Unit 03b : Advanced Hydrogeology
Engineering Implications
Engineering Implications
• Groundwater flow systems have
important implications for certain
engineering activities and can be
significantly modified by others.
– Large reservoir impoundments
– Induced seismicity
– Excavation inflows and stability
– Landslides and slope stability
Reservoir Filling
Flooded
Natural
• Dams are generally
constructed in groundwater
discharge areas (because
aquifers predominantly
discharge to river valleys).
• Reservoir heads are
generally greater than the
aquifer heads.
• Groundwater discharge to a flooded valley is usually
inhibited as a reservoir fills.
• Recharge continues unaffected by flooding.
Transient Readjustment
Reversal
Flooded
New Springs
Natural
• The flow regime adjusts by filling groundwater storage
until a new steady state is established.
• Where the water table was near the surface, new
discharge zones become established. (e.g. Flathead
Reservoir, Mt.)
• Flow direction reversals in the subsurface are likely to
occur. (e.g. Lake Diefenbaker, Sk.)
Valley Bottom Stability
Post-Reservoir Head
Zone of potential uplift and slope failure
Pre-Reservoir Head
Aquitard
Aquifer
• Reservoir impoundments can also lead to stability problems.
• Beneath the reservoir, increased pore-pressures are partially
compensated by the total stress increase due to the water loading.
• Downstream of the impoundment, pore-pressures are increased to
similar levels with no total stress compensation.
Valley Wall Stability
Piezometric Surface
Slide Block
Water Table
River
Slide Debris
Regional Aquifer
• Pore-pressures can reactivate bedrock shears, faults and gouge
(mylonitic) zones
• Increased uplift pressure can cause heave of the valley floor.
• Bedrock slide blocks and landslides can be reactivated or initiated
by large changes in pore-pressures in valley walls.
Induced Seismicity
• Many human activities are known to induce or
increase seismic activity (Simpson,1986):
– fluid injection for various purposes:
•
•
•
•
–
–
–
–
waste disposal,
solution mining,
geothermal power generation, and
secondary oil recovery;
deep underground mining;
removal of large volumes of rock during quarrying;
fluid extraction in petroleum production; and
impoundment of large reservoirs behind high
dams (Simpson, 1986).
Seismic Risk
• Of all causes, reservoir impoundment has
produced the largest earthquakes.
• There is evidence linking earth tremors and
reservoir operation for more than 70 dams.
• Reservoirs are believed to have induced five
out of the nine earthquakes on the Indian
peninsula in the 1980s which were strong
enough to cause damage.
• Reservoir induced seismicity (RIS) is well
documented but relatively poorly understood.
RIS
• The mechanisms of RIS are not sufficiently well
understood to predict accurately which dams will
induce earthquakes or how strong the tremors are
likely to be.
• Most of the strongest cases of RIS have been
observed for dams over 100 metres high - but dams
just half this height are also believed to have induced
quakes.
• Reservoirs filling can
– increase the frequency of earthquakes in areas of already
high seismic activity and
– cause earthquakes to happen in areas previously thought to
be seismically inactive.
t
Mechanism I
sn
• The most widely accepted
explanation of how dams
cause earthquakes is related
to the extra water pressure
created as the reservoir fills.
• When the pressure of the water (u) in the rocks increases, it acts
to reduce the normal load (sn) on fault planes thereby reducing
the frictional resistance mobilized and increasing the tendency
to shear.
t = c + (sn – u) tan(f)
where
t
is the shearing resistance
c
is the cohesion and
f
is the friction angle
• Note that the principal stresses remain in the same orientation.
t
Mechanism II
• It is also possible to cause
failure by increasing the
vertical principal stress as a
result of the weight of
impounded water.
sn
• In this case the normal load (sn) on fault plane has increased
but the deviatoric stress has changed more to induce failure
t = c + sn tan(f)
where
t
is the shearing resistance
c
is the cohesion and
f
is the friction angle
• Note that in this case the orientation of the principal stresses are
changed and that this mechanism will only trigger normal faults
where s1 is vertical.
