February 23rd 2009

JACKED BOX TUNNELLING      

                   

     given by Dr Douglas Allenby

After having had to postpone his talk earlier in thed month because of inclement travelling conditions , Dr Doug Allenby gave an enthralling talk to a large and appreciative audience on the technique of "Jacked Box Tunnelling". He asked his listeners to imagine they were in the situation of having to try to put in a new road or railway across the path of an existing heavily used transport system e.g. the M1 motorway. Do they bridge it or do they tunnel or do they put in a controlled intersection? All are possible but he pointed out that the first and last are very expensive both in capital cost and severe disruption to the existing traffic flows.

If the soil conditions are right often tunnelling is the best option particularly if the jacked box tunnelling is used. In this case a box section is slid along the tunnel line that formers the outer casing of the new facility. If suitable precautions are taken about anchoring the pushing points and care is taken over the alignment of the box when it is being pushed little or no disruption occurs on the existing traffic flows.

           Aerial view of the Boston Big Dig                                                       Close up view of driving the tunnel under the railway

Clearly problems could occur if the local soil structure is poor; this implied that good soil surveys are required and that appropriate methods have to be used to 'strengthen' the soil e.g freezing it.

Dr Allenby outlined the methods involved e.g. building up good anchor points and having efficient spoil removal systems. To give substance to his talk he illustrated it by describing several applications from driving simple culverts under railway lines, putting a new trunk road under the M1, putting in new tunnels for the Docklands light railway and finishing with the 'daddy' of them all i.e. the 'Big Dig' in Boston Mass (USA) where massive new tunnels were put in so that a new interstate highway was driven under the city centre replacing a rather ugly, outdated, poor maintenance double decker road.

After Dr Allenby had answered many questions from the floor he was eventually thanked for a very stimulating talk by the President.

For those interested in finding out more Dr Allenby has provided us with a a copy of a paper he and his co-workers presented to the IMechE.
Click here to access a pdf of the paper