Construction Design Assignment

Q 2

Write short notes on the following types of foundation commonly used for buildings.

i) Strip foundation

Detailed Drawing of Strip Foundation

Strip Foundation

Strip foundations are used where the soil is of good bearing capacity. The key sizes of a strip

foundation for concrete cavity wall construction and timber frame cavity wall construction are

similar. 

The size and position of the strip is directly related to the overall width of the wall.

The principle design features of a strip foundation are based on the fact that the load is

transmitted at 45 degrees from the base of the wall to the soil.

The depth of a strip foundation must be equal to or greater than the overall width of the wall.

The width of the foundation must be three times the width of the supported wall.

It is vital that the strip foundation is strengthened through the inclusion of steel reinforcement.

A compacted hardcore base of minimum 150mm is installed to form a platform for the subfloor

and the subsequent loads of the dwelling.

The 150mm concrete subfloor is poured on the hardcore in order to provide a strong, smooth

platform for the insulation.  A radon barrier is installed to form a continuous seal on the entire

footprint of the house. 

A Damp Proof Course is installed in order to repel any rising moisture. It is vital that the DPC is

carried up into the blockwork to form a water tight seal over the entire floor area.

The DPC must run through the blockwork at a minimum of 150mm above finished ground level.

100mm of rigid insulation is installed below the finished floor to ensure that there is no heat lost

through the foundation. 75mm concrete screed then provides the finished floor. 

ii) Pad foundation

Pad foundations are used to support individual or multiple columns, spreading the load to the

ground below. They are generally square or rectangular in plan, with the plan area being

determined by the permissible bearing pressure of the soil. The shape in plan will be dictated by

the arrangement of the columns and the load to be transferred into the soil.

The thickness of the slab must be sufficient to ensure distribution of the load. The top of the pad

may be sloping (i.e. the pad is thicker in the centre than it is at the edge). This is an economic

solution, although there may be construction problems involved with casting the slope.

In simple cases the pad may be constructed from mass concrete. In general, some reinforcement

(either welded steel fabric or reinforcing bars, depending on the loads involved) will be required

in both directions. For design purposes, the pad is treated as if it were an inverted cantilever

carrying the soil pressure and supported by the column.

iii) Raft foundation

Detailed Drawing of Raft Foundation

The concrete raft is supported on a compacted hardcore base which extends out past the edge of

the raft.

The edge of the raft is stepped, creating a toe, which allows the outer leaf of the wall to continue

below ground level.

It is vital that the concrete slab is strengthened by the use of steel reinforcement. A radon barrier

is installed to form a continuous seal on the entire footprint of the house. 

A Damp Proof Course is installed in order to repel any rising moisture. It is vital that the DPC is

carried up into the blockwork to form a water tight seal over the entire floor area. The DPC must

run through the blockwork at a minimum of 150mm above finished ground level.

iv) Combined foundation

b) What are the main reasons for setting up temporary timbering support during trench

excavation works?

A trench is defined as an excavation when its length greatly exceeds its depth. Shallow

trenches are usually considered to be less than 6m deep and deep trenches greater than 6m.

Depending on the dimensions of a trench, excavation can either be carried out by hand or

using a mechanical digger. Trenches are commonly required to allow services, pipelines

orfoundations to be laid.

Water ingress into the trench is often a major issue and ground water table locations and

soil strata should be investigated before any extensive excavation takes place. Over short

periods of time for relatively shallow depths most soil types will stand almost vertically without

any problems. However, trenches other than those which are relatively shallow may require

a trench support scheme.

Timber Supports

Historically, trenching involved using timber to support horizontal and vertical soil loads and this

technique is still used today. Timber trenching is generally used for low risk, narrow trenches,

shafts or headings. The timber solutions require good workmanship and are reasonably labour-

intensive, however they are versatile and the equipment required is easy to handle and transport.

Trench boxes

Trench boxes are suitable for low-risk situations in stable, dry ground and can be placed in pre-

excavated trenches or installed using the ‘dig and push’ technique. The system requires at least

two struts at each panel for stability which must be considered when access is required for

construction work or piping.

Trench sheets

Trench sheets are the most adaptable of the systems available, and are most commonly used to

retain poorer soil. They can support deeper trenches with larger surcharges and provide a

continuous support. They require multiple levels of strut support and the slenderness of the

sheets can often limit the depth of the trench as they are installed by light machinery and could

buckle under large vertical loads.