Total stress 
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Unit weight ranges are:
dry soil  g_{d}  14  20 kN/m³  (average 17kN/m³) 
saturated soil  g_{g}  18  23 kN/m³  (average 20kN/m³) 
water  g_{w }  9.81 kN/m³  (» 10 kN/m³) 
See Description and classification
Any change in vertical total stress (s_{v}) may also result in a change in the horizontal total stress (s_{h}) at the same point. The relationships between vertical and horizontal stress are complex.
Total stress in homogeneous soil 
total stress 
Total stress below a river or lake 
total stress 
Total stress in unsaturated soil 
total stress 
Just above the water table the soil will remain saturated due to capillarity, but at some distance above the water table the soil will become unsaturated, with a consequent reduction in unit weight (unsaturated unit weight = g_{u})
s_{v} = g_{w} . z_{w} + g_{g}(z  z_{w})
The height above the water table up to which the soil will remain
saturated depends on the grain size.
See Negative pore pressure (suction).
Total stress with a surface surcharge load 
total stress 
Pore pressure 
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Groundwater and hydrostatic pressure 
Pore pressure 
Under hydrostatic conditions (no water flow) the pore pressure at a given point is given by the hydrostatic pressure:
Water table, phreatic surface 
Pore pressure 
Negative pore pressure (suction) 
Pore pressure 
The height above the water table to which the soil is saturated is called
the capillary rise, and this depends on the grain size and type (and
thus the size of pores):
· in coarse soils capillary rise is very small
· in silts it may be up to 2m
· in clays it can be over 20m
Pore water and pore air pressure 
Pore pressure 
Pore pressure in steady state seepage conditions 
Pore pressure 
The hydralic gradient, i, between two points is the head drop per
unit length between these points. It can be thougth of as the "potential"
driving the water flow.
Hydralic gradient PQ,  i =   dh ds 
=  du ds 
.  1 g_{w} 
Thus  du = i . g_{w} . ds 
But in steadystate seepage, i = constant
Therefore the change in pore pressure due to seepage alone, du_{s}
= i . g_{w} . s
For seepage flow vertically downward, i is negative
For seepage flow vertically upward, i is positive.
Effective stress 
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In fact, it is the combined effect of total stress and pore pressure that controls soil behaviour such as shear strength, compression and distortion. The difference between the total stress and the pore pressure is called the effective stress:
effective stress = total stress  pore pressureor s´ = s  u
Note that the prime (dash mark ´ ) indicates effective stress.
Terzaghi's principle and equation 
Effective stress 
All measurable effects of a change of stress, such as compression, distortion and a change of shearing resistance are due exclusively to changes in effective stress. The effective stress s´ is related to total stress and pore pressure by s´ = s  u.
The adjective 'effective' is particularly apt, because it is effective
stress that is effective in causing important changes: changes in strength,
changes in volume, changes in shape. It does not represent the exact contact
stress between particles but the distribution of load carried by the soil
over the area considered.
Mohr circles for total and effective stress 
Effective stress 
Mohr circles can be drawn for both total and effective stress. The points
E and T represent the total and effective stresses on the same plane. The
two circles are displaced along the normal stress axis by the amount of pore
pressure (s_{n} = s_{n}'
+ u), and their diameters are the same. The total and effective shear
stresses are equal (t´ = t).
The importance of effective stress 
Effective stress 
Changes in water level below ground (water table changes) result in changes in effective stresses below the water table. Changes in water level above ground (e.g. in lakes, rivers, etc.) do not cause changes in effective stresses in the ground below.
Changes in effective stress 
Effective stress 
If both total stress and pore pressure change by the same amount, the effective
stress remains constant. A change in effective stress will cause: a change
in strength and a change in volume.
Changes in strength 
Changes in effective stress 
Therefore, if the pore pressure in a soil slope increases, effective stresses will be reduced by Ds' and the critical strength of the soil will be reduced by Dt  sometimes leading to failure.
A seaside sandcastle will remain intact while damp, because the pore pressure is negative; as it dries, this pore pressure suction is lost and it collapses. Note: Sometimes a sandcastle will remain intact even when nearly dry because salt deposited as seawater evaporates slightly and cements the grains together.
Changes in volume 
Changes in effective stress 
The rate of change of effective stress under a loaded foundation, once it is constructed, will be the same as the rate of change of pore pressure, and this is controlled by the permeability of the soil.
Settlement occurs as the volume (and therefore thickness) of the soil layers change. Thus, settlement occurs rapidly in coarse soils with high permeabilities and slowly in fine soils with low permeabilities.
