Soil Mechanics Finals
type of K values
1. at rest, 2. active, 3. passive
Types of compression
1. elastic (instant/elastic deformation/no change in moisture content) 2. consolidation (primary: volume changes due to the expulsion of water from voids, Vv changes with Vt)
Failure mode depends on the type that foundation bears upon
1. general shear failure: dense sand and stiff cohesive soil, 2. local shear failure: medium compacted sand or clay, 3. punching shear failure:loose soil
Pore water pressure 1. is the hydrostatic pressure 2. changes as the height of the soil column increases 3. is different in vertical and horizontal directions
1. is the hydrostatic pressure
speed up the consolidation
1. put more stress applied (larger embankment) 2. wick drain - cloth drains all throughout, lets water flow up quickly
Total stress at a point is 1. resisted by the soil skeleton and the water in the voids 2. resisted by the water in the voids 3. resisted by the soil skeleton
1. resisted by the soil skeleton and the water in the voids
Total stress at a point is due to 1. the weight of everything acting above that point 2. the weight of the water in the voids above that point 3. the weight of the soil solids above that point
1. the weight of everything acting above that point
Select the incorrect statement. 1. Clay soils are unstable because they shrink and swell considerably with changes in moisture content. 2. Cohesive soils, such as silt, maintain their strength when unconfined. 3. Coarse-grained soils are more stable as a foundation material than silt or clay.
2. Cohesive soils, such as silt, maintain their strength when unconfined.
Void ratio is: 1. ratio of the volume of the voids to the total volume 2. ratio of the volume of the voids to the volume of the solids 3. ratio of the volume of water to the volume of the voids
2. ratio of the volume of the voids to the volume of the solids
effective stress is 1. the stress resisted by the water in the voids and the soil skeleton 2. the stress that is resisted by the soil skeleton 3. the stress resisted by the water in the voids
2. the stress that is resisted by the soil skeleton
#200 sieve
200 opening per 1 linear inch sieve
Which soil classification would be expected to have the highest bearing capacity? 1. Silt 2. Sand w/ fines 3. Clean gravel 4. Clay
3. Clean gravel
Which of the following statements is incorrect? 1. Clay soils can be unstable because they may shrink and swell with changes in moisture content. 2. Clays tend to be impervious to fluids. 3. In general, bearing capacity decreases with increased soil density. 4. The water table is the level beneath which the soil is saturated with groundwater.
3. In general, bearing capacity decreases with increased soil density.
Soil is a three-phase system consisting of what phases? 1. clay, silt, sand 2. solids, voids, water 3. water, solids, air
3. water, solids, air
Soil collected through test pits or test borings can be used to understand: 1. Extent of soil consolidation under loading 2. Permeability of soil 3. Water content of soil 4. All answers provided are correct
4. All answers provided are correct
soil classification systems
AASHTO (roads), USCS - Unified Soil Classification System (structure)
Failure Envelope, Mohr Circle
Above Failure envelope (failed - technically impossible to reach that area) As it touches failure envelope (fails) determine if shear stress exceeds the strength Failure does not occur at max shear stress
Rankine Model
Assumption: 1. wall is vertical, 2. soil is horizontal at front and back, 3. no friction between soil and wall 4. wall is rigid, 5. soil starts at rest
As construction materials
Bearing pressure = force/area If the soil cannot withstand the bearing pressure, the building will sink. (solution: drill down to bed rock for stronger supports)
USCS soil classification based on grain size
Boulder (>12''), Cobbles(>3"), Gravel(>#4), Sand(>#200), Fine (clay, silt)
Unconfined Compression Tests
Cohesive soil, clay is self standing, quick test, no excess pore water pressure measured, excess pore water pressure > 0, short term failure envelope
Void Spaces
Continuous and occupied by water, air, or both
D60,D30,D10
Diameter associated with 60%,30%, and 10% respectively
Compression
Due to increase in applied load (change in stress), top surface of soil moves downward
Factor of Safety
Force Resisting/Force Driving (Ex: Weight of the dam, uplift force)
ASTM Unified Soil Classification System
Gravel (6.4-76.2mm), sands (0.05-6.4mm), silts (0.002-0.05mm) and clays (<0.002mm) based on physical composition and characteristics
Direct Shear Test
Noncohesive soil and dry soil (usually), no excess pore water pressure, long term failure envelope
Normally consolidated v over-consolidated stress
Normally consolidated (current stress at max stress) Overconsolidated (current stress < max stress, experienced the max stress in the past)
Effective Stress
Rest of total stress carried by the soil grains at their points of contact. The sum of vertical components of the forces developed at the points of contact of the solid particles per unit cross-sectional area of the soil mass.
