Cong. SS. Sh. and engineering

Organization:Sandstones and Conglomerates

Shales and MudstonesBoth sandstones and shales

Engineering properties

ExplorationLandslide HazardsExcavations FoundationsUnderground worksMaterial properties

Exploration need to determine: Physical properties:

geometrybeddingshear zonesjointsfaults

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 at a specific temperaturetemperature;

• it takes into consideration the viscosityviscosity of the liquid or gas.

permeability or conductivityWhy is this important with respects

to groutability?

Question

?? expected permeability of sandstone and conglomerate?

??What physical properties affect permeability?

• pores pores – sizesize– numbernumber– unconnecteunconnecte

dd– open open – cementcement

Porosity <> Permeability

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 Clastic dikes form when sediment is

partially consolidated but under high partially consolidated but under high

pressure. pressure.

•If a water-laden layer can find a weak If a water-laden layer can find a weak

spot in the overlying layers, it squirts spot in the overlying layers, it squirts

upward. upward.

•Earthquakes are a common trigger.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 hazardsFriction 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 hazardsFriction 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 p-wave 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 problems1. scour – erosion by running water2. poorly cemented ss not suitable for

concrete dams3. uplift pressure due to permeability can

cause problems4. strength of the ss must be greater than

the stress applied5. piping can occur due to internal erosion

Dam foundations – associated problems6. 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  concrete embankment,

earth filldifferential settling

  withstands

deformability ability

very low extensive deformation

seepage path gradient

high – greater risk for piping

low – less risk for piping

uplift pressure

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

ExplorationLandslide HazardsExcavations DamsTunnelsFills and embankments

Exploration need to determine: Physical properties:

geometrybeddingshear zonesjointsfaults

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 excavationsFig 4.30especially 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 monitorc. uplift pressure difficult to control by either

grouting or drainage holesd. location of bentonites and mylonites are

difficultse. faults, joints and other such dislocations are

difficult to locatef.   calcareous shales can give rise to piping and

solution cavities

Tunnels

1. squeezing ground approximately the same as heaving

a. inward creep of rockb. damage of supports c. lining brokend. depth dependent, occurs at depths,

h1/2 qu/, where qu is the compressive strength and is the weight

Tunnels

1. squeezing ground approximately the same as heaving

e. expansive clays are more likely to squeezef. slaking can also occurg. bolting difficulth. short creat difficulti. lining may be necessary immediatelyj. block fall common in cemented shale along

joint systems

Fills and embankment problems 1. deterioration of the slopes

continuous and causes compaction

a. expansive clay stone & shaleb. highly slaking clay stone & shalec. weathered clay stone & shale d. fissil clay stone & shale

2. slides common due to low shear strength

Chapter 4.7 Engineering properties of sites with both sandstone and shale

ExplorationLandslide HazardsExcavations 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 described3. differences in weathering

Landslides

• block slides Fig. 4.33

excavation

1. blasting causes damage easily2. slides3. payment – rock or soil 4. classification difficult, rippability

etc.

foundations

1. differential settling2. differential expansion3. difficult to predict rock type at

depth – sandstone or shale

Chapter 4.8Case histories

Portage Mountain Dam and PowerhouseDamage to a housing development by mustone expansiionShale foundations in TVA damsFoundation in Melange – scott damExcavaations 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 levelgrouting attempted but little grout accepted

by the rockrock had differential strength and settlementRemedy – 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 limestoneuniform bed dipbedding plane cavities filled with

insoluble yellow clayrecurrant down to 40 feetRemedy – concrete filled grout trench,

cavities filled with grout

Shale foundations in Tennessee valley d. South holston dam - folded shales,

calcareous sandstone and conglumerateFew outcrops – pre investigations importantexploration results: significant core hole

loose, either drill wash out or solution cavies, numerous slickensides

Problemsslip into tunnels resulting in considerable

overbreakstrong 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