CIVE40009 Geotechnics Flashcards
Water content ?
w = mass of water / mass of dry soil
= Mw / Ms
= ρwVw / ρsVs
=(Sr*e) / Gs
Define soft ground
exhibits significant displacement in response to an applied load
Describe the relationship between void ratio and resistance to load
higher void ratio (e) = lower resistance
Unit Weight ?
force per unit volume in kN/m^3
Specific Gravity
Gs = ρ solid / ρ water
(ratio of densities of soil mineral and water)
Specific Gravity
Gs = ρ solid / ρ water
(ratio of densities of soil mineral and water)
Void Ratio ?
e = volume of voids / volume of solid
(Vv/Vs) or (Vw+Vg)/Vs
Specific Volume
v = total volume / volume of solid
=(Vv +Vs)/Vs
=1 +e
Solid Volume Fraction ?
= volume of solid / total volume
= Vs / (Vv + Vs)
= 1/v
Porosity ?
n = volume of voids / total volume
=Vv / (Vv+Vs)
=e/(1+e)
=(v-1)/v
Degree of Saturation ?
Sr = Vol of liquid / vol of voids
= Vw / Vv
= Vw / Vw + Vg
Air Void Ratio ?
a = vol gas / total volume
= Vg / Vv + Vs
= (1-Sr)e / 1 + e
Mass of Liquid Content ?
Mw = Sreρw*Vs
Bulk Density ?
(Sr*e + Gs)/(1+e) * ρw
Saturated Density ?
ρ sat = (e + Gs)/(1+e) * ρw
Dry Density ?
ρd = Gs/(1+e) * ρw
= Gs*ρw/v
Specific Volume ?
v = ρs/ρd
Buoyant Weight ?
y’ = y - yw
(unit weights)
- effective weight of a soil sample when it is submerged in a fluid
- calculated by subtracting the weight of the displaced fluid from the weight of the soil sample
Vertical Stress Differential ?
dσz / dz = ρg = y( unit weight )
Gradient of Increase in Pore Water Pressure over Depth ?
du / dz = ρw*g = yw (unit weight)
u = pore water pressure
Effective Stress ?
difference between total (vertical) stress & pore pressure
σ’z = σz - u
(in equilibrium)
- stress that is transmitted through the solid skeleton (as opposed to water-filled voids) of a soil, and it governs the shear strength, compressibility, and settlement characteristics of soil
How can you link effective stress & buoyant weight ?
dσ’z/dz = dσz/dz - du/dz = y - yw = y’
how to find water content of a soil ?
using a ratio of masses
method : soil moisture analysis
1. Weigh sample and record the weight as initial dry weight.
2. Place the soil sample in an oven and dry it at constant temp.
3. allow it to cool in a desiccator (using drying agent e.g. silica gel)
4. Weigh the soil and record the weight as final dry weight
Water content (%) = [(Initial dry weight - Final dry weight)/Final dry weight] x 100%
how do you determine the specific gravity of soil minerals ?
pycnometer method
1. weigh pycnometer (small glass bottle with stopper & narrow tubular hole, known volume) M1
2. place some dry soil particles in bottle & weigh, M2
3. fill with water - ! ensure no trapped air ! & weigh M3
4. empty & clean the pycnometer
5. fill with water only, & weigh M4
Gs = (M2-M1) / [ (M4-M3) + (M2-M1) ]
= mass of dry soil / mass of displaced water - mass of dry soil
this method provides an average specific gravity for the soil sample, and it assumes that the soil particles have a uniform density
how do you determine degree of saturation
- determine density of soil :
- remove soil, insert flexible membrane lining, fill hole with water to identify volume - using mass of displaced soil & volume, calculate in-situ density
- using specific gravity, calculate void ratio : e = (Gs * (ρw/ρd))-1
- hence calculate degree of saturation : Sr = [((1+e) * ρ)/(ρw -Gs)]/ e
compaction ?