U: average consolidation
S at the time of interest/ S total *100% Function of Drainage Distance, time , k e (void ratio) = Av*Hv/Vs if (Av doesn't change)
Volume of Soil
Solid particle distributed randomly with void spaces in between
Distribution of Stress along a given cross section of soil profile (Effective stress concept)
Some fraction of the Normal stress at a given depth in a soil mass is carried by water in void space, other carried by the soil skeleton at points of contact of the soil particles
Primary Consolidation: After
The applied load starts to resisted by the soil skeleton (the effective stress) The speed of transfer: fast (sand (high K)), slow (clay (low K)) Final effective stress = initially effective stress + change in stress (applied loads) water table level do not change (voids still filled with water)
Pore Water Pressure (hydrostatic pressure)
The portion of total stress carried by water in continuous void spaces (equal intensity in all direction)
total stress
The total vertical stress acting at a point below the ground surface is due to the weight of everything lying above
Chart method
Use only if 1. homogeneous soil, 2. constant unit weight, cohesion, angle of internal friction, 3. grand surface (straight above and below the slope), 4. groundwater table well below the toe of the slope
triaxial test- consolidated drained
any type of soil, consolidated (adjusted for in-situ stress), drained, long time for the experiment, long term behavior (excess pore water pressure = 0), long term behavior
triaxial test
any type of soil, control vertical and horizontal stress, pore water pressure, change in head, confining pressure, deviator stress, expensive to conduct to the test
triaxial test- consolidated undrained (CU)
any type of soil, saturation (S) = 100%. undrained, short term, excess pore water pressure >0, short term behavior
Non cohesive soil (sand/gravel)
c = c' = 0, the effective angle = angle = 25 - 45 (excess pore water pressure exit quickly)
c' value
c' = 0 (Gravel, sand, inorganic silt, normally consolidated clay), c' > 0 (over consolidate clay)
The example: Normally consolidated clay
change in stress (x psi). stress 1 = stress 3 + change in stress Normally consolidated clay (c=0, stress 1' = stress 1 - excess pore water pressure, stress 3' = stress 3 - excess pore water pressure)
Primary consolidation
change in volume due to expulsion of water from voids
secondary consolidation
change in volume due to plastic readjustment of soil fabric
USCS
classification based on 1. size of soil grain, 2. distribution or gradation of soil grains (well graded: well distribution of different soil grain size), 3. organic components (organic: behave weird - remove for the stable supports), 4. behavior of fine particles (sticky or not)
Cohesive soils
clay - retain strength when unconfined
two broad classes of soils
coarse grained soils and fine grained soil
infinite slope
cohesionless soil, shallow failure plane due to bedrock
soil type
cohesive (clay - resist some tension) vs noncohesive (sand and gravel)
Distribution of Stress along a given cross section of soil profile helps to analyze
compressibility of soils, bearing capacity of foundation, stability of embankments, lateral pressure on earth-retaining structures
The soil underlying a building site
consist of superimposed layer (mix of soil types due to weathering or deposition)
Voids
consists of water (bottom) and air (top)
The integrity of building structure
depends on stability and strength under loading of the soil underlying the foundation
Distribution or gradation of soil grains
determine permeability and strength of the soil
Density
determine the bearing capacity of granular soils
solids
different size, shape, angularity
foundation
distribute weight of structure to the soil, not overstress the soil/shear failure/damage to the structure
Coarse grained soil
gravel and sand = relatively large particles (visible to naked eye)
foundation system
groundwater should be removed to avoid reducing the bearing capacity of the soil and to minimize the possibility of water leaking into a basement
Bearing capacity
happen at shallow foundation type: shallow and deep
Soil failure
inability of soil to perform as intended. Soil fail when Driving force > resisting force (FS<1)
Construction of the foundation
increase in net stress (depends on load per unit area, depth at estimation of stress made, other factors)
Clay soils
is unstable as it shrinks and swells with changes in moisture content.
