Transcript Chapter 4.5 Engineering properties of sandstones and conglomerates

Cong. SS. Sh. and
engineering
Organization:
Sandstones and Conglomerates
Shales and Mudstones
Both sandstones and shales
Engineering properties
Exploration
Landslide Hazards
Excavations
Foundations
Underground works
Material properties
Exploration need to
determine:
Physical properties:
geometry
bedding
shear zones
joints
faults
tests and observations at
the site
• groutability - the ability to pump or
inject a mixture of grout into the rock an
thus make it impervious.
This is often difficult in fine-grained
sandstone
• morphology of the sandstone; is the
assumption of equal thickness true or does
it thin or thicken in some direction
tests and observations at
the site
• degree of cementation – related to
rock durability and permeability
• stability of cementation – is the
cement soluble or reactive
• moisture content – poorly cemented/high moisture content
– well cemented/low moisture content
permeability
• permeability is a property of the rock
or soil,
• the ease of which liquids or gas can
move through the formation
• related cohesion and friction size
• volume of pores and
• degree of openness or connection
between pores and fractures
conductivity
• conductivity is a
property of rock or
soil together with a
given liquid or gas at
a specific
temperature;
• it takes into
consideration the
viscosity of the
liquid or gas.
permeability or conductivity
Why is this important with respects to
groutability?
Question
?? expected permeability of
sandstone and conglomerate?
??What physical properties affect
permeability?
Porosity <> Permeability
• pores
–
–
–
–
–
size
number
unconnected
open
cement
Permeability
cement > unconnected
Joints frequency and
interconnection
problems
associated with
field tests:
1.
orthoquartzite - is often fractured and
extremely hard
•
•
drill water is lost in fractures – need to case the hole
quartz content wears heavy on the drill bit
• loss of diamonds
• frequent drill bit replacement required
2. miss identification – granite is similar
to arkose sandstone in sandstone dikes
fig 4.23
3. case hardening – occurs in dry climates,
the upper 25 cm is extremely hard
This results in the misinterpretation of
the rock hardness and durability
4. cross bedding – misinterpretation of
the orientation of bedding can result in
3d projection problems
Questions
?? How are sandstone dikes formed? In what type of
rocks (metamorphic, sedimentary, igneous) do they
occur?
•Clastic dikes form when sediment is partially
consolidated but under high pressure.
•If a water-laden layer can find a weak spot
in the overlying layers, it squirts upward.
•Earthquakes are a common trigger.
slopes
–
–
–
–
Sheet joint development in sandstone along cliffs
Compare to exfoliation of granite,
heaving of shale in excavations,
popping rock or squeezing ground in tunnels.
Landslide hazards
Friction material – thus in
general risk is uncommon
Exception:
• When the beds are
underlain by “weaker” rock
• Slab formation due to
sheet jointing and bedding
planes
Landslide hazards
Friction material – thus in
general risk is uncommon
Exception:
• When the beds are
underlain by “weaker” rock
• Slab formation due to
sheet jointing and bedding
planes
Surface excavations
• rippability the ability to
break the rock without
blasting;
• rippability is related to pwave velocity which is
related to hardness and
durability of the rock; fast
p-waves/strong rock/not
rippable vs slow p-waves
/weak rock/rippable
Surface excavations
• Blasting can damage the rock, create
boulders which are difficult to handle
Surface excavations
Foundations
– bearing capacity usually good in sandstones and
conglomerates, compressive strength test
inversely proportional to moisture content
– friable sandstones - erosion and weathering
risk, durability is proportional to cement
Surface excavations
Foundations
– bearing capacity usually good in sandstones and
conglomerates, compressive strength test
inversely proportional to moisture content
– friable sandstones - erosion and weathering
risk, durability is proportional to cement
Dam foundations
All types of dams have been founded on
sandstone.
Dam foundations –
associated problems
1. scour – erosion by running water
2. poorly cemented ss not suitable for
concrete dams
3. uplift pressure due to permeability can
cause problems
4. strength of the ss must be greater than
the stress applied
5. piping can occur due to internal erosion
Dam foundations –
associated problems
6. bearing capacity vs erodability – even if
the rock is strong enough to support the
weight it may be very susceptible to scour
7. under seepage causes high uplift pressures
– this can be remedied by a grout curtain
8. bank storage – if the rock is highly
permeable a great part of the water that
fills in the reservoir will move into the
rock, up to 1/3 to total inflow volume for
highly permeable sandstones
Dam foundations
Question:
Which type of dam would be most suitable in
an area with
1. porous, friable un-cemented sandstone
and siltstone?