mechanically increasing the density of soil by reducing its volume’
usually by a roller or falling weight
how does ease of compaction vary with moisture content
compaction energy required to achieve a certain density increases as the degree of saturation increases, as at lower moisture contents the soil particles can more easily be displaced
until the optimum moisture content - moisture content at which this maximum dry density is achieved
over- and under- compaction can have considerable structural consequences :
beyond the optimum moisture content may result in reduced strength and stability (over)
under-compaction may lead to excessive settling and deformation
how are soil particles formed
processes of erosion, transport & weathering from parent rocks
different minerals have varying resistance to temperature, humidity & chemical weathering
what are the means of transport of soil particles
gravity (e.g. landslide), ice (e.g. glaciers), water, air (wind)
riffling ?
process of dividing a soil sample into smaller representative samples
According to the BS1999 classification system what is the boundary between fine and coarse-grained soils
75µm
what are the two soil behaviour framworks ?
Nature
- essential, intrinsic characteristics of the soil due to the materials in the composition
- e.g. particle size distribution (PSD), mineralogy (plasticity)
State
- physical conditions in the environment the soil is in
- e.g. exposure to weathering
what is the coefficient of uniformity
Cu = d60 / d10
poorly graded : Cu<4
describes degree of uniformity or variation in the soil particle sizes
high coefficient of uniformity = narrow range of particle sizes = well-graded
low coefficient of uniformity = wider range of particle sizes = poorly graded
what is the coefficient of curvature
Cz = (d30)^2 / d60*d10
well graded: 1<Cz<3
gap graded outside of this range
identifies gaps
When would you use dry and wet sieving respectively
Dry - for soils containing insignificant amounts of silt & clay ONLY (due to flocculation & agglomeration)
Wet - for most coarse-grained soils
at what PSD would sedimentation & seiving techniques be used respectively ?
Sedimentation : fine-grained soils, such as silt and clay
PSD of < 425 microns
Seiving : coarse-grained soils
sedimentation methods ?
hydrometer & pipette :
- relies on application of Stokes’ Law
- assumed smooth spherical particles, in reality angular & rough
- through the settiling of soil particles suspended in a liquid
- uses settling velocity
- not applicable to particles < 0.2 in diameter
laser diffraction :
- based on the principle that the intensity of light scattered by particles is proportional to their size
seiving methods ?
- soil passed through series of stacked seives with decreasing aperture
- limitation : not ideal for non-spherically shaped soil particles
undisturbed soil ?
soil in its natural state
remoulded soil ?
disturbed from its natural structure with some/most of its natural structure removed due to mechanical means (eg mixing)
intact soil ?
somewhat disturbed, some loss of natural structure
reconstituted soil ?
all of its natural structure has been removed due to mechanical mixing
- at 1.5-2x liquid limit
reconstituted soil ?
all of its natural structure has been removed due to mechanical mixing
- at 1.5-2x liquid limit
liquidity index ?
used for clays to quantify soil state :
IL = (w-wp) / (wL-wp)
w = water content
wp = water content at plastic limit
wL = ‘’ at liquid limit
relative density ?
used to quantify state of sands :
Dr = (emax - e) / (emax - emin)
e = void ratio
how do you find the plastic limit ?
roll to about 3 mm diameter in a given number of rolls
subjective
Atterberg Limits ?
set of tests used to determine the properties of fine-grained soils, such as silt and clay
Liquid limit (LL): lowest water content at which soil cannot maintain its shape (soil changes plastic state –> liquid state).
Plastic limit (PL): Lowest water content where soil is deformable without cracking/crumbling ( changes semi-solid state to a plastic state )
Shrinkage limit (SL): The water content at which further loss of moisture will not cause any more volume reduction.
classifying fine grained soils
plasticity index ?
Ip = wL - wP
how to you find the liquid limit ?
cone penetrometer (less subjective & more repeatable)
casagrande cup (more subjective & more dependant on operator)
which parameters describe nature & state conditions ?
Nature :
clays : mineral composition, wP, wL, Ip
sands : mineral comp, PSD, emin, emax
State :
clays : w, e, LI (liquidity index), clay structure
sand : grain structure, e , Dr, State parameter
soil fabric ?
arrangement of soil particles in a soil mass, including their shape, size, orientation, and distribution
(non-scalar quantity)
relative compaction ?
