2 - Effective stress Flashcards
For an element of saturated soil, why is the water in the pores under pressure?
Due to physical location and external forces.
What is the pressure within the pore water called?
The pore pressure
What 2 things can pore water be?
Either static or seeping
What is static pore pressure?
Where the hydrostatic pressure depends simply on the depth below the water table
What is seeping pore pressure?
When the pore water seeps through the soil under a hydraulic gradient.
If you put a standpipe in soil with water in it, how could the pore pressure be shown?
As the height to which water would rise to, in a standpipe open to atmospheric pressure
What does pore pressure, u equal?
u = yw X d
or
Pore pressure = unit weight of water X depth of water
What is hw?
The pore water pressure head, or simply the pressure head.
What is the natural, static level of water in the ground called?
The water table
Below the water table, are the pore pressures positive or negative?
Positive
Above the water table, if the soil is saturated, is the pore pressure positive or negative?
The pore pressure is negative
In dry soil, is the pore pressure positive or negative?
Pore pressure will equal zero
Why do the pore pressures with a soil sample need to be monitored?
To know about changes in the water table, fluid flow and stress levels.
How are pore pressured traditionally measured?
In the drainage pipes outside a triaxial cell using a large pore pressure inducer
Why is it important that a pore pressure measurement system is completely free of air?
So that minimal pore water flow is required to register changes in pore pressure.
What is the simplest way of determining the ground water level and pore water pressure?
By observation in a borehole that is fitted with a perforated pipe.
What are the benefits of using a standpipe?
They are cheap and easy to install.
What are the negatives of using a standpipe?
Require manual effort to take readings, therefore remote readings are impossible.
What does the Casagrande Piezometer consist of?
A filter tip joined to a riser pipe
What are the benefits of using a Casagrande Piezometer?
They are economical, simple to use and have long term reliability
What is a negative of using a Casagrande Piezometer?
Remote readings are impossible, as manual labour is required.
How does a pneumatic piezometer work?
The piezometer is pressurized with air, and the change in air pressure is used to measure the pore water pressure. As the pressure changes, it exerts a force on the air with the chamber, which is measured by a gauge.
What are the advantages of using a pneumatic piezometer?
They are sensitive, detect small changes, and do not directly contact the ground water.
What is the disadvantage involved with using a pneumatic piezometer?
They are very expensive, and the tube can be vulnerable to damage.`
What does a vibrating wire piezometer compromise of ?
A vibrating wire pressure transducer and a signal cable. It converts water pressure to a signal using a diaphragm, a steel wire and an electromagnetic coil.
Where can a vibrating wire piezometer be used?
In a borehole, suspended in a standpipe, or embedded in a saturated soil.
What are the negatives of using a vibrating wire piezometer?
Expensive and require electrical protection.
What do pressure transducers consist of?
A diaphragm, strain gauges, and a data logger. Water pressure acts on the diaphragm of the transducer, and produces a signal output proportional to the pressure.
What are the benefits of using a pressure transducer?
Easy to utilise and allow high frequency changes.
What are the negatives of using a pressure transducer?
Expensive and the reading can ‘drift’ over time.
What are geostatic stresses?
Stresses caused by gravity acting on the soil or rock.
Why are geostatic stresses always present in soil?
Because there is always gravity.
What are induced stresses?
Stresses caused by external loads, such as foundations
Due to soil being a granular material, we make assumtion to formulate stresses within a soil body, we use CHILE, what does this stand for?
Continuous, Homogeneous, Isotropic, Linear, Elastic
What is the total vertical stress acting on a point of soil?
The weight of everything above it, including the soil itself.
What does the friction between soil grains control?
A lot of the soil behaviour
What is the friction between soil particles controlled by?
The interphase resistance (u) and the reaction force between particles.
Why is it impossible to measure the exact load at every particle interaction in soil?
There is millions, therefore we take an effective load.
What causes a difference between total stress and effective stress in soil?
The presence of water
What happens to the effective stress as the water pressure increases?
The effective stress decreases, as water pressure in the voids acts to push soil particles apart from each other, therefore reducing the load at the particle contacts.
What does the effective stress (‘) equal?
The total stress - water pressure (u)
What is Terzaghi’s principle?
That effective stress = total stress - water pressure (u)
What does change in effective stress equal?
Change in total stress - change in water pressure
What will a change in effective stress cause?
A change in volume and strenth
What will a change in total stress cuase?
Will often not cause a change in effective stress
What will happen immediately after the construction of a foundation on a fine soil (e.g. clay) ?
The pore pressure increases, but immediately begins to reduce as drainage occurs. The rate of change of effective stress under a loaded foundation will be the same as the rate of change of pore pressure.
What is the rate of change of pore pressure controlled by?
The permeability.
What will a change in vertical effective stress also cause a change in? (hint, horizontal….)
A change in the horizontal effective stress.
What is the ratio of horizontal to vertical effective stress defined as?
The coefficient of the lateral earth pressure, K
