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summary of the theory behind the use of bentonite and his clear descriptions of constructional techniques. With regard to the sections of the Paper concerned with structural diaphragm walls and particularly water cut-off walls, I am sure he will be interested to learn of the methods adopted to build such as part of the Derby inner ring road.42. A road close to the River Derwent was required to be reconstructed up to 4 m below original ground level. The whole area of the work below water table was bounded by a cut-off wall, most of which was constructed under bentonite.43. It was decided to adopt the simplest form of wall panel with no spigot and socket joints nor primary and secondary construction techniques. The wall was constructed in alternate panels generally 7.2 m long X 0.5 m wide using steel tube stop ends. Excavation took place from ground level resulting in trench depths of up to 8.5 m to obtain a 0.6 m cut-off in the Keuper mar1 (Fig. 9).44. By using a Poclain grab to scrape the majority of the contaminated concrete from the ends of the previously cast panels a successful cut-off has been achieved and less than 0.03 l/s passes into an enclosed area of 3200 ma despite the hydraulic head regularly exceeding 3 m. The methods described by the Author of forming a junction between bays of cut-off walling are obviously important when watertightness is essential but it is clear that for many purposes an adequate seal can be obtained by less costly procedures.45. Elsewhere on the perimeter of the enclosed area at Derby, the cut-off wall has been used as a continuous foundation for bridge abutments. In designing these foundation walls to withstand the compression and bending resulting from forces imparted by the bridge decks, the bentonite trenches were widened to 0.8 m. M r A. S. Braithwaite, Engineering and ManagementConsultant, Arthur S. Braithwaite I wish to refer to the use made of bentonite in the construction of some large foundations over the Mersey Tunne1.I The problem was to construct the foundations for a 16 storey building situated immediately above the Mersey Tunnel in such a manner as to prevent any of the load being transmitted to the tunnel lining.47. This problem was deemed to have been solved by boring holes through the rock down to near the level of the bottom of the tunnel structure and belling-out the base. In these holes were constructed reinforced concrete columns so that a cavity was left between the column and the rock which was later filled with bentonite.48. Two reasons were given for the use of bentonite in this situation: first, the designers considered that there was a possibility that the gap or cavity could become filled with rubble, thereby transmitting load from the pile to the rock and thence to the tunnel lining, and second, the cavity might act as a vertical drain, with unforseeable consequences. Bentonite was considered to prevent both occurrences.49. How efficient would bentonite be as a solution to these problems? Is any evidence available to enable the designer to judge t...

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