Transportation of Excavated Material

The 6,000,000 T of material excavated by Crossrail is generated by the construction of:

• 36km of 7.1m OD tunnelling in Drives X, Y, Z and H using EPBs to tunnel mostly through over-consolidated clays and water bearing sands.
• 6km of 7.1m OD TBM tunnelling in Drive G using slurry machines to tunnel through a weak limestone (Chalk).
• There are significant lengths of SCL tunnel at 5 stations, at 2 crossovers and at the tunnel junction at Stepney Green where the line splits.
• There are significant box excavations at 6 Crossrail stations. There are 3 permanent shafts and many temporary shafts. There are 5 portals. Two Stations are not being constructed by Crossrail and these stations have their own excavated material disposal arrangements.

This excavated material is taken from the full range of geological materials present in London. These strata units and the types of construction they are found in are set out in Table 1.

The geotechnical properties of these materials that are most relevant to transportation are those that govern stability during transportation. These are moisture content, plasticity and undrained shear strength for the EPB, SCL, and box/shaft/portal excavations. Typical values of natural moisture content, undrained shear strength (Su) and liquid limit are given in Table 2.

London Clay & Lambeth Group clay in-situ properties at the tunnel horizon

• These units are mostly over-consolidated very stiff silty clays and clays of intermediate to high plasticity with in-situ moisture contents close to their plastic limits.

Lambeth Group sands and Thanet Sand in-situ properties

• These units are mostly fully saturated silty sands.

In-situ Chalk properties

• The Chalk encountered in slurry TBM drive (Drive H) is typically a medium density, weak to moderately weak fractured Limestone, comprising mostly silt sized particles of calcium carbonate with bands of gravel to boulder sized siliceous nodules known as ‘Flint’. Fracturing within the Chalk is typically found as two orthogonal sets and the fracture spacing ranges from closely spaced to medium spaced. The in-situ moisture content is normally about 30%. The Chalk is considered to be an aquifer, with the available water being held in open facture systems. The Chalk encountered by Crossrail is all below the water table.

Classification of Excavated Materials

The excavated material can be divided in to 4 classes according to how the excavation method alters the in-situ properties of the material. These classes are listed below from the greatest to the least induced alteration:

1. Slurry TBM excavated material
2. EPB TBM excavated material
3. Material excavated from SCL Tunnels.
4. Material excavated from Shafts, Boxes and Portals

	Geological Strata in stratigraphic order	Typical Thickness	EPB TBM Tunnels Drives X, Z and G	EPB TBM Tunnels Drive Y	Slurry TBM Tunnels Drive H	SCL Tunnels	Portals, Boxes and Shafts
Older Strata                                                                     Younger Strata	Alluvium: Soft Clays and organic soils.	<5m					Yes
	Fluvio-glacial Deposits: Sands and Gravels	<7m	Yes, but not in large volumes	Yes, but not in large volumes	Yes, but not in large volumes		Yes
	London Clay: Over-consolidated silty Clay	0 to 40m	Yes	Yes		Yes	Yes
	Harwich Formation and Lambeth Group: Over-consolidated Clay and water bearing Sands	20m	Yes, but not in large volumes	Yes		Yes	Yes, but not in large volumes
	Thanet Sands: Water bearing silty Sands	15m		Yes
	Chalk: Weak high moisture content Limestone	30m+			Yes

Table 1 – Geological materials excavated by Crossrail

 

TBM Slurry

For Drive H (Slurry TBMs) the output from the TBM is a slurry produced by adding water during tunnelling. Most of the slurry is derived from the Chalk. Close to the portals the slurry also contains the shallow overlying sands and gravels. In these areas bentonite and water were added during tunnelling.

After tunnelling and treatment the Chalk slurry typically comprises a Silt with a moisture content between 30% and 35%.

EPB arisings

For Drives X, Z and G (EPB TBMs) the excavated material is mostly London Clay. This material is conditioned at the TBM by the addition of foam based conditioners (water + air + conditioner). Dosage rates vary but typically involve the injection of fluid equivalent to 10% of the volume excavated for every segment ring in the tunnel.

