Module 5 Review Questions Flashcards
What activities provide information for a ground profile
- soil geology
- site investigation
- soil description
Define the discipline of engineering geology
The application of geological sciences to engineering practice
Investigations commonly performed by an engineering geologist
- geological hazards
- geotech and material properties
- landslide and slope stability
- erosion, flooding dewatering
- seimsic hazards
what is the greatest impediment to obtaining soils information
access to subsurface is limited and soil samples are usually collected from a very limited number of locations and are generally disturbed
Under what conditions does engineering on slopes become problematic
all slopes have potential to be problematic, but conditions of increased pore pressure (reduced effective stress) generally promote slope instability
what aspect of the ground is most important when considering the stability of slopes
groundwater (pore pressure)
what aspects of the ground are important to consider when selecting a dam site
- strength and stability of abutments
- dam footprint and its permeability
what aspects of the ground are most important for foundation design of a multi-storing building
bearing capacity of soils and rocks within the building footprint
why do deep excavations require consideration of the groundwater regime
- base of excavation may be beneath groundwater table
- risk of flooding or base heave due to high pore pressure
what aspects of river flood plain soils give rise to common geotechnical problems
- high water content and presence of organic soils both give rise to large settlements upon loading
what measurements can be taken to improve the tensile strength of soils in earth embankments
- soil mixing with the addition of ‘binder’ material (cement)
- placement of tensile geogrid layers between ‘lifts’ of soil
why are tunnel portals often areas of significant engineering problems
located on slopes in near surface materials that have been subject to weathering and stress relief
- therefore weaker than expected and give rise to slope instability
Why do geologically recent glacial periods control the soils in shallow marine environments
recent glaciations responsible for a drop in global sea level
- exposed much of shallow marine environment to subaerial processes which dominate soil found in these areas
What are the difference between natural, in situ soils and the soil samples commonly tested in the laboratory
- natural, in situ soils are undisturbed
- in most lab tested soils the material has been homogenised to remove the effects of structure and bonding
Two common human activities that lead to ground subsidence
- intensive agriculture
- mining
- tunneling
What type of dam would you construct in a region of suspected surface fault rupture hazard
embankment dam
What rock cycle processes lead to the formation of soil
- weathering
- transportation
- deposition
What soils are found in the region in and around Christchurch
- Loess
- alluvium
- peat
- silts sands and gravels
What generally controls the strength of a sedimentary rock
the cemented material
Of the three main rock types, which would you expect to have the greatest permeability
Sedimentary
What are the three main chemical reaction processes involved in chemical weathering
- oxidation
- hydrolysis
- dissolution
What are the common products of chemical weathering of crystalline rocks
- clay
- potassium ion
- bicarbonate ion
- silica
Why is the rockhead (boundary between unweathered rock and overlying residual soil) irregular under conditions of chemical weathering
- permeability of the rock
- spacing of discontinuities
What controls the rate of chemical weathering
- original rocks
- climate
- surface area
what are some of the common engineering problems in karst terrain
limestone
- caves
- lead to sinkholes
what rocks commonly contain pyrite
coal and shale deposits
Describe the nature of the contact between rock and residual soil
rockhead
What are the common effects of liquefaction on foundations
- loss of bearing capacity
- loss of frictional strength
- differential settlement
What conditions give rise to the majority of debris flow
heavy rainfall on colluvial slopes
which soils exhibit expansive behaviour
mixed-layer clays
Name three common causes of ground subsidence
- natural cavern collapse
- mining-induced or tunneling-induced settlement
- tectonic movement
- dewatering
Erosion is common in what geological environments
ALLL - anywhere where there is sufficient energy to transport loose, non-bonded gemoaterial
As well as case histories what improves our general understanding of soil behaviour in geotechnical engineering
experience
Name and describe the effects of three agents of erosion
- Water - transport soils of differing grain sizes along the course of a river
- Wind - transport of soils of a limited grain size (sand or smaller)
- Ice- glacial transport of soils - grinding bedrock to produce rock flour
What measures can be taken to mitigate the hazards with slope instability
- Drainage
- Slope Grading
- Improve slope materials
- Anchoring
What type of dam would you construct in a region of suspected surface fault rupture hazard?
zoned earth embankment
- sufficient freeboard to accommodate vertical displacement
- large clay core and filters (to accommodate fault movement without complete dislocation)
What mitigation measures would you reccommend for an area subject to episodic debris flow activity
- AVOIDANCE
- debris dams and levees
- controlled slope drainage
what common soil type is prone to internal erosion
loess
What physical characteristics control the overall mechanical behaviour of rock?
Discontinuities (fractures, joints, faults)
What physical characteristic control the overall mechanical behaviour of the soil
grain size and geometry
Differences between oceanic and continental crust
Oceanic - thin, dense, composed of basic igneous rock (basalt)
Continental - thick, less dense, composed of acidic igneous rock (granite)
What geological time period is of most importance for engineering
the most recent - Quaternary era
What are the prominent Quaternary soil types in NZ
- loess
- alluvial
- volcanic ash
- glacial
Differences between intrusive and extrusive rocks
- extrusive = rapid cooling, fine crystals
- intrusive = slow cooling, coarse crystals
Differences between acidic and basic igneous rocks
- acidic contain free silica
- colour
Would metamorphic rocks make a good aggregate source? Why?
No
- alignment of crystals means that when crushed, metamorphic rock tends to have tabular shape, leading to strongly anisotropic strength when the particles align
Bedrock in South Island?
Greywacke (sedimentary rock = mostly sandstone)
Where would you expect thermal expansion to occur as a weathering process
anywhere where there is a high diurnal temperature range e.g. high altitude desert areas
What are the three main chemical reaction proccesses involved in chemical weathering
- Hydrolysis
- Dissolution
- Oxidation
What rocks are most susceptible to chemical weathering
- high temperature (basic) igneous rocks and soluble sedimentary rocks
what are the common products of chemical weathering of crystalline rocks
clays
what soil would result from complete chemical weathering of granite
clay-rich soil containing quartz grains (sand to gravel size)
what controls rate of chemical weathering
rock composition, climate and surface area
what is the result of chemical weathering of limestone
Karst terrain (caves etc.)
What measures would you take to minimise the impact of pyrite on foundations
prevent exposure to atmosphere or remove all pyrite-containing materials
What influences the structure of residual soils?
the parent rock
what is the most common terrestrial soil transport mechanism?
rivers (fluvial environment)
