This paper shows methods and factors for the calculation of optimal annular gap dimensions and presents the current state of the art for optimal lubrication.
Pipe jacking has made enormous progress in the past 30 years through the development of so-called microtunnelling.
Starting with the non-accessible tunneling, for which this remote-controlled technology was developed in the early 1980’s, tunnels with an outside diameter of more than 4 m are now being driven without operating personnel in the tunnel using this remote-controlled technology.
Driving 1,000 m and longer is no longer a rarity today. Apart from the interesting innovations in the field of mechanical engineering, among other things with regard to drill heads for loose ground up to the hardest rock, in the field of surveying and conveying technology, which made such long tunnelling routes possible in the first place even with many curves in the horizontal and vertical axis, the better knowledge of the pipe statics with corresponding online control measurements with continuous overload control must also be mentioned here.
In past years, these topics have been repeatedly reported on in the technical literature.
However, much less has been reported so far on the subject of the annulus, which is particularly important for long-distance tunnelling.
Today, the use of fully automatic lubrication systems often results in skin friction of less than 1 kN/m² surface area. Under good conditions, such long tunnelling operations can therefore be carried out exclusively with the main station.
Nevertheless, for safety reasons, intermediate jacking stations must be used every 80-120 m, which can be activated if necessary.
This paper shows methods and factors for the calculation of optimal annular gap dimensions and presents the current state of the art for optimal lubrication.
It also shows which mistakes can be avoided and which other, so far not sufficiently discussed risks in the annular gap can additionally occur and who is responsible for this from the author’s point of view.
Dimensioning of the annular gap
In general, the principle applies: the larger the outside diameter, the larger the overcut. In the following, the overcut is defined according to the valid German DWA A 125, chapter 3.1.19 with reference to the radius.
The overcut is usually between 5 and 50 mm. 5 mm applies to small diameters up to approx. OD = 300 mm, 50 mm applies to large pipe jacking with OD from 3,000 mm.
In individual cases, in addition to the topic of skin friction, further project-specific boundary conditions such as cover, possible settlements, pipe length, routing with or without bends as well as possible deviations caused by the control system must always be taken into account.
Whether the annulus is actively specified by the drill head itself – usual for slurry machines – or by the shield with internal drill head should be decided on a project-specific basis. In particular with strongly rolling material, the alternative with an internal drill head should always be discussed.
An incorrect dimensioning of the annular gap can have a strong influence on the skin friction. However, especially in rock jacking, it can also lead to a complete stop of the jacking or to damage of individual pipes.
Figure 1 shows an example of the effect of a too small annulus in a straight rock tunneling.
Reason: every controlled drive inevitably has deviations from the nominal axis.
The permissible deviations are limited to max. permissible values in the aforementioned DWA A 125, among others.
In the present case, the overcut was not sufficient to push the following reinforced concrete pipes through the cavity without forcing due to the executed control movements with the resulting deviations.
Of course, in addition to the size of the annular space, the pipe length and the quality of the control system play a decisive role here.
If a jacking pipe is subsequently “jammed” – usually the first one behind the machine – the jacking either gets stuck or the pipe is damaged.
As a rule, so-called radial cracks occur approximately in the middle of the pipe, especially in rock drifts.
Reduction of skin friction
In addition to proper dimensioning of the annular space, the recommendations presented here follow the goal of achieving the best possible skin friction.
This is an important parameter for successful tunnelling, especially in case of sandy or pebbly soils. It is crucial that the annular gap created by the drill head or the shield is immediately filled with bentonite.
The bentonite fills the annular gap and reduces the skin friction by not only acting as a lubricating film but also by supporting the surrounding soil.
In the case of a small pore size of the subsoil, such as in silt, the supporting effect can already be produced by a relatively thin outer filter cake.
However, if the pores are large enough to allow the bentonite to penetrate into the surrounding soil, for example when driving into coarse sand or gravel, the required supporting effect must be achieved by means of stagnation.
For this purpose, the bentonite suspension is mixed and prepared according to the specific project, so that the properties are available to allow stagnation in the corresponding pore size.
Here, the static yield point (according to DIN 4126) is particularly important as the decisive reference value, which is measured with the “Kugelharfe”.
