Car park basements can be built using two different methods, the top-down process or the bottom-up method. Both have their advantages, but the bottom-up method tends to be the most cost-effective.
Definition
The bottom-up method refers to the order in which the subfloors are constructed. This method makes the most sense on the initial examination. The contractors in charge of the project dig down to the deepest level of the car park basement, and construct that bottom level first, along with the primary foundation. The other layers are built up, one by one, as the contractors lay the necessary supports for the next floor while they work. In the end, the ground floor is reached and construction on the building itself is begun.
Material
Concrete is the material of choice for bottom-up construction of car park basements. Of course, not all concrete is the same, and this concrete needs to be especially sturdy to hold the weight of the building and the basements themselves. Durable, small-aggregate concrete mixed with chemical strengtheners and bonded fibers is often used. However, a number of tricks can lend extra support. Steel rebar and larger metal columns can provide extra support for larger projects that need greater stability.
Benefits
The bottom-up method tends to be the most cost-effective for the contractors working on the project. It is also the easiest to plan for, since the stages of construction progress from the foundation upward. Dirt can be quickly hauled away from the foundation site before work begins, and mistakes can be spotted and rectified before any floors are built on faulty supports, preventing the possibility of extra damage.
Limitations
While the bottom-up method may be inexpensive and quick, it is not always practical. In downtown areas, building construction often faces very limited space options and a strict deadline. In some cases it is better for the overall plan to start construction of the building itself and build it up floor by floor while excavating the basement down level by level instead of using the bottom-up approach.