RC = pd / pdmax
pd = dry density
used for state quantification of soil mixtures
consolidation ?
for saturated soils :
- densification of soil by applying external loads
- to remove water
- hence decreased void volume
why compact soils ?
- increase stiffness (reduces settlement)
- increase strength ( bearing capacity, liquefaction resistance etc.)
- reduce permeability
- reduces air voids
porosity ?
volume of void spaces (pores) within a soil or rock
n = Vv / Vt
volume of voids / total volume
expressed as percentage
note void ratio is relative to total volume of solid, porosity to total volume
specific volume ?
V = 1 + e
volume containing 1 unit Volume of solid material
examples of uses of compacted soils
- retaining wall backfill
- road embankment
- earth dam
- offshore pipeline backfill
field compaction methods ?
1 static loading/ pressure (FG)
2 kneading (CG)
3 impact (FG)
4 vibration (CG)
FG fine grained
CG coarse grained
quality control methods for compaction ?
- Sand Cone
- low cost/accurate
- slow/low control - Balloon
- moderate cost/fewer steps
- slow/difficult to obtain accuracy - Nuclear Density Meter
- quick/direct measurements
- high cost/radiation uses/ water content error
how does total vertical stress vary with depth ?
for homogenous soil deposits :
dry : σ z = γ d × z = ρd × g × z
saturated : σ z = γ sat × z = ρsat × g × z
linear variation with depth due to soil’s self weight
how does pore pressure vary in hydrostatic situations ?
u = γ w × z = ρw × g× z
Pore pressure increases
linearly with depth
hydrostatic = no flow / seepage
how does the total stress, pore water pressure and effective stress relate ?
Total stress = pore water pressure + effective stress
σ = u + σ’
effective stress ?
- stress that is transmitted through the soil skeleton, which is the solid particles and their contacts with each other, as opposed to the total stress which includes the pore water pressure
- net stress supported by
the soil skeleton - governs the strength and deformation behavior of the soil.
how does effective vertical stress vary in homogenous soil deposits
Effective vertical stresses
increase linearly with depth in
homogeneous soil deposits
σʹz = (γ satz) − (γ wz)
= (γ sat −γ w)z
= (ρsat − ρw) gz
capillary rise ?
water is drawn upward through small spaces or pores in a soil medium due to the cohesive forces between water molecules and the soil particles
- surface tension of water creates an adhesive force between the water and the soil particles, which pulls the water upward against gravity
- height to which water can rise in a soil medium due to capillary action depends on the pore size distribution of the soil, the surface tension of water, and the contact angle between the water and the soil particles
- think of varying difficulty of running on a beach depending on distance from sea
How does water flow affect the effective stresses in soil?
- pore water pressure generated by the flow of water can change the effective stress state of the soil
- (water flow/seepage exerts pressure on soil particles)
- upward seepage (flow), causes increased pore pressure, & decreased effective stress (hence, saturated soils are often more susceptible to liquefaction)
is downward or upward seepage more desirable ?
**downward seepage
- drains excess water from the soil, reducing the pore water pressure (u) and increasing the effective stress
how is bernoulli’s theroum applied to soil mechanics
H = u / γw (pressure head) + z (elevation head) + ν^2/2g (velocity head)
note : velocity head assumed to be negligible for most soils
how is bernoulli’s theorum applied to soil mechanics
H = u / γw (pressure head) + z (elevation head) + ν^2/2g (velocity head)
note : velocity head assumed to be negligible for most soils
what is the hydraulic gradient ?
head loss per unit length
i = Δh / l
discharge and seepage velocities ?
q = v ⋅ A = vs ⋅ Av
[L3/T]
where vs = seepage velocity,
v = discharge velocity,
Av = area of voids
how is darcy’s law applied to geotechnics ?
states that the rate of flow of water through soil is proportional to the hydraulic gradient (i.e., the change in water pressure over a distance) and the hydraulic conductivity of the soil
v ∝ k ⋅i = k Δh/l
i = hydraulic gradient,
k = hydraulic conductivity (coefficient of permeability)
how is the permeability of soils determined ?
laboratory environment
fine-grained : falling-head test
-constant hydraulic head for specified time period, then water allowed to fall, time taken for drop in water level
coarse-grained : constant-head test
- constant hydraulic head, flow rate measured
ultimate limit state (ULS) ?
