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Piling: Small but perfectly formed

nine elms aerial

Piling work currently underway in Battersea will put the new into the former New Covent Garden Market site as it is transformed into a new residential district.

A 1.4ha plot of land that was home to the New Covert Garden Market in Battersea, London will become part of 1,800 residential units, retail and leisure complex called Nine Elms Square.

For the first phase of the scheme, Expanded Geotechnical has been undertaking the piling for three 37 to 54 storey towers with its part of the work to be completed in the New Year.

“It’s an incredibly tight site,” explains Expanded Geotechnical project manager Joe Curtis who has been working on the project since June 2018.

“Of the 12 blocks planned for the entire scheme, this first phase includes the three tallest at 37, 47 and 54 storeys.”

Ground investigation of the site was undertaken by Concept and found the ground conditions were made up of Chalk, Thanet Sands, Lambeth Group, London Clay, River Terrace Gravels with a made ground and alluvium.

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Nine Elms

There are a total of four 120t piling rigs squeezed on site in rotary and CFA mode

Work on site started with Keltbray excavating a minimum of 3m from the existing ground level across the site to leave the pile mat at 1.5m below previous ground level before Expanded set about the piling.

Each tower has differing specifications and, as a result, there are a total of four 120t piling rigs squeezed on site in rotary and CFA mode with attendant excavators, dumpers and cranes, as well as over 50 Expanded operatives.

“All the tower piles are drilled to roughly the same level, around 54m with a minimum 2.5m socket into the Thanet Sands,” says Curtis.

For the tallest of the towers, Expanded has completed 275 1,200mm diameter piles to around 54m depth, with six additional king posts.

The shortest will be supported by 82 900mm diameter piles to around 54m depth, with 16 using king posts too.

The 47 storey tower call for 93 piles with a mixture of 900mm and 1,050mm diameter , drilled to 54m.

So far, so straightforward. But what this site does have that Curtis hasn’t personally seen before is a scour feature infilled with clay running across the north east section – exactly where tallest tower will be.

“In this area, we have had to accommodate a drastic change in the depths of clay ,” explains Curtis. “The clay is very fissured which can cause issues with stability of the bore.

“The depth of clay has ranged from 3m to 12.5m in about 45m [horizontal distance]. It’s a big change in a very short distance and not something I have seen before in London.

“Sometimes we came across differing bands of material within the Lambeth Group - bands of sands and gravels at 40m, where in other piles you can get down to 50m.

“The piles were flooded [with support fluid] prior to the bore becoming unstable within the granular material, once flooded we would often then encounter cohesive bands of material which impacted drilling speeds significantly.

“Cohesive material drilled under support fluid with our specialised digging buckets proved to be tough going compared to the traditional granular strata. Due to the differing nature of the Lambeth Group the flood depth of piles varied by up to 10m across a very small area of site.

“It is very strange ground conditions in that area, but as you come away from the scour feature the ground conditions become a bit more uniform.”

In addition to the tower piles, across the site there are 510 podium piles which vary from 750mm to 900mm diameter . These will be CFA drilled to the around 25m below the existing level for the landscaping around the feet of the towers.

When GE visited the site late last year, 85 of the podium piles were completed, with the rest planned for the New Year. A change in the design led to Expanded using CFA piles over the rotary ones originally planned.

It is very strange ground conditions in that area, but as you come away from the scour feature the ground conditions become a bit more uniform

“We were going to use 600/750mm diameter rotary pile,” says Curtis, “but the change in design with the new load information, it was within the capability of a CFA rig. We’ve upped the diameter and shortened the piles and this has had obvious programme benefits.

“Due to varying pile cut off levels and to reduce the amount of redundant reinforcement in the CFA podium piles, we are plunging our reinforcement cages up to 5m below pile mat level. This is while maintaining installation tolerances of -50/+150mm.

“Traditionally the reinforcement would be terminated at pile mat level but the plunging methodology adopted is reducing waste .”

This is one of many changes that Expanded has undertaken during the scheme, including piles for an extra tower crane late on bringing the total to six for the site.

“In terms of changes, the design is still evolving. Originally it was a pile cap scheme, but it is now a raft slab which has different effects on loads on the podiums piles with a lot of tension loading on the podium piles.

“It can be hard on site for us to keep up with the changes, but we are managing it well.”

We are pouring over 450m³ of concrete a day on this site which is some going on a piling job.

A secant pile wall formed of 375 1,180 and 880mm diameter rotary bored and 900mm diameter CFA bored piles installed around the site with a deflection criteria of 10mm to some parts that are close to an existing Thames Water sewer.

The depths ranging from 12m to 36m with the kingpost piles installed at 1,650mm and 900mm diameter the increase in diameter was due to the significant shear loads subjected to the piles acting as prop support to the secant wall.

“We have put in the king post piles so we can maintain the deflections ,” says Curtis.

“The secant wall is propped in parts and the rest is a fully cantilevered wall away from the Thames Water sewer.

“The retained height varies significantly where podium piles are pretty shallow. Nearer the towers it is 8.5m retained and in other areas of the site it is 3.5m.”

With the current job due to end at the beginning of 2019, Curtis is confident that they will keep to the schedule.

“We are doing well on programme,” he says. “We are pouring over 450m³ of concrete a day on this site which is some going on a piling job. Factor in the 100 muck away wagons a day, three reinforcement deliveries a day, and no health and safety incidents, and we are really pleased with the progress.

“It’s an incredibly tight site and how we are managing it deserves some credit.”

Putting polymer to the test

All the tower piles are drilled under polymer and the first time that Expanded has used this method on a site of this size.

“Traditionally we would use a bentonite slurry so the polymer is something new for us. Not only does the polymer have environmental benefits, it is cheaper to dispose of,” explains Curtis.

Bentonite can be reused but it does need to go through a cleaner. With polymer the solids are in a suspension and easier to clean.

“We are fairly new to this and at the start we had to undertake testing before we concreted the piles.

“At first we were struggling to break down the polymer in the testing facility - differentiating between sand and mixed silt/clay was a difficult interpretation when carrying out the sand content test from the base of the piles.

“Occasionally we would need to wait for a period of time following the sample being taken from the bore to allow the solids to drop out of suspension whilst the silty/clay particles would remain in suspension for a significant period.

“On breaking down the polymer using a domestic bleach a clear dark line was visible which clearly defined the percentage of sand within the test sample.

Expanded has been cleaning the polymer on site with five aeriated holding tanks each holding 54m³ with a polymer plant on site too.

“Sometimes there is a delay in taking your sample from the bottom of the pile, breaking the polymer down and getting rid of the slurry,” adds Curtis.

Expanded has used the polymer before, but not to the depths on the Nine Elms Square project.

“Any job that Expanded will be looking at from now on that needs support fluid, the estimating department will be going down the road of polymer . We have done a lot of sonic logging testing and the results are good.”

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