5.1 Settlement: What It Is and How Much Is Acceptable
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| Settlement of Soil |
· When a soil mass is subjected to a compressive force, like all other materials, its volume decreases.
· The property of the soil due to which a decrease in volume occurs under compressive forces is known as the compressibility of soil.
· The decrease in volume of soil, under stress is because of:
o Compression and Expulsion of Pore Air.
o Compression and Expulsion of Pore Water
o Gradual readjustment of clay particles into more stable configuration.
· Reduction in volume of soil mass results in change of lateral and vertical dimensions of soil mass.
· As soil being infinitely large in the lateral direction, hence the change in dimension in this direction is considered to be negligible, but there is significant change in vertical direction which is termed as settlement of soil.
· Settlement refers to the vertical movement of a structure due to the compression of soil beneath its foundation. While all buildings experience some degree of settlement over time, excessive settlement can cause cracks, tilting, or even structural failure.
· In other words, settlement of soil is the gradual sinking of the structure due to compression of the soil below.
· Total settlement of soil is expressed as three components:
o Immediate settlement: Occurs right after construction which is called initial compression, mostly in granular soils. It is immediate phenomenon.
o Primary consolidation: Gradual compression in clayey soils due to expulsion of excess pore water pressure generated due to increase in total stress. It is time dependent phenomenon
o Secondary compression (creep): Long-term deformation under constant load, usually in organic soils (Peat).
Allowable Settlement: Engineers must design foundations to keep settlement within safe limits. For example:
- Residential buildings: typically, ≤ 25–40 mm
- High-rise buildings: stricter limits, often ≤ 25 mm
Salient Points about Consolidation
In the analysis of consolidation theory, we consider only one-dimensional consolidation. 1D dimensional consolidation means that deformation occurs in only vertical direction.
The characteristics of soil during 1-D consolidation or swelling can be determined by the mean of Oedometer test/ Consolidation Test. Apparatus is known as Odeometer or consolidometer.
o A soil is said to be normally consolidated when the existing effective stress is the stress it has ever experienced in its stress history.
o The maximum value of stress that the soil has ever experienced is called preconsolidation stress σ’o.
o If existing effective stress σ’> σ’o, soil is said to be normally consolidated.
o If σ’< σ’o soil is said to be over consolidated soil/ preconsolidated soil.
o σ’o/ σ’= Over consolidation ratio.
o For Normally consolidated soil, OCR = 1
o For over consolidated soil, OCR >1. This is due to erosion of overburden soil, permanent rise in water table, melting of ice sheets after glaciation.
o Normally consolidated soil is much more compressible than over consolidated soil.
o Amount of deformation per unit increase in stress is called compressibility.
o e-log for Normally consolidated soil is always a straight line.
o e-log for Over consolidated soil has convex curvature upwards.
Compressibility of Clay can be represented by three coefficients.
1. Compression Index (Cc): It is the slope of linear portion of e-log σ’ plot and is a dimensionless number. It has its significance inly for Normally consolidated soil.
o Constant value for given type of soil.
o Not a function of effective stress.
o Higher value Cc resulting vertical deformation of clay.
2. Coefficient of compressibility (av):It decreases with increase in effective stress. It will be -ve as the void ratio decreases with increase in stress.
3. Coefficient of volume compressibility (mv):It is not a constant value but depends on stress range over which it is calculated.
Oedometer test can also be used to calculate coefficient of consolidation Cv, which indicates the rate of consolidation and is represented by:
Practise Questions:
1.
Applied Stress (kN/m2) | Sample Thickness (mm) |
0 | 19.7 |
50 | 19.3 |
100 | 19.15 |
200 | 18.85 |
400 | 18.56 |
800 | 18.36 |
0 | 18.90 |
Water content of the sample at the end of test was 28% and Gs= 2.72. Find the void ratio at each applied stress.
2. In a laboratory consolidation test the void ratio of the samples reduced from 0.85 to 0.73 as the pressure was increased from 1 to 2 kg/cm2. If the coefficient of permeability of the soil be 3.3 X 10-4 cm/s. Determine
a) Coefficient of volume change
b) Coefficient of consolidation
3. In a normally consolidated clay stratum of 3m thickness has two permeable layers at its top and bottom. The liquid limit and the initial void ratio of the clay are 3.6% and 0.82 respectively while the initial overburden pressure at the middle of the clay layer is 2kg/cm2. Due to the construction of the new building this pressure increases by 1.5kg/ cm2. Compute the probable consolidation settlement of the building.
4. The subsoil profile of a proposed site of construction is shown below, a square footing of 2m X 2m carries a load of 1000KN and is laid with its base at 1m depth below ground surface assuming that post construction settlement in sand is negligible determine the consolidation settlement of clay layer on account of construction. This is geological evidence, clay is NC, use 2 vertical and 1 horizontal for load dispersion to estimate the stress increase in clay layer.
Answers:
1. Values ranging 0.712-0.836
2. a) 0.065 cm2/kg. b) 5.077 cm2/sec
3. 9.37cm
4. 86.20mm
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