How to determine Earth Pressure Balance EPB and Tunnel Boring Machine TBM
Operating modes of EPM & TBM
EPB & TBMs control the stability of the tunnel face and subsidence of the ground surface by monitoring and adjusting the pressure inside the cutterhead chamber to achieve a balance with the water and ground pressure in front of the cutterhead, hence the name ‘pressure balance’. The way these machines operate is called ‘close mode’ – here, the whole excavation chamber is under pressure. Tunnel boring machines (TBMs) are highly specialized machines designed to bore through the earth to construct a variety of infrastructure, from super-highways to water-overflow systems. Different types of TBMs can be used, depending on the geological conditions. So what is an earth pressure balance (EBP) tunnel boring machine, and what advantages does it bring?
Due to the widespread need for the urban transportation network, construction of underground railway lines by mechanized shield-tunneling increases in metropolises. The case of the Shiraz (Iran) subway twin tunnel construction is an example of transport network extension. In the present paper, a 3D numerical model was developed to investigate the tunnel's lining behavior and the ground displacement surrounding the second line of the Shiraz subway tunnels. All the excavation phases of EPB-TBM (Earth pressure balanced, Tunneling Boring Machine) were simulated using the Finite Difference Method. Furthermore, a new simple method were proposed to take account of the TBM shield conical shape. In most numerical simulations, the segmental joints were assumed parallel to the tunnel axis, despite the fact that the longitudinal joints make an oblique angle with the tunnel. Three kinds of lining pattern were then considered: continuous, straight and oblique joints. The numerical simulation results were compared to highlight the influence of the oblique segmental joints on the tunnel lining behavior. The numerical results indicated the necessity of using segmental lining with oblique joints pattern to obtain an accurate evaluation of the structural forces applied in segmental lining. In this work, the influence of the longitudinal distances between the tunnel faces on the surrounding ground displacement and the induced lining forces was investigated. The excavation of a new tunnel near to an existing tunnel with high lagging distance between twin tunnels leads to increase in the normal and the longitudinal displacements, in the internal lining forces and to a decrease in the ground surface settlement above the tunnels. Basic structure of a TBM with highlight to the progressive diameter reduction from the cutterhead
(A), to the front (B) and back (C) of the shield and the lining extrados (D).
As the soil flows through the openings at the cutterhead, into the excavation
chamber (B), it blends in the supporting mixture, which is kept pressurized to support
the face. The system that controls how the spoils are extracted from the chamber is also
responsible for keeping the pressure stabilized between the tunnel face and the
bulkhead. In EPB machines, the mixture is composed of the excavated soil and
additives, and is removed from the chamber mechanically, through a screw conveyor.
In slurry pressure balance (SPB) and mixshield machines, the mixture is mostly
composed of a slurry suspension, and is removed through a hydraulic circuit. The
chambers of mixshield machines are divided by a submerged wall, in a working
chamber, completely filled with slurry, and a pressure chamber, partially filled with a
pressurized air bubble that controls the pressure at the chamber.
The diameter of the cutterhead determines the size of the excavated boundary (A),
which tends to be larger than the front of the TBM shield (B). The shield is always
tapered, so the front diameter (B) is slightly larger than the diameter at the back (C).
After the lining segments are combined in a ring inside the shield, the machine thrusts
itself forward, leaving the lining in contact with the ground (D). From the moment the
cutterhead passes a cross-section, the structure from the TBM to the tunnel lining
presents a progressively smaller diameter to support the excavation. To mitigate the
convergence of the soil from A to D (see Figure 1), grout is injected from the back of
the shield. To prevent water or grout from coming in the TBM, the contact between the
shield and the lining is sealed with steel brushes filled with pressurized grease [1].
These basic principles are essential to discuss how the interaction mechanisms
between the soil and the TBM are often interpreted within idealized frameworks that do
not account for some observed features of the soil response. In response to that, the
path from field measurements to theoretical developments is traced in Section 2. Some
ideas on how these developments can be applied into practice and even incorporated in
the usual methods to estimate the tunneling induced displacements and the lining forces
are presented in Section 3. Finally, it is argued that an unbalanced concentration of.
The second step is to quantify the gradient inducing the flow from the face. An
analytical formulation can be derived [4], based on the approximation that there is an
infinitesimal constant hydraulic source all over the tunnel face. This distributed head is
defined with reference to the in-situ water pressure. By equating the volumetric flow
rate from the source (A=dr.r.dθ) with the one at a certain radial distance (s) along a
semi-spherical domain in front of the tunnel (A=2.π.s²), one obtains:
2
..4.....
2 s
ds
d drq dr k (1)
where q is the discharge from the point source, assumed constant all over the
tunnel face.
By integrating Eq. 1 along the following limits: ϕ=[ϕ(S),∞]; s=[S,∞]; r=[0,R];
θ=[0,2π], and defining 22 rxs , one obtains:
xRx R x
0 22 (2)
where ϕ0 is the incremental piezometric head at the tunnel face (x=0).
From Eq. 2 it is possible to calculate the hydraulic gradient at the tunnel face as:
Rx R
x
dx R
d
x x
0
0
22
0
0
1
(3)
The penetration velocity can then be defined as:
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40
excess pore pressure - φ (kPa)
distance from tunnel face - x (m)
SPB
SPT - Fit
EPB
EPB - Fit
SPB EPB
φ0 55 120
R 4.25 4.325
xRx R
0 22
T.G.S. Dias and A. Bezuijen / TBM Pressure Models – Observations, Theory and.
Tunnel boring machines (TBMs) are highly specialized machines designed to bore through the earth to construct a variety of infrastructure, from super-highways to water-overflow systems. Different types of TBMs can be used, depending on the geological conditions. So what is an earth pressure balance (EBP) tunnel boring machine, and what advantages does it bring?
Simply put, earth pressure balance machines (EPBs) are shield TBMs specially designed for operation in soft ground conditions containing:
- Water under pressure
- Loose sedimentary deposits
- Sands, gravels, silts, clays
- Formations with large boulders
- High water table
These machines are usually referred to as ‘Single Shield EPB TBMs’. First, let’s look at exactly how EPB TBMs work, before explaining how our products support efficient excavation in this tunneling technique.
Parts of EPB and TBM
- Mixing arms: They mix the soil to obtain the required rheology
- Cutterhead: It removes the soil from the tunnel face
- Bulkhead: Transfers the thrust force to the soil paste in the excavation chamber where it is controlled using pressure sensors
- Excavation chamber: The plastic soil in the excavation chamber transfers the support pressure at the tunnel face
- Screw conveyor: The rotation speed determines how much material is removed from the excavation chamber, thus regulating the required support pressure
- Erector: To lift and position the segments during ring building
- Tail shield: Wire brushes seal the gap between the inside of the tail shield and the outside of the segmental lining
- Backfilling: The annular gap between the excavated surface of the ground and the outside of the tunnel lining is filled with grout
- Tunnel lining: The precast concrete segmental lining is installed with precision.
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