Soil Behavior
This chapter briefly presents the background information on soil mechanics.
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Soil Behavior
1.1
Introduction
This chapter briefly presents the background information on soil mechanics. The earth’s crust comprises both rock and soil, of which the former is a natural aggregate of minerals connected by a strong and permanent cohesive force, and the later is a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water [2]. In soil dynamics/mechanics, rock is often treated as an elastic or rigid medium, while the modeling of soils usually requires more detailed consideration, as will be discussed in this book. As shown in Fig. 1.1, soil can be regarded as a gathering of particles/grains. Most of these particles originate from the degradation of rocks and are referred to as mineral particles. Some, meanwhile, originate from residues of plants or animals (rotting leaves, pieces of bone, etc.), and these are called organic particles or organic matter. There are also spaces/voids between particles, and they contain water and/or air, making the soil a three-phase material. If one digs tens of meters down into soil, one will observe various layers (also called horizons or strata) of soil with different colors and compositions. This succession of soil layers is called the soil profile, as illustrated in Fig. 1.2.
1.2
Soil Classification
Soil can generally be categorized as granular or cohesive. Granular soil consists of gravel, sand, and cohesionless silt. Cohesive soil is normally referred to as clay. These soils can be clearly distinguished by their sizes, as shown in Table 1.1. Granular classification (i.e., cohesive, granular, or mixed) enables engineers to predict the soil’s behavior, which has important implications for the mechanical and erosion properties of sediment [3, 4]. © Springer International Publishing AG 2018 J. Jia, Soil Dynamics and Foundation Modeling, https://doi.org/10.1007/978-3-319-40358-8_1
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1 Soil Behavior
Fig. 1.1 Soil can be realized as a skeleton of solid particles enclosing spaces/voids
B A
Fig. 1.2 Illustration of soil profile
Water level
Table 1.1 Classification of soil type based on particle sizes and shapes Soil type Granular soil
Cohesive soil
Gravel Sand Silt Clay
Particle size (mm)
Particle shape
Distinguishable with naked eye
2–60 0.06–2 0.002–0.06 766
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1 Soil Behavior
The shear failure criterion in terms of total stress parameters can be expressed as: sclay ¼ c þ rn tanð/Þ
ð1:17Þ
where c is the total stress cohesion intercept or cohesion strength; / is the total stress friction angle. For saturated clays, / = 0, the undrained strength is then: sclay ¼ c
ð1:18Þ
For normally consolidated clays that have not been subject to greater compression than that at the existing site, the cohesive part is small. However, for over-consolidated clay that has been subject to increased compression in the past (caused by, for instance, the erosion of higher soil layers), the cohesion may be significant [27]. In practice, rather than measuring the r0n and c0 individually, sclay is directly obtained by un
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