Polymers are commonly used to condition soils excavated by EPB tunnel boring machines (TBMs). Laboratory tests are required to determine the appropriate polymer dosage and conditioning parameters for different soil types. Polyacrylamides (PAs) and partially hydrolyzed polyacrylamide (PHPA) polymers are often used due to their ability to form gels when hydrated and interact with soil particles. PHPA polymers can disperse clays and flocculate clayey soils. Proper conditioning with polymers helps maintain pressure at the excavation face, reduces cutter head friction, and facilitates soil removal.
2022_TEST INVESTIGATE SOIL CONDITIONING FOR EPB-compressed (1).pdf
1. Polymers used for the conditioning of soil coming from mechanized excavation with EPB
technology
by Lamanna Luigi Franco [*]
a)-Introduction
During the excavation of underground tunnels with TBM of the EPB type (Earth Pressure Balance), with
the possibility of being convertible into a Slurry TBM (for use with bentonite mud) to be used for
excavation in rock, it is one of the largest problems for the conditioning of the soil, along its entire route,
in particular in front of the rotating excavation head, especially when one is in the presence of mixed
alluvial soils of the sandy-gravelly type and many times, under the aquifer.
Alluvial soils are composed of a variously assorted mixture of clay, silt or silt, sand and gravel.
These types of soils are very present in river valleys, alluvial plains and in the areas of the mouth of rivers.
These are formed thanks to the deposition, during episodes of flooding, of the sediments transported by
the flooded watercourses outside their riverbed.
Fig. 01 – Typical diagram of a TBM of the EPB type
In general, EPB-type TBMs are suitable for making tunnels in the presence of low-permeability soils and
with a content of fine particles (diameter <0.075 mm) of at least 15-20%, which require an equilibrium
pressure of less than 5 -6 bar. In addition, the EPB-type mechanized milling cutter is more suitable than
the bentonite mud [slurry] face counter-pressure machine in the event that numerous inspections in the
excavation chamber are required, as the emptying of the excavation chamber takes place through the screw
conveyor takes place, in a much faster time.
b) -Bentonite (Quintilio Napoleoni, et al. 2020)
"Bentonite" is the trade name of a series of natural clays characterized by the ability to absorb water by
swelling.
-The main component of bentonite is montmorillonite: a clayey mineral belonging to the class of
phyllosilicates called smectites.
-In bentonite, montmorillonite is always accompanied by other clayey minerals (such as kaolin, mica, illite,
etc.) and non-clayey minerals (such as quartz, feldspar, calcite and gypsum).
-The presence or absence of these minerals can affect the quality of bentonite and make it more or less
suitable for certain applications.
b.1) -Gellifying and waterproofing properties
The absorption of water by bentonite leads to the formation of semi-solid gels capable of withstanding
even strong hydrostatic pressures.
A montmorillonite particle can be considered to be made up of a thin packet of negatively charged
elementary layers. Because of this charge, each particle is capable of rejecting another, giving rise to the
penetration and absorption of water molecules attracted around the elementary lattice; for this reason,
while the packet expands giving rise to swelling, a stable envelope is formed around each particle which,
having reached its saturation limit, rejects more water even when subjected to pressure.
The formation of a gel is also linked to the filtration rate of the bentonite in the soil: when it is reduced,
the bentonite solution will tend to gel (and therefore to form the filter-cake).
2. b.2) - Binder properties
This characteristic of bentonite finds its main application in the solid transport capacity of water and
betonite solutions
Properly hydrated bentonite covers the grains of soil and acts as the connective tissue of the entire mass.
This coating is substantially homogeneous and there is a tendency to form flakes of hydrated bentonite
+ soil that tend to "float" within the fluid.
The speed of fluid transfer within the sludge circuit prevents the formation of gel and, therefore, allows
the transport of the muck in suspension even over great distances.
c) -Technical characteristics of the EPB excavation system
The "EPB" system foresees that the advancement of the machine takes place with the excavation
chamber constantly and completely full of the excavated material suitably conditioned, in order to
guarantee a homogeneous and uniform distribution of earth pressure at the face and without pressure
drops between one push and the next. The conditioning of the soils in the excavation chamber must take
place, according to the granulometric characteristics of the soils to be excavated, with the addition of
foams and, if necessary, polymers and fine material (fillers), in the case of granular fractions containing low
percentages of fine . Advances with an empty chamber or with a chamber that is not completely full
cannot be accepted in any way.
