The consolidation settlement is induced due to volumetric change. Since soil particles are practically incompressible, consolidation settlements is caused by a reduction in voids due to gradual squeezing out of water.
What are the key aspects of consolidation settlement?
Some key aspects of consolidation settlement are as follows. It is a three-dimensional process sine movement of water could be any direction. However, the confinement of the soil in the lateral direction, consolidation could be considered as vertical and one dimensional. The permeability of soil impact the consolidation of soil.
What determines the rate of settlement of a saturated soil?
The rate of settlement of a saturated soil is expressed by the coefficient of consolidation (c v). The rate of settlement of a soil, and hence the value of c v, is governed by two factors: the amount of water to be squeezed out of the soil; and the rate at which that water can flow out.
What causes consolidation stress in soil?
The increment of applied stress that causes consolidation may be due to either natural loads (e.g. sedimentation processes), or human-made loads (e.g. the construction of a building or an embankment above a soil mass) or even the decrease of the ground water table.
What are the two major problems with soil settlement analysis?
Two Major Problems with soil settlement analysis are: Obtaining a reliable stress profile from the applied load. ΔH = total settlement, ΔHc = consolidation settlement, ΔH = secondary compression, U = average degree of consolidation.
What causes consolidation settlement?
The consolidation settlement is induced due to volumetric change. Since soil particles are practically incompressible, consolidation settlements is caused by a reduction in voids due to gradual squeezing out of water.
What properties of soil are important for consolidation?
The consolidation characteristics include compressibility modulus, Poisson's ratio, coefficient of permeability and the OCR. These characteristics are needed for anchor capacity calculations under long-term loads.
What are the factors that influence the ground settlement?
Natural factors include: 1) Topography: the steep terrain, if drainage facilities undeserved or geologic conditions poor, it is easy to decrease the strength and stability of embankment; 2) Climate: seasonal climate makes moisture of the soil in the state of liquid or gas move from hot to cold and amass, which leads to ...
How do you determine if a soil is normally consolidated?
Overconsolidation. If the current state of soil is on the normal compression line it is said to be normally consolidated. If the soil is unloaded it becomes overconsolidated.
What are the factors that affect the consolidation?
The factors which affect the consolidation are: (e) Magnitude of the consolidating pressure and the manner of its distribution across the thickness of the layer. (a) Thickness of clay layer: If the thickness is more the consolidation of the layer will be more due to self overburden pressure.
What is consolidation settlement?
When soil is loaded undrained, the pore pressures increase. Then, under site conditions, the excess pore pressures dissipate and water leaves the soil, resulting in consolidation settlement. This process takes time, and the rate of settlement decreases over time.
How is settlement of soil determined?
Therefore, to determine the settlements, it is necessary to know: the course of vertical stresses σz with depth. The settlement-generating base stress σ1 = σ0 - γ • h must be used, taking into consideration the stress reduction by the excavation unloading for the embedment depth of the foundations.
What is the difference between settlement and consolidation?
Debt consolidation and debt settlement are strategies for making debt manageable, but they are different methods and bring different results. Debt consolidation reduces the number of creditors you'll owe. Debt settlement tries to reduce the amount of debt you owe.
What are the factors that determine the selection of sites for settlement?
Determining where to settle land has always depended on a variety of factors, including proximity and accessibility to needed resources. Locations of landforms such as rivers, mountains, and bays has influenced where towns and cities were built.
What is meant by consolidation of soil?
The term consolidation describes the process by which a soil mass decreases in volume in response to either natural or man-made loadings. Natural loadings that induce consolidation result from geological processes such as sedimentation.
What is the purpose of consolidation of soil?
Consolidation plays an important role in stability analysis for the embankment, footings or a column constructed on clayey soils. The settlement behaviour of these structures and the pore pressure needs to be investigated for construction on these soils.
What is primary consolidation settlement?
Primary consolidation is a process of dissipation of increased pore water pressure due to the application of loads on the soil layer and increases in the effective stress in the soil skeleton. This results in a volumetric change in the soil skeleton.
What type of soils undergo consolidation?
