Another skyscraper II

There has been some progress since the job started. To date, however, there's been no progress upwards; all recent activity has remained up to four metres below the original ground level.

The piling is finished. I missed the pile test, where a selected pile is loaded with large numbers of eight-tonne concrete cubes and the amount of settlement is measured. The next stage is to expose the steel reinforcement to provide something for the concrete ground slab to attach itself to. Normal procedure is to break the concrete at the tops of the piles and thread foundation slab steel into the ground slab steel before pouring the foundation concrete.

The contractor has installed steel I-columns around the perimeter of the building footprint. These act as underground fence posts to hold back adjacent earth while the soil around the piles is excavated. Exactly the same technique was being used in Washington DC, as photographed by nzm.

Not many piles visible. Most are below ground

It is a little unfortunate that the steel columns were installed using a crane with a vibrator attachment; the columns are vibrated so that they glide into the ground, and will presumably be removed using the same technique. Everyone living or working nearby spent a couple of weeks having their dental fillings loosened by the all-pervasive vibration. Still, it's less noisy than a steam hammer donking away all day.

A crane

Now that dewatering has been installed, an excavator can trundle around the site without sinking into a noisome muddy pool. In fact, provided the pump keeps on running the ground will likely remain dry. Groundwater slurps up the little green pipes by suction and is ultimately discharged into a nearby storm sewer.

The consequences of inadequate dewatering

As the excavation started, it quickly became apparent that of all those piles that were installed, initially only two were visible, poking out of the bottom of the excavation. There were several suspicious-looking bent bits of reinforcing steel. I suspect that the tops of some of the piles were inadvertently demolished during the excavation process. Further excavation to around three metres below original ground level finally revealed several other piles. So they didn't all sink without trace into the quagmire. We are now regaled by jackhammers as the concrete is broken out to expose the steel bars.

I have finally managed to upload some pictures, courtesy of Picasa. That's on 22nd July, a week after uploading the text.