Soil structure
By soil structure is meant the arrangement of the solid phase in the earth mass. As is known, soil consists of solid particles and pores filled with water, air, or water vapor, most often partly air and partly water. The structure is determined by the arrangement of the solid particles in the soil. The structures of cohesionless and cohesive soil differ significantly from one another.
Structure of loose soil
The structure of unbound soil develops under the action of gravity. The solid particles of this soil are coarser and move downward, settling under gravity, while the intermolecular attractive forces of such particles do not exist or are negligible and are overcome by gravity. For this reason, in unbound soil the solid particles arrange themselves next to one another, occupying the free space between the arranged particles (fig. 1_a_). If the particles of unbound soil are approximately round and of the same size, as may be the case with uniformly graded sand, then the structure may be densest (fig. 1_b_) or loosest (fig. 1_c_), depending on the arrangement of the solid particles in the soil. The densest structure corresponds to the greatest possible compactness of unbound soil and its porosity is nmin=26%. The loosest structure corresponds to the smallest possible compactness of unbound soil and its porosity is nmax=48%. Thus, for example, loose dune sand has n=47%.
Fig. 1 Structure of unbonded soil
If the cohesionless soil has grains of different sizes, so that finer particles fill the voids between coarser ones (fig. 2), the soil porosity is considerably smaller (n=20% and less).
Fig. 2. Structure of unbound soil of different grain sizes
Structure of bonded soil
In cohesive soil, the structure is different from that of noncohesive soil. In cohesive soil, cohesion forces appear, which are stronger than the gravitational force of fine particles and bind them at the contact surfaces. Because of this, these particles do not fall downward in sequence, but remain stuck one to another and one beside another (fig. 3_a_), thus creating a chain-like network structure (fig. 3_b_), a flaky structure (fig. 3_c_), etc., which are called higher-order structures. Such structures have high porosity, which can reach 60-80% of the total mass, which is not the case with noncohesive soil.
Fig. 3. Structure of bound soil
Porosity and porosity coefficient
Soil porosity is defined as the ratio of the pore volume to the total soil volume (fig. 4).
Fig. 4. Porosity and porosity coefficient of soil
If we denote by
- Vp = the volume of pores in the soil
- Vm = volume of solid particles
- V = the total volume of soil,
by definition, the porosity of the soil n is
n = VP / V = VP / (VP + Vm),
or in % of the total soil volume
n = 100 * VP / (VP + Vm) %.
The porosity coefficient of soil is defined as the ratio of the volume of pores to the volume of solid particles:
e = VP / Vm.
From the porosity formula n we have:
nVP + nVm = VP, from which Vm = VP(1-n) / n.
By substituting this value into the formula for e we obtain
e = n / (1-n).
From this equation we obtain
e - ne = n; n = e / (1 + e).
The porosity coefficient indicates the density of the soil. The greater the coefficient e, the lower the soil density, and vice versa. If a compressible soil settles under load, the amount of settlement Δ_h_ can be determined using the porosity coefficient before and after settlement.
Let us consider a layer of compressible soil of thickness h (fig. 5_a_). If this layer is loaded, it will settle by a height Δ_h_, so that its thickness due to the load will decrease to h1. Therefore
Δ_h_ = h – h1.
Since settlement of compressible soil takes place at the expense of the pores, because the solid particles are practically incompressible, and also the water, if the soil pores are partially or completely filled with it, this settlement can be schematically represented in fig. 5_b_.
Fig. 5. Settlement Δh of a compressible soil layer of thickness h under load
If we denote by h0 the imaginary height of a layer of solid particles without pores, called the reduced height, then we can express the settlement Δ_h_ in relation to the reduced height h0 by the ratio:
If we consider a soil layer of unit width and length, then the heights h, h1 and h0 represent volumes, and therefore
where e is the porosity coefficient before, e1 after settlement. From the above, settlement is obtained
Δ_h_ = h0 (e-e1).
Settlement can be expressed in relation to the layer height h before settlement if we substitute the reduced height value h0:
h0 = h – nh = h (1-n) = h / (1+e),
since n = VP / V = VP / h_,_ hence VP = nh.
Thus one obtains
Individual soil types have the following average values of porosity n and void ratio e:
Soil moisture
Types of water in soil
Water in soil occurs in the form of water in the pores between solid particles, so-called pore water, then as adsorbed water on the solid particles, and as constitutional water.
Pore water consists of free water between solid particles, whose movement follows Darcy’s law; then gravitational water, which moves under the action of gravity from top to bottom in all directions, but whose movement does not follow Darcy’s law; capillary water, which moves under the action of capillary forces; and water of surface tension, which is held by surface tension in the corners between the solid soil particles and is also called angular pore water. All this water can be completely removed by drying the soil at a temperature of 110oC.
Adsorbed water is the water that surrounds the solid particles to which it is bound by molecular forces. This is the so-called water film, whose thickness ranges from 6-80 µµ (1µµ=10-6 mm, milimicron). The influence of adsorbed water on coarser particles is negligible; however, the finer the particles are, the greater its influence, because the total surface area of the particles increases and thus the ratio of adsorbed water to solid particles per unit volume of soil. This water can be removed only partially by drying at 110oC.
Constitutional water is chemically combined in the crystals of the minerals of the solid soil particles. There is very little of this water, and it cannot be removed by drying the soil. Therefore, this water can be regarded as an integral part of the solid particle.
Water content in the soil
By definition, the amount of water in the soil, or soil moisture, is the ratio of the weight of water contained in the soil to the weight of its solid constituents. According to the amount of water in the soil, three cases are distinguished: soil completely saturated with water, soil partially saturated with water, and completely dry soil.
Soil fully saturated with water
If we denote soil porosity by n, the unit weight of all particles by γs, and the unit weight of water by γw, then the moisture content of saturated soil wZ can be expressed by the following equation
Soil partially saturated with water
If we denote by Ww the weight of the water contained in the soil, and by Wd the weight of the solid particles, i.e. the dry weight of the soil, then the soil moisture w
w = Ww / Wd , or, in percentages of the dry weight of the soil: w = 100 Ww / Wd.
In this case, soil moisture can be expressed by the degree of saturation Sr, which represents the ratio of the actual weight of water in the soil to the weight of water in the same soil that would be present if all pores were filled with water, i.e. to the weight of water in saturated soil:
respectively for _γ_w = 1,00 Sr = w*γs / e.
The limiting values for the degree of moisture are:
- for completely dry soil w = 0; Sr = 0
- for fully saturated soil w = e/γs; Sr = e/γs * γs / e = 1,0.
Fig. 6. Soil sample partially saturated with water
The degree of saturation (sometimes also called saturation or degree of moisture) is also expressed as the ratio of the volume of pores filled with water to the total pore volume (fig. 6):
Sr = V2 / V1
where V2 is the volume of pores filled with water, V1 the total volume of pores filled with water, air, and gases. If w is the soil moisture in % of the dry weight of the soil Wd, then we have
V2 * γW = w Wd, hence V2 = w Wd / γw.
On the other hand, we have that V1 = V – Vd, and since Vd * γs = Wd, it follows that
Vd = Wd / γs and V1 = V – Wd / γs.
Therefore, the degree of soil saturation Sr will be
or as a percentage of the dry weight of the soil
The soil completely dry
In this case w = 0. All pore water has been removed, while adsorbed water has only been partially removed, i.e. the water film still exists, but its thickness has been reduced.