Evidence
• For most well-studied cases of RIS, the intensity of
seismic activity increased within around 25 km of the
reservoir as it was filled.
• The strongest shocks normally occurred relatively
soon - often within days but sometimes several years
- after the reservoir reached its operating level.
• After the initial filling of the reservoir, RIS events
normally continued as the water level rose and fell
but usually with lower frequency and magnitude than
the initial events.
• The pattern of RIS is, however, unique for every
reservoir.
Interpretation of Evidence
t
• The evidence is consistent
with a mechanism involving
stress-relief.
• The early events release
the initial stresses more
quickly the nearer they are
t
to the critical level for slip.
• Later changes in stress
trigger less violent releases
as the fault plane weakens
(c,f approaches residual )
with each successive
event.
sn
sn
Another Perspective
• Seismologists have published a list of about 100
cases of RIS.
• These cases show that after the completion of a dam,
the reservoir area experienced earthquakes of microlevel magnitude - 2.0 or 3.0 on the Richter scale.
• Dense seismic networks have increased the
detection potential and increased the number of
cases cited as instances of RIS.
• The earthquakes that the Indian peninsula has so far
experienced may not be attributable to dams.
• Construction of dams should be done in such a way
as to withstand anticipated seismic activity and minor
stress adjustments are inevitable.
Indian Earthquakes
• India is unique as far as earthquakes are concerned.
The northern part of India, the Himalayan frontal arc,
is one of the seismically most active regions in the
world.
• Four great earthquakes (>8.0M) have occurred in a
the period 1897-1950 – the largest subsequent
earthquake occurred in Gujarat in 2001.
• A catalogue of Indian earthquakes from the earliest
times has been compiled.
• The 1967 earthquake at Koyna M6.3 in Western India
confirmed that peninsular India, believed until then to
be aseismic, is vulnerable to earthquakes.
• The more recent 1993 Killari earthquake M6.4 in the
Deccan Traps was unexpected and devastating.
Seismicity of India
Gujarat
Killari
Koyna
• The map shows
the location of the
Koyna and Killari
earthquakes in
the largely
aseismic Indian
penninsula.
• The recent M7.9
Gujarat quake is
also shown.
Koyna Dam Earthquake
• The area between the Koyna
and the Warna dams, in the
vicinity of the Shivaji Sagar
and Vasant Sagar reservoirs,
is unique for its ongoing, high
level of seismic activity.
• Seismicity at Koyna has close
correlation with the filling
cycles of the Koyna reservoir.
• The 1967 Koyna event, in the watershed of the Krishna River in
Maharashtra state, is a classic example of earthquake activity
triggered by reservoir.
• The world's worst confirmed reservoir-induced earthquake was
triggered by the Koyna Dam.
• Nearly 200 were killed in the magnitude 6.3 tremor.
Koyna Dam Background
• Since its first impoundment
in 1962, more than 150
earthquakes of magnitude
4.0 have been recorded.
• Events are mostly restricted
to an area 40 × 25 km2 south
of the Koyna-Dam.
• This marks the area as
probably the best in the
world to study the
phenomenon of reservoir
induced/triggered seismicity
(RIS).
• The height of the Koyna-Dam is 103 m, reservoir volume is 2.78×109 m3.
• Seasonal fluctuations of the lake level are typically 30 to 35 m and are
dominated by monsoon rainfalls.
• The site is now highly instrumented and the subject of active research
Killari Event
• The most puzzling event in Peninsular India is the
Killari earthquake.
• The devastating magnitude 6.4 earthquake struck
Killari, Maharashtra in 1993, killing 10,000 people.
• The event was totally unexpected as it was located in
the Deccan Trap-covered stable Indian Shield. There
was no record of any historical earthquake in the
region.
• The Killari earthquake is considered the most
devastating SCR (Stable Continental Region) event
in the world.
• Some seismologists believe that the Killari event was
triggered by a nearby (Tirna) reservoir.
Tirna Reservoir
• The Killari earthquake was about 10 km from the
Lower Tirna Reservoir.
