- Email: [email protected]
Uses and limitations of ground freezing with liquid nitrogen
Engineering Geology, 13 (1979) 485--494 485 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
USES AND LIMITATIONS OF GROUND FREEZING WITH LIQUID NITROGEN
K. STOSS and J. VALK Deilmann-Haniel GmbH, Dortmund (G.F.R.) (Received June 15, 1978)
ABSTRACT
Stoss, K. and Valk, J., 1979. Uses and limitations of ground freezing with liquid nitrogen. Eng. Geol., 13: 485--494. The question whether ground freezing by direct gasification of liquid nitrogen (LN 2) is an alternative to conventional freezing by circulation of liquid coolants is discussed. Also discussed are: the technical characteristics of and basic differences between the two methods; the specific advantages and disadvantages of ground freezing with LN2; the relevant cost factors and their influence on the overall cost of a freezing project; and the criteria for the technical and economic feasibility of LN 2 freezing in construction work. A description is given of four typical applications: for temporary foundations; sealing of a defective diaphragm wall; final sealing of a designed gap in a bored pile retaining wall; and driving of a water tunnel under a railway embankment. INTRODUCTION R e c e n t l y , t h e use o f liquid n i t r o g e n (LN2) for g r o u n d freezing has b e e n increasingly r e p o r t e d . W h e t h e r this indicates t h e d e v e l o p m e n t o f a general alternative t o c o n v e n t i o n a l freezing b y circulating cold salt s o l u t i o n s is a q u e s t i o n o f t e n asked in t h e i n d u s t r y . O n an a t t e m p t t o answer this q u e s t i o n , this p a p e r discusses t h e uses a n d limitations o f freezing with LN2 against a b a c k g r o u n d o f freezing with brine. CHARACTERISTICS AND RELEVANT DIFFERENCES BETWEEN THE TWO METHODS Table I lists in brief t h e characteristics o f t h e t w o m e t h o d s a n d t h e relevant d i f f e r e n c e s b e t w e e n t h e m . F o r LN2 freezing an o p e n s y s t e m is assumed. With t h e c o n v e n t i o n a l m e t h o d o n l y MgC12 and CaCI2 brines are regarded as suitable, and o t h e r c o o l a n t s such as k e r o s e n e are v e r y s e l d o m used. TECHNICAL
ADVANTAGES
AND DISADVANTAGES
OF FREEZING WITH LN 2
F r o m T a b l e I t h e m o s t i m p o r t a n t t e c h n i c a l advantages o f LN2 freezing can be p e r c e i v e d as follows:
486
TABLE I Characteristics of LN2-freezing and brine freezing Quality
electric water
L N
power
for
cooling
refrigeration plant o
storage
tank
circulation
pumps
pipe system distribution coolant
for of
Brine
not
required
required
not
required
required
not
required
required
required
required
not
required
required
supply
only
supply
and
return
.4 .4
low temperature material for surface pipes, valves etc.
required
not
required
low temperature material for freeze pipes
not
not
required
physical condition of coolant
liquid/vapour
minimum temperature achievable (theoret.)
-
required
196 ° C
c re-use
of
control shape wall
of
coolant
of
system
frost
34 ° C M g C 1 2 55 ° C C a C l 2
impracticable
standard
difficult
easy
often
irregular
regular
great ces
differen-
small ces
freeze
temperature profile in f r e e z e w a l l
M
liquid
differen-
penetration
fast
slow
impact on freeze w a l l in c a s e o f d a m a g e to f r e e z e pipe
none
thawing
noise
none
little
effect
(1) On-site equipment and installations are relatively simple and easy to erect. (2) The time required to form a frozen earth wall of given thickness is comparatively short because o f the high temperature gradient available.