Consolidation is generally related to fine-grained soils such as silts and clays. Coarse-grained soils, such as sands and gravels, also undergo consolidation but at a much faster rate due to their high permeability. Saturated clays consolidate at a much slower rate due to their low permeability.
What is meant by consolidation of soil?
The term consolidation describes the process by which a soil mass decreases in volume in response to either natural or man-made loadings. Natural loadings that induce consolidation result from geological processes such as sedimentation.
How do you increase the rate of consolidation?
Consolidation can be accelerated by installing vertical drains to a depth of up to 35 meters, shortening its settlement rate from several years to several months.
What factors affect the compaction of soils?
Factors affecting compaction Nature and type of soil, i.e. sand or clay, grading, plasticity. Water content at the time of compaction. Site conditions, e.g. weather, type of site, layer thickness. Compactive effort: type of plant (weight, vibration, number of passes)
What is the process of soil consolidation?
The consolidation (as compression) of soil usually occurs in three stages: 1. Initial consolidation — It is a comparatively a sudden reduction in soil volume on application of the load mainly due to expulsion and compression of air present in soil pores. 2.
How to do a consolidation test?
By applying a series of load increments to the soil sample and recording settlements at selected time intervals, the consolidation test proceeds. Observations of compression versus time for each load interval are taken and the data are plotted on either a semi-logarithmic plot or square root of time. The purpose of these plots is to obtain the values of t 50 (time at 50 per cent consolidation) and t 90 respectively.
What is the process of deposition?
In the natural process of deposition, fine-grained soils, like silt and clay undergo the process of consolidation under their own weight of overburden pressure. A state of equilibrium is reached after an elapse of several years and the compression ceases.
What is primary consolidation?
2. Primary consolidation — It is the main phase of compression due to expulsion of pore water and dissipation of excess pore pressure. It is a time settlement phenomenon.
How to find load transferred to soil grains?
The load transferred to the soil grains at any time, i.e., ΔP̅ can be worked out by subtracting the area of that curve from the total area ABCD. The ratio of this area to the area ABCD would eventually indicate the extent or the degree of consolidation at that time, and is usually expressed as percentage.
What is a consolidated clay deposit?
If the present effective overburden pressure in the deposit is the maximum pressure to which the deposit has ever been consolidated at any time in the past , such a deposit is called a normally consolidated clay deposit. There is no reliable procedure available to predict the in situ effective stress-void ratio relationship.
What does "consolidation" mean?
1. Meaning of Consolidation: ADVERTISEMENTS: Consolidation is the gradual reduction in the volume of a partly or fully saturated soil under sustained loading and is mainly due to the expulsion of water from the soil pores. When a static load is applied to a saturated (or nearly saturated) clay.
Which soil has higher permeability?
In particular, cohesionless soils have higher permeability than cohesive soils that have small voids blocking the water movement. In geotechnical design, the total settlement of the soil has to be properly predicted and must meet the requirements of the project. If the soil’s characteristics are not adequate to meet the aforementioned requirements ...
What is the term for the movement of soil in the vertical direction?
Settlements refer to the soil’s movement in the vertical direction typically induced by stress changes. The total settlement of the ground consists of 3 components: immediate settlement (commonly referred to as elastic settlement, although this is a misnomer), consolidation settlement (or primary settlement) and creep settlement (or secondary settlement).
What is the difference between creep and immediate settlement?
Since soil particles are practically incompressible, consolidation settlements is caused by a reduction in voids due to gradual squeezing out of water. Finally, creep settlement occurs under a constant load and is depended on the stress history, the type of soil and the anisotropy of the soil.
How long does creep settlement last?
The settlement process may be completed almost immediately or may last for a significant amount of time (even decades) depending on the soil’s permeability and water drainage paths.
Consolidation and Settlement
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Summary
The settlement of soils in response to loading can be broadly divided into two types: elastic and time-dependent settlement. Elastic settlements are the simplest to deal with; they are instantaneous, recoverable, and can be calculated from linear elastic theory.
What is Soil Consolidation?
