HARBOUR WORKS.

Dominion, Volume 3, Issue 781, 2 April 1910, Page 5

HARBOUR WORKS.

THE DOCK AT TE ARO. ■ A DIFFICULT PROBLEM. For practically six months no work has been done ia connection with the construction of the Wellington Harbour Board'e dock at Tβ' Aro, and when waited yesterday by a Dominion representative there was an air of desolate calm about the idle works. Sk months of the best summer.Wellington has ex-' perienced for a decade have passed 1 and gone without the dock being any nearer' completion. It is not the'fanlt of the contractors, Messrs. M'Lean. and Sons, Ltd. It has, been occasioned by'- a disease" that , , is said to afflict concrete in the soft at a certain depth of water, called "laitance"—a mechanical adtion, caused by the abnormal pressure of water destroying the adhesive properties of the cement in tie concrete. The contractors, it :■ is said, would be quite content to proceed with toe work • laving down the concrete (OX specined consistency) in' the manner set out in. the specifications, but in view of the fact that the concrete'could : not be got to set solid in 40ft. of water the Harbour Board intervened. It is conoeded by most people who know anything about it, .that"to lay concrete at a depth of. 40ft. under water is not a job to be tackled witii impunicy. Up to a certain depth conbetter in water than in the air, but water pressure, as-every engineer knows, causes more trouble in engineering works than any. other obstacle to be overcome. It has been' suggested, that a coffer-dam should have been specified in the first instance and. some members of-the board:some months ago thought it was not then. too late_to alter the specifications to provide for that mode of construction. Nothing, however, was done,. except that it was decided to experiment, and month after month has dragged alone, and the experts are still experimenting - There are regiments of concrete blocks lying about the "dockyard," every cm showing the ravages of "laitance" more or less. . . ■..;■■. ..... ..."

Jf 1 F ■WM'-'Md* as to-what was nS g u°T ? t , the Present time. Two more blocks had been.raised a few days before after weeks' submersion! They were, still boxed -in, and wire blanketed,with sacks oi, the top. They had not yet been tested, so our renri sentatave took on the contact abe, with tho aid of a pocket-knife, Z cut away two inches of "concrete" off the; top—that was the '!laitanne blight," and could still scrape away ?he rotten concrete deeper than that. How 1 such concrete would stand the pounding of a- 101b. hammer he could not say but wa 3 assurre j that ; it - & m & better sample than many of its p«£ $&$&'&•■&& iA? ■ b ? g s -experiment, however, ml? Z e T^ d ' "*■■ P resenfc ' the suit of which is awaited Some time ago it was decided to lay down a big section of 145 cubic yards of concrete, using three instead of two bags of cement to every, cubic yard. This has now. beendown about a fortnight, and wiU be given some time yet before being thoroughly examined and tested.:;"..'.- i- .'■:■.■■■ ■;. -:.' : - '■ ■'■ : "' - "--'

. The trouble has occurred in laying down the flooring of the dock: It is essential.that a dock built in the:harbour and surrounded by newly-reoiaim-ed land (which admits of : tho, freest tidal soakage), should have a floor or bottom as .nearly, water-resistant as' possible, and this is-what, up to the present; it : hae>been : found 'impossible tO get. ■.■:■ ,:(.■'.-■:.!.'" ■ •■■■■■:,■".■..■.■■'-.■■ ■' " In the meantime the. contractors are receiving offers of ideas as to how the' dock could or should be constructed from a vanetj of people " M '- Loa