For Drive Y (EPB TBMs) the excavated material is a mixture of London Clay, the underlying Lambeth Group clays and importantly water bearing sands from the Lambeth Group and Thanet Sands. The clays are conditioned at the TBM in a similar way to the Drives discussed above. Where the TBM has a full or near full face of saturated sand much smaller volumes of fluid are injected, these can be as low as 2% of the ring volume.

SCL arisings

Mostly London Clay with some limited volumes of underlying clays and sands from the Lambeth Group. As an excavated material both are mixed with excavated fibre reinforced sprayed concrete temporary lining fragments. The lumps or fragments of excavated geological materials are all broadly at their in-situ moisture contents and undrained strengths.

Box, shaft and portal arisings

Mostly London Clay with some limited volumes of over and underlying clays, sands and gravels. As with the SCL arisings the lumps or fragments excavated materials are all at or close to their in-situ moisture content and undrained strengths.

Most of the materials excavated from the tunnels, shaft and box excavations are over-consolidated silty clays (London Clay and Lambeth Group clays). A significant but minor constituent of the EPB TBM arisings is this clay mixed with water bearing sands.

This mixed material mostly arises on Drive Y where the geology is very varied and the two TBM’s typically run together with a chainage offset of several hundred metres. This means that it is normal for the two TBMs to be tunnelling in differing combinations of Clay and water bearing Sand. The conveyors from these two TBMs then combine the arisings from the two TBMs into a single supply of mixed material at a dock loading facility called Instone Wharf for onward transportation.

In this situation the Sands typically have an in-situ moisture content of >20%. The addition of this water to the arisings has a significant impact on the properties of those arisings. The effect of mixing these conditioned clays and wet sands on the conveyor system is to produce a very low strength, low plasticity and high moisture content sandy clay. The nature of this mixed material is such that the moisture content is usually very close to the much reduced liquid limit, and the low plasticity means the undrained strength of the material is very sensitive to small changes in the moisture content.

Typical properties of TBM arisings

Drives X, Z and Drive Y ‘clay only’ material

• Undrained Shear Strength: 20 to 30kPa
• Natural Moisture Content: 35% and Liquid Limit: 75%

Drive G

At the time of writing Drive G hadn’t begun, but it was expected to produce mostly ‘clay only’ material as above.

Drive Y mixed clay and sand material

• Undrained Shear Strength: 5 to 15kPa
• Natural Moisture Content: 20 to 30% and Liquid Limit: 25 to 35%

Typically this mixed Drive Y material is very close to behaving like a liquid and could potentially have insufficient strength to sustain slope angles of the height and inclination that might be found in the hold of a ship pitching and rolling in a rough sea.

Safe transportation – strength and moisture content limits

Transportation by River – Barges

The barges used by Crossrail are designed to carry liquids and as such have V shaped cross sections for their holds. These are known as Hopper Barges. As they are designed to carry liquids there are no moisture content or shear strength limits for arisings carried in their holds. However barges can only legally operate within defined limits within the River Thames. They cannot operate beyond these limits in what is defined as the open sea of the English Channel. This means they cannot be used to transport arisings to Wallasea Island.

Transportation by Sea – Ships

Transportation of material to Wallasea Island by ship involves leaving the limits of the River Thames and travelling via what is defined as the open sea, a journey of approximately 120km. Only ships can be used to transport material via the sea. Ships designed to carry liquids (those with V or W shaped cross sections for their holds) can carry arisings with any moisture content and /or undrained shear strength. Unfortunately ships like these that could also cope with the relatively shallow water at one of the Crossrail loading points (Instone Wharf) are not readily available.

The ships used by Crossrail have rectangular cross section holds. This raises the issue of the stability of the arisings in the ship’s hold and the consequent stability of the ship itself in rough seas. To address these risks there are limits set on the moisture content and undrained shear strengths of the materials that can be carried in these ships.

