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1 FACE SUPPORT OF SLURRY PRESSURE BALANCE (SBP) SHIELDS
Among the mechanized tunneling machines, slurry pressure balance (SPB) shields are known as
a reliable excavation technology in non-cohesive soils under groundwater table and unstable
tunnel face conditions (Fig. 1). Utilizing pressurized bentonite suspensions as support medium
in the excavation chamber, the main advantage of a SPB shield is the exact control of the
support pressure. In such conditions, the tunnel face is actively supported preventing surface
deformations and other ground movements.
To achieve a stabilised tunnel face, two key conditions must be fulfilled. First, a sufficiently
large slurry pressure in the excavation chamber of the shield has to be applied with reference to
the acting boundary conditions. The required pressure in the excavation chamber can be
determined by various methods, e.g. Anagnostou & Kovari (1994), Jancsecz & Steiner (1994)
or following the recommendation of the German Tunnelling Committee for the calculation of
face support pressure (DAUB 2016). Second, the slurry excess pressure in terms of the
difference between the slurry pressure and the groundwater pressure must be transferred at the
soil skeleton to counteract the earth pressure. In practice, the German standard DIN 4126 (2013)
is often used to predict the support pressure transfer at the soil skeleton.
As summarized by Zizka et al. (2018), Morgenstern & Amir-Tahmasseb (1965) conducted
one of the first attempts to analyse the interaction between bentonite slurry and soil for the
Design of the support medium for slurry pressure balance (SPB)
shield tunnelling in demanding ground conditions
B. Schoesser
Ruhr University Bochum, Bochum, Germany
M. Straesser
Herrenknecht AG, Schwanau, Germany
M. Thewes
Ruhr University Bochum, Bochum, Germany
ABSTRACT: The application range of slurry shield machines aims primarily non-cohesive
soils. In addition, slurry shields are preferably utilized in tunnelling projects with challenging
ground conditions, e.g. coarse and highly permeable soils, extremely heterogeneous geology as
well as very high ground water table. These boundaries impose particularly high requirements
on the support fluid. Here, bentonite suspensions are applied, which are characterised by the
rheological parameters yield point and viscosity. For special applications, further characteristics
and quality standards are important, e.g. density and the interaction with the in-situ soil at the
tunnel face. In this paper, the fundamental rheological parameters of bentonite suspensions are
discussed, including widely used measuring devices such as the Marsh-funnel, ball-harp
rheometer and rotational viscometer. The parameters gained from these devices are supple-
mented by advanced rheometric tests. The measurement profiles provide further information on
the flow behaviour. As an example, the flow curve is a valuable tool to constitute the yield point
and the fluid flow under very low and high shear rates. This extended analysis allows a detailed
discussion within the suspensions design phase. Finally, the suspension requirements are
summarized and related to practical experience for the needs of a construction site. These
settings are transferred to suitable suspension design supported by the findings of research work
and laboratory test.
purposes of open trench stabilization. The authors pointed out that at the time of publishing their
paper, several mechanisms such as hydrostatic pressure, arching of the soil and electro-osmotic
forces were discussed as the main mechanisms responsible for the slurry support of non-
cohesive open trenches. Weiss (1967) performed laboratory experiments dealing with
stabilization of non-cohesive soils by bentonite slurry. In his experiments, he visualized the
penetration behaviour of slurry into the pores of soil and suggested that the stabilisation
behaviour depends on the equivalent pore diameter of soil and the yield point of the slurry.
Weiss (1967) summarizes the stability condition of a trench in non-cohesive soils as three
complementary conditions.
yield point of slurry is required to achieve the equilibrium of forces on a single soil grain.
slurry pressure must exceed the groundwater pressure.
slurry excess pressure must counterbalance the earth pressure.
Figure 1. Application range of slurry pressure balance (SPB) shields (Thewes 2009).
