tability of a Raft During Construction and Use
Unlike individual load-bearing footings, which support each building element, raft foundations (also known as mat or cellular foundations) are poured as a continuous slab that spans the footprint of a construction. The large surface area distributes loads evenly, and helps to reduce the overall amount of pressure on underlying soil. This makes them a great choice for structures that have heavy loads or are situated in areas with variable soil conditions.
Raft foundations are also easier to construct than isolated footings, which can require complex shoring to prevent them from collapsing. As a result, they can help to save on time and money. Similarly, they are often cheaper to excavate than other foundation options. Furthermore, they often require less concrete than other foundation types.
In addition, a Floßbau provides a solid base for walls and other structural elements, making them an ideal option for extensions and conservatories. However, if you’re thinking of adding an extension or conservatory to your property, it’s important to understand how much they will cost. There are a number of factors that can affect the price of a raft foundation, including how big your extension or conservatory is and the site’s accessibility.
How to Secure the Stability of a Raft During Construction and Use
While raft foundations are typically more expensive than other shallow foundations, they can be very cost-effective for larger structures and sites with difficult access. In addition, they can help to reduce the overall construction programme, as they allow for quicker installation and offer a more efficient solution to traditional shallow foundations.
A raft can be combined with piling to create a hybrid solution that is both cost-effective and durable. The raft and piles work together to distribute loads, reduce soil pressure and control differential settlement. During the design process, the optimum number of piles and their diameter, reinforcement and length are determined to ensure stability.
Unlike traditional footings, which are connected to the ground via steel bars, a raft is supported by a structural fill ‘cushion’ which redistributes the load to the surrounding piles. In turn, this can minimise the influence of ineffective piles on the overall performance of the foundation.
It is important that the Engineer checks that the raft foundation will remain stable during construction and use. This is achieved through the raft and pile design calculation (BS 8110:Part1:1997). The calculation will determine the required width of raft, perpendicular to the edge load, in order to distribute the load without exceeding the allowed bearing pressure and check that the raft is stable against shear and bending moments. If these requirements are not met the design should be rejected by the Engineer.
