Nicole Highway incidence

[zaren]

I believe LTA Engineers only ensure that the contractors work within the standard specifications/levels, like inspector, not involved in the actual design and construction.

Maybe my guess shld be:
1)The thickness of the retention wall should be thicker
2)The steel beam structure wasn't designed to withstand the excessive soil load bearing.

Doesn't look like a structural design issue to me, based on the observation that other sections of similarly-designed retaining wall & steel struts have not collapsed. The most critical components of steel structures are usually the connections. One of the workers interviewed on TV yesterday said in Cantonese that he heard "boom boom boom" sounds which sounded like the steel welds (which join steel structural members together) popping out one by one hours before the collapse. Structural failure of steel struts is usually completely instantaneous and catastrophic, so the reports of warning sounds and signs before the failure are quite unusual.

 

[StreetShooter]

My guess is unusually high rainfall in recent months may have set off soil movement in the area. Stresses in the soil could have been induced by shifting groundwater levels due to the interacting of the infiltrating rainwater and existing saltwater levels.

Not to mention the unusually high temperatures lately, causing soil to expand. If there's an empty space to expand into, guess what happens?

 

[mervlam]

Not to mention the unusually high temperatures lately, causing soil to expand. If there's an empty space to expand into, guess what happens?

I personally don't think that land temperature would be a significiant contributing factor of the ground subsidence. Otherwise, there may be similar incidents sparking all over the island's excavation sites.

The main issue here is mainly the tricky soil conditions of the affected area. The area is reclaimed land, topped with mainly sand on soft marine clay. The recent high rainfall and seawater from the basin could have affected the watertable level of the area, inducing stresses. That sets off soil movement.

Structural failure of steel struts is usually completely instantaneous and catastrophic, so the reports of warning sounds and signs before the failure are quite unusual.

I'm not so sure if the on-site engineers would monitor the steel struts with strain gauges. Any signs of abnormal strain and stress on the structure would show on gauge, wouldn't it? Don't think structure failure is completely instantaneous without telltale signs.

 

[StreetShooter]

I personally don't think that land temperature would be a significiant contributing factor of the ground subsidence. Otherwise, there may be similar incidents sparking all over the island's excavation sites.

Hmm, true true. Enough armchair amateur engineering from me for this morning.

I'm not so sure if the on-site engineers would monitor the steel struts with strain gauges. Any signs of abnormal strain and stress on the structure would show on gauge, wouldn't it? Don't think structure failure is completely instantaneous without telltale signs.

I remember reading a quote from a geologist once, in National Geographic. He said the state of the art in earthquake prediction goes something like this: "We're going to have an earthquake.... NOW!".

 

[mervlam]

I remember reading a quote from a geologist once, in National Geographic. He said the state of the art in earthquake prediction goes something like this: "We're going to have an earthquake.... NOW!".

:bsmilie:

nono... it doesn't work that way for steel struts. Stress and strain is "accumulative" over time till the point of failure, depending on the material properties of the strut and loading pattern. The point of failure of the strut can be calculated, give or take with a safety factor. The structure is then designed to meet specifications.

Correct me if I'm wrong.

 

[ralph]

.........Don't think structure failure is completely instantaneous without telltale signs.

that depends on what you mean by "telltale",
if you mean visual signs...it doesnt happen all the time.

thats what ndt is for.
anyway...the properties of materials & mechanics of solids is a profound subject. :bigeyes:

 

[mervlam]

that depends on what you mean by "telltale",
if you mean visual signs...it doesnt happen all the time.

thats what ndt is for.
anyway...the properties of materials & mechanics of solids is a profound subject. :bigeyes:

i dont mean visual signs.... i meant signs from instruments like strain gauge.

 

[Linkster]

SINGAPORE - The body of a Land Transport Authority engineer was found at the Nicoll Highway collapse site on Thursday morning, bringing to three the number of dead from the accident and leaving one more person listed as missing.

Mr Tan Lock Yong's body was found at 9.45am and removed from the rubble two hours later.

The family has been informed and have gone to the mortuary to claim the body.

From: http://straitstimes.asia1.com.sg/latest/story/0,4390,247133,00.html?

 

[yowch]

My condolences to the people affected. Excavations are always risky jobs.

I would like to comment, but I cannot, as my boss is one of those nominated in the investigation panel, and any knowledge that I come across is evidence.

In general, these few days has been exceptionally hot. And that there should be sufficient strain gauges and monitoring on the system, as that's the standard contract requirement. As for design sufficiency, how would the authorities and client approve of the system if it was under designed in the first place.

Let the investigation be carried out, let the cause be ascertained and we can then work towards less accidents such as this.

Again, my deepest condolences to those involved, knowing that I can easily be one of them, too, some other time and place.

 

[Linkster]

Before LTA can open Crawford Underpass soon, the section of Merdeka Bridge nearest to the incident site will have to be cut off. This will ensure that there will be no stress on Merdeka Bridge when Crawford Underpass opens.

We would like to emphasise that the Merdeka Bridge is safe. LTA has been monitoring the area around the Merdeka Bridge and Crawford Underpass regularly and there are no further ground movements in the area around the Merdeka Bridge and Crawford Underpass.

For more information please click http://app.lta.gov.sg/corp_press_content.asp?start=1006

 

[hoppinghippo]

I think for such a large project like this, there would be adequate homework done before and during the works. be it structural designs, monitoring, etc. until we know the full story, we should give them the benefit of the doubt.

My personal opinion is that its prob a miscalculation on the estimated stresses, perhaps the reclaimed land and its stability or the effects of excavation work on it weren't as anticipated. usu land effects are based on empirical data, like different soil types, etc, so its not an exact science. or perhaps the supporting structures experience failure due to again underestimated stresses, insufficient safety factor, poor installation / welding, etc.