Further research on the stability of slurry-stabilized trenches was conducted by Müller-
Kirchenbauer (1972). The author performed both, experiments and theoretical analysis. He
stated that penetration of slurry in the pores of soil influences the slurry pressure transfer on the
soil skeleton. The penetration behaviour of slurry is determined by the grain size distribution of
soil and the stagnation gradient of slurry. He distinguishes two cases of slurry-soil interaction
(Fig. 2). The first is the formation of a filter cake at the soil’s surface (Type I) and the second is
slurry penetration inside soil´s pores without any aggregation of slurry particles at the surface
(Type II). In Type II, the shear resistance of the pore channel wall is activated. The shear
resistance is in equilibrium with the excess hydraulic head of the slurry in the trench at the end
of the penetration process. The pressure transfer Type III consist of both, penetration zone and
the filter cake (Zizka et. al. 2018).
Figure 2. Different types of pressure transfer in diaphragm wall stabilization according to Müller-
Kirchenbauer (1972) (Zizka et. al. 2018).
For tunnelling in coarse and permeable soils, the pressure transfer according to type II is the
applicable principle. Here, the bentonite suspension penetrates into the ground. The movement
is induced by the acting pressure and leads to the interaction between the suspension and the
surface of the soil particles. Depending on the flow velocity, a certain shear rate acts within the
suspension. At the contact areas, shear stresses of the magnitude of the yield point of the
suspension are transferred to the soil particles. When the penetration depth has become so large
that the integral of the transferred shear stresses is in equilibrium with the difference between
the suspension pressure and the prevailing earth pressure, the penetration process stagnates.
For the design of a bentonite suspension, the rheological parameters need to be adapted to the
boundaries of the in-situ soil. Here, the relevant geological characteristics of non-cohesive soils
cover the description of the pore space in terms of porosity, permeability, compactness and
effective or characteristic grain size d10 gained from the particle size distribution.
2 SUSPENSION RHEOLOGY AND RHEOMETRY
Bentonite suspensions are classified as non-Newtonian fluids. They inhibit the rheological
parameters yield point, viscosity and thixotropy. The parameters can be determined constitu-
ently but they interact with each other. For practical application, the flow model according to
Bingham is used for the rheological description of bentonite suspensions.
For the determination of the rheological parameters, different technical devices are available.
Marsh funnel, ball-harp rheometer and rotational viscometer belong to the proven rheometric
technologies utilised on construction sites.
2.1 Rheology
Rheology describes the deformation and flow behaviour of materials and fluids in general. The
flow behaviour of a fluid is caused by the introduction of an external force. The flow starts
when the outer energy is bigger than the inner energy. Overcoming the inner physical structure
allows the fluid to flow. This inner resistance is described as viscosity. Water and oil show
varying viscosity values indicating different flow velocities at ident boundary conditions. In
addition, the specific parameter of a bentonite suspension is the yield point. Below the yield
point, the bentonite suspension acts as a solid (including a thixotropic effect); above the yield
point it acts as a fluid. The transition between solid and liquid behaviour is marked by the yield
point.
2.1.1 Viscosity
As mentioned before, viscosity is the internal resistance of the fluid to flow. Temperature as
well as type, amount and duration of the shear exposure influences the behaviour of the fluid
flow. The smaller the viscosity, the lower the resistance and the faster the flowing motion.
Viscosity values are gained directly using the viscometer (Chap. 2.2.3) or indirectly using the
Marsh funnel (Chap. 2.2.1)
The general law describes the relation between viscosity 𝜂 [Pa s]; shear stress 𝜏 [Pa] and
shear rate 𝛾 [1/s] as follows:
𝜂
(1)
2.1.2 Yield point
After reaching the yield point, fluids that show a linear flow relation are categorized as Bingham
plastic fluids. The following equation represents the mathematical model of the Bingham type.
Figure 3 shows the theoretical context.
𝜏𝜏
𝜂𝛾 (2)
where 𝜏 = shear stress [Pa]; 𝜂 = viscosity [Pa s]; 𝛾 = shear rate [1/s]; and 𝜏 = yield point [Pa]
The transferable shear stress 𝜏 at a shear rate 𝛾 of 0 provides the yield point of the Bingham
fluid. Overcoming the yield point, the shear stress 𝜏 increases linearly with increasing shear
rates 𝛾. The slope of the straight line describes the viscosity 𝜂 of the fluid (Fig. 3).