If the bentonite stagnates in the pores of the subsoil, the supporting pressure can be applied in the annular space. Due to the penetration of the bentonite into the pores, a certain additional quantity of bentonite – the so-called multi grouting quantity (Bentonithandbuch Praetorius & Schößer, 2016) – is required to stabilize the annular space.
Automatic bentonite lubrication systems control, for example, the initial injection directly at the machine in order to inject the required amount of bentonite at the first injection point.
This is an important prerequisite for sufficient lubrication of the annular gap and thus for the lowest possible skin friction.
Along the remaining pipes, this built up lubricating film then only has to be “maintained”. It is important here that the primary lubricating film is maintained continuously and does not break off.
This is the state of the art when the common lubrication systems are operated properly. With a real-time display of the grouting quantities along the pipeline route, it is possible, for example, to immediately detect if the primary lubricating film has not been applied in the volume-controlled lubrication system from Herrenknecht AG –see Figure 2-.
This system is also designed to automatically correct the error at the next bentonite station.
A subsequent injection in every third to fifth pipe maintains and cares for the built up lubricating film.
Depending on the advance speed and geology, the bentonite is injected in the required quantity at the appropriate points around the pipe string.
The automatic delivery of defined quantities of bentonite for the individual sections avoids over- or undersupply and ensures efficient lubrication.
Interaction between start-up seal, annular gap lubrication and working face support
In order to be able to keep the bentonite once introduced and thus a qualified annular gap lubrication in the annular gap continuously, a start-up seal is absolutely necessary.
If this is not available or if it leaks strongly -see figure 3-, the bentonite can be flushed out of the annular gap if ground water is present.
A functional start-up seal is therefore an elementary prerequisite for qualified annular gap lubrication.
The interaction between the working face support and the annular gap support is similar.
The working face, mining chamber and annular gap are hydraulically connected.
If the annular gap is not sufficiently filled with bentonite, part of the drilling mud can migrate into the annular gap when the supporting pressure is applied to the working face –see figure 4- and deposit undesirable fine material there –see figure 5–.
This case can be recognized by the control of the fluid circulation. In principle, the quantity in the production line should always be greater than the quantity in the feed line.
If the annular gap is sufficiently filled with lubricating bentonite, the migration of drilling mud into the annular gap is prevented and the lubricating effect is not adversely affected.
Correct mixing and swelling
To ensure that the bentonite suspension can reliably fulfill its purpose, special attention must be paid to its preparation. The advantages of well-mixed and sufficiently swollen bentonite are good yield and greater stability against impurities in the subsoil.
The mixing water, mixing time, mixing efficiency and swelling time have a great influence on this. The mixing water should have a low hardness and a pH value of 8 to 10.
With sodium carbonate (Soda Ash), both pH value and hardness can be corrected if necessary. The mixing water should also be free of other impurities such as high salt concentrations or chlorine.
Mixing time and mixing efficiency are strongly related and always depending from the temperature. It is important to apply a high shear energy in order to disperse the bentonite platelets as well as possible in the water, i.e. to distribute them.
In small mixers with shear impeller, such as when using a Häny IC1100, mixing times of 10 to 20 minutes are required for 250 L of suspension.
Larger mixing plants with Venturi mixers, such as the 20-foot mixing plant from Herrenknecht with 18 m³ tank size, require at least 20 – 30 minutes for mixing.
The necessary mixing and especially the subsequent swelling times vary depending on the type of bentonite. The bentonite supplier will provide the relevant details.
Some modified bentonite types, for example, swell after 2 hours, some natural bentonite require at least 10 hours. In most cases natural bentonite is cheaper, but require larger tank capacities.
In addition to machine diameter and driving speed, the existing subsoil has a major impact on the required bentonite quantities per pipe or day and thus on the necessary installed pumping capacity.
Together with the mixing and swelling times, this results in requirements for the swelling tank sizes that must be maintained.
If too small swelling tanks are used, there is a risk that an insufficiently swollen bentonite suspension will be pressed into the annular gap. In this case, the suspension would not only have a lower resistance to unfavourable conditions such as high water hardness.
It would also result in poorer filter cake formation at the working face or in the annular space. Compared to a well-swollen bentonite suspension, higher quantities of bentonite powder would also have to be used to achieve the set target values, such as a certain static yield point.
Table 1 gives an overview of the recommended swelling tank or mixing plant sizes depending on machine size and soil. The parameters are shown for a modified bentonite type which is ready for use relatively quickly.
