Bearing Capacities (Ch16) COMBINED
ultimate bearing capacity
The ultimate load per unit area of a shallow foundation which will cause shear failure of the soil supporting the foundation is called
it is located at the base of the footing
The water table will have no effect on the ultimate bearing capacity when
Two-Way Eccentricty
When foundations are subjected to loads with two-way eccentricity, as shown, the effective area is determined such that the centroid coincides with the load. The procedure for finding the effective dimensions, B' and L' are beyond the scope of this class.
peak and critical state, decrease
___ and ___ friction angle ______ with increase in stress
(c' and friction angle') or (c and friction angle)
both: we decide
FS on shear strength parameters
can do FS on shear properties instead of bearing capacity cd=c/FS (shear) d: design angle of friction (d) = tan^-1 (tan (angle of friction)/FS (shear)) FS for qall = 3 to 4 FS (shear) = 1.5
continuous footing:
continuous strip of concrete that spreads the load across soil
different types of shear failure
different types of shear failure
settlement
for large foundations, do analysis on settlement or bearing capacity?
if GWT is below depth of influence
gamma and q
what does Terzaghi assume about footing base?
has friction
the ___ of the two effective dimensions is the effective width and the other is the effective length
lower
for piles, what is failure done by?
punching or "cavity expansion"
consolidation
settlement analysis for foundation on SOFT soils will be based on ____ theory
1mm/storey + 10%
settlement of large foundation on a strong soil
Continuous Footing
shallow foundation used to support wall loads
Settlement acceptability rules of thumb
shallow foundations 1" deep foundations 4-6" more of case by case basis
small (light) structures have what types of foundation?
shallow or mat
shape factor accounts for:
shape of footing, either rectangular or circular
Design Foundations against
shear failure --> Bearing Capacity settlement
depth factor accounts for :
shearing resistance along failure surface in soil above base
increasing dimension, or the embedment depth of a footing
to shift from general shear failure to punching shear failure
settlement
total load on a foundation governs _____ rather than how the load is transferred to the ground
punching shear
typically high compressibility soils, loose sands, drained clays
what is general shear
typically low compressibility soils, dense sand, saturated clay, undrained clays
local shear
typically medium compressibility soils, medium sense sands
Footings on layered soils non conservative approach
use weighted c, angle of friction and gamma only if properties are not too different
if GWT is under bottom of footing but within depth of influence
weighted gamma and q
increases
young's / deformation modulus ____ as stress increases
Environmental Issues that Cause Foundation Problems - Seepage/Erosion
- Burst pipes or cracked sewers - Problem on sandy soils - Eroded fines can lead to collapse settlements - Salt mining in Cheshire - High water flow can increase pwp lowering effective stress
Environmental Issues that Cause Foundation Problems - Seasonal Changes
- Cyclic wetting and drying - Swell and Shrinkage of clays - Swell potential function of PI - Minimum depth (0.9m) suggested to get away from seasonal zone, can be deeper
Environmental Issues that Cause Foundation Problems - Temerature
- Low Temp leads to freezing and expansion and loss of strength on thaw (min depth again), - When heated some clays in areas under kilns and boilers may dry out and bake soil leading to shrinkage
Environmental Issues that Cause Foundation Problems - De-Watering
- Water abstraction can lead to problems
Permissable Settlement
- buildings are designed to allow for the expected settlement - defined limits
Piles Foundations
- columns constructed in ground to support structure - transfer loads to deeper strata - can be end bearing or frictional or combination of both - used where shallow foundations are uneconomic or won't work due to: - high load - poor soil - seasonal changed to depth - excessive settlements critical
what are the 7 considerations of selecting a foundation type?
