TREATMENT OF CHALK SPOILS FROM THE LEE

and CROSSRAIL C 310 THAMES TUNNELS Yves Chouanard, Commercial Director at MS

MS sas www.m-s.fr

ABSTRACT:

The Thames Water Lee tunnel, with its 8.88m diameter, 6,904m length and its geology of more than 98% in chalk, is a real challenge in terms of spoil treatment. The total volume of the chalk slurry after separation of the flints (5 to 25%, average 15%) represents more than 1,200,000m3 of liquid slurry. In such geology, where 95% of the solids could be transformed into extra-fines, well below the cut off point for cyclones or centrifuges, several method have been tried in the past to facilitate the disposal of the spoils, but none met the requirements of the contractor, both in terms of costs and final result. This presentation will describe the methods and results achieved by the Morgan Sindall Vinci Construction Grands Projets Bachy Soletanche Joint Venture MVB in reaching a consistency of more than 75% solids by weight, and thus drastically reducing the volume of spoils and easing their disposal.

The presentation will also give an overview of the results obtained on another tunnel site in similar geology, the Crossrail Thames Tunnel, by Hochtief Murphy Joint Venture HMJV. After their success in the award winning CTRL 320 tunnel, where the discharge of the centrifuges was mixed with cement before placement, HMJV has accepted to reconsider its excess mud treatment method, as they has been convinced by the arguments in favour of filter-presses. This enable them to experience a different approach to the slurry management.

SUMMARY : THE PROJECTS THE GEOLOGY THE PRELIMINARY LABORATORY TESTS : dilution and pressing tests THE TECHNICAL CHALLENGES THE SOLUTIONS IMPLEMENTED:

Adaptability to Chalk paste with flints and to clayey Thanet sands Slurry management Excess mud treatment

CONCLUSION

 

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PRESENTATION OF THE PROJECTS : LEE TUNNEL : The Lee Tunnel is part of a scheme to store and send to treatment the sewage and rainwater from London, which presently overflows into Thames River. The 6.9 km long Lee Tunnel runs underwater and underground through East London, to carry sewage from London's largest combined sewer overflow from the Abbey Mills Pumping Station in Stratford to the Beckton Sewage Treatment Works (STW) in Newham. Tunnel Work started in February 2012, and the entire project is expected to be completed in 2015.

CROSSRAIL C 310 : The 2.6 km long twin tubes Thames Tunnel is the only Crossrail route which crosses the river Thames. It is also the only slurry shield TBM of this project, because of the chalk and Thanet sand, elsewhere EPB machines run in London clay, sand and gravel. Boring started in October 2012, and is due to be completed early 2014.

 

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GEOLOGY : The Lee tunnel runs predominantly in Seaford Chalk formation, with a small section of overlaying fine clayey Thanet sands. This Chalk consists of white, sometimes light grey, low to medium dense, weak to moderately weak chalk, with layers of flints ranging from 5 to 25%, with an expected average value of 15%. A bulk density of 2 kN/m3 has been assumed for the design. At the time of the sizing of the separation plant, we pointed out that there were two questions which were not fully answered in the GIBR and which were of major importance for the sizing of the Separation Plant : one was the ability of the chalk to go into suspension, and the other was the filterability of the resulting slurry. We have therefore received 2 tons of samples that we tested in our laboratory.

The Crossrail Thames Tunnel is in the same geology : only the proportion of Thanet sand in the drive is slightly greater :

 

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PRELIMINARY WORK AND LABORATORY TESTS : 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 also helps to study the consequences of any modification of a parameter, such as density, viscosity, or instant and daily boring progress target in terms of sizing of the components or in terms of consumables. It includes the following sections : • a summary of the geological profile of the tunnel, • the VMB calculations for each geology, • a summary showing the critical values given by the VMB in these different geologies - values which will become sizing criteria, • a calculation of the spoils volumes and of the consumables for the entire tunnel.

These calculation sheets have already given a number of key parameters for the sizing of the STP at bidding stage, but certain parameters needed confirmation.

 

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We received a 500 kg big bag of samples in September 2010 and another in April 2011, both from from a nearby quarry, and we performed another set of tests in April 2012, after start of the work, with samples from Abbey Mills, and Pumping shaft. The aims of the tests were :

• to check the % of chalk going into suspension, as a function of the attrition time in a concrete mixer which simulates the transport duration,

• to measure the liquid and plastic limits,

• to carry out rheologic tests : VP (plastic viscosity), VA (apparent viscosity), Yv (Yield Value), Filtrates API,

 

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• to perform pressing tests, with chamber and membrane plates of different thicknesses, and with different additions : lime, polymer, and other "drying" products.

