soils. Therefore, we performed five experiments using different mixes of Na-bentonite and glass beads (100, 50, 25, 12 and 0% clay) to determine if any of these mixes exhibited membrane properties and to investigate what effect clay content had upon the membrane properties of the soil. Each mixture was compacted to 345 kPa and the sample mixtures were 0.58-0.97 mm thick. All the experiments used an approximately 35 ppm Cl ) solution under an average 103 kPa hydraulic head. Experimental results show that all the simulated clay-sand mixtures do exhibit measurable membrane properties under these conditions. Values of the calculated reflection coefficient ranged from a low of 0.03 for 12% bentonite to 0.19 for 100% bentonite. Solute rejection ranged from 5.2% for 12% clay to a high of over 30% for the 100% clay. The 100% glass bead sample exhibited no membrane properties.
Previous work has demonstrated that suspended clay accumulating on filter paper can act as a membrane and affect chemical concentrations in the filtered water. For this reason, we looked at the possibility of membrane effects altering water chemistry during filtering for Missouri Rivers. Membrane effects during filtering could cause an initial decrease in sample concentrations as the filter cake began acting as a membrane, with a corresponding increase of concentration as the concentration polarization layer was formed behind the filter cake. Samples from five Missouri rivers were tested: the Mississippi River at St. Louis, the Missouri River at Kansas City, the Gasconade River at Jerome, the Osage River at the junction of Highway 63 and 50, and the Meramec River one mile downstream from springs. Three 1-l samples were filtered from each river using a 0.45 lm filter. An unfiltered sample from each river underwent dialysis to determine the actual ion concentrations of the overall sample. None of the filtered samples demonstrated a statistically significant alteration of water chemistry using current filtering techniques in this preliminary study, suggesting that membrane effects due to accumulation of clay particles on filter paper may not be a common problem in Missouri and similar regions.
Plastic lumber is being used to replace wooden lumber in some construction applications, especially in outdoor applications where the plastic lumber is presumed to weather better than the wood. However, the structural properties of the plastic lumber are not well understood, and the use of plastic lumber in structural applications is not authorized in the common building codes. Contractors who use plastic lumber in structural applications such as outdoor decks are in most cases violating the building codes. In this research effort, standard 1 Â 6 tongue-in-grove plastic lumber planks were tested for many different structural properties. The tests were conducted at 723.3 C to simulate winter conditions, and at 40.6 C to simulate summer conditions. In all cases the high temperature strength and stiffness was lower than at low temperature, so the high temperature values would determine the allowable strength and stiffness for design. The conclusion was that the plastic lumber is a good structural material, but that it is not appropriate to simply substitute plastic lumber for wooden lumber pieces of the same size in structural applications. The plastic lumber is not as strong and stiff as the wooden lumber, and so larger sizes must be used to obtain the same strength and stiffness. Because of the much lower modulus, compression members made from plastic lumber may need to be of much larger size to resist buckling.
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