Transcript Slide 1
Construction Technology: Substructure DW57 34 Outcome 2 Methods of groundwater control Water in the ground These kids probably think there is some kind of magic happening here ... they pull down a lever and out of the ground below their feet comes clear, cool freshwater. They (and maybe you) may not realize that there is an immense amount of water in aquifers below the earth's surface. In fact, there is a hundred times more water in the ground than is in all the world's rivers and lakes. Water in the ground • Some water underlies the Earth's surface almost everywhere, beneath hills, mountains, plains, and deserts. • It is not always accessible, or fresh enough for use without treatment, and it's sometimes difficult to locate or to measure and describe. • This water may occur close to the land surface, as in a marsh, or it may lie many hundreds of feet below the surface, as in some arid areas of the West. • Water at very shallow depths might be just a few hours old; at moderate depth, it may be 100 years old; and at great depth or after having flowed long distances from places of entry, water may be several thousands of years old Water in the ground • Groundwater occurs only close to the Earth's surface. • There must be space between the rock particles for groundwater to occur, and the Earth's material becomes denser with more depth. • Essentially, the weight of the rocks above condense the rocks below and squeeze out the open pore spaces deeper in the Earth. • That is why groundwater can only be found within a few miles of the Earth's surface. • The water within the ground slowly moves towards streams, lakes, marshes and oceans as shown above. Most rivers receive about half of their flow from the water in the ground. This is why rivers can keep flowing for weeks after no rain. Water in the ground • The presence of water in soil voids and rock fissures has a major impact on the engineering behaviour of the ground. • Difficulties with soils can be due to the water contained in the voids and not the actual soil itself. • A high water-table can present difficulties with foundation excavations and reduce the bearing resistance of the ground. • The presence of chemicals in groundwater, such as acids and sulphates, can cause damage to foundation concrete if it is not of an appropriate quality. • Groundwater can cause unacceptable humidity levels around a building or structure. So where does all this water come from? The stages of the hydrological cycle Rainfall Ultimate source of all fresh water. When it lands on the ground it can be dispersed in three ways; • Evapotranspiration – evaporation from open water and transpiration by plants • Runoff – surface water flowing into streams and rivers • Infiltration – seepage into ground to become groundwater The principal source of groundwater is precipitation (rainfall). While much of this water may be lost as surface run-off or by evaporation and transpiration to the atmosphere, some of the water enters the ground and moves downwards by infiltration through the continuous soil voids. The relationships between the processes of infiltration, run-off, evaporation and transpiration are influenced by climate, topography, vegetation and geology. Groundwater • all water flowing through or stored within the ground, in both rocks and soils • derived from infiltration • lost by flow to surface springs and seepage out through seabed Water Table • Level in the rocks below which all voids are water filled • Generally follows the surface topography, but with less relief • It meets the ground surface at lakes and most rivers Water Table • The water table can change depending on a number of factors – Seasons (dry summers, wet winters) – Vegetation such as trees – How the water occurs, eg liquid, solid or gas Water Table • Rain – the more rain the higher the water table. Flooding can occur where there is excessive rain. Some rain evaporates soon after it falls and some drains on the surface to join watercourses Water Table • Snow – semi solid format of rain. More snow would increase the water table. Water Table • Sea – the water table moves close to the sea as the tide comes in or out. • Watercourses such as rivers or burns – the water table will be higher the closer it is to a watercourse. • Marsh land – the soil is retaining water. Water table is either on the surface or just below it Water Table • Soil type – if the soil retains the water, such as clays, this could disturb the water table. Dependent on how permeable the soil is • Underlying geological structures – the nature of the bedrock below could push the water table to peaks if it is folded, faulted or fractured depending upon pressure. • Burst water main – this would lead to a high water table Water Table • Floodplain – area of land around a river where the river encroaches onto when it exceeds its channel capacity Groundwater that is in direct contact vertically with the atmosphere through permeable ground with continuous open spaces, such as pores and voids in soils and fissures in rock, is termed unconfined. Confined water is separated from the atmosphere by impermeable ground. Confined water is sometimes described as artesian when the pressure in the groundwater corresponds to a head of water which is above ground level. The first unconfined water to be encountered may be at a perched watertable; the perched water is isolated from groundwater at deeper levels by an impermeable layer. Tutorial • What are the sources of water in the ground? • What is the water table and how might it be affected? • Draw a section indicating where you might find perched water, a confined aquifer and an unconfined aquifer.