4-1
GROUNDWATER FLOW, PERMEABILITY, AND
PRESSURE
Lesson 4
4-2
LESSON 4 – GROUNDWATER FLOW, PERMEABILITY, and PRESSURE
Learning Outcomes -
Discuss effects of groundwater on rock slope
stability;
Define groundwater flow in rock masses –
permeability and head (pressure) distribution;
List factors influencing measurement of water
pressure with piezometers.
4-3
Effect of Groundwater on Slope Stability
Reduces shear strength of sliding surface
Induces thrust forces in tension cracks
Changes in moisture content accelerate
weathering
Freezing induces wedging in open fissures
Erosion of surface soils and weak infillings
4-4
Factor of Safety Calculation
pp
pP
VW
VUWcAF
cossin
tan)sincos(
5-2
4-5
Effect of Ground Water on Stability
Water Force V Acts in Tension Crack - Adds to Driving Force
(5-5)
Water Force U Acts on Sliding Surface - Decreases Normal Force
(5-4)
psww cosec)ztanbH(z½U
ww
2
z½V
4-6
Hydrologic Cycle
4-7
Hydrologic Cycle – Climate Effects
4-8
4-9
Groundwater Discharge and Recharge Zones
4-10
Effect of Permeability on Water Table
4-11
4-12
Flow Systems in Rock
4-13
4-14
Definition of Permeability - Darcy’s Law
4-15
Darcy’s Law - Calculation of Permeability, Head, and Flow
)hh(A
1Qk
21
K = permeability (m/s)
Q = flow quantity per unit time (m3/s)
L = path length (m)
A = path cross-section area (m2)
(h1 - h2) = head difference (m)
4-16
Range of Permeability for Rock and Soil Intact rock - shale, dolomite, granite:
- 10-7 to 10-10 cm/s
Fractured rock, clay-filled joints: - 10-3 to 10-6 cm/s
Jointed rock, clean sand: - 10-1 to 10-2 cm/s
Open jointed rock, karstic rock: - 10 cm/s
Heavily jointed, blasted rock, clean gravel: - 101 to 102 cm/s
4-17
Range of Permeability for Rock and Soil
4-18
Permeability of Intact and Jointed Rock
Permeability of intact rock (primary
permeability) - very low
Permeability of jointed rock mass (secondary
permeability) - depends on joint spacing and
aperture
4-19
Permeability of Joint Systems
)spacing(
)aperture(fk,tyPermeabili
3
e.g. Aperture = 1 mm, spacing = 1 m,
k = 8.1 x 10-4 m/s
Aperture = 0.2 mm, spacing = 1 m,
k = 6.5 x 10-6 m/s
4-20
Flow Nets - Equipotential and Flow Lines
4-21
Effect of Anistropic Permeability on Pressure Distribution
Isotropic
Horizontally Bedded
Inclined Bedding
4-22
Measurement of Water Pressure
132041A – Geotechnical Instrumentation
132031A – Subsurface Investigations
4-23
Stand
Pipe
CMT
Data
packs
I-90 Snoqualmie Pass East
Hyak to Keechelus Dam
Pro
ject
No
. 0
62-2
002
Date
: Jan
uary
2009
Figure 29
Piezometeric Data for Jenkins’ Knob Vicinity
0
20
40
60
80
100
120
11/04/06 02/02/07 05/03/07 08/01/07 10/30/07 01/28/08 04/27/08 07/26/08 10/24/08
Dep
th (f
t BG
S)
Date
H-201-06: VWP @ 122.2 ft
H-201-06: VWP @ 73.2 ft
H-203-06: VWP @ 97.2 ft
H-203-06: VWP @ 54.9 ft
VWP installation depths
H-201-06 in Design Sector V, H-203-06 in Design Sector VI
EXAMPLE of VIBRATING WIRE PIEZOMETER RECORD
4-25
Factors Influencing Installation of Piezometers in Rock
Orient hole to intersect discontinuities
Position completion zone in jointed rock
Effect of faults - high and low permeability
zones
Effect of rock types (e.g. sandstone and shale)
with differing permeabilities
Hydraulic time lag - standpipe, pneumatic
piezometers
Cost and reliability, access for readings
4-26
Field Measurement of Permeability
Must disturb the groundwater regime by adding
or removing water from the system and
measuring the rate of recovery
4-27
4-28
Field Measurement of Permeability
Variable head tests - e.g. falling head test in
standpipe piezometer (Table 4-3, Page 4-13)
Constant head tests – e.g. packer test (Fig 4-9,
Page 4-12)
Pump tests - pumped well and observation
well(s)
4-29 Groundwater Control – suit to geology
4-30
Ground Water Control
Passive Methods
Horizontal drains Adits/Galleries
Active Methods
Pumped wells
Pumped shafts
Combinations
4-31
Passive Horizontal Drains
4-32
Groundwater Control
Summary of
options
4-33
LESSON 4 – GROUNDWATER FLOW, PERMEABILITY, and PRESSURE
Learning Outcomes -
Discuss effects of groundwater on rock slope
stability;
Define groundwater flow in rock masses –
permeability and head (pressure) distribution;
List factors influencing measurement of water
pressure with piezometers.