Underpinning is often considered as a solution for subsidence however the need to stabilise ground could warrant underpinning for other reasons therefore underpinning is not a solution exclusive to subsidence. For example, the use of a building may change which would add load. The original foundation design may not have been designed for this additional load, this could warrant underpinning. Adding an additional storey/s to a building would be a good example of this. Also, the construction of nearby structures may warrant additional support to an existing foundation for which underpinning may be selected. For whatever reason underpinning is used it is important that the correct method is selected. Traditional underpinning was discussed in last week’s article however this week will I discuss needle beam underpinning, cantilever beam underpinning and an underpinning raft.
Needle beam underpinning is used where traditional underpinning is not appropriate due to the existing foundations being too deep, or good bearing strata is so deep that it is uneconomical to dig (depths greater than 1.5m). Concrete mini piles are typically installed in pairs at 1.0m-1.5m intervals and approximately 1.0m-1.5m apart, although this can vary with design. The advantages of this system include suitability for restricted access, the needle beams can be constructed at a higher level if the existing foundations are too deep, it is often faster than traditional underpinning, it is more economical at greater depths, the system has a high load capacity and there is less disruption and spoil produced compared to traditional underpinning.
Many of the advantages of cantilever beam underpinning compared to traditional underpinning are the same as needle beam underpinning described above in terms of speed, more economical at greater depth etc. A further significant advantage of cantilever beam underpinning is where access is particularly restricted as the mini-piles are cast from just one side of a wall/structure.
|Underpinning Raft before concrete is poured. Source: http://www.larsenpiling.com/|
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