Foundations

Question 1

Outline some different types of foundation. When would each be used?

Answer 1

Strip Foundation – Shallow foundations used typically for housebuilding. Strip foundations are used to support a line of loads, either due to a load-bearing wall, or a line of columns.

Raft foundations – Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure.

Pad foundations – Pad foundations are used to support an individual point load such as that due to a structural column.

Displacement Piled foundation – A pile shaft is driven or jacked into the ground resulting in the soil to be displaced radically. Piled foundations transfers the load to solid ground a certain depths depending upon the type of building.

Non displacement piled foundation - Soil is removed and the resulting hole filled in with concrete or a pre cast concrete pile is dropped into the hole and grouted in.

Question 2

What is an underpinning sequence and why is it necessary? Describe the process of underpinning.

Answer 2

Underpinning involves digging under the present foundation to a depth where firm strata exists and replacing the excavated material with mass concrete.
This cannot be done all in one go or it would undermine the wall therefore it has to be undertaken in a sequence of bays.

Bays, generally 1.0m – 1.2m in length are marked out and given a latter between A-D. The bays are then excavated and filled with mass concrete in sequence i.e. all of the A’s, then when they have gained sufficient strength all of the C’s and so on.

Question 3

With regards to foundations, walls and roof: What are the differences in methods of construction between an early 20th century house and a modern house?

Answer 3

Foundations
A house built in 1900 would most likely be built on stepped brick foundations.
A house built now would most likely be built off concrete strip foundations.

Walls
A house built in 1900 would most likely be solid wall construction or have a small non insulated cavity.
A house built now would most likely be cavity wall construction with a minimum 100mm cavity filled with insulation. It could also be timber framed.
Roof

A house built in 1900 would most likely be rafter and purlin or king or queen post with a covering of stone, slate or clay tiles depending on the locality.
A house built now would most probably have a trussed rafter roof fabricated off site. It could be covered in a variety of finishes.