487 (3) The high rate of frost penetration reduces the formation of ice lenses in soils sensitive to freezing. (4) Because of the very low operating temperature the m e t h o d is less liable to groundwater flow. (5) Rupture or leakage of freeze pipes cannot cause local thawing of the frozen earth wall. The main disadvantage is that it is difficult to design and control the system in such a way that a regular shape of the freeze wall, as well as a minimum consumption of LN2, is achieved. IMPORTANT FREEZING
COST FACTORS
AND THEIR INFLUENCE ON THE TOTAL COST OF
The list of technical advantages and disadvantages of LN2 freezing seems to indicate that this m e t h o d is superior to freezing with brine wherever a regular growth of the freeze wall is of less importance. However, as everywhere in the construction business, here again the final judgement on t w o competing methods does n o t depend only on their technical advantages and disadvantages: On the contrary, the decision for one or the other is usually made for economic reasons. Cost overrules technical elegance! The following main cost items contribute to the overall cost of a groundfreezing operation: (1) The cost of drilling and casing of freeze holes and thermistor holes. (2) The cost of moving in and site erection of the installations on the site and the cost of those materials and installations which have to be fully depreciated against the job. (3) The cost of the building of the freeze wall. (4) The cost of the maintenance of the freeze wall. (5) The cost of dismantling and moving out the installations. In the overall cost of drilling and casing of the freeze holes and thermistor holes there are no significant differences b e t w e e n freezing with LN2 and with brine. In b o t h cases outer freeze pipes and inner downpipes are required, and the necessary borehole diameters are almost the same. The cost of moving in, on-site installation, dismantling and moving o u t of the necessary equipment -- such as the pipe system for the distribution of the coolant on the surface, the LN2 or brine tanks, the freeze plants, the monitoring devices, etc. -- with b o t h freezing methods greatly depends on the t y p e and magnitude of the specific freeze job. The only general statement which can be made is that with LN2 freezing there is an almost linear relation between cost and number of freeze pipes installed, whereas with brine freezing a rise in costs occurs whenever another freeze plant has to be added to meet increasing demand. To the cost of the building and maintenance of the frozen earth wall, must be added the cost of the personnel for operation and maintenance of the system, the equipment rent, the cost of instrumentation and measurements,
488 and, as a major item, the cost of nitrogen or the cost of electric energy and water respectively. Again, general figures cannot be given. However, there is no doubt that the specific consumption of nitrogen in relation to the designed volume o f frozen earth is a most important factor. This itself is directly dependent on h o w effectively the freezing process is designed and can be controlled, and h o w much of the inherent cooling potential of the LN2 is wasted. The existing problems in this respect explain why the figures on LN2 consumption per cubic meter of soil given in the literature show a wide variation. In order to arrive at reasonably reliable cost relations between freezing with LN2 and freezing with brine, a comparison on the basis of specific projects is necessary. To this purpose the authors examined -- based on their extensive experience with both methods of ground freezing -- three fictitious jobs. The basic data of these jobs and the results of the comparison are summarized in Table II. The cost relations given there refer to jobs in the Rhine--Ruhr area where liquid nitrogen is available at a favourable price. CRITERIA FOR THE FEASIBILITY OF FREEZING WITH LN2 From Table II and from the graph in Fig.1 showing the cost relation as a function o f maintenance time, it is evident that, in respect of volume to be frozen and maintenance time, there is only a rather small area where LN2 freezing is economic. Of course, the specific technical advantages of LN2 freezing can overrule the higher cost in special cases. However, all in all, one has to conclude -- and this is in full accordance with the personal experience of the authors -- that the use of liquid nitrogen for ground freezing is then and only then advantageous if: TABLE II Examples for cost relations LN~-freezing/brine freezing Designed dimensions of frozen earth body Type of Project
depth
i.d.
