Soil consolidation
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Soil consolidation refers to the
mechanical process Mechanics (from Ancient Greek: wikt:μηχανική#Ancient_Greek, μηχανική, ''mēkhanikḗ'', "of machine, machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among Ph ...
by which
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
changes volume gradually in response to a change in
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
. When soil saturated with water is subjected to an increase in pressure, the high volumetric stiffness of water compared to the soil matrix means that the water initially absorbs all the change in pressure without changing volume, creating excess
pore water pressure Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles ( pores). Pore water pressures below the phreatic level of the groundwater are measured with piezometer ...
. As water diffuses away from regions of high pressure due to
seepage Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and wat ...
, the soil matrix gradually takes up the pressure change and shrinks in volume. The theoretical framework of consolidation is therefore closely related to the
diffusion equation The diffusion equation is a parabolic partial differential equation. In physics, it describes the macroscopic behavior of many micro-particles in Brownian motion, resulting from the random movements and collisions of the particles (see Fick's la ...
, the concept of
effective stress The effective stress can be defined as the stress, depending on the applied tension \boldsymbol_ and pore pressure p, which controls the strain or strength behaviour of soil and rock (or a generic porous body) for whatever pore pressure value or, ...
, and
hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on th ...
. In the narrow sense, "consolidation" refers strictly to this delayed volumetric response to pressure change due to gradual movement of water. Some publications also use "consolidation" in the broad sense, to refer to any process by which soil changes volume due to a change in applied pressure. This broader definition encompasses the overall concept of soil compaction,
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
, and heave. Some types of soil, mainly those rich in
organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
, show significant creep, whereby the soil changes volume slowly at constant effective stress over a longer time-scale than consolidation due to the diffusion of water. To distinguish between the two mechanisms, "primary consolidation" refers to consolidation due to dissipation of excess water pressure, while "secondary consolidation" refers to the creep process. The effects of consolidation are most conspicuous where a
building A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fun ...
sits over a layer of soil with low
stiffness Stiffness is the extent to which an object resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is. Calculations The stiffness, k, of a b ...
and low permeability, such as
marine clay Marine clay is a type of clay found in coastal regions around the world. In the northern, deglaciated regions, it can sometimes be quick clay, which is notorious for being involved in landslides. Marine clay is a particle of soil that is dedica ...
, leading to large
settlement Settlement may refer to: *Human settlement, a community where people live *Settlement (structural), the distortion or disruption of parts of a building * Closing (real estate), the final step in executing a real estate transaction *Settlement (fin ...
over many years. Types of construction project where consolidation often poses technical risk include
land reclamation Land reclamation, usually known as reclamation, and also known as land fill (not to be confused with a waste landfill), is the process of creating new land from oceans, seas, riverbeds or lake beds. The land reclaimed is known as reclamati ...
, the construction of embankments, and
tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...
and
basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...
excavation in clay.
Geotechnical engineer Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It al ...
s use
oedometer An oedometer test is a kind of geotechnical investigation performed in geotechnical engineering that measures a soil's Consolidation (soil), consolidation properties. Oedometer tests are performed by applying different Structural load, loads to a s ...
s to quantify the effects of consolidation. In an oedometer test, a series of known pressures are applied to a thin disc of soil sample, and the change of sample thickness with time is recorded. This allows the consolidation characteristics of the soil to be quantified in terms of the coefficient of consolidation (C_) and hydraulic conductivity (K). Clays undergo consolidation settlement not only the action action of ''external'' loads (surcharge loads) but also under its own weight or weight of soils that exist above the clay. Clays also undergo settlement when dewatered (groundwater pumping) because the effective stress on the clay increases. Coarse-grained soils do not undergo consolidation settlement due to relativity high hydraulic conductivity compared to clays. Instead, Coarse-grained soils undergo the immediate settlement.