2. hard sandstone, well-cemented with silica
cement?
3. calcite cemented sandstone?
What are the main risks??
Dam foundations
differential
settling
deformability
ability
seepage path
gradient
uplift pressure
concrete
embankment,
earth fill
withstands
very low
high – greater
risk for piping
extensive
deformation
low – less risk
for piping
not good
OK
piping – internal erosion due to upward
directed flow lines
Underground works in
sandstone
problems:
soft rocks:
• collapse
• subsidence in
overlying material
• water inflow
• “making ground caves”
hard rocks
• wear on drill
• silicoses
Questions
??What tunnel problems are associated
• with hard sandstone or conglomerates
• with soft sandstone?
• What measures can be taken?
Aggregate material /
dimension stone
hardness important
extremely soft rocks are not suitable as
aggregates or dimension stone
Good in general for both concrete and
asphalt are:
hard / strong / wear resistant /durable /
resistant to weathering
Aggregate material /
dimension stone
Good in general for concrete
• free mica content should be low to insure
good rheology in concrete
• reactive minerals such as flint, gypsum, salt,
pyrite can cause problems in concrete
Corrosion of metal and concrete by
acid and sulfate ions
Aggregate material /
dimension stone
Good in general for asphalt
• quartz rich rocks often do not have an
excellent grip in asphalt – additives make it
possible to use
• light color desired – safety
Aggregate material /
dimension stone
Good in general for dimension stone
• few fractures and bedding plane
discontinuities
Chapter 4.6 Engineering
properties of shales and
mudstones
Exploration
Landslide Hazards
Excavations
Dams
Tunnels
Fills and embankments
Exploration need to
determine:
Physical properties:
geometry
bedding
shear zones
joints
faults
Exploration need to
determine:
classify
–
–
–
–
–
–
–
–
–
cemented
compacted
expansive
slaking
weathering effects
mylonite
bentonite
gassy potential
conductivity
Exploration problems:
– breakage and deterioration
– core recovery difficult
– field moisture needs to be
preserved by bagging or
coating the cores
Landslide hazards:
Landslide hazards – two types common in
argillaceous rocks
1. cemented shale –
a. glide along bedding planes when the planes dip less
than the slope, enhanced by the occurrence of
bentonite layers or mylonite zones (dip < 5 degree
required)
b. dislocation common between weathered and non
weathered zones
c. topple when bedding is very steep, often in more
brittle rocks
Landslide hazards:
Landslide hazards – two types common in argillaceous rocks
2. compacted shale and clay soils – slump; their weight is
greater than their strength
a. slaking – a continuous process. Surface material
slakes and is eroded exposing new fresh material. The
process is repeated
Landslide hazards: slaking
Question:
?? Which glacial sediment has a problem with
slaking in surface excavations?
Tills that are rich in silt are notorious for slaking.
They flow in open cuts, especially when there is
a high groundwater pressure due to the
excavation slope.
Heaving and rebound
Heaving – upward and inward
into excavations
Fig 4.30
especially common in
expansive mudstone,
expands due to the
removal of the confining
stress not due to swelling
with added water
inward expansion is common
in areas with high initial
horizontal stress
Dams – generally clay and
shale are not ideal
1. earth-fill or embankment dams –
several successful dams even on
expansive compacted shale
Dams – generally clay and
shale are not ideal
2. concrete dams – very difficult
a. seepage difficult to determine – and is
generally high
b. hydraulic gradient – can be difficult to monitor
c. uplift pressure difficult to control by either
grouting or drainage holes
d. location of bentonites and mylonites are
difficults
e. faults, joints and other such dislocations are
difficult to locate
f. calcareous shales can give rise to piping and
solution cavities
Tunnels
1. squeezing ground approximately the
same as heaving
a.
b.
c.
d.
inward creep of rock
damage of supports
lining broken
depth dependent, occurs at depths,
h1/2 qu/, where qu is the compressive
strength and  is the weight
Tunnels
1.
squeezing ground approximately the same
as heaving
e.
f.
g.
h.
i.
j.
expansive clays are more likely to squeeze
slaking can also occur
bolting difficult
short creat difficult
lining may be necessary immediately
block fall common in cemented shale along joint
systems
Fills and embankment
problems
1. deterioration of the slopes
continuous and causes compaction
a.
b.
c.
d.
