Soil Properties

Definition

Soil in broad terms for foundation engineering is the ground supporting a structure. Soil is considered to be any loose sedimentary deposit, such as gravel, sand, silt, clay or a mixture of these materials.

Granular soils (e.g. sand & gravel)

  • Granular soils (non-cohesive soils) e.g. sand
    Sand and gravel have no shear strength. An apparent cohesion in sand can be noticed when water is present. Sand grains stick together due to negative pore pressure (building sandcastles is an example). Sand stand in slopes when wet but will not stand when dry or saturated.
  • Strength, bearing capacity and slope stability all derived from internal friction (Phi). for granular soils (sand & gravel) range from 30° to 45°. increases due to grading, packing density and grain angularity.
  • Courser grained soils are more permeable to water and, unless saturated, may have very little water in their voids.
  • If well consolidated and confined, they form a foundation that is almost as stable as rock.
  • If loosely consolidated or with high percentage of organic matter, the site must be classified as a problem site.

Cohesive soils (e.g. silts and clays)

  • Cohesive soils have shear strength. It is possible to make a vertical cut in silts and clays and it remain standing, unsupported, for some time. This cannot be done in dry sand. In clay and silts, therefore, some other factor must contribute to shear strength. This factor is called cohesion. It results from the mutual attraction, which exist between fine particles and tends to hold them together in a solid mass without the application of external forces.
  • Clay consist of very fine microscopic particles which hold water to increase their volume, and release moisture to decrease their volume.
  • Special precaution needs to be taken in the design of footings to resist or avoid the forces caused by shrinking and swelling.

Soil consists of a mass of solids particles separated by spaces or voids. The voids of a soil are usually a mixture of air and water (soil is partially saturated). In certain circumstances the voids may be completely filled with water or air only. If only air is present the soil is dry and if only water is present the soil saturated. The Figure1 shows a cross-section through a granular soil.

(a)(b)
Figure 1   Cross-section through granular soil

In order to study the properties we will adopt the idealised form of the diagram shown in Figure 1. The soil mass has a total volume V, containing a volume of solid particles Vs and a volume of voids Vv. In most cases the void volume exists of a mixture of water (Vw) and air (Va).
The designation for the weight is W and the weight of water is Ww and the weight of air is zero as shown on the right side in Figure 1 (b).
Figure 2 shows the three cases in diagrammatic form
      (a) soil & air
      (b) soil with water and
      (c) soil with water & air
Figure 2 Water and air contents in a soil


Void ratio (e)

The volume of voids, Vv, is obviously equal to V - Vs.

(Vv (volume of voids = volume not occupied by solids.)
For ease of calculating we will assume that all solids are compressed together and their volume considered equal to a unit volume as shown in Figure 1 and Figure 2.

Porosity (n)



usually Vv = Va + Vw

Moisture content (m)

The specific gravity of solids is Gs and the density of water is w
Mass of solids, M = Gs w
Mass of water, Mw = Sr e w
The quantities in the above formula are used in the opposite equation.


Degree of saturation (Sr)

When both air and water are present the soil is said to be partially saturated. The degree of saturation is simply

Air-void ratio

Percentage air voids
      Also    
The ratio of the volume of air to the total volume of soil multiplied by 100 is known as the percentage air voids.

 

Density () and unit weight ()

We can express the amount of material in a given volume , V, in two ways:
                   1.   the amount of mass, M, in the volume, or
                   2.   the amount of weight, W, in the volume.



The more common way is to use the amount of mass instead of the amount of weight. Using the weight necessitates in a conversion as shown below:

The weight is:    weight = mass × gravitational acceleration
[Gravitational acceleration = 9.81 m/s (for ease use 10 m/s)]
The unit for the force or weight is the newton (N) (1 N = 1kg × 1 m/s)
the weight of 1 kg on earth = 10 N
(Density of water at 4C, w = 1000 kg/m³ = 1 Mg/m³)

Specific gravity (Gs)

The specific gravity of a material is the ratio of the weight or the mass of a volume of the material to the weight or mass of an equal volume of water.


Bulk density()

Dry density(d)


Void ratio (e)
The volume of voids, Vv, is obviously equal to V - Vs.

For ease of calculating we will assume that all solids are compressed together and their volume considered equal to a unit volume as shown in Figure 1 and Figure 2.

Porosity (n)

Degree of saturation (Sr)
When both air and water are present the soil is said to be partially saturated. The degree of saturation is simply

Percentage air voids

The ratio of the volume of air to the total volume of soil is known as the percentage air voids.

Density () and unit weight ()
We can express the amount of material in a given volume , v, in two ways:
the amount of mass, M, in the volume, or
the amount of weight, W, in the volume, or

The weight is:
                        weight = mass &times gravitational acceleration
[Gravitational acceleration = 9.81 m/s (for ease use 10 m/s)]
The unit for the force or weight is the newton (N) (1N=1kg×1m/s)
        the weight of 1 kg (mass) = 10 N
Density of water at 4C, w = 1000 kg/m³ = 1 Mg/m³
hence the weight of water, w = 10,000 N/m³ = 10 kN/m³. (1 Mg/m³ = 1 Tonne/m³)
(Where w = unit weight of water at 4C)

Specific gravity (Gs)
The specific gravity of a material is the ratio of the weight or the mass of a volume of the material to the weight or mass of an equal volume of water.


Unit weight of soil
The unit weight of a material is its weight per unit volume. In soil work the most important unit weights are as follows:
Bulk unit weight (b)
This is the natural in-situ unit weight of a soil

Dry unit weight (d)