History and terminology

According to the "father of
soil mechanics Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and wat ...
",
Karl von Terzaghi Karl von Terzaghi (October 2, 1883 – October 25, 1963) was an Austrian mechanical engineer, geotechnical engineer, and geologist known as the "father of soil mechanics and geotechnical engineering". Early life In 1883, he was born the first c ...
, consolidation is "any process which involves a decrease in water content of saturated soil without replacement of water by air". More generally, consolidation refers to the process by which
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
s change volume in response to a change in
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
, encompassing both compaction and swelling.


Magnitude of volume change

Consolidation is the process in which reduction in volume takes place by the gradual expulsion or absorption of water under long-term static loads. When
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
is applied to a soil, it causes the soil particles to pack together more tightly. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil. The magnitude of consolidation can be predicted by many different methods. In the classical method developed by Terzaghi, soils are tested with an
oedometer test An oedometer test is a kind of geotechnical investigation performed in geotechnical engineering that measures a soil's consolidation properties. Oedometer tests are performed by applying different loads to a soil sample and measuring the deforma ...
to determine their compressibility. In most theoretical formulations, a logarithmic relationship is assumed between the volume of the soil sample and the effective stress carried by the soil particles. The constant of proportionality (change in void ratio per order of magnitude change in effective stress) is known as the compression index, given the symbol \lambda when calculated in natural logarithm and C_C when calculated in base-10 logarithm. This can be expressed in the following equation, which is used to estimate the volume change of a soil layer: \delta_c = \frac H \log \left( \frac \right) \ where :δc is the settlement due to consolidation. :Cc is the compression index. :e0 is the initial
void ratio The void ratio of a mixture is the ratio of the volume of voids to volume of solids. It is a dimensionless quantity in materials science, and is closely related to porosity as follows: :e = \frac = \frac = \frac and :\phi = \frac = \frac = \f ...
. :H is the height of the compressible soil. :σzf is the final vertical stress. :σz0 is the initial vertical stress. When stress is removed from a consolidated soil, the soil will rebound, regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will consolidate again along a recompression curve, defined by the recompression index. The gradient of the swelling and recompression lines on a plot of void ratio against the logarithm of effective stress often idealised to take the same value, known as the "swelling index" (given the symbol \kappa when calculated in natural logarithm and C_S when calculated in base-10 logarithm). Cc can be replaced by Cr (the recompression index) for use in overconsolidated soils where the final effective stress is less than the preconsolidation stress. When the final effective stress is greater than the preconsolidation stress, the two equations must be used in combination to model both the recompression portion and the virgin compression portion of the consolidation processes, as follows, \delta_c = \frac H \log \left( \frac \right) + \frac H \log \left( \frac \right)\ where σzc is the preconsolidation stress of the soil. This method assumes consolidation occurs in only one-dimension. Laboratory data is used to construct a plot of
strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...
or
void ratio The void ratio of a mixture is the ratio of the volume of voids to volume of solids. It is a dimensionless quantity in materials science, and is closely related to porosity as follows: :e = \frac = \frac = \frac and :\phi = \frac = \frac = \f ...
versus
effective stress The effective stress can be defined as the stress, depending on the applied tension \boldsymbol_ and pore pressure p, which controls the strain or strength behaviour of soil and rock (or a generic porous body) for whatever pore pressure value or, ...
where the effective stress axis is on a
logarithmic scale A logarithmic scale (or log scale) is a way of displaying numerical data over a very wide range of values in a compact way—typically the largest numbers in the data are hundreds or even thousands of times larger than the smallest numbers. Such a ...
. The plot's slope is the compression index or recompression index. The equation for consolidation settlement of a normally consolidated soil can then be determined to be: The soil which had its load removed is considered to be "overconsolidated". This is the case for soils that have previously had
glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
on them. The highest stress that it has been subjected to is termed the " preconsolidation stress". The "over-consolidation ratio" (OCR) is defined as the highest stress experienced divided by the current stress. A soil that is currently experiencing its highest stress is said to be "normally consolidated" and has an OCR of one. A soil could be considered "underconsolidated" or "unconsolidated" immediately after a new load is applied but before the excess
pore water pressure Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles ( pores). Pore water pressures below the phreatic level of the groundwater are measured with piezometer ...
has dissipated. Occasionally, soil strata form by natural deposition in rivers and seas may exist in an exceptionally low density that is impossible to achieve in an oedometer; this process is known as "intrinsic consolidation".


