Lee Tunnel Project

Diaphragm Walls

London, U.K.

Scope of Works: Beckton Overflow Shaft: 1500mm thick diaphragm wall shaft 89m deep approximately 23m internal diameter pre liner wall. 79m deep excavation. Beckton Connection Shaft: 1500mm thick diaphragm wall shaft 92m deep approximately 28m internal diameter pre liner wall. 81m deep excavation Tideway Pumping Shaft: 1800mm thick diaphragm wall shaft 98m deep approximately 40m internal diameter pre liner wall. 82.5m deep excavation Abbey Mills Station Shaft: F 1200mm thick diaphragm wall shaft 84m deep approximately 28m internal diameter pre liner wall. 72m deep excavation. Ab-Abbey Mills Station Shaft:G 1500mm thick diaphragm wall shaft 86m deep approximately 28m internal diameter pre liner wall. 72m deep excavation. Beckton Overflow Shaft Grouting Works: Curtain Grouting - 83nr holes to 11m depth below the dia-phragm wall through reservation tubes Break Out Grout Block - 16m x 8m x 16m block, compromising of 148 boreholes at 1.5m c/c to a depth of 76m. In total 71276m3 C50/60 concrete and 5143 tonnes of rein-forcement steel installed including glass fibre in TBM portals.

INTRODUCTION:

The Thames Tideway Tunnels programme is a key component

of the upgrade of London’s combined sewer system. The first

section to be constructed is the Lee Tunnel between the

Beckton Sewage Treatment Works and the Abbey Mills

Pumping Station.

This comprised the construction of five shafts and a 7.2m

internal diameter tunnel running 6.9km from Abbey Mills to

Beckton. The shafts are some of the largest ever constructed in

Europe and range from 20 to 38m finished internal diameter

with diaphragm wall depths from 84 to 98m. Each shaft is

formed from a reinforced concrete diaphragm wall with a

secondary fibre-reinforced concrete lining and conventionally

reinforced concrete base slab.

MVB, a joint venture of Morgan Sindall, Vinci Construction

Grands Projets & Bachy Soletanche delivered the Lee Tunnel

project for Thames Water. They worked with Thames Water

and their Project Management Team in a collaborative manner

under an NEC contract with Mott MacDonald and Morgan

Sindall Underground Professional Services as designers.

GEOLOGY AND HYDROGEOLOGY

The stratigraphy was similar to that encountered under much of

east London, comprising a sequence of superficial deposits

overlying Tertiary clays, sands and chalk. The chalk (weak

limestone of 3 to 6 MPa) was encountered over approximately

50% of each shaft towards the base and was of grade A. The

degree of fracturing was variable, but greater near the Thanet

Sand interface. Bands and nodules of extremely strong flint

were encountered.

DIAPHRAGM WALLS DESIGN :

The design followed EC2 & EC7 and had to consider an

assessment of chalk stiffness, high hoop stresses, large

multiple openings and non-axisymmetric loadings. A subgrade

reaction programme “Paroi 2”, developed by BSL, was used for

the design. Plaxis version 9 was also used to undertake

axisymmetric finite element calculations and a Mohr Coulomb

soil model without strain hardening used.

A608

Five circular diaphragm wall shafts up to 40m in diame-ter with walls up to 98m deep and 1800mm thick

CLIENT: Thames Water

MAIN CONTRACTOR: Morgan Vinci Bachy JV

MAIN CONTRACTOR’S DESIGNERS: Mott MacDonald & Morgan Sindall Underground

Professional Services (UnPS) Ltd

www.bacsol.co.uk

DURATION OF WORKS: 20 months

Fig. 1: Hydrofraise Excavation Tool Pumping & Connection Shafts

A608

Diaphragm Wall Panel Excavation:

The diaphragm wall panels were

excavated using the Hydrofraise unit

(reverse circulation milling machine) as a

series of overlapping rectangular panels

forming a faceted circle. Primary panels

were up to 7.2m wide comprising 3 “bites”.

Secondary single bite panels overlapped

and cut into the concrete of the primaries.

Instrumentation informed the operator of

the plan position, inclination and twist of

the cutter body during excavation.

Hydraulic rams and pressure plates were

fitted to the cutter bodies and jacked

against the side of the panels to change

the inclination/orientation of the cutter to

achieve the required tolerances. A Koden

ultrasonic echo sensor device was also

used to survey the position of the panel

once complete. Results were

electronically transferred into 3D CAD

producing an as-built of the excavation.

This acted as a calibration of the onboard

instrumentation and provided confidence

in its accuracy. The 3D surveys provided

confidence to the excavation team that

there would not be any excess inflows of

soil and water and the verticality observed

achieved and in many cases exceeded

the specification of 1 in 300 for all panels.

Cage Design & Placement:

Each diaphragm wall panel contained

reinforcement over its full depth. In the

primary panels three separate cages were

placed and, since 22.5m was the

maximum cage length which could be

transported to site, a three-bite primary

panel required 15 separate pieces of

reinforcing cage to be installed. Cages

were connected using threaded couplers.

The cages underwent temporary works

design checks for safe handling, lifting,

splicing and placing. The shape of the

overlap between the panels was

trapezoidal. To even up the shape of the

overcuts, polystyrene panels were

connected to the outside of the primary

panel cages and the cages were spaced

apart using steel leaf springs so that the

polystyrene was removed during the

excavation of the secondary panels.

Panel Concreting:

Concrete was supplied from an onsite

batching plant.

Placement rates for concrete pours up to

1300m3 varied from 35 to 100 m3/hr

depending on panel size and location. The

high strength concrete required a complex

mix design in order that the required

workability was maintained throughout.

Completed Shaft Excavation:

www.bacsol.co.uk

Sustainability √ All spoil from the shafts was removed by marine transport taking around 6000 lorry movements off the roads from the diaphragm walling alone. √ Rather than tankering waste bentonite

offsite the desanding plant included equip-ment capable of reverting it back to its constituent parts so that the water could be re-used or put into the sewage works and the solid powder taken away with the other spoil.

√ A high level of cement replacement by

GGBS was used in the high strength C50/60 concrete.

√ The project was registered with Consid-

erate Contractors Scheme achieving scores of up to 38/40 and liaison meetings took place with local residents.

Fig. 3: Hydrofraise Instrumentation Output

Safety √ Due to the width and depth of the panels a bespoke system of panel covers & edge protec-tion was developed by the site team for all the various stages of construction includ-ing excavation, caging and concreting.

Fig. 2: Close up of Hydrofraise Cutter Drums

Fig. 4: Example 3D Koden Output

Fig. 5: Primary Panel Cage Installation

Fig. 6: Triple Bite Primary Panel Concreting

Fig. 7: Abbey Mills Shaft Excavation