Introduction ––
Soil compressibility is defined as the ability of soil to shrink in volume under mechanical load, whereas consolidation is the compression of saturated soil under constant pressure and that results from the expulsion of water from soil voids. The compressibility and cohesion of the soil are important parameters required in the design consideration.
Soil consolidation is divided into three phases which include initial consolidation, primary consolidation and secondary consolidation. Soil consolidation is time-dependent and its analysis is usually based on Terjaghi's principle.
It is important to know the rate of consolidation as well as the total consolidation to be expected for the design of structures. In this way, the designer will be able to determine the necessary precautions and design considerations to keep the disposal to a tolerable extent, otherwise the willingness to use the structure may be impaired and the design life of the structure may be reduced.
Contents ––
compressibility of soil
soil consolidation —
1. Initial Consolidation
2. Primary Consolidation
3. Secondary Consolidation
compressibility of soil ––
When a clay mass is subjected to a compressive force, its volume decreases i.e. gives the volume of settlement. The property of soil due to which there is a decrease in volume due to compressive force is called compressibility of the soil.
Soil compression can be caused by compression of solid particles and water in voids, compression and expulsion of air in voids, expulsion of water in voids.
Soil consolidation ––
The compression of saturated soil under constant static pressure is called consolidation which is entirely due to the expulsion of water from the voids. Consolidation is generally related to fine-grained soils such as silt and clay.
Coarse-grained soils such as sand and gravel are also stronger, but at a much faster rate because of their high permeability. Saturated soils consolidate very slowly due to their low permeability.
The process of consolidation is often confused with the process of compaction. Compaction increases the density of an unsaturated soil by reducing the amount of air in the voids. However, consolidation is a time-related process in which some of the water is removed from the voids to increase the density of a saturated soil. Consolidation theory is needed to predict both the magnitude and the rate of consolidation settlements to ensure the serviceability of structures installed on a compressed soil layer.
Soil consolidation is made up of three components which include initial consolidation, primary consolidation and secondary consolidation:
Principle of consolidation
When a saturated clay is subjected to compressive load, excess pore pressure develops and water drains out of the clay layer into an adjacent layer of relatively more permeable soil like sand. The drainage of pore water takes place as long as there is excess pore water pressure. This process is known as primary consolidation. Once the excess pore pressure becomes zero, that is excess pore pressure gets fully dissipated, the primary compression under the applied stress ends. It is observed that some compression takes place after primary consolidation. It is known as secondary consolidation.
1. Initial Consolidation
When a load is applied to partially saturated soils, there is a reduction in volume due to the expulsion and compression of air in the voids. The slight decrease in volume is due to the compression of the solid particles.
The reduction in the volume of the soil just after the load is applied is known as initial consolidation or initial compression. For saturated soils, the initial consolidation is mainly due to the compression of the solid particles.
2. Primary Consolidation
After initial consolidation, the volume further decreases due to the removal of water from the voids. When a saturated soil is subjected to pressure, initially all applied pressure is taken by the water as the pressure of the excess pore water. A hydraulic gradient will develop and water will begin to flow and decrease in volume.
This reduction in volume is called primary consolidation of soil. In fine-grained soils, primary consolidation occurs over a long period of time. However, in coarse-grained soils, primary consolidation occurs early due to high permeability.
3. Secondary Consolidation
Volume reduction continues at a very slow rate even after the excess hydrostatic pressure developed by the applied pressure is completely gone and primary consolidation is complete. The additional reduction in quantity is called secondary consolidation. Secondary consolidation becomes important for some soil types, such as peat and soft organic soils.
Spring analogy
A saturated soil mass consist of soil particles and voids filled by water. Fig. shows the skeleton formed of soil particles can be assumed to be replaced by a number of springs and the water filling voids in soil mass by the water filling the cylinder. A compressive load is applied on the top of the piston. An outlet with valve is provided to control drainage of water from out of the cylinder.
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| Spring analogy |
Let Z0, be the length of springs under pressure of 10 units as shown in Fig. (a). Let the length decreases to Z1, when the pressure is increased by 2 units as shown in Fig. (b). In Fig. (c), (d), (e) & (f) springs with piston is shown placed in a container filled with water. For soil mass by analogy
o = total stress, o'+u
u = excess pore pressure
For fig (d) 0 = d'+Å« or 12 =10+2 For fig (e) o=0'+Å«
or 12 = (10+ V O')+(2-Vo')
VoÊ»= part of additional stress transferred to springs at that state.
