Lecture #14 - Geotechnical: Shallow and Deep Foundations Flashcards
Name the 3 parts building can be divided into.
superstructure, substructure and foundation system
What is superstructure?
part of building above ground level
what is substructure
includes basements if provided
What is foundation
lower part of building that supports the weight of structure and transmits it to the ground
A building without a basement only has….
superstructure and foundations
What is foundation typically made of
concrete, masonry or a combination of both
Explain shallow foundations
used for buildings with light loads and consist of a reinforced concrete pad or strip footing that spreads the weight of the building over a larger area of soil
Explain deep foundations
used for buildings with heavy loads or unstable conditions and consist of driven piles, drilled shafts, or auger-cast piles that extend deep into the ground
Explain combined foundations
used for buildings that require both shallow and deep foundations and consist of a combination of pad or strip footings with piles or drilled shafts
What does foundation type choice depend on?
soil characteriestics and load pressure
Which foundation type is cheaper?
shallow
Explain footing.
widened base of a column or wall, widening is essentail because soil strength lower than material strength, footing distributes the superimpose load on a large area of soil so pressure on soil is less than soils bearing capacity
What are most footings constructed of? What about lightly loaded walls?
reinforced concrete, plain concrete
Explain isolated/independent footings.
used where the superimposed load is a point load, ex column
Explain continuous wall footings.
also called strip footings, are commonly used where the superimposed loads are linear, generally from a load-bearing wall
What is sometimes used to save concrete?
stepped footing with a pedestal
Which has higher bearing capacity: column or wall footing? Why?
column footing because of concentrated load on column
Explain combined footing.
combination of two isolated column footing, used where 2+ adjacent columns closely spaced and heavily loaded, combining reduces excavation cost and distributes load over larger area
Explain alternatives to combined footing.
strap footing and cantilevered footing in which a cantilevered grade beam spans between the two isolated column footings
3 types of monolitchic concrete foundations
slab-on-ground foundation, mat foundation, raft foundation (also called a floating foundation)
Explain slab-on-ground foundation.
commonly used, low cost and ease of construction, good for low-rise, light frame residential or commercial buildings, slab fucntions as foundation system and ground floor of building
Explain mat foundation.
columns and wall of building bear on large and thick reinforced concrete slab, used where soil has low bearing capacity so that independant column footings really big
When is it more economical to use a mat foundation?
if excavation required for isolated footings is more than 50% of the footprint of building, also where bedrock is so deep that use of deep foundation system is uneconomical
Explain raft/floating foundation.
consists of hollow mat formed by a grid of thick reinforced concrete walls between two thick reinforced concrete slabs, weight of soil excavated from ground is equal to weight of entire building so that the pressure on soil is unchanged
What do deep foundations include?
Piles and drilled piers that are like slender columns buried in the ground
Why are deep foundations prevented from buckling
confied by soil
What do piles and drilled piers transfer the load to
either bedrock or soil of high bearing capacity while passing thorugh unsuitable soil conditions
Typical pile materials?
steel, timber, concrete elements driven into gorund, except concrete piles which can also be site-cast in pre-drilled holes
what are site-cast piles called
drilled piers or caissons
What are pile material selection criteria
availability, cost, below-grade environment, type of soil, load magnitude, equipment required to drive the piles
What pile material is typically used under heavier loads
steel and concrete
Explain wood piles
limited in load baering capacity becasue of nature pf material and limitations on cross sectional area of tree trunks, wood piles treated with creosote or chromated copper arsenate (CCA) as a preservative
Explain friction pile
resists most of the loads by skin friction and by the end bearing
Explain end-bearing pile
ressits most of the loads by end bearing and some by skin friction
What are caissons
concrete cylinders poured into drilled holes (cast-in-place), reach through weaker soil to bear on competent soil beneat
explain end bearing caisson
belled when additional bearing capacity is required
Explain socketed caisson
drilled into a hard stratum and transfers its load primarily bu friction between the soil or rock and the side of the caisson
Explain caisson construction
auger on telescoping bar brings up load of soil from caisson hole, auger will be rotated rapidly to spin off soil before being reinserted in the hole
What is the caisson drilled equipped with for cutting through hard material
carbide toothed cutting haed
How is caisson bell formed
belling bucket with retractable cutters
Explain precast concrete piles
made of precast concrete and are generally solid, subject to attack by sulfur present in some soils, requiring use of sulfate-resistant portland ccement, concrete piles can be provided with a steel tip extension for easier penetration
Explain steel piles
may be H-shaped or hollow pipers, hollow are filled with concrete after being driven, covered with protective coatings if used in corrosive environments, for longer piles ends of cylinders are flanged so that they can be coupled to make longer sections
Explain screw/helical pile
rows of pile transfer loads from the shallow footing, located in a relatively weak soil into the stronger underlying stratum
Explain group pile
end reaction from tip bearing may be ignored for piles driven in weak soils, because they are driven piles are smaller in cross section than drilled piers and load capacity of one pile also small so therefore theyre used in clusters of three, pile cap functions like footing
Explain pressure bulb
as pile cluster increases, pile act together to create single larger bulb of higher pressure that reaches deeper into the ground, need to be careful so that this doesn’t overstress soil or cause excessive settlemtn of foundation
