A Unique Metro Accident in Brazil Caused by Multiple Factors 1
MAIN CAUSES OF ACCIDENT Ridge of jointed rock exactly along cavern roof Ridge of rock missed by drilling due to low spot Weathering of sides of ridge of rock preventing arching above cavern roof Discontinuity under Rua Capri (favourable orientation in normal circumstances) Water cracked sewage pipe/flowing after collapse Unusually high precipitation in December 2006 Undetected clay below and behind part of left wall Coincidences of location and time (also for victims) 2
Location (next to 7 traffic lanes and railway) 3
Eleven boreholes around shaft and eastern station cavern Exceeds international norm: L borehole / L tunnel by a factor of 2 to 4 4
WHAT WAS EXPECTED ON AVERAGE CONCERNING ROCK COVER 5
Borehole SM 8704 drilled near centre of (future) station cavern. Rock encountered at 18 m depth, at elevation 706m. (3 m above cavern roof) Low rock cover confirmed same as mean of five holes nearest cavern (see next two holes also!) 6
Note consistent 17 to 18 m of soil and saprolite in closest boreholes 8702 and 8703 7
EXTRAORDINARY REALITY: SUB SURFACE RIDGE OF ROCK UNDETECTED (grossly simplified here) Most of collapsed rock in centre of cavern fell 10 m, reaching elevation 704 707 m, i.e. 1 to 4m above the (original) cavern arch. (Rock elevation contrast omitted from public institution report) 8
SCHEMATIC OF WHY RIDGE of ROCK WAS MISSED WHEN DRILLING (drawings of fallen top 5 6 m omitted from public institution report) 9
SOME NECESSARY CHAINAGES FF marks the eastern boundary a steep discontinuity 10
Principal components of the collapse 11
View of cavern floor 15 months later 12
Some details of the complex geology 13
Example of facemapping in cavern Notably RMRparameter recordings, and joint set descriptions including clay in principal and secondary joints 14
ROCK QUALITY LOGGING (Six of the face logs) The core of better quality rock was indistinct close to the shaft Increased volume of good rock in direction of Rua Capri (towards the East) 15
Lattice girder count at two of the mapped sections RMR rock class values of the core (B) and the surrounding rock (A) on right. Tassometer deformations were 15 mm and 21 mm at these locations. 16
Increased volume of Class III rock as Rua Capri was approached. (Reduced grout take consistent with this). (String model, 8 th Feb 2007) 17
Independent Q logging, following collapse; range 0.1 to 4, similar to cavern logging, and similar to IPT logging for São Paulo Metrô from 1996 1997. 18
Q logging of 7 nearest holes, see numbers 1 to 7 with Q calculation for 5 holes along station cavern 19
CROSS HOLE VELOCITY : SOLUTION with LESS NOISE AT INSTRUMENT HORIZONS. ONLY JUSTIFIED WHERE PROBLEMS ANTICIPATED DRILLING AT PINHEIROS DID NOT GIVE EVIDENCE OF PROBLEMS. (Vp/km gradient = 1/s = 2.0/0.01 = 200 (= exceptionally high). 20
Approximate correspondence between IPT cross hole seismic at the Marginal, and Q logging results for five nearest boreholes 21
HEAVY PRIMARY SUPPORT WAS USED FOR THE STATION ARCH Cambota and recessed elephant feet support the top heading 22
Due to assumed low rock cover (3 m): lattice girders @ 0.85 m c/c + 35 cm S(fr) for temporary support of top heading (A cheaper B+S(fr) design was rejected) 23
One of the most robust support methods from the Q system: RRS (rib reinforced shotcrete) would also have failed under the Pinheiros ridge of rock loads and there was assumed to be insufficient rock cover for efficient bolt action 24
Possible clue concerning Pinheiros collapse geometry, from distant IPT seismic profile: with Rua Capri, houses, and boundary discontinuity superimposed. (Note lack of velocities due to problems with noise, differentially weathered steep structures). 25
An imagined sequence of increasing sub surface differential weathering 26
An advanced stage of weathering: a wedge shaped ridge surrounded by clay 27
Relic joint structures in overlying saprolite assumed to have contributed to loading 28
Core stone phenomena in massive granites (drawing from Linton, 1955 more jointing deeper saprolite) 29
