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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Pipe jacking is a construction process for the no-dig laying of pipes. Successful pipe jacking demands low skin friction between the ground and the jacked pipe. This is achieved with bentonite lubrication. The bentonite slurry fed into the annular gap fulfils several purposes. It stabilises the annular gap by supporting the surrounding ground and reduces friction contact between ground and jacked pipe. The Bentonite Handbook deals comprehensibly with the relevant aspects of annular gap lubrication: starting with the ground conditions, which are of decisive importance for lubrication, through the rheological properties of the bentonite slurry to the technical components of lubrication technology and lubrication strategy. The use of standardised measuring apparatus is described as well as mixing equipment and the automatic lubrication system. Overview tables with calculations and suggested values for bentonite consumption quantities depending on the prevailing ground conditions and the pipe jacking parameters complete the recommendations.
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Table of Contents
Cover
Title
Copyright
Dedication
Acknowledgement
Foreword
List of symbols used
1 Basics
1.1 Basics and technical implementation of bentonite lubrication systems
1.2 Annular gap lubrication in pipe jacking
1.3 Preliminary remarks about the ground
2 Bentonite and bentonite suspensions
2.1 Composition and structure.
2.2 Hydration behaviour
2.3 Card house structure and thixotropy
2.4 Yield point, viscosity and gel strength
2.5 Stability
2.6 Usual types of bentonite
3 Additives
3.1 Polymers
3.2 Types of polymer additives
3.3 Overview of polymer additives and their functions
4 Correct preparation of bentonite suspensions
4.1 Mixing
4.2 Mixing equipment
4.3 Mixing instructions
4.4 Hydration tank
5 Properties of the suspension and measurement processes
5.1 Viscosity: Marsh funnel
5.2 Yield point: ball-harp rheometer
5.3 Viscosity and gel strength: Rotational viscometer
5.4 Filtrate water and thickness of filter cake: filter press
5.5 Density
5.6 Water hardness: test strips
5.7 pH value
5.8 Conductivity
5.9 Temperature
6 Ground and groundwater
6.1 Geology of solid rock
6.2 Geology of soils
6.3 Stability and stand-up time
6.4 Hydrogeology
6.5 Influence of various rock properties on the use of bentonite
6.6 Contamination in the ground, groundwater or mixing water
7 Bentonite suspensions for annular gap lubrication
7.1 Size of the annular gap
7.2 Vertical position of the pipe string in the driven cavity
7.3 Functions of the lubricant in the annular gap
7.4 Adaptation of the bentonite suspension to the ground
7.5 Rheological parameters of the bentonite suspension
7.6 Suspension quantities
7.7 Time-dependant loss volumes
7.8 Lubrication strategies
8 Lubrication technology
8.1 Arrangement and spacing of the lubrication points in the pipe string
8.2 Number and arrangement of the injection fittings per lubricating point in the pipe cross-section
8.3 Non-return valves
8.4 Lubrication circuit
8.5 Interaction between the support pressure at the face and the annular gap pressure
8.6 Bentonite supply in the starting area
8.7 Lubricant pressure, lubricant quantity and pressure losses
9 Reporting
9.1 Which parameters should be documented for bentonite lubrication?
9.2 Forms
10 Lists of the required injection quantities
10.1 Explanation of the lists.
Literature
End User License Agreement
List of Illustrations
1 Basics
Fig. 1.1 Principle of construction of the standard Herrenknecht bentonite lubrication system
Fig. 1.2 Herrenknecht system lubrication station.
Fig. 1.3 Lubrication point in pipe string and distribution of the injection fittings around the cross-section.
Fig. 1.4 Branch from the compressed air feed pipe to an individual lubrication point.
2 Bentonite and bentonite suspensions
Fig. 2.1 Crystal structure of montmorillonite [8].
Fig. 2.2 Charge distribution of a montmorillonite elementary layer. The edges are positive (red), the surfaces negative (blue).
Fig. 2.3 Card house structure. Positive edge charge red; negative surface charge blue.
Fig. 2.4 Course of thixotropic solidification with time [23].
Fig. 2.5 Flow curves for Newtonian flow, Bingham flow with yield point and structural viscose flow [100].
Fig. 2.6 Flow curve of a thixotropic fluid (bentonite suspension) [100].
