Exam 2 Flashcards
Flow Lines
Lines of Fluid Flow
Equipotential Lines
Connects points of equal total head (include both ground lines)
Rules for Flow Net
- Lines must cross at 90 degrees
2. Must have equal lines (circle inside)
Flow Net is what equation graphically
La Place
Total Head
(Elevation Head) + (Pressure Head)
Change in Head (delta h)
(Elevation of top water) - (Elevation of Bottom water)
Hydraulic Gradient
i=(Change in head) / (# of drops)
Total Head at certain point equation
Htotal,c=(total head) - (i)*(# of drops)
Elevation Head
Helevation= elevation at point
Pressure Head
Amount of pressure at point, use Total Head formula to plug and chug pressure and total
Total Flow Equation
q=kh(Nf/Nd)
- k:given
- h:Change in head
- Nf: # elements to bottom of pile
- Nd: # of drops
What does a Sheet Pile do
-sheet pile lengthens the flow path, this creates more equipotential drops, more drops lessens the total head which lessens the pressure head and therefor lessens pressure under the damn
Where is hydraulic gradient the greatest
i=(change in head) / (length)
Change in head stays the same, and the smaller the length means larger gradient so the smallest element has the greatest (usually under the pile)
4 ways to deal with groundwater
- Open pumping
- Pre-draining
- Cutoffs
- Exclusion
Open Pumping
- least expensive solution to dewatering
- pump water as it fills the hole
Pre-Draining
-look up better definition from book
Cutoffs
-look up from book
Exclusions
-look up from book
Ways to construct a Cutoff wall
- diaphragm walls
- slurry trench
- secant drilled shaft wall
- tremie seals
- permeation grouting
- ground freezing
- *get definitions from book
Total Stress, sigma
-weight of everything on top of that point
=(density of material) * (height of that material) + …..
Porewater Stress, u
u=(density of water) * (height of water table)
Effective Stress, sigma bar
=(total stress) - (porewater stress)
=sigma - u (in psf)
Relations for void ratio and specific gravity
e=(w*Gs)/(Sr)
(Unit weight)d=(Gs*Unit weight water)/(1+e)
Unit weight=(Unit weight)d * (1+w)
(Unit Weight)d= (unit weight)/(1+w)
Relative Compaction, Cr
Cr=[(unit weight)d]/[(Unit weight)d max] x 100%
What is a Heave
-When excavation is lifted
When does a heave occur
When effective stress=0 which means totals stress=porewater stress
Capillary Rise
Ability of water to move upwards against gravity due to adhesion of the fluid and the solid wall
Quick Condition
upward water pressure gradient and water flow reduce the effective stress (quick sand)
Forms of Compaction
-Runways, Earth Damns, Roadways, Embankments, Levees
4 Types of Compaction rigs
- Sheepsfoot Roller
- Rubber Tired Roller
- Steel Wheeled Roller
- Vibratory Roller
Why compact?
- increases engineering properties of the soil
- stress applied to lower amount of air and void space in soil
- Densifies the soil
What does compaction do to Engineering Properties?
- Higher strength
- Lower compressibility
- Lower permeability
- Less tendency to absorb water
What does compaction do to Performance of the fill?
- Greater stability
- Less settlement
- Less seepage
- Reduced Shrink/Swell
Laboratory Compaciton techniques vs. field use
- Impact compaction=None
- Static compaction =Steel/rubber wheeled roller
- Kneading Compaction=Sheepsfoot roller
- Vibratory=Vibratory Roller
Calculate Pore Pressure at a Point
Pore Pressure=[(Nd/Delta h) + Height of water above] * (Unit weight of water)
Compute moisture content for 80% and 100%
W=S( (unit weight of water) / (dry unit weight) - (1/Gs)) x 100%
Compute Relative companion, Cr
(Dry unit weight)=(unit weight) / (1+w)
Cr=(dry unit weight) / (dry unit weigh max)
Standard is Cr>90%
Sand Cone Steps
- weight of soil
- Volume of soil
- Density of soil
- water percent of soil
- dry unit weight of soil
- Relative compaction of soil
Pressure in Water Formula
Height of head * Unit weight of water
How does compaction differ for a landfill and that of a fill to support a building
Fill= based on percent of dry unit weight, Has high dry unit weight which means low compressibility and high strength (range near 10%)
Landfill=concerned in k value, may specify compacted WET of optimum
What is a sand cone and how is it used
- device to measure volume in the field
- Used to get dry unit weight
What type of soil is best compacted with a sheepfoot compactor
Clay and will also do silt
What type of soil is best compacted with a vibratory compactor
Granular (Sand)
How is a Piezometer reading calculated
-find the total head at that area
h(total)=Height of water - (change per drop * # of drops)
What happens when an excavation has a negative effective stress?
Clay will “blow out” or heave
Four types of Field Compaction Tests
- Sand cone
- Rubber Balloon
- Drive Cylinder
- Nuclear Density
Four ways to compact soil in the laboratory
- Impact (Dropping weight on it)
- Static (Load is applied and held)
- Kneading (small foot is loaded and unloaded multiple times)
- Vibratory (for sand and gravel, vibrated)
Why in a lab is impact compaction most popular?
-been around the longest so a large data base
Standard proctor test vs. Modified
Modified has a larger 10 lb weight dropped from a higher 18” distance with 5 layers instead of 3
Why does dry density lower after optimum water content?
At optimum water content the soil is considered saturated, so adding more water would just replace soil and decrease the dry density
