Lecture #14 - Geotechnical: Shallow and Deep Foundations

Question 1

Name the 3 parts building can be divided into.

Answer 1

superstructure, substructure and foundation system

Question 2

What is superstructure?

Answer 2

part of building above ground level

Question 3

what is substructure

Answer 3

includes basements if provided

Question 4

What is foundation

Answer 4

lower part of building that supports the weight of structure and transmits it to the ground

Question 5

A building without a basement only has….

Answer 5

superstructure and foundations

Question 6

What is foundation typically made of

Answer 6

concrete, masonry or a combination of both

Question 7

Explain shallow foundations

Answer 7

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

Question 8

Explain deep foundations

Answer 8

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

Question 9

Explain combined foundations

Answer 9

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

Question 10

What does foundation type choice depend on?

Answer 10

soil characteriestics and load pressure

Question 11

Which foundation type is cheaper?

Answer 11

shallow

Question 12

Explain footing.

Answer 12

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

Question 13

What are most footings constructed of? What about lightly loaded walls?

Answer 13

reinforced concrete, plain concrete

Question 13

Explain isolated/independent footings.

Answer 13

used where the superimposed load is a point load, ex column

Question 14

Explain continuous wall footings.

Answer 14

also called strip footings, are commonly used where the superimposed loads are linear, generally from a load-bearing wall

Question 15

What is sometimes used to save concrete?

Answer 15

stepped footing with a pedestal

Question 16

Which has higher bearing capacity: column or wall footing? Why?

Answer 16

column footing because of concentrated load on column

Question 17

Explain combined footing.

Answer 17

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

Question 18

Explain alternatives to combined footing.

Answer 18

strap footing and cantilevered footing in which a cantilevered grade beam spans between the two isolated column footings

Question 19

3 types of monolitchic concrete foundations

Answer 19

slab-on-ground foundation, mat foundation, raft foundation (also called a floating foundation)

Question 20

Explain slab-on-ground foundation.

Answer 20

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

Question 21

Explain mat foundation.

Answer 21

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

Question 22

When is it more economical to use a mat foundation?

Answer 22

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

Question 23

Explain raft/floating foundation.

Answer 23

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

Question 24

What do deep foundations include?

Answer 24

Piles and drilled piers that are like slender columns buried in the ground

Question 25

Why are deep foundations prevented from buckling

Answer 25

confied by soil

Question 26

What do piles and drilled piers transfer the load to

Answer 26

either bedrock or soil of high bearing capacity while passing thorugh unsuitable soil conditions

Question 27

Typical pile materials?

Answer 27

steel, timber, concrete elements driven into gorund, except concrete piles which can also be site-cast in pre-drilled holes

Question 28

what are site-cast piles called

Answer 28

drilled piers or caissons

Question 29

What are pile material selection criteria

Answer 29

availability, cost, below-grade environment, type of soil, load magnitude, equipment required to drive the piles

Question 30

What pile material is typically used under heavier loads

Answer 30

steel and concrete

Question 31

Explain wood piles

Answer 31

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

Question 32

Explain friction pile

Answer 32

resists most of the loads by skin friction and by the end bearing

Question 33

Explain end-bearing pile

Answer 33

ressits most of the loads by end bearing and some by skin friction

Question 34

What are caissons

Answer 34

concrete cylinders poured into drilled holes (cast-in-place), reach through weaker soil to bear on competent soil beneat

Question 35

explain end bearing caisson

Answer 35

belled when additional bearing capacity is required

Question 36

Explain socketed caisson

Answer 36

drilled into a hard stratum and transfers its load primarily bu friction between the soil or rock and the side of the caisson

Question 37

Explain caisson construction

Answer 37

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

Question 38

What is the caisson drilled equipped with for cutting through hard material

Answer 38

carbide toothed cutting haed

Question 39

How is caisson bell formed

Answer 39

belling bucket with retractable cutters

Question 40

Explain precast concrete piles

Answer 40

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

Question 41

Explain steel piles

Answer 41

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

Question 42

Explain screw/helical pile

Answer 42

rows of pile transfer loads from the shallow footing, located in a relatively weak soil into the stronger underlying stratum

Question 43

Explain group pile

Answer 43

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

Question 44

Explain pressure bulb

Answer 44

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