ASCE OC Geo Institute - Dinner Presentation - 08-15-2017

Presented by:

David Albus, RCE, GE

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


Hydraulic Gradient

TOTAL HEAD=

PORE PRESSURE +

ELEVATION

SATURATED FLOW


AdjacentElements

Element

K·i (X-direction)

K·i (Y-direction)

Boundary Flux

Change in Water Content

Infiltration in a Basin


Impounding layer caused by soil with lower permeability


Other “Basin” Systems

Arched Chamber Systems Stackable Block Systems

Dry Well System

Types of Percolation Testing

Shallow Percolation Pit - Falling Head

Deep Percolation Pit - Falling Head




Single-Ring Infiltrometer – Constant Head

Single-Ring Infiltrometer – Constant Head

Double-Ring Infiltrometer – Constant Head



Well Permeameter - Constant Head

USBR 7300-89

Well Permeameter - Constant Head

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


Depth of Ponding (in)

Percent of Infiltration for 12 inches of ponding

9 75%

6 50%

3 25%



Design of Dry Well

Well Permeameter Equations







Find Q at ave. water level and Volume of segment

T1=V1/Q1

Find Q2 and V2 T2=V2/Q2





Add up for Total Time

Ave. water level of segment


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


Chamber Section

Gravel Section

95’ Invert

80’

60’


TIME= 0 HR

Groundwater


TIME= 1.7 HR


TIME= 5.5 HR


TIME= 9.7 HR


TIME=13.6 HR


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

Questions & Answers

ASCE OC Geo Institute - Dinner Presentation - 08-15-2017

Engineering