Figure 3: Flow model of Bingham.
2.2 Rheometry
2.2.1 Marsh funnel
The Marsh time tM is measured according to DIN 4127 (2014) for 1,000 cm³ and according to
API 13B (1997) for 948 cm³ with a standardised Marsh funnel (Fig. 4 left). The Marsh funnel is
easy to use on site and provides rapid detection of any change in suspension properties relative
to an initial value. The Marsh funnel reaches its limits with very viscous suspensions, since the
run-out times become impracticably long or the suspension does no longer flow out of the
Marsh funnel (Praetorius & Schösser 2017).
The functionality of a certain Marsh funnel can be examined by determining the run-out time
of water. When the funnel contains 1,500 cm³ of water, the amount of 1,000 cm³ should spill in
28 s (DIN 4127, 2014) and in 26 s for an amount of 948 cm³ (API 13B 1997).
Figure 4: Rheometric devices for the determination of suspension parameters: Marsh funnel (left), ball-
harp rheometer (middle), and rotational viscometer (right).
2.2.2 Ball-harp rheometer
The static yield point stat τf is determined by the process of von Soos as laid down in DIN 4127
(2014) with a ball-harp rheometer. Ten balls (Fig. 4 middle) hang from a disc on nylon threads.
The balls have different diameters and consist of either glass or steel. Each of these balls
corresponds (depending on their specific weight and the density of the suspension) to a yield
point.
The balls are arranged round the disc with an equivalently increasing yield point and each is
marked with a number. The disc is fixed to an apparatus, which enables vertical dipping of the
disc and thereby the balls into the surface of the suspension. When the disc is lowered, all balls
corresponding to a lower yield point than that of the suspension float on the surface, and all
balls corresponding to a higher yield point sink into the suspension (Praetorius & Schösser
2014). When the balls are dipped, the weight of the balls is opposed by the floating force and
the yield point of the suspension. Balls that sink into the suspension can be recognised by their
tight threads, and the balls that float can be recognised by their slack threads (Fig. 5).
Since the balls are arranged in a sequence of increasingly assigned yield points, the actual
yield point of the suspension is located between the values of the last floating ball and the first
sunken ball (Schulze et. al. 1991).
Figure 5: Determination of the static yield point with the ball-harp rheometer
2.2.3 Rotational viscometer
To determine the plastic viscosity ηp and the Bingham yield point τB, the rotational viscometer
according to API 13B (1997) is used. In this apparatus, the suspension is filled into the annular
gap between two rotationally symmetrical and coaxially mounted cylinders (Fig. 4 right). One
of the cylinders rotates with the angular velocity Ω and the other remains stationary. The
required force to overcome the flow resistance of the suspension in the annular gap can be
determined as a function of torque and rotation velocity (DIN 53019 2008).
In the case of apparent viscosity, the measurement of shear stress is performed at a shear rate
of 600 rpm (A 600) (Fig. 6). In the case of plastic viscosity, the shear stresses are determined at
shear rates of 600 rpm (A 600) and 300 rpm (A 300). Subtracting the A 300 value from the
A 600 result in the plastic viscosity value. The Bingham yield point is calculated by subtracting
the plastic viscosity from the value of A 300 (API 13B 1997).
Figure 6: Determination of Bingham parameters using rotational viscometer
Addressing the Bingham model, further analysis can be derived by means of the flow curve,
as it represents the relation between the shear stress and the shear rate. Here, systematic
derivations can be drawn for very low and very high flow velocities. They help to assess the
behaviour of the bentonite suspensions within the complete slurry circuit including excavation
chamber as well as feeding and transportation lines during tunnelling works.
3 RELATION BETWEEN SUSPENSION RHEOLOGY AND SOIL MECHANICAL
PARAMETERS
For the selection of a suitable bentonite suspension, the relevant geotechnical characteristics of
the ground must be known. The objective is to find the optimal bentonite suspension depending
on the geotechnical conditions of compactness, permeability and pore space to fulfil all
requirements regarding the face support.
In the following, formulae are summarized to calculate the required value of the yield point,
which suits the certain geological condition. On this basis, a target value for different bentonite
products and varying solid contents can be derived (Praetorius & Schösser 2017).