Table 1 illustrates that the type of ground has a much greater influence on the required size of the mixing plant than the diameter of the tunnelling machine.
This correlation must be taken into account anew for each project in order to prevent that source tank sizes proven in the past turn out to be too small for a subsequent project.
Recommended measuring methods for the lubricated bentonite
To ensure that the bentonite suspension consistently exhibits the required properties, the static yield point (DIN 4126) and the Marsh viscosity (DIN 4126) should be measured at regular intervals.
If unfavourable chemical conditions exist for the bentonite, the filtration property should also be determined using a filter press (API 13 B-1 or DIN 4126).
The measured values in Table 2 can be regarded as rough guide values for different types of soils. These must be adapted to the special conditions of the respective construction project.
Table 2: Reference values for static yield point and Marsh viscosity for lubricated bentonite suspensions (source: Herrenknecht AG)
|Typ of underground / dominating grain sizes||Marsh- viscosity [sec.]||static flow limit“ball harp” [N/m²] resp. [# ball]|
|Clay, siltstone, claystone, slate, water sensitive stones||35 – 45||10 – 13 (# 2 – 3)|
|Solid rock (no larger gaps)|
& wide graded loose rock (z.B.till)
|40 – 55||13 – 20 (# 3 – 4)|
|Fine sand to medium sand||50 – 80||20 – 38 (# 5 – 7)|
|Coarse sand, well sorted||80 – 120||38 – 60 (# 6 – 9)|
|Loose rock dominated by gravel||80 – 120||38 – 60 (# 6 – 9)|
|Loose rock dominates with stones||120 – 140||60 – > 70 (# 9 – 10)|
|Gravel, stones with very little sand and fine particles||140 – 170|
(difficult to measure)
|60 – > 70 (# 9 – 10)|
Special conditions require special measures
By adding different additives, the bentonite can be adapted to the respective soil conditions. Various manufacturers offer, for example, admixtures for projects in saline subsoil or with salty groundwater.
In addition, there are suitable additives for high degrees of hardness or high calcium values, which occur, for example, when concrete is driven through.
The admixtures prevent decomposition of the bentonite suspension and thus the loss of the desired properties.
A special aspect must be taken into account for pipe jacking in swellable clay: The introduction of a bentonite suspension that has not yet matured or that has not yet swollen properly can cause the soil to swell.
This, in turn, can lead to high skin friction and can even cause the pipe string to jam.
The lubricated bentonite has a long residence time in the annular gap. It must therefore be avoided that the suspension releases water over time and releases it to the surrounding soil.
This can first be supported by the complete swelling of the lubricating bentonite before injection and can be prevented with the aid of special additives.
It is important to know the swelling potential of the soil in advance. According to the German ATV DIN 18319, this must be determined in the course of the soil investigation and described in the soil mechanics report for subsoil types with potentially swellable minerals. In this way, these conditions can be taken into account in planning and execution.
Interaction of bentonite lubrication and intermediate jacking stations
In professional bentonite lubrication, skin friction of less than 1 kN/m² is not uncommon today when using fully automatic lubrication systems.
Nevertheless, it is recommended for safety reasons to carry ready-to-use intermediate jacking stations approx. every 80 – 120 m, with the first intermediate jacking tool as close as possible to the drill head.
These are urgently needed in case of partial failure of the lubrication or other friction problems (see below). Under such unfavourable and possibly unforeseen conditions, skin friction of far more than 10 kN/m² can occur, in exceptional cases even up to 100 kN/m².
Monitoring of the actual skin friction during driving
Especially for long drives, geologically challenging routes and/or particularly challenging project parameters, it is recommended that all the intermediate jacking joints used be brought into operational readiness and driven a few centimeters – even if the intermediate jacking joints are currently not actively supporting the drive.
In this way it is possible to measure and document the respective forces for each intermediate jacking station. In this way, the forces acting on the pipes in front of the respective jacking station can be determined and continuously compared with the permissible forces for the pipes in front of the jacking station, which are determined by a jacking monitoring system – OLC, Jackcontrol, CoJack, etc-.
If significant deviations are found, the cause must be investigated. Possible causes include improper lubrication, the problems described above with the starting shaft seal or working face support or other reasons described below, such as underground cavities, eccentric stones or strongly varying hydrostatic pressures of the ground or fissure water.