- geological conditions - loading conditions - construction methods - design codes and regulations - environmental conditions - impact on neighboring structures - construction times and cost
Bulbs of Stress
- lines of equal stress can be plotted for the increase in stress beneath a loaded foundation, creating a bulb like shape - curves plotted down to value of 0.2q as stresses are considered low below this point - lots of small adjacent footings can be considered as one overall footing for this - give an idea of max depth of soil that is stressed - shape of bulb depends on foundation type: - strip footings stress to 3 times width - square pads stress to 1.5 times width
Rule of Thumb
- method of calculating change of stress - for rectangular or circular foundations
Types of Shallow Foundations - Pads (combined or single)
- point loads from single to multiple columns - can be reinforced
gross allowable load bearing capacity
- qall=qult/FS individual footing
what causes vertical stresses in the ground
- self-weight of the soil ( the overburden pressure) -from external loading
Settlement varies with:
- soil type - mechanism and magnitude of loading - with time
Total Settlement
- structure moves as a whole - problem for connecting services
Immediate (Elastic) Compression, Si
- when soil experiences a change in stress due to loading - caused by elastic compression and re-orientation of the soil particles - (in partially saturated soils expulsion of air) - tend to be immediate settlements that occur before the soil changes volume
Secondary Compression, Ss
- when soil experiences a change in stress due to loading - re-orientation of particles - not well understood - typically small - often ignored
Primary (Consolidation) Compression, Sc
- when soil experiences a change in stress due to loading - time dependant - caused by flow of water in soil
Differential Settlement
- where one part of a structure moves relative to another - limits set by visual appearance - can be caused by: -- variations in strata/soil conditions -- variations in load -- variations in the time of construction of areas of structure
net allowable load bearing capacity
-(qall)net = qult-unit weight*Depth)/FS Deep footings (mat foundations)
examples of small (light) structures:
-1-2 story commercial -apartments -hotels/motels -elevated water tanks -warehouses
Foundations: factors that reduce environment impact
-3-4 times as much energy is delivered in heat than is used in electricity -uses geothermal mass of ground and high thermal storage capacity of concrete in piles
Foundation: Waste
-different types of foundations create different types of waste material -landfill tax introduced in 1996 to encourage reduction in waste production/ disposal
Foundations: Consideration of what materiel is needed
-embodied carbon materials - concerns an estimation of the carbon production associated with extraction of raw material, processing transportation to site and use of plant during site operation
local shear failure:
-footing undergoes settlement -some soil bulges -slip surface ends beneath soil surface
Foundation: Vibration, noise, air quality
-sensitivity of the area to the above should be considered -major concern for piling operations -sensitivity of an area may limit certain foundation construction methods
what may the distribution of stress below the ground depend on ?
-stiffness of the foundation -compressibility & stiffness of the soil -loading conditions -natural variations within the soil properties below the foundation
Load applied to soil through foundations must be...
... less than an allowable value the soil can cope with, which is a function of: - soil strength - settlement potential
Foundations must...
... spread load (p) and give applied stress (q)
Examples of Small Structures
1-2 story commercial buildings Apartments Hotels/motels Elevated Water Storage Industrial warehouse buildings
Requirements for foundation to function properly
1. settlement of soil caused by load must be within tolerable limit 2. shear failure of the soil supporting the foundation must not occur 3.
problems concerned with regarding the bearing capacity of soils are:
1. what size must the foundation be to carry a given load 2. what load may safely be carried by a foundation of a given size 3.what is the factor of safety of a particular foundation under a given load
Primary Design Considerations
1.) Avoid Shear Failure of the Soil 2.) Prevent Excessive Settlements
Loads are transmitted to deeper layers by?
1.) Friction 2.) End Bearing on stiffer layers or bedrock 3.) Combination of Friction & End bearing
3 basic modes of bearing capacity failure
1.) General Shear Failure 2.) Local Shear Failure 3.) Punching Shear Failure
3 Types of Foundations
1.) Shallow (spread) Footings 2.) Mat Foundations 3.) Deep Foundations: Piles, Drilled Shafts
general shear, local shear, punching shear
3 failure modes of soil supporting foundations
what is the depth to width ratio for small (light) structures?
<4
large (heavy) structures have a depth to width ratio of what?
>4
qnet(u)=0
A fully compensated mat foundation is one in which the net increase in soil pressure below the mat is
(a) a flat plate thickened under columns. (b) a flat plate with pedestals. (c) a slab with basement walls. (d) all of the above
A mat foundation can be
The Foundation
A part of an engineering system which transmits loads to the underlying soil or rock
pile foundation
A system of piles, pile caps and straps that transfers the structural load to the bearing stratum (lower levels) into which the piles are driven. Timber, concrete or steel Used for heavy structures
Mat Foundation
AKA Shallow Foundation. A large footing under an entire building, which distributes the building load over the entire area. It is also known as 'raft foundation'.