Furthermore, the TBM-EPB is more suitable than a Slurry TBM in the case of excavation in sticky clay.
These soils can cause adhesion problems (clogging) in the cutter head and in the transport system of the
spoil which are solved very well thanks to the use of appropriate conditioning agents.
The TBM Slurry cutter, on the other hand, presents problems linked to the difficulty of treating adhesive
cohesive soils in the treatment plants at its disposal. The use of Bentonic Slurry can be effective in the case
of coarse soil with little presence of fine material.
d) -Technical characteristics of the Slurry excavation system
The following description is intended only to be an indication of the main characteristics of the
excavation system indicated; it is not to be considered exhaustive as the configuration of a “Slurry Shield”
TBM differs from manufacturer to manufacturer, although its main characteristics remain unchanged.
The “Slurry Shield” system is based on the use of a pressure bentonite suspension that fills the excavation chamber
placed in front of a watertight metal bulkhead (front septum), which divides the pressure zone from the
non-pressurized one.
A tiller head, placed on the front of the chamber containing the sludge, digs the soil, simultaneously
mixing the excavated soil with the bentonite suspension, and making it homogeneous. This [milling head]
must be equipped with excavation tools suitable for tackling the following types of soil: clays, sands,
Fig. 02 - TBMs tipology of mechanized
3. gravel with pebbles, mixed soils and lithoid material. The milling head, also this type of cutter, is designed
to equip suitable excavation tools (cutters) to overcome any stratifications of very abrasive lithoid
material with resistance that can reach up to 100 MPa.
The mud is then pumped up to a separation plant placed outside and the pressure of the mud (and
therefore on the excavation face) is exerted by a "cushion" of compressed air which floats on the surface
of the mud. This cushion is placed behind a second diaphragm (pressure septum) which divides the upper
area of the excavation chamber. To correctly counterbalance the thrust created by the pressure of the
groundwater, the ground of the excavation face and the bentonite mixture, the air pressure is adjusted with a
tolerance of the order of ± 0.05 bar.
The "Slurry Shield" system can be considered, at least theoretically, as a closed circuit system where the
fluid follows the following cycle: accumulation - pumping to the excavation chamber - excavation -
separation - accumulation.
However, the lack of corrections of the density of the conveying fluid in the excavation process would
lead to an increase in the density of the fluid itself, due to the dissolution of the finer particles, up to
intolerable values for the system. It is therefore necessary to eliminate part of the fluid from the closed
circuit and reintegrate it with water and bentonite. The quantity of fluid to be reintegrated depends on
the geotechnical characteristics of the soil affected by the excavations. In practice, in the presence of
granular soils where the portion of fine material (silts and clays) is very limited, the quantity of water to be
reintegrated is minimal. Conversely, soils with a consistent percentage of fine material (silts, clays) involve
a considerable consumption of make-up water, compared to a lower consumption of bentonite, especially
in the presence of clay.
e) -Conditioning system
Therefore, to condition the ground, when a TBM-EPB is used for the excavation phase, the following
products are used to avoid subsidence associated with the excavation phase:
- surfactants [dosage = Cf (%)];
- polymers [dosage = Cp (%)];
- sealants of the terminal part of the TBM-EPB shield, commonly called "tail";
- sealant of the main bearing of the TBM-EPB rotating and milling head, commonly called "head";
- bentonite;
- water.
"Polymers" are fundamental when you are in the presence of alluvial soils and under water tables because
they have the function of acting on the "foam", on the "soil" and on the "conditioned" soil in order to modify
the consistency of the latter [ the ground], maintaining a correct pressure at the excavation face [to avoid
subsidence]; reduce the friction between the ground and the tiller head [including the cutters] and finally
facilitate the removal of the excavated soil [waste material to be removed] towards the outside of the tunnel.
A preliminary assessment of the dosages of the use of these products, in particular, of the "surfactants",
"polymers", and / or "bentonite" and "water", are often indicated by the suppliers of these products or by the
previous experience of excavations carried out in similar conditions, while, in reality, laboratory tests must
be carried out in advance to evaluate, on the basis of the geotechnical characteristics of the stretch to be
crossed, providing at least no. 3 Slump tests for each conditioning agent chosen and the same stability tests
over time of the conditioned soil, as well as half-life tests of the foams.
This test campaign is used to define the characteristics of the conditioning parameters: FIR Foam Injection
Ratio and FER Foam Expansion Ratio.