The aim of slurry assessment is to specify the requirements concerning the suspension
rheology (yield point 𝜏) depending on the geotechnical conditions (characteristic grain size
𝑑). The standardised approaches for determination of slurry properties can be found in DIN
4127 (2014) and are summarized in the DAUB recommendation (DAUB 2016).
3.1 Required yield point of the suspension depending on the pressure gradient fso
The interaction of the bentonite suspension with the ground can be described with the help of
an existing pressure gradient 𝑓
, which represents the decrease in slurry excess pressure over a
meter of the penetration distance into subsoil. The pressure gradient 𝑓
is a theoretical variable,
since the penetration depth is usually much smaller than 1 m. According to DIN 4126 (2013),
the existing pressure gradient can be calculated from Equation (3).
𝑓
, ⋅
(3)
where fso = pressure gradient [kN/m³]; 𝜏 = yield point of the suspension [kN/m²]; and d10 =
characteristic grain size of the soil [m].
According to DIN 4126 (2013), Equation (3) is valid for a pure suspension without sand if
the pressure gradient fso > 200 kN/m³. Substituting fso = 200 kN/m³, the required yield point 𝜏
of the pure suspension can be derived from Equation (4).
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝜏 . ⋅
, 𝑁/𝑚² (4)
where 𝜏 = yield point of the slurry [kN/m²]; and d10 = characteristic grain size of the soil [m].
The calculated values of the “required yield point 𝜏” according to Equation (4) are listed in
Table 1. Even with a reduced particle size of the soil d10, the value of the required yield point
increases rapidly, e.g. it is a challenging task to put a yield point of 40.0 N/m² into practice
within the slurry circuit. For highly permeable or heterogeneous soils, additional requirements
concerning the support medium need to be defined.
Table 1. Calculation of the “required yield point τf” [N/m²] of the bentonite suspension depending on the
pressure gradient fso = 200 kN/m² (Eq. 4).
d10 [mm] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
τf [N/m²] 5.7 11.4 17.1 22.8 28.6 34.3 40.0 45.7 51.4 58.3
3.2 Required yield point of the suspension depending on micro-stability
From a local pressure transfer perspective, face stability is considered at the soil particle scale
(micro-stability). Micro-stability is the stability of a single grain or group of grains that prevents
them from falling out from the soil skeleton under gravity. According to DIN 4126 (2013),
Equation (5) defines the “required yield point 𝜏” to satisfy the criterion for micro-stability
(fresh suspension).
⋅ ⋅
⋅ 1𝑛
⋅ 𝛾 𝛾
⋅ 𝛾
𝜏
(5)
where 𝑑 = characteristic grain size of the soil [m]; 𝑛 = soil porosity [-], 𝛾 = unit weight of
soil grains [kN/m³], 𝛾 = partial safety factor for permanent load case in GZ1C acc. to DIN
1054 (=1.00) [-], 𝛾 = unit weight of fresh slurry [kN/m³], 𝛾 = partial safety coefficient for
drained soil within the status GZ1C in load case LF2 acc. to DIN 1054 (=1.15) [-]; 𝜑′ =
characteristic drained friction angle of the soil [°]; 𝜂 = safety factor accounting for deviations
in the yield point of suspensions (= 0,6) [-]; and 𝜏 = yield point of the slurry [kN/m²].
The calculated values of the “required yield point 𝜏” according to Equation (5) are listed in
Table 2 applying a soil porosity of n = 0.26 [-]; unit weight of the soil 𝛾 = 2.65 [kN/m³]; unit
weight of the fresh slurry 𝛾 = 1.025 [kN/m³]; and a characteristic drained friction angle of the
soil 𝜑′ = 37 [°].
Table 2. Calculation of the “required yield point τf” [N/m²] of the bentonite suspension depending on the
micro-stability at the tunnel face (Eq. 5).
d10 [mm] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
τf [N/m²] 2.4 4.8 7.2 9.6 12.0 14.4 16.8 19.2 21.6 24.0
Comparison of the calculated values of the yield point depending on the pressure gradient fso
(Tab. 1) and on the micro-stability (Tab. 2) shows that the criterion of micro-stability leads to
significantly lower yield point values (< 50%). Based on micro-stability, they appoint lower
limit values, which must be kept as an absolute minimum during the tunnelling works. In the
practical use on site, a warning range for the yield point should be implemented.
