W.A. Take

Effect of GCL Properties on Shrinkage When Subjected to Wet-Dry Cycles

The potential shrinkage of eight different geosynthetic clay liners (GCLs) subjected to wetting and drying cycles is examined. It is shown that the initial (e.g, off-the-roll) moisture content may affect the initial shrinkage but did not notably affect the final equilibrium shrinkage. For GCLs with granular bentonite and wetted to a moisture content of about 60% (or greater) in the hydration phase, the actual moisture content did not appear to affect the magnitude of the final equilibrium shrinkage. However, it did affect the rate of shrinkage. Specimens brought to about 100% moisture content in each cycle reached a constant shrinkage value much faster than those brought to about 60% in each wetting cycle. GCLs containing powdered bentonite generally shrank more than those containing granular bentonite. All of the powdered bentonite specimens continued a slow accumulation of strain with increasing cycles, even up to 75 cycles. The shrinkage of a needle-punched GCL with a thermally treated scrim-reinforced...

Characterization of Transparent Soil for Unsaturated Applications

Experimental characterization of unsaturated soils is of primary importance to further understanding of fundamental behavior, as well as allow for accurate modeling and predictions, of constitutive and field behavior. In the laboratory, the most common research methodology used to investigate the hydraulic behavior of unsaturated soils involves placing the unsaturated soil in a column apparatus with measurements of pore pressure and moisture content being made at discrete locations distributed along the elevation of column. These types of tests have provided many valuable insights into unsaturated flow phenomena; however, there are some limitations with this methodology including the discrete nature of the measurement points. In this paper, an alternative method is proposed which aims to combine the use of digital image analysis with a transparent soil to avoid the ambiguity of traditional boundary image measurements of moisture content in column experiments. At 100% saturation, the transparent soil particles appear invisible and allows for the ability to see through the soil mass. Any air bubbles will be visible within the soil voids and as a result, at varying degrees of saturation less than 100%, the soil will become progressively non-transparent. The relationship between pixel intensity of the unsaturated soil and degree of saturation is defined and validated. This relationship allows definition of the degree of saturation throughout the column profile thus giving the opportunity to verify and further develop constitutive models for unsaturated hydraulic behavior.

Thermal Expansion and Contraction of Geomembrane Liners Subjected to Solar Exposure and Backfilling

AbstractGeomembranes (GMBs) are widely used as advective barriers in landfill liner systems. When exposed to the sun, GMBs exhibit a network of wrinkles as a result of thermal expansion. These wrinkles disrupt the intimate contact between the GMB and the underlying layer. If a hole is coincident with a GMB wrinkle then the space under the wrinkle has the potential to act as a preferential pathway for flow of contaminants. Thus, the size and shape of GMB wrinkles have implications for leakage rates through the composite liner system. However, wrinkles are only a concern if they persist after placement of backfill, which is currently a subject of debate. In this paper, wrinkles are induced in a 1.5-mm-thick, black high-density polyethylene strip GMB specimen overlying a geosynthetic clay liner using natural solar and laboratory energy sources. Particle image velocimetry techniques are employed to record cross-sectional wrinkle geometry during growth and subsequent backfilling. This cross-sectional geometry ...

Ground Displacements from a Pipe-Bursting Experiment in Well-Graded Sand and Gravel

Pipe bursting is a construction technique that involves the replacement of an existing buried pipe with potentially much less surface disturbance than traditional cut and cover construction. However, excessive ground movements associated with pipe-bursting operations may lead to damage to surrounding infrastructure. A static pipe-bursting experiment was performed in sand and gravel within an 8-m-long, 8-m-wide, and 3-m-deep test pit to quantify the ground displacements from pipe bursting. An existing unreinforced concrete pipe buried 1.385 m below the ground surface was replaced with a high-density polyethylene pipe. Pulling force and the three-dimensional nature of surface displacements associated with pipe bursting are examined. The 4-m wide surface response had a peak vertical displacement of 6 mm. In addition, transverse displacements of 1.2 mm resulted in the formation of a tension crack in the ground above the concrete pipe. This experiment offers data that improves the understanding of the mechanisms of ground disturbance, and provides unique experimental data for calibration of numerical models.

Analytical solution for pile hammer impact

Abstract The drivability study of piling requires modelling of the hammer impact to calculate the input force wave at the pile head. This force wave can be computed numerically or by using simple analytical solutions. In this paper, analytical solutions are presented for hammer impact using a model which uses lumped masses for the ram and anvil, a spring for the anvil cushion, and another spring for the cap cushion on the top of a pile. The pile is modelled as a dashpot. The solutions account for the separation of the anvil mass from the pile cap cushion as well as the separation of the ram mass from the anvil cushion. The developed solutions are used to perform a parametric study to illustrate the influence of the pile cushion on the maximum force transmitted to the pile head.

Experimental Test of Theory for the Stability of Partially Saturated Vertical Cut Slopes

AbstractThis paper extends Culmann’s vertical-cut analysis to unsaturated soils. To test the extended theory, unsaturated sand was compacted to a uniform porosity and moisture content in a laboratory apparatus. A sliding door that extended the height of the free face of the slope was lowered until the vertical cut failed. Digital images of the slope cross section and upper surface were acquired concurrently. A recently developed particle image velocimetry (PIV) tool was used to quantify soil displacement. The PIV analysis showed strain localization at varying distances from the sliding door prior to failure. The areas of localized strain were coincident with the location of the slope crest after failure. Shear-strength and soil-water-characteristic parameters of the sand were independently tested for use in extended analyses of the vertical-cut stability and of the failure plane angle. Experimental failure heights were within 22.3% of the heights predicted using the extended theory.

Physical modelling of rainfall-induced flow failures in loose granular soils

The tragic consequences of the March 2014 Oso landslide in Washington, USA were particularly high due to the mobility of the landslide debris. Confusingly, a landslide occurred at that exact same location a number of years earlier, but simply slumped into the river at the toe of the slope. Why did these two events differ so drastically in their mobility? Considerable questions remain regarding the conditions required to generate flow failures in loose soils. Geotechnical centrifuge testing, in combination with high-speed cameras and advanced image analysis has now provided the landslides research community with a powerful new tool to experimentally investigate the complex mechanics leading to high mobility landslides. This paper highlights recent advances in our understanding of the process of static liquefaction in loose granular soil slopes achieved through observations of highly-instrumented physical models. In particular, the paper summarises experimental results aimed to identify the point of initiation of the chain-reaction required to trigger liquefaction flow failures, to assess the effect of slope inclination on the likelihood of a flowslide being triggered, and to quantify the effect of antecedent groundwater levels on the distal reach of landslide debris with the objective of beginning to explain why neighbouring slopes can exhibit such a wide variation in landslide travel distance upon rainfall-triggering.