Calculating vertical stress in the ground 
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Simple total and effective stresses 
Calculating vertical stress 
The figure shows soil layers on a site.

(a) At the top of saturated sand (z = 2.0 m)  
Vertical total stress  s_{v} = 16.0 x 2.0  = 32.0 kPa 
Pore pressure  u = 0  
Vertical effective stress  s´_{v} = s_{v}  u  = 32.0 kPa 
(b) At the top of the clay (z = 5.0 m)  
Vertical total stress  s_{v} = 32.0 + 20.0 x 3.0  = 92.0 kPa 
Pore pressure  u = 9.81 x 3.0  = 29.4 kPa 
Vertical effective stress  s´_{v} = s_{v}  u = 92.0  29.4  = 62.6 kPa 
Effect of changing water table 
Calculation of vertical stress 
The figure shows soil layers on a site. The unit weight of the silty sand is 19.0 kN/m³ both above and below the water table. The water level is presently at the surface of the silty sand, it may drop or it may rise. The following calculations show the effects of this:  
Stresses under foundations 
Calculation of vertical stress 
From an initial state, the stresses under a foundation are first changed by excavation, i.e. vertical stresses are reduced. After construction the foundation loading increases stresses. Other changes could result if the water table level changed.
The figure shows the elevation of a foundation to be constructed in a homogeneous soil. The change in thickness of the clay layer is to be calculated and so the initial and final effective stresses are required at the middepth of the clay.
Unit weights: sand above WT = 16 kN/m³, sand below WT = 20 kN/m³, clay = 18 kN/m³.
Calculation of vertical stress 
Shortterm and longterm stresses 
The figure shows how an extensive layer of fill will be placed on a certain site.
Shortterm and longterm stresses 
Initially, before construction 
Initial stresses at middepth of clay (z = 2.0m)
Vertical total stress
s_{v} = 20.0 x 2.0 = 40.0kPa
Pore pressure
u = 10 x 2.0 = 20.0kPa
Vertical effective stress
s´_{v} = s_{v}
 u = 20.0kPa
Initial stresses at middepth of sand (z = 5.0 m)
Vertical total stress
s_{v} = 20.0 x 5.0 = 100.0 kPa
Pore pressure
u = 10 x 5.0 = 50.0 kPa
Vertical effective stress
s´_{v} = s_{v}
 u = 50.0 kPa
Shortterm and longterm stresses 
Immediately after construction 
The construction of the embankment applies a surface surcharge:
q = 18 x 4 = 72.0 kPa.
The sand is drained (either horizontally or into the rock below) and so there is no increase in pore pressure. The clay is undrained and the pore pressure increases by 72.0 kPa.
Initial stresses at middepth of clay (z = 2.0m)
Vertical total stress
s_{v} = 20.0 x 2.0 + 72.0 = 112.0kPa
Pore pressure
u = 10 x 2.0 + 72.0 = 92.0 kPa
Vertical effective stress
s´_{v} = s_{v}
 u = 20.0kPa
(i.e. no change immediately)
Initial stresses at middepth of sand (z = 5.0m)
Vertical total stress
s_{v} = 20.0 x 5.0 + 72.0 = 172.0kPa
Pore pressure
u = 10 x 5.0 = 50.0 kPa
Vertical effective stress
s´_{v} = s_{v}
 u = 122.0kPa
(i.e. an immediate increase)
Many years after construction 
Shortterm and longterm stresses 
After many years, the excess pore pressures in the clay will have dissipated. The pore pressures will now be the same as they were initially.
Initial stresses at middepth of clay (z = 2.0 m)
Vertical total stress
s_{v} = 20.0 x 2.0 + 72.0 = 112.0 kPa
Pore pressure
u = 10 x 2.0 = 20.0 kPa
Vertical effective stress
s´_{v} = s_{v}
 u = 92.0 kPa
(i.e. a longterm increase)
Initial stresses at middepth of sand (z = 5.0 m)
Vertical total stress
s_{v} = 20.0 x 5.0 + 72.0 = 172.0 kPa
Pore pressure
u = 10 x 5.0 = 50.0 kPa
Vertical effective stress
s´_{v} = s_{v}
 u = 122.0 kPa
(i.e. no further change)
Steadystate seepage conditions  Calculation of vertical stress 
The figure shows seepage occurring around embedded sheet piling.
In steady state, the hydraulic gradient,
i = Dh / Ds = 4 / (
7 + 3 ) = 0.4
Then the effective stresses are:
s´_{A} = 20 x 3  2 x 10 + 0.4 x
10 = 44 kPa
s´_{B} = 20 x 3  2 x 10  0.4 x
10 = 36 kPa