Sorry Hokie, I've used screw piles for big loads... There's a transformer yard in Saskatchewan that is supported on a 1m thick slab and supports six 400,000lb transformers. Still standing and first time I used screw piles. Had a geotechnical report and design parameters...
The only oversight was that I bevelled the steel tube shaft instead of putting a bottom plate and steel 'spike'. When they were installed the bevelled tube allowed the pile to 'wander' a bit prior to going down. This was a problem because the adjacent building was also on screwpiles with a gradebeam. These piles wandered a bit. Only a couple of other issues were that I insisted that torque values be recorded... this wasn't done. There is a good correlation on torque and pile capacity. The other thing that became a small issue was that the transformer suppliers drawings showed the transformers anchored to the concrete slab and the transformers set in grout. When queried, I said the manufacturer's drawings noted that; I told them to check with the manufacturer. Normally for transformers of that size, I wouldn't have thought about anchorage.
Another project that I used Sorbweb containment.
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?
When a client insists that there's no need for geotechnical investigations and the Engineer accepts to continue with the work, the Engineer should be sure that his experience into things is pretty a good one to counter these Clients. Clients always look for ways of cutting down costs and remember that political reason usually defeats engineering reasoning even when it is wrong.
Nonetheless, in such situations, one has to increase their perceived design factor of safety to 5 and beyond. Subsoils always come with surprises. Helical piling should have been driven much much deeper because as you go down water follows thereby affecting further the bearing capacities.
Looking at the links, I've done a photo analysis (see photo attached). I guess part of this was covered in the 1st thread.
Nonetheless, a quick photo-forensics analysis of the photos (before and after collapse) shows two number 5-pile-groups. A "5-pile-group" is this case is a group of 5-number piles joined by a pile-cap. The pile cap was instead a horizontal I-beam steel member upon which the bridge deck sat. For ease of reference, the 5-pile-group shall be referred to as a Pile-group.
To note, each pile group was well braced width-wise but not length-wise. Or we could say, length-wise, the deck itself was acting at the bracing as well as being supported. Kind of risky though as there would not be enough robustness which would be a necessity on such a project.
Further examination of the available photos, the pile-cap looked like a 400x300 H-Beam (a close match would be 400x300x124kg/m). The Steel pipe used as piles was close to a 400mm diameter (a close match= CHS400x16).
The bracings looked like Steel Angle sections of 75x6 (say EQA 100x100x15).
The Bridge itself was reported to be 7.3m wide and 36m long.
So, when you look closely in the photos, the bracings for the pile group closer to the observer is slightly into the water whereas the other one further away is not. This is a clear indication of substructure settlement in that pile group and absence of a bedrock within the vicinity. And take note that any slight settlement is just enough to induce new out-of-plan forces and moments coming from the supported bridge deck and onto that pile group's weaker direction (length-wise) which eventually widens the span and leads to the collapse, just like in the photo.
So the idea was that the Bridge's weaker longitudinal direction (length-wise) had to be kept in position by the bridge-deck itself on assumption that the piling system was itself well anchored deep into the river bed for a good pile-soil stifness effect and cantilever control effect. This wasnt the case, hence a substructural failure. Therefore a deeper awareness of the geotechnical conditions was a necessity (officially) or a better look into the conditions during construction.
What should have been expected!
1) It'd have been safer to carry out geotechnical investigations to be informed of the soil profile to some 10m deep into the riverbed or where bedrock is encountered for piles to be driven into (say 5m) past the river bed surface or an additional say, 1m into bedrock. I photo-estimated a height of about 8m from the riverbed to the deck level. Longitudinal bracing would have been included to reduce on the cantilevel effect.
Or
2). Simply via experience & ingenuity, no geotech results (...must be goodly-good without doubt), in the helical/screw piling process, the torque readings would have been recorded and analysed for soil's cohesive resistance vis-a-vis the depth screwed into the riverbed for good anchorage and the longitudinal cantilever effect reduced by a bracing feature in addition to the existing bridge deck bracing effect.
This would have countered the following:
- Substructural settlement via well anchorage into the ground
- Longitudinal Cantilever effect via a proposed longitudinal bracing system in addition to the bridge-deck.
It seems like someone is getting a lesson about when to say "No thank you." and walk away.
All clients are not good clients can be the hardest lesson of all to learn.
A black swan to a turkey is a white swan to the butcher ... and to Boeing.
In a given area, there's usually a correlation between the installed torque and the load capacity. On a major project where I used them for supporting six 400,000 lb transformers, I'd asked that the torque be recorded... never happened, but they've been in place for nearly 10 years and no feedback... I guess they're OK.
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
It's obviously important to interview the company and the crew who installed the piles.
Since it's a matter of public record (in Canada, right?), it might prove to be informative.
I notice that the charges against the designer are that he didn't do his design in a "careful and diligent manner".
This would imply that the pile installers did exactly what he said, and noticed nothing amiss, even with their years of experience. And did it anyway, without complaint.
