A Unique Metro Accident in Brazil Caused by Multiple Factors

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 face‐
mapping in cavern Notably RMR‐
parameter 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, 8th 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
•
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•
•
•
•
•
•
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
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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
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