Soil consolidation

Soil consolidation refers to the mechanical process by which soil changes volume gradually in response to a change in pressure. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually groundwater. When soil saturated with water is subject 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. As water diffuses away from regions of high pressure due to seepage, 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 concept of effective stress, and hydraulic conductivity.

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, and heave. Some types of soil, mainly those rich in organic matter, 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 sits over a layer of soil with low stiffness and low permeability, such as marine clay, leading to large settlement over many years. Types of construction project where consolidation often poses technical risk include land reclamation, the construction of embankments, and tunnel and basement excavation in clay.

Geotechnical engineers use oedometers 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 () and hydraulic conductivity ().

Oedometer
Two oedometers at the University of Cambridge

History and terminology

According to the "father of soil mechanics", Karl von Terzaghi, 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 soils change volume in response to a change in pressure, encompassing both compaction and swelling.[1]

Mechanism

Consol curve plain
The experimentally determined consolidation curve (blue dots) for a saturated clay showing a procedure for computing the preconsolidation stress.

Consolidation is the process in which reduction in volume takes place by expulsion of water under long-term static loads.

When stress is applied to a soil that 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 to determine their compression index. This can be used to predict the amount of consolidation.

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 soil which had its load removed is considered to be "overconsolidated". This is the case for soils that have previously had glaciers 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" immediately after a new load is applied but before the excess pore water pressure has dissipated.

Consolidation analysis

Spring analogy

The process of consolidation is often explained with an idealized system composed of a spring, 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.

Consolidation spring analogy
  1. The container is completely filled with water, and the hole is closed. (Fully saturated soil)
  2. 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)
  3. 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)
  4. 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)

Primary consolidation

This method assumes consolidation occurs in only one-dimension. Laboratory data is used to construct a plot of strain or void ratio versus effective stress where the effective stress axis is on a logarithmic scale. 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:

where

δc is the settlement due to consolidation.
Cc is the compression index.
e0 is the initial void ratio.
H is the height of the compressible soil.
σzf is the final vertical stress.
σz0 is the initial vertical stress.

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,

where σzc is the preconsolidation stress of the soil.

Secondary consolidation

Secondary compression is the compression of soil that takes place after primary consolidation. Even after the reduction of hydrostatic pressure some compression of soil takes place at slow rate. This is known as secondary compression. Secondary compression is caused by creep, viscous behavior of the clay-water system, compression of organic matter, and other processes. In sand, settlement caused by secondary compression is negligible, but in peat, a soil with very high organic content, it is very significant. Due to transfer of stresses to points of contact of soil grains some of the highly viscous water between the points of contact is forced out.

Secondary compression is given by the formula

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

Time dependency

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 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 is greater than normal because it is carrying part of the applied stress (as opposed to the soil particles).

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

or the root method with

t50 time to 50% deformation (consolidation) and t95 is 95%

Where T95=1.129 T50=0.197

See also

References

  1. ^ Schofield, Andrew Noel; Wroth, Peter (1968). Critical State Soil Mechanics. McGraw-Hill.

Bibliography

  • Coduto, Donald (2001), Foundation Design, Prentice-Hall, ISBN 0-13-589706-8
  • Kim, Myung-mo (2000), Soil Mechanics (in Korean) (4 ed.), Seoul: Munundang, ISBN 89-7393-053-2
  • Terzaghi, Karl (1943), Theoretical soil mechanics, John Wiley&Sons, Inc., p. 265
Donald Cousens Parkway

Donald Cousens Parkway or York Regional Road 48, also referred to historically as the Markham Bypass or Markham Bypass Extension, is a regionally maintained arterial bypass of Markham in the Canadian province of Ontario. Named for former Markham mayor Don Cousens in April 2007, the route initially travelled northward from Copper Creek Drive in Box Grove, south of Highway 407, to Major Mackenzie Drive (York Regional Road 25). A southern extension to Steeles Avenue was later completed and the name Donald Cousens Parkway applied along the extension to Ninth Line. In addition to its role of funneling through-traffic around downtown Markham, the route serves as a boundary to residential development as land to the north and east are part of the protected Rouge National Urban Park and southwest limits of the planned Pickering Airport.

