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.