4. SOIL FIR %
Clay 30 - 80
Sandy Clay-Silt 40 - 60
Sand Clayey Silt 20 - 40
Sand 30 - 40
Clayey Gravel 25 - 50
Sandy Gravels 30 - 60
Tab. 01 – FIR indicative for different types of terrain (EFNARC)
It is believed that up to a minimum of 3 trials are required for each proposed conditioning agent. The
concentration of conditioning products, as usual, is defined on the basis of the indications provided by
the producers. There are numerous advantages associated with the use of additives, all of equal
importance, to highlight the following conditions:
- type of tests to be performed;
- choice of products that are biodegradable [verification through biodegradation tests - WGK 1] and relative
dosages;
- their plasticity by verifying that the final conditioning mixture that will be used has a fluid-plastic
consistency, both in front of the milling head and in the excavation chamber;
- measurement of the limit hydraulic load that the mixture can sustain due to water retention;
- Slump Test to measure the acceptable consistency values of the limits on the size of the aggregates [fine
grain and coarse grain] present in the soil by lowering the Abrams cone;
- Mortar Flow Table Test spreading test to verify the plasticity of the conditioned soil.
I remember that, on this last point, the verification of the fluid-plastic consistency is essential to indicate
the necessary pressure value, to be imposed on the milling head. In particular, in the TBM-EPB excavation
chamber there is the risk of clogging, therefore it is essential to measure the adhesiveness [Mixing Test and Pull-
Out Test] and the resistance [Vane Test and Fall Cone Test], which the mixture could oppose the normal
rotation of the cutter head, due to the presence of fine-grained aggregates. While, it is also necessary to check
the hydraulic seal of the soil, always in the excavation chamber when there is the presence in the soil of
coarse-grained aggregates [Abrasion and Permeation].
So, as you can well understand, the Laboratory Tests, to be done in the preliminary phase, before
excavation, are fundamental, because they can be extremely variable, compared to estimates and dosages,
initially indicated by suppliers.
These tests require highly specialized personnel with significant experience in the field and in similar
Experimental Studies. In fact, it may happen that the granulometric and mineralogical nature (e.g. presence
of clays containing montmorillonite) can influence both the chemical determinations of the surfactant and the
results of the eco-toxicological tests.
Our objective of the experimentation is to evaluate whether the soils conditioned with the selected foam
products representative of the excavation, can produce a significant effect of the Tests, compared to the
same unconditioned control soils, providing a range of conditioning parameters useful to define the type
of additive to be used and the quantities to be used.
f) -Polymer for conditioning the excavation grounds
Polymers are macromolecules made up of a large number of smaller repeating molecules (monomers)
chemically linked in long chains. The properties of polymers vary, depending on their chemical
composition and structure. The size of the polymer molecules (characterized by the molecular weight),
the branches or groups attached to the polymer chain, the cross-linking between the chains and the
intermolecular forces affect all the physical properties of the polymers (e.g. Bailar et. Al., 1989). A wide
range of polymers are used as soil conditioning agents in tunneling by TBM with EPB technology
(Milligan, 2000).
5. Fig. 03 – Soil conditioning examoples – EPB, foam production [The products: Chemsurf-H-38, Chemsurf-A-22,
Chemsperse and Chemcryl, for the tests, were kindly provided by the Italian chemical industry CHEMIX SRL
of Golasecca located in the province of Varese, Italy]
One of the most important groups of polymers used for soil conditioning and lubrication are
polyacrylamides (PAs) and their derivatives.
They maintain their solubility in water thanks to their high degree of linearity; the introduction of high
degrees of crosslinking produces materials that do not dissolve but swell and absorb large quantities of
water.
The partially hydrolyzed polyacrylamide derivatives are copolymers (PHPA) and are most commonly used as
soil conditioning agents.
PHPA is a linear copolymer of anionic acrylate and non-ionic acrylamide monomers whose molecular charge and
weight can be varied to create polyacrylamide polymers with different properties.
This type of PHPA polymer can be used for use in the presence of fine sands, silts and clays with a high
water content and has powerful structuring and viscosifying effects by binding soil and water to improve
the consistency of the spoil providing excellent lubrication.
PHPA products, with anionic (negative), non-ionic (neutral) or cationic (positive) charges, which
influence their interactions with linear-chain and highly water-absorbing soil particles, forming a soft gel
when hydrated .
Furthermore, the molecular weight can vary by several orders of magnitude, influencing the viscosity of the
polymer in solution and their dispersing or flocculating action.
