Footing founded on sandy soils are subjected to settlement during the loading.
Can be estimated using the elastic compressibility properties of soils,
- Elastic modulus (E)
- Poisson's ratio
There are many methods to estimate the elastic (immediate) settlement. before that,
We can divide foundations in to 2,
1). Rigid foundations - Foundations are not subjected to any deformation
2). Flexible Foundations - Foundations are also subjected to deformations
In the theory of elasticity following assumptions are made,
1. Soil medium is semi-infinite
2.Soil is linear elastic
3.Soil is homogeneous and isotropic
( Homogeneous : same properties everywhere, Isotropic : same properties in every direction)
Empirical methods to calculate immediate settlements
1. Timoshenko and Goodier method
B = width of the footing
qnet = net pressure on soil ( q net = Q col/B*L)
Es = elastic modulus of compressible soil under footing
v = poisson ratio of the soil
If = influence factor
2. Janbu et al method
where,
I1 and I2 = influence factors
3. Schemertmann method
C1 = depth correction factor
C1 = 1 - 0.5 (q0'/qnet) , q0' - effective vertical overburden pressure at the foundation level
C2 = creep factor
C2 = 1 + 0.2 log ( t / 0.1 ) , t = time in years - after the loading when you need
Iz = vertical strain influence factor
delta z = thickness of elemental layer
Es = 2.5 qc ( for squire footings )
3.5 qc ( for strip footings )
qc - cone penetration resistance from CPT (check my before blogs)
4. Terzaghi and peck method
where,
ro B = settlement in inches
B = width of the foundation in feets
q = applied bearing pressure in tons/ft^2
N = average SPT value over a depth of 2B
5. Parry method
where,
ro = settlement in mm
q = pressure from the foundation in MN/m^2
B = width of the footing in m
a = 200 inches in SI units
CD = factor for excavation
CT = factor for the thickness of the compressible layer
CW = factor for depth of the water table
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