Geology for Engineers: Groundwater, Rainfall, and Climate - PDF

GEOLOGY FOR ENGINEERS GROUNDWATER AND CLIMATE GROUNDWATER MOVEMENT NEXT PAGE  The movement follows D’Arcy’s Law, amended to allow for the particular hydraulic properties of pervious media through which flow takes place. Well established as is the study of groundwater hydro...

GEOLOGY FOR ENGINEERS GROUNDWATER AND CLIMATE GROUNDWATER MOVEMENT NEXT PAGE  The movement follows D’Arcy’s Law, amended to allow for the particular hydraulic properties of pervious media through which flow takes place. Well established as is the study of groundwater hydrology, it can never be forgotten that the theory assumes certain specified subsurface conditions and that the practical application of the theory demands accurate information about subsurface conditions- in other words, an appreciation of local geology. NEXT PAGE NEXT PAGE  The water level in a producing well will naturally fall when pumping begins and will continue to do so until hydraulic gradient in the material surrounding the well in such that flow into the well will occur at the rate equal to that at which water is being removed by pumping. NEXT PAGE GROUNDWATER SURVEY NEXT PAGE  Determination of groundwater conditions becomes a vital part of preliminary site investigation. Groundwater surveys generally call for trained and expert hydrological advice and a working knowledge of local geology. The engineer’s problem usually to find the water depth, in such quantities, and of such quality that it can be utilized economically in the public service. NEXT PAGE  Plants were the first indicator of groundwater to be used regularly, and vegetarian still constitutes a fairly reliable guide. In the first century B.C., Vitruvius listed plants known to him to indicate the presence of groundwater near the surface of the ground and even endeavored to suggest when they were reliable guides. O. E. Meinzer suggested the term phreatophytes for those plants that habitually grow in arid regions only when they can send their roots down to the groundwater surface. NEXT PAGE  Contour maps provide one means of estimating the quantity of groundwater available. Various method may be used; their relative suitability is dependent on local conditions and the data available. One method is to attempt to prepare a water balance sheet for the area considered, measuring the total runoff and rain, estimating the losses due to evaporation and absorption by plants, and determining the remaining balance, which may normally be assumed to be the addition to the groundwater supply. NEXT PAGE SPRING AND ARTESIAN WATER NEXT PAGE SPRING  Spring sometimes provides useful and pure water supply. They may give a continuous supply, but some spring can be affected by the pumping of groundwater in adjacent areas. Springs often accompany instability of ground and especially that of steep natural slopes, requiring that their origin and course must always be determined in advance of design. NEXT PAGE Water is said to be artesian when the groundwater rises either up to or above ground level as soon as the water- bearing bed is pierced. The name artesian is one of the few in connections with subsurface conditions that is not elf- descriptive. It is originated from that of the province of Artois, France. NEXT PAGE  Sub artesian wells are those in which the groundwater is under hydrostatic pressure but not to such an extent that it is forced to ground level. As soon as it is freed it comes to rest at some point below ground level but above the top level of the water-bearing bed. NEXT PAGE GROUNDWATER UNDER THE SEA NEXT PAGE  Sodium chloride is a common impurity in groundwater, developed through contact of groundwater with rocks having soluble content. Another source of sodium chloride and allied salts in groundwater is the serious problem of encroaching seawater in coastal area. NEXT PAGE NEXT PAGE NEXT PAGE  Rainfall patterns are influenced of the local geology. There is, therefore, an intimate relationship between rainfall intensity and local geology. Study of local records, therefore, must be an early part of all site investigations. Where no local recording station exist, expert advice should be taken as to how the nearest available records can be best extrapolated for the site being studied. In such cases, for major projects, early installation of a recording rain gauge should be seriously considered. NEXT PAGE  Rainfall is a major contributor to flood flows in rivers and all smaller watercourses. If a site under investigation includes any watercourse, however small, every effort must be made to determine the extent of the floodplain and to correlate this with relevant rainfall records so that the necessary allowance can be made for possible future flooding. Floodplain mapping is now well developed in many urban and developing areas, nut the danger of building on known floodplains remain. NEXT PAGE TEMPERATURE AND WIND NEXT PAGE  Seasonal temperature variations are usually taken for granted and seldom though about to any degree except by those with special interest in the weather. These variations are, however, of great importance in civil engineering works and they influence geological phenomena such as weathering. NEXT PAGE  Temperatures are therefore an important factor in the local climate and are recorded at all weather stations, usually as the maximum and minimum, for each day. When these are plotted on a chart which already contains a record of the long-term average temperatures. NEXT PAGE CLIMATIC RECORDS NEXT PAGE  Every major country today has a national weather service responsible for recording all weather features throughout the country and for publishing these in convenient form. Typical are the series of fine bulletins entitled Climates of the States prepared by the National Oceanic and Atmospheric Service. NEXT PAGE  These are available for each state from the Superintendent of Documents in Washington D.C. Information given in a typical bulletin includes “freeze data” long term average air temperatures and precipitation by month for locations throughout the state, as well as the extreme values so far recorded. Helpful maps with isolines for air temperatures and mean annual precipitation are also included. NEXT PAGE