Pre-reading - soil mechanics Flashcards
What are soils in an engineering sense and how are they distinguished from rocks?
Soils are either man made or naturally occurring and unlike rock the individual particles are not sufficiently bonded
What are some examples of soil as a waste material from construction, industry and domestic?
Construction - excavation spoil
Industry - ash, slag, pulversied fuel ash
Domestic - landfill sites
What are selected soil materials?
These are materials that do not have the negative associated to waste i.e. pollution
What are selected soil materials like?
might be spread in thin layers, well compacted may be called engineered fill
Why are selected soil materials used in engineering (with examples)?
High shear strength and low compressibility providing adequate stability and ensure volume changes are small (highway embankments and earth dams)
What are the general problems associated with contaminated soils?
contaminants could comprise of metals such as mercury or lead which could cause harm to plant, animal and human health and could also be dmamging to building mterials embedded in the soil
What 2 groups of naturally occuring soils are there?
In-situ - formed at that location
and those
transported to their current location
What 2 catergories of in-situ soil are there?
Weathered rocks and peat
How might transported soils be moved?
Water
Wind
Ice
Volcanics
Gravity
WHat does weathering produce?
Decomposition and desintergration of rocks
What is the process that causes disintergration of rock?
Mechanical weathering
Where is mechanical weatrhing most intense and what does this lead to?
Cold environments resulting in the fragmentation of the rock and its mineral grains
What causes decomposition and what does this lead to?
Chemical alteration and thus decomposition turning the hard rock into softer clay
Where is chemical weathering/ decomposition most intense?
Hot, wet climates like the tropics
What are peats?
Almost entierly organic matter being composed of partly decmoposed and fragmented plant remains
How much of earths land surface is peat?
5 to 8%
What can glacial drifts (glacial deposits) be divided into?
Soil deposited directly by ice
Soils deposited by melt waters
What is soil depsosited by galciers called?
Till
How is lodgement clay formed and what is often reffered to as?
At the glaciers base and often boulder clay
How was ablation till formed?
Debris on the ice surface which is lowered as ice melts
What is the typical composition of ablation tills?
Sands, gravels, cobbles and bouldrers
What might soils deposited by outwash plains be reffered to as?
outwash deposits
Where is wind blown soil production most prevalent?
dry areas where there is little moisture to hold
What is the name of wind blown soils?
Aeolian deposits
What are some examples of areas where you can find aeolian deposits?
deserts, coastlines and periglacial regions
What are the 2 forms of wind blown deposits?
Dunes
Loess
What are dunes?
mounds of sand with different shapes and sizes
How much will sand dunes migrate in a year?
Over 10m/yr
What are loess deposits?
they cover a large area mostly in the NH where silt are mixed with sand and clay particles
What is the minerology of soil particles determined by?
original rock and degree of alteration
What are the different types of groundwater recharge with an example?
Artificial - irrigation losses
Natural - excess rainfall
Indirect - aquitard leakage
What is hydrogeology?
study of the movement, occurence, distribution and geological interaction of water with earths crust
What is porosity?
fraction of given volume which is void or interstices
What is hydraulic conductivity?
measure for the ease of movement of water through a porous material
What is an aquifer?
layer or layered sequence of rock or sediment with one or more layers containing water and can transmit significant quantities of water under normal hydraulic gradient
What rock types will “good” aquifers form in?
Sands
Gravels
Solutionally weathered limestone
Fractured sandstones
What is an aquitard?
formation of lower permeability that may transport regionally large quantities of groundwater flow but negligible supplies of groundwater can be obtained
What are some examples of aquitards?
fluvial and glacial-fluvial silts and sandy clays
What is an aquiclude?
saturated geological unit that has very low permeability and can act as a barrier for regional groundwater flow
What are some examples of aquiclude rocks?
Clays
Shales
Metamorphic rocks
How does the British criterion distinguish between coarse and fine soils?
Coarse soil wont stick together when wet will Fine soil wont
What is the typical shape of clay particles?