The test results confirm different assumptions made by calculation sheets and lead to the following choices after discussion with the Contractor :

to maintain a safety margin, it was decided to consider for the sizing - the possibility that 100% of the chalk will come into solution, and - only 5% of flints,

to choose 40mm cakes, as tests have delivered a final water content of 25 to 30%, with an addition of 0.5 to 1% of lime (many other products were tested, but lime demonstrates a higher performance). In Chalk, chamber plates have been chosen to maximize the useful volume, and a minimum number of filter-presses equipped with membrane plates will be kept for the treatment of the bentonitic mud recovered in Thanet sands,

to minimize the volume of excess slurry and to optimize the cycle time of the filter-presses, a high value of return slurry density was chosen, originally up to a maximum of 1.42, but finally reduced to 1.30 due to the viscosity of the real mined chalk.

 

 

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THE TECHNICAL CHALLENGES : Flexibility of the STP : The two geologies encountered not only lead to a different behaviour of the slurry, but also require different rheologic properties, which demand a very flexible design of the separation plant, in order to be able to handle :

• high density chalk pulp, containing very abrasive flints, where the main concern was a control of the density and an optimization of the spoils volume,

• clayey Thanet sand, possibly coming in sticky conglomerates, where the concern was the stability of the face.

A first stage with a revolving trommel (cut size 6mm) was chosen, as well as a double cycloning stage, with a lower cut at 45µm (d50). Different wear protection methods have been tested - rubber, wear resistant steel, ceramic linings (tiles or moulded parts). Slurry management : With an instant boring speed of 8 to 10 cm/minute, the slurry management in terms of density becomes a critical issue, and the MS patented "slurry management" proved its efficiency.

 

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If we take the example of Crossrail C310 working conditions, we can summarize the specification of the slurry as follows :

In Thanet Sand :

Density Plastic Viscosity (mPa.s)

Baroïd Filtrates (ml)

Baroïd Cake thickness (mm)

pH Value

Min Max Average Min Max Average Min Max Average Min Max Average Min Max

1.17 1.25 23.00 16.79 62.50 46 33 67 3 1 5 8.5 7.86 10.87

In Chalk :

Density Plastic Viscosity (mPa.s)

Filtrates (ml) Baroïd Cake thickness (mm)

pH Value

Min Max Average Min Max Average Min Max Average Min Max Average Min Max

1.21 1.25 6.78 5.94 8.51 197 160 226 14 10 18 11.01 10 12.15

In addition, the other possibilities given by this slurry management, such as the "hyperbaric intervention mode" also facilitate the maintenance of the TBM cutting tools :

 

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Excess mud treatment :

This is the major challenge of this Tunnel Work, which becomes even more challenging due to the fact that such a very small difference in moisture content gives a huge difference in the chalk spoil :

Water content = 26.6% (ó humidity ratio = 21%)

Water content = 35,1% (ó humidity ratio = 26%)

Lee Tunnel : with an average return density of 1.21 and an excess mud density of 1.253 , we have 245 m3 of excess mud for one ring, of which 140 tons of solids and 290 m3 of water. With the current weekly progress of 200 m, it represents 29,000 m3 of excess mud to be processed !

On the entire tunnel drive, we should recover 624,400 tons of solids <63µm, which gives the following comparison between the different solutions :

- Decanter centrifuge (based on the figures recovered from CTRL 320*), and

- Filter-presses (present results obtained on this site) :

Excess mud Decanter centrifuge (*) Filter-presses

Solids 624,400 tons 624,400 tons 624,400 tons

Water 1,300,000 m3 275,000 m3 200,000 m3

Pulp/Cake 1,535,000 m3 510,000 m3 435,900 m3

∆ = -75,000 m3

Water content 208% 44% 32%

Consistency Liquid Pasty Solid cake

(*) Warren Phear Schultheis Gregg 2003 : "Treatment and Placement of Chalk Spoil from the CTRL Thames Tunnel".

In addition to the reduction of the total volume of the spoil, and to the drastic change in its consistency, we should notice another big savings, which is the possibility to re-use the filtrate water from the Filter-presses for dilution of the slurry, after correction of the pH, instead flocculants used with centrifuges would make it difficult.

 

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Crossrail C310 : with the same average return density of 1.21 and an excess mud density of 1.25, we have 180 m3 of excess mud for one ring, of which 73 tons of solids and 152 m3 of water. On the entire tunnel drive, we should recover about 108,000 tons of solids <63µm, which gives the following comparison between decanter centrifuge and Filter-presses (water content figures are slightly different from Lee Tunnel values, as they are average values of the different soil conditions and the proportion of chalk with regards to Thanet sand and mixed soils is different):

Excess mud Decanter centrifuge Filter-presses

Solids 108,000 tons 108,000 tons 108,000 tons

Water 225,000 m3 55,000 m3 39,000 m3

Pulp/Cake 266,000 m3 96,000 m3 80,000 m3

∆ = -16, 000 m3

Water content 208% 51% 36%

Consistency Liquid Pasty Solid cake

CONCLUSION :

The results obtained on these two sites have demonstrated :

• the advantages of a precise management of the slurry characteristics in real time, adjusting density & viscosity independently,

• the superiority of filter-presses for the treatment of Chalk spoils, in terms of final water content of the cakes, and also in terms of consistency.