(m)
temporarY
volume
(m)
3.0
8.0
3.0
4.0
20.0
15.0
17.4
12.0
excavation
relation LN2/brine
t0 30 60
0.52 0.66 0.79
0.5
110
i0 30 60
0 . 76 0.93 I .09
1.2
733
10 30 60
1.33 I .84 2.23
freeze shaft large open
cost
tenance period (days)
57
foundation small
main-
489
2.6 2.4
2.2 c .0 • 4J .d
open excavetlon {733 m s ) ~
/
2.0
1.8 ou
1.6 1.4 1.2 smell freeze shaft (II0 m ' ) ~
1.0
==~=
0.8 0.6
temporary ( 57 m')
foundation
0.4 0.2 i
10
20 ,
30
40
50
60
70
maintenance
period
i
80 (days)
Fig.1. Cost relation LN2/brine.
(1) A rapid formation of the frozen earth wall is important (e.g., in an emergency); or (2) The soft and groundwater conditions require particularly low temperatures (e.g., heavy groundwater flow); or (3) Only small volumes of earth have to be frozen; and also (4) Only a short maintenance period is required. In other words, freezing with LN2 is obviously not an overall alternative to freezing with circulated brine, but it is a most important addition which is extending the range of applications for ground freezing in general. The decision which method should be used has always to be based on the conditions of the specific case. However, it is the opinion of the authors that the potential of LN2 for rapid and deep temperature ground freezing is not yet fully utilized. In this respect a lot remains to be done.
490 EXAMPLES OF APPLICATIONS The following brief reports on some freeze jobs executed using liquid nitrogen as coolant illustrate the variety of applications where LN2 freezing is the ideal solution for the particular problem.
Temporary foundation The first example refers to a case where LN2 freezing solved a problem which could n o t be dealt with successfully by other construction methods. A special 450-ton crane hired for positioning the reactor containment for a new nuclear power station in Georgia required an adequate foundation. Because of the bad ground on site this foundation was to rest on six largediameter caissons, three of which were drilled as planned; however the remaining three caissons could n o t be sunk to the required depth because of repeated inrush of sand. The holes were then backfilled with sand and this was frozen by using LN2. In this manner frozen sand piles 11 m deep and 3.3 m in diameter were formed. After covering them with concrete the crane was erected. A loading test run with 1.4 times the weight of the containment produced only a settlement of 2 mm, whereas 13 mm would have been tolerable. The containment was placed in position without difficulty and no further settlements occurred.
Sealing o f a defective diaphragm wall The second example describes the application of LN2 in an emergency (Fig.2). The connection between the t w o sections of a diaphragm wall which was to stabilize the excavation for a hospital extension at Geneva was defective. As a result sand and water penetrated through a gap approximately 0.5 m wide in such volume that the work had to be stopped and the adjacent portion of the excavation had to be backfiUed. In spite of this it was impossible to stop the water seepage through the gap completely, and a groundwater flow with a velocity in the order of several meters per hour remained. Time-consuming and expensive b u t unsuccessful efforts to solve the problem finally resulted in an acute danger of intolerable settlements of the adjacent buildings. Only then was freezing considered. Despite of extremely unfavourable conditions it was possible to stop the water seepage and completely seal the gap by applying LN2 in several rows of pipes inside and outside the diaphragm wall. After four days of freezing excavation was continued and the wall could finally be repaired.
Sealing o f a designed gap in a bored pile retaining wall In this case freezing with LN2 was used to supplement a conventional construction m e t h o d (Fig.3).
Fig.2. E x a m p l e s o f a p p l i c a t i o n o f L N 2 in e m e r g e n c y cases.
~D k.t
~ig.3. F r e e z i n g w i t h LN 2 t o s u p p l e m e n t c o n v e n t i o n a l c o n s t r u c t i o n a l m e t h o d s .