Construction of the route began in 2002 north of 16th Avenue. In 2004, an interchange with Highway 407 was constructed along with a connection north to Highway 7. Both segments and the interchange were opened by December of that year. The following year, construction began to connect these two segments as well as on the Box Grove Bypass along Ninth Line; the former opened in October 2006 and the latter in the spring of 2007. Construction of the most recently opened segment, connecting the Box Grove Bypass to the interchange with Highway 407, began in 2009. It opened after several delays in 2012 and included a realignment of 14th Avenue.

Donald Cousens Parkway and a planned connection with Morningside Avenue in Toronto form an "East Metro Transportation Corridor", originally envisioned by the province in the 1970s as a six lane municipal expressway. During the mid-1990s, the Ministry of Transportation of Ontario (MTO) conducted studies and identified the need for the corridor by 2011. Although York Region had intended for a continuous alignment, the City of Toronto government opposed the direct connection between Morningside and Donald Cousens Parkway. As a result, it is now proposed to connect Morningside Avenue and Donald Cousens Parkway via a widened Steeles Avenue. However, no timeline has been announced as of 2015.

Irrigation

Irrigation is the application of controlled amounts of water to plants at needed intervals. Irrigation helps to grow agricultural crops, maintain landscapes, and revegetate disturbed soils in dry areas and during periods of less than average rainfall. Irrigation also has other uses in crop production, including frost protection, suppressing weed growth in grain fields and preventing soil consolidation. In contrast, agriculture that relies only on direct rainfall is referred to as rain-fed or dry land farming.

Irrigation systems are also used for cooling livestock, dust suppression, disposal of sewage, and in mining. Irrigation is often studied together with drainage, which is the removal of surface and sub-surface water from a given area.

Irrigation has been a central feature of agriculture for over 5,000 years and is the product of many cultures. Historically, it was the basis for economies and societies across the globe, from Asia to the Southwestern United States.

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 deformation response. The results from these tests are used to predict how a soil in the field will deform in response to a change in effective stress.

Oedometer tests are designed to simulate the one-dimensional deformation and drainage conditions that soils experience in the field. The soil sample in an oedometer test is typically a circular disc of diameter-to-height ratio of about 3:1. The sample is held in a rigid confining ring, which prevents lateral displacement of the soil sample, but allows the sample to swell or compress vertically in response to changes in applied load. Known vertical stresses are applied to the top and bottom faces of the sample, typically using free weights and a lever arm. The applied vertical stress is varied and the change of the thickness of the sample is measured.

For samples that are saturated with water, porous stones are placed on the top and bottom of the sample to allow drainage in the vertical direction, and the entire sample is submerged in water to prevent drying. Saturated soil samples exhibit the phenomenon of consolidation, whereby the soil's volume changes gradually to give a delayed response to the change in applied confining stresses. This typically takes minutes or hours to complete in an oedometer and the change of sample thickness with time is recorded, providing measurements of the coefficient of consolidation and the permeability of the soil.

Offshore embedded anchors

Offshore embedded anchors are anchors that derive their holding capacity from the frictional, or bearing, resistance of the surrounding soil, as opposed to gravity anchors, which derive their holding capacity largely from their weight. As offshore developments move into deeper waters, gravity-based structures become less economical due to the large size needed and the consequent cost of transportation.

Each of several embedded-anchor types presents its own advantages for anchoring offshore structures. The choice of anchoring solution depends on multiple factors, such as the type of offshore facility that requires mooring, its location, economic viability, the lifetime of its use, soil conditions, and resources available.

Examples of facilities that may need mooring are floating production storage and offloading (FPSO) units, mobile offshore drilling units, offshore oil production platforms, wave power and other renewable energy converters, and floating liquefied natural gas facilities.

San Pietro Caveoso

San Pietro Caveoso, also known as "Saint Peter and Saint Paul Church" is a Catholic worship place situated in the Sassi of Matera.

