Analysis of drying and saturating natural gypsum samples for mechanical testing

The stability of underground abandoned gypsum mines is dependent on the gypsum pillar's strength, and most abandoned mines are in a fully saturated condition. Moisture affects the strength of gypsum and is therefore commonly measured when testing rock strength. For most rocks, this is a simple task of weighing the rock's mass before and after oven-heating at a specified temperature and duration. For natural gypsum, however, this is not a straightforward process. Heating natural gypsum can result in dehydration and transformation of gypsum to hemihydrate and anhydrite, thus changing the physical characteristics of the gypsum such as its particle density which in turn affects the moisture content and strength measurements. To prevent transformation when determining the moisture content of gypsum, the American Society for Testing Materials (ASTM) recommends lowering the drying temperature from 110 °C to 60 °C. To investigate the temperature at which gypsum transforms to hemihydrate, we used a helium pycnometer to measure the particle densities of gypsum, hemihydrate and anhydrite. In this research, we suggest that a higher drying temperature of 80 °C can be used for drying gypsum without transforming gypsum to hemihydrate. Further, preparing saturated samples for mechanical testing, which is required in stability analyses of abandoned mines, is challenging due to the dissolution of gypsum when placed in water. To address this problem, we investigated the following methods to saturate gypsum cores taking into account the solubility of gypsum: (1) water immersion, (2) vacuum saturation, and (3) improved vacuum saturation. The research indicates that all the three methods are acceptable but they should be conducted using a saturated gypsum-water solution to minimize dissolution. Further, the research found that the improved vacuum saturation method saturated the test samples within 24 h, while duration of 30 h was required for the other two methods. Keywords: Gypsum-hemihydrate-anhydrite transformation, Dehydration, Rock core saturation, Moisture content, Helium pycnomete

Tropical storm-induced landslide-dammed lakes and debris flow hazards at Ocotepeque, Western Honduras

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. One of the deadliest tropical storms on record in Central America is the 1934 tropical storm that resulted in over 3000 fatalities; the majority of fatalities caused by floods, debris flows, and landslides. The hardest hit region was in western Honduras near the city of Ocotepeque, where 64 cm of rain fell on June 4, 1934. The rainfall caused a rock landslide forming a natural dam in a mountain valley above Ocotepeque. The dam failed 3 days later on June 7. The ensuing debris flow destroyed Ocotepeque killing an estimated 486 people, over 10% of the city’s population. There was little to no reporting of this disaster due to the city’s remote location and lack of adequate communications. Following this event, the city was relocated 4 km north of Ocotepeque and renamed Nueva Ocotepeque. Over time, however, the old location, Ocotepeque, was resettled and called Antigua Ocotepeque. In this study, we examine the 1934 event and the effects of a similar recurrence on both Antigua Ocotepeque and Nueva Ocotepeque. Landslide hazard maps of the area (for shallow landslides) were generated and used to investigate the possibility of landslide dams forming. The potential debris flow inundation areas were predicted, and the effects of potential debris flows were investigated. Deterministic slope stability analyses conducted on the new location indicated that potential landslides could be more significant than suggested by the current hazard maps

A dynamic damage growth model for uniaxial compressive response of rock aggregates

A model that combines damage evolution theory with dynamic crack growth is developed to investigate the uniaxial compressive response of rock aggregate. A damage parameter that determines the time at which the rock looses its ability to transmit the stress completely is introduced into the model. Flaw distribution in rocks is described by a two parameter Weibull distribution. The model correlates the damage parameter with the dynamic growth of wing cracks from the pre-existing microcracks. Influences of model parameters on stress-strain response and failure strength are studied systematically. The results of the dynamic damage evolution model are compared to the experimental observations on three types of rock and a good correlation is obtained. © 2002 Elsevier Science Ltd. All rights reserved

Creating 3D models of tractor tire footprints using close-range digital photogrammetry