I'm really guessing here but looking at the other excavation sites, if I recall correctly several sites have surrounding buildings which developed jialat cracks and all which shows just how much vibration and all from such works can affect nearby structures. apparently they started works without anticipating such damage to nearby buildings, so what more nicoll highway?

but having said this, all in all I think its just a misfortunate accident. accidents will always occur, murphy's law. I think we should just learn from it and move on.

 

[Aton]

I must agree with what yowch said, excavation really is one of the most risky works in the building construction industry.

I have no doubt about the capability of those companies and authorities involved in such works, its one of the precedence that will benefit future design of supporting structures. I believe they have stuck within the safety guidelines, its just that no one expected the increase in load on these structure by the soil. (how it happened will have to see from the inquiry report)

I think most of us when passing by road excavation felt inconvenient at times, whether on public or private transport, because the road diversion certainly have caused slower traffic flow, even pedestrian are affected.

Now we understand the risk these people faced everyday, more respect should be given to them for scultping our landscape and aid in improving the way we commute everyday.

 

[zaren]

poor construction methods are a more likely cause of the collapse than "increased load from the soil". although you can get "soil heave" from the bottom of the excavation, i've yet to see active pressures from soil (marine clay or otherwise) increase unexpectedly for no apparent reason. All it takes is, for example, one of the steel bracing members not to be fixed and the buckling length of the steel struts is doubled, thereby reducing the steel struts' strength. When one steel strut fails, the remaining struts in its vicinity will all be subjected to increased loads. (think log PT, one guy doesn't carry the log, everyone else carries a heavier load). the steel struts may then fail one by one due to the "domino effect", and eventually lead to the collapse of the wall and the highway. nevertheless, it's a tragedy that lives have been lost, and i'm sure the building control regulations will be tightened after this incident.

 

[Prismatic]

Quite unlike to be heaving, heaving normally isn't castastrophic, and the signs are quite obvious unless the pressure built-up is sudden and large in magnitude. And from what I've seen at sites, the factor of safety is very high for excavations. So unless it's gross undercutting over a substantial number of struts, it's unlikely that a failure on one particular strut will affect such a wide area. I don't think active pressure built-up can be dismissed just yet, considering the unusually high rainfall this past month. Groundwater movement is a really tricky affair. Also a point to note (THOUGH VERY UNLIKELY) is an increase in passive pressure from saltwater intrusion in the soil.

It's all highly speculative at this stage and it's will be quite hard to ascertain the real problem if it's a soil-related case, because by now, the soil in area is probably disturbed by all the rescue excavation, injection of concrete etc etc.

 

[zaren]

Quite unlike to be heaving, heaving normally isn't castastrophic, and the signs are quite obvious unless the pressure built-up is sudden and large in magnitude. And from what I've seen at sites, the factor of safety is very high for excavations. So unless it's gross undercutting over a substantial number of struts, it's unlikely that a failure on one particular strut will affect such a wide area. I don't think active pressure built-up can be dismissed just yet, considering the unusually high rainfall this past month. Groundwater movement is a really tricky affair. Also a point to note (THOUGH VERY UNLIKELY) is an increase in passive pressure from saltwater intrusion in the soil.

It's all highly speculative at this stage and it's will be quite hard to ascertain the real problem if it's a soil-related case, because by now, the soil in area is probably disturbed by all the rescue excavation, injection of concrete etc etc.

wouldn't the "active pressure build-up" apply to all sections of the wall and not only a localised section? furthermore, a rise in groundwater table due to heavy rainfall is almost immediate, and not a delayed reaction of several days or weeks. i also fail to see how "saltwater intrusion in the soil" can possibly increase passive pressure. marine clay is practically impermeable anyway. in any case, an increase in passive pressure will counteract the active pressure and improve the stability of the retaining wall, not hasten its collapse.

 

[Aton]

The area have quite a good mix of soil, sand etc all different layer. Prior to the MRT construction, excavation was done for the crawford underpass few years ago, which happen to be adjacent to the site. Contractors don't just use marine clay to backfill excavated site materials like hardcore, sand, clay, gravel, tarmac, brick, etc are also used. Hardcore is a general term used for demolition debris. But of course they will use different material to different layer during backfill.
Thats all I know, I'm a building guy, not in civil so limited knowledge. hehehe.

 

[Prismatic]

wouldn't the "active pressure build-up" apply to all sections of the wall and not only a localised section? furthermore, a rise in groundwater table due to heavy rainfall is almost immediate, and not a delayed reaction of several days or weeks. i also fail to see how "saltwater intrusion in the soil" can possibly increase passive pressure. marine clay is practically impermeable anyway. in any case, an increase in passive pressure will counteract the active pressure and improve the stability of the retaining wall, not hasten its collapse.

You are assuming that the soil is homogenous which is definitely not true in a real-life situation because soil is not a fluid. And you are considering a change in groundwater level due to infiltration only. Groundwater movement also include seepage through soil from other regions of the whole basin. The process is especially complicated in a built-up area because of all the foreign objects we introduce into the ground. Rise in groundwater level due to a rainfall event is can only be considered a local peak, the rise of groundwater levels due to lateral seepage is more gradual.

Saltwater intrusion cause crystallisation of mineral in the soils which increase effective stress. The direction of how the active and passive pressure acts depends on what kind of retaining wall is being used anyway.

 

[Prismatic]

We are probably boring the rest of the CS guys to death. ;p

 

[mysonlovenemo]

Prismatic,
to be frank hor, pretty chim leh....... see star liao.....,
but this goes to show that we have got talent from multi-discipline at CS

any news on the last victim in the incident, he is a brave man who risk his own life to save others, hope that miracle will happen........

 

[zaren]

ok ok... back to photography! :bsmilie: