Vertical stresses 1 Flashcards
_________ along a given cross section
of the soil profile is key to analyzing the
compressibility of soils, bearing capacity of
foundations, stability of embankments and
lateral earth pressure on earth retaining
structures.
Nature of stresses
______ is a multi phase system
Soil
To perform any kind of analysis - we must understand
_________
stress distribution
Due to the self
weight of the soil mass.
Geostatic Stresses
From structures
Excess Stresses
___________- stresses increase with
depth
Vertical geostatic
3 types of
geostatic stresses
a Total Stress,
b. Effective Stress,
c Pore Water Pressure, u
The rate of drainage depends on the _________ of the soil.
permeability
The strength and compressibility of the soil depend on the _________ within the solid
granular fabric. These are called _______.
stresses, effective stresses
is the stress applied to the soil by its own
weight
Effective Stress
soil carries the stress in 2 ways
- A portion is carried by the water (acts equally in all directions)
- A portion is carried by the soil solids at their point of contact.
The sum of the vert. components of the forces at their
points of contact per unit of X- sectional area is the
________
effective stress
_________ controls the deformation of the soil
Effective stress
The interaction between soils and percolating water has an
important influence on:
The design of foundations and earth slopes,
The quantity of water that will be lost by percolation through a
dam or its subsoil.
As water flows through soil it exerts a frictional drag on the soil
particles resulting in head losses. The frictional drag is called
_________ in soil mechanics
seepage force
It is often convenient to define seepage as the________
seepage force per
unit volume
If the seepage direction is downwards, then the resultant seepage
stresses are in the same direction as the gravitational effective
stresses. T or F
T
Foundation failures due to ‘__________’ are
quite common.
piping
_______ is a phenomenon by which the
soil on the downstream sides of some
hydraulic structures get lifted up due
to excess pressure of water.
Piping
The pressure that is exerted on the soil due to the seepage of water is called the ____________.
seepage force or pressure
_______ of soils are reduced due
to the development of neutral stress or pore pressures.
Shear strengths
In the stability of slopes, the _______ is a very
important factor.
seepage force
A detailed understanding of the _________ is
therefore essential for a satisfactory design of structures.
hydraulic conditions
The computation of seepage loss under or through a dam,
the uplift pressures caused by the water on the base of a
concrete dam and the effect of seepage on the stability of
earth slopes can be studied by _________.
constructing flow nets
At any depth z, zy_b is the pressure of the submerged soil acting
downward and izy_b is the seepage pressure acting upward. T or F
T
equal to the ratio of the submerged unit weight of soil and the
unit weight of water.
Maximum value of hydraulic gradient
________ soils lose all of their shear strength
and bearing capacity and a visible agitation of soil grains is
observed. This phenomenon is known as __________
cohesion less, boiling or a quick
sand condition
For all
practical purposes the specific gravity of granular materials can
be assumed as equal to ______
2.65
__________ are common in excavations below the ground
water table. This can be prevented by lowering the ground water
elevation by _______ before excavation.
Quick conditions, pumping
Quick conditions occur most often in__________ and
cannot occur in coarse soils.
fine sands or silts
The larger the particle size, the lesser is the porosity. T or F
F, greater
§ Zone between the land surface and water
table
§ Pore contains water and air
§ Also called as __________
Unsaturated zone, vadose zone or the zone of
aeration
§ pores are completely filled with water
§ Contains water at greater than atmospheric
pressure
§ Also called ________-
Saturated zone, phreatic zone
§ Surface where the pore water pressure is
atmospheric
§ Divide between saturated and unsaturated
zone
Water table
Zone immediately above the water table that
gets saturated by capillary forces
Zone Capillary Rise/Fringe
Soil Water
Three categories
- Hygroscopic water
- Capillary water
- Gravity water
· Microscopic film of water surrounding soil particles
· Strong molecular attraction; water cannot be removed by natural forces
· Adhesive forces (>31 bars and up to 10,000 bars!)
Hygroscopic water
· Water held by cohesive forces between films of hygroscopic water
· Can be removed by air drying or plant absorption
· Plants extract capillary water until the soil capillary force is equal to the
extractive force – Wilting point: soil capillary force > plant extractive force
Capillary water
· Water that moves through the soil by the force of gravity
Gravity water
Amount of water held in the soil after excess water has drained
is called the field capacity of the soil.
Field capacity
Below the water table, pore pressures are ______. In dry soil,
the pore pressure is _____. Above the water table, when the soil
is saturated, pore pressure will be _______
positive, zero, negative
The height above the water table to which the soil is saturated
is called the __________
capillary rise
Capillary rise
· in coarse soils capillary rise is very small
· in silts it may be up to _____
· in clays it can be over ______
2m, 20m