Fig. 2.7 Flow curve of a bentonite suspension [69].
Fig. 2.8 Different types of gel strength [80].
Fig. 2.9 Stable bentonite suspension with evenly distributed solid particles in water [97].
Fig. 2.10 types of segregation of bentonite suspensions: sedimentation (left), consolidation (middle), filtration (right) [97].
Fig. 2.11 Schematic diagram of sodium carbonate activation [7].
4 Correct preparation of bentonite suspensions
Fig. 4.1 Principle of a charge mixer.
Fig. 4.2 Example of a mixing system with water jet pump.
Fig. 4.3 Hydration tank with two agitators.
5 Properties of the suspension and measurement processes
Fig. 5.1 Viscosity measurement with a Marsh funnel [94].
Fig. 5.2 Determination of the dynamic yield point dyn τF and the differential viscosity η′ with the Marsh funnel according to DIN 4127 [23].
Fig. 5.3 Determination of the static yield point with the ball-harp rheometer [94].
Fig. 5.4 Layout of a rotational viscometer [94].
Fig. 5.5 Scheme of a filter press [94].
6 Ground and groundwater
Fig. 6.1 Uniaxial rock compression strength of a slate depending on the compression direction relative to the direction of the cleavage [74].
Fig. 6.2 Principle of a tensile splitting test [59].
Fig. 6.3 Dependency of the tension strength on the orientation of an anisotropic sample [59].
Fig. 6.4 Performance of a Cerchar test. The tip of the pin is drawn over the rock sample [72].
Fig. 6.5 Construction of a Cerchar apparatus: 1 + 3 clamp for the sample, 2 lever, 4 test pin, 5 test pin fitting, 6 weight [72].
Fig. 6.6 Classification of the surface condition of interfaces into the classes stepped, corrugated and planar/flat according to ISRM [40].
Fig. 6.7 Typical grading distribution curves for various soil types [43].
Fig. 6.8 Rounding scale for non-cohesive soil grains. 1: very angular, 2: angular, 3: sub-angular, 4: slightly rounded, 5: rounded, 6: well-rounded [48].
Fig. 6.9 Definition of porosity n and void ratio e [74].
Fig. 6.10 Model diagram of the loosest possible (a) and densest (b) compactness of uniform soil particles [30].
Fig. 6.11 Stand-up time as a function of the RMR and the Q value as well as the size of the cavity. Red data points represent tunnels, green points mines [77].
Fig. 6.12 Hydrogeological terms [52].
Fig. 6.13 Pore cavities (top left), joint cavities (top right) and karst cavities (bottom left) [14].
Fig. 6.14 Darcy’s flow law [52].
Fig. 6.15 Coefficient of permeability k
f
depending on the effective grain diameter d
10
and the coefficient of uniformity U [61].
Fig. 6.16 Hydraulic conductivity of rocks (rock mass permeability) [87].
7 Bentonite suspensions for annular gap lubrication
Fig. 7.1 Required jacking force with and without the use of intermediate jacking stations (interjacks).
Fig. 7.2 Uplift force and weight from the jacked pipe.
Fig. 7.3 Variable positioning of the bentonite outlets.
Fig. 7.4 Grinding effect due to uplift on the pipe string.
Fig. 7.5 Pure penetration based on Kilchert and Karstedt [57].
Fig. 7.6 External filter cake based on Kilchert and Karstedt [57].
Fig. 7.7 Internal filter cake based on Kilchert and Karstedt [57].
Fig. 7.8 Schematic diagram of fluid friction [79].
Fig. 7.9 Tunnelling machine blocked in rock due to entrained rock particles.
Fig. 7.10 The main function of the suspension depending on rock mass classification.
Fig. 7.11 Parameters of the suspension depending on the fissure opening width.
Fig. 7.12 Additives for the suspension volumes depending on the permeability of the rock mass.
Fig. 7.13 Weighting between lubricating and support functions in cohesive soils according to consistency.
Fig. 7.14 Weighting between lubricating and support functions in non-cohesive soils according to the compactness.
Fig. 7.15 Optimisation of the lubricant with regard to the activity/swelling of the soil.
Fig. 7.16 Supplements to the suspension volume according to the permeability of the soil.
Fig. 7.17 Parameters of the bentonite suspension dependant on effective grain diameter.
