TBM selection example in (London) Chalk

The essence of selecting the suitable type of TBM lies in “Know thy enemy; know thy self” – 1) the analysis of the engineering characteristics of the given ground and 2) the functionality of each type of TBM. This blog post extends the previous series of TBM selection further and uses Chalk in the London area as an example.

For chalk tunnelling, the choice between EPB and slurry TBMs depends on how the chalk breaks down, its groundwater conditions, flint content and the presence of dissolution features. The following paragraphs each looks at a specific engineering feature of chalk and how it affects the selection of TBM.

1) Chalk is a sedimentary rock consisting mostly of fine grained particles and therefore can progressively break down into fine particles and form ‘slimes’ or ‘toothpaste’ during excavation, mixing and pumping.

EPB TBM:

• EPB is well equipped to deal with fine grained ground. Fine material can help form the plastic paste needed for chamber support and the screw-conveyor pressure plug. However, fines alone do not guarantee a suitable paste. Water content and conditioning must also be controlled to maintain the required consistency in the chamber balance face pressure.

• Sticky, wet chalk paste discharged from the screw can be difficult to transport on belts or in skips, with sticking, spillage and handling problems. This leads to high risk of transporting excavated spoil being the bottleneck of the who chain of workflow.

Slurry TBM

• Hydraulic transport contains the chalk spoil within a closed pipeline system, avoiding routine handling of sticky paste inside the tunnel

• However, the difficulty is transferred to the surface. Suspended fines are difficult to remove and accumulate in the recycled slurry, increasing its density and thickening the medium. Fine-solids treatment can become the controlling production bottleneck.

2) Chalk is a water-bearing formation with variable permeability. In the London area White Chalk as a principal aquifer, with potential hydraulic continuity to the river Thames. Permeability and water inflow can vary along the tunnel alignment, so reliable groundwater control is central to selection.

EPB TBM

• Can provide effective support where the conditioned spoil forms a reliable, sufficiently impermeable pressure plug. However, high water pressure or flow can compromise that plug. Loss of pressure through the screw creates risks of uncontrolled soil/water discharge and inundation.

Slurry TBM

• The pressurised slurry system and mix-shield air cushion provide accurate face-pressure control, with less dependence on spoil-plug behaviour. However, this advantage depends on maintaining suitable slurry properties and effective pressure transfer at the face.

3) Certain Chalk layers tend to contain hard, abrasive flint bands and nodules. Flints introduce a strong contrast with the surrounding chalk in properties. Cutting in these layers produces hard fragments and shards that can cause significant wear and damage to excavation tools.

EPB TBM

• Disc cutters are typical used to allow the machine to deal with hard ground, but flint-bearing chalk paste acts as a grinding medium and inhibit disc rotation and lead to high wear. More wear and frequent interventions for cutting tool maintenance should be expected.

Slurry TBM

• Slurry lubricates and cushions rock fragments, reducing contact wear. A jaw crusher can also be installed at the bottom of the excavation chamber to limit fragment size before hydraulic transport, although it can be difficult to repair if damaged.

4) Chalk may contain dissolution features, fissures and karstic voids. These can create pathways for support fluid loss.

EPB TBM

• EPB has relatively lower risk and better capability to continue through karst depending on void volume and water inflow. However open, water-bearing features can still compromise the spoil plug and face control. EPB therefore should not be automatically assumed to be suitable for karstic chalk.

Slurry TBM

• Slurry may escape into solution features, groundwater or the surface/riverbed. Pressurised slurry can also exacerbate pathways through these features. This is a major qualification to the slurry preference.

In terms of case history, Highspeed 1 (Channel Tunnel Rail Link) generally used EPB TBM, when tunnelling in a mixture of chalk and other soft ground conditions. Thames Tideway East, Lee Tunnel and Crossrail Eastern Running Tunnels used slurry TBM, in predominantly chalk conditions.