Spread Footing
AKA Shallow Foundation. A wide shallow footing usually constructed of reinforced concrete. depth of ebedment to width ration <4
qu-=6.68c'
According to the Terzaghi's ultimate bearing capacity equation, the ultimate bearing capacity of a square and circular footings resting on a clayey ground in undrained condition is
qu=5.7c'
According to the Terzaghi's ultimate bearing capacity equation, the ultimate bearing capacity of a strip footing resting on a clayey ground in undrained condition is
General Shear failure
Brittle soils Well defined failure wedge Adjacent ground bulges upwards Abrupt Possible foundation tilting
Effect of Groundwater
Case I: If groundwater is located a distance of D above the bottom of the foundation, q = γ(Df - D) + γ'D Case II: If groundwater table coincides with the bottom of the foundation, the magnitude of q is equal to γDf. However, γ in the third term should be replaced with γ' Case III: If groundwater table is at a depth d below the bottom of the foundation, the magnitude of q = γDf. However, γ in the third term should be replaced with γav
Effective Area
Effective width: B' Effective Length: L' B' = X or Y, whichever is smaller L' = X or Y, whichever is larger A' = B' x L'
Piles and drilled shafts
For heavier structures when great depth is required for supporting the load, which of the following foundation is generally recommended?
Failure modes
General Shear Local Shear Punching Shear
Shallow Foundations
Generally Have an embedment ratio less than 4
qnet(u)=qu-q
If the difference between the unit weight of concrete used in the foundation and the unit weight of soil surrounding the foundation is assumed to be negligible, then the net ultimate bearing capacity,
General Shear
Occurs in Dense Sand or Stiff cohesive coil
Punching Shear
Occurs in dense soil layers overlying loose of soft soils
Examples of Large Structures
Professional Office Buildings Health care facilities Condominiums Multi-story education/research Bridges
glaciated
STRONG soils are typically those which have been ____
Local Shear failure
Slight bulging of the ground Large settlement Transition mode of failure
Types of footings
Square Rectangular Continuous Combined
Combined footings
Support multiple columns Useful when columns are too close together for their own footings
which bearing capacity equation is more conservative
Terzaghi is more conservative than general
Ultimate bearing Capacity
The avg load per unit area that causes a shear failure of the supporting soil
(a) shape factors. (b) depth factors. (c) load inclination factors. (d) all of the abov
The general bearing capacity equation considers the following:
(b) spread footings. (c) piles and drilled shafts. (d) both (b) and (c)
The load of a structure is transferred to the foundation soil through
Allowable bearing Capacity
The maximum design value to ensure an adequate Fs
3
The minimum factor of safety used to get net allowable bearing capacity from the known ultimate bearing capacity is generally
Bearing Capacity Failure - Transcona
The pressure exerted on the soil beneath the elevator caused the failure. Clay, silt, and glacial deposits sit beneath the structure. The clays were of the stratified type, meaning that pockets of silt sat between thin sheets of actual clay. (Baracos, 1957) The present clay types lack the unconfined compressive strength of other soil types. The foundation pressed downward with a force of 3.06 tsf (tons per square foot) while the clay beneath it could only support approximately 1.13 tsf based on samples
e>B/6
The tension develops in the soil below the footing when the eccentricity
Drill Shaft foundations
Used for heavy structures. Shaft drilled into the subsoil and then filled with concrete. Metal casing may be used while shaft being drilled. Diameter of casing much larger than the pile
Rectangular footings
Useful when obstructions prevent a square footing Can resist some moment loads
Environmental Issues that Cause Foundation Problems - Vibration
Vehicles loads, construction work, earthquakes - Lead to particle re-oriantation or build up of PWP
Punching shear failure
Very plastic soil Large settlement Low capacity No surface bulging
When are Deep Foundations used?