refers to the point at which a soil structure or system has reached its maximum capacity and cannot withstand any further loading
- point of failure/structural collapse
servicability limit state (SLS) ?
soil structure or system has reached a level of deformation or displacement that exceeds the allowable limit for the intended use
which design approaches are used to determine the stess, load & resistance properties
allowable stress design or load & resistance factor design
give examples of shallow foundations
- strip foundation
- raft foundation
soil is strong enough to support the building loads without the need for deeper excavation - typically low-rise
give examples of deep foundations
pile foundation
piled raft/mat
drilled shaft foundation
soil is weak and cannot support the building loads with shallow excavation - for high-rise
what is the effect of external loads on the soil fabric
can cause :
- compaction
- consolidation
- liquefaction
- shear
due to changes in vertical stresses, pore pressure & vertical displacements
what are the two basic types of mass movements ?
Large Scale :
- large area, slope stability analysis required
- retaining structures may be too expensive
- alternative stabilisation methods/monitoring may be implemented
Small Scale :
- small, contained area
- reataining structures often applied
- additional stabilisation not typically required
types of large scale mass movements ?
- Falls (rapid free fall)
- Topples (forward rotation about pivot, usually predictable)
- Slide (shear failure) (circular/rotational)
- Flow Slide (shear failure usually due to seismic effects)
- Translational Movement
- Lateral Spread (shear failure/liquefaction along soil boundaries, elevation change)
retaining structures ?
purposes : avoid limit states
1. create temporary space
2. create permanent space
3. retain soil
4. stabilise slopes/excavations
examples of retaining structures ?
externally stable :
- gravity (stability from self weight)
- cantilever (embeded into foundation soil)
- tieback (tieback interaction with retained soil)
- braced (inclusion of additional bracing elements)
internally stable :
- reinforced soil (soil nails, geotextiles/geogrids, metalic strips)
at-rest soil state ?
(no external loading)
- zero shear stress (τ)
- purely hydrostatic, with equal stresses acting in all directions
σ’h = K0 σ’v
(K0 is at-rest lateral earth pressure coefficient)
active soil state ?
(subjected to an external load or deformation)
- effective stress not balanced –> soil particles are no longer at rest (τ not zero)
- leads to deformation or failure due to shear stress (τ)
σ’hA = KA * σ’v
active effective horizontal stress = active lateral earth pressure coefficient * vertical effective stress
Lateral Earth Pressure Coefficient for Active Soil State ?
Lateral Earth Pressure Coefficient for Active Soil State ?
KA = (1−sinφ) / (1+sinφ)
since σ’h = KA * σ’v, suggests decrease in lateral stress
φ = angle of internal resistance to shearing of soil
Mohr-Coulomb Failure Envelope ?
- straight line that relates the shear stress (τ) to the normal stress (σ) acting on the soil
- soil stress state when failure occurs
passive soil state ?
- soil is constrained laterally and cannot deform laterally due to the presence of a rigid structure
- high horizontal/lateral effective stresses
(wall moves towards soil)
Lateral Earth Pressure Coefficient for Passive Soil State ?
KP = (1+sinφ) / (1-sinφ)
since σ’h = KP * σ’v, suggests increase in lateral stress
φ = angle of internal resistance to shearing of soil
activity ?
Activity (A) is the ratio of the plasticity index (Ip, in %) to the clay
fraction of the soil (C, in %) that is smaller than 2 m (i.e., A = Ip/C).
A-Line Graph ?
graph of plasticity limit - liquid limit
A-Line represents definition between silts & clays where silts are on the bottom (low wp high wl), clays on top (high wp low wl)
cycle of geology ?
cycle of geology ?
encompasses all major geological processes
Land : mainly erosion & rock distruction
Sea : deposition & formation of new sediments
Underground : new rocks created & deformed
driven by earth (tectonic) movements
minerals ?
Naturally occurring inorganic substance which has a
definite chemical composition and presents an ordered atomic arrangement
rocks ?
Aggregates of one or more mineral. The nature and
properties of a rock are determined by the minerals in it and by the manner in which the minerals are arranged relative to each other
igenous rock origin ?
crystallised from molten magma , found underground
types of igneous rock ?
