3. Soil Mechanics Flashcards
What is Total Stress?
the total weight of soil along a soil cross section divided by the area of the cross section. Include pore water pressure.
What is Effective Stress?
sum of the stress transmitted through the contracting soil grains along a soil cross section. Does not included pore water pressure.
Pore Water Pressure?
pressure transmitted through water in soil voids (pore water).
What are the deformation of soils a function of?
Effective stress.
What type of stresses does effective stresses apply to?
Normal stresses only (not shear stresses)
What is another name for effective unit weight?
Buoyant weight.
How is the effective stress effected by the water table?
As water table goes down, effective stress increases; conversely, as water table rises, effective stress decreases.
What influence does the water table have if it fluctuates ABOVE ground?
The total and pore pressure in the soil changes, but NOT the effective stress.
How do vertical loads at ground surface affect the soil beneath the loads?
Stress increase in the soil; spreading laterally and decrease with depth but affecting a larger plan area.
What type of material is suitable to be analyzed with the Boussinesq equation?
Homogeneous and isotropic material.
What are two methods for computing stress increases below a uniform load?
(1) Approximate Method (2:1 method) (2) Elastic Theory using influence charts
What type of loading is the Boussinesq equation valid for?
Point Loads.
What are the three types of Pressure Coefficient, K?
K0 = at rest; Ka = active; Kp = passive
What is Earth pressure?
When the wall and pressure applied by the material is known is static (at rest).
What is a Active earth pressure?
When the wall moves AWAY from the backfill. There is a decrease in pressure on the wall which continues until a minimum value is reached after which no further reduction in pressure.
What is Passive earth pressure?
When the wall moves TOWARDS the backfill. There is an increase in the pressure on the wall which continues until a maximum value is reached after which no further increase in pressure.
Between active and passive earth pressure, which requires the greater force before the wall exhibits movement?
Passive.
In what situation could active and passive earth pressure occur at the same time?
After excavation of soils on one side of a sheet pile. Top half becomes active (away from the soil) while the bottom becomes passive (into the soil).
What form of stress distribution does earth pressure take?
Linear from the surface (shape = triangle)
What is the assumption of Rankine’s Theory?
Frictionless wall with a vertical back and horizontal backfill of granular soil.
Identify and describe the three types of settlement?
- Immediate Settlement: caused by elastic deformation (distortion of soil volume but no volume change)
- Primary Consolidation Settlement: reduction of soil volume as a result of pore water expulsion
- Secondary Consolidation Settlement: reduction of soil volume as a result of plastic adjustment of the soil structure
What are the differences between Compaction and Consolidation?
Compaction: Increases density of a unsaturated soil by reducing the volume of air.
Consolidation: increases density of a saturdate soil by draining out some of the water.
Compaction: Instantaneous process
Consolidation: TIme dependent
Compaction: Water content does not (or little) change
Consolidation: Water content decreases
Consolidation of Sand
Drainage occurs almost instantaneously; thus settlement is immediate. Elastic and consolidation process cannot be isolated therefore, primary consolidation is incorporated in the elastic parameters.
Describe the consolidation of clay
- Drainage occurs slowly; thus settlement and strength gain are gained are delayed.
- Settlement can be separated (elastic, primary and secondary).
- Clayey soils undergo consolidation settlement with not only under the action of external loads but also under its own weight.
- Clayey soils undergo settlement when dewatered because the effective stresses increases.
What is the main purpose of the consolidation test?
To obtain soil properties which are used in predicting the rate and amount of consolidation settlement of structure founded on clay. Know as Oedometer Test
What is Normally Consolidated clay?
When the maximum past effective stress is equal to the existing effective stress.
What is Over Consolidated (Preconsolidated) clay?
When the maximum past effective stress is GREATER than the existing effective stress.
What is Under Consolidated clay?
When the maximum past effective stress is LESS than the existing effective stress.
What is the Overconsolidation Ratio (OCR)?
The ratio of past effective stress to the existing effective stress. If OCR = 1, the soil is normally consolidated. OCR > 1 = Over consolidated; OCR < 1 = Under consolidated.
What does the rate of consolidation provide?
(1) information on time required to a specific settlement, (2) a function of compressible soil properties, thickness, and top and bottom drainage condition.
Coefficient of Consolidation, Cv is proportional to?
(1) coefficient of hydraulic conductivity (2) the distribution of pore water pressure (3) effective stress in a consolidating clay layer between two layers.
What are the functional differences between coefficient of consolidation (Cv) and compression index (Cc)?
Cv is a function of time; Cc is a function of pressure
How does doubling the drainage distance effect the consolidation?
It increases the consolidation time by a factor of 4 (since Hdp is squared)
What is considered a shallow foundation?
When the foundation’s depth to width ratio, D/B, is less that or equal to 1 (i.e. D is less than or equal to B).
How are shallow foundations classified?
Isolated Footing, Wall Footing, Combined Footing, or Mat Foundation.
How are shallow foundations designed?
Design is based on maintaining bearing (contact) stress below the allowable bearing pressure of the soil.
How is the ultimate bearing capacity calculated?
It is calculated as “gross”, thus the weight of the concrete footing and any soil covering the footing need to be included in the calculation of the bearing stress caused by applied forces.
What are the two components of shallow foundation settlement?
(1) Immediate (short term) and (2) Consolidation (long-term)
How does the rigidity of a foundation vary the settlement?
For rigid foundation the settlement is uniform, where as a flexible foundation, more settlement occurs at the center than the edges.
What is slope failure?
the overstressing or reduction in shear strength of soil which may cause rapid or progressive movement of the slope.
What are some causes of slope failure?
- Change in the slope profile such as excavation when widening a highway.
- Reduction in shear resistance due to freeze and thaw cycles and weathering
- Increase in the driving force acting on the slope
- Increase in ground water pressure due to rain or seasonal fluctuations that may reduce frictional resistance or cause swelling of clay
- Vibration caused by earthquake, blast or pile driving
For cohesionless soil (drained sands) what is the maximum slope cut controlled by?
the angle of internal friction (phi).
What are the three different slope failures used in NAVFAC and Taylor analyses?
Base Circle, Toe Circle, and Slope Circle.
What is the density of water in both metric and US customary units
62.4 pcf (USCS); 1 g/cm^3