bO
493 The open excavation for a section of the D o r t m u n d subway was stabilized by a bored pile retaining wall. The only exception was a 3.2-m wide area in which a large sewer line intersected the tunnel above the present tunnel roof. In this area no piles could be placed. Because of the bad ground conditions it was impossible to close this gap b y conventional means, and it was decided to apply LN2 freezing. Before excavating below the existing groundwater level on both sides of the pit one row of slightly inclined freeze pipes were placed which overlapped the gaps and reached into stable rock. After only three days of freezing an impervious and sufficiently thick freeze wall was formed, and the excavation was completed. The gap was then finally closed by reinforced concrete. Sewer tunnel under a railway e m b a n k m e n t
This example refers to a project which was initially designed on the basis of LN2 freezing (Fig.4). For the new sewer system of the c o m m u n i t y of Charrat, Switzerland, a tunnel with a diameter of 1.5 m and a length of 14 m had to be driven under the e m b a n k m e n t of a main railway line. The tracks were only 5 m above the tunnel axis. Movements of the rails were acceptable only within very narrow limits. Soil investigations showed alternating layers of silt, sand and gravel. The groundwater level - - f o u n d 2.5--3.0 m above the tunnel axis - - w a s strongly influenced by the level of an open canal running parallel to the railway line at a distance of a b o u t 10 m. This also caused rapid and unpredictable changes in groundwater movements. To secure the tunnel during excavation it was decided to use ground freezing. Under the circumstances rapid freezing was advantageous in order to minimize heave or settlement of the rails, and particularly low temperatures were required to compensate for the detrimental effect of the groundwater flow. Therefore it was decided to use LN2. Furthermore, a comparison of the estimated cost proved that LN2 freezing was also more economical than freezing with brine. On both sides of the railway e m b a n k m e n t access shafts were sunk from which a total of 13 freeze holes and 4 thermistor holes were drilled parallel to the tunnel axis. The freeze holes were situated on a circle with a diameter of 2.4 m. Within three days after beginning of freezing a watertight frozen earth cylinder of sufficient thickness was formed. The tunnel was excavated b y hand, the sewer pipes pulled in from the shafts and the overbreak afterwards backfilled by cement--bentonite injections. During freezing and excavation the rails were checked daily, and no noticeable m o v e m e n t caused either b y frost heaving or soil settlement were reported.
494
A
Fig.4. L N 2 freezing for sewer t u n n e l s u n d e r a r a i l w a y e m b a n k m e n t .
USES AND LIMITATIONS OF GROUND FREEZING WITH LIQUID NITROGEN
K. STOSS and J. VALK Deilmann-Haniel GmbH, Dortmund (G.F.R.) (Received June 15, 1978)
ABSTRACT
Stoss, K. and Valk, J., 1979. Uses and limitations of ground freezing with liquid nitrogen. Eng. Geol., 13: 485--494. The question whether ground freezing by direct gasification of liquid nitrogen (LN 2) is an alternative to conventional freezing by circulation of liquid coolants is discussed. Also discussed are: the technical characteristics of and basic differences between the two methods; the specific advantages and disadvantages of ground freezing with LN2; the relevant cost factors and their influence on the overall cost of a freezing project; and the criteria for the technical and economic feasibility of LN 2 freezing in construction work. A description is given of four typical applications: for temporary foundations; sealing of a defective diaphragm wall; final sealing of a designed gap in a bored pile retaining wall; and driving of a water tunnel under a railway embankment. INTRODUCTION R e c e n t l y , t h e use o f liquid n i t r o g e n (LN2) for g r o u n d freezing has b e e n increasingly r e p o r t e d . W h e t h e r this indicates t h e d e v e l o p m e n t o f a general alternative t o c o n v e n t i o n a l freezing b y circulating cold salt s o l u t i o n s is a q u e s t i o n o f t e n asked in t h e i n d u s t r y . O n an a t t e m p t t o answer this q u e s t i o n , this p a p e r discusses t h e uses a n d limitations o f freezing with LN2 against a b a c k g r o u n d o f freezing with brine. CHARACTERISTICS AND RELEVANT DIFFERENCES BETWEEN THE TWO METHODS Table I lists in brief t h e characteristics o f t h e t w o m e t h o d s a n d t h e relevant d i f f e r e n c e s b e t w e e n t h e m . F o r LN2 freezing an o p e n s y s t e m is assumed. With t h e c o n v e n t i o n a l m e t h o d o n l y MgC12 and CaCI2 brines are regarded as suitable, and o t h e r c o o l a n t s such as k e r o s e n e are v e r y s e l d o m used. TECHNICAL
ADVANTAGES
AND DISADVANTAGES
OF FREEZING WITH LN 2
F r o m T a b l e I t h e m o s t i m p o r t a n t t e c h n i c a l advantages o f LN2 freezing can be p e r c e i v e d as follows:
486
TABLE I Characteristics of LN2-freezing and brine freezing Quality
electric water
L N
power
for
cooling
refrigeration plant o
storage
tank
circulation
pumps
pipe system distribution coolant
for of
Brine
not
required
required
not
required
required
not
required
required
required
required
not
required
required
supply
only
supply
and
return
.4 .4
low temperature material for surface pipes, valves etc.
