INTRODUCTION:
Engineers work with large volumes of soil and rock which will contain variable amounts of fluid in their pores and fractures. It is helpful to distinguish the material from which these volumes are made from the mass which they form. Sediments are made from particles, big and small, and “rocks” are made from rock!
Sediments: The coarsest sediments are those produced by land sliding and glaciation which maytransport fragments of rock so large that an examination at close range may fail torecognize that they have been displaced. More commonly, rock fragments found beloweroding cliffs may be many tones in weight. Such very large fragments may befurther eroded during river transportation to gravel and boulder size. These fragmentsare recognizably rock but, as they disintegrate to yet smaller and sand-size grains, thegrains tend to be largely of single minerals. The type of mineral of which they are composed will depend upon the source rock and the degree of abrasion suffered during transportation.
Thus the most common sand-forming mineral is quartz, but in limestone areas the grains may be predominantly calcareous. If there are local sources of less erosion-resistant minerals, such as mica, these may be found mixed with more resistant minerals transported from distant sources. Grains of all sizes will be, to some degree, rounded by abrasion during transportation and the degree of roundness achieved is of geotechnical significance, for angular grains tend to interlock and give greater shear strength than more rounded grains. Uniformly graded sediments comprise more or less equally distributed representatives of many grain sizes. Well graded sediments are mostly of one grain size while gap graded sediments lack a range of grain sizes. Well graded sediments tend to have greater porosity and thus greater permeability than uniformly graded sediments because there are fewer finer particles to fill pore spaces between larger particles. At silt size, the particles and the pore spaces between them are very small so that permeability is very low and movement of water slows. For this reason, attempts to compact water saturated silt may result in raised pore water pressures and the subsequent liquefaction of the deposit.All of the sediments so far described are granular and the grains do not adhere to each other. The yet finer grained sediments, clays, are formed of particles less than 0.002 mm in diameter and often much smaller, and are commonly very small plates of clay minerals bonded together by electro-chemical forces. This bonding gives the clay cohesion allowing the material to be moulded. Clay minerals result mainly from the weathering of other rock forming minerals. Thus kaolinite results from the weathering of feldspars in granitic rocks.
Intact Rock Materials: ‘Intact’ rock is commonly taken to mean a piece of rock about the size of a laboratorytest specimen (usually a cylinder of core no larger than about 100 mm long and 50 mmdiameter) without obvious cracks or breaks. Most rocks are formed of mineral grainsor other rock fragments bonded together in some way. The amount of pore spacepresent, the size of the pores and the nature and quantity of the cement has a majoreffect on the mechanical properties of the intact rock material. In general terms, thegreater the porosity, the weaker the rock and, of course, the weaker and less abundantthe cementing mineral, the weaker the rock also.No rock can be stronger than the minerals of which it is composed and the natureof the rock forming minerals has a dominating influence on the behaviour of some rocks. Thus rocks formed from soluble minerals such as calcite and gypsum, pose problems from past solution and future solubility. Evaporites are effectively monomineralic rocks whose formation is associated with the evaporation of mineral charged water. They include such minerals as halite (rock salt), gypsum and anhydrite and potash salts (such as a carnallite and polyhalite). Evaporites, particularly rock salt, will flow under pressure and may be found in salt domes pushed, and still moving from their original position, upward into overlying strata. The susceptibility of these rocks to creep gives problems in mines and tunnels.
Fluids and Gasses: The main fluids of importance in engineering geology are water and oil. It is almostimpossible to over-emphasize the importance of water in determining the engineeringbehavior of geological materials and masses. Water is almost incompressible andwhen present in the pore spaces of a material, if only in small amounts, will modifythe behavior of that material under stress. The behavior of clays, in particular, isvery much dependent on moisture content. Water is seldom pure and contains dissolvedminerals, such as sulphates, which may react with engineering materials. Salinityand acidity limits its use in such processes as the manufacture of concrete. Onfreezing, water expands and ground heave and ice wedging are important mechanismscausing ground disruption and slope instability.
Oil as a fluid in the ground is not often of direct importance in civil engineering unless there are projects proposed at great depth in which oil may be inadvertently encountered. However, it is a common cause of contamination in industrial sites and becomes a component of sediments that has to be considered when such sites are redeveloped.
0 comments:
Post a Comment