What would happen if we neglected soil information that we built our foundations on? (5)
- 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
Frost Heave
Lifting of the ground due to expansion caused by water freezing in the soil. To avoid, foundations should be at least 600mm below ground
Swelling and shrinkage
Clay soils are subject to shrinking in summer and swelling in winter. Foundations should be 900mm below ground to account for this.
What is the difference between shallow and deep foundations?
Shallow is between 0.2 and 2m, and deep is more than 2m
Four types of shallow foundations
Strip foundations, ground beams, pad, raft
Strip foundations
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.
How is shear failure avoided in strip foundations?
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.
Key facts Strip foundations
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
Advantages of strip foundations (3)
- Economical for the depth with limited excavation. 2. Simple, 3. Can support a two storey domestic structure
Disadvantages of strip foundations (4)
- Wider strips required to support greater loads on poor ground, 2. Trench may require side supporting, 3. Requires backfilling, 4. Limited loading capacity
What is the difference between DPM and DPC
DPM is horizontal, underneath the floor slab to prevent rising moisture. DPC are installed in the walls to prevent rising damp.
Stepped Strip Foundations
These are strip foundations but for sloping ground. Add steps into the foundation to account for the slope to distribute the load down
Trench fill foundations
Where loading or ground conditions permit narrow trenches, trench fill is cost effective. This is just filling the trench with concrete basically
Deep strip, or trench fill foundations?
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
Trench fill foundations advantages (4)
- Can cope with groundwater, displaced by poured concrete. 2. Fast method of construction, 3. No trench support required. 4. No backfilling required
Trench fill foundations disadvantages (2)
- Higher cost associated with larger volumes of concrete. 2. Lower sustainability in terms of higher cement content
Pad foundations
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
Factors to consider with pad foundations (4)
- 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
Pad foundations advantages (3)
- 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
Pad foundations disadvantages (2)
- Can use lots of concrete which is bad for cost/sustainability, 2. Large pads pay be required to resist uplifting forces from wind
Raft foundation
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
When are raft foundations used? (3)
- 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
What are the steps to construct a raft foundation?
- 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
Raft foundation advantages (2)
- Economical solution with minimal excavation depth. 2. Floor and foundations are cast as one, saving time.
