Upload
vankiet
View
222
Download
0
Embed Size (px)
Citation preview
page : 1
Treatment of C.D.G. with slurry shield TBM- Case
study in Hong Kong
Yves Chouanard Commercial & Marketing Director
MS sas [email protected]
ABSTRACT:
Slurry TBMs have been chosen in decomposed granite alignments so as to ensure safe boring conditions, both in Hong-Kong and in Singapore. Despite the fact that, in both cases, the GIBR gives a very wide variety of grain-sizes distributions, it is of major importance for the contractor at bidding stage to come to a sensible evaluation of the quantities of spoils which will be recovered by the Separation Plant, especially the extra-fines portion - 63µm. The aim of this presentation is to compare the figures extrapolated from the GIBR with the ones obtained on the sites, and to adjust the simulations tools used for the sizing of the different components of the Separation Plant by the STP supplier, especially the excess mud treatment section.
1. PRESENTATION OF THE PROJECTS : 1.1. MTR WIL 703 :
Hong-Kong MTR 703
These two tunnels (boring Ø 6.35m, 770m + 490m long) are part of the West Island Line (Sheung Wan to Sai Ying Pun), situated in a very congested urban area. Contract was awarded by MTR Corporation to Dragages, Maeda & Bachy Solétanche Joint Venture. Its first ring was bored in September 2011, and the tunnel was completed in november 2012. The geological profile was showing about 31% of the drive in CDG, 42% in mixed face, and 21% in rock, with a small section representing 6% in Alluvium.
1.2. XLR 820/821:
Hong-Kong MTR 820/821 – 2x 1 800 m3/h
These twin tubes, 3.5 + 0.75 km long tunnels on Kowloon side (boring Ø 9.32m) belong to Guangzhou-Shenzhen-Hong Kong Express Rail Project XLR. Contract has been awarded by MTR Corporation to Dragages Bouygues Joint Venture. Its first ring was bored in October 2011, and the tunnels are due to be completed in 2015. The geological profile was showing about 52% of the drive in CDG, 14% in mixed face, and 35% in rock.
2. NTERPRETATION OF THE GEOLOGICAL FILE One of the difficulties in the interpretation of the GIBR is the extrapolation of two critical values of the CDG section, - its dry density (γd), and - its % of extra-fines, because of their direct influence on the sizing of the STP excess mud treatment section. The GIBR was giving the following information in terms of particle size distribution :
which is a very wide range… from 8 to 80% !
ð an average value of 30% was chosen for MTR 703 and 32% for XLR 820/821.
The in-situ densities (γh) were also given within a wide range; from :
ð an average value of 19kN/m3 was chosen for MTR 703 and 2 for XLR 820/821.
3. TOOLS TO SIMULATE THE PROCESS & THE VOLUMES OF SPOILS GENERATED:
The "Volume & Mass Balance" calculation sheets, or "VMB", have been developed by MS to help in the sizing of a Slurry Treatment Plant with regards to the geology of one particular tunnel project, and to the technical choices of the contractor in terms of instant boring speed and daily progress target. It includes the following sections :
• a summary of the geological profile of the tunnel, • the VMB calculations in each geologies, • a summary showing the critical values given by the VMB in these different
geologies, values which will become sizing factors, • a calculation of the spoils volumes and of the consumables for the entire
tunnel.
4. SUMMARY OF RESULTS OBTAINED ON XLR 820 JOB SITE :
Data GIBR Results from XLR 820
Soils bulk density
γh
Bedrock 2.6 2.6
Mixed face 2.2 2,2
Soft CDG 1.9 2
< 63µm Soft CDG 32% (extrapolation) 40%
Soil bulk density :
CDG until ring 250, then mixed face, and full-face granite for last 50 rings.
mostly in CDG. % of fines -63µm : First tube :
The average value of fines < 63µm measured on site in CDG is close to 25%, but the conglomerates of clay recovered with the sand/or the gravels, which represents
another 5%, should be added, which means that the total is effectively close to 32% on this drive. % of fines in the CDG : Second tube :
On the second tube, the average value of fines < 63µm measured on site is close to 35%, with peaks on the last 50 rings up to 39%, but again, the conglomerates of fines recovered with the sand/or with the gravels should be added, what represents another 5%, i.e. the total is this time close to 40% with peaks up to 45%. These two examples demonstrate the variability of the ground, and especially the % of fines. Abrasivity of the CDG : One major issue in the treatment of CDG is the proper desanding of the slurry, while resisting to it very high abrasiveness : in these CDG, 56% of the rock is considered as "Very Abrasive" (Cerchar Abrasiveness Index 2 to 4), and 41% "Extremely Abrasive" (CAI 4 to 6).
