CIVE40009 Geotechnics Flashcards

Question

how do you determine degree of saturation

Answer 1

1. determine density of soil : - remove soil, insert flexible membrane lining, fill hole with water to identify volume 2. using mass of displaced soil & volume, calculate in-situ density 3. using specific gravity, calculate void ratio : e = (Gs * (ρw/ρd))-1 4. hence calculate degree of saturation : Sr = [((1+e) * ρ)/(ρw -Gs)]/ e

Answer 2

mechanically increasing the density of soil by reducing its volume' usually by a roller or falling weight

Answer 3

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

Answer 4

processes of erosion, transport & weathering from parent rocks different minerals have varying resistance to temperature, humidity & chemical weathering

Answer 5

gravity (e.g. landslide), ice (e.g. glaciers), water, air (wind)

Answer 6

process of dividing a soil sample into smaller representative samples

Answer 7

**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

Answer 8

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

Answer 9

Cz = (d30)^2 / d60*d10 well graded: 1

Answer 10

Dry - for soils containing insignificant amounts of silt & clay ONLY (due to flocculation & agglomeration) Wet - for most coarse-grained soils

Answer 11

Sedimentation : fine-grained soils, such as silt and clay PSD of < 425 microns Seiving : coarse-grained soils

Answer 12

**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

Answer 13

- soil passed through series of stacked seives with decreasing aperture - limitation : not ideal for non-spherically shaped soil particles

Answer 14

soil in its natural state

Answer 15

disturbed from its natural structure with some/most of its natural structure removed due to mechanical means (eg mixing)

Answer 16

somewhat disturbed, some loss of natural structure

Answer 17

all of its natural structure has been removed due to mechanical mixing - at 1.5-2x liquid limit

Answer 18

all of its natural structure has been removed due to mechanical mixing - at 1.5-2x liquid limit

Answer 19

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

Answer 20

used to quantify state of sands : Dr = (emax - e) / (emax - emin) e = void ratio

Answer 21

roll to about 3 mm diameter in a given number of rolls **subjective**

Answer 22

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

Answer 23

Ip = wL - wP

Answer 24

**cone penetrometer** (less subjective & more repeatable) **casagrande cup** (more subjective & more dependant on operator)

Answer 25

**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

Answer 26

arrangement of soil particles in a soil mass, including their shape, size, orientation, and distribution (**non-scalar quantity**)

Answer 27

RC = pd / pdmax pd = dry density used for state quantification of soil mixtures

Answer 28

for saturated soils : - densification of soil by applying external loads - to remove water - hence decreased void volume

Answer 29

- increase stiffness (reduces settlement) - increase strength ( bearing capacity, liquefaction resistance etc.) - reduce permeability - **reduces air voids**

Answer 30

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

Answer 31

V = 1 + e volume containing 1 unit Volume of solid material

Answer 32

- retaining wall backfill - road embankment - earth dam - offshore pipeline backfill

Answer 33

1 static loading/ pressure (FG) 2 kneading (CG) 3 impact (FG) 4 vibration (CG) FG fine grained CG coarse grained

Answer 34

1. Sand Cone - low cost/accurate - slow/low control 2. Balloon - moderate cost/fewer steps - slow/difficult to obtain accuracy 3. Nuclear Density Meter - quick/direct measurements - high cost/radiation uses/ water content error

Answer 35

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

Answer 36

u = γ w × z = ρw × g× z Pore pressure increases linearly with depth | hydrostatic = no flow / seepage

Answer 37

Total stress = pore water pressure + effective stress σ = u + σ'

Answer 38

- 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.

Answer 39

Effective vertical stresses increase linearly with depth in homogeneous soil deposits σʹz = (γ sat*z) − (γ w*z) = (γ sat −γ w)z = (ρsat − ρw) gz

Answer 40

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

Answer 41

- 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)

Answer 42

****downward seepage - drains excess water from the soil, **reducing** the pore water pressure (u) and **increasing** the effective stress

Answer 43

H = u / γw (pressure head) + z (elevation head) + ν^2/2g (velocity head) | note : velocity head assumed to be negligible for most soils

Answer 44

H = u / γw (pressure head) + z (elevation head) + ν^2/2g (velocity head) | note : velocity head assumed to be negligible for most soils

Answer 45

head loss per unit length i = Δh / l

Answer 46

q = v ⋅ A = vs ⋅ Av [L3/T] where vs = seepage velocity, v = discharge velocity, Av = area of voids

Answer 47

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)

Answer 48

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

Answer 49

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

Answer 50

soil structure or system has reached a level of deformation or displacement that exceeds the allowable limit for the intended use