Soil Consolidation refers to the process in which the volume of a saturated (partially or fully) soil decreases due to an applied stress. The term was introduced by Karl von Terzaghi also known as the "father of soil mechanics and geotechnical engineering". Terzaghi established the one-dimensional consolidation theory and changed the definition of the term since it was previously associated (and still is, in geosciences) with the compaction of clay sediments that formed shales.
What is the simplest case of soil consolidation?
The simplest case of consolidation examined is the one-dimensional consolidation. In this case, the lateral strain of the soil mass is neglected. The testing procedure to quantify the critical soil properties associated with soil consolidation is the Oedometer Test. The term “Oedometer” derives from the Ancient Greek language and means “to swell”. The test is one of the most commonly conducted, and important, laboratory tests in geotechnical engineering. The Oedometer Test aims at measuring the vertical displacement of a cylindrical, saturated soil sample subjected to a vertical load while it is radially constrained. In the subsequent test, the incremental loading consolidation test is described. Note that there is also a constant rate of strain (CRS) test, that nowadays is becoming more popular
How long does the consolidation process last?
In general, consolidation in sandy soils is a quick process (occurring possibly immediately during construction) whereas the process may last for many years or even decades in clay soils.
What is the pre-consolidation pressure?
The Pre-consolidation Pressure: The maximum effective stress that the soil specimen has sustained in its geological history.
Why do you put filter papers between stone and soil?
Filter papers, placed between stone and soil sample to prevent soil from clogging the pores of the stone
Why is a reservoir filled with water?
Reservoir, filled with water to ensure that the soil remains essentially saturated
How many stages are there in consolidation?
The consolidation procedure is commonly separated into 3 stages:
What is secondary consolidation?
Secondary consolidation may be the larger component if settlement in some soils, particularly in soils with a large organic component. Secondary consolidation is associated with both immediate & consolidation type settlements, although it is usually not of much significance with immediate settlements.
What happens after primary consolidation?
After primary consolidation the soil structure continues to adjust to the load for some additional time. This settlement is termed secondary consolidation/secondary compression. At the end of secondary consolidation the soil has reached a new K o -state (at-rest state).
Why is the leaning tower of Pisa undergoing consolidation settlement?
The lean is caused by consolidation settlement being greater on one side. This, however, is an extreme case. The principal settlements for most projects occur in 3 to 10 years.
What is the difference between total settlement and differential settlement?
Total settlement is the magnitude of downward movement. Differential settlement is non-uniform settlement. It is "the difference of settlement between various locations of the structure. Angular distortion between two points under a structure is equal, to the differential settlement between the points divided by the distance between them.
How much settlement is acceptable for a road embankment?
A fixed-end arch would suffer greatly if the abutments settle or rotate. For road embankments, storage silos and tanks a settlement of 300mm - 600mm may be acceptable, but for machine foundations the settlement may be limited to 5mm 30mm. Different types of construction materials can withstand different degrees of distortion. For example, sheet metal wall panels do not show distress as readily as brick masonry.
What happens if soil shears fail?
A soil shear failure can result in excessive building distortion and even collapse. Excessive settlements can result in structural damage to a building frame nuisances such as sticking doors and windows, cracks in tile and plaster, and excessive wear or equipment failure from misalignment resulting from foundation settlements.
What is the value of m in a settlement?
m = number of corners contributing to settlement ΔHi. At the footing center m= 4; and at a corner m = 1, at a side m = 2.
Why does soil volume decrease?
When soil is loaded due to the load coming from superstructure, the soil volume will decrease due to the change of particle arrangement in the soil. If both soil particle and water within the soil voids are assumed to be incompressible and soil is completely saturated, then volume change will occur due to the removal of water from ...
How does volume change in soil?
When soil is loaded due to the load coming from superstructure, the soil volume will decrease due to the change of particle arrangement in the soil. If both soil particle and water within the soil voids are assumed to be incompressible and soil is completely saturated, then volume change will occur due to the removal of water from the soil voids as a result of externally applied loading. Due to the volume change a downward deformation will take place which causes settlement of the superstructure (as shown in Figure 21.1 and Figure 21.2). The rate of volume change depends on the permeability of soil. Thus, consolidation is a major issue in case of clay due to its low permeability. Consolidation is a time-dependent phenomenon of soil. In the Figure 21.1, the vertical strain e can be written as:
What is total settlement?
