Guest Post By James L. Gordon, P. Eng. (Ret'd)
On May 1st, I read with mounting astonishment the aDB Engineering report dated April 20, 2017, on their investigations into the causes of the collapse, which occurred on 29th May, 2016.
The level of incompetence described in the report is so widespread that it brings into question the integrity of all other structures in the development. All staff, from carpenters to engineers are included within the ranks of the incompetent. The sub-standard construction was clearly visible to all, but nothing was done to rectify the errors.
The collapse occurred in the lower portion of the draft tube as shown on the following Figure1, copied from Figure 2 in the aDb report. In this area, the water passage changes from a vertical expanding cone to an almost horizontal expanding rectangle as shown in Figure 2. It is the most complex double-curved shape in the project and very difficult to build.
For small turbines, the shape is formed in timber, with an east coast shipwright engaged as the carpenter foreman. A shipwright carpenter is the only person capable of cutting and shaping the timber to the required complex shape.
For large turbines, the shape is usually formed with collapsible steel forms, since wood does not possess the required strength. Unfortunately, the contractor elected to use timber.
FIGURE 1 – LOCATION OF FAILURE AND LATTICE WOOD TOWERS
The forms were supported with untreated southern pine softwood timber lattice towers as shown in the Figure 2 obtained from the Independent Engineer’s report dated July 2016.
FIGURE 2 – VIEW LOOKING DOWN THE DRAFT TUBE AT THE LATTICE WOOD TOWERS.
The tower height is about 5.4m. The vertical lattice tower portions are formed with built-up 2” x 10” timbers which were exposed to weathering, with the bottom immersed in stagnant water, fungus was growing at the bottom, parts of the wood were rotten, assembly was deficient with inadequate shims, alignment was off with beams only partially supported by the towers, all as shown in the following photos copied from the aDB report.
FIGURE 3 – FAILURE SCENE AND WEATHERED TOWERS (above) – NOTE STAGNANT WATER (below)
FIGURE 4 – BUCKLED TIMBER AND GAP IN TIMBER
FIGURE 5 – DRY ROT AND INADEQUATE SHIMS. TIMBERS ARE 2” X 10”
From the photographic evidence it is obvious to any observer that the timber lattice towers were deficient and should never have been used to support a heavy load of concrete.
At the time of the collapse, there were 5 workers at the site. All fell into the concrete mix during the collapse, and one was fully submerged but luckily managed to extricate himself from the liquid concrete with the aid of another worker.
FIGURE 6 – FUNGUS GROWTH (Left) AND DAMAGED TOWER TOP (Below)
There were 6 towers supporting the forms, and all collapsed. The failure destroyed all evidence, hence the report was based on observations of the condition of the lattice towers in the other units.
The list of unacceptable work is long, with details as follows –
1. Flooding up to about 3 feet above the tower foundation.
2. Fungus and decayed wood in towers with mushroom growth.
3. Exposure of the untreated wood to rain and snow.
4. Severe weathering, with some weathering occurring during storage at site.
5. Ice built up between ribs, in June!
6. Buckling of tower timbers.
7. Compression failure in some towers.
8. Gaps between timbers and lack of shims at top.
9. Wood saw marks indicating no quality control at the fabrication plant.
10. Incorrect alignment of some towers.
11. Inadequate nailing of braces.
12. Splices not conforming to CSA standards.
13. No protection of the untreated lumber on site prior to installation of the towers
from fabrication in summer of 2014, to use in the spring of 2016.
14. Tower load calculated at 57,700 lbs, and capacity at 40,500 lbs.
15. No allowance for expansion of the timber due to moisture retention, compromising structural integrity.
From the foregoing, it is obvious that there was no quality control at the fabrication plant and on site, and no NALCOR staff with sufficient experience to realize that the towers were totally inadequate. This is astonishing, since any carpenter looking at the towers would conclude that something was wrong.
Also, an engineer looking at the towers would immediately observe the errors in fabrication, installation and deterioration due to weathering. This indicates that either no engineers inspected the structure, an almost impossible conclusion, or, more likely, they were totally inexperienced to such an extent that their competence to practice engineering in Newfoundland should be questioned!
Jim Gordon, PEng. (Retired)