When Greater load capacities or lower settlements are required
the ground surface
When the general shear failure of the foundation soil takes place, the failure surface in soil extends to
Spread footings and mat foundations
Which of the following foundations are generally referred to as shallow foundations?
eccentricity
accounts for moment changes effective are of footings turns square into rectangle B' = B-2eB L' = L-2eL area of footing changes due to eccentricity
what are shallow foundations
also known as spread foundations and are typically considered to be where either: - the depth of the foundation is less than the width of the foundation -depth is less than 3m (within reach of normal excavation plant) -if soil near the surface can support structural load (raft, strip,pad) raft:spreads the load over a large area strip:supports lines of loads pad:support individual point load
decrease
angle of internal friction ___ as pressure increase
terms used to describe the amount of pressure imposed on the ground
applied pressure bearing pressure contact stress
what are deep foundations
are those which apply pressure some distance below the finished ground surface, transferring the load deeper at depth if unsuitable soils are present near the surface. they are often used for reasons relating to poor bearing capacity of the ground but also other factor.
converges, greater increase
as load increases the amount of settlement (converges/diverges) because of _____ of deformation modulus E beneath the small footing
increases
as the distance from the bottom of the core increases, water content ____
local shear to general shear
as the ring radii ratio increases, the failure mode tends to move from ____ to _____
settlements will be elastic, mostly during construction
assumption for deformation analysis
shallow foundations must
be safe against shear failure in the soil that supports them not undergo excessive settlement (immediate and secondary) need both
decreases
bearing capacity factors ___ with increased size of footing (BUT may not be accounted by the decrease of internal friction as pressure increase)
reduced strength parameters for punching failure
c' = 2/3c tan(angle of friction) =2/3tan(angle of friction)
bearing capacity definition:
capacity of soil to support loads applied to the ground, aka the max average contact pressure bw foundation and soil where shear failure won't occur
combined footings applicability
columns to closely spaced support of column too close to obstruction or property line column spacing < 7 m
foundation construction process
compact subgrade/fill place reinforcement and forms pour concrete and place connections for columns to distribute load attach columns
large (heavy) structures have what type of foundation?
deep foundation
undistrubed
disturbes/undisturbed samples are impossible
Terzaghi's Ultimate Bearing Capacity
evaluate shallow strip footings A foundation may be defined as shallow if depth (Df) is less than or equal to width (B) weight of soil can be replaces by a uniform surcharge (q=(unitweight)*(Df) qu = qc + qq + qy
compression index
for STRONG soils, elastic settlement will be predominant and consolidation tests are carried out to determine ____
N > 30
for cohesionless soils to be considered as strong soils
must have been deposited or overriden by a glacier have N > 15 blows for 0.3m or undrained strength 100 kPa
for cohesive soils to be considered as strong soils
q allowable equations
for shallow foundations qall = qult/FS deep foundations with mat footing (qall)net = (qult-unit weight of soil*depth)/FS
Nc, Nq, Ngamma are functions of what
friction angle (dimension-less)
if water table above bottom of footing
gamma' and q'
types of shear failure
general shear failure dense/stiff smooth curve at surface local shear failure medium dense ripples at surface punching shear failure loose soil foundation sinks
the outer skin of the core
in core sample, the moisture is highest at
general bearing capacity equation
includes: shape factors depth factors load inclination factors
consolidated has what effect on strength
increases strength
what does depth of footing below ground surface contribute to?
increasing bearing capacity
what is bearing capacity
is the value of applied pressure that causes shear failure of supporting soils. there is different modes of shear failure depending on the soils properties
mat foundation
large foundation that supports columns and walls
what two things are FOS selected to do?
limit settlement avoid bearing failure
inclination factor accounts for:
load application inclined at angle
End-bearing pile
load carried by pile is transmitted to a firm stratum
what does ultimate bearing capacity mean?
loading intensity at which soil suffers shear failure
Effective stress analysis
long term conditions stiff clay c', ang of fric' CD test drained condition consolidated clay
3/4" or 20 mm
maximum allowable differential settlement recommended by Terzaghi and Peck
what determines the depth of footing below ground surface?