Figure 6 below shows such an evaluation by an OLC Online Load Control System. Here, using the example of an intermediate jacking station, the current force on the pipes -green- and the permissible jacking force – red- which is determined by continuous buckling measurement based on the load history is plotted.
The permissible force -green – calculated according to the pipe statics is also shown for information.
Problems of uplift during tunnelling underground water
When tunnelling in groundwater, the pipe string is subject to vertical forces in addition to the axial driving forces and the forces resulting from the soil or rock. These are the dead weight of the pipes with the installations inside, which act downwards according to the earth’s gravitational pull.
The uplift, which is calculated by multiplying the pipe volume by the specific weight of the surrounding liquid, acts in the opposite direction. Especially with larger pipes, there is therefore almost always a considerable resulting force component upwards.
Care should be taken when making calculations, especially with regard to the specific weight of the surrounding liquid.
Depending on the mixture, pure bentonite already has a specific weight of approx. 1.05 – 1.1 t/m³. If you add to this a contamination by the surrounding soil, the specific weight is already approx. 1.10 – 1.15 to/m³.
When used in salty soils or near the sea, the salty water alone can already have a specific weight of up to 1.05 to/m³. All together must be counted therefore in salty ground water with up to 1.20 – 1.25 t/m³ in extreme cases.
This calculation shows the importance of a previous calculation and, if necessary, also the execution of necessary tests to determine the specific weight of the bentonite liquid.
What are the effects of uplift on pipe jacking? In the following 2 cases are shown where uplift can have negative effects on the pipe jacking.
Uplift in case of too much soil extraction: “Over-Excavation”:
Especially in case of insufficient working face support and rolling/flowing soil conditions, the so-called “over excavation” can occur. In this case the drill head extracts more soil than corresponds to the volume of the excavated cross-section. This results in loosening of the soil above the pipes, which can lead to smaller caverns that are immediately filled with bentonite. As a result, the uplift of the pipes can cause them to shift upwards from the excavated axis as the drive progresses.
To prevent such scenarios, it is important that the machine is driven at a speed adapted to the ground conditions. This can be ensured if, in addition to maintaining the pre-calculated hydraulic support pressure, the front pressure and torque of the drill head are constantly monitored and do not fall below pre-determined minimum values.
A quantity measurement of the excavated material and coordination with the advance speed, which is often demanded by clients, unfortunately still fails today due to the insufficiently accurate measuring methods. In addition, there is the inaccuracy due to the necessary estimation of the “in situ” density of the soil to be excavated.
Due to the aforementioned shortcomings, errors of approx. 5 – 10 % can easily occur in such quantity measurements with the current state of the art.
Over excavation at this height would not be acceptable under any circumstances, neither with regard to the floating of the pipes nor to the inevitable subsequent subsidence. In this case, the aforementioned monitoring of front pressure, hydraulic support pressure and torque of the drill head is more effective until a qualified measuring technique is available.
Uplift due to abrasion:
This is a process that occurs only in relatively few drives, although uplift is present in all drives under water. Especially when driving in soft rock, uplift due to abrasion occurs more frequently. The author’s experience in this area has been gained in the Arabian Peninsula and in various sea outfalls.
In such soft rock – mostly sand or siltstone – the jacking pipes now constantly rub with the ridge against the rock due to the uplift.
As a result, the soft rock is rubbed off by the jacking pipes during the jacking process and this fine material – specific weight approx. 2.5 to/m³ – sinks downwards under the force of gravity despite the bentonite filling of the annular gap due to the dynamics of the jacking process.
Of course, this process is particularly favoured by the strong wall roughness of the jacking pipes, poor pipe connections – partly with protrusions – and the use of unsuitable bentonites – see chapter 2.
The fine material is deposited under the jacking pipes, while abrasion continues to take place at the top, so that the pipes drift further and further upwards. Figure 7 shows the survey results that document this floating up during such a drive by means of frequent geodetic tunnel surveys. It is clearly recognizable that the vertical deviation of the pipes from the axis increases with continuous driving.
Fixed points here are of course the first pipes at the start shaft – these are held by the extension structure – and the front pipes directly behind the machine, which are pulled down due to their connection to the machine. The machine itself of course does not float up due to its heavy weight. Interesting is also the wavy course of the vertical change of position in the present example -see figure7-. It can be seen here that the soft rock had a somewhat higher strength, so that the abrasion and thus the change of position was lower. This fact – different rock strength – was also recognizable according to the geological preliminary investigations.