For sands and other materials that are not plastic, the moisture content limits for transportation are set via a series of tests which establish the moisture content at which the material liquefies in a set of specific test conditions. The limiting safe moisture content is known as the Transportable Moisture Limit (TML). These test and analysis procedures are set out in international maritime law (the International Maritime Solid Bulk Cargoes (IMSBC) Code^[2], which became mandatory on January 1, 2011 under the International Convention for the Safety of Life at Sea (SOLAS), 1974).

However these international maritime laws do currently not set out criteria for the safe carriage of wet plastic clay based materials, probably because this is an unusual cargo without any normal commercial benefit.

The liquefaction based test methods in the IMBSC Code do not work on clayey materials as these are not normally prone to liquefaction. As a result Crossrail, in conjunction with the UK Marine and Coastguard Agency (the MCA – the organisation responsible for implementing international maritime law in UK waters) established Crossrail specific processes for determining a TML for each load of clay transported by ship to Wallasea Island. This TML was based on applying a factor of safety to the liquid limit of the particular load of clay.

Crossrail also recognised that in some cases this TML alone could indicate a low plasticity clayey material was safe to ship when the excavated clayey material actually had a very low undrained strength and as such could be unstable in a ship’s hold in a rough sea. This situation arises where TBM arisings from a pair of TBMs tunnelling in different geology (clay and wet sand) could be mixed on the conveyor system to produce very wet low plasticity, low strength clays at the point of loading on to a ship. This is a particular issue for Drive Y where the addition of wet sand to the clay both increases the moisture content and decreases the liquid limit of the mixed material by reducing the proportion of clay. To address the issue of potentially unstable wet, weak mixed clay / sand arisings Crossrail carried out some simple slope stability analyses using:

• The hold dimensions of the ships bring used;
• Assumed excavated material shapes as tipped in the holds;
• A conservative view on the maximum roll angle the ship might experience in a rough sea.

These analyses were used to establish limiting undrained shear strengths for safe transportation of clayey material by sea to be used in conjunction with a TML. Whilst the TML varies with the plasticity of the spoil the minimum undrained shear strength limit was set at a uniform 20kPa. For clayey materials TML was defined as the liquid limit minus 0.15.

Transportation by Lorry

In the UK lorry drivers are responsible for assessing the safety and stability of their loads and have the power to refuse to carry anything they consider unsafe, like very wet, near slurry like material. In practice this has not been a significant issue for Crossrail. The slurry TBM contract has additional contractual requirement to reduce the slurry produced by the TBM to a moisture content of 35% or less before transportation and disposal.

Transportation by Train

There are no particular requirements that are more onerous than those set out above.

 

Transportation Strategy

Table 2 sets out the transportation strategy used for all the sources of excavated material.

		Land Transport & Destination			Marine Transport & Destination
Source	Type of arisings	Train to Northfleet interim stockpile site	Lorry to Tilbury interim stockpile site	Train/Lorry to other licensed waste recovery or disposal site.	Barge to other licensed waste recovery or disposal site.	Barges cannot be towed to Wallasea	Ship to Wallasea
Drive X	TBM Clay	Yes					Yes
Drive Y & G*	TBM Clay				Yes		Yes*
Drive Y & G	TBM Clay/Sand				Yes
Drive Z	All arisings			Yes
Drive H	Treated Chalk slurry			Yes
SCL tunnels	As dug clay and sand + SCL		Yes				Yes
Portals, Boxes and Shafts	As dug clay and sand	Yes**	Yes**				Yes

Table 2 – Transportation Strategy

*In practice all of Drive Y was barged to other licensed waste recovery or disposal sites

** Arisings from Paddington Box were trained to Northfleet and then shipped to Wallasea

Table3 summarises the changes in the relevant geotechnical properties imposed by the excavation methods for each of the significant geological materials encountered during construction and sets out the resulting transportation options.