Answer 51

allowable stress design or load & resistance factor design

Answer 52

- strip foundation - raft foundation soil is strong enough to support the building loads without the need for deeper excavation - typically low-rise

Answer 53

pile foundation piled raft/mat drilled shaft foundation soil is weak and cannot support the building loads with shallow excavation - for high-rise

Answer 54

can cause : - compaction - consolidation - liquefaction - shear due to changes in vertical stresses, pore pressure & vertical displacements

Answer 55

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

Answer 56

- **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)

Answer 57

purposes : **avoid limit states** 1. create temporary space 2. create permanent space 3. retain soil 4. stabilise slopes/excavations

Answer 58

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)

Answer 59

(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)

Answer 60

(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

Answer 61

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

Answer 62

- straight line that relates the shear stress (τ) to the normal stress (σ) acting on the soil - soil stress state when failure occurs

Answer 63

- soil is constrained laterally and cannot deform laterally due to the presence of a rigid structure - high horizontal/lateral effective stresses ## Footnote (wall moves towards soil)

Answer 64

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

Answer 65

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).

Answer 66

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)

Answer 67

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

Answer 68

Naturally occurring inorganic substance which has a definite chemical composition and presents an ordered atomic arrangement

Answer 69

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

Answer 70

crystallised from molten magma , found underground

Answer 71

**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

Answer 72

**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

Answer 73

igneous rock : - higher silica content (>65%) -rich in light-coloured minerals - granite, rhyolite

Answer 74

igneous rock : - lower silica content ~< 50% - basalt - ultramafic igneous exists

Answer 75

- magma released from long, narrow crack -> fissure - extensive lava fields

Answer 76

crystals approximately equal size

Answer 77

irregular crystal size distribution

Answer 78

large crystals surrounded by much smaller crystals

Answer 79

fine-grained : individual mineral grains are too small to be seen with the naked eye (commonly extrusive rocks)

Answer 80

**coarse-grained texture** : in which the individual mineral grains are large enough to be seen with the naked eye

Answer 81

**coarse-grained texture** : in which the individual mineral grains are large enough to be seen with the naked eye, typically intrusive

Answer 82

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

Answer 83

- 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

Answer 84

mosaic of interlocking crystals : fineness is determined by rate of cooling - other textural properties depend on cooling conditions (e.g. glassy or vensicular)

Answer 85

generally uniform high strength & reistant to weathering and erosion

Answer 86

**silicate minerals** **non-silicate minerals** (carbonates, oxides, halides, sulphides, sulphates, phosphates)

Answer 87

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)

Answer 88

silicate mineral group medium-high susceptibility to weathering 2 x common : 1. Alkali feldspar (Pink or white) 2. Plagioclase feldspar (White, colourless or grey)

Answer 89

produced by the surface processes of the rock cycle including weathered rock fragments, minerals precipitated from water, and organic material

Answer 90

**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

Answer 91

widely varied : rivers, lakes, beaches, tidal flats, shallow maring, deep maring (with increasing energy levels) rock texture & deposition dependant on environment

Answer 92

uniform particle size distribution - typically indicates that the sediment was transported a significant distance

Answer 93

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

Answer 94

has a range of particle sizes arranged in a sequence

Answer 95

shale, sandstone & conglomerate

Answer 96

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

Answer 97

- fine-grained sedimentary rock - finely laminated - tendency to break along parallel side fragments - formed from compact muds/ clay minerals

Answer 98

bedding (horizontal layers that are visible in many sedimentary rocks)

Answer 99

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

Answer 100

(i) Thermal or contact metamorphism (temperature) (ii) Dynamic or dislocation metamorphism (stress) (iii) Regional (temperature + stress)

Answer 101

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

Answer 102

contact metamorphism of recrystallised carbonate minerals = metamorphosed limestone

Answer 103

Gneiss, schist, slate

Answer 104

Mostly deep inside mountain chains

Answer 105

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

Answer 106

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

Answer 107

- 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

Answer 108

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

Answer 109

UCS (unconfined compressive strength) test ~strong >50MPa ~weak < 5Mpa

Answer 110

usually very hard and dense, with high compressive and tensile strengths - due to crystalline structure & interlocking mineral grains

Answer 111

high strength variability, often with planar weaknesses due to foliation & layering

Answer 112

typically lower strength & high susceptibility to weathering and erosion susceptible to planar weakernesses

Answer 113

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

Answer 114

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

Answer 115

( 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

Answer 116

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)

Answer 117

* Suitability of the site for the proposed project; * Site conditions and ground properties; * Potential geotechnical/geological issues; * Ground characterisation