Time dependency


Spring analogy

The process of consolidation is often explained with an idealized system composed of a
spring Spring(s) may refer to: Common uses * Spring (season) Spring, also known as springtime, is one of the four temperate seasons, succeeding winter and preceding summer. There are various technical definitions of spring, but local usage of ...
, a container with a hole in its cover, and water. In this system, the spring represents the compressibility or the structure of the soil itself, and the water which fills the container represents the pore water in the soil. # The container is completely filled with water, and the hole is closed. (Fully saturated soil) # A load is applied onto the cover, while the hole is still unopened. At this stage, only the water resists the applied load. (Development of excess pore water pressure) # As soon as the hole is opened, water starts to drain out through the hole and the spring shortens. (Drainage of excess pore water pressure) # After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excess pore water pressure. End of consolidation)


Analytical formulation of consolidation rate

The time for consolidation to occur can be predicted. Sometimes consolidation can take years. This is especially true in saturated clays because their
hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on th ...
is extremely low, and this causes the water to take an exceptionally long time to drain out of the soil. While drainage is occurring, the
pore water pressure Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles ( pores). Pore water pressures below the phreatic level of the groundwater are measured with piezometer ...
is greater than normal because it is carrying part of the applied stress (as opposed to the soil particles). T_v=\frac \ Where Tv is the time factor. Hdr is the average longest drain path during consolidation. t is the time at measurement Cv is defined as the coefficient of consolidation found using the log method with C_v=\frac or the root method with C_v=\frac t50 time to 50% deformation (consolidation) and t95 is 95% Where T95=1.129 T50=0.197


Creep

The theoretical formulation above assumes that time-dependent volume change of a soil unit only depends on changes in effective stress due to the gradual restoration of steady-state pore water pressure. This is the case for most types of
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
and clay with low amounts of organic material. However, in soils with a high amount of organic material such as
peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficien ...
, the phenomenon of creep also occurs, whereby the soil changes volume gradually at constant effective stress. Soil creep is typically caused by viscous behavior of the clay-water system and compression of organic matter. This process of creep is sometimes known as "secondary consolidation" or "secondary compression" because it also involves gradual change of soil volume in response to an application of load; the designation "secondary" distinguishes it from "primary consolidation", which refers to volume change due to dissipation of excess pore water pressure. Creep typically takes place over a longer time-scale than (primary) consolidation, such that even after the restoration of hydrostatic pressure some compression of soil takes place at slow rate. Analytically, the rate of creep is assumed to decay exponentially with time since application of load, giving the formula: S_s=\frac C_ \log \left( \frac \right) \ Where H0 is the height of the consolidating medium
e0 is the initial void ratio
Ca is the secondary compression index
t is the length of time after consolidation considered
t95 is the length of time for achieving 95% consolidation


Deformation characteristics of consolidation

Coefficient of compressibility a_v = \frac. The compressibility of saturated specimens of clay minerals increases in the order kaolinite Cornell University, 1950
The more compressible the clay, the more pronounced the influences of cation type and electrolyte concentration on compressibility. Coefficient of volume compressibility m_v = \frac=\frac


See also

*
Compaction (geology) In sedimentology, compaction is the process by which a sediment progressively loses its porosity due to the effects of pressure from loading. This forms part of the process of lithification. When a layer of sediment is originally deposited, it con ...
*
Settlement (structural) Settlement in a structure refers to the distortion or disruption of parts of a building due to * unequal compression of its foundations; * shrinkage, such as that which occurs in timber-framed buildings as the frame adjusts its moisture content; ...
*
Soil mechanics Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and wat ...
* Vacuum consolidation


References


Bibliography

* * * {{DEFAULTSORT:Consolidation (Soil) Soil mechanics Sedimentology