On Negative Skin Friction

An unusual case of piling

Many years ago I designed a building in central London, which was later occupied by a well known tech giant. There were many interesting features to the building, however perhaps the most interesting is concealed from view below ground level.

Like many large buildings in London its weight is too heavy to be borne on shallow foundations therefore concrete piles embedded deep into the soft London clay were required.

The structure was to be built on the site of a prior building which had also been piled. We soon discovered that is was going to prove difficult to match new pile positions with proposed column positions, because the existing ones were always in the way, though never quite in the right place we needed them. Grubbing out the existing piles was very expensive and would have loosened the soils somewhat. We decided to build a deep raft on top of the existing piles that would allow us to transfer new load into them from our planned column positions. Justification of the the existing pile’s load capacity was rather interesting, but that is not the topic on this occasion.

The footprint of the new building was required to extend beyond that of its predecessor and therefore a line of new piles was required in front of the existing ones. This was to prove an opportunity to use some creativity, because we knew that this part of the site fell within the influence zone of the planned Cross Rail underground tunnels. The route of the tunnels had been secured with legislation for many years.

To understand the influence this would have it is first beneficial to understand how friction piles work and what the effect of future tunnelling would be.

Friction piles, as the name would suggest, resist load by generating friction between the surface of the pile and the surrounding ground. The larger the circumference of the pile and the longer the pile is the more friction will be generated. Consider trying to push a tent peg into the ground or hammering in a fence post. It becomes progressively harder the deeper they penetrate into the ground; this is friction in action. It is a powerful force.

Tunnelling is a complex operation, but in principle it involved pushing a circular shield into the ground and digging the ground out within the safety shield, which prevents the ceiling from collapsing. A structural ring is then constructed inside the shield before it is advanced to the next section of the tunnel. When the shield moves on the ground fills the gap that is left behind by settling on to the newly constructed structural ring. As the tunnel progresses a wave of settlement follows the head of the tunnel.

An interesting thing happens when concrete piles fall within the zone of settlement. As the ground settles and drops it tugs down on the embedded piles. This action pulls the piles downwards due to friction between the ground and the pile shaft. In other words the tunnelling has caused friction to work in reverse. Instead of friction pushing against the weight of the structure it is now acting in concert with the weight of the building ‘sucking’ it into the ground. This is known as ‘negative skin friction’ and it would be bad for the building, particularly of the rest of the building, which is out with the tunnelling zone, stays where it is and does not settle.

 

The conventional way to overcome this problem is to sleeve the piles in steel tubes over the full length of the tunnel’s zone of influence. To ensure the piles still have capacity to resist the building’s weight they extend beyond the sleeves and are embedded below the depth of the tunnel. This means that when friction reverses the steel sleeve is tugged downwards, but the pile remains static inside the sleeve while load resistance continues to be provided by friction generated below the tunnel.

This is a rather expensive option, because each pile needs a long steel sleeve. We therefore decided to think about whether we could design the foundations without sleeves. If we could it would save both time and money for the client.

We ‘war gamed’ many scenarios, but eventually reasoned that, providing the piles extended below the depth of the planned tunnels, the most likely scenario was that the expected negative skin friction would cause an ‘apparent’ increase in the weight of the building. We were confident that we could design the piles to deal with that ‘apparent’ increase and so that’s what we did. It took a while to design, and being an unconventional approach, it also took a while to obtain formal approval.

It all seemed very theoretical at the time. Cross Rail had been spoken of for years, but had never been realised. Many thought it would never happen. Of course that view has since changed. Cross Rail is due to become operational in 2022. So far the building is still standing quite happily; our hard work appears to have paid off.

Something also worth mentioning; the route of the tunnels originally passed beneath the existing piles too, which would not have done them any good at all. Within the approval process for our new design we therefore proposed that they be altered to avoid damaging the existing foundations. Our proposal was eventually accepted and the layout of the tunnels was changed. In this location they run on top of each other instead of side by side.