The water table
level beneath which the soil is saturated with groundwater
Stress state in soil mechanics
loads only applied in x and y direction (no applied shear stress), stress in x and y direction = primary stresses (x direction = confining stress from adjacent soil, y direction = effective stress)
Coarse Grained soils
low percentage of void spaces, more stable as foundation materials than silt or clay, more permeable and drain better than fine grained soil, less susceptible to frost action
The allowable bearing capacity of soil
maximum unit pressure that foundation can withstand vertically or laterally on soil mass
shearing strength of a soil
measure of its ability to resist displacement when an external force is applied, due largely to the combined effects of cohesion and internal friction
the number of blows required by a hammer to advance a standard soil sampler
measures the density of granular soils and the consistency of some clays at the bottom of a borehole (the Standard Penetration Test)
factor of safety for finite slope
methods of slices and chart method
Soil Classification
methods vary depending on grain size, huge difference in physical properties between soil types, classification helps defining behaviors
lateral earth pressure
need to know for retaining wall design and abutments
at rest (basement walls)
no wall displacement, soil in elastic equilibrium, soil in elastic equilibrium
bearing failure of shallow foundation
occurs when stress exerted by foundation > ultimate bearing capacity of the soil
total flow
q (flow Rate/1 unit length) * the parameter of the water on plan view (water before the dam)
Compaction vs consolidation: compaction
quick, drive at air and water, under dynamic loads
Compaction
quick, driving out air and water, under the dynamic loads
Granular soils (gravel, sand, or some silts)
require confining force for shear resistance. Have a shallow angle of repose
slope fail
resisting force (shear strength) < driving force (a slope to slide: soil surface strength - soil weight, surface loads, seepage): slope fail do not know failure surface location (trial + error: guess the location)
cohesive soil or non cohesive mix
short term (c != 0), long term (c' = 0)
Classification of Coarse grained soil
sieve analysis: separate particle by sizes - Distribution: plot % finer (% passing) vs grain size (log size)
Fine Grained soil
silt and clay = smaller particles
wall failure
sliding, overturning, bearing capacity
Compaction vs consolidation: consolidation
slow (for clay): function of permeability, drive out water, under static loads
Active
soil pushes against the wall, wall moves away from soil, stress when soil failure happens (not wall failure)
Short term (total stress analysis)
soon after stress is applied, the effective stress =0 The change in pore water pressure = the change in stress. The effective stress < total stress - pore water pressure Undrained condition (change in u) > 0
Soil strength
stress = force/area, soil strength comes from friction and cohesion. Failure Envelope equation = Mohr Coulomb failure (effective cohesion = 0 for granular materials, normally consolidated clay)
A subsurface investigation includes
the analysis and testing of soil disclosed by excavation of test pit up to 3m deep or deeper test boring
Long term (effective stress analysis)
the change in stress = the change in effective stress. The effective stress = total stress - pore water pressure. Non cohesive soil, non saturate soil = the effective stress = total stress Undrained condition (change in u) = 0
soil profile
the diagram of vertical section of soil from the ground surface to the underlying material
Reconsolidation stress
the max stress experienced by soil
Suitability of soil as a foundation materials
the stratification, composition, and density of the soil bed & variations in particle size, and the presence or absence of ground water
Compaction (rolling, tamping, or soaking)
to achieve optimum moisture content, increase the density of soil bed
3 main shear strength tests
to define failure envelope (need cohesion & angle of internal friction) 1. direct shear test, 2. unconfined compression, 3. triaxial test
Moisture content and void volume is important
to determine the unit weight
Sloped sites & excavation of a flat site
unconfined soil displace laterally
A subsurface investigation is conducted to
understand the structure of soil, its shear resistance and compressive strength, its water content and permeability, and the expected extent and rate of consolidation under loading
Passive
wall moves toward the soil, wall pushes against the soil
USCS gradation classification
well graded, poor graded, gap graded
Primary consolidation: Initially
when loads, stress (delta sigma) is applied, soil wants to compress (water in voids). Initially the applied loads are resisted by "excess pore water pressure" (delta u)