Fig. 7.18 Pure penetration for non-cohesive soil with d
w
> 6 mm or rock with 2a > 6 mm (high permeability).
Fig. 7.19 Bearing function of particles in bentonite suspension.
Fig. 7.20 Required lubricant quantities consisting of annular gap volume (left), extra suspension volume (middle) and subsequent injection (right).
Fig. 7.21 Required lubricant quantity of the initial injection volume for the tunnelling machine (from annular gap volume V
annular gap
and extra suspension volume V
extra suspension
).
Fig. 7.22 Required lubricant quantity for the lubrication points in the pipe string (subsequent injection volume V
pipe string
).
Fig. 7.23 Theoretical model for varying lubricant quantities V
machine
of the initial injection from the tunnelling machine under varying geological conditions over the course of the drive.
Fig. 7.24 Theoretical model for varying lubricant quantities V
pipe string
of the subsequent injection along the entire pipe string under varying geological conditions over the course of the drive.
Fig. 7.25 Variable initial injection volume V
machine
and subsequent injection volume V
pipe string
depending on the geological conditions, in each case per metre advance. For comparison: the annular gap volume V
annular gap
is constant over the course of the drive.
Fig. 7.26 Variable initial injection volume V
machine
, variable subsequent injection volume V
pipe string
and – for comparison – the constant annular gap volume V
annular gap
, in each case summed over the course of the drive.
Fig. 7.27 Individual suspension quantities at the individual points along the drive length over the course of the drive.
Fig. 7.28 Typical grading curves with associated value d
10
for various soil types [43].
Fig. 7.29 Uneven distribution of injected bentonite over the length of the drive when only normal cycle is used.
Fig. 7.30 Uniform distribution of injected bentonite over the length of the drive when normal and extra cycles are used.
Fig. 7.31 Route-related bentonite distribution with the use of a volume-controlled bentonite lubrication ssytem.
Fig. 7.32 Display of the system monitor of a volume-controlled lubrication system.
Fig. 7.33 Principle of initial and subsequent injection.
Fig. 7.34 Transfer of the initial injection: left normal initial injection, right initial injection in case of great danger of the shield rolling.
8 Lubrication technology
Fig. 8.1 Arrangement and spacing of the bentonite station in the pipe string.
Fig. 8.2 Arrangement of three or six injection fittings distributed around the pipe section.
Fig. 8.3 Offset arrangement of several injection fittings in one jacked pipe (HK).
Fig. 8.4 Offset arrangement of the injection fittings at adjacent lubrication stations.
Fig. 8.5 Installation of shut-off valves to the injection fittings.
Fig. 8.6 Non-return valve; left conical, right flap non-return valve.
Fig. 8.7 Relationship between the support pressure at the face and the pressure for annular gap lubrication.
Fig. 8.8 Annular gap supply from a static tank to the starting seal.
Fig. 8.9 Pressure losses per 100 m in a bentonite line depending on pipe diameter [73].
9 Reporting
Fig. 9.1 Report sheet for bentonite lubrication for standard sites.
Fig. 9.2 Report sheet for bentonite lubrication for more challenging sites.
10 Lists of the required injection quantities
Fig. 10.1 Explanation of injection quantities and pumping rates in rock.
Fig. 10.2 Explanation of injection quantities and pumping rates in soil.
Guide
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Table of Contents
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Bentonite Handbook
Lubrication for Pipe Jacking
Steffen Praetorius
Britta Schößer
Dipl-Geol. Steffen Praetorius Herrenknecht AG Business Unit Utility Tunnelling Schlehenweg 2, D-77963 Schwanau-Allmannsweier
Dr.-Ing. Britta Schößer Ruhr-Universität Bochum Lehrstuhl für Tunnelbau, Leitungsbau und Baubetrieb Universitätsstr. 150, D-44801 Bochum
Translated by David Sturge, Kirchbach, Germany
Cover: Principle of construction of the Standard Herrenknecht bentonite lubrication System (Source: Herrenknecht AG)
Library of Congress Card No.: applied for
British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.
