W. Andy Take

Water-Retention Behavior of Geosynthetic Clay Liners

The hydration and subsequent hydraulic performance of geosynthetic clay liners (GCLs) depend on the water-retention curve (WRC) of the GCL. Because of the inherent difficulty in obtaining the WRC for these materials, limited data exists regarding the WRCs of GCLs in the literature. In this study, high-capacity tensiometers and capacitance relative humidity sensors were used to quantify the water-retention behavior of GCLs for four different GCL products that vary both in materials (woven and nonwoven geotextiles) and in fabrication detail (thermal treatment and needle-punching). The water-retention behavior was investigated under wetting and drying paths; we present results in terms of gravimetric and volumetric moisture content and bulk GCL void ratio. The WRCs of the different GCL products showed significant variation among wetting and drying curves, indicating that both needle-punching and thermal treatment have a significant effect on the swelling behavior of the GCL and its WRC. Theoretical equations...

Optimum Accuracy of Two-Dimensional Strain Measurements Using Digital Image Correlation

AbstractFoil and vibrating wire strain gauges have an optimum strain measurement accuracy of one microstrain. However, they can only provide discrete strain readings over a single fixed-gauge length. Digital image correlation (DIC) offers an alternative to conventional strain gauges because a two-dimensional (2D) surface strain field can be obtained from a single sensor (camera). However, the benefits of 2D strain measurements are only worthwhile if a similar level of measurement accuracy to conventional strain gauges can be achieved. This paper presents the results of an investigation into the optimum strain measurement accuracy that can be achieved by using the 2D technique on artificial images (which eliminate errors associated with cameras and lighting). The principle of the 2D DIC technique and its historical development will be introduced. Then, three potential techniques for taking strain measurements will be presented and compared: single readings, averaged linear readings, and an approach on the ...

Curvature Monitoring of Beams Using Digital Image Correlation

A method for measuring longitudinal strains with the height at a section, and thus the curvature, using a technique based on digital image correlation (DIC), is presented. The background to this technique is introduced as well as previous work in this area. The accuracy of DIC under ideal conditions is established using artificially generated images that represent beams with various curvatures. The practical accuracy of DIC is established by comparing the strains measured using DIC to those predicted by elastic theory and measured using strain gauges for a steel beam. The correlation between these results is found to be excellent. DIC is then used to measure curvatures in RC beams and these results are compared with analytically predicted results with good agreement. The choice of an appropriate gauge length for RC is discussed and is shown to be one of the significant advantages of using DIC as opposed to strain gauges in both laboratory testing and field monitoring of bridge structures.

Effect of antecedent groundwater conditions on the triggering of static liquefaction landslides

Real-time early warning systems for shallow landslides are typically built upon real-time measurements and forecasts of rainfall and empirical correlations between past patterns of rainfall and landslide occurrence. Whereas these relationships describe whether certain combinations of rainfall and preexisting groundwater levels are of elevated risk of landslide triggering, not all combinations leading to landslide events necessarily have the same consequences in terms of landslide mobility (velocity and distal reach of the landslide). In this paper, the technique of geotechnical centrifuge modeling is used to quantitatively evaluate the hypothesis that the mobility of a landslide triggered under elevated antecedent groundwater conditions is higher than scenarios under drier antecedent conditions. Five identical slope models with a shallow depth to bedrock were subjected to different antecedent conditions ranging from zero groundwater flux to nearly saturated conditions prior to rainfall. The results from these scenarios show that higher antecedent groundwater conditions can result in landslides with velocities about three times higher and travel distances about eight times higher than low antecedent conditions due to static liquefaction of the soil at the base of the slope.

A Large-Scale Triaxial Apparatus for Prototype Railroad Ballast Testing

Shear strength and compressibility of large grain-size materials are critical parameters for the geotechnical design of road bases, rock fill embankments, and railroad sub-base. However, due to the correspondingly large scale of triaxial specimens necessary for rock-fill and railroad ballast material testing, the numbers of facilities that are capable of testing these materials are few. In this paper, a cost effective design is documented for a triaxial apparatus capable of testing prototype railroad ballast material containing particle sizes up to 63.5 mm (2.5 in.). Unique to this testing apparatus is the use of vacuum as confinement to allow an unobstructed digital image measurement of specimen volume change during testing. The specimen preparation methodology, manufacture of latex membrane, and instrumentation are also discussed. Finally, the results of a cyclical triaxial test are presented to demonstrate the quality of the testing data from this triaxial apparatus.

