Foundation Design

Question 1

What would happen if we neglected soil information that we built our foundations on? (5)

Answer 1

1. weight overloading from the structure to the soil, 2. Sub-stratum could be uneven and lead to unequal settlement, 3. Building surface might not be level, 4. As the structure transmits the load to the ground, stability compromised, 5. Potential lateral movements around the building

Question 2

Frost Heave

Answer 2

Lifting of the ground due to expansion caused by water freezing in the soil. To avoid, foundations should be at least 600mm below ground

Question 3

Swelling and shrinkage

Answer 3

Clay soils are subject to shrinking in summer and swelling in winter. Foundations should be 900mm below ground to account for this.

Question 4

What is the difference between shallow and deep foundations?

Answer 4

Shallow is between 0.2 and 2m, and deep is more than 2m

Question 5

Four types of shallow foundations

Answer 5

Strip foundations, ground beams, pad, raft

Question 6

Strip foundations

Answer 6

II shaped, loads downwards are distributed at a 45 degree angle, creating a defined area of loading. If foundation is too wide, shear failure may result.

Question 7

How is shear failure avoided in strip foundations?

Answer 7

The foundation is designed so that it does not extend outside the zone of downward loading. As the width is increased, the depth is also increased to avoid shear failure.

Question 8

Key facts Strip foundations

Answer 8

Most common type, weaker the soil/heavier the building, wider the strip. Designed to spread load from uniformally loaded bricks. Mainly used for 2-3 storeys, not suited for seasonal movement soil or poor GC

Question 9

Advantages of strip foundations (3)

Answer 9

1. Economical for the depth with limited excavation. 2. Simple, 3. Can support a two storey domestic structure

Question 10

Disadvantages of strip foundations (4)

Answer 10

1. Wider strips required to support greater loads on poor ground, 2. Trench may require side supporting, 3. Requires backfilling, 4. Limited loading capacity

Question 11

What is the difference between DPM and DPC

Answer 11

DPM is horizontal, underneath the floor slab to prevent rising moisture. DPC are installed in the walls to prevent rising damp.

Question 12

Stepped Strip Foundations

Answer 12

These are strip foundations but for sloping ground. Add steps into the foundation to account for the slope to distribute the load down

Question 13

Trench fill foundations

Answer 13

Where loading or ground conditions permit narrow trenches, trench fill is cost effective. This is just filling the trench with concrete basically

Question 14

Deep strip, or trench fill foundations?

Answer 14

Strip foundations require the bricks to go further down, so space for them to lay will also need to be dug/supported. Trench fill foundations means you dont need to make this space which is good

Question 15

Trench fill foundations advantages (4)

Answer 15

1. Can cope with groundwater, displaced by poured concrete. 2. Fast method of construction, 3. No trench support required. 4. No backfilling required

Question 16

Trench fill foundations disadvantages (2)

Answer 16

1. Higher cost associated with larger volumes of concrete. 2. Lower sustainability in terms of higher cement content

Question 17

Pad foundations

Answer 17

Spread concentrated loads (like steel beams). A resting point for ground beams. Foundation to brick piers, reinforced concrete columns, steel columns, ground piers. Area of pad is determined by load/soil strength

Question 18

Factors to consider with pad foundations (4)

Answer 18

1. Thickness, 2.Depth (need to reach adequate bearing, avoid moisture movements, avoid frost heave). 3. Position of loading (needs to be centrally in both directions 4. steel reinforcement

Question 19

Pad foundations advantages (3)

Answer 19

1. Economical foundations for steel columns, 2. Can be reinforced to withstand greater axial loads, 3. Can be linked with a continuous beam that provides external wall foundations

Question 20

Pad foundations disadvantages (2)

Answer 20

1. Can use lots of concrete which is bad for cost/sustainability, 2. Large pads pay be required to resist uplifting forces from wind

Question 21

Raft foundation

Answer 21

Constructed in the form of a continuous slab extending beneath the whole building. Loadings from building are spread over a wide area. Think multiple pads, with a gigantic plate underneath all

Question 22

When are raft foundations used? (3)

Answer 22

1. Poor soils, building load needs to be spread more, 2. Where soil is variable, 3. Where total area of pad foundation is greater than 75% of total building area

Question 23

What are the steps to construct a raft foundation?

Answer 23

1. Identify the desired depth, 2. Excavate soil, 3. Pour 50mm weak contract as compaction, 4. Erect formwork, 5. Place reinforcement on spacers, 6. Pour concrete, 7. curing

Question 24

Raft foundation advantages (2)

Answer 24

1. Economical solution with minimal excavation depth. 2. Floor and foundations are cast as one, saving time.

Question 25

Raft foundation disadvantages (2)

Answer 25

1.Requires formwork and steel reinforcement design, 2.edge of slab has to be protected from the elements

Question 26

Deep foundations

Answer 26

Generally referred to as deeper than 3m below ground level - unlikely to be affected by changing conditions at the surface of the ground

Question 27

Piles

Answer 27

Columns within the ground, transfer loads from superstructure to the ground.

Question 28

Displacement Piles

Answer 28

Driven into ground displacing the surrounding earth. Driven shell (casing) > place the reinforcement > pour concrete

Question 29

Replacement Piles

Answer 29

AKA Bored piles. Removing the volume of the soil. The holes are bored by a hollow weighted grab or by rotary borer or auger. Ground is removed by boring and replaced by pouring concrete into void and loweing reinforcmeent cage into the concrete

Question 30

Pile foundations advantages (3)

Answer 30

1. Economical solution to reach greater bearing capacity soils. 2. Can be pre-fabricated off-site, maintaining quality. 3. Can cope with variable ground conditions

Question 31

Pile foundations disadvantages (1)

Answer 31

Noise during installation

Question 32

Basement foundation disadvantages (4)

Answer 32

1. Problems with excavation, 2. Construction problems, 3. Achieving watertightness, 4. Leading to cost implications and risk

Question 33

When to use a basement? (4)

Answer 33

1. Needing additional accommodation, 2. to meet planning reqs on building height, 3. to reach good bearing soil, 4. To make use of overburden removal

Question 34

4 Grades of basement

Answer 34

1. Basic utility (car parks etc) 2. Better utility (Retail storage etc.) 3. Habitable (offices, residential) 4. Special (Archive storage etc)

Question 35

Basement walls reqs (4)

Answer 35

1. Need durability (below ground is a hostile environment) 2. Cope with axial and lateral loads 3. Exclude moisture, 4. Design related to grade

Question 36

Tanked basement walls

Answer 36

Tanking involved applying a membrane to the inside or outside of the structure. This makes the basement watertight. Defined as ‘Type A’ waterproofing

Question 37

4 types of waterproofing basement strategies

Answer 37

1. Lowering the water table 2. Drained cavity 3. Integral waterproofing 4. tanking

Question 38

Drained cavity

Answer 38

Water is collected through the walls in a cavity beneath the basement and pumped out. Need to be wary of possible silting in the pumps

Question 39

Integral waterproofing

Answer 39

The concrete keeps out the water. Achieved by optimising the concrete mix design with traditional additions of mineral and chemical admixtures

Question 40

Basement construction process (5)

Answer 40

1. Excavate 2. Raft or slab 3. Structural walls 4. Floor and tanking 5. backfilling