Implications of environmental conditions on arid gypseous soils for geotechnical engineering applications

[摘要] Loosely packed sandy soil, created via aeolian deposition and stabilised by interparticle bonding via gypsum crystals (dispersed within the soil fabric), are a significant geohazard in hot, arid environments such as the Middle East (for example, these deposits cover a large area of Iraq). Gypsum, being a moderately soluble salt, can have a substantial influence on the engineering properties of soils, and changes in water content can result in rapid collapse of the soil fabric, damaging surrounding structures; i.e. these deposits are metastable. A search of the literature illustrated that no research focused on the behaviour of an aeolian gypseous soil in environmental conditions. Also, employing electrical resistivity, which might be a helpful tool to monitor water movement in the soil deposit, has not been considered previously. This study, therefore, examined two interconnected experiments. The first aimed to study the impact of groundwater movement (the dimensions of the samples were 144 mm diameter and 300 mm height, with 10% and 20% gypsum content) due to evaporation through an unsaturated column of gypseous soil. These samples were exposed to various conditions, including changing gypsum content, groundwater, temperature gradients, and a breeze across the upper surface of the samples. The second study aimed to determine the stress-settlement characteristics of a pad footing resting on gypseous soil deposit (the dimensions of the samples were 349 mm diameter and 300 mm height, with 20% gypsum content) when exposed to groundwater flow, with and without exposure to the environmental conditions used in the previous study (temperature gradient, and breeze). Electrical resistivity was used to monitor the movement of salty water through the samples in ‘quasi-real-time’, without having to dissect the samples. Obtaining undisturbed samples of this soil to study in the laboratory can be problematic for logistical reasons. Therefore, a method has been developed that produces repeatable samples with the desired properties to facilitate a systematic investigation of these soils without the need for sampling. This method offers flexibility so that large and small samples can be created for investigation. Samples with a range of gypsum content (from 5% to 30%) were investigated with an oedometer and with unconfined compressive strength apparatus, and it is clear that increasing the gypsum content results in stronger material that exhibits greater iii collapse potential when inundated with water. Secondary compression and creep appear to be important mechanisms when considering settlements with inundation of water, suggesting that longer duration load steps should be used in compression tests (90 days were used herein) than the normal 24-hour period if the metastable nature of these materials is not to be underestimated. It is apparent from the evaporation test that, over time, manufactured soil experiences partial dissolution and re-precipitation of its gypsum, with precipitation taking place in the upper layers of the soil as the water evaporates from the soil under increasing temperatures. While the dissolution of gypsum does not appear to be the dominant mechanism dictating the metastable response of the soil samples in the three footing tests, where partial dissipation of suction and possible softening of cementitious bonds via the gypsum crystals, appears to have a more pronounced impact due to both the time required for a collapse to occur and the volume of water required to trigger a collapse event. The soil resistivity was very sensitive to moisture, temperature, and load-induced variations rather than crystal gypsum variation. Conditions of stress, temperature, and moisture substantially affected the current passing through the soil fabric. It was found that increasing the gypsum content provides more ions for electrical conduction, leading to an increase in the pore water conductivity and therefore diminishing soil resistivity. However, the soil resistivity for the three footing tests was completely different upon failure, and this due to the test conditions (amount of water, densification of the upper layer, temperature, and test time).

[发布机构] University:University of Birmingham;Department:School of Civil Engineering

[关键词] Q Science;QE Geology