4 SLURRY ASSESSMENT FOR DEMANDING GROUND CONDITIONS
In slurry pressure balance (SPB) shields, temporary support to the tunnel face is provided by the
bentonite suspension under pressure. The viscosity of the suspension and the associated reduced
flow capability lower the risk that support medium is lost through sudden uncontrolled escape
into the surrounding ground. The slightly higher density of the regular bentonite suspension
compared to that of water means that the overpressure of the support medium at the top of the
face is low and blowouts are thus avoided (Anagnostou & Kovari 1994).
The stability of the tunnel face is maintained by a suspension with a proper rheology (yield
point, viscosity) adapted to the prevailing geotechnical conditions and a sufficiently high
support pressure in the excavation chamber. The latter is achieved through the capability of the
support medium to develop a mechanism in the ground, which enables the support pressure to
be transferred into the grain structure of the soil in the form of effective stresses.
For demanding ground conditions e.g. higher permeability, coarse to very coarse soil,
heterogeneous particle size distribution, a support medium is required that develops a suitable
pressure transfer mechanism reliably. For these in-situ ground, the presented types of pressure
transfer (Fig. 2) are only applicable to a limited extend.
Facing that challenge, the concepts of Low Density Support Medium (LDSM) and High
Density Support Medium (HDSM) was developed in the laboratory of the Ruhr University
Bochum (RUB) in cooperation with Herrenknecht AG for the world’s first use of the VD-TBM
(variable density) at Klang Valley MRT Project in Kuala Lumpur, Malaysia. This concept was
also proven in slurry shield tunnelling for undercrossing the river Spree with very small
overburden at project Metro U5 in Berlin, Germany.
The basic idea is to increase the density of the basic bentonite suspension up to a certain
value. With increasing density, the viscosity and yield point of the support medium also
increase. This enables or improves the support pressure transfer at the tunnel face. The theory is
shown in Figure 7.
Figure 7: Pressure gradients of the support pressure in the excavation chamber for LDSM (low density
support medium) and HDSM (high density support medium) according to (Straesser et. al. 2016)
The pressure distribution of the earth and groundwater pressure increases linearly with depth
(Fig. 7). This distribution is reproduced by the pressure gradients in the excavation chamber.
Via compressible air cushion in the working chamber, the support pressure is adjusted slightly
above the earth and groundwater pressure, including a safety factor. This positive pressure is
also acting in vertical direction and must be compensated by the static imposed load from the
cover depth. If this imposed load is raised by decreasing density or increased permeability of the
ground, the risk of blowout or heave arises (Straesser et. al. 2016). According to the DAUB
recommendation (DAUB 2016) the safety factor for the blowout verification should be > 1,1.
Applying a suspension with high density, the pressure gradient curve is flatter with altogether
higher values (Fig. 7). The pressure ordinates in the crown are slightly shorter compared to
LDSM. This improves the safety factor concerning incidents as blowout and soil heave. For a
constant support pressure, the high density of the support fluid results in a smaller pressure
ordinate in the crown. This value has a direct effect on the verification of safety against heave.
A low value here increases the factor of safety or enables passing through a zone with shallow
cover without additional safety measures such as ballasting (Straesser et. al. 2016).
Increasing the density through the addition of inert solids, the fines content in the suspension
raises and the HDSM becomes altogether more viscous. As mentioned before, this viscosity is
caused by internal friction between adjacent layers (Praetorius & Schösser 2017). The higher the
viscosity, the stronger is the bonding between the molecules and the lower is the flow
capability. Through the reduced flow ability of HDSM, the risk of a sudden loss of support
medium through uncontrolled escape into an extensive network of pores or fissures is
considerably reduced. At the same time, the fine particles in the HDSM help to mechanically
block or clog smaller pores or fissures through successive accumulation (Praetorius & Schösser
2017).
