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Foundations > Final Assessment > Flashcards

Final Assessment Flashcards

0
Q

Construction process of deep pile foundations:

A

1st. Geotechnical investigation
2nd. Dewatering
3rd. Decision on type of pile
4th. For in-situ: driven or bored.

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1
Q

Characteristics of PILE FOUNDATIONS

A
  • relative small diameter, 300-350mm.
  • piles are usually driven in groups
  • vertical, horizontal or combination of both loads
  • they increase stability of structure
  • for each pile, usual load capacity is 1500-1700 kN.
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2
Q

Piles in groups

A
  • They distribute load uniformly
  • They make it easier for designer to find center of load beneath foundation, otherwise if unplanned eccentric load occurs, then connection between pile and column would fail.
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3
Q

Pile caps types

A

3 to 6-pile group.

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4
Q

Slopes

A
  • definition of slope: an exposed ground surface that stands at an angle to the horizontal.
  • slopes are required in the construction of highways, dams, bridges.
  • slopes are generally less expensive than constructing walls.
  • attention has to be paid to geology, surface drainage, ground water condition and the shear strength of soil.
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5
Q

Two types of slopes:

A
  • Natural slopes created by landslide, erosion, sedimentation.
  • Artificial slopes created to build dams, highways, canals.
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6
Q

Slope failure triggering mechanism:

A
  • intense rainfall
  • water-level change
  • seepage
  • volcanic eruption
  • earthquake
  • human activity (construction)
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7
Q

Causes of slope failure:

A

Erosion: the wind and flowing water causes erosion of top surface.

Seepage: seepage forces in the sloping direction add to gravity force and make the slope susceptible to instability.

External loading: additional loads placed on top of slope.

Construction activity: at the bottom of the slope.

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8
Q

Types of slope failure

A
  1. Face (slope) failure
  2. Toe failure
  3. Base failure
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9
Q

Types of slope (dependent on length)

A
  1. Infinite slope: they have dimensions that extend over great distances and soil mass inclined throughout.
  2. Finite slope: slopes with limited base, toe and height.
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10
Q

Slope factor of safety

A

It is defined as the ratio of average shear strength of a soil to the average shear stress developed along the potential failure surface.

Minimum value should be greater than 1.

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11
Q

Coefficient of friction is controlled by:

A
  • mineralogy
  • grain shape (angular v/s rounded)
  • packing arrangement (loose v/s tight packing)
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12
Q

Types of piles

A

> by construction material: concrete, steel, masonry, timber.
by cross-section: circular, square, hexagonal.
by mode of load transfer: bearing pile, friction pile.
by method of forming: precast, prestressed, cast-in-situ.
by method of installation: driven, bored.

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13
Q

Soils can be divided into 2 main types:

A
  • Clay/silt: which have capacity to absorb water, thus providing more settlement of the structure.
  • Sand/gravel: have a low compaction capability and a high drainage capacity.
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14
Q

Settlement refers to…

A

the vertical downward displacement at the base of a foundation or other structure due to ground movement.

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15
Q

The two types of settlement are:

A
  • uniform settlement, which occurs on fine soil and it is not critical.
  • differential settlement, which occurs only in parts of the soil under the surface. These can be avoided with a geotechnical report in order to treat weak areas.
16
Q

Compaction occurs when…

A

soil particles are forced into a closer state of packing with reduction in volume and expulsion of air.

Vibration due to traffic movement, heavy machinery and certain construction operations such as pile driving can also cause compaction settlement.

17
Q

Consolidation is…

A

a long-term process. In saturated soils, the effect of loading is to squeeze out pore water, this process is called consolidation.
The most susceptible soils are clay and silt.

18
Q

Moisture moment

A

Some clay soils show a marked increase or decrease in volume as the water content is respectively increased or decreased, these are alternatively called shrinkage or expansive clays.

19
Q

Effect of vegetation

A

The radial extent of some tree root system is greater than the height of the tree, they may also reach depths of several meters, therefore swelling in soil will occur.

New planting at a distance away from the building of at least 1.5 times the mature height of the tree.

20
Q

Effect of ground water lowering

A

As water is pumped from an excavation, the water table in the surrounding ground may be lowered. Settlement can result from this change in hydrostatic conditions due to processes:

a. Firstly, decrease in water content will result in a decrease in volume.
b. Secondly, a reduction in hydrostatic pore pressure results in an increase in the effective (actual current) over burden stress.

21
Q

Effect of seepage and scouring

A

In certain sandy soils such as fine dry sands, the movement of water is high.

Scouring is the removal of material by surface water and streams. This can also occur where sewers or water mains have been fractured.

22
Q

The process of consolidation

A

When a saturated mass of soil is loaded by a foundation, an immediate increase in pore pressure occurs and a hydraulic gradient is set up so that seepage flow takes place into surrounding soil.
This excess pore pressure dissipates as water drains from the soil (very quickly in coarse soil; slowly in fine soil).
As the water leaves the soil, a change in volume occurs, the rate gradually diminishing until it reaches the steady point. This process is called consolidation.

23
Q

Basic assumptions of Terzaghi’s theory:

A
  1. Soil is fully saturated and homogenous.
  2. Both water and soil particles are incompressible.
  3. The change in volume is one-dimensional in the direction of the applied stress.
  4. The coefficient of permeability and volume compressibility remains constant.
  5. The change in volume corresponds to the change in void ratio.
24
Q

Open layer

A

The layer that drains water freely from its upper and lower surface is called open layer. The length of a drain path (d) is equal to half the thickness (H).

25
Q

Half-closed layer

A

If the drainage takes place at one boundary, then the layer is called half-closed and the length of the drainage path is equal to the thickness (H).

26
Q

Scope and purpose of site investigation and in situ testing:

A
  • to assess general suitability of site and environment
  • to prepare adequate and economic design, including temporary work design
  • to plan best method of construction
  • to determine the changes that may arise in the ground and the environment.
27
Q

Steps for site survey:

A
  • desktop study
  • preliminary site survey
  • detailed site exploration and sample collection
  • laboratory testing of samples
  • in situ testing
  • reporting
28
Q

Two types of sites:

A
  • compact site: enclosed urban, mountain region, off-shore.

- extended site: roads, railways, pipelines.

29
Q

Four factors to be considered when choosing method of soil/site exploration:

A
  1. Geological nature of site
  2. Topographical nature of site
  3. Type of information required
  4. Cost and time
30
Q

Four types of sampling:

A

A) trial pits: for cohesive soils (clay and silt) and limit is 2-3 meters.

B) hand auger: for both cohesive and cohesionless soils, and it goes up to 5-6 meters.

C) rotary auger: can also be used for hard soils and goes up to 50 meters.

D) core drilling: is used for rocks and has a 30-100mm diameter core.

31
Q

In situ tests:

A
  • standard penetration test (SPT) is used to estimate relative density and shear strength properties.
  • plate bearing test is used to estimate bearing capacity and soil settlement. Diameter of plate is 300mm-1m. Trial pit is necessary.
  • California bearing ratio: used for roadworks using a dynamic cone penetrometer. For sand in Perth, Perth Sand Penetrometer (PSP) is used.
32
Q

Site investigation report includes:

A
  • introduction
  • site description
  • geology of site
  • soil conditions
  • construction materials
  • comment and recommendations
  • references
  • appendices
33
Q

What are deep foundations?

A

A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground.

Reasons for a deep foundation over a shallow foundation are very large design loads, a poor soil at shallow depth, or site constraints (like property lines).

34
Q

What are pile foundation?

A

Group of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear.

Foundations (2 decks)

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