Introduction to shallow foundations- Eurocode 7

Question 1

What are the 4 design considerations for a foundation?

Answer 1

• Spatial variability of soil type

Spatial variability of soil properties

• Sensitivity of structure to the movement
• Limits to the movement (e.g. machinery hazardous work)

Question 2

What does a foundation need to be?

Answer 2

Safe and Economical

Question 3

List 4 types of loads that foundations experience.

Answer 3

• Self-weight of the structure
• Environmental loads, e.g. snow, wind, earthquake
• Service loads, e.g. fixtures, fittings, people
• Loads due to adjacent structures

Question 4

Complete the sentence:

The distribution of loads can change……..

Answer 4

During the life of the structure

Question 5

What should be considered when designing the depth of a shallow foundation (foundation level)?

Answer 5

1. The foundation base should be located at a depth such that frost action (~0.5m in UK) and seasonal swelling and shrinkage are minimised
2. The foundation base should be located below made ground (~0.5 ÷ 1.5m) and safe from groundwater level oscillations
3. Different parts of the structures should ideally be built onto the same foundation level
4. The construction/excavation on existing structures should be possible.

Question 6

What are the considerations when designing the width of a shallow foundation?

Answer 6

• Exceeds width of the element it supports - Ideally load distributed through the foundation at about 45˚ to give optimum foundation
• If L is the width of the structural element and H is the depth of the base of the structural element, the optimum foundation width is: B = L + (D - H) x 2 3
• Foundations wider than optimum have to be reinforced
• Foundations narrower than optimum may be uneconomic

Question 7

Following Eurocode 7, what are the serviceability limit states that must considered? (2)

Answer 7

• Excessive settlement (or heaving) leading to excessive angular distortion
• Vibrations resulting in unacceptable effects such as settlement and soil liquefaction.

Question 8

Following Eurocode 7, what are the ultimate limit states that must be considered? (3)

Answer 8

• Bearing resistance failure caused by shear failure of the surrounding soil
• Failure by sliding under inclined loading
• Loss of overall stability due to the development of a deep slip surface within the surrounding soil

Question 9

What is a bearing capacity analysis used for?

Answer 9

Initial sizing of the foundation

Question 10

How do we calculate the design bearing resistance?

Answer 10

The design bearing resistance R_d is calculated using the factored shear strength parameters.

Question 11

How do we calculate the vertical design action

Answer 11

The vertical design action Fd is calculated from factored loads without subtracting the weight of the overburdened soil.

Question 12

What has to be checked in case of clays?

Answer 12

Both undrained and drained conditions have to be checked in the case of clays, although the undrained condition is usually more critical.

Question 13

When are long term analyses considered (drained conditions)?

Answer 13

In Cases of granular soil deposits.

Question 14

According to Eurocode 7, What is the 5 step procedure when designing a foundation?

Answer 14

1. Calculate the loads to be applied through the foundation throughout the life of the structure
2. Determine the zone of influence – characterisation of soil properties
3. Estimate initial foundation size and depth based on prescriptive method and the minimum depth
4. Calculate the bearing pressure - Check the bearing resistance
5. Adjust foundation size and depth until bearing resistance meets criteria

Question 15

Determine the size of a square pad foundation to support a central load on stiff clay (C_u = 95 kPa). Assume undrained conditions. The permeant load G_k= 1450 kN and the variable load Q_k=350 kN. Use Design Approach 1 of EU code.

Answer 15

See notes