Breadth - Geotechnical Engineering Flashcards
what is total stress?
total stress is effective stress + pore water stress
what is effective stress?
effective stress is total stress - pore water stress
what is the unit weight of water?
62.4 lb/PCF
what happens when a point load is applied to a soil
the stresses spread laterally with depth and stress decreases with depth
what is lateral earth pressure and how many types are there?
lateral earth pressure is the pressure that soil exudes on walls. there are 3 types of pressures and they are at rest, active, and passive pressure.
what is active earth pressure?
when wall moves forward due to the earth’s pressure pushing on it
what is passive earth pressure?
when the wall moves towards the soil (falls or pushes back)
what kind of failure is experienced during passive earth pressure?
the soil behind the wall experiences shear failure
if a wall with lateral earth pressure is unable to yield or cannot yield then what kind of pressure is it experiencing?
it is an at rest earth pressure
what is rankine’s theory?
it applies to active and passive pressure for a frictionless wall with a vertical back and a horizontal backfill of soil
what is the total force equation for active and passive lateral forces?
the total force is P=kgammah^2half + ksurcharge*H
what is consolidation?
a stress increase caused by construction of foundations or loads
what can soil settlement be classified as (the stages of consolidation)?
immediate settlement, primary consolidation, and secondary consolidation
what happens during the phases of soil settlement?
elastic deformation happens which is the initial loading, primary consolidation is when pore water pressure starts to be expulsed.
what is the difference between consolidation and compaction?
compaction is an instantaneous process and water content does not change while increasing the density of unsaturated soils. consolidation is a time dependent process and water content decreases while increasing the density of a saturated soil by draining the water
when looking at consolidation what are the 2 questions we ask and study?
how much will it consolidate and for how long
when working with TWO sand for drainage layers what can we assume about it?
half the thickness in the equation because it is double pervious and will drain faster
what is an example of an at rest pressure?
cantilever wall
what is a braced sheet wall an earth pressure example of?
passive earth pressure
what is a shallow foundation?
the footing is close to the surface or ground level
what is a tie down and when do you use it?
tie downs are used with spread footings when they will need to resist tension and have to be anchored down
what happens when a load is applied to a soil that is too heavy?
shear failure will occur when the load exceeds the allowable bearing stress
what happens when there is uneven settlement underneath loads and footings?
shear forces and moments can occur underneath
when looking at clays and bearing stress in shallow footings, what should we keep in mind about cohesion?
cohesion can also be used to find the undrained shear strength of clay
what is eccentricity?
its the effect of moment on spread footings
what is the eccentricity moment on spread footings?
e = M /p
what component in the footing design will cause the settlement?
the rigidity of the spread footing and whether the foundation of the footing is rigid or flexible
what determines whther the foundation of the footing is flexible or not?
the depth of the footing will determine how rigid it is. a thick foundation = rigid. thin = flexible.
is there a formula for calculating the initial settlement in footings? where can it be found
under the settlement (elastic method) formula.
what 2 features cause slope failure?
overstressing or reducing the shear strength of soil
what are causes of slope failure?
- change in slope profile such as excavation
- increase in ground water pressure due to rain causing reduction in frictional resistance or swelling of clay
- freezing and thaw cycles
- vibrations caused by earthquakes
- increase in driving force on slope
what is the maximum slope for cuts in cohesionless soil?
it is the angle of internal friction
how do you find the factor of safety in slopes with a stability number?
you must go to the table that shows deep/shallow circles and the formula can be found there
what are the dashed numbers on the slope stability FoS chart??
the dashed values on the chart are the values for d
d=D/H
can you use a submerged soil when funding slope stability?
no you need to subtract the submerged soil with the unit weight of water
what happens if the slope angle is greater than 53 in a stability slope problem?
it will be toe failure
what is the different between Taylor’s Chart and the other slope failure method?
Taylor’s chart requires you to know the height before the slope. you will use the equation n*H
can you find the formula for FS in dry sand?
yes you must look for it and it will be the formula that has tan phi / tan b
what are the 3 protective systems from OSHA?
shoring, shielding, and sloping
what is tsf?
ton / square foot
what is the minimum distance that temporary spoils should be placed at?
2’
what does cumulative mass retained mean?
it means how much is sitting on that particular sieve or how much soil has passed through at that pan
what does cumulative percent retained mean?
it is the cumulative divided by total M
what does D60 mean in sieve analysis?
D60 means that the particle size where 60% of it passes is finer
what is D60 in the soil classification tests?
