asce oc geo institute - dinner presentation - 08-15-2017
TRANSCRIPT
Definitions
Infiltration Rate- The “straight-path” velocity at which water enters into soil Is dependent on the water pressure More pressure= higher infiltration
Permeability Rate- Also known as Hydraulic Conductivity Is an intrinsic value - does not vary due to pressure
Percolation Rate- Rate that water falls in a percolation test Dependent on water pressure and geometry of test
All three have units of length/time- leads to confusion
Flow of water through Soil
AdjacentElements
Element
K·i (X-direction)
K·i (Y-direction)
Boundary Flux
Change in Water Content
Laboratory Testing for Comparison
Kozeny-Carmen Equ. from Chapuis & Aubertin (2003)
A2 a constant in the range of 0.29-0.51; commonly 0.29 is used. Gs the specific gravity of solids, 2.65. S the specific surface (m2/kg) e void ratio k hydraulic conductivity in m/s
Design of Chamber or Basin
Ratio of Clearance to Aquitard/width of BMP
Effective Infiltration Rate
2.25 0.75U + 0.25L
1.5 0.5U + 0.5L
0.75 0.25U + 0.75L
U= Upper Permeability L= Lower Permeability
Design of Chamber or Basin
Depth of Ponding (in)
Percent of Infiltration for 12 inches of ponding
9 75%
6 50%
3 25%
Well Permeameter Equations
Find Q at ave. water level and Volume of segment
T1=V1/Q1
Find Q2 and V2 T2=V2/Q2
Find Q3 and V3 T3=V3/Q3
Find Q4 and V4 T4=V4/Q4
Find Q5 and V5 T5=V5/Q5
Find Q6 and V6 T6=V6/Q6
Add up for Total Time
Ave. water level of segment
Well Permeameter Equations
K=saturated permeability T1 & t2= Times of interest H1 & H2= height of water in well at t1 & t2 L= Length of well screen r= Well radius
Computer Model of Dry Well
Water Surface
More permeable layer
Less permeable layer
Impermeable Layer (fill)
Radial Distance of Saturation
Upper Chamber
Lower Well
Computer Model of Dry Well
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14 16 18 20
He
igh
t o
f W
ate
r in
We
ll (f
t)
Time (hrs)
40-foot deep, 4-foot diameter: Height of Water in Dry Well
Design Parameters
Infiltration Basin/Chamber- Infiltration rate = permeability if ponding is 12” or more Reduce infiltration rate for ponding less than 12” using linear reduction Adjust rate in consideration of impeding layers or GW If impeding layer or GW more than 3X width, minimal effect If shallower than 3X Width, roughly weighted to proportion of distance Provide a second infiltration rate for down draw determination Use ½ of the value appropriate to the max ponding depth
Dry Well- Infiltration rate ≠ permeability Need Q value for specific well configuration Use computer software or closed-form equation if suitable Effective Infiltration rate = Q/wetted area Effective Infiltration Rate will be much greater than permeability Don’t be surprised to get rate that is 5X to 20X of permeability Use finite step method or Horslev equation for explicit draw down Or use computer model to determine drawdown time