The total settlement of the soil is the summation of three settlements (i) immediate settlement (ii) primary consolidation settlement and (iii) secondary consolidation settlement or settlement due to creep. Immediate (or elastic) settlement occurs almost immediately after the loading is applied due to the distortion of the soil without any volume change due to removal of water. The time–dependent settlement due to the removal of water from a loaded saturated soil is known as primary consolidation settlement. The primary consolidation depends on the permeability and compressibility of the soil. Some soil (such as peat or soft organic clay) shows time-dependent settlement under constant effective stress during the post primary consolidation period. The settlement during post primary consolidation period is known as secondary consolidation settlement or creep. Thus, the total settlement (St) can be written as:
Is compaction a time dependent phenomenon?
Compaction is almost an instantaneous phenomenon, whereas consolidation is a time-dependent phenomenon. In case of consolidation soil is always saturated, whereas in case of compaction soil is always unsaturated. Consolidation is the reduction of water voids, whereas compaction is the reduction of air voids. For compaction specified compaction techniques are used, whereas consolidation occurs due to application load on the soil.
What is the degree of consolidation?
The degree of consolidation, also known as consolidation ratio, indicates the progress of consolidation at any time at the given location z in the compressible layer. The degree of consolidation (U) at any time during consolidation is defined as the percentage of compression that has been completed at that time relative to the total consolidation settlement under the given effective stress increment. It is usually expressed in percentage.
What is the one dimensional consolidation equation?
The one-dimensional consolidation equation, Eq. (11.27), given by Terzaghi is a partial differential equation of second order. Its solution can be obtained by Fourier series. It is evident from the equation that the excess pore water pressure (u) is a function of depth (z) and time (t). It may be represented by the following equation –
Why does compression take place?
8. Compression takes place only due to expulsion of pore water, that is, the initial and secondary consolidation is neglected.
What is the equation for the total pore water pressure to be dissipated?
Evidently, u the total pore water pressure to be dissipated = Δσ = applied stress increment.
How to find the rate of flow of pore water?
We know that the rate of flow (i.e., volume per unit time) of pore water is given by q = v × A , where v is the velocity of flow and A is the area of flow. Hence, volume of water entering the soil element per unit time is –
What are the three non-dimensional parameters of Terzaghi's 1D consolidation equation?
The solution of Terzaghi’s 1D consolidation equation consists of equations relating the three non-dimensional parameters T, U, and d.
What is the velocity of flow of pore water?
As per Darcy’s law, velocity of flow of pore water v z = k x i. Substituting the value of i from Eq. (11.21) we get –
What is the OCR of NC soils?
oFrom the definition of NC soils, they always have OCR=1.
Who suggested a simple graphic construction to determine the preconsolidation pressure s’?
Casagrande (1936) suggested a simple graphic construction to determine the preconsolidation pressure s’
Is soil constant?
for a particular soil is not constant but depends on the stress range over which it is calculated.
Consolidation
- When the construction of the foundation is done(application of additional load), pore water pressure in the saturated clay increases as the hydraulic conductivity of the clay is very small. Some time is required for excess pore water pressed to dissipate and increase the stress to be t…
Primary Consolidation
- The definition of primary consolidation is discussed above under the terminology. Let’s the method of primary Consolidation settlement calculation There are two different stages of soil. 1. Normally Consolidated Soil 2. Over Consolidated Soil It is required to assess the consolidation of soil is done based on the above states. In simple terms, the normally consolidated soil is experie…
Secondary Consolidation
- Secondary consolidation settlement occurs after the completion of dissipation of the excess pore water pressure developed immediately after the application of load. The settlement caused due to the plastic adjustment of the soil fabrics at the end of the primary consolidation is called secondary consolidation. Secondary Consolidation also can be explained as slippage and reorie…
Types of Tests to Find Consolidation Settlement
- One dimensional consolidation test
- Oedometer Test
- Incremental Loading Oedometer