For (b) or o'=12
It is clear from the analogy model that the saturate soil mass subjected to an initial pressure and when no drainage is occurring o=o' =12.
Terzaghi's one dimensional consolidation theory -
Assumption ––
Terzaghi (1923) derived the basic differential equation of consolidation which represents the first step of analysis of consolidation process.
a) The soil mass is homogenous and fully saturated.
b) The soil particles and water are incompressible.
c) Darcy's law for flow of water through soil mass is applicable during consolidation.
d) Coefficient of permeability is constant during consolidation.
e) Load is applied in one direction only and deformation occurs only in the direction of load applied.
f) The drainage of pore water occurs only in one direction. g) A boundary drainage face offers no resistant to flow of water from soil.
h) The time lag in consolidation is due entirely to permeability of soil.
Consolidation Test –
The apparatus used is shown in Fig. It is known as consolidometer or oedometer two types of consolidation cells.
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| Fixed ring consolidation cell |
If void ratio e is plotted as ordinate on natural scale again effective stress ó as abscissa on logarithmic scale, the virgin compression curve and expansion curve become nearly straight line.
According to Terzaghi, the virgin compression curve can be defined by following empirical relation
e=eo -Cc log10.o'/o'0
eo = Initial void ratio corresponding to initial effective stress
e= void ratio corresponding to increased effective stress .
Differentiate compaction and consolidation of soil. Whether there is any point of similarity between the two ——
Differences: —
Compaction of soil ––
(1). Compaction is the process by which solid soil particles are packed more closely together together by mechanical means.
(2). It is achieved through reduction of air voids.
(3). It is a rapid process.
(4). It is an artificial process.
(5). Proper compaction of soil is achieved at optimum moisture content.
consolidation of soil. -—
(1). Consolidation is the process by which soil soil particles are packed more closely together by under the application of static loading.
(2). It is achieved through gradual drainage of water from soil pores.
(3). It is a gradual process. In some soils it takes many years.
(4). It is a natural process.
(5). Consolidation is strictly applicable for saturated or nearly saturated clays or soils. with low permeability.
Similarities:
At the end of the process a closer packing of soil grains results. Shear strength increases.
Compressibility & permeability decreases.
Short questions
(1) When the compactive energy is increased —
(a) Both OMC and maximum dry density increase
(b) OMC decreases but maximum dry density increases
(c) OMC increases but maximum dry density decreases
(d) OMC remains same but maximum dry density increases
Answer: (c)
(2). Identify the correct option -
Statement 'a': Normally consolidated soil is the one whose present stress is never exceeded in its stress history.
Statement 'b': The greater the coefficient of permeability, the less. is the rate of settlement.
(a) only statement ‘a' is wrong
(b) only statement ‘b' is wrong
(c) both statements are
(d) none is wrong
Answer: (b)
(3). Coefficient of consolidation a soil is affected by –
(a) compressibility
(b) permeability
(c) both compressibility and permeability
(d) none of the above.
Answer: (c)
4 . Degree of consolidation is –
(a) directly proportion to time and inversely proportional to drainage path
(b) directly proportional to time and inversely proportional to square of drainage path
(c) directly proportional to drainage path and inversely proportional to time
(d) directly proportional to square of drainage path and inversely proportional to time
Answer: (b)
(5). Time factor for a clay layer is ––
(a) a dimensional parameter (b) directly proportional to permeability of soil
(c) inversely proportional to drainage path
(d) independent of thickness of clay layer
Answer: (b)
6. In consolidation process the soil is –
(a) fully dry
(b) fully saturated
(c) partly saturated
(d) none of theabove
Answer: (b)
7. The ratio of settlement at any time t to the final settlement is known as
(a) coefficient of consolidation
(b) degree of consolidation
(c) compression index
(d) none of the above
Answer: (b)
8. Coefficient of consolidation is used for evaluation –
(a) total settlement
(b) time rate of settlement
(c) stress in soil
(d) over consolidation ratio
Answer: (b)
9. Settlement at any time to final settlement in a clay is known as ––
(a) settlement ratio
(b) degree of consolidation
(c) coefficient of volume decrease
(d) time factor
Answer: (b)
10. A fully saturated clay was subjected to a load of 500 kN/m². After 10 hours, the average pore water pressure was 200 kN/m². The degree of consolidation at the time is —
(a) 20%
(b) 35
(c) 60%
(d) 70%
Answer: (c)
11. For routine consolidation test in laboratory, the thickness of the specimen is –– (a) 15 mm
(b) 20 mm
(c) 25 mm
(d) 40 mm
Answer: (b)
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