Bibliographic Information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2017 Wilhelm Ernst & Sohn, Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Rotherstraße 21, 10245 Berlin, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Coverdesign: Sophie Bleifuß, Berlin, Germany Production management: pp030 – Produktionsbüro Heike Praetor, Berlin Typesetting: Reemers Publishing Services GmbH, KrefeldPrinting and Binding:
Print ISBN: 978-3-433-03137-7
ePDF ISBN: 978-3-433-60655-1
ePub ISBN: 978-3-433-60653-7
eMobi ISBN: 978-3-433-60654-4
oBook ISBN: 978-3-433-60652-0
For Angela, Lucia and Luana
S.P.
For Holger, Leo and Ole
B.S.
Acknowledgement
Intensive specialist discussion about challenges in pipe jacking practice and scientific findings has found its way into this Bentonite Handbook and sprouted new (research) ideas. For their valuable contributions, we wish to thank the After Sales staff of the Utility Tunnelling department at Herrenknecht AG as well as the experienced and motivated specialists on pipe jacking sites. For their detailed discussions, we wish to thank Ms. Dipl.-Ing. Geotechnik/Bergbau Christel Flittner, Mr. Dipl.-Ing. Tiefbohrtechnik Oliver Knopf, Ms. Irmhild Lauter, Mr. Ulrich Schröder, Mr. Hermann Spengler and Mr. Dipl.-Geol. Björn Zenner.
Mr. Dipl.-Geol. Matthias Botzenhardt made a particular contribution with his expert assistance regarding the subject of additives. The production of drawings and text was supported by Mr. Dipl.-Ing. Nick Biermann, Ms. Melanie Ruff, Mr. Roman Duda and Mr. Tobias Bucher.
We wholeheartedly thank Mr. Prof. Dr.-Ing. Markus Thewes and Mr. Dr.-Ing. Marc Peters as well as the research and development colleagues of the Utility Tunnelling department at Herrenknecht AG and at the Chair of Tunnelling and Construction Management at the Ruhr University, Bochum for their sympathetic support and the pleasant working atmosphere. The publisher Ernst & Sohn and particularly Mr. Dr. Helmut Richter, Ms. Esther Schleidweiler, Mr. Dr. Michael Bär and Mr. David Sturge deserve our gratitude for their support in the implementation and design of the book.
We would be pleased to receive feedback and suggestions.
Steffen Praetorius and Britta Schößer
Foreword
Pipe jacking is an indispensable process for the installation of underground pipes. Constant improvement of the machinery in recent decades has led to pipe jacking projects being successfully completed in almost any geology and hydrogeology, with challenging routes. The success of a pipe jacking project is ensured by smooth interaction of the tunnelling technology and the process operations. The main challenges, which are met daily on pipe jacking projects, are to minimise potential risks and to increase the practical distances.
The development of the jacking force over the length of the drive – and particularly the skin friction along the pipe string – is of central importance for the implementation of pipe jacking projects. Improved working methods can avoid increased jacking forces and the resulting delays to progress or stoppages. One essential element in the reduction of skin friction is well functioning annular gap lubrication, with the lubricant and the lubrication technology being adapted to suit the constraints of the jack and particularly the ground conditions. Both components – lubricant and lubrication technology – depend on important details and demand a good basic understanding on the part of the construction staff.
The lubricant mostly consists of a bentonite suspension, whose rheological parameters yield point and viscosity have to be adapted to suit the prevailing geological conditions on each pipe jacking project. It has to be correctly prepared and the rheological parameters checked according to standards. The lubrication technology supplies the lubricant continuously in sufficient quantity into the annular gap. In advance, the required quantities of lubricant over the course of the jack have to be determined, prepared in good time and kept available in sufficient volume. These figures depend directly on the size of the tunnelling machine and the jacked pipe as well as the soil mechanics parameters grading distribution, compaction and permeability. When an automatic bentonite lubrication system is used, the number of injection fittings in the pipe section at a lubrication point has to be decided as well as the spacing of the lubrication points and their injection intervals in the tunnelling machine and in the pipe string.
Precise matching of the individual aspects makes it possible to hold the pipe string in the correct position, considerably reduce the coefficient of friction between pipe and ground and finally keep the skin friction controllable as jacking proceeds.
The Bentonite Handbook deals with the various aspects of annular gap lubrication comprehensively, and should serve well as a design aid and a guideline for site practice. It is of course not possible to exhaustively deal with all practical problems of pipe jacking. Responsible action by well trained engineers will always remain the basis of good and successful construction even with the use of this book.