Platy
What can be used to quantify soil particle shape?
origin - (OG material)
Closest geometrical form - cube etc
Particle edges -angular, sub angular etc
What type of soil are most natural soils?
Composite soils
What are composite soils?
mixture of different particle sizes and varing distribution
What is the water table?
The uppermost part of a zone of saturation underground
Why might dewatering be employed as a technique for excavation?
when pumping risk cannot be accpeted due to the surrounding rocks being too permeable so wells/ bore holes are used to decrease the water table
What is running sand?
A man made event where water carrying soil particles flows out the sides of an excavation
What is the distribution of saline to fresh water?
Saline - 97.5%
Fresh - 2.5% (majority in ice caps)
How many people depend on groundwater for their daily supply?
2 billion
What are the negatives associated with autographic recording gauges of rainfall?
They work by having a bucket of known volume which when it overflows takes a reading however this can lead to missing out on small rain fall evenst but does provide info on intensity
What is rock mechanics?
theorhetical and applied science of the mechanical behaviour of rock and rock masses
What are the 2 very general catergories of physical parameters in mechanics?
Dynamic quantities which give rise to motion
Kinematic quantities which describe motion geoetrically
What is the main purpose of studying rock mechanics?
anticipate motion of a structure and see whether the resultant displacements are acceptable
What are the benefits of slope angle in excavation?
Steep slopes favour economics
Shallow slopes favour stability
What is one of the primary objectives of slope stability analysis?
estimation of safety factor for considered slope and slope mass
What are the 2 types of important translational slope failures?
Planar block slides
Wedge slides
How are planar blocks slides possible?
when joints dip into a surface excavation and the excavation is long relative to height
What are wedge failures?
Translational slides that occur when joint planes combine to form a rock block that may slide down the intersection line of the joints
What is the actual name for the wedge in wedge failures?
A tetrahedron
What are 2 important types of rotational slope failure?
Coventioanl reverse rotation
Forward rotation associated with toppling of rock blocks
What surface will conventional reverse rotation occur?
often approximated as a circular arc transecting a soil-like material
What types of shaft are used in mines?
Vertical shafts are more common but incline are also present
What must the design of mine shafts be like?
Long stable life
Safe at all points
What might be done with mine shafts in areas where the rock is strong and there is low insitu stress?
Unlined shaft is acceptable for example in use for ventilation but some provisions like meshes are used to prevent impact of small rock falls
What is subsidence?
downward motion of a ground surface
What are the most common causes of subsidence?
Withdrawal of fluids from pores and fractures
Disturbance of ground by excavation
What is the majority of earths crust composed of?
silica (95%)
Why might failures occur in geotechnical engineering?
TYpical sample of 0.001% of soil in an area leading to uncertainties in calculations
Once in a life time event (1 in 100 year flood)
How are soils created?
By the exposure of rocks to weather
How are particle size of soil and stability related?
Coarser grained soil (sands) tend to be more stable
What are faults?
fractures in rock mass that has experinced different movement
What effects can faults have?
elevation differences
erosional differences
weaker bearing capacity
What are escarpments?
assymetric hills formed when an outcrop is eroded unevenly or when edge of rock layers is not flat
What is the ground water table?
surface of water in rock or soil where the water stress is equal to atmospheric pressure
What is the difference between dry and saturated soil?
Dry - pore spaces filled with air
Saturated - pores filled with water
what is the definition of soil structure?
Arrangement of the soil grains
What soil structure is assocciated to coarse grain soils?
Loose or dense
What soils structures are associated to fine grained soils?
flocculated and dispersed
What are karsts?
underground landscape when limestone is eroded or dissolved by ground water in limestone this creates sinkholes
What are folds?
when rocks layers are curved or bent by the earths crust movements
What are synclines and anticlines?
Synclines - concave features (valleys)
Anticline- convex features (hills)
What are alluvium and alluvial fans?
soil deposits transported to the bottom of the steep slope by the erosion of a river flowing down that slope
