Monday, 17 May 2010

A Hole in the Masterplan

ENVIRONMENT Minister Freddie Cohen has bowed to public pressure and ruled out any building on the Millennium Town Park - despite winning yesterday's vote to hold off decisions until the debate on the Masterplan for the north of town in six weeks. In the States yesterday Members effectively rejected St Helier Constable Simon Crowcroft's proposition to force work to start this year on a park without any buildings on it. That appeared to set up a clash in six weeks' time, with the Masterplan proposing building on the eastern end of the site and a counter-proposal ruling out the plan for a four-storey development on the eastern end. But this morning Senator Cohen made a concession ruling out any building on the site, although he says that some underground parking will still be needed.  (1)

But is this the only reason for dropping the houses? If we cast out minds back years ago, to when Dereck Carter was President of the Public Services Committee, on 7th November 1995, the States minutes report this question:

Would the President inform the States at what stage the present works of the `cavern' and main line from the Gas Works to the Weighbridge are, and the works on the shaft at the Gas Works end  of the drainage system?

He replied as follows:

The present stage of construction is as
        follows -

(a)  the storage tank, or `cavern', has been excavated to 90 per cent of its size;
(b)  the surface water tunnel from the Weighbridge to Snow Hill is approximately 50 per cent complete;
(c)  following breakdown and repairs,  the tunnel boring machine (TBM) constructing the surface water               tunnel from Snow Hill to the Gas Works car park has just re-started cutting the rock, and is still going through commissioning stages;
(d)  the shaft at the Gas Works car park is almost fully excavated, but the TBM will have to be removed before this shaft can be finished.

Now that shaft - which is massive - was finished around 1997, and is located at the eastern end of the Gas Place car park - just where the foundations of the four storey development would be. The report (2) presented to the States on 17 April 2002 goes into more detail.

Various options were considered, and it was concluded that a relief sewer was required to take surface water from the Gasworks area to the Weighbridge, and that overflows of foul sewage would have to be picked up from the Bath Street sewer system and at the Weighbridge, from the overflow.

So under the proposed housing for Gas Place - which would be at the eastern end of the car park - would be partially on top of the access shaft to the tunnel from the Weighbridge.

The entire 1,100 metre length of the three-metre diameter tunnel from the Weighbridge to Oxford Road would be constructed in rock, at least ten metres below the top of the rock and a minimum of 20 metres below ground. This would ensure that there would be no significant effect on buildings and structures along the route. This was a prime concern of the scheme, as there was a high water table, with the foundations of the older buildings being in the soft ground. It was essential to have a tunneling method that did not cause dewatering of the ground and settlement of the buildings.

The northern end of the route would be a shaft (S4) in the north-east corner of the Gasworks car park. This shaft would provide the connection point for the Gasworks Brook and for a planned surface water sewer from Oxford Road to Springfield.

I imagine that it would cost a considerable sum to relocate this shaft, which, by a happy coincidence of Freddie Cohen dropping the housing, will no longer prove a problem - especially as none of the wonderful Hopkins Masterplans seemed to even be aware of its existence. Did they do their homework? It would have certainly caused massive problems if they had not.

Whether the tunnel will be deep enough not to cause problems for an underground car park is something which has also to be considered. Again, none of the planned "parkscape" seems to make any mention of this. Obviously, an underground car park may not need as deep foundations as a four storey building, but engineers need to be able to ensure that the tunnel is not damaged, and that the access shaft area is excluded.

What is more, and this will also need to be taken account of with any underground parking - is problem that the tunnellers encountered with the groundwater conditions - with a high water table in the Gasworks vicinity:

Considerable delays occurred in the sinking of the deep shaft at the Gasworks (S4). Work started in October 1994 and the sinking of the shaft was programmed to be completed in four months, then to await the arrival of the TBM tunnel from Snow Hill. A problem was encountered in sinking the shaft through the interface between the soft ground and the rock, with a high water table present in the soft ground. This resulted in a very long delay, while the Contractor attempted to inject grout into and around the interface. After negotiating the interface, further delays occurred in sinking the shaft through the hard rock, and this was not completed until October 1996.

Shaft at the Gasworks (S4). Unforeseen ground conditions - higher than foreseen magnitude of groundwater inflow at soft ground/rock interface and through fractures in rock, increased hardness of rock, and additional specialised grouting of rock.

Has this been factored into the cost of an underground car park? Let's hope it is before any work actually starts on the site, or the cost could rocket into the stratosphere.


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