Groundwater & Seepage

Question 1

Flow Nets - which are flow lines which are equipotential?

Answer 1

“Drops” are flow lines
-seepage force is same along flow lines

Equipotential lines
-total head same at all points along equipotential lines

Question 2

Darcy’s Law = ?

Answer 2

Q = kiA

where i = change in head / change in water travel length
A = cross sectional area of flow (not same as water travel length)
ex. if given length of a dam with cross-section, A would be thickness of permeable layer*length of dam
Water travel length would be distance from U/S to D/S

Question 3

What causes seepage force in soil?

Answer 3

Viscous drag of water flowing through soils

Question 4

What reference to find seepage force equations?

Answer 4

UFC 10N - pg 262
Seepage force PER UNIT VOLUME = i*unit wt of water

Total seepage force on pg 260

Question 5

Effective Depth and soils for Dewatering Methods

Answer 5

Sump & Pump
-suitable for fine grained soils, limited application for coarse grained
-can lower about 15 ft

Wellpoint
-Suitable for sand and gravels
-can lower water about 15-20 ft

Vaccuum Wellpoint
-Suitable for silts and sany silts
-Difficult to maintain vaccuum greater than 15 ft depth

Deep Well
-Pervious sands or fissured rock
-Suitable for dewatering as deep as 300 ft

Ejector Wellpoint
-stabilize fine grained soils by reducing pwp
-limited to about 100 ft below pump

Cutoff Walls***
-very common
-usually sheetpiles, slurry walls, shafts, grout curtain, deep well, etc

Question 6

What is critical gradient, Ic?

Answer 6

Ic = (total unit wt - unit wt of water) / unit weight of water

I is change in head / change in water travel length

Question 7

What happens to permeability if compacted wet or dry of optimum?

Answer 7

If clay compacted WET of optimum –> lower permeability
DRY of optimum –> higher permeability

Question 8

Uplift pressure = ?

Answer 8

Uplift pressure = pressure head*unit wt of water

Question 9

Overall apparent permeability if given Kh and Kv?
Avg permeability?
Effective permeability?

Answer 9

Overall apparent, effective permeability = Sqrt (Kv*Kh)
Avg permeability = (Kh + Kv)/2

Question 10

Potential side effects on adjacent facilities from dewatering?

Answer 10

If not carefully controlled and designed, dewatering can result in settlement of adjacent structures