required
not
required
low temperature material for freeze pipes
not
not
required
physical condition of coolant
liquid/vapour
minimum temperature achievable (theoret.)
-
required
196 ° C
c re-use
of
control shape wall
of
coolant
of
system
frost
34 ° C M g C 1 2 55 ° C C a C l 2
impracticable
standard
difficult
easy
often
irregular
regular
great ces
differen-
small ces
freeze
temperature profile in f r e e z e w a l l
M
liquid
differen-
penetration
fast
slow
impact on freeze w a l l in c a s e o f d a m a g e to f r e e z e pipe
none
thawing
noise
none
little
effect
(1) On-site equipment and installations are relatively simple and easy to erect. (2) The time required to form a frozen earth wall of given thickness is comparatively short because o f the high temperature gradient available.
487 (3) The high rate of frost penetration reduces the formation of ice lenses in soils sensitive to freezing. (4) Because of the very low operating temperature the m e t h o d is less liable to groundwater flow. (5) Rupture or leakage of freeze pipes cannot cause local thawing of the frozen earth wall. The main disadvantage is that it is difficult to design and control the system in such a way that a regular shape of the freeze wall, as well as a minimum consumption of LN2, is achieved. IMPORTANT FREEZING
COST FACTORS
AND THEIR INFLUENCE ON THE TOTAL COST OF
The list of technical advantages and disadvantages of LN2 freezing seems to indicate that this m e t h o d is superior to freezing with brine wherever a regular growth of the freeze wall is of less importance. However, as everywhere in the construction business, here again the final judgement on t w o competing methods does n o t depend only on their technical advantages and disadvantages: On the contrary, the decision for one or the other is usually made for economic reasons. Cost overrules technical elegance! The following main cost items contribute to the overall cost of a groundfreezing operation: (1) The cost of drilling and casing of freeze holes and thermistor holes. (2) The cost of moving in and site erection of the installations on the site and the cost of those materials and installations which have to be fully depreciated against the job. (3) The cost of the building of the freeze wall. (4) The cost of the maintenance of the freeze wall. (5) The cost of dismantling and moving out the installations. In the overall cost of drilling and casing of the freeze holes and thermistor holes there are no significant differences b e t w e e n freezing with LN2 and with brine. In b o t h cases outer freeze pipes and inner downpipes are required, and the necessary borehole diameters are almost the same. The cost of moving in, on-site installation, dismantling and moving o u t of the necessary equipment -- such as the pipe system for the distribution of the coolant on the surface, the LN2 or brine tanks, the freeze plants, the monitoring devices, etc. -- with b o t h freezing methods greatly depends on the t y p e and magnitude of the specific freeze job. The only general statement which can be made is that with LN2 freezing there is an almost linear relation between cost and number of freeze pipes installed, whereas with brine freezing a rise in costs occurs whenever another freeze plant has to be added to meet increasing demand. To the cost of the building and maintenance of the frozen earth wall, must be added the cost of the personnel for operation and maintenance of the system, the equipment rent, the cost of instrumentation and measurements,