must get below: top soil depth of frost penetration potential depth of erosion seasonal moisture variations
compensated foundations
(deep foundation) -foundations in which the relief of stress due to excavation is approx balanced by the applied stress due to the foundation -net stress applied is therefore very small -normally comprises a deep basement
Environmental Issues that Cause Foundation Problems - Lack of Confinement (Lateral Support)
- Adjacent excavation can undermine
Environmental Issues that Cause Foundation Problems - Compaction
- Building on fill - Poorly compacted materials can settle and consolidate
soft rock, dense sands and gravel
(3) can be considered as strong soils
saquare/rectangular footing units
q--> psf
Allowable overall bearing capacity
qall lower value of: -qall=qult/FS from shear failure -q of allowable settlement FS approximately = 3 lots of unknowns sometime FS = 2 slope stability FS approximately = 1.5
Allowable load bearing capacity
qall = qult/FS
Relationship between applied foundation pressure and soil settlement
qfail = Max pressure at which soil fails in shear qs = Safe Bearing Pressure (qfail/FoS) qa = Allowable Bearing Pressure (to limit settlement to permissible levels) Actual FoS once built = qfail/qa
Ultimate bearing capacity
qult max stress at which shear failure occurs in soil under the foundation qult =Qult/Area of footing
bearing capacity equation
qult = qc + qq + qgama c: cohesion q: overburden pressure gamma: unit weight
Isolated footings applicability
relatively high ratio of soil resistance to structural loads
mat foundation applicability
relatively low ratio of soil resistance to structural load soils to weak/soft can use with deep foundations
Special site condition: foundation on layered soil
rigid base at shallow depth stronger soil underlain by weaker soil
Depth of influence and shear strength parameters
sand--> angle of friction Depth of influence =2B angle of friction >0 Clay --> c and angle of friction Depth of influence saturated clays =B angle of friction in saturated clays = 0
Total stress analysis
short term condition weak saturated clay cu, ang of fric = 0 UU test
Local Shear Failure
similar to general shear failure but slip surface end somewhere inside soil
what type of footings can small (light) structures have?
spread or continous
Terzaghi equations can be used for
square footings circular footings strip footings Punch failure square and circular footings have additional slip surface area on sides
cohesive, N>15. non cohesive N>30
strong soils can be defined as ___ soils
Strap footing applicability
support of column too close to obstruction or property line column spacing > 7m
settlements
tall buildings on strong soils tends towards similar _____ irrespective of foundation type
ring radii ratio, lateral loading
the bearing capacity of ring footings is dependent on ___, ring foundations have high capacity to resist ____
Bearing Capacity
the capacity of soil to support loads applied to the ground maximum average contact pressure between he foundation and the soil which should not produce shear failure in the soil
Environmental Issues that Cause Foundation Problems - Subsidence (mining etc)
- Collapse of old mines or settlements during tunnelling
Foundation: Contamination
- Foundation construction can lead to generation of contaminated spoil and the creations of pathways for movement of contaminate
what is the middle third rule?
the resultant force should be within the middle of the third of the footing to avoid zones where the footing will not transmit loads to the soil
Friction Pile
the superstructure load is resisted by the shear stresses generated along the surface of the pile
why is a lower F used for extreme loads?
to avoid failure (collapse), anticipate significant settlements
angle of friction = 0 behavior
unconsolidated undrained weakest state of clay c=cu saturated undrained conditions & unconsolidated
Footings on layered soils conservative approach
use lowest c, angle of friction and gamma
Environmental impacts of foundations
waste contamination vibration, noise, air quality foundation that reduce environment impacts consideration of what materiel is needed
centrifuge modelling
what kind of modelling for large structure on strong soils
what is an eccentrically loaded footing?
when resultant load is not coincident with the centroid of the foundation, the foundation dimension need to be theoretically adjusted to align with the load center with the centroid
General Shear Failure
when uniformly sidtrbuted load (q) = (qu) footing undergoes very large settlement failure soil on one or both sides of footing bulges and the slip surface extends to the ground surface at failure soil reached plastic equilibrium and failure occurs by sliding
Environmental Issues that Cause Foundation Problems - Vegetation (Planting and Removal)
- Trees remove water, rules on proximity of trees to buildings (NHBC and BRE) guidance - Can cause settlement or heave - Critical for clays (settlement after long hot summers)
Environmental Issues that Cause Foundation Problems
- compaction - seasonal changes - vegetation (planting and removal) - vibration - de-watering - seepage/erosion - temperature - loss of confinement (lateral support) - subsidence (mining etc.)
