Soil Settlement and Consolidation

Vocabulary and key concepts covering soil st compressibility, settlement, 1D consolidation, oedometer testing, Terzaghi's theory, and associated engineering parameters.

Main problems of soil mechanics

A division into problems of settlement (stress-strain and strain-time relationships) and problems of stability (shear strength).

Young's modulus of elasticity ($E$)

A measure of the soil's elastic behavior, used in the relationship $\triangle l = \frac{\sigma}{E} \times l$ where $\triangle l$ is deformation and $\sigma$ is normal stress.

Boussinesq distribution

An assumption used to calculate soil settlement ($w$) at the surface based on the distribution of stress within the soil mass.

Effective stress principle

The relationship defined as $\sigma' = \sigma - u_w$, where $\sigma'$ is effective stress, $\sigma$ is total stress, and $u_w$ is pore water pressure.

Oedometer (Edometar)

An apparatus used to determine soil compressibility by measuring height changes ($h$) in a cylindrical sample under incremental loading ($P$).

Porous ceramic stone (PKK)

A component in the oedometer test used to facilitate water drainage from the soil sample during consolidation.

Coefficient of compressibility ($a_v$)

The ratio of the change in void ratio to the change in pressure, calculated as $a_{vi} = \frac{\Delta e_i}{\Delta p_i}$.

Modulus of compressibility ($M_v$)

Also known as the oedometer modulus ($E'_{oed}$), it is defined as $M_{vi} = \frac{1 + e_{i-1}}{a_{vi}}$ or $M_{vi} = \frac{1}{m_{vi}}$.

Coefficient of volume change ($m_v$)

A measure of soil compressibility defined as $m_{vi} = \frac{a_{vi}}{1 + e_{i - 1}}$.

Pre-consolidation stress ($\sigma'_p$)

The maximum effective vertical stress that a soil has experienced in its geological history, often estimated using the Casagrande method (1936).

Overconsolidation Ratio (OCR)

The ratio of maximum historical effective stress to the current in-situ effective vertical stress: $OCR = \frac{\sigma'_{max}}{\sigma'_{v0}}$.

Normally consolidated soil

Soil that is currently experiencing its maximum historical effective stress, indicated by an $OCR = 1$.

Overconsolidated soil

Soil that has been subjected to higher effective stresses in the past than it is currently experiencing, indicated by an $OCR > 1$.

Compression index ($C_c$)

The slope of the linear part of the void ratio ($e$) vs. $\log(\sigma')$ curve during loading, calculated as $C_c = \frac{e_A - e_B}{\log(\frac{\sigma'_B}{\sigma'_A})}$.

Recompression index ($C_r$)

Also known as the swelling index, it represents the slope of the void ratio vs. $\log(\sigma')$ curve during unloading and reloading.

Primary consolidation

The phase of consolidation characterized by the dissipation of excess pore water pressure ($u_w$) from the soil.

Secondary consolidation

Also known as creep or flow, this phase occurs after primary consolidation is complete while effective stress remains constant ($\sigma' = \text{const.}$) and $\Delta u_w = 0$.

Terzaghi's 1D consolidation theory (1943)

A theory assuming soil is homogeneous, fully saturated, and that water flow and compression are strictly one-dimensional (vertical).

Coefficient of consolidation ($c_v$)

A parameter representing the rate of consolidation, defined as $c_v = \frac{k}{\gamma_w \times m_v}$ where $k$ is permeability and $\gamma_w$ is the unit weight of water.

Time factor ($T_v$)

A dimensionless factor used to track the progress of consolidation over time, calculated as $T_v = \frac{c_v \times t}{d^2}$.

Drainage path ($d$)

The longest distance water must travel to exit a soil layer; $d = \frac{H}{2}$ for double-sided drainage and $d = H$ for single-sided drainage.

Degree of consolidation ($U_v$)

The ratio of dissipated pore water pressure to the initial pore water pressure, ranging from $0$ (start) to $1$ (end of primary consolidation).

Isochrones

Curves representing the distribution of pore water pressure ($u$) with depth ($z$) at specific points in time ($t$) during the consolidation process.

Total settlement ($\Delta h$)

The final volume change of a soil layer under a pressure increment, calculated as $\Delta h = m_v \times \Delta p \times h$.