488 and, as a major item, the cost of nitrogen or the cost of electric energy and water respectively. Again, general figures cannot be given. However, there is no doubt that the specific consumption of nitrogen in relation to the designed volume o f frozen earth is a most important factor. This itself is directly dependent on h o w effectively the freezing process is designed and can be controlled, and h o w much of the inherent cooling potential of the LN2 is wasted. The existing problems in this respect explain why the figures on LN2 consumption per cubic meter of soil given in the literature show a wide variation. In order to arrive at reasonably reliable cost relations between freezing with LN2 and freezing with brine, a comparison on the basis of specific projects is necessary. To this purpose the authors examined -- based on their extensive experience with both methods of ground freezing -- three fictitious jobs. The basic data of these jobs and the results of the comparison are summarized in Table II. The cost relations given there refer to jobs in the Rhine--Ruhr area where liquid nitrogen is available at a favourable price. CRITERIA FOR THE FEASIBILITY OF FREEZING WITH LN2 From Table II and from the graph in Fig.1 showing the cost relation as a function o f maintenance time, it is evident that, in respect of volume to be frozen and maintenance time, there is only a rather small area where LN2 freezing is economic. Of course, the specific technical advantages of LN2 freezing can overrule the higher cost in special cases. However, all in all, one has to conclude -- and this is in full accordance with the personal experience of the authors -- that the use of liquid nitrogen for ground freezing is then and only then advantageous if: TABLE II Examples for cost relations LN~-freezing/brine freezing Designed dimensions of frozen earth body Type of Project
depth
i.d.
(m)
temporarY
volume
(m)
3.0
8.0
3.0
4.0
20.0
15.0
17.4
12.0
excavation
relation LN2/brine
t0 30 60
0.52 0.66 0.79
0.5
110
i0 30 60
0 . 76 0.93 I .09
1.2
733
10 30 60
1.33 I .84 2.23
freeze shaft large open
cost
tenance period (days)
57
foundation small
main-
489
2.6 2.4
2.2 c .0 • 4J .d
open excavetlon {733 m s ) ~
/
2.0
1.8 ou
1.6 1.4 1.2 smell freeze shaft (II0 m ' ) ~
1.0
==~=
0.8 0.6
temporary ( 57 m')
foundation
0.4 0.2 i
10
20 ,
30
40
50
60
70
maintenance
period
i
80 (days)
Fig.1. Cost relation LN2/brine.
(1) A rapid formation of the frozen earth wall is important (e.g., in an emergency); or (2) The soft and groundwater conditions require particularly low temperatures (e.g., heavy groundwater flow); or (3) Only small volumes of earth have to be frozen; and also (4) Only a short maintenance period is required. In other words, freezing with LN2 is obviously not an overall alternative to freezing with circulated brine, but it is a most important addition which is extending the range of applications for ground freezing in general. The decision which method should be used has always to be based on the conditions of the specific case. However, it is the opinion of the authors that the potential of LN2 for rapid and deep temperature ground freezing is not yet fully utilized. In this respect a lot remains to be done.
490 EXAMPLES OF APPLICATIONS The following brief reports on some freeze jobs executed using liquid nitrogen as coolant illustrate the variety of applications where LN2 freezing is the ideal solution for the particular problem.
Temporary foundation The first example refers to a case where LN2 freezing solved a problem which could n o t be dealt with successfully by other construction methods. A special 450-ton crane hired for positioning the reactor containment for a new nuclear power station in Georgia required an adequate foundation. Because of the bad ground on site this foundation was to rest on six largediameter caissons, three of which were drilled as planned; however the remaining three caissons could n o t be sunk to the required depth because of repeated inrush of sand. The holes were then backfilled with sand and this was frozen by using LN2. In this manner frozen sand piles 11 m deep and 3.3 m in diameter were formed. After covering them with concrete the crane was erected. A loading test run with 1.4 times the weight of the containment produced only a settlement of 2 mm, whereas 13 mm would have been tolerable. The containment was placed in position without difficulty and no further settlements occurred.