© 2017 ISTVS Close-range digital photogrammetry is utilized to construct the 3D models of an agricultural tire footprint. These models were then analyzed to obtain the tire footprint depth, area and volume. The procedure of using the photogrammetry technique for developing 3D models of a tire footprint on soil as well as an assessment of the accuracy of the 3D models are discussed in this paper. Testing was conducted using a tractor tire in a large soil bin in a lab to generate a single tire footprint along with a rolling tire test to simulate a longer tire rut. Our experiments showed that the close-range digital photogrammetry provides an efficient and accurate method to assess the depth and volume of the tire footprint in soil

Automated soil particle-size analysis using X-ray absorption

While the hydrometer method (ASTM 422-63) has proven to be an accurate method of particle-size analysis of soil and is relatively inexpensive to conduct, it is prone to operator error as well as taking a minimum of 24 hours to conduct. Recent research has shown that an automated X-ray absorption instrument can closely approximate hydrometer test results for soils. The X-ray absorption instrument determines the change in mass of a sedimenting soil-water suspension through the absorption of X-rays while utilizing Stokes\u27 law to determine particle sizes in the range from 300 μm to 0.1 μm. A number of hydrometer tests on three different soils were conducted and compared with particle-size analysis using an X-ray absorption instrument. Controlled testing between the hydrometer and the X-ray absorption instrument produced almost identical results for silts while tests on clay show good agreement from 75 μm through about 10 μm. However, below 10 μm the X-ray absorption test results start to show a slightly higher percent of fines than the hydrometer test, with the percent difference increasing as the particle size decreases. The increase appears to be related to the plasticity of the clay, with low plasticity clay showing an average increase of only 1 to 3 percent, while the higher plasticity clays vary from an average of 3 to 6 percent

Hazard assessment of rainfall-induced landslides: a case study of San Vicente volcano in central El Salvador

© 2014, Springer Science+Business Media Dordrecht. The San Vicente volcano in central El Salvador has a recurring and destructive pattern of landslides and debris flows occurring on the northern slopes of the volcano, and in recent memory, there have been at least seven major destructive debris flows. There has been no known attempt to study the inherent stability of these slopes and determine the factors that might lead to slope instability. Past events on the volcano were used to perform a 2D slope stability back analysis and to estimate the unknown model parameters. This analysis confirmed that the surface materials of the volcano are highly permeable and have very low shear strength. Additionally, the analysis provided insight into the groundwater table behavior during a rainstorm. Slope geometry, rainfall totals and initial groundwater table location were found to have the greatest effect on stability. A methodology is outlined for creating a stability chart to be used during rainfall events for monitoring slope stability. This chart could be used by local authorities in the event of a known extreme rainfall event to help make evacuation decisions. Finally, recommendations are given to improve the methodology for future application in other areas as well as in central El Salvador

Application of anchored geosynthetic systems for in situ slope stabilization of fine-grained soils

The use of anchored geosynthetic systems (AGS) was proposed by Koerner et al. for the stabilization of slopes at or near their failure state. AGS provides in-situ stabilization of soil slopes by combining a surface-deployed geosynthetic with an anchoring system of driven reinforcing rods similar to soil nailing. Installation of the system involves tensioning a geosynthetic over a slope\u27s surface by driving anchors through the geosynthetic at a given spacing and distance. By tensioning the geosynthetic over the slope\u27s surface, a compressive load is applied to the slope. Benefits of AGS are described to include the following: (a) increased soil strength due to soil compression, including increased compressive loading on potential failure surfaces; (b) soil reinforcement through soil nailing; (c) halt of soil creep; (d) erosion control; and (e) long-term soil consolidation. Following installation of the AGS and 1 year of monitoring, it was found that the anchored geosynthetic system provided only some of the reported benefits and in general did not function as an active stabilization system. This was in part because the system could not provide and maintain loading on the geosynthetic. The geosynthetic, however, did tension when slope movement occurred, preventing the slope from failing. Thus, the system functioned more as a passive restraint system and appeared to function well over the monitoring period