SOIL WATER CHARACTERISTIC CURVES FOR SOLID WASTE

Transcription

1 SOIL WATER CHARACTERISTIC CURVES FOR SOLID WASTE by Craig H. Benson and Xiaodong Wang Environmental Geotechnics Report Environmental Geotechnics Program Dept. of Civil and Environmental Engineering University of Wisconsin-Madison Madison, Wisconsin September 30, 1998

2 1. SCOPE The purpose of this study was to determine a soil water characteristic curve (SWCC) for a solid waste from California. Two points on each SWCC were of particular interest: (1) field capacity and (2) wilting point. Field capacity is the volumetric water content 1 (θ) at a matric suction (ψ) of 33 kpa 2. Wilting point is usually defined as θ corresponding to ψ = 1500 kpa (Hillel 1998). Samples of the solid waste were shipped to the University of Wisconsin-Madison in 120- L drums. A representative specimen was selected from the drums and tested following methods described in ASTM D Field capacity and wilting point were then determined from the SWCC. This report describes the composition of the solid waste, the experimental methods, and the SWCC. 2. COMPOSITION OF SOLID WASTE The solid waste in each drum was thoroughly mixed in the laboratory on receipt. After a representative portion of the waste was selected for testing, the remaining waste was sorted into the following categories: (1) plastic, (2) paper, (3) wood, (4) metals, (5) cloth, and (6) soillike material. The weight of these components was then determined. A summary of the weights is listed in Table 1. The soil-like material was similar to silty sand with occasional rock fragments. Representative specimens of the waste were collected and tested to determine their water content. Methods in ASTM D 2216 were used. The gravimetric water contents were 18.4% (Drum 1) and 6.7% (Drum 2). 1 Volumetric water content = volume of water/total volume. 2 1 psi = 6.9 kpa. 1

3 Table 1. Composition of Solid Waste. Drum 1 Drum 2 Component Mass (kg) Fraction (%) Mass (kg) Fraction (%) Plastic Paper Wood Metals Soil Cloth Total METHODS TO DETERMINE THE SWCC The SWCC was measured in a pressure plate extractor using methods described in ASTM D A schematic of the extractor is shown in Fig. 1 and a photograph is shown in Fig. 2. The principal components of the extractor are the (1) pressure vessel, (2) acrylic cylinder, (3) ceramic plate, (4) rubber bladder, and (5) outflow tube and graduated cylinder. The pressure plate operates on the principle of axis translation, which is based on the definition of matric suction, ψ. That is ψ = u a u w, where u a is the pore air pressure and u w is the pore water pressure. Different matric suctions are obtained by varying u a (via changes in air pressure in the pressure vessel) while maintaining u w at zero (Fredlund and Rahardjo 1993). The specimen of solid waste was first compacted in an acrylic cylinder (200 mm diameter by 100 mm tall) to a unit weight of 7 kn/m 3 (45 pcf). Prior to compaction, large sheets of plastic in the waste were shredded into pieces so that they could be included in the specimen. A sharp knife was used to cut the plastic sheets. The specimen was then saturated by clamping it between end plates and inducing flow using a hydraulic gradient of 5. 2

4 Pressure Readout Pressure Regulator Pressure Chamber Sample Ceramic Plate Rubber Bladder Bolt Outflow Tube Fig. 1. Schematic of Pressure Plate Extractor. Fig. 2. Photograph of Pressure Plate Extractor. 3

5 After saturation, the acrylic cylinder was placed on the saturated ceramic plate (air entry pressure = 1500 kpa). Prior to placing the specimen, the bladder beneath the plate and the outflow line were saturated. The lid and cylinder for the pressure vessel were then placed and the bolts clamping them were tightened. The effluent end of the outflow tube was raised to the center of the specimen resulting in an average pore water pressure of zero. A small air pressure was initially applied to the vessel to induce a matric suction. Outflow transmitted by application of the air pressure was then recorded. After the outflow stopped, indicating that the specimen had equilibrated with the applied matric suction, the air pressure was increased. This process was repeated to develop an entire SWCC. Water contents were calculated for each matric suction as follows: θ(ψ i ) = θ(ψ i -1 ) ( V w /V) (1) where θ(ψ i ) is the volumetric water content at the i th matric suction, θ(ψ i-1 ) is the volumetric water content corresponding to the previous matric suction (i.e., ψ i-1 ), V w is the water expelled by applying the air pressure to raise the matric suction from ψ i-1 to ψ i, and V is the total volume of the specimen. 4. RESULTS The SWCC for the solid waste is shown in Fig. 3. van Genuchten s function (van Genuchten 1980) was fit to the data using the program RETC (van Genuchten et al. 1991). The van Genuchten function is: θ θ θ s r θ r = 1+ 1 ( αψ) n m (2) 4

6 where θ s is the saturated water content, θ r is the residual water content, and α, m, and n are fitting parameters. The parameter m was constrained as 1-1/n following standard convention. The van Genuchten parameters, the wilting point, and the field capacity are summarized in Table Data van G. - Mualem Excel solver fit Suction Head (cm) θ r = 11% θ s = 53% α = 0.26 cm -1 n = Volumetric Water Content (%) Fig. 3. Soil Water Characteristic Curve for Solid Waste. 5

7 Table 2. van Genuchten Parameters, Wilting Point, and Field Capacity. Parameter Value α (1/cm water) 0.26 n 2.22 θ s (%) 53 θ r (%) 11 Wilting Point (%) 11 Field Capacity (%) REFERENCES Fredlund, D. and Rahardjo, H. (1993), Soil Mechanics for Unsaturated Soils, John Wiley and Sons, New York. Hillel, D. (1998), Environmental Soil Physics, Academic Press, New York. van Genuchten, M. (1980), A Closed-Form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils, Soil Science Society of America J., 44, van Genuchten, M., Leij, F., and Yates, S. (1991), The RETC Code Quantifying the Hydraulic Functions of Unsaturated Soils, Report No. EPA/600/2-91/065, US Environmental Protection Agency, Office of Research and Development, Washington, DC. 6