POST COLLAPSE EXCAVATION REVEALS LIKELY COLLAPSE MECHANISMS 30
Nominal cota 704 m on either side of the cavern. Fallen materials towards centre 31
Cota 705 707 m, estaca 7.085 m. Example of fallen core material. Has fallen 9 to10 m but still has a top elevation of 706.5 m (approx.) Previously at elevation 716, or 10 m above assumed (drilling determined) levels. 32
The smooth and weathered appearance of the edge of the fallen rock 33
Folded lattice girders due to footing failure next to left wall (early chainage only). Pre grouting tubing holds block together. 34
Evidence for elephant footing failure, due to inwards displacement of wall S(mr). 35
Crushed excavator Damage is indirect evidence of the many thousands of tons of fallen rock 36
Folded arch/wall/arch support 37
TENSILE FAILURE OF CAMBOTA STEEL (along Abril: left side of cavern) 38
Some investigations of the potential failure mechanisms using numerical models 1. Elephant footing failure (cracking of rock) using FRACOD (Dr. Baotang Shen, Australia) 2. Overall cavern failure modelling (ridge of rock loading) using UDEC (Dr. Stavros Bandis, Greece) 39
FRACOD (BEM fracture mechanics) modelling of possible rock cracking below elephant-feet (from 100, 250 or 500 tons/m loading) Examples of cracking WITHOUT and WITH joints present 40
FRACOD results: three loading levels (2.5, 6 and 12 MPa) three UCS assumptions (16, 10 and 5 MPa) three deformation moduli assumptions (8, 5, 2.5 GPa) 41
Modelled footing displacements. Thinnest lines for the weakest rock (5MPa) Thickest lines for the strongest rock (16 MPa) 42
Input data suggestions to numerical modellers: Dr. Shen, Dr. Bandis 43
The UDEC model on the left did not cause collapse: the increasingly thick wedge of weathered material (red colour) seen on the right was required. 44
Preliminary modelling without rock support 45
Recognisable maximum deformation of 21 mm. Heavy loading of support is (of course) modelled. 46
Axial forces (blue fence ) and bending moments (red fence ). Deformation only 26 mm, until plastic hinges were softened. ( Elephant footing failure also commencing) 47
N M interaction diagram. Points beyond the red curve are treated as plastic hinges and are softened. General failure commences. 48
Ultimate failure of support: deformation vectors 49
This block diagram now shows loss of contact on left of ridge 50
Finally the assumed triggering mechanism Water and water pressure from cracked pipe Located at rear discontinuity, beneath R. Capri Change of cross section just before discontinuity D=1000mm 700mm: A1/A2 = 2.0 Assumed source of water pressure rise Water flowing from cracked pipe after collapse (Unusually high rainfall in December 2006) Clay deformation softening pore pressure effect? 51
700 mm diameter sewage pipe presumed cracked by down dip shearing, when the cavern arch approached and passed 20 m below this discontinuity. 52
The fractured pipe that may have supplied the various adverse geological structures with water and water under pressure. (We now know that FF was not a fault, just a major discontinuity) 53
The storm drain that fate determined should cross the geological discontinuity surface (red) at the rear 54
CONCLUSIONS and RECOMMENDATIONS 1. The physical impossibility of performing necessary but unreasonable levels of sub urban site investigation will prevent the execution of shallow city metro projects, unless a limited level of risk is accepted. 2. Elimination of risk would involve socially and commercially unacceptable degrees of disturbance beneath too many roads and buildings. 3. Deeper construction from the underground, as practiced of necessity in many cities lacking suitable geology, could be a future, cheaper, and safer solution for São Paulo, and would also result in less settlement damage. 4. Rock conditions for tunnelling are invariably more favourable at depth, whereas the near surface is more unpredictable due to the effects of deep weathering and locally reduced rock quality. 5. It is too optimistic to expect almost zero risk just because of numerous prior projects in a city, or because of the insight of talented geologists. 55
END 56