& examples
extrusive
- magma extruded to surface (volcano)
- basalt
intrusive
- magma solidifies below earth surface
- later exposed due to erosion (through dykes vertical, batholiths & sills horizontal)
- granite
types of igneous rock ?
& examples
extrusive
- magma extruded to surface (volcano)
- fine-grained/glassy texture from rapid lava cooling
- basalt
intrusive
- magma solidifies below earth surface
- later exposed due to erosion (through dykes vertical, batholiths & sills horizontal)
- large crystals due to slow cooling
- granite
felsic rock ?
igneous rock :
- higher silica content (>65%)
-rich in light-coloured minerals
- granite, rhyolite
mafic rock ?
igneous rock :
- lower silica content ~< 50%
- basalt
- ultramafic igneous exists
fissure eruptions ?
- magma released from long, narrow crack -> fissure
- extensive lava fields
equigranular ?
crystals approximately equal size
inequigranular ?
irregular crystal size distribution
pophyritic ?
large crystals
surrounded by much smaller
crystals
Aphanitic texture ?
fine-grained : individual mineral grains are too small to be seen with the naked eye (commonly extrusive rocks)
Phaneritic texture ?
coarse-grained texture : in which the individual mineral grains are large enough to be seen with the naked eye
Phaneritic texture ?
coarse-grained texture : in which the individual mineral grains are large enough to be seen with the naked eye, typically intrusive
Goldich’s dissolution series ?
series that describes which minerals in igneous rocks are weathered and dissolved when exposed to chemical weathering processes
- most stable (more silica = more covalent bonds) = quartz
- least stable (most ionic bonds) = olivine
rock cleavage ?
- break along flat, parallel planes of weakness
- atoms in these minerals are arranged in a regular, repeating pattern that creates planes of weakness
- producing smooth, flat surfaces
igneous rock texture ?
mosaic of interlocking crystals : fineness is determined by rate of cooling
- other textural properties depend on cooling conditions (e.g. glassy or vensicular)
strength of igneous rock ?
generally uniform high strength & reistant to weathering and erosion
primary rock forming minerals ?
silicate minerals
non-silicate minerals (carbonates, oxides, halides, sulphides, sulphates, phosphates)
mica minerals ?
group of minerals that are characterized by their sheet-like structure and excellent cleavage (readily splits into flakes)
medium-high susceptibility to weathering
2x common :
1. Muscovite (colourless/silvery)
2. Biotite (dark brown to black)
feldspar minerals
silicate mineral group
medium-high susceptibility to weathering
2 x common :
1. Alkali feldspar (Pink or white)
2. Plagioclase feldspar (White, colourless
or grey)
sedimentary rocks
produced by the surface processes of the rock cycle
including weathered rock fragments, minerals precipitated from water, and organic material
sedimentary rock principal groups ?
clastic - accumilation & lithification of pre-existing rocks, mineral & organic materials from erosion, transport and deposition ‘standard group’
non-clastic - formed from the precipitation of minerals from water, or by the accumulation and lithification of organic material
sedimentary rock environment ?
widely varied : rivers, lakes, beaches, tidal flats, shallow maring, deep maring (with increasing energy levels)
rock texture & deposition dependant on environment
well-sorted rock ?
uniform particle size distribution
- typically indicates that the sediment was transported a significant distance
well-graded rock ?
has a range of particle sizes arranged in a sequence
-typically indicates that the sediment was deposited by a process that fluctuated in energy over time, such as a river or a beach
well-graded rock ?
has a range of particle sizes arranged in a sequence
key types of sedimentary rock ?
shale, sandstone & conglomerate
sandstone ?
from accumulation and cementation of sand grains
- cemented together by minerals precipitated from groundwater
high variability -> classified by Dot Classification
- triangular graph with axes representing grain size (in millimeters) and sorting (measured as the standard deviation of the grain size distribution)
sedimentary
shale ?
