Canal failure blamed on planning faults

Press, 9 March 1982, Page 3

Canal failure blamed on planning faults

PA Tauranga A failure to recognise the sensitive nature of the underlying ground was the basic fault which led to the collapse of the Ruahihi'Canal, the Ministry of Works and Development’s investigation into the incident has found. A report was compiled by senior engineers of the Ministry who investigated the failure of the canal, in an area known as Fill A, in September, 1981. The canal was part of the Tauranga Joint Generation Committee’s Wairoa development scheme and fed the last, and biggest, of three new power stations. The report said that the scheme had been conceived by Mr Lloyd Mandeno, a founder of the consulting engineering partnership of Mandeno, Chitty, and Bell.

This firm had been the consultant to the committee for the investigation, design, and construction of the three stations. The firm had had a continuing association with the committee since at least 1968. The firm had amalgamated with another firm and had become part of the new engineering practice of Worley, Downey, Mandeno, Ltd, in April, 1981. The Ministry’s engineers who comprised a committe of inquiry into the collapse, Mr A. V. Hatrick (chairman), Mr J. H. H. Galloway, and Mr A. Howarth, said in the report > that the engineering investigations for the canal had been appropriate in most respects and had given adequate indication of the type of material'to.be expected. “However,’ these indications were 1 not recognised and the additional work

necessary to confirm the existence and behavioural aspects of highly structured, sensitive soils and their extent and distribution under and adjacent to the canal, was not carried out,” they said. “No exploration drilling was done between chainage 2500 metres and the forebayquate indication of the type of material to be expected. “However, these indications were not recognised and the additional work necessary to confirm the existence and behavioural aspectand the additional work necessary to confirm the existence and behavioural aspects of highly structured, sensitive soils and their extent and distribution under and adjacent to the canal, was not carried out,” they said. “No exploration drilling was done between chainage 2500 metres and the forebayquate indication of the type of material to be expected. “However, these indications were not recognised and the additional work necessary to confirm the

existence and behavioural aspects of highly structured, sensitive soils and their extent and distribution under and adjacent to the canal, was not carried out,” they said.

“No exploration drilling was done between chainage 2500 metres and the forebay and the instability of the natural country was not identified. (Chainage is taken from the Lake McLaren end of the 3350-metre canal and the breakout from the canal was estimated to have happened round chainage 3250 metres, or 100 metres from the forebay).

“This carried over into the engineering design and resulted in a failure to consider the hazards of a canal built in ground siisceptibld to loss of strength or internal erosion if disturbed or subjected to changed groundwater conditions.

“The works were adequately executed and there is no reason to believe that the failure was due to deficiencies of construction.”