basement extention extra liveing space

[frenaros]

Hi I have recently been appointed to a refurbishment project in London, London clay, the employer now as been asked to construct a new basement / living accom. Under the existing terraced house, I have been asked to give my ideas but I told then I have little to no basement experience. Of what I can see one would need to underpin 70cm up to 1.5meters under the existing footings as the existing footings seem to be on different levels, so I am not sure if one could get away with mass concrete or have to use reinforced? Also could one attach the floor slab to the underpin sections and how? As I am new to this type of work are there any sites / books /drawings giving typical derails of this type of work?

 

[ishvaaag]

The classical approach would have been to draw along the perimeter a number of sections in plan to be partially unsupported, then supported in some specific order to the required depth till having the whole perimeter supported. Then proceed with the same care for inner supports if any, what may require provisional foundations-shorings. Once everything is supported to the required depth enough excavation would be made as to permit the completeness of a inner layer of basement wall, and then the inner supports be brought to its final shape, keeping everything shored whilst in the process. If you have not external access to the walls you will need to distroy the ground level slab and proceed in similar way. Monitorize movements and any crack appearing, specially if neighbor property is being affected.

Now the trend is to use helical piles to shore the existing foundation. Even so be sure to concrete enough part of the new walls before having the whole of the loads just on a number of freestanding shafts of the helical piles.

 

[PEinc]

Rarely are concrete underpinning piers reinforced. The underpinning piers are usually plain concrete, about 3' x 4' in plan area. The vertical compressive load per square inch on concrete underpinning piers is usually very low due to the large area of the piers.

Helical piers may vertically support the existing foundation, but you would still need to laterally support the earth load. Therefore, skip the helical piers and just install, and dry pack, concrete underpinning piers around the area to be deepened. The underpinning piers and the building wall and original foundation need to act as a gravity wall. Check for overturning and sliding. If the safety factors are too low, the underpinning and/or the original foundation wall may need to be laterally supported.

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