The front is in baroque style and presents three portals. Over each portal there is a niche with statues. They show the "Madonna of the mercy", "Saint Peter" (over the left portal) and "Saint Paul" (over the right portal). The side niches are surmounted by two rectangular windows and the central one by two single-lancet windows. There is a rose window and a bell tower with a pyramidal cusp on it.

The central nave ceiling is adorned with pictures of "Jesus and Saint Peter" and "Saint Paul's conversion". The 18th century altar has a wooden polyptych dating back to 1540, painted by an anonymous artist from Matera. The church originally had eight chapels, but the right four were demolished to build the oratory. In the fourth left chapel there is a baptismal font from the 13th century. It is 17.2 m width and 43 m long and has a deep choir.

The church has been recently consolidated, with a project about soil consolidation and general anchorage of the macro-elements of the building, and between the building and the foundation rock.

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; or

undue loads being applied to the building after its initial construction.Settlement should not be confused with subsidence which results from the load-bearing ground upon which a building sits reducing in level, for instance in areas of mine workings where shafts collapse underground.

Some settlement is quite normal after construction has been completed, but unequal or differential settlement may cause significant problems for buildings. Traditional green oak-framed buildings are designed to settle with time as the oak seasons and warps, lime mortar rather than Portland cement is used for its elastic properties and glazing will often employ small leaded lights which can accept movement more readily than larger panes.

Soil compaction (agriculture)

Soil compaction, also known as soil structure degradation, is the increase of bulk density or decrease in porosity of soil due to externally or internally applied loads. Compaction can adversely affect nearly all physical, chemical and biological properties and functions of soil. Together with soil erosion, it is regarded as the "costliest and most serious environmental problem caused by conventional agriculture."In agriculture, soil compaction is a complex problem in which soil, crops, weather and machinery interact. External pressure due to the use of heavy machinery and inappropriate soil management can lead to the compaction of subsoil, creating impermeable layers within the soil that restrict water and nutrient cycles. This process can cause on-site effects such as reduced crop growth, yield and quality as well as off-site effects such as increased surface water run-off, soil erosion, greenhouse gas emissions, eutrophication, reduced groundwater recharge and a loss of biodiversity.Unlike salinization or erosion, soil compaction is principally a sub-surface problem and therefore an invisible phenomenon. Special identification methods are necessary to locate, monitor and manage the problem appropriately.

Top soil compaction is considered partly reversible and its occurrence controllable. Subsoil compaction, however, is regarded as the major problem because it can be permanent, meaning the pore functions can potentially not be restored after deterioration. Since farmers in modern intensive agriculture depend on heavy machinery and therefore cannot completely avoid compaction, soil compaction management approaches focus on mitigation. Attempts to mitigate soil compaction include biological, chemical and technical approaches. Long-term public policies can tackle the underlying reasons for soil compaction. For instance, subsidies for low-tech agriculture may decrease heavy machinery use on the field, and educational programs aiming at slowing population growth can lower the pressure on agriculture caused by population size.

Urmia Lake Bridge

The Urmia Lake Bridge or Urmia Lake Causeway is a road bridge in northern Iran. It is the largest and longest bridge in Iran, and crosses Lake Urmia, connecting the provinces of East Azerbaijan and West Azerbaijan. The project was completed in November 2008.

The bridge reduced the driving distance between Tabriz and Urmia by 135 kilometres (84 mi), saving time and fuel consumption, and reducing road accidents. It has helped stimulate cultural exchanges, tourism and trade between the provinces of East Azerbaijan and West Azerbaijan.

Zvenigorodskaya

Zvenigorodskaya is a station of Saint Petersburg Metro, on Frunzensko-Primorskaya Line, between stations Sadovaya and Obvodny Kanal.

It was opened on December 20, 2008 as one of the first stations on the new Frunzensko-Primorskaya Line. It is connected with foot passages to the Pushkinskaya station, serving the Kirovsko-Vyborgskaya Line. Upon the opening it was without an independent surface exit; all traffic had to go through Pushkinskaya. Escalators and a surface lobby were added later.

Soil
Foundations
Retaining walls
Stability
Earthquakes
Geosynthetics
Numerical analysis

Languages

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.