Annual Dinner 1987

Tensile stresses in concrete Mr J, T. Chase of Worcester continues our correspondence, following Mr Pannell’s letter in July 1986, on the queried justification for reliance on tensile stresses in design in unreinforced concrete. Mr Pannell referred to the subject specifically in relation to the use of minipiles to strengthen ground slabs where settlement has occurred. Mr Chase believes that there is no need in such cases to appeal to tensile strength, since in practice arching action offers support. He writes: In the particular case of minipiles supporting a plain concrete slab on a grid of 1.0-1.5 m in dwellings, I believe the slab is supported by arching action within its thickness. In practice, the slab acts as a series of mutually supporting domes until an edge is reached, tensile strength playing no essential role except, perhaps, at the slab edges. At the edge, restraint is provided by: (i) the piles themselves reacting against the fill above the footing and in bending; (ii) the surrounding brickwork, supported in its turn by the external ground; (iii) the stiffness provided by the edge strip of the slab between the last row of piles and the wall: here the slab is acting as a deep beam spanning horizontally between piles. Verulam Verulam

A steady rise in groundwater levels in the aquifer beneath London was noted as long ago as 1972, but only recently has attention been focused on the potential engineering implications of this rise. Sponsored by the Department of the Environment and other interested parties, the Construction Industry Research & Information Association (CIRIA) has organised a study into the problem. This paper describes some of the effects which a rise in groundwater levels might have on structures and is based on work currently being carried out under contract for CIRIA. T. Blower

I am indebted to the engineers who have responded to my views, and I do not wish to take issue with any contra views; we all have our own perspectives on important matters. David Wardle