In fact, the anionic charge of the PHPA molecule depends on the ratio between the monomers of acrylamide and
acrylate combined in the polymerization reaction.
The most common additives that are used for the conditioning of soils, as already indicated above, are:
free water, foams composed of air and a solution in water of a foaming agent, to which a polymer, composed of a
solution of water and polymer typically anionic at 30%, having a molecular weight of the order of 105 g / mol.
The molecules have a linear structure, resulting in high water solubility of PHPAs with a high molecular
weight.
It is possible to obtain similar results with polymers that have the characteristic of interacting with water
forming long chains of molecules, thus creating a mixture capable of compensating for the scarce presence
of fine soil and helping to keep the soil grains in suspension larger.
Another feature of PHPA polymers with molecular weights higher than 105 g / mol, are used to act as
flocculants in the presence of clayey soils through a clay disintegration mechanism.
In fact, anionic polymers establish a chemical-physical interaction with each other on the surface of the
clay particles through electrostatic attractions on the edges of the particles and through other chemical-
physical separation bonding mechanisms, between two different phases, forming a structure of
flocculated clay.
CHEMSPERSE
- CHEMSURF-A-22
- CHEMSURF-H-38
- CHEMCRYL
ADDITIVE 2
ADDITIVE 1
WATER
SOLUTION
AIR
FOAM
INJECTED
AVERAGE COMPOSITION FOR A NORMALLY USED FOAM:
- CHEMSURF-A-22 AND CHEMSURF-H-38 : 0.5 – 1 %
-- WATER: 5 – 10 %
-- AIR: 90 -95 %
-- CHEMCRYL (EVENTUAL): < 0.1 %
FOAMING AGENT CAN HAVE INSIDE A SMALL AMOUNT OF POLYMER TO
STABILIZE THE FOAM BUBBLES
FER
FIR
6. In construction site practice and on natural soil samples prepared for laboratory tests, it was possible to
highlight that numerous chemical factors influence the interactions between polymers and clay particles, such
as the molecular weight and the charge of the polymer, the pH and the ionic strength of the solution, which affect
the shape of the polymer molecule and the surface charge of the clay particles in solution on soils containing
varying fractions of clay [composed of montmorillonite, mica, vermiculite and kaolinite], silt and sand and siliceous
sands with variable particle size.
The flocculating action of PHPA polymer solutions on these types of excavated soils form a paste with
greater plasticity capable of absorbing large amounts of water.
In fact, the use of this type of polymeric formulation is able to be injected [also through a foam generator]
as an aqueous-based solution or as an additive to foamy solution and bentonite sludge, where they can be used to
complement the properties of these agents, forming a combined conditioning treatment improving the
control of the soil flow allowing a reduction of the friction coefficient, a better lubrication of the auger,
of the excavation chamber and of the tools [Cutters] and a reduced wear and clogging of the TBM-EPB
, as often happens in the presence of clay in alluvial excavated soils.
Fig. 04 – Conditioning layout showing injection at cutter face, working chamber and screw conveyor
f.1)-Anti-clay polymer additives
Clay is a natural rock or soil material which is fine grained. It consisting of one or more clay minerals
with metal oxides and organic matter. The property of clay changes due to its water content. The plasticity
of clay increases with higher amount of water; when it is drier, it becomes hard, brittle and non-plastic.
Excavation in clay formation has many disadvantages both for operation efficiency and tool wearing such
as:
- Clogging and adhesion in cutter head;
- Trouble with the Cutters;
- Difficulty in the movement of the excavated material;
- Blocking and aggregation;
-Reduction in torque efficiency;
-Reduction of rate of excavation.
In general, substantial modification of soil suitable for EPB is also possible in clayey soil. As in porous
soils, the use of foam alone will not be successful in most cases. To correctly create the homogeneous
soil paste to the excavation it is necessary to use disintegrating additives for the clay.
All
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WEB
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FER
FIR
7. f.2) -Soil Conditioning Laboratory Test through a “Disgregation Test”
The disintegration tests were carried out with the aim of verifying the capacity of the dispersing agent of
the “Chemsperse” type.
They were identified 2 test procedures [fig. 05 and 06]:
1) - Addition of dispersant solution;
2) - Immersion in the dispersant solution.
