Smith's Elements of Soil Mechanics. Ian Smith
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clay sample after drying = 4.323 kg
Calculate the water content, the bulk, and the dry densities.
If the particle specific gravity was 2.69, determine the void ratio and the degree of saturation of the clay.
Answer w = 28%, ρb = 1.90 Mg/m3, ρd = 1.49 Mg/m3, e = 0.82, Sr = 93%
Chapter 2 Permeability and Flow of Water in Soils
Learning objectives:
By the end of this chapter, you will have been introduced to:
water in soils and the differences between aeration and saturation zones;
the groundwater table and groundwater flow;
hydraulic head, the hydraulic gradient, Darcy's Law, and saturated flow;
the laboratory and field determination of the coefficient of permeability;
the use and construction of flow nets to determine flow quantities;
measurement of suction pressures in unsaturated soils;
flow of water through earth dams and deposits of different permeabilities.
2.1 Subsurface water
This is the term used to define all water found beneath the Earth's surface. The main source of subsurface water is rainfall, which percolates downwards to fill up the voids and interstices. Water can penetrate to a considerable depth, estimated to be as much as 12 000 m, but at depths greater than this, due to the large pressures involved, the interstices have been closed by plastic flow of the rocks. Below this level, water cannot exist in a free state, although it is often found in chemical combination with the rock minerals, so that the upper limit of plastic flow within the rock determines the lower limit of subsurface water.
Subsurface water can be split into two distinct zones: saturation zone and aeration zone.
2.1.1 Saturation zone
This is the depth throughout which all the fissures and voids are filled with water under pressure. The upper level of this water is known as the groundwater table (GWT), phreatic surface or groundwater level (GWL), and the water within this zone is called phreatic water or, more commonly, groundwater.
The water table tends to follow, in a more gentle manner, the topographical features of the surface above (Fig. 2.1). At groundwater level, the hydrostatic pressure is zero, so another definition of water table is the level to which water will eventually rise in an unlined borehole.
Fig. 2.1 Tendency of the water table to follow the earth's surface.
The water table is not constant in depth but rises and falls with variations of rainfall, atmospheric pressure, temperature, and proximity to tree roots. In coastal regions, the GWL can be affected by tides and is said to be tidal. At locations where the water table reaches the ground surface, springs, lakes, swamps, and similar features can be formed.
2.1.2 Aeration zone
Sometimes referred to as the vadose zone, this zone occurs between the water table and the surface, and can be split into three sections.
Capillary fringe
Owing to capillarity, water is drawn up above the water table into the voids of the soil. Water in this fringe can be regarded as being in a state of negative pressure, i.e. at pressure values below atmospheric. The minimum height of the fringe is governed by the maximum size of the voids within the soil. Up to this height above the water table, the soil will be sufficiently close to full saturation to be considered as such. The maximum height of the fringe is governed by the minimum size of the voids. Between the minimum and maximum heights, the soil is partially saturated.
Terzaghi and Peck (1948) give an approximate relationship between the maximum height and the grain size for a granular soil:
where C is a constant depending upon the shape of the grains and the surface impurities (varying from 10.0 to 50.0 mm2), and D10 is the effective size expressed in millimetres.
Intermediate belt
As rainwater percolates downward to the water table, a certain amount is held in the soil by the action of surface tension, capillarity, adsorption, and chemical action. The water retained in this manner is termed held water and is deep enough not to be affected by plants.
Soil belt
This zone is constantly affected by precipitation, evaporation, and plant transpiration. Moist soil in contact with the atmosphere either evaporates water or condenses water into itself until its vapour pressure is equal to atmospheric pressure. Soil water in atmospheric equilibrium is called hygroscopic water and its water content (which depends upon relative humidity) is known as the hygroscopic water content.
The various zones are illustrated in Fig. 2.2
Fig. 2.2 Types of subsurface water.
2.2 Flow of water through soils
The voids of a soil (and of most rocks) are connected together and form continuous passageways for the movement of water brought about by rainfall infiltration, transpiration of plants, imbalance of chemical energy, or a variation of the intensity of dissolved salts.
When rainfall falls on the soil surface, some of the water infiltrates the surface and percolates downward through the soil. This downward flow results from a gravitational force acting on the water. During flow, some of the water is held in the voids in the aeration zone and the remainder reaches the groundwater table and the saturation zone. In the aeration zone, flow is said to be unsaturated. Below the water table, flow is said to be saturated. Flow of water through soils is often referred to as seepage.
2.2.1 Saturated flow
The water within the voids of a soil is under pressure. This water, known as pore water, may be static or flowing. Water in saturated soil will flow in response to variations in hydraulic head within the soil mass. These variations may be natural or induced by excavation or construction.
2.2.2 Hydraulic head
The head of water acting at a point in a submerged soil mass is known as the hydraulic head and is expressed by Bernoulli's equation:
(2.1)