Introduction On the 20th of April 2004 at around 3:30 pm, the Nicoll Highway collapsed when a tunnel constructed for use y !"# trains collapsed$ #he temporary lateral support system of the tunnel failed, resulting in the formation of a %00 ft$ deep ca&e, which spread across si' lanes of the Nicoll Highway$ Highway$ #he collapse (illed four people and in)ured three$ #he incident resulted in disruption of the gas, water and electric lines, which a*ected nearly %+000 people in the area$ #wo #wo spans of a neary ridge had to e demolished demol ished and reconstructed due to the damage in soil conditions the collapse had done in the neary areas$
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http://www.meehaesong.com/project/!"/
ac(ground of the #unnel near the Nicoll Highway #here are three asic types of tunnel construction in common use: %$ -ut.and.co&er tunnel, constructed in a shallow trench and then co&ered o&er$ 2$ ored tunnel, constructed in situ, without remo&ing the ground ao&e$ #hey are usually of circular or horseshoe cross.section$ 3$ Immersed tue tunnel, sun( into a ody of water and layed on or uried )ust under its ed$ #he tunnel near the Nicoll Highway elongs to the cut and co&er type$ #o e speci/c, ottom up method was used to construct that tunnel$
http://www.railsystem.net/cut-an#-co$er/
http://www.railsystem.net/cut-an#-co$er/
enerally, to construct a cut and co&er tunnel using ottom.up method, we /rst install temporary e'ca&ation support walls, such as soldier pile and lagging, slurry walls or sheet piling, followed y dewatering within the trench if re1uired$ e then e'ca&ate and install temporary wall support elements such as struts or tie ac(s$ Ne't we start constructing the oor of the tunnel, walls and then the roof$ aterproo/ng is applied as re1uired$ astly, ac(/lling is done and we restore the ground surface$ #he tunnel structure is a it more complicated than the schematic description shown efore$ It has more struts and piles to support the tunnel$
http://failures.wikispaces.com/Nicoll+Highway+-+Subway+Tunnel+Collapse
As shown in the picture is the typical cross section of the failed tunnel section$ #he diaphragm walls were 0$56% m thic( and were aout 4064+ m deep$ alls e'tended aout %63 m into the sti*er strata 7the ol d allu&ium layer8$ idth of the tunnel is aout %9620 m and width of each diaphragm wall panel was m$ #here were two layers of interloc(ing )et grout piles 7;<8$ #he upper layer of the ;< was %$+ m thic( and was temporary and the lower layer of the ;< was 2$+ m thic( and formed the ase of the tunnel$ ;< layers were uilt to minimi=e the deection of the walls while tunnels was eing e'ca&ated$ ored piles were constructed to support the rail o'es$ >'ca&ation was supported y a system of steel (ing post and %0 le&els of struts placed at 4m center to center$ As the e'ca&ation progressed the struts were constructed and efore the construction of the %0th le&el of strut the temporary layer of the )et gout pile was remo&ed$
"easons for -ollapse %$ #he main reason for the collapse is the erroneous soil analysis which leads to under design of the diaphragm walls$ #he use of ?!ethod A@ or ?>*ecti&e tress !ethod@ of
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http://www.hssmge.gr/H%&HT'The()*Nicoll()*Highway()*Collapse.p#f
ithout these splays, the load e'erted on the waling eam from the wall was much larger than intended$ Instead, they sustituted -. channel steel sections with -.channel sti*eners to reinforce the strut.waler connections which later uc(led from the strain$ A ris( analysis which dictated the need for reassessing this aspect of the design in light of uc(ling oser&ed at the sti*eners, was ignored to (eep costs down$ #he collapse e&entually occurred when the load e'erted on the ninth of the ten le&els of the entire racing system e'ceeded the capacity of the strut.waler connections there, causing a phenomenon called ?forced sway failure@ C a &iolent, rapid deformation of the connections which hastened the collapse$ #he remaining le&els of racing could not withstand the increased load, and suse1uently failed as well$ 3$ esides, lac( of toe in depth of the diaphragm wall into competent strata was also found to e a factor that caused the collapse$ In the in&estigations following the collapse it was found that certain parts of the tunnel went through a uried &alley of old allu&ium under the layers of marine clays$ In&estigation re&ealed that the upper portion of the old allu&ium &alley consisted of sands, silts and clay with organic material with &ery low <# 7tandard ngineer and re&iewed y an Accredited -hec(er$ or(s going deeper than m must also e designed and chec(ed y a ngineer and an Accredited -hec(er who are specialists in eotechnical >ngineering$ eotechnical specialists are also re1uired for tunnels greater than 2m in diameter, underground structures greater m and comple' foundations of uildings more than 30 storey high$ #here ha&e een changes too with the industry le&el$ hen it comes to uilding for e'ample, itEs the de&eloperEs responsiility to hire a sucontractor to measure ground mo&ement or the tilting of uildings$ As ingapore continues to uild underground, especially in a densely uilt en&ironment, it is &ery important to ensure the e'ca&ation wor(s are carried safely$
