Foundations

Question 1

Loads supported by foundations

Answer 1

Dead; live; wind; horizontal pressure from Earth; horizontal thrust from arches, rigid frames, domes, vaults; uplift from underground water; earthquakes.

Question 2

2 types of foundation settlement

Answer 2

Uniform and differential. Main objective of foundation is to minimize differential settlement. Most failures are due to it. Gross failure of a foundation is rare.

Question 3

Types of soils

Answer 3

Rock
Continuous mass of solid mineral; strongest and most stable; only removed by drilling and blasting.

Soil
General term for Earth material that is particulate

Boulder
Particles too large to lift with one hand

Cobble
Particles that take whole hand to lift

Gravel
Particles lifted easily with thumb and forefinger

Sand
Particles can be seen but are too small to be picked up individually

Silt
Equidimensional; low surface area to volume ratio; smaller than sand; similar to course grained sand.

Clay
Plate shaped; smaller than silt; very high surface area to volume ratio; cohesive soil.

Layers = strata

Question 4

Least stable type of soil

Answer 4

Clay because it is dimensionally unstable under changing subsurface moisture conditions

Question 5

Soil with best drainage

Answer 5

Gravel and coarse sands. Worst: fine silt and clay (don’t put drainage pipe in these soils)

Question 6

2 Ways to test soil and what do lab tests do?

Answer 6

Test pits or test borings. Pits better when foundation is no deeper than 8’ (reach of small excavator). Lab tests pass soil through set of sieves with graduated mean sizes to determine particles size. They test liquid limit, plastic limit and water content. Soils report shows results and recommendations.

Question 7

Water table

Answer 7

Elevation at which pressure of ground water is atmospheric

Question 8

2 types of excavation slope support

Answer 8

Benched or sheeting. Benched used when you have room; must be shallow angle with particulate soils.

Question 9

Sheeting

Answer 9

Soldier beams and lagging; sheet piling; slurry walls

Question 10

Soldier beams and lagging

Answer 10

Wide flange beams driven vertically into Earth at close intervals before digging begins. As earth is removed, heavy wood planks are placed against flanges of beams.

Question 11

Sheet piling

Answer 11

Vertical planks of wood, steel or precast concrete driven into earth to form solid wall before excavation occurs. Slurry walls are more complicated and expensive and used if they become part of permanent foundation.

Question 12

Types of excavation bracing

Answer 12

Crosslot bracing, raking and tiebacks. Tiebacks don’t get in the way of excavation.

Question 13

Ways of dewatering

Answer 13

Pump water from sumps with well points or build watertight barrier. We’ll points lower water table below level of excavation but may disturb neighboring lots.

Question 14

What is the difference between shallow and deep foundations?

Answer 14

Shallow foundations transfer the load to the earth at the base of a column or wall of the substructure. Deep foundations (piles or caissons) penetrate through upper layers of bad soil to transfer the soil to competent bearing soil or rock.

Question 15

Primary factors that affect the choice of foundation type

Answer 15

Subsurface soil and groundwater conditions; and structural requirements such as foundation loads, building configuration and depth.

Question 16

3 Ways to build a foundation on a slope

Answer 16

1. Stepped footing
2. Tie beams between columns
3. Grade beam

Question 17

2 types of foundations used when the bearing capacity of the soil is low compared to the weight of the building

Answer 17

1. Mat foundation (footings merged together)
2. Floating foundation (The building weights about the same as the soil that was removed (1 story of soil removed is equal to 5-8 stories of superstructure))

Question 18

4 types of deep foundations

Answer 18

1. Caissons (straight or belled; poured; belled must be excavated in cohesive soil)
2. Socketed caisson (into bedrock; poured)
3. End bearing pile (rests on bedrock; driven)
4. Friction pile (gets capacity from friction between the soil and the sides of the pile; driven)

Piles are driven close together and concrete grade beams at the top tie them together to support columns and loadbearing walls.

Question 19

Seismic base isolation

Answer 19

Base isolators allow lateral movement caused by earthquakes. Layers of rubber and steel deform in shear to become a parallelogram. A lead core keeps the sandwiched materials together.

Question 20

3 failures mechanisms in retaining walls

Answer 20

1. Overturning
2. Sliding
3. Undermining

Question 21

3 things that will drastically affect foundation cost

Answer 21

1. Building below the water table
2. Building close to an existing structure
3. Increasing the wall or column load from a building beyond what can be supported by a shallow foundation

Question 22

Kelly ball test

Answer 22

Tests workability of freshly poured concrete by dropping a ball and measuring the depression and comparing to slump tests.

Question 23

Electrical impedance test

Answer 23

Tests moisture level of a slab by running electricity through it. Moist slabs conduct more electricity.

Question 24

Compressive Strength Cylinder test

Answer 24

Lab: tests concrete’s compressive strength at prescribed intervals (7, 14, 21, 28 days). The cylinders are created in the field during concrete placement and are chosen at random points during the concrete pouring process.

Question 25

During which period does concrete gain the most compressive strength?

Answer 25

First 0-3 days. Must be protected from freezing and evaporation. It continues to gain strength after 28 days, but for purposes of testing and design strengths, compressive strength refers to its strength at 28 days.

Question 26

Batter boards

Answer 26

Temporary supports that hold wires to indicate excavation limits. Set back from edge so as not to disturb operations.

Question 27

Grade stakes

Answer 27

Indicate how much cut our full is required at a specific location to meet finished grade.

Question 28

Transit tool

Answer 28

Surveying tool used to determine elevations points on site. Mounted on tripod. Either a scope or laser.

Question 29

Proctor compaction test

Answer 29

Lab geotechnical testing method used to determine the soil compaction properties, specifically, to determine the optimal water content at which soil can reach its maximum dry density.

Question 30

Slump test

Answer 30

Field test used to determine the consistency as well as workability of fresh concrete. Slumps of 4-5 inches indicate ideal consistency.

Question 31

Schmidt Rebound Hammer Test

Answer 31

Field test (non-destructive) measures the elastic properties or strength of existing concrete or rock, mainly surface hardness and penetration resistance. Compare results to graph to determine compressive strength of concrete.

Question 32

Air Content Testing

Answer 32

Field air content testing of concrete is conducted to determine if delivered concrete is within the air content specifications established by the engineer.

Question 33

Unit Weight Testing

Answer 33

Field: Fresh concrete is placed inside a container of known volume and weighed to provide the unit weight or density of the concrete. This is typically reported in pounds per cubic foot in the United States.

Question 34

Destructive Concrete Testing

Answer 34

The existing concrete is cored, and a cylinder removed, which is then delivered to a laboratory for testing using the same method to that applied to cylinders from recently poured concrete. More accurate than Schmidt Rebound Hammer test.

Question 35

Tensile or Flexural Testing of Concrete

Answer 35

Required for concrete installed in aircraft runway and highway applications. The testing specimens are created in the shape of a rectangular beam, which when cured is subject to a load on both ends until it snaps in the center, providing engineers with a measure of the concrete’s ability to withstand bending forces.