• The maximum water depth is about 20m, which is at
the low end of the range of depths of reservoirs
where induced seismicity has been documented.
• The reservoir level was low at the time of the main
shock, which is consistent with the expected negative
effect of the loading by the reservoir on an underlying
thrust fault.
• Several other recent earthquakes in peninsular India
appear to be located close to reservoirs.
• Whether the Killari earthquake was triggered by the
Lower Tirna reservoir is not known, but it cannot be
ruled out at this time.
Narmada Valley
• Indian seismologists have noted an increase in seismic activity
in the Narmada Valley over the past 20 years, which may be
linked to reservoir impoundment.
• In the Narmada Valley, a series of tremors were felt soon after
the completion of the Sukta Dam.
• A strong earthquake hit the Narmada Valley on May 22, 1997,
killing around 50 people and injuring 1,000 in the city of Jabalpur
in the state of Madhya Pradesh.
• The epicentre of this magnitude 6.0 earthquake is believed to
have been about 20-40 kilometers from Bargi Dam, which
completed filling in 1990.
• The recent earthquake has focused attention on the seismic
risks faced by the large dams planned for the Narmada Valley,
and on the risk of reservoir-induced earthquakes.
Seismic Hazard Assessment
• Seismic hazard assessments are an integral part of
site investigation for large dams and reservoirs.
• In order to interpret the recorded seismicity of a
region, a thorough review of the available previous
seismicity and seismotectonic studies is performed.
• The analysis is further deepened through the
integration of three-dimensional velocity structures
and inversion studies beneath this area.
• The compilation of all these data makes it possible to
define and gain considerable insight concerning the
major seismic sources active in the region.
Talembote Case History
• The assessment of seismic hazard within the
Talembote area, Morocco, is a study of a dam located
within the actively deforming intermountain belt of the
Rif region, considered the most active zone in
Morocco.
• The historical seismic data available on Morocco
extend to about 11 centuries back in history.
• Of more importance is the 20th century seismicity
data, which reveals the occurrence in 1909 of a M6.4
event about 50 km away from the dam.
Location of Talembote, Morocco
Talembote Seismic Setting
• Of particular importance are shallow surface features;
mostly normal and strike-slip faults, which are
identified as local faults that are running right next to
the dam-site.
• However, most of the seismic activity seems to be
related to reverse faults along Rif-nappes connected
to a detachment surface at about 20 km-depth.
• This detachment runs right underneath the dam-site.
The detachment zone may coincide with a low
strength layer that decouples the overlying sediments
from the basement of the African Plate.
• As a result, there is a high level of small magnitude
earthquakes.
Talembote Seismic Analysis
• The analysis of seismic hazard of the site of the
Talembote dam has shown that the Maximum
Credible Earthquake (MCE) is in the order M6.8,
risking to produce a maximum acceleration of 0.5g.
• This event could possibly be generated once every
ten thousand years by one of the faults passing in the
immediate proximity of the dam.
• When considering the much shorter design life for the
dam-structure, it is normal to use an earthquake
return period 7 or 8 times the 75-year design life.
• An acceleration of 0.085 g, corresponds to a return
period of 550 years. This acceleration is rounded to
predict an operational basic earthquake of 0.1 g.
Induced Seismicity
• Many human activities are known to induce or
increase seismic activity (Simpson, 1986):
– fluid injection for various purposes:
•
•
•
•
–
–
–
–
waste disposal,
solution mining,
geothermal power generation, and
secondary oil recovery;
deep underground mining;
removal of large volumes of rock during quarrying;
fluid extraction in petroleum production; and
impoundment of large reservoirs behind high
dams.
Fluid Injection and Extraction
t
t
sn
• It is easy to see that
increases in pore
pressure due to fluid
injection may reduce
the effective strength of
faults below the critical
shear stress causing
failure and earthquakes.
sn
• It is also apparent that a
reduction in pore
pressure due to fluid
extraction will have the
opposite effect,
effectively strengthening
the fault.