Professor Markus Thewes
List of symbols used
I. Greek symbols
γ
specific weight
γ
concrete
specific weight of reinforced concrete
γ
suspension
Specific weight of suspension
γ
particles
Specific weight of solid particles
η
(dynamic) viscosity
η
′
differential viscosity
η
s
apparent viscosity
η
p
plastic viscosity
λ
Darcy friction factor
μ
coefficient of friction
ρ
density
ρ
f
density of suspension
ρ
s
density of solid particles
ρ
suspension
density of suspension
ρ
particles
density of solid particles
σ
c
rock strength
τ
shear stress
τ
B
Bingham yield point
τ
F
yield point
φ
internal angle of friction (shear strength)
φ
′
angle of shear resistance (dynamic probing);drained friction angle (shear strength)
φ
u
undrained friction angle (shear strength)
χ
adaption parameter from
Slichter
(
Eqn. 6.13
)
II. Latin symbols
a
half fissure opening width
A
adaption parameter from
von Soos
(
Eqn. 6.17
)
A
pipe string
developed area of the pipe string
B
adaption parameter from
von Soos
(
Eqn. 6.17
)
c
form coefficient from
Kozeny
(
Eqn. 6.14
)
c
′
drained cohesion (shear strength)
c
particles
c
u
undrained cohesion (shear strength)
c
w
resistance coefficient
C
proportionality factor from
Hazen
(
Eqn. 6.15
); adaption parameter from
von Soos
(
Eqn. 6.17
)
C
joint space
joint volume in rock
C
casing
supplement factor for the developed area of the pipe for injection into the surrounding ground
C
porosity
porosity of soils
d
void spacing
d
10
grain diameter at 10% passing (effective diameter)
d
60
grain diameter at 60% passing
d
50
grain diameter at 50% passing
d
s
diameter of solid particles
d
particle
diameter of a soil particle
d
w
effective grain diameter
D
compaction; velocity gradient
e
void ratio; void opening width
e
max
maximum possible void ratio
e
min
minimum possible void ratio
f
filtrate water loss
f
s
local skin friction (dynamic probing)
F
area; force
F
A
uplift force
F
uplift
uplift force on the jacked pipe
F
borehole
developed area of the excavated section
F
G
weight force
F
weight
weight force of the jacked pipe
F
weight installations
weight force of installations (cables, pipes etc.) in the jacked pipe
F
R,spec
specific skin friction
F
jacking
jacking force of the pipe string
F
W
resistance against sinking of a soil particle in the suspension
g
acceleration due to gravity
h
pressure head difference
I
A
activity
I
C
consistency index
I
D
relative density
I
P
plasticity index (Atterberg)
J
hydraulic gradient, fall
J
a
joint alteration number (RQD)
J
n
joint set number (RQD)
J
r
joint roughness number (RQD)
J
w
reduction factor for groundwater
k
f
permeability, coefficient of permeability
k
k
fissure permeability (
Eqn. 6.18
)
k
s
sand roughness height
k
T
rock permeability with a fissure set
K
coefficient
l
length, distance
l
overcut
overcut
L
reference
length of the reference drive
m
D
dry mass of grains with a diameter greater than 0.4 mm
m
T
dry mass of grains with a diameter less than 0.002 mm
M
ballasting
mass required to ballast the jacked pipe
n
porosity
n
e
usable porosity
n
max
maximum possible porosity
n
min
minimum possible porosity
N
0
adaptation ramming: number of impacts for the first 15 cm penetration depth (dynamic probing)
N
10
number of impacts for 10 cm penetration depth (dynamic probing)
N
30
number of impacts for 30 cm penetration depth after the adaptation ramming (dynamic probing)
p
pressure
q
c
tip pressure (dynamic probing)
Q
Q-value (measure of rock mass quality); flow quantity of a fluid
Q
machine
pumping rate at the tunnelling machine
Q
pipe string
pumping rate at the pipe string
Re
Reynolds number
s
penetration depth (of the suspension into the surrounding ground)
t
time; temperature
t
10′
gel strength after 10 min
t
10″
gel strength after 10 s
t
M
Marsh time
t
M1500
Marsh time for 1500 ml of suspension to run out
w
water content
w
L
water content of a soil at the transition from liquid to plastic consistency (liquid limit)
w
P
water content of a soil at the transition from stiff to semi-solid consistency (plastic limit)
w
S
water content of a soil at the transition from semi-solid to solid consistency (shrinkage limit)
U
coefficient of uniformity
v
flow velocity
v
f
filter rate
v
advance
advance rate
V
(total) volume
V
H
volume of voids
V
machine
initial injection volume
V
extra injection
extra suspension volume
V
annular gap
annular gap volume
V
pipe string
subsequent injection volume
V
t
volume of solids
w
s
sinking speed
1Basics
1.1 Basics and technical implementation of bentonite lubrication systems1)
Two basic types of bentonite lubrication systems are differentiated:
– Interval-controlled bentonite lubrication systems, in which the valves are controlled in a defined sequence.
