Chalk is a variable material, the properties of which are dependent upon its composition, textural features and diagenetic history. With the exception of certain horizons in the Lower Chalk that contain appreciable amounts of clayey material, the English Chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions. The latter comprises 70–80% of chalk. Cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures. Hence, high values of porosity were retained. Chalk varies appreciably in density and hardness. The harder chalks are the result of diagenetic processes and bioturbation that brought about densification. In soft chalks the grains are only bound together at the points of contact by thin films of calcite.
The latest classification of chalk is based on an assessment of intact dry density, discontinuity aperture and discontinuity spacing. Chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation. Under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel-shaped failure occurs indicating plastic deformation and textural disaggregation. Similarly, at low loading, chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded.
Chalk undergoes dissolution and so solution features are found throughout its outcrop.
Mineworkings in the Chalk extend back into the distant past, the most ancient being those excavated in the Neolithic Age for flint. Several types of workings exist. Collapse of old mineworkings, most of which are unrecorded, is difficult to predict. The potential for subsidence, caused by the collapse of both mineworkings and dissolution features, affects development and its occurrence can lead to the abandonment of property or, worse, the loss of lives. 相似文献
Studies of the respiratory health effects of different types of volcanic ash have been undertaken only in the last 40 years, and mostly since the eruption of Mt. St. Helens in 1980. This review of all published clinical, epidemiological and toxicological studies, and other work known to the authors up to and including 2005, highlights the sparseness of studies on acute health effects after eruptions and the complexity of evaluating the long-term health risk (silicosis, non-specific pneumoconiosis and chronic obstructive pulmonary disease) in populations from prolonged exposure to ash due to persistent eruptive activity. The acute and chronic health effects of volcanic ash depend upon particle size (particularly the proportion of respirable-sized material), mineralogical composition (including the crystalline silica content) and the physico-chemical properties of the surfaces of the ash particles, all of which vary between volcanoes and even eruptions of the same volcano, but adequate information on these key characteristics is not reported for most eruptions. The incidence of acute respiratory symptoms (e.g. asthma, bronchitis) varies greatly after ashfalls, from very few, if any, reported cases to population outbreaks of asthma. The studies are inadequate for excluding increases in acute respiratory mortality after eruptions. Individuals with pre-existing lung disease, including asthma, can be at increased risk of their symptoms being exacerbated after falls of fine ash. A comprehensive risk assessment, including toxicological studies, to determine the long-term risk of silicosis from chronic exposure to volcanic ash, has been undertaken only in the eruptions of Mt. St. Helens (1980), USA, and Soufrière Hills, Montserrat (1995 onwards). In the Soufrière Hills eruption, a long-term silicosis hazard has been identified and sufficient exposure and toxicological information obtained to make a probabilistic risk assessment for the development of silicosis in outdoor workers and the general population. A more systematic approach to multi-disciplinary studies in future eruptions is recommended, including establishing an archive of ash samples and a website containing health advice for the public, together with scientific and medical study guidelines for volcanologists and health-care workers. 相似文献
