Secondary Settlement/Creep (ΔHc) Occurs under constant effective stress due to continuous rearrangement of clay particles into a more stable configuration. Predominates in highly plastic clays and organic clays.
Full Answer
What is meant by creep settlement?
Creep settlement. After the consolidation settlement is done, i.e. enough water has been squeezed out, the soil particles will now take on the full compression. Similar to concrete, soil undergoes creep as well when subject to compression.
What is the creep of soil?
Similar to concrete, soil undergoes creep as well when subject to compression. In geotechnical engineering, the word ‘creep’ is always associated with the solid particles of the soil (note ‘consolidation’ in contrast is associated with ground water).
What happens to the soil after consolidation settlement?
After the consolidation settlement is done, i.e. enough water has been squeezed out, the soil particles will now take on the full compression. Similar to concrete, soil undergoes creep as well when subject to compression.
What is the difference between immediate settlement and 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 is creep settlement?
Immediate (undrained) settlement. Primary consolidation settlement. Creep settlement. Past experience indicates that, if the embankment is constructed with good quality material and is well compacted, the long-term settlement within the embankment can be controlled to within 0.2% of the height of the embankment.
What is creep in consolidation?
Since creep is defined as the solid material slowly deforming to resist the applied external stress in the long term, creep is one type of time dependent behavior, which is ongoing all the time, regardless the change of the applied external stress. However, the rate of change of the applied external stress.
What is secondary settlement?
Secondary settlement areas developed outside the central city district after the end of World War 1 and have established their own distinct identities over time.
What is secondary settlement of soil?
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 ...
What is the difference between primary and secondary consolidation settlement?
When the rate of densification is controlled by the time required for expulsion of fluids, this volume change is called primary consolidation. Secondary consolidation is the volume change controlled by anything else, such as the sliding of particles over each other or compression of particles.
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 is primary and secondary settlement?
The overall process of geotechnical settlement is made up of three main components occur at different stages: Immediate settlement (also known as elastic settlement) Consolidation settlement (or primary settlement) Creep settlement (or secondary settlement).
What does primary settlement mean?
Primary consolidation settlement rc occurs in cohesive or compressible soil during dissipation of excess pore fluid pressure, and it is controlled by the gradual expulsion of fluid from voids in the soil leading to the associated compression of the soil skeleton.
What are the types of settlements?
The four main types of settlements are urban, rural, compact, and dispersed. Urban settlements are densely populated and are mostly non-agricultural. They are known as cities or metropolises and are the most populated type of settlement.
What is the difference between primary and secondary compression?
One-dimensional compression of a saturated soil, subjected to an increment of total vertical stress, consists of a primary compression that takes place during the increase in effective vertical stress and a secondary compression that follows at constant effective vertical stress.
What are the types of soil settlement?
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 consolidation settlement in soil?
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.
What is creep in geotechnical engineering?
Creep is a time dependent process in which materials accumulate strains (deformations) under the influence of constant (effective) stresses. Creep of geomaterials can be often observed in slopes where creep manifests as slow mass wasting, that is slow downhill movement of soil and rock mass - see picture below.
What is consolidation settlement in soil?
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.
Abstract and Figures
The paper focuses on long-term oedometer tests lasting up to 42 years and performed on silty sand, (organic) clayey silt, peat and (pre-treated) sewage sludge. Secondary consolidation (creep) could be observed in all cases, lasting over many years and occurring widely linear with the logarithm of time.
References (5)
The phenomenon of almost double increasing of a peat secondary consolidation coefficient throughout the oedometer tests was noticed by a number of the researchers. An analysis of a long-term laboratory experiment results has shown that the reason of this effect is not caused by natural processes occurring in a peat.
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 is the final settlement of a foundation?
Generally, the final settlement of a foundation is of interest and U is considered equal to 1 (i.e. 100% consolidation) 1. Immediate Settlement. Immediate settlement takes place as the load is applied or within a time period of about 7 days. Predominates in cohesion less soils and unsaturated clay.
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.
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 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).
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.
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 creep in soil?
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.
Why does soil creep?
However, in soils with a high amount of organic material such as peat, 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.
What is overconsolidated soil?
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" or "unconsolidated" immediately after a new load is applied but before the excess pore water pressure 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".
What happens when stress is removed from a soil?
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.
What is the process of a soil change volume?
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.
How long does it take for a soil to consolidate?
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).
What is 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.
What is the theory of consolidation and creep?
Consolidation and creep of clayey soils is an extensively studied topic in soil mechanics. Significant advancements have occurred since the formal inception of the classical theory of consolidation for soils (Terzaghi, 1923). The theory disregards time dependent deformations during consolidations. Thus it was already discovered as early as 1936 that the theory cannot capture observed field measurements due to existence of continued deformations after consolidation was finished (Buisman, 1936). The immediate modifications that followed were to divide the total deformations into the primary and secondary consolidation phases where the primary consolidation phase is computed according to the classical consolidation theory and a creep deformation is added afterwards as a secondary consolidation phase. However, shortcoming of such formulation was recognized by earlier researchers that indicated existence of creep during primary consolidation (e.g. e.g. Šuklje 1957; Bjerrum 1967; Janbu 1969). These assertions are substantiated with extensive long-term field measurements that clearly evidenced existence of creep during Primary consolidation (e.g. Larsson, 2003; Leroueil 2006).
What is the numerical calculation of settlements of field conditions?
Numerical calculation of settlements of field conditions are normally based on soil parameters interpreted from laboratory tests. Hence, it is crucial that the laboratory samples have the desired level of quality and be representative to give acceptable prediction of field performances.
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 transferred to the soil skeleton.
Terminology
Consolidation of the compressible soils occurs in two stages defined previously.
Primary Consolidation
The definition of primary consolidation is discussed above under the terminology.
Secondary Consolidation
Secondary consolidation settlement occurs after the completion of dissipation of the excess pore water pressure developed immediately after the application of load.
Worked Example: Primary Consolidation Settlement Calculation in Normally Consolidated Clay
Consolidation of soil (normally consolidated) can be evaluated from the following equation as discussed in this article.
Soil Settlement and Soil Shear
Components of Settlement
- The components of settlement of a foundation are: 1. Immediate settlement 2. Consolidation Settlement, and 3. Secondary compression (creep) ΔH = ΔHi + U ΔHc + ΔHs ΔH = total settlement, ΔHc = consolidation settlement, ΔH = secondary compression, U = average degree of consolidation. Generally, the final settlement of a foundation is of interest and ...
Immediate Settlement Calculations
- Where q0 = intensity of contact pressure in units of Es (Undrained Modulus of Elasticity) B’ = least lateral dimension of contributing base area in units of ΔHi Es, μ = Elastic Soil Parameters. A major problem is of course to obtain correct stress-strain modulus Es. Es can be found from laboratory tests like unconfined compression tests, Triaxial compression tests, and in-situ tests like SPT, C…
Consolidation Settlement Calculations
- 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 Ko-state (at-rest state). Secondary consolidation may be the larger component if settlement in some soils, particularly in soils with …
Settlement Limits
- 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. Theoretically, no damage will be done to a structur…