Introduction
Soil mechanics is a relatively new discipline in civil engineering, which entails the study of the engineering properties of soil relative to the design of various engineering structures that are constructed in or from the earth. Here, civil engineering structures include embankments, earth retaining walls, sub-surface water repositories, dams, and basements among others (Powrie, 2004, p. 1; Mitchell & Soga, 2005). Studies note that the behavior of soils can be examined in reference to soil permeability, soil consolidation, and the shear strength, which entail major soil concepts elaborated in this article.
Soil permeability
Soil permeability refers to the ability of water to move through the soil particles in either one or two dimensions. As a result, at any one time, soil can be regarded to as being partially saturated, saturated or dry. Moreover, saturated soil is said to consist of two phases, that is, the soil particles and pore water. Therefore, the influence of the two phases on various civil engineering procedures warrants the determination of various aspects of soil permeability.
Here, studies note that the movement of ground water through the pores between interconnected soil particles is caused by the hydraulic gradient (i). Furthermore, the rate of ground water movement (q) through the soil pores is measured by the coefficient of permeability (k), which is approximately equivalent to the size of soil (D10). Accordingly, in practice, Darcy’s law states that, the flow rate of groundwater (q) through a soil sample with a cross-sectional area (A) will be given by; q=Aki (Powrie, 2004, p. 121; Aysen, 2005, p. 67).
Conversely, the coefficient of permeability has been determined through experimental procedures in the laboratory and the field, and thus, studies note that k ranges from 1m/s for clean gravels to below 10-9 m/s for clay soils. Here, it is worth noting that the value, k is influenced by the particle size, particle shape, particle orientation, viscosity of water, and the general soil structure. Accordingly, in most soil mechanics applications, the laboratory measurement of k involves different methods including constant head method (using a flexible wall permeameter) and the permeability test (using the Rowe type consolidation cell). On the other hand, in-situ permeability tests include the pumping test (involving an unconfined aquifer), Auger hole test, Piezometer test, Pneumatic displacement test, and the Tracer test (Aysen, 2005, p. 76). Overall, considering that laboratory tests involve a small soil sample, it is practical to use both the laboratory and in-situ tests to come up with an average coefficient of permeability for different projects.
Soil consolidation
In simple terms soil consolidation entails removing water from the soil particles. Considering one dimensional consolidation, the consolidation settlement involving fully saturated soils such as clay results from the gradual reduction of the soil volume as water is drained through the soil voids. Here, due to external pressure on the soil sample, water is forced to move out of the pores as the pore pressure increases. As a result, an increase in pore pressure (excess pore pressure) can vary relative to time and position within the soil sample. Initially, the external pressure (load) can be resisted by the pore water, and thus, in case a drainage condition is present, the external pressure disperses, and the soil mass remains saturated. Thus, the resultant decrease in soil volume is roughly equivalent to the amount of water drained. Conversely, an increment in effective stress on the soil mass is equivalent to a decrease in pore pressure (Powrie, 2004; Aysen, 2005, p. 210).
Therefore, in a typical time-settlement curve, the effect of a vertical load on a saturated soil mass is represented by three segments; the elastic settlement, the primary consolidation, and the secondary consolidation. Here, the compression index is given by;
Cc=e0-e1/logδ’0 –logδ’1=e0 – e1/log (δ’1/δ’0) whereby 0=the initial value of the void ratio and vertical stress, and 1=the final value of the void ratio and vertical stress (Aysen, 2005, p. 213). In most soil mechanics applications, the oedometer is used in a one-dimensional consolidation test in which case studies note that the value of Cc for fully saturated clays ranges from 0.1-0.5 relative to the plastic characteristics of the soil mass. Moreover, the consolidation coefficient increases as the soil plasticity increases (Aysen, 2005).
The shear strength of soils
Apparently, the shear strength of soils entails the ability of a soil mass/structure to resist collapse/failure as a result of shear stress. In most soil mechanics applications, soil structure can collapse due to tension or shear, but most engineers tend to overlook the failure due to tension. As a result, in failures related to shear, the strength of soil regarding any particular plane is measured using two major parameters, that is, cohesion and the degree of internal friction. Here, studies note that in case the shear stress equals the highest shear strength of soil along a specified plane, then it follows that the soil structure fails/collapses resulting into huge shear strains (Aysen, 2005, p. 109). Consequently, the side of the plane referred to as the failure plane tends to slide against the intact plane, thereby collapsing the entire soil structure.
Accordingly, a failure criterion, which shows the link between the highest shear stress and the peak shear strength of soils, is determined using the Mohr-Coulomb’s circle of stress. For instance, consider that the soil mass is exposed to two-dimensional stresses. Here, the typical and shear strains (stresses) on a specified plane (P) inclined at an angle α relative to the x-axis are determined using the equations given below, whereby s= the center of Mohr’s circle of stress, and t= the radius of the circle;
s = δ1 + δ3 /2 = δz + δx /2, and t= √ [(δz – δx /2)2 +t2xz] (Aysen, 2005, p. 110). Therefore, accurate determination of the failure criterion is critical to a geotechnical engineer in that it complements other aspects of solid mechanics particularly in case of stability problems to calculate the likelihood that a particular load will cause the failure of a soil structure (Mitchell & Soga, 2005). Overall, soil permeability, soil consolidation, and the shear strengths of soil entail critical parameters in civil engineering, which require adequate consideration before embarking on any construction project.
Reference list
Aysen, A. (2005). Soil Mechanics: Basic concepts and engineering applications. London: Taylor & Francis Group plc, UK.
Mitchell, J.K. & Soga, K. (2005). Fundamentals of soil behavior (3rd ed.). New York: John Wiley and Sons, Inc.
Powrie, W. (2004). Soil mechanics: Concepts & applications (2nd ed.). New York: Taylor & Francis Group.