Tsunamis generated by long and thin granular landslides in a large flume

In this experimental study, granular material is released down slope to investigate landslide-generated waves. Starting with a known volume and initial position of the landslide source, detailed data are obtained on the velocity and thickness of the granular flow, the shape and location of the submarine landslide deposit, the amplitude and shape of the near-field wave, the far-field wave evolution, and the wave runup elevation on a smooth impermeable slope. The experiments are performed on a 6.7 m long 30° slope on which gravity accelerates the landslides into a 2.1 m wide and 33.0 m long wave flume that terminates with a 27° runup ramp. For a fixed landslide volume of 0.34 m3, tests are conducted in a range of still water depths from 0.05 to 0.50 m. Observations from high-speed cameras and measurements from wave probes indicate that the granular landslide moves as a long and thin train of material, and that only a portion of the landslide (termed the “effective mass”) is engaged in activating the leading wave. The wave behavior is highly dependent on the water depth relative to the size of the landslide. In deeper water, the near-field wave behaves as a stable solitary-like wave, while in shallower water, the wave behaves as a breaking dissipative bore. Overall, the physical model observations are in good agreement with the results of existing empirical equations when the effective mass is used to predict the maximum near-field wave amplitude, the far-field amplitude, and the runup of tsunamis generated by granular landslides.

Laboratory Study of Downslope Erosion for 10 Different GCLs

AbstractThe potential for downslope bentonite erosion from a geosynthetic clay liner (GCL) subjected to an imposed daily surface flow of deionized water is examined for 10 GCLs with different carrier and cover geotextiles, bentonite (powdered and fine and coarse grained and some with a polymer-enhancement additive), and a polypropylene coating on the carrier geotextile. The erosion was similar but more visible in products with a white geotextile than in those with a black geotextile. Also, the erosion features were similar but more visible with nonwoven geotextiles than with woven geotextiles. Polymer-enhanced sodium bentonite substantially slowed the formation of erosion features. A polypropylene coating over the upper geotextile prevented erosion features from developing by keeping water from penetrating into the bentonite layer when subjected to 60 daily imposed-flow/air-drying cycles.

Laboratory Simulation of Bentonite Erosion by Downslope Flow on a GCL

AbstractUnder some circumstances, leaving a composite geomembrane/geosynthetic clay liner (GCL) exposed to solar radiation in the field has been shown to cause shrinkage of the underlying GCL. Recent field studies have shown that leaving a composite liner exposed can also lead to erosion of bentonite from the GCL due to downslope moisture migration. This paper reports an experimental technique that reproduced similar erosion in the laboratory on a typical landfill side slope of 3H:1V. The test method simulates the features that occur with the erosion of bentonite caused by downslope migration of evaporative water in the field. The laboratory tests demonstrate that erosion features can be present but may not be visible unless the appropriate back lighting is used. Erosion features measuring over 25 mm in width were produced. The test method simulated the features that were observed with bentonite erosion in the field and has the potential for use in examining other factors that may affect this type of eros...

Quantification of Optical Clarity of Transparent Soil Using the Modulation Transfer Function

Transparent synthetic soils have been developed as a soil surrogate to enable internal visualization of geotechnical processes in physical models. Transparency of the soil dictates the overarching success of the technique; however, despite this fundamental requirement, no quantitative framework has yet been established to appraise the visual quality of transparent soil. Previous approaches to assess and optimize transparency quality included an eye chart assessment method, although this approach is highly subjective and operator-dependent. In this paper, an independent method for quantitatively assessing the optical quality of transparent soil is proposed based on the optical calibration method, Modulation Transfer Function (MTF). The work explores this hypothesis and assesses the potential for MTF to quantify the optical quality of transparent soils for a number of aspects including (i) optimum oil blend ratio, (ii) depth of viewing plane, and (iii) temperature. The results confirmed that MTF offers a robust and reliable method to provide an independent quantitative measure of the optical quality of transparent soil. The impact of reduced soil transparency and the ability to track speckle patterns—thus accuracy and precision of displacement measurement—was correlated with MTF to evaluate the permissible viewing depth of transparent soil.

Comparison of Wrinkles in White and Black HDPE Geomembranes

AbstractWrinkles were quantified and compared for 1.5-mm-thick, high-density polyethylene (HDPE) geomembranes in 20-m-long and 13-m-wide control sections that were identical except for color—one ha...