Ground Water (CON4341) PDF Jan 2022

CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 5 GROUND WATER 5.1 Formation of Ground Water Water commonly gets into the soils and rocks when surface precipitation (rain or snow) percolates (move downwards) thr...

CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 5 GROUND WATER 5.1 Formation of Ground Water Water commonly gets into the soils and rocks when surface precipitation (rain or snow) percolates (move downwards) through the interconnecting channels made by the voids in soils or the cracks in rocks. It is then termed ground water. Figure 5.1 Formation of ground water 5.2 Aquifer and Aquiclude A body of soil or rock which holds ground water and allows water to move through itself is called an aquifer; the opposite of an aquifer is an aquiclude (Figure 2). Figure 5.2 Aquifer and aquiclude 1 of 7 CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 5.3 Types and Zones of Ground Water In soil, there are two types of ground water and they occur in distinct zones separated by the water table or ground water level or phreatic surface (Figure 3). Ground water level (symbol g.w.l.) or phreatic surface is the top level of saturation of a body of soil. Types of ground water (a) Phreatic or Gravitational water, which: is subject to gravity forces saturates the pore spaces (voids) in the soil below the water table has a positive internal pore pressure (i.e., greater the atmospheric pressure) tends to flow laterally. (b) Vardose water, which : may be transient percolating water moving down to join the phreatic surface (or watertable). may be capillary water held above the water table by surface tension having a negative internal pore pressure Figure 5.3 Zones of ground water 2 of 7 CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 Zones of ground water (a) Saturated Zone: the top surface is at atmospheric pressure and is known as the water table or the phreatic surface. Below this surface the soil is saturated with water subject to positive hydrostatic pressure. (b) Aeration zone: this zone is subdivided into 3 subzones. Immediately above the phreatic surface, the soil remains saturated with water due to capillary action which holds water below atmospheric pressure. Hence, the pressure in the pore (or pore water pressure) is negative. Above the capillary saturated one is a partially saturated subzone where water is held by capillary action (surface tension) and absorption. The top subzone occurs only when there is continuous upward evaporation and/or downward percolation. 5.4 Unconfined Aquifer and Confined Aquifer Unconfined aquifer The surface water usually moves downwards through the void channels of a layer of pervious (permeable) soil or rock to the top level of saturation or the ground water level (Figure 5.4). An aquifer fed by direct downward percolation which establishes a water table is called an unconfined aquifer. Water will normally fill a pit or borehole to the level of the ground water table. Aquifer Impermeable layer Figure 5.4 Unconfined aquifer 3 of 7 CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 Confined Aquifer An aquifer, which is sandwiched between two impermeable layers of soil or rock, is fed with water from a source at some distance away is termed a confined aquifer. Water in a confined aquifer is often subject to the pressure (artesian pressure) of a head of water higher than the ground level immediately above and, if tapped by a cased borehole, a column of water will rise above the ground surface level. The level to which the water will rise in such a situation is known as the piezometric surface or piezometric level. (Note that there is no water table inside a confined aquifer.) Aquifer Figure 5.5 Confined aquifer 5.5 Pore Water Pressure The water in an aquifer, whether confined or unconfined, has pressure at any point due to the head of water above it. Since the water is in the void spaces in the soil or rock, i.e., in the pores, this pressure (shown diagrammatically by the manometers in Figure 5.6) is known as the pore water pressure (symbol u). 4 of 7 CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 Pore water pressure at A u = h1 γw (a) Unconfined aquifer Pore water pressure at B u = h2 γw (b) Confined aquifer Figure 5.6 Pore Water Pressure in confined and unconfined aquifers 5 of 7 CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 Positive Pore Water Pressure Normally, water pressure increases positively with depth below the ground water level (g.w.l.)as shown in Figure 5.7. Pore water pressure at A u = h1 γw u Figure 5.7 Normal increase of pore water pressure with depth In some soils, particularly those with very small void spaces, the top of saturated (g.w.l.) is modified by the surface tension effect of the very small channels formed by the interconnecting pores. This is demonstrated by a simple experiment as shown in Figure 5.8. Above the ground water leveling a soil, where it is partially saturated, the narrow channels formed by interconnecting small voids act like a system of capillary tubes which suck up water by surface tension. This phenomenon is termed soil suction. Pore water pressure u =- h γw Figure 5.8 capillary rises due to surface tension and resulting pressure distribution 6 of 7 CON4341 Soil Mechanics and Geology GROUND WATER Jan 2022 Monitoring (Measurement)of ground Water Level Where normal positive water pressure exists below the water table it may be measured by the use of instruments called standpipe and piezometer. (Negative pore water pressure is measured using a tensiometer.) Figure 5.9 shows a typical standpipe and an open-hydraulic piezometer which are commonly used in Hong Kong. 7 of 7