– Volume-controlled bentonite lubrication systems (since 2014), in which the valves are controlled according to configured demand along the route; alternatively, the valves can also be controlled in a defined sequence.
Both systems exist both as systems integrated into the control container or as standalone systems.
In general, a lubrication system consists of the parts shown in Fig. 1.1. The first station in the lubrication circuit is the mixing tank, in which the bentonite suspension is dispersed before it is pumped into the storage tank. The bentonite pump supplies the individual lubrication points in the tunnelling machine and in the pipe string.
Fig. 1.1 Principle of construction of the standard Herrenknecht bentonite lubrication system1: control unit;2: mixing tank;3: storage tank;4: tunnelling machine;5: lubrication ring;6: advance pipe;7: lubrication point;8: injection fitting;9: bentonite pump;10: compressed air supply;11: control cable;12: bentonite feed.
In an interval-controlled lubrication system, lubrication cycles are used according to the strategy of the machine driver. A lubrication point (see Fig. 1.2) consists of several injection fittings. The lubrication cycle starts these one after another (e.g. valve 1 – valve 2 – valve 3); thus only one valve is open at any one time. Then the next lubrication point is started.
Generally, normal cycle and extra cycle are differentiated. The normal cycle serves to lubricate the entire tunnel drive. The extra cycle permits in contrast additional control of separately selected lubrication points using the appropriate valves or injection fittings. A larger volume of lubricant can be supplied to the machine using the extra cycle. In addition, each lubrication point sends a feedback signal to the control unit, enabling a check whether the individual lubrication point is actually connected.
In a volume-controlled system, the tunnel length is divided into sections each 1 m long. Each of these sections is assigned a configured ideal quantity of suspension depending on the ground conditions. The lubrication system automatically ensures that the connected lubrication points fill these quantities in the corresponding tunnel sections. The individual components of the lubrication system are basically the same for both systems; they are now described in more detail blow.
1.1.1 Control unit
The control unit is installed in the container or as a stand-alone unit next to the launching shaft. From here, the machine driver controls the tunnel drive and the lubrication cycle. In principle, the machine driver can select each valve in the entire tunnel drive individually. The (maximum) pump pressure is set directly at the pump.
In interval-controlled operation, the control unit enables two different presets for the valve setting. The first method is called “preset quantity”. In this case a defined bentonite quantity is provided, which should be fed through each valve. As soon as the given quantity has been reached, the valve closes and the next valve is opened. The opening time of the valve in this case follows from the flow rate of the bentonite suspension, so a flow meter and pressure measurement unit is required for this control variant, which is connected directly to the control unit. It has the task of recording the flow quantity and sending it to the control unit. For this purpose, a magnetic-inductive flow meter (MID) is often used. This instrument is based on the fact that the suspension flows through a magnetic field and thus induces a voltage, which is recorded by two electrodes.
The second method of valve control is called “preset time”. This permits the valves to be opened for a defined time. In this case it does not matter what volume of bentonite flows through the valve in this time; this can be different for each valve.
Another important setting, which the machine driver undertakes from the control unit, is the selection of normal or extra cycle.
1.1.2 Mixing tank
The mixing tank can be set up either separately or directly next to the control unit. It mixes the bentonite suspension (lubricant). Its size depends on the quantity of bentonite suspension needed in the course of the tunnel drive. The mixer is connected to the mixing tank or directly integrated into it. The mixer consists of a shear impeller, rotating shear arms or a venturi system.
The mixing tank can be fitted with electronic flow meters and/or modules for electronic data logging for better control and monitoring.