In addition to the sometimes considerable change in the vertical position due to the floating of the pipes, the result can be a jamming of the pipe string or even the destruction of individual jacking pipes due to constraints. Below is the explanation for this in an exemplary sequence:
- the drill head moves within the permissible tolerances in height and side, initially through very soft rock, then in harder rock
- the following pipes are deflected upwards in the soft rock area due to the processes described above
- more and more fine material is deposited under the pipes
- at the transition from soft to hard rock – or, of course, when passing through intermediate shafts – the following pipes cannot float up and have to follow the drilling path of the tunnel boring machine, so that they are “forced” onto the target axis. In this process, the pipes are forced downwards and the fine material underneath is partly pulled along with them
- this causes the fine material under the pipes to become extremely dense, so that the pipes are “squeezed” by vertical forces in the sole and roof
- this results in an extreme increase in skin friction over a relatively short distance and high constraining forces on the pipes located in this area. As a result, there are only 2 possibilities: either the tunnel drive jams due to excessive skin friction or individual pipes fail due to the external load, which produces fracture patterns similar to those of a peak pressure test, see figure 8.
How can the floating of the pipes be prevented?
First of all it is important to know this phenomenon. Based on the geological and hydrogeological investigations, which should be carried out before any construction project, an initial risk assessment can be made.
A risk always exists when extremely soft rock, or even highly compacted sand is present in the driving horizon with a simultaneous groundwater level above the ridges. Particularly critical are conditions in which the excavated cavity would not collapse even without a pipe.
In such conditions it is important to use pipes with the smoothest possible walls and precise pipe connections without any protrusions. The only real possibility known to the author so far to counteract the uplift is ballasting. This must be dimensioned on the basis of the simple calculation shown above.
Figure 9 shows 2 examples of such ballasting.
The occurrence of so-called “eccentric stones” can occur in all tunnelling operations, in large or small – here however less frequently – diameters, in long or short tunnelling distances.
The effects/damage caused by such stones usually occur in the roof areas of the pipes. If these stones are not firmly embedded in their matrix, they can sink onto the pipe due to gravity, since even proper annular gap lubrication does not permanently support a single stone.
A prerequisite for “eccentric stones” is, in addition to the presence of such stones in the vicinity of the pipes, a relatively firm, densely bedded sandy/gravelly or solid cohesive soil or rock. Such an “eccentric stone” can be very small – a few cm – or large – up to 1 m and larger.
According to experience, “eccentric stones” usually occur in the roof area between 10 and 14 o’clock.
Such “eccentric stones” are usually not touched by the drill head at this time, or at most they are slightly machined. In the course of the jacking, it can happen that such a stone comes into contact with the jacking pipe due to strong wall roughness of the jacking pipes, small offsets at the pipe outer diameter, mostly at pipe joints or at the intermediate jacking stations.
If the stone gets stuck with a protruding part, a nose, e.g. at such an offset, it is pressed into the soil/rock by the driving force. However, if the surrounding soil/rock is so firm that the stone cannot be displaced, then the stone and the pipe will be subjected to high loads.
Since most stones are not spherical, the stone usually tries to turn. Then it acts as an “eccentric” and an extreme point load on the jacking pipe can occur. In the following jacking there are 3 possible scenarios:
- the pipe string gets stuck
- the stone is destroyed or displaced
- the pipe is destroyed
Re 1: If the jacking gets stuck, you can do an elaborate search for the damaged area. Here one can roughly orientate oneself by the forces of the intermediate jacking stations, provided that these are always a few cm extended –see above, chapter 3.6- even when pushing with the main station, in order to determine in which section of the drive the pipes get stuck. Afterwards, it is only possible to determine to where the string can still be moved by means of plaster marks or exact joint measurements. Finally, core drillings are necessary to explore the exact position of the stone, if at all, and to remove it.
Re 2: If the stone is destroyed or displaced into the surrounding soil, normally nobody notices.
Re 3: If the pipe is destroyed at certain points, the damaged area can be discovered immediately, at least in the accessible tunnel drive. An underground repair is usually possible, in groundwater however sometimes critical or very costly –see figure 10-. In non-accessible tunnelling, only a mining shaft can be made at the damage site.
