A Novel Technique for Distributed Measuring of Strain in the Vadose Zone

Characterizing water content and pressure changes in the vadose zone is important to understanding a variety of geologic processes, ranging from storage and drainage, to evapotranspiration, and recharge. Changes in water content, or pressure, cause strain in the solid porous medium of the vadose zone, and it is possible that measuring those strains could be used as a characterization tool. The Coherence-length-gated Microwave Photonics Interferometry (CMPI) technique measures strain at high resolution along many intervals defined by pairs of reflectors distributed along an optical fiber. This technique has recently been developed at Clemson University, and it appears to have the spatial and temporal resolution to characterize strains that are expected to occur with hydrologic changes in the vadose zone. However, the technique has never been used to measure strain in porous media, so its capabilities remain uncertain. The objective of this thesis is to evaluate the ability of using CMPI to measure strain changes in the vadose zone. The research approach consists of conducting laboratory tests using a column filled with sand that is subjected to changes in water content and pressure. An optical fiber with CMPI reflectors was laminated in a high-surface-area ribbon and deployed along the axis of the column. The column was made from 8-inch, Schedule 40 PVC pipe (75 cm tall, 20 cm inner diameter) and filled with medium-grained sand (K = 2.5x10-6 m/s, porosity = 0.28, Coefficient of Uniformity = 2, van Genuchten (n = 4.34, = 0.00039 m-1, θs = 0.28, and θr = 0.08), Young’s Modulus = 18 –30MPa). Five pressure and four temperature sensors were inserted through the wall of the column. The optical fiber strain measurements include five reflectors spaced 10 cm apart in a ribbon on the inside, and 5 addition reflectors on the outside along the axis of the column. Strain measurements are made between pairs of reflectors. The reflectors are created in 250-micron-diameter acrylate-coated single mode Corning SMF28e+ optical fiber using a femtosecond laser. The optical fiber was laminated between two pieces of polyester film creating a large surface area to transfer strain from the porous media to the optical fiber. The experiments were conducted by filling the column from the bottom or infiltrating water from the top. The water was allowed to equilibrate to room temperature prior to each test in order to limit thermoelastic strain. Five injection tests with a rate of 250 ml/min and three infiltration tests at varying rates were conducted, and the results show patterns of strain and pressure are generally similar. Hydrologic conditions define three zones based on the pressure magnitude and distribution. 1.) Ambient Zone where the pressure heads are quasi-static, and the pressure gradient is roughly unity (head gradient of zero). This is the uppermost zone and is characterized by negative pressures. The pressure head gradient is approximately 1. 2.) Transition Zone where the pressures increase from ambient to zero, are changing relatively rapidly and the pressure gradient is relatively steep (pressure head gradients of 2). The Transition zone is 10 to 15 cm thick. 3.) Positive Pressure Zone where the pressure is positive, the rate of change is slower than in the transition zone and the gradient is flatter (pressure head gradient 1.1 to 1.2). This is the lowest zone in the column. Injection of water causes the pressure to increase and the three pressure zones to move upward. The Transition zone moves at a rate of approximately 2x10-4 +/- 0.5x10-4 m/s, according to analyses of pressure profiles. This is the same as the ratio of the volumetric flux (5.4x10-5 m/s) and the porosity (0.28). Strain signals in the range of 10s of were observed with a noise level of generally less than 0.1 (signal to noise ratio of greater than 100) during injection and drainage. The strain generally increases during injection and decreases during drainage. The strain rate, and how the strain relates to changes in pore pressure varies considerably, however. These changes are repeatable and were used to define four stages in the strain time series during filling and four stages during draining. An additional three stages were identified in the time series of tests that involved filling the column until water ponded on the surface of the sand. The spatial and temporal distributions of strain caused by injection depend on the location of the moving pressure zones. The locations of the different pressure zones were determined from pressure profiles at different times and these data were transferred to strain time series. This shows that the pressure zones occur in consistent locations relative to the strain sensor during the different stages, which indicates that the changing pore pressure is controlling the strain. This was expected, but how the pressure controls the strain was unexpected. In some locations, the strain increases with increasing pressure, which is consistent with Hooke’s Law. In other locations, however, the strain is unchanged or decreases when the pore pressure increases. It is inferred that this is a result of stress transfer, where an increase in stress in one location causes a decrease in stress in another location to maintain equilibrium. An interesting effect occurs when the upper surface of the sand becomes saturated. Significant compression (several 10s of ) occurs as the pressure and saturation increase at the upper surface. Compression occurs throughout the column, but the effect is greatest at the top of the column. The rate of compression is fastest slightly before ponding occurs, but it slows markedly and nearly stops when water starts to accumulate at the surface (ponding). This effect reverses when the surface of the sand is drained, resulting in tension throughout the column. The compression caused by this effect can be as large or larger than the tensile strain that accumulated during filling. This effect was unexpected because increasing pressure is normally associated with tensile strain. Nevertheless, this effect was observed consistently in all tests where the pressure changes at the upper surface, including tests where water was injected from below or infiltrated from above. This effect behaves as if the pore pressure at the upper surface of the sand exerts a normal force on the boundary (increasing pore pressure exerts a downward compression on the boundary). Results of the laboratory experiments indicate that CMPI can measure strain caused by fluid pressure changes in the vadose zone with a signal to noise ratio of 100 or more. Injection and drainage cause a strain signal that is complex, but repeatable. The magnitudes of the strain signal are consistent with magnitudes that are expected based on poroelastic calculations using independently measured properties of the sand. The strain signal can be explained with a conceptual model that recognizes three basic effects: 1.) strain in proportion to pressure change (Hooke’s Law); 2.) strain independent of local pressure change due to stress transfer; 3.) strain caused by a load applied at the ground surface that is proportional to the pressure change. These results indicate that the CMPI technique with an optical fiber laminated in a polyester ribbon generates data that represent the strain distribution during hydrologic processes of imbibition and drainage in variably saturated sand. This suggests that distributed strain measurements using CMPI could be a viable approach for evaluating processes in the vadose zone, laying the groundwork for future field implementation

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