	Typical in-situ properties	Typical post excavation properties	Ship to Wallasea	Hopper Barge on sites along the River Thames	Train / Lorry
Slurry TBM Chalk material	Weak fractured Limestone. Moisture content = 30% to 35%	Silt Moisture content 30% to 35%	No, potentially unstable in a ship’s hold and not considered safe to ship	Yes, but barges cannot go to Wallasea, so not used	Yes – used to transport to various licensed waste recovery or disposal sites
EPB clayey material	Moisture content: 20 to 30%. Liquid Limit: 55 to 80%. Su: 100 to 300kPa	Moisture content: 30 to 40%. Liquid Limit: 70 to 80%. Su: 20 to 30kPa (by shear vane)	Yes – Crossrail specific TML and Su tests are routinely passed	N/A	Used for locations where shipping to Wallasea is impracticable
EPB mixed clay and sand material	As EPB Clayey material. Sand moisture content: 20 to 30%	Moisture content: 20 to 30%. Liquid Limit: 25 to 35%. Su: 5 to 15kPa (by shear vane)	Yes for Drives X, Y and G if Crossrail specific TML and Su tests are passed.	Yes if TML and Su tests are failed – mostly a Drive Y issue	Yes for Drive Z
SCL material	As EPB Clayey material.	No Significant change	Yes. TML and Su tests are routinely passed.	N/A	Yes, transported to Tilbury or Northfleet for loading on to Ships for Wallasea
Box Shaft and Portal material	As EPB Clayey material.	No Significant change	Yes – as SCL material	N/A	Yes – transported to Tilbury or Northfleet for loading on Ships for Wallasea

Table 3 –  Summary of the excavation induced changes in material properties and the impact on transportation options

Consents for the deposition of the excavated material ^{[3 to 8]}

This part of the paper concentrates on the legal framework and permission processes that allow the deposition of the excavated material at Wallasea Island.

The main items of EU legislation that control the deposition of Crossrail’s excavated material are the: Landfill Directive (Directive 1999/31/EC), the Waste Framework Directive (Directive 2008/98/EC) and EU Council Decision 2003/33/EC which establishes criteria and procedures for the acceptance of waste at landfills. A variety of UK regulations are used to implement these EU Directives (see references). These regulations establish permit systems that control the deposition of Waste in England and Wales. The permits are administered by the Environment Agency (EA).

In the Waste Framework Directive a ‘waste operation’ is defined by reference to recovery and disposal operations. Any recovery or disposal of waste is a waste operation. The Wallasea Island Permit is a waste recovery permit rather than a disposal permit. Crossrail had to demonstrate the following in order to be granted a recovery permit for Wallasea:

• That there is a clear benefit from the activity and that it has a beneficial use;
• That the recovered waste is suitable in terms of chemical composition and engineering properties for its intended use ( Wallasea Island is a sensitive marine habitat);
• That the amount of waste proposed was no more than was required for the scheme;
• That the waste is being used as a substitute for a non-waste material; and
• That the proposal will be completed to an appropriate standard.

The permit allows the deposit of waste to create an intertidal coastal habitat at Wallasea Island. The permit allows for the deposition of non-hazardous and inert mixtures of excavated soil and other defined materials. Wastes that are excluded and that cannot be sent by the Crossrail project to Wallasea Island include wastes with the following characteristics:

• Consisting solely or mainly of dusts, powders or loose fibres;
• Hazardous wastes; and
• Wastes that are in a form which is either sludge or liquid.

Following an extensive testing programme on samples recovered from the ground investigations the following Permit framework was established.

Waste acceptance procedures – material not suspected of being contaminated

London Clay is naturally present at Wallasea Island, the Permit allows any similar excavated materials to be deposited without any further chemical testing as long as there is no suspicion that the material is contaminated. It is expected that most of the excavated material will fall into this category.

Excavated material from areas with <5m clay cover requires a desk study to identify any current or historic sources of pollution that might have resulted in the release of contaminated liquids (such as gas works, petrol stations, landfills etc.). If no such sources are present then the material can be deposited at Wallasea without further testing. If pollution sources are identified then additional testing is required prior to deposition.

Waste acceptance procedures – material where there is risk of contamination

At Wallasea the aquatic environment is critical, so waste acceptance is based on leachate testing against inert Waste Acceptance Criteria (WAC). Detailed quantitative risk assessments have been undertaken that take account of the environmental setting and specific receptor characteristics at Wallasea.

