foundation system of burj al arab

Construction Sequence of Raft or Mat Foundation

Site selection, geotechnical investigation and structural analysis and proportioning of raft or mat foundation are done in design phase. Construction phase includes

• Site preparation
• Clearing construction site
• Excavation
• Fill placement 
• Providing stiffening beams 
• Providing vapor barriers
• Placement of reinforcing steel
• Pouring concrete
• Site finishing
• Follow up during or after construction 

Site preparation:

The construction site should be maintained in such way that minimum moisture variation is occurred by providing sufficient drainage to improve runoff of rainwater which facilitates to reduce subsequent differential settlement. Well drained site provides a dry working conditions producing firm surface.

A layer of granular fill of up to 1 foot thick is very effective to improve trafficability providing a temporary road for easy and quick transportation of materials and mechanized equipment into or out of construction site. The advantage of providing fill is manifolds; with improved trafficability, it helps

• To have improved grade of better drainage
• To provide surcharge loading on foundation soil beneath it
• To arrest swell pressure of soil generated due to wetting for long period.

It is recommended to complete site preparation before wet season. Soil preparation is a continuous process until site is protected by foundation to reduce ill effect of wetting and drying cycle due to seasonal variation. Sometimes lime with or without cement mixed with surface soil improve trafficability of problematic soil.

Clearing construction site:

During clearing site, removal of vegetation and trees is often leave depressions. Such depressions, trenches and holes are often filled with soil compacted at natural moisture content to reach density of in-situ natural soil and graded to initial level of ground.

Cutting of soil, if required, is kept minimum as it reduces overburden pressure on soil underneath over the cut area, cutting also reduces pore water pressure of the soil. When soil underneath is of cohesive type (i.e. less permeable), it takes considerable time to reach an equilibrium condition consistent with surrounding soil.

What is Development Length for Steel in Reinforced Concrete?

Development length can be defined as a length of reinforcing bars into embedment of concrete required to develop full tensile strength of them. 

This length depends on either splitting or pull-out failure. According to figure 01 the moment is obviously maximum at point a (not considered weight of beam); maximum moment means maximum stress on steel. As the beam is simply supported, end restraint is zero which means zero moment at support. Let’s try to analyze this problem. 

Development length

If the stress in bar at a is fs and area of bar is Ab , the tension force in bars is Abfs; this means, this force has to be transferred to concrete from bar. To transfer this force by bond force, l length of bar is required.
Shear force and bending moment diagram
The strength of bar is Abfy; to develop this strength, the length of transferring force, (here l) must be equal or more than development length. If development length is ld, when lld , bond failure occur after mobilizing full strength of bar (Abfy) i.e. premature failure is avoided.

Thus, the beam may fail due to other type of failure like shear or bending, but bond failure is restricted.The beam is safe even when local slip is occurred near cracks across small regions on the beams.

Notice that, the main safety requirement for failure due to bond is only length of reinforcing bar. This length from a point of given stress in steel (fs or anything ≤ fy) to nearest free end should be equal or more than development length of it.

 Bond force per unit bar length,
 Bond force

Where jd=Lever arm between resultants of compression and tension force.

v= Shear force

If proper development length is provided, magnitude of nominal flexure bond force, as in equation (1), is not very important as integrity of beam is ensured; this is valid even under local minor bond failures.

When actual length provided is not adequate i.e. full development length, however, cannot be provided,special anchorage like by hooks, should be provided.