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I work in the middle of London, in an area full of large office blocks. Across the road from my office they have started construction of a large building (10 stories plus). Over the last few weeks, diggers have dug a large (and vertical walled) hole. Lorries have taken the resulting dirt and old concrete away, leaving a very neat hole.

In the last day or so, the lorries have returned with new dirt (or the old dirt crushed) and diggers have been putting it back in the hole (and compacting it).

Why put the dirt back? Surely leaving the hole deeper would allow for deeper basement (or digging it shallower would be cheaper)?

I'm not a structural engineer, so this is all lost on me, but I'm fascinated.

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  • $\begingroup$ One reason is that central London is brownfield meaning soil is contaminated by pollution. They may have had to remove that soil due to contaminated soil not meeting the requirements for building use type. Of our is an accommodation block you cannot build on land with historical episodes of heavy metal poisons being leached into the ground. People tend to get sick and die horrible deaths over many years of living above such ground. $\endgroup$
    – Rhodie
    Commented Jun 27, 2019 at 16:50

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This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that.

It also ensures the foundation is uniform under the building to avoid a new tower of Pisa.

Given the age of the city it may have been to scour the land for potential archaeological finds (fervently hoping they would come up empty).

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    $\begingroup$ Why not just sink piles then? $\endgroup$
    – Joe
    Commented Mar 6, 2015 at 16:15
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    $\begingroup$ @Joe The longer the pile, the larger (diameter) it has to be, or the more of them you have to sink. Further, they can be hard to sink through old structures already in place. It's better to remove everything down to the soil that has been in place for centuries, and sink piles from there than it is to assume anything and sink piles from newer fill where the composition and construction is uncertain. $\endgroup$
    – Adam Davis
    Commented Mar 6, 2015 at 16:34
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    $\begingroup$ @Joe You sink piles when you know what you sink them into. This was the case with the church I visit (8.5-metre piles to get to the ground of the pond that was where the church is now), but it's not the case in the centre of London I suppose :-) $\endgroup$
    – yo'
    Commented Mar 6, 2015 at 23:27
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There are a few reasons why the footprint of large buildings are excavated and replaced.

  • All of the old building must be removed - Typically, older buildings will have been built with weaker materials or less stringent building standards than are currently used. Depending on the age of the existing structure, complete plans may not even exist. This means that enough material needs to be removed to ensure that the soil can be assured to have the required strength for the new structure. Also this ensures that new piles will not be driven into old concrete, steel, or masonry.
  • Pollutant or Hazardous Waste removal - Especially if the old building was a factory (recently or at one time), there might be hazardous materials in the soil. Environmental regulations have not always been as strict as they are now. A lot of bad things were just buried and forgotten.
  • Historical artifacts - If the area is thought to have had any sort of history, digging below existing buildings may uncover some of this history. Historical societies love this, but owners and contractors hate the delay and added cost.
  • Groundwater removal - If groundwater is close to the surface or close to the basement of the new building, additional excavation may be required so that a sealing layer of concrete or clay can be placed. This can seal the site so that it isn't a muddy mess while the rest of the construction finishes. Dewatering wells may also be installed.
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If one side of foundation has soil packed more loosely than the other side, the foundation may cause the ground to sink, and building may crumble. Even if it has a steel-reinforced concrete foundation, the building may tip sideways, causing uneven floors, or wood to shift and split, even if concrete remains in same shape.

In my area we have a lot of caves, some hidden, that collapse under weight and cause sinkholes. Large buildings (Like a Tesco) have cracks in the walls EVEN AFTER the soil was repacked, and is still re-settling and causing NEW cracks 17 years later. One of the oldest houses in town has the foundation 17 feet deep, so that it hits bedrock. (5.1816 meters)

Repacking the soil with power-hammers, or using a DIFFERENT filler material that is less prone to settle over time, is also a common practice. As is digging deeper just to pour a thicker foundation (or a wider one with a smaller footprint on the surface, like a submerged pyramid).

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In addition to the other answers, some soils (e.g. clay) expand and contract as their moisture content changes, and wet soil expands as it freezes. This movement is rarely even across site, and differential movement causes a building to crack.

Foundations are dug down to below an expansive soil and below the frost line to avoid movement. They are then built up with crushed rock or concrete, which doesn't expand with moisture or freezing temperatures.

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