Rock Abrasiveness
Percentage Distribution Class (after Cerchar)
CAI (Cerchar Abrasiveness Index)
Not Very < 0.5 0%
Slightly 0.5 to 1 0%
Medium to Abrasive 1 to 2 2.6%
Very 2 to 4 56.4%
Extremely 4 to 6 41%
Quartzitic > 6 0%
A proper and "integral" desanding and desilting of the slurry is therefore required, and a special care should be brought to the wear protection.
Quality of the slurry MTR 703 : low cover, urban area lead to high quality of slurry in terms of rheologic properties :
Density YV Filtrates
CDG 1,147 7,5 Pa 52 mL
Rock 1,094 1,9 Pa 147 mL
Mixte 1,138 2,7 Pa 49 mL
XLR 820 : less constraints, but natural viscosity due to the activity of the fines (up to 6 Pa, without bentonite addition), bigger diameter and other constraints such as the cement contamination of the slurry.
Sand content
(elutriometer) Density YV Filtrates Ring / Day
Average 1,5% 1,136 4 Pa 80 mL 8 r/d
Max 3,5% 1,23 6 Pa 170 mL 12 r/d
Min 0,5% 1,03 1 Pa 60 mL 6 r/d
In both cases, the MS' patented "Slurry Management System" enable to manage independently the slurry rheologic properties, in terms of density and viscosity.
Treatment of extra-fines 40 to 45% fines < 65µm in the ground, means for a Ø 9.3m tunnel 120 to 140 m3 of excess mud per bored meter ending in the excess mud tank. For two tunnels bored concurrently with a total daily progress target of 40m, it is 4,800 to 5,600 m3 of sludge to be processes daily. Only Filter-presses enable a transformation of this liquid sludge volume into solid cakes, with an average moisture content of 27% (water weight on total cake weight) :
Filter-Press cakes
Filter-press productivity in CDG :
Bentonite
concentration in excess mud
Filtrates (API-30 min)
Average Yield Value
Lime addition
Excess mud average density
FP15-C80-M16 production
in m3/h of excess mud
in t/h of solids
XLR 820 0 kg/m3 ~80 ml 3 Pa
(natural viscosity of
fines)
2% (+coagulant)
1.207 30 - 36 10 - 12 t/h
MTR 703 0 to 40 kg/m3 ~50 ml 6~7.5 Pa 5% 1.167 30 - 37 8 - 10 t/h
Total spoils volume at the end of the tunnels : Taking the example of XLR 820/821, the original assumption of 30% fines <63µm in the CDG lead to a total of 175,000 tones of solids < 63µm after completion of the two tunnels, and to the following volumes: Excess mud
volume Excess mud
density Reduction
ratio
Original volume in excess mud tank : 600,000 m3 1,182
In case of treatment by decanter centrifuge 325,000 m3 1,335 / 1.8
After treatment by Filter-Press 132,500 m3 1,822 / 4.5
Disposal of soils by barges / storage on ground
5. COMPARISON BETWEEN HONG-KONG & SINGAPORE CDGS :
COMPLETELY DECOMPOSED GRANITE COMPARISON HONG KONG vs SINGAPORE
Parameter Hong Kong Singapore
ROCK
Names of granites Kowloon and Sha Tin granites
Bukit Timah and Jurong granites
In situ density (γh) 2,5 to 2,6 2,4 to 2,6
Dry density (γd) 2,4 to 2,5 2,25 to 2,5
Permeability 10-7 to 10-9 m/s 10-6 to 10-9 m/s
Abrasivity Very abrasive to quartzitic Very abrasive to quartzitic
CDG grades V&VI
Densities Min Medium Max Min Medium Max
In situ density (γh) 1,7 2 2,1 1,7 2 2,1
Dry density (γd) 1,2 1,6 1,8 1,2 1,6 1,8
Grain size distribution Min Medium Max Min Medium Max
% > 6 mm 0,0% 10,0% 25,0% 0,0% 5,0% 15,0%
Sand (63 µm - 6 mm) 35,0% 50,0% 90,0% 30,0% 55,0% 75,0%
Extra-fines (< 63 µm) 10,0% 40,0% 60,0% 25,0% 40,0% 55,0%
Liquid limit 30 - 70% 40 - 80%
Plastic limit 10 - 45% 10 - 35%
Plastic index 10 - 50% 10 - 60%
Permeability 10-5 to 10-8 m/s 10-6 to 10-8 m/s
6. CONCLUSION :
Boring with a Slurry Shield TBM in Completely Decomposed Granite is challenging as it requires:
• a perfect separation of the excavated solids, • a mastery of the rheologic properties of the slurry, • a proper solution to treat the big volume of extra-fines spoils.
The experience gained in Hong Kong could be perfectly applicable to Singapore geology and environment.