Some soils naturally contain levels of certain elements that exceed inert WAC limits. For these substances separate hydrogeological risk assessments have been carried out to set Wallasea acceptance levels above the inert WAC levels. For instance leachate results for London Clay often exceed inert WAC levels for sulphate and selenium. Hydrogeological risk assessment WAC values have been established for Chloride, Fluoride, Molybdenum, Zinc, Antimony, Lead, Sulphate and Selenium.

Crossrail maintains a Baseline WAC Levels Database of soil leachate testing results which can be used to determine whether any high concentrations found are consistent with natural ground or whether they are likely to be due to industrial contamination.

If the material is confirmed as contaminated with hazardous substances, or elevated levels of non-hazardous substances of man-made origin that are above the Wallasea Acceptance Levels then its suitability for recovery at Wallasea must be considered further.

For sites expecting to generate arisings that include Alluvium, which may include a proportion of naturally occurring organic matter, hydrocarbon results may be elevated due to natural organics and so it may be necessary to provide details of the type of hydrocarbon to distinguish between naturally derived organic compounds, such as humic acids, and man-made contaminants such as diesel, motor oil and bitumen. These distinctions are made using an additional suite of specialised tests and assessment procedures. At the end of these processes any excavated material that is considered unsuitable for Wallasea is disposed of at an appropriately licensed Landfill.

Additives

Additives are often used to facilitate excavation works and material handling and these become mixed with the excavation arisings and as such need to be considered in the permitting process.

Acceptable Additives

The following additives used to facilitate excavation are considered to be inert and therefore are deemed suitable for use:

• Polyethylene (or similar products), glass, plastic or steel fibre and dowels used to reinforce Sprayed Concrete Lining (SCL).
• Bentonite used in tunnelling or in constructing diaphragm walls.
• Concrete may make up to a proportion of the loads and may arise as:
  • Sprayed Concrete Lining (SCL), permitted if <150mm in any one dimension
  • Lumps of concrete, permitted if <150mm in any one dimension
• Primary or recycled aggregates used in constructing diaphragm walling <150mm.

Additives that require additional approval

The use of additives such as foams, lubricants and polymers are subject to an EA permitting process based on a Water Risk Assessment. The Water Risk Assessment has to do two things:

• Assess the use of additives in-situ for risk to groundwater during tunnelling; and
• Assess the use of additives based on the destination and water environment/receptors at Wallasea.

Where contractors are mixing additives with waste after excavation process to better manage subsequent materials handling, these additives also need a water risk assessment for Wallasea. An example would be the use of flocculants in drilling fluids used to assist in the separation of solid arisings from liquids.

Disposal sites other than Wallasea Island

Crossrail has an undertaking to the UK Government to make beneficial reuse of the excavated material from the construction of the tunnels and stations. The Crossrail excavated material strategy is to send material to Wallasea Island. As set out in the first part of this paper this is not always possible and in some cases material has to be sent to alternative disposal sites. Crossrail continues to meet or exceed its targets for beneficial reuse or recycling of waste and excavated material with current rates reporting 99% of clean excavated material being beneficially reused against a target of 95%.

Examples of beneficial reuse could include engineering uses such as capping and lining landfills, creating wildlife reserves or farmland, land raising and recreational land creation (e.g. a golf course). Any non-Wallasea Island waste destination site used by Crossrail must have an appropriate Environmental Permit and be able to demonstrate that the material will be beneficially reused.

References

[1] Società Italiana Gallerie Convegno “ Terre e rocce da scavo nelle Opere in Sotterraneo: Un problema o una Opportunità? “ Samoter 2014 – VeronaFiere (VR) 8 – 9 Maggio 2014

[2] The International Maritime Solid Bulk Cargoes Code (2011)

[3] The Landfill Directive (Directive 1999/31/EC)

[4] The Waste Framework Directive (Directive 2008/98/EC)

[5] EU Council Decision 2003/33/EC

[6] The Environmental Permitting (England and Wales) (Amendment) Regulations 2013

[7] The Landfill (England and Wales) Regulations 2002 (as amended)

[8] The Environmental Permitting (England and Wales) (Amendment) Regulations 2013