Relevance of membrane properties of clay-rich geologic materials to water-related engineering issues

A number of laboratory studies have shown that clays exhibit membrane properties. However, the vast majority of these studies have been done using high compaction pressures, high hydraulic heads, and pure clays. Such high compaction pressures and high heads are not representative of shallow subsurface conditions encountered in engineering applications. Therefore, the goal of this research was to obtain experimental data more representative of situations encountered in the shallow subsurface --Abstract, page iv

Simple Gravity-Fed Piston Pump

Constant head pumps have a number of uses in the laboratory, and are often used to provide hydraulic heads for permeability measurements of lithologic materials. Since our lab suffers periodic power outages that interfere with long-term experiments, we designed and built a gravity-driven piston pump that requires no electricity. The pump is composed of a piston inserted into a vertical cylinder. The piston travels under the influence of iron weights added to the attached piston rod. Our pump used schedule 40 PVC pipe for the cylinder and acrylic for the piston. In eight months of constant use, there have been no pump failures. This pump operates over a pressure range of 20 kPa to 210 kPa. A small head loss occurs when using this pump due to falling water level in the cylinder. The calculated 5.97 kPa loss is not significant for our purposes and is less than the 13.8 kPa observed pressure variance due to asperities in the cylinder. Experiments in our machine shop indicate that honing the cylinder will significantly reduce the pressure variations. Design criteria and cost estimates for machining parts for this type of pump are included. This pump may be useful for long-term experiments at relatively low fluid pressures

Membrane Effects during Filtering Investigation of Membrane Effects during Filtering of Natural Surface Waters in Missouri

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 μm 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

Reverse Osmosis Properties of Bentonite/Glass Bead Mixtures at Low Compaction Pressures

It is well-known that clays and shales can be membrane-functioning. When a hydraulic head difference exists across a membrane-functioning barrier, a portion of the solute is rejected from solution passing through the membrane. Many engineering structures such as landfill liners, mixed soil augered barriers, and retention pond liners consist of soil-clay mixes, which may have clay contents as low as 12%. Some shallow geologic environments may also contain similar clay/soil mixes. No previous testing has been performed to investigate hydraulic-head-induced solute rejection in such mixed soils. Therefore, we performed six experiments using five 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 determine what effect clay content had upon the membrane properties. All the mixtures were compacted to 690 kPa and the compacted soil samples were 0.56-1.40 mm thick. An approximately 35 ppm Cl- solution and a hydraulic head of approximately 90 kPa were used for all experiments. Experimental data shows that these simulated clay-sand mixtures do exhibit measurable membrane properties with as low as 12% clay by weight under these conditions. Calculated reflection coefficient values ranged from a low of 0.07 for 12% Na-bentonite to a high of 0.26 for 100% clay. Solute rejection ranged from 12.6% for 12% clay to a high of more than 40% for the 50% and 100% clay samples. The glass beads represent fine-grained sand and were shown to have no membrane properties

Hyperfiltration of NaCl Solutions Using a Simulated Clay/Sand Mixture at Low Compaction Pressures

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

Structural Properties of Recycled Plastic/Sawdust Lumber Decking Planks

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. In this research effort, standard 2×6 plastic lumber planks were tested for many different structural properties. The plastic lumber tested was a blend of recycled plastic and sawdust. The tests were conducted at −23.3°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 high temperature modulus of the plastic lumber was 5.79, 1.03, and 1.12 GPa in compression, flexure and tension respectively. High temperature strength values were 16.8, 12.0, and 1.45 MPa in compression, flexure and tension respectively. The high temperature shear strength of the plastic lumber was 5.31 MPa. Strength tests were also performed for nail and screw connections typically used with lumber, and the pull-out and lateral load were comparable to wooden lumber. The plastic lumber performed well under sustained load tests at high temperature. Slip resistance tests were performed, and it was found that the plastic lumber is more slippery than wooden lumber, but probably does not represent a safety hazard. The conclusion was that the plastic lumber is a good structural material, but it is not appropriate to simply substitute plastic lumber for wooden lumber pieces of the same dimension in structural applications. Plastic lumber structures must be designed using the structural properties of the plastic lumber

Structural Properties of Recycled HDPE Plastic Lumber Decking Planks

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 6 tongue-in-grove plastic lumber planks were tested for many different structural properties. The tests were conducted at -23.3°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