3 Types of Compression experienced in soil when there is a change in stress
- elastic - primary (consolidation) - secondary
Foundation type used is related to:
- soil type - structure type - loading - constructibility - time and money
Types of Shallow Foundations - Rafts
- spread load over area of whole building - used in areas of weak soil to reduce pressures or accommodate settlement
Types of Shallow Foundations - Buoyant Rafts and Basements
- spread load over area of whole building - used in areas of weak soil to reduce pressures or accommodate settlement - used where low bearing pressure required
Types of Shallow Foundations
- strip footings - pads (combined or single) - rafts - buoyant rafts and basements
Foundations are designed for with 2 factors in mind
- the supporting soil must not *shear* (ULS) - the supporting soil must not *settle* (SLS)
Settlement - 2 elements that must be considered
- total amount of settlement - time for that settlement to occur
Types of Shallow Foundations - Strip Footings
- used to support walls (strip loads) - typically used for houses - can be reinforced to span small soft spots and for larger loads - width normally governed by room to construct
Settlements
- when soil beneath a structure 'sinks' during and after construction - soil must not squash to an excessive level under loads - must be predicted and managed
examples of large (heavy) structures:
-3 or more story office building -3 or more story health care facilities -condos -multi-story edu/research facility -bridges
general shear failure:
-footing has large settlement -soil on sides bulges -slip surface reaches ground surface
Factors that control failure modes
1.) Soil Strength 2.) Soil Compressibility
construction
80% settlement happen during ____
soft clayey foundation soils
A fully compensated foundation is generally recommended on
what is an eccentric foundation?
An eccentric foundation is a foundation whereby the wall sits on the outer edge of the foundation
Assumptions for Terzaghi's bearing capacity equation
Depth <width Rough bottom (no sliding) Homogeneous soild General failure mode No consolidation Very rigid foundation Soil above the footing is only considered as a surcharge load (no strength) Centric compression loads only
General Bearing Capacity Equation Modification
Depth factor-accounts for the shearing resistance developed along the failure surface in soil above the base of the footing Shape factor-accounts for shape of footing, either rectangular or circular Inclination factor-accounts for load application inclined at a certain angle for the vertical
Shallow Foundation
Depth-of-embedment-to-width ratio <4
Deep Foundation
Depth-of-embedment-to-width ratio >4
Factor of Safety
FS of 3 or more applied to ultimate soil-bearing capacity to find allowable soil bearing capacity q(all) = q(u)/FS q(net) = q(u) - q
Eccentrically Loaded Footing
Footing for which loads are not in the center
4% to 10% of the width of foundation
For foundations of a shallow depth, the ultimate load with a general shear failure of soil may occur at a foundation settlement of
e=M/Q
If a foundation is subjected to a total vertical load Q axially along with a moment M, the eccentricity of the load system in the effective area method of calculating the ultimate bearing capacity of eccentrically loaded foundations is estimated as
Nq = 1.
If the angle of friction of the foundation soil is 0°
Nγ = 0
If the angle of friction of the foundation soil, φ′ = 0°,
30 t/m2
If the net ultimate bearing capacity of a foundation is 30 t/m2, its net allowable load bearing capacity will be
Total Settlement, St =
Immediate Settlement, Si + Primary Settlement, Sc + Secondary Settlement, Ss
q=yDf
In Terzaghi's ultimate bearing capacity theory, the effect of soil above the bottom of the foundation may be assumed to be replaced by an equivalent surcharge,
Punching shear failure
In fairly loose foundation soil, what type of failure mode is expected?
less than 4
In general, shallow foundations are those foundations that have a depth-of-embedment-towidth ratio of approximately
mat foundation.
In soil with low load-bearing capacity, it is more economical to construct the entire structure over
(b) Local shear failure (c) Punching shear failure (d) both (b) and (c)
In which of the following failure modes, the failure surface in the foundation soil does not extend to the ground surface?
Types of shallow foundations
Isolated footing Combined footings -rectangular -trapezoidal Strap footings Mat foundation
Square footings
Most common Support single centrally loaded column
Local Shear
Occurs in Compressible Soils, Soft Clays, and Loose Sands
(b) shear failure of the soil. (c) both (b) and (c) (d) functional failure of the structure.
Overstressing the foundation soil can result in
Strip footings
ROT (L/B)> 10 q--> lb/ft per foot of wall
Mat footing
Structural loads are erratic and not evenly distributed Mat foundations provide more uplift capacity as they are heavier
Continuous footings
Support bearing walls
Ny=2(Nq+1)tanφ
The bearing capacity factor Nγ is given as
Df=Q/Ay
The depth of embedment of a fully compensated foundation is