The procedure of Example 05 was applied to a second soil previously prepared for sieving means at the
ASTM n. 4 in order to completely eliminate the loop. Subsequently, 3 well-known soil samples by weight
(1 kg) were collected to which a different quantity of dispersing solution was added (increase in geometric
progression). The solution was prepared in the ratio of 1% of the volume of the tested medium (35 ml
"Chemsperse" for a total volume of estimated medium equal to 3500 ml). After the addition of the solution,
the weight of the material passing through an ASTM # 4 sieve was determined, correcting the value to
take into account the addition of the solution itself.
The procedure of Example 06 was applied to the soils 2a) and 3) and provides for the immersion of 3
soil samples of known weight (about 75 g and 100 g for the second and 3 respectively) land in a constant
volume of solution (500 ml) prepared with variable percentages (0.5, 1.0 and 1.5%) of “Chemsperse”
compared to the estimated volume of the samples (about 50 ml and 60 ml for the soils 2a and 3
respectively).
f.3)-Evaluation parameters required
It was asked to evaluate and propose possible soil conditioning solutions for land consisting of "mixture
of sand" and "clay".
• Soil conditioning product: Foam + Polimer
Typically, Foam is used at 2 to 3% (range 0,5 – 4%) in water to make a solution Polimer liquid polymers
can be jointly used with Foam to strengthen the foam or to adjust the properties of the excavated soil.
What are the benefits of Polimers
• Reduced permeability and increased sealing at the face.
• Even and controlled support pressure and increased face stability.
• Lower inner friction and lower abrasiveness of the soil at the cutterhead through to the screw
conveyor, reducing power consumption and wear to the cutters.
• Increased cohesion of coarse, clean sands and gravels – smoother soil extraction.
Fig. 05 - Res
ultsof thetestsperformedfor thec
ombination of
s
lump2A
Fig. 06 - Res
ultsof thetes
tsperformedon theslump2A and
2B c
ombinations
OPTIMAL AREA
8. • Water soaking and swelling effect, turning wet soil into a more manageable consistency.
• Improving the yield of bentonite slurries. Suitable for bentonite slurries in saline conditions.
Typically the dosage of liquid Polimer is betwee, 0,3 to 3 % on the foam solution (0,2 to 2 kg/m3
) on the
volume of soil excavated.
f.4)-Parameter characteristics of the foam
The main features that describe the Foam used in the field of soil conditioning TEST for excavation
with EPB are:
- concentration of the foaming agent: we have seen during the TESTS that much depends, besides
the choice of the value to be used during the generation of the foam, the characteristics of the type of
foam generator and the type of water that is used.
Therefore we have come to the conclusion that the project value must be exclusively defined at
the operational level.
- expansion ratio (Foam Expansion Ratio - FER): we must consider a "wet" range of 8-10, and a
"dry" range of 14-20.
- soil injection value (Foam Injection Ratio - FIR).
- time of "half-life" of the foam: we have seen during the TEST that much depends on the expansion
ratio FER.
In the tests the concentration of the foaming agent was 2%.
g)-Conclusions
From the analysis of the test results it can be concluded that the addition of Foam [containing a certain
percentage of "Polymer"] is necessary for conditioning the soil to give the material a homogeneous
behavior.
It should also be emphasized that the results obtained from the tests and reported in this brief illustration,
the reports refer to tests carried out in the laboratory and in a second phase, verification tests were carried
out directly on site because, those indicated by the laboratory tests, they are to be considered indicative
and preliminary.
In fact, during the excavation phase, the values obtained in the laboratory were calibrated and verified,
on a daily basis, during the continuation of the excavation with the TBM-EPB.
NOT ACCEPTABLE
CONDITIONING,
TOO DRY
OPTIMAL CONDITIONING
NOT ACCEPTABLE
CONDITIONING, TOO
FLUID
TEST
TEST
TEST
9. In conclusion, I would like to underline that today, the products used as conditioning agents present on
international markets are all biodegradable and there are no problems for the disposal of the waste
material.
However, for some products, it has been found that it is necessary to make a pre-washing of the same
conditioned waste material, so that it can be 100% environmentally friendly and non-toxic, before its
removal to landfill.
I remember that it is necessary that all products must be checked, through preventive laboratory tests, so
that it is shown that they respond positively to environmental regulations, before their use.
[*] Luigi Franco, LAMANNA
Independent Technical Consultant in the sector of Tunnelling, Mining and Underground Technology
President of the Fondazione Internazionale di Centro Studi e Ricerche, No-Profit
132, via dei Serpenti, 00184 ROMA, Italy, U.E.
Email: lamannaluigifranco1@gmail.com