The Effect of Moisture Content and Particle Size on Direct Shear and Oedometer Tests
The general term “soil” is usually defined as the product of the weathering of the Earth’s crust. Different professions will consider those properties of soil that are most relevant to them. Where construction is concerned, the engineering properties of soil are paramount.
These properties include moisture content, specific gravity, porosity, particle size, consistency limits, etc. They are based on the concept of the three-phase system of soil – the fact that soil consists both of solid particles and of pores that are filled with either water or air.
Types of soil
The particle size distribution of soil describes the percentage of soil particles in different size ranges within a certain soil mass. According to the USCS classification, soil is divided into two basic groups: coarse-grained (incoherent) and fine-grained (coherent). Fine-grained soils show a greater intensity of attraction between particles. This is because there are more particles per unit of volume, and therefore more points of contact. By contrast, the distance between particles in coarse-grained soils is the cause of their lower coherence.
Incoherent soils have no consistency, while coherent soils (silt and clay) will differ in physical properties depending on their water content. The different consistency limits are the liquid limit, plastic limit and shrinkage limit. The plasticity index represents the difference between the liquid and plastic limits, while the consistency index is the difference between the liquid limit and the moisture content of the soil, divided by the plasticity index.
Soil settlement is the downward vertical movement of the Earth’s surface caused both by natural processes and human activity. Stress in soil is mostly the result of the soil’s weight, as well as additional loads from buildings. Deformations that occur depend on the type of soil and its properties. Considering the previously mentioned three-phase system of soil, we can differentiate the stress taken on by solid particles from the stress taken on by water, which is called pore pressure. Settlement in incoherent materials occurs simultaneously with the application of the load. Coherent soils have lower permeability, so the pore pressure that results from load can dissipate slowly, and in turn, stress increase and settlement also happen slowly.
It has been established that soil strength – one of the most important mechanical properties of soil used in construction work – depends on effective stress, cohesion, friction between particles, the breakage of individual grains and many other factors. Different types of soil have different soil strengths, depending on their classification as well as their physical and mechanical properties.
Shear strength is the maximum amount of shear stress that can be applied to the soil in a certain direction. In a direct shear test, the soil is first loaded vertically, and after complete consolidation, shearing is initiated. When peak shear stress is reached, accompanied by plastic deformations, the soil experiences failure.
By testing the shear strength, it’s possible to determine the cohesion and angle of the internal friction of soil. Cohesion is not a constant soil parameter; its value depends on moisture content. At a higher moisture content, the strength of electrochemical driving forces is lower, which contributes to lower cohesion in the soil. Incoherent soils have a higher value of the angle of internal friction than coherent soils. The angle of internal friction in coherent soils decreases with increasing moisture content and plasticity index, as well as with decreasing particle size.
The shear rate must be low enough to allow complete drainage of the sample. Negative pore pressure increases the shear strength in the shear zone, which often results in higher cohesion. Incoherent soils usually have a higher shear rate, while coherent soil shear rates are lower.
The deformability, or the compressibility of soil, is a parameter that is extremely important in obtaining a complete overview of the physical and mechanical properties of soil and is needed when laying foundations. Soil deformability is measured by applying a vertical load on the soil sample, while lateral deformation is prevented. The deformability properties of soil differ depending on its coherence, compaction and state of consistency – soft samples show significantly lower resistance and have higher compressibility than hard samples. Also, the primary consolidation of a hard sample will be quicker than that of a soft sample, and its secondary consolidation will be less pronounced.
The bulk modulus of soil is not a constant but changes as the load does – the modulus can only be determined for an interval. The higher the value of the modulus, the lower the compressibility of the soil and vice versa. Measurements show that a very small portion of soil deformation is related to elastic deformation of solid particles or water. A large portion of the deformation is caused by the movement of solid particles, which is an irreversible process. In incoherent soils, the process of pore pressure change is not complete, and significant settlement can be expected even without additional loads. Therefore, the particle size distribution of the soil will significantly determine some of its most important mechanical properties and physical parameters, and indirectly condition the type and complexity of the engineering solution required in construction work.
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Read more: Grain size analysis