Geotechnical design - A review

Question 1

What are the 6 types of geotechnical uncertainty and what is the cause of them?

Answer 1

1) the risk of encountering an unknown geological condition (technical)
2) the risk of using the wrong geotechnical criteria e.g. designing to the wrong failure mechanism or failure to anticipate the eventual failure mechanism (technical)
3) the risk of bias and/or variation in the design parameters being greater than estimated
4) human error
5) design changes
6) over conservativism

Question 2

What can the ‘factor of safety be defined as’?

Answer 2

Ratio of the resistance of the structure to the applied loads in order to ensure freedom from danger, loss or risks

Question 3

What three things does the magnitude of the factor of safety depend upon?

Answer 3

• severity of failure
• probability of failure
• reliability of the design data

Question 4

What is limit state design?

Answer 4

In limit state design, a single FoS is not stated, but certain parameters or calculated values are factored at certain points in the design calculation.

Question 5

What are the three main purposes for partial factors?

Answer 5

1) to guard against uncertainty in material properties, actions and calculation models
2) to ensure acceptable deformations
3) to achieve compatibility with past practice which has proved to be safe

Question 6

What are partial factors applied to?

Answer 6

Loads, soil properties and resistances

Question 7

Three causes of uncertainty concerning site characterisation?

Answer 7

• natural heterogeneity
• uncertainty due to lack of data
• measurement errors

Question 8

Uncertainty in soil properties can be divided into 2 main groups

Answer 8

• data scatter

- systematic error

Question 9

Data scatter, as a source of uncertainty in soil properties, can be split into:

Answer 9

• real spatial variation and

- random testing errors

Question 10

Systematic error, as a source of uncertainty in soil properties, can be split into:

Answer 10

• statistical error in the mean

- bias in measurement procedures

Question 11

A single factor of safety works best when ______ ________ is the greatest uncertainty and the (actual) FoS varies enormously with uncertainty on ______

Answer 11

Material strength

Load

Question 12

Describe the graphical 3 sigma rule in 4 steps

Answer 12

1) Draw regular trend line
2) Discount anomalies
3) Draw highest and lowest conceivable bounds
4) Draw a line one third of the way from the trend line to the bounds. This represents one SD.

Question 13

What are the four types of settlement?

Answer 13

1) Overall settlement
2) Tilt (local and overall)
3) Angular distortion (or relative rotation)
4) Relative deflection