- fine-grained sedimentary rock
- finely laminated
- tendency to break along parallel side fragments
- formed from compact muds/ clay minerals
sedimentary rock structure ?
bedding (horizontal layers that are visible in many sedimentary rocks)
metamorphic rock ?
solid-state transformation of pre-existing rock into texturally or mineralogically distinct new rock as a result of high temperature, high pressure, or both
origin is therefore sedimentary, igneous or other
three types of metamorphic rocks ?
(i) Thermal or contact metamorphism (temperature)
(ii) Dynamic or dislocation metamorphism (stress)
(iii) Regional (temperature + stress)
metamorphic rock texture ?
foliated : planar arrangment of minerals forming bands, lineation is linear arrangement from regional metamorphism
increased metamorphism (heat & pressure ) = high banding
non-foliated : regional / contact metamorphism
marble ?
contact metamorphism of recrystallised carbonate minerals = metamorphosed limestone
metamorphic rock major examples ?
Gneiss, schist, slate
metamorphic rock environment ?
Mostly deep inside mountain chains
rock weathering
breakdown or alteration of rock materials due to natural physical, chemical, and biological processes
2x types :
1. mechanical
- natural agents (temp, water, wind), e.g. freeze-thaw cycle
2. chemical
- chemical reactions (with water, O2, CO2, acids)
also biological weather : caused by living organisms, e.g. plant roots can grow into cracks in rocks
unconfined compression test
rock strength
UCS
- cylindrical or cubical rock sample is loaded under compression until it fails
- measuring the maximum load that the rock can withstand before failure occurs
- careful :provides only a single-point measurement of rock strength and does not account for the variability of rock properties
- apply Mohr Circle
point load test
rock strength
- applying a concentrated load to a small rock specimen and measuring the force required to break the rock
Is50 = P/De^2
Is50 is the uncorrected point load strength index
P is the failure load
De is the equivalent core diameter
commonly applied on field
brazilian tensile test
rock strength
cylindrical rock sample is loaded in a diametrical plane until it fails. The strength of the rock is then determined by measuring the maximum load that the rock can withstand before failure occurs
common way of describing rock strength ?
UCS (unconfined compressive strength) test
~strong >50MPa
~weak < 5Mpa
typical rock srength of igneous ?
usually very hard and dense, with high compressive and tensile strengths
- due to crystalline structure & interlocking mineral grains
typical rock srength of metamorphic ?
high strength variability, often with planar weaknesses due to foliation & layering
typical rock strength of sedimentary rocks ?
typically lower strength & high susceptibility to weathering and erosion
susceptible to planar weakernesses
unconformity
geology
boundary or contact between two rock formations that represents a period of erosion or non-deposition, e.g. from erosion, tectonic/volcanic activity or change in sea level
angular unconformity : horizontal strata of sedimentary rock deposited on tilted and eroded layers
disconformity : parallel layers of sedimentary rocks which represents a period of erosion or nondeposition
non-conformity : sedimentary rocks overlie an erosion surface cut into igneous or metamorphic rocks
unconformity
geology
boundary or contact between two rock formations that represents a period of erosion or non-deposition, e.g. from erosion, tectonic/volcanic activity or change in sea level
angular unconformity : horizontal strata of sedimentary rock deposited on tilted and eroded layers
disconformity : parallel layers of sedimentary rocks which represents a period of erosion or nondeposition
non-conformity : sedimentary rocks overlie an erosion surface cut into igneous or metamorphic rocks
apparent / true dip ?
( dip refers to the angle at which a rock layer is inclined from the horizontal plane )
apparent : as it appears to an observer looking perpendicular to the strike of the layer
true : actual angle of inclination of a rock layer or structure relative to the horizontal plane
3 distinctive characteristics in the description of rocks ?
MATERIAL CHARACTERISTICS
those free from discontinuities.
Strength, bedding/layering, colour etc and Name
DISCONTINUITY CHARACTERISTICS
those of bedding, jointing & shear.
Orientation, spacing, roughness, strength etc
MASS CHARACTERISTICS
rock material + rock discontinuities = Rock mass
overall structure particularly discontinuities.
Fracture state (TCR, SCR, RQD, FI)
objectives of ground investigation ?
- Suitability of the site for the proposed project;
- Site conditions and ground properties;
- Potential geotechnical/geological issues;
- Ground characterisation
