Geology textbook Flashcards
What is the primary goal of the assesment of ground conditions?
- Intact rock
- rock mass
What is intact rock?
An unfractured, small block
How do we measure the strength of an intact rock?
Uniaxial compression test
Unconfined compression test
(They’re the same thing)
What is the definition of a rock mass?
A large mass of fractured rock in the ground
What is the value that we would expect from a strong rock in a uniaxial compression test?
> 100 MPa
What is the value that we would expect from a weak rock in a uniaxial compression test?
<100MPa
How is a uniaxial compression test carried out?
Material loaded to cause failure, this load is applied by 2 flat plates with no lateral restraint
What does SBP stand for?
Safe bearing pressure
What does SBP mean?
The load that can be imposed upon rock in the ground
What are pyroclastic rocks formed from?
A material collectively known as tephra
What are the properties of tephra?
Similair to those of sedimentary rocks
What is an extrusive ignous rock?
A rock made from magma that is extruded onto the earths surface to form a volcano
How are intrusive ignous rocks formed?
When magma solidifies below the surface of the earth.
What is a batholith?
A large blob shaped intrusion that is roughly equidimensional and btween 5 and 50 km in diammeter
What are batholiths usually made from?
Granite
What is a dyke?
A smaller sheet intrusion formed when magma has flowed into a fissure, between 1 and 50m wide
What are dyke’s usually made from?
Dolerite
What is cleavage?
The natural splitting of a mineral along parallel planes
What determines the mineral strength?
It’s intrinsic hardness and it’s cleavage
What is a mafic ignous rock?
An igneous rock that is relatively low in silicates (between 45 and 55 %)
What percentage of silicates do ultramafic rocks have?
<45%
What kind of rock has a silicate content of between 55 and 65%?
intermediate
What kind of rock has a silica content of greater than 65%?
felsic
What is the name of the process involving the laying of sediment is beds and planes?
lithification
What moves sedimentary rocks above sea level?
Earth movements
What is the most important factor in sediment transport?
Water
What are some of the less common types of sediment transport?
Gravity (steeper slopes)
Wind (small, fine, dry particles)
Ice
Volcanoes
What is lithification?
The process by which loose or weak sediment is turned into a stronger sedimentary rock
What causes lithification?
High temperature and/ or pressure
What are the results of lithification?
Increase in strength
What are the three main stages of lithification?
- Cementation
- Recrystallization
- compaction
- consolidation (this isn’t usually counted)
What is cementation?
The filling of the intergranural pore spaces by a natural mineral cement
What is recrystallization?
The small scale solution and redeposition of mineral, so that some grains become smaller and some larger. The result may be similar to that of cementation
It can produce a mosaic texture, which reduces bedding and therefore increases the rock strength
What is compaction?
The restructuring and change of grain packing, with decrease in volume
This reduces porosity - this increases strength by increasing the amount of grain to grain contact
What is consolidation?
The increase of strength in clays due to the reduction in their pore water content
It can include some compaction and new mineral growth too
All rocks containing clays contain overconsolidated clays
What influences sedimentary rock strength?
- The strongest cementing medium
- The amount of grain to grain contact (porosity)
- The amount of crystallisation (lack of bedding or interbedding minerals)
What are the 2 main types of sedimentary rocks?
- Clastic
- Non-clastic
What is regional metamorphism?
The strengthening of rock in mountains due to the collision of plate boundaries (high pressure)
What is thermal or contact metamorphism?
High temperature only
What are the metamorphic changes in rock?
- Recystallization
- growth of new minerals
What are some of the properties of metamorphic rock?
- Directional pressure causes minerals to grow in the easiest way, perpendicular to the maximum pressure
- This causes foliation and therefore planar weakness (much less than sedimentary rocks due to interlocking crystal structure)
What mineral is often present in metamorphic rocks?
Micas, the mineral that forms foliations - high amounts are associated with weakness in the rock
What is the grade of a metamorphic rock?
The overall extent of change in the sequence/
i.e.
slate –> schist –> gneiss
What is an outcrop?
The exposure of rock at the surface, or the area of a rock lying directly beneath a soil cover
What is dip?
The angle in degrees below the horiontal - direction goes down the dip
What is strike?
The direction of horizontal line on a dipping surface - cam be used on bedding or any hrozontal surface
What is a fault?
A fracture that has had rock displaced along it
What are joints?
Rock fractures with no movements along that are formed by tectonic stressing and are developed in nearly all rocks
What are bedding planes?
The dominant fracture withing sedimentary rocks
What are some common features of faults?
- badly broken ground
- weaker and less stable than the adjacent rock
- implications for foundation bearing capacity and slope stability
- Also important in the integrity of tunnel roofs
What are the defining feature of a horizontal bed?
Outcrops that follow the contours because they are at contstant altitude
What are the defining features of verticle beds?
Straight outcrops that ignore the contours
What are the defining features of dipping beds?
Curved outcrops that cut across and respond to the contours - because outcrops shift down - dip as erosion lowers the surface
How do you work out the dip direction?
The V -rule
An outcrop of dipping rock bends around a v shape where it corsses a valley, and the V at the outcrop points (like an arrowhead) in the direction of the dip
What are the constituents of soil?
Mixture of mineral debris and plant material
Consists of topsoil and possibly a clay subsoil
What is engineering soil?
A weak material that can be excavated without ripping or blasting, includes soil driftm weak rocks and weathered rocks
What is rockhead?
The buried drift/rock interface; commonly a consicuous boundary between weak soils and driftand the underlying strong rock
What is drift?
transported superficial sediment on top of the bedrock
What are the maint types of weathering?
- Frost shattering (cooler latitudes and higher altitudes)
- Salt crystalization
- chemcial processes accelerate in hot wet chlimates - acids from dense plant cover
- production of clay minerals from silicates (weakens igneous rock
- temperature weathering produce ilite as the dominant clay mineral
- hot, wet weathering of igneous rocks produces smectite
- speriodal weathering
What is important to look out for in the building of tunnels?
- Faults - can cause change in rock, increased water flow, broken rock
- Groundwater - may need flow diversion
- Overbreak in hard fracture rocks and sedimnetary rocks along vertical strike
- squeezing ground - platic flow, mostly in clays and shales wehere UCS/ overburden stress <2
- Swelling ground - collapse due to increased water content inc calys
- temeprature
- Rockhead
- Stress reduction - can cause delayed failures
- vertical stress within tunnel walls rises to 3 times overburden load
What can we use to support tunnels?
- steel ribs and concrete casings that are premanuactured and then brought in
- rock bolts are applied through shotcrete and wire mesh through into rocks forming an arch or to much stronger rocks