Fluid Extraction
• It is more complex to explain how fluid extraction can
induce seismicity.
• In fact, earthquakes are not induced in reservoirs
undergoing extraction.
• The failures occur in the surrounding low permeability
rocks where high pore pressure gradients are
generated.
• The analysis of Grasso (1995) and Segall (1992)
shows that seismicity occurs near the reservoir
magins, both above and below the depressurized
formation.
• The analysis is an example of the application of the
theory of poroelasticity (Segall et al., 1994).
Fluid Injection
• For fluid injection to
induce seismicity
several conditions
must be satisfied:
– Differential stresses
must be high
– Absolute stress level
may be low
– Pore pressure build up
must occur so low
permeability reservoirs
are more susceptible
t
t
sn
sn
Excavations
t=0
Excavated face
x
t=t
HH
h(x,t)
Seepage face
L
• Ibrahim and Brutsaert
(1965) provide a simple
analytical solution for
predicting inflows and
water-level decline in the
vicinity of excavations.
• For the simple 2D case
with an initial head H, in an
isotropic homogeneous
region, length L, the
solution, h(x,t) depends on
h/H and x/L.
Transient Excavation Heads
1
t=0
x
t=t
h(x,t)
10
0.1
0.5
HH
1
1.0
h/H
c
0.1
5.0
0
L
• The transient response
depends on the
dimensionless Fourier
number
NF = t / T*
where T* = KH/SyL2
x/L
1
0.01
0
NF
4
• The dimensionless discharge
is given by:
c = T*q’ / HL
where q’ is the inflow per unit
length of excavation.
Transient Excavation Heads
1
10
0.1
1
0.5
c
1.0
h/H
0.1
5.0
0
x/L
1
0.01
0
NF
4
• At the seepage face (x=0) the
• For example, let Sy =
predicted head is 0.2H and at
0.26, L = 100 m, K = 10-4
the boundary (x=L), 0.7H
m/s, H = 5 m, so T*= 60 • The dimensionless discharge
days. After 30 days NF =
is about 0.35 so the inflow per
0.5.
unit length is about 350 m3/d
Trench Problems
• One of the most
common problems in
construction is bottom
heaving or boiling in
trenches.
• Heave occurs because
the material removed
was providing a normal
load preventing upward
displacement.
• Boiling results from
removal of granular
particles by high
seepage exit velocities
Heaving
• Before trenching the total
stress at A is:
g s h1
• After trenching the total
stress is:
g s h2
• The maximum uplift
pressure at A is thus:
gs(h1 – h2)
• Uplift pressures can result
in heaving at the base of
trenches, particularly in
plastic cohesive soils.
h1
gs
h2
A
Boiling
• Before trenching the
effective stress at A is:
gs(h1 - h2) - gwhw
• After trenching before any
pore pressure dissipation
the effective stress is:
gsh2 – gwhw
• If the effective stress at A is
zero the soil is fluid and
boils when:
h2 < (gw/gs)hw
h1
hw
gs
h2
gw
A
• For boiling to occur the materials must be both cohesionless and
have a relatively low hydraulic conductivity (high gradient).
• Fine sands and silts are most prone to boiling.
Flow Net
• Boiling is entirely a
groundwater seepage
phenomenon.
• Consider a The flow net
around the base of a trench
as a result of seepage.
• The head distribution is
such that the flow per unit
cross sectional area is the
same for all net elements.
• The velocities are a
maximum at the seepage
face, where the head
gradient is highest.
Piping
• Now consider a single flow
tube. All the flux through
this tubes exits through a
small segment at the base
of the trench.
• For a soil particle on the
base of the trench, the exit
velocity may be sufficient to
move the particle.
• Particle removal results in a
shorter flow path and a
higher exit velocity so the
next particle is more easily
removed.
• Erosion backwards along a
flow tube is called piping.