With the increased viscosity of the HDSM due to the addition of fines, the yield point of the
HDSM also rises. The higher the yield point of a suspension, the higher is the magnitude of the
shear stresses that can be transferred to the grain structure of the soil (Praetorius & Schösser
2017). This achieves a stagnation of the HDSM in highly permeable ground, such as coarse soil
(gravel) and highly karstic rock, at a much smaller penetration depth.
Beside the density and the yield point of the suspension as well as the characteristic grain size
d10 of the soil, the slurry assessment considers also the actual flowing properties of the support
medium. These include the penetration behaviour into the surrounding ground and the capability
of transferring the support pressure applied in the excavation chamber to the tunnel face. For
investigating the interaction of HSDM with certain ground condition, injection tests were
developed and performed at the laboratory of Ruhr University Bochum. Aim of the test series
was to evaluate the capability of the support medium to develop a mechanism with the ground
for reliable support pressure transfer.
For the injection tests, a basic bentonite suspension (LDSM) with a solid content of 50 kg/m³
was modified. The density was increased from 1.025 t/m³ up to 1.7 / 1.8 / 1.9 t/m³. To
intentionally enlarge the scope of application of the HDSM, special additional aggregates (sand
with particle size 2-4 mm) were added to selected suspensions and then injected into coarse-
grained soil. During the injection tests, the development of the support pressure and the
penetration depth were monitored.
A supporting effect in the subsoil can only be achieved if the difference between the
supporting pressure in the excavation chamber and the sum of the earth pressure and the
groundwater pressure at the working face can be transferred as effective stress to the grain
skeleton. For this purpose, the suspension must be able to build up a zone of lower permeability
in the soil. The penetration behaviour of the suspension into the soil plays a decisive role here.
Within the framework of the injection tests carried out, the coarse soils contain particle sizes of
16-25 mm and 25-63 mm with high permeability values.
Figure 8 shows an overview of the results of the injection tests in reference to HDSM density,
the size and amount of additional sand and the particle size of the injected soil including the
permeability coefficient kf.
For the evaluation of the injection test series, three categories were laid down (Fig. 8):
(1) Small penetration depth < 10 cm transferring support pressure ≥ 2.0 bar
(2) Limited penetration depth < 50 cm transferring moderate support (≥ 0.8 bar)
(3) Theoretically infinite penetration depth without any support pressure transfer
Figure 8: Overview of the results of injection tests in reference to HDSM density and particle size of
the injected soil – classification in categories 1, 2, 3 (Straesser et. al. 2016)
Figure 8 shows that the density of the HDSM is the basic need for face support pressure, In
the soil with particle size of 16-25 mm, a minimum density of 1,700 kg/m³ is necessary. With
increasing amount of additional sand, the transferred support pressure is increased (HDSM
1,800 kg/m³ + additional sand 125g and 225g). Furthermore, the same modified suspension
(HDSM 1,800 kg/m³ + additional sand 225g) transfers high support pressure (cat. 1) in soil 16-
25 mm, in the soil 25-63 mm the support pressure decreases to cat. 2.
In summary, the challenge of the injection test is the adjustment of the suspension design in
terms of optimal density of the HDSM and the right amount of additional sand to improve the
support pressure transfer at the tunnel face. The extreme soil conditions were tested to
investigate the potential of the HDSM slurry. For application on the construction site the HDSM
should be assessed in the individual case whether, having regard to the financial and technical
feasibility.
5 CONCLUSION
With the introduction of the Variable Density-TBM, the terms Low Density Support Medium
(LDSM) and High Density Support Medium (HDSM) are used to differentiate the support
medium used. LDSM is a conventional bentonite suspension as used in slurry pressure balance
(SPB) shield tunnelling. The properties of LDSM are defined through the physical parameter
density and the rheological parameters yield point and viscosity. A HDSM is a support medium
with a significantly increased density. It is based on a standard bentonite suspension, whose
density has been intentionally increased by the addition of inert, chemically inactive solid
materials. The quality of a HDSM is also evaluated with the parameters density, yield point and
viscosity. The interaction with the soil needs further investigations to evaluate the transferable
support pressure and the in-situ penetration depth.
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