D60 is sieve 1/2 inch or 12.7 mm
what is D30, in sieves, in the soil classification tests?
is sieve 40
what is D10 in the soil classification tests?
sieve 200
when is darcys equation used?
darcys equation is used to find permeability
what requirements must drilling techniques meet?
penetrate the material, collect appropriate samples, and prevent caving of the hole
what is a hollow stem auger?
a hollow stem auger is a type of drill that captures soil samples.
what is the SPT test and what does it provide?
the SPT test provides an N value result which correlates to its relative density and angle of internal friction
what is the N value?
the N value is the sum of the number of blows counted over the second and third 6” increments
when calculating for the correction factor what must we remember when finding the effective pressure?
we must remember that if the depth goes below the depth of the groundwater table then we must deduct the unit weight of water for whatever depth is beneath the water table
does passive or active forces require more movement to mobilize?
passive forces will require much more movement
what is the coulomb theory?
the coulomb theory differs from the rankine theory because it uses 3 variables and is a more complete consideration of all forces acting on the mass
what variables does coulomb take into consideration?
coulomb theory takes into consideration backfill, friction between wall and backfill, and the vertical face
what characteristics in construction does soil take when being borrowed/cut?
it has total unit weight and $/CY
what characteristics does soil take when being transported?
unit weight is loose, it’s at its max weight/volume, and $/trip
what characteristics does placement and compaction take?
its soil is max dry density, optimum water content, and percent compaction
what is the area ratio in percentage?
inner diameter^2 - outer area^2 / inner diameter^2
when doing phase diagram problems what can we assume about the degree of saturation?
we can assume that the degree of saturation is 1
what is the formula for total unit weight when given the saturated unit weight and the moisture content %?
total unit weight = saturated unit weight * (1+moisture content)
what is the formula for coefficient of at rest pressure?
total stress horizontal / total stress vertical
what is the formula for factor of safety for bearing capacity?
FS = q(allowable)/ vertical stress
where is the heel and the toe of cantilever retaining walls?
the toe is at the furthest left of the footing and the heel is the furthest right point
what is the factor of safety against sliding?
its the ratio of resistance against sliding to the sliding force
what forces contribute to overturning?
the passive and active forces contribute to overturning
what is the resisting forces?
the resisting forces are the weight of the soil off the edge, the weight of the retaining wall, and the base of the footing
what is the factor of safety for the overturning?
it is the resisting moment divided by the sum of the vertical forces
what is the force of friction?
summation of all the weights * 0.5
what are the load combination for LRFD?
U=1.2D + 1.6L
when dealing with the slenderness ratio what must you remember about the length?
when dealing with the length you have to use the weakest axis. the weak axis is the smallest value and can be either the x or y axis
what is the LRFD version of the flexural strength equation?
Mu=0.9*Mn
what causes a concrete foundation to lift?
when surface layers of soil freeze it causes the soil to expand and once the soil freezes into the deeper layers that soil will also expand causing pressure on the foundation causing it to lift
what can prevent foundations from shifting and moving?
you shape it trapezoidal
what are the 3 components of exploration?
boring, testing, and sampling
what is wash boring?
wash boring includes removing water from the hole and it is placed in a settling tank to allow the drill to bore a hole
what are disturbed samples?
a soil that has experienced large structural disturbance during the sampling. this sample will not be able to be used for some testing
what are undisturbed soil samples?
a minimum damage has been done to the undisturbed sample. can be used for many tests
what is the atterberg limits also referred to?
atterberg test can also be named the consistency limit
if a clay is in the granular materials what is it good for?
subgrade soil
what is the effective unit weight?
effective unit weight is the saturated weight minus the unit weight of water
when looking at a permeability problem, total head can be represented along any point by what?
elevation head and pressure head
what is the formula for FOS on safety?
the formula should be Fy / (Fx)
what is the formula for the force of sliding aka Fy?
F = w1+w2 * tan (angle of friction)
how do you find the slope on a point on a vertical curve?
you find the derivative of the curve elevation formula
when dealing with gravity wall problems with water tables, what is the first thing you should do before solving the problem?
draw the forces that are acting on the wall in the soil. from there you can solve the force components
cohesion is also known as what?
cohesion can also be referred to as shear strength
when given a problem regarding undrained soil and cohesion what formula must we use to solve the problem?
we must use the mohr’s columb formula
how do you find the angle of internal friction during a shear strength test?
you find the difference between the shear stress and divide it by the difference of the horizontal stress
when using the square footing contour charts, how do you use the chart?
for square footings square divide the sides by 2 and then square them. once you’ve added and squared the size of the footing divide that number by the original size of the footing and use that number along the x axis of the chart
what is another word for buoyant unit weight?
aka effective unit weight
what is direct runoff?
direct runoff is inflow that has a direct stormwater connection to sanitary sewer and causes an increase in wastewater flowrates
is clay subject to long term settlement?
yes clay will participate in long term settlement
what is the factor of safety in rotational failure for permanent slopes
1.5
what can you find from the coefficient of AT REST pressure?
you can find the horizontal and vertical pressure. the formula is total stress = horizontal stress / vertical stress
what is the formula for the height of a pile?
H=r*tanB
what is the formula for the adjustment of the weight of aggregates to saturated surface dry?
divide weight by moisture content plus 1. then multiply the moisture content of the aggregates and add 1
Wssd = (W / 1+1)*(1+wssd)
when calculating horizontal forces for retaining walls with groundwater table, how do you split up the retaining forces?
you have to calculate the soil pressure above the groundwater table, calculate the soil pressure below the groundwater table (rectangular), calculate the buoyant force against the soil, then the pore water pressure (just water)