Sealing o f a defective diaphragm wall The second example describes the application of LN2 in an emergency (Fig.2). The connection between the t w o sections of a diaphragm wall which was to stabilize the excavation for a hospital extension at Geneva was defective. As a result sand and water penetrated through a gap approximately 0.5 m wide in such volume that the work had to be stopped and the adjacent portion of the excavation had to be backfiUed. In spite of this it was impossible to stop the water seepage through the gap completely, and a groundwater flow with a velocity in the order of several meters per hour remained. Time-consuming and expensive b u t unsuccessful efforts to solve the problem finally resulted in an acute danger of intolerable settlements of the adjacent buildings. Only then was freezing considered. Despite of extremely unfavourable conditions it was possible to stop the water seepage and completely seal the gap by applying LN2 in several rows of pipes inside and outside the diaphragm wall. After four days of freezing excavation was continued and the wall could finally be repaired.
Sealing o f a designed gap in a bored pile retaining wall In this case freezing with LN2 was used to supplement a conventional construction m e t h o d (Fig.3).
Fig.2. E x a m p l e s o f a p p l i c a t i o n o f L N 2 in e m e r g e n c y cases.
~D k.t
~ig.3. F r e e z i n g w i t h LN 2 t o s u p p l e m e n t c o n v e n t i o n a l c o n s t r u c t i o n a l m e t h o d s .
bO
493 The open excavation for a section of the D o r t m u n d subway was stabilized by a bored pile retaining wall. The only exception was a 3.2-m wide area in which a large sewer line intersected the tunnel above the present tunnel roof. In this area no piles could be placed. Because of the bad ground conditions it was impossible to close this gap b y conventional means, and it was decided to apply LN2 freezing. Before excavating below the existing groundwater level on both sides of the pit one row of slightly inclined freeze pipes were placed which overlapped the gaps and reached into stable rock. After only three days of freezing an impervious and sufficiently thick freeze wall was formed, and the excavation was completed. The gap was then finally closed by reinforced concrete. Sewer tunnel under a railway e m b a n k m e n t
This example refers to a project which was initially designed on the basis of LN2 freezing (Fig.4). For the new sewer system of the c o m m u n i t y of Charrat, Switzerland, a tunnel with a diameter of 1.5 m and a length of 14 m had to be driven under the e m b a n k m e n t of a main railway line. The tracks were only 5 m above the tunnel axis. Movements of the rails were acceptable only within very narrow limits. Soil investigations showed alternating layers of silt, sand and gravel. The groundwater level - - f o u n d 2.5--3.0 m above the tunnel axis - - w a s strongly influenced by the level of an open canal running parallel to the railway line at a distance of a b o u t 10 m. This also caused rapid and unpredictable changes in groundwater movements. To secure the tunnel during excavation it was decided to use ground freezing. Under the circumstances rapid freezing was advantageous in order to minimize heave or settlement of the rails, and particularly low temperatures were required to compensate for the detrimental effect of the groundwater flow. Therefore it was decided to use LN2. Furthermore, a comparison of the estimated cost proved that LN2 freezing was also more economical than freezing with brine. On both sides of the railway e m b a n k m e n t access shafts were sunk from which a total of 13 freeze holes and 4 thermistor holes were drilled parallel to the tunnel axis. The freeze holes were situated on a circle with a diameter of 2.4 m. Within three days after beginning of freezing a watertight frozen earth cylinder of sufficient thickness was formed. The tunnel was excavated b y hand, the sewer pipes pulled in from the shafts and the overbreak afterwards backfilled by cement--bentonite injections. During freezing and excavation the rails were checked daily, and no noticeable m o v e m e n t caused either b y frost heaving or soil settlement were reported.
494
A
Fig.4. L N 2 freezing for sewer t u n n e l s u n d e r a r a i l w a y e m b a n k m e n t .