• Piping occurs mainly in fine
sands and silts (cohesionless
soils with low K)
Landslides
•
Terzaghi (1950) provided the classic
treatment of landslide development and
attributed rapid movements to:
1. External changes (surcharge loading of crest,
toe erosion, undercutting, etc)
2. Earthquake shocks (through horizontal loads
due to increased g-forces)
3. Lubrication by water (dismissed as wet soils
tend to have more friction mobilized than dry
soils)
4. Groundwater level rises (increases in pore
pressure in the Mohr-Coulomb equation)
Mohr-Coulomb Equation
t = c + (sn – u)tan f
where t is the shearing resistance (FL-2)
c is cohesion (FL-2)
sn is the normal stress (FL-2)
u is the pore pressure (FL-2)
f is the friction angle.
•
As pore pressure increases the frictional resistance (second
term on RHS) is reduced
•
For a cohesionless soil, c=0 so all shearing resistance is lost
when u = sn
•
For a cohesive soil, failure will occur when pore pressure
overcomes both the cohesion and friction components.
•
Notice that suction increases shearing resistance
•
Some believe that “cohesion” does not exist in soils and that
what is observed is a suction phenomenon induced by
dilation.
Periodicity in Slope Movements
• From the Mohr-Coulomb equation it is clear that pore
pressure is a critical factor in slope stability.
• Pore pressure varies seasonally as precipitation and
evapotranspiration rates change
• Slopes thus become more susceptible to failure in the
wetter seasons (no suction) and failures show
periodic movements.
• Controlling and preventing slope movements are thus
largely groundwater control problems.
• Drainage and dewatering are the first line of defence
for both prevention and remediation of slope stability
problems.
Soil Slope Remediation
Infiltration Control
Crest Drain
Well Control
Toe Drain
Rock Slopes
• Rock slopes present some different
problems because of the presence
of discrete fractures or joints.
• Water pressures can build much
more quickly because of the much
lower storage characteristics of
fractured rock masses (<1% to 5%)
compared to soils (20% to >50%).
• Joint orientation and persistence are
significant factors in stability.
• Inclined “daylighting” joints that are
not free-draining (plugged) are often
most troublesome.
General Case For Rock Wedge
• Consider the forces on a wedge
bounded by a dipping joint and
a vertical tension crack.
• The weight of the block, W
V
• The water pressure due to the
tension crack, V
• The water pressure due to the
joint, U
U
W
y
q
• The forces resisting sliding are:
cA + (W cosy – U – Vsiny)tanf
• The forces tending to induce
sliding are:
Wsiny + Vcosy
Factor of Safety
• The factor of safety against
sliding is:
F = cA + (W cosy – U – Vsiny)tanf
Wsiny + Vcosy
zw
z
V
U
H
A
W
y
q
where
A = (H – z)/siny
U = ½gwzwA
V = ½gwzw2
W = ½grH2[(1-(z/H)2coty – cotq]
and
gr and gw are the specific weights of
the rock and water respectively
Some “cohesion” may be provided
by interlocking joint asperities.
Factor of Safety
• The factor of safety against
sliding (ignoring cohesion) is:
F = (W cosy – U – Vsiny)tanf
Wsiny + Vcosy
zw
z
where
U = ½gwzwA
V = ½gwzw2
W = ½grH2[(1-(z/H)2coty – cotq]
and
gr and gw are the specific
weights of the rock and water
respectively
V
U
H
A
W
y
q
Prudent conservative design
assumes zero cohesion.
Water in Rock Masses
•
Hoek and Bray (1974) list the following concerns
with groundwater around open pits:
1. Water pressure reduces slope stability.
2. High moisture content results in increased specific
weight and moisture content changes in shales leads
to accelerated weathering.
3. Freezing of water in fractures can result in ice wedging.
4. Erosion of particulates can lead to plugging of fracture
drainage.
5. Liquifaction of saturated overburden or waste tips can
occur as a result of excess pore pressures.
6. Discharge of water into pits requires pumping to avoid
difficulties with mining equipment in wet conditions.
•
The first item is by far the most important
consideration in pit wall design
Rock Slope Remediation
Infiltration Control
Crest Drain
Well Control
Weep Hole Drains