expansive clay stone & shale
highly slaking clay stone & shale
weathered clay stone & shale
fissil clay stone & shale
2. slides common due to low shear
strength
Chapter 4.7 Engineering
properties of sites with both
sandstone and shale
Exploration
Landslide Hazards
Excavations
Foundations
Chapter 4.7 Engineering
properties of sites with both
sandstone and shale
two different types of rocks
are more difficult and create
more problems than does one
rock type alone
Exploration
The combination of rhythmic bedded sandstone and
shale is common - Flysch
Exploration different for
each rock type
1. ground water relation in each rock
2. contacts described
3. differences in weathering
Landslides
• block slides Fig. 4.33
excavation
1. blasting causes damage easily
2. slides
3. payment – rock or soil
4. classification difficult, rippability
etc.
foundations
1. differential settling
2. differential expansion
3. difficult to predict rock type at
depth – sandstone or shale
Chapter 4.8
Case histories
Portage Mountain Dam and Powerhouse
Damage to a housing development by
mustone expansiion
Shale foundations in TVA dams
Foundation in Melange – scott dam
Excavaations in shales for Bogata, Colombia
Portage mountain dam &
powerhouse
•
•
•
•
•
•
•
•
peace river, Canada
embankment dam
200 m high
2 km long
underground chamber
46m high
300 m long
27 m wide
Portage mountain dam &
powerhouse
• Gething Formation, Cretaceous sandstone
and shale with coal beds. The coal had
burnt naturally and still had cavities where
there was ash and cavities and was still
burning
• Moosebar Formation, black shale, highly
weathered up to 70 m deep
• Dunlevy Formation, thick bedded sandstone
Portage mountain dam &
powerhouse
• The dam site selection was finally on the
Dunlevy Formation and Gething Formation
• The shales did not swell but did slake
slightly
• Problems occurred in the underground
powerhouse – deflection of up to 20cm of
the roof strata
• This was supported by bolts and grout
Damage to a housing development
by mudstone expansion Fig 4.35
Unprecedented wetting of expansive clay
inter bedded with sandstone resulted in 15
cm heave
The claystone was impervious but highly
fractured. Fractures conducted water into
the rock and thus swelling occurred down
to more than 2.5 m depth
Remedy – drainage, exclude claystone in
embankments, foundations on beams 10 to
15 m deep
Shale foundations in
Tennessee valley
lower to middle Paleozoic
limestone/dolomite sandstone and
shale with some metamorphic rocks.
Dams founded on the shale –
foundations difficult
– open joints
– mud filled joints
– pyrite rich black shales
Shale foundations in
Tennessee valley
a. Chickamauga project
folded limestone with some shale layers
and bentonite
Shale layer – impervious, protected from
weathering it did not slake badly
Shale foundations in
Tennessee valley
b. Watts Bar dam
Rome formation – sandstone, shaley sandstone,
sandy shale, compacted 1.5 Mpa, limb of an
anticline
Clean up to a sound bearing level
grouting attempted but little grout accepted by
the rock
rock had differential strength and settlement
Remedy – steeped foundation so that each of the
monoliths would be on a “Bearing” layer
Shale foundations in
Tennessee valley
c. Fort Loudoun – limestone and
dolomite with some calcareous shales
and argillaceous limestone
uniform bed dip
bedding plane cavities filled with insoluble
yellow clay
recurrant down to 40 feet
Remedy – concrete filled grout trench,
cavities filled with grout
Shale foundations in
Tennessee valley
d. South holston dam - folded shales,
calcareous sandstone and conglumerate
Few outcrops – pre investigations important
exploration results: significant core hole loose,
either drill wash out or solution cavies,
numerous slickensides
Problems
slip into tunnels resulting in considerable
overbreak
strong when unweathered, but weathered rock
slaked quickly
Foundation in melange – scott
dam, eel river California
Franciscan melange predominately graywacke
and shale with sheared serpentine
construction started on right bank – but
after 2/3 complete the proposed stable
left bank slid
Stability is still a question – the dam was not
complete at the time the book was written
Excavation in shales, Bogata,
Columbia, 2600 m above sea level
• dam and 70 km long conveyance system, sewage
and power supply
• Rocks – intensely folded Paleozoic and Cretaceous
massive orthoquartzite sandstone interbedded
siliceous shale and siltstone with bituminous black
shale overlain by tertiary coal bearing sediments.
Chemical weathering has softened the sandstone
in the upper 30 m and the shale has changed to a
sticky clay soil.
• Landslides common on the steep slopes
Excavation in shales, Bogata,
Columbia, 2600 m above sea level
• Moved the site several times but landslides
continued to threaten the construction.
• Attempt to lower the pore pressure in the shale –
difficult due to the low permeability – proved to
be successful.
• Years later – leakage was noted from a steel
pipeline and a slide diagnosed
• The pads of the pipeline were greased and thus
allowed the slide to slip without damaging the
structure