There are few measures to avoid the effects of such “eccentric stones”. Basically, the pipes should be as smooth as possible on the outside and have minimal offsets at the pipe connections.
A solution for intermediate jacking stations to avoid the extreme misalignment in the area of the intermediate jacking follower pipe would certainly be a task for qualified pipe manufacturers.
Clamping wedges in the annular space
Such “clamping wedges” -see figure 11- occur exclusively in rock drifts. They are usually individual wedge-like pieces of rock that break away from the existing geology, fall into the annular gap and subsequently jam the jacking pipes. In this respect they are quite comparable with the aforementioned “eccentric stones”.
Due to gravity, these stones also usually fall onto the pipes in the area of the ridges. The loosening from the rock matrix is promoted by flat or slightly inclined bedding of layers, slate surfaces or fissures. Measures to prevent such occurrences are not known to the author.
Special features concerning the importance of the annulus for pipe jacking
a) Different hydraulic pressures:
Uncontrolled water flows in the annular gap can flush out the bentonite. This problem rarely, if ever, occurs in rock drifts. In principle, the danger exists if the jacking passes through several fissures or other water-bearing geological faults. If these water-bearing geological faults have very different water pressures, a strong flow of water in the direction of the lower pressure via the annular gap occurs at the moment when the cutting head of the machine hit the second fault.
The same can also happen if the jacking is driven out of a rock with a high water pressure inside cleavings leads to a highly water-permeable soil without groundwater or if unknown old cavities are approached. In such cases, the lubricating bentonite can be washed out of the annular space, resulting in increased skin friction. Countermeasures can only be taken if this danger situation is recognized in time by means of appropriate injections before the start of driving.
b) Steel inside the annular space
Steel guide rings cannot withstand the tensile force that occurs due to complication and fail –see figure 12-. Reasons can be insufficient material thickness and/or insufficient anchoring in the concrete.
After damage, these steel rings can become jammed in the annular space, especially in the case of rock jacking, and hamper driving. In this case, the author recommends defining corresponding requirements in the relevant standards – e.g. German DWA A 125- for steel and anchoring in the future.
In addition to the enumeration of the problem definitions and the pointing out of possible solutions it is to be listed here (see table 3) in summary, which of the parties involved in the project is responsible in the opinion of the author for the respective topic.
Table 3: Responsibilities
|Provision of results of underground and groundwater investigation for the selection of the bentonite lubrication||x|
|Implementation of adapted bentonite lubrication||X|
|Default measuring method for the bentonite suspension||x|
|Note on uplift protection||x||X|
|Implementation of uplift protection||X|
|Expenses for unforeseen “eccentric stones” and clamping wedges||x|
|Requirement constant monitoring of the intermediate jacking station forces||x|
|Execution Monitoring of intermediate jacking station forces||x|
|Expenses for unknown strong water flows in the annular space||x|
|Cooperative behavior towards the other parties||x||x|
The distribution of responsibilities contained in Table 3 has been the subject of repeated discussions and disputes. For this reason, when preparing the award documents, the client should prepare this table or an extended/even modified table, which is bindingly introduced as an integral part of the contract.
This table can be extended with reference to further responsibilities which have to be regulated in microtunnelling.
This ensures more clarity, fewer disputes and serves a more effective execution of the tunnelling project.
As described at the beginning, pipe jacking has made enormous progress in the past 30 years. The limits of what is technically feasible have been pushed further and further and both -quality and reliability- have been continuously optimized.
Modern and efficient measuring technology improves quality assurance and innovative online monitoring systems allow the site management and the client to have an up-to-date insight into what is happening on the construction site at any time.
The identification of still possible problems and the attempt to explain these interrelationships in this article should help clients and planners as well as the executing construction companies and manufacturers of machines, pipes and materials to improve their level of knowledge and help to consistently continue the successful path of further development of the environmentally friendly pipe jacking technology.
By Dr. Ing. Hans-Peter Uffmann, Public appointed and sworn expert for pipe jacking and microtunnelling, Aachen
The author would like to thank Dipl. geologist Steffen Praetorius, Head of the Geotechnical Engineering Team, Business Unit Utility Tunnelling, Herrenknecht AG, Schwanau, for his intensive collaboration on this technical paper and for contributing his expertise and practical experience on the subject of bentonite.
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