Publisher’s version / Version de l'éditeur:
Technical Note (National Research Council of Canada. Division of Building Research), 1961-05-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.4224/20359199
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Instrumentation on the Ottawa Outfall Sewer Tunnel - Clay Section
Eden, W. J.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=996d5f40-7529-4c20-b9bc-fc58072e59ea https://publications-cnrc.canada.ca/fra/voir/objet/?id=996d5f40-7529-4c20-b9bc-fc58072e59ea
Purpose
In the initial phase it is expected that the measurements will be of assistance in deciding whether it will be necessary to use air pressure in the
tunnelling operations. The general information to be gained from the study will
At its eastern end the Ottawa outfall sewer tunnel will be driven
through clay for approximately one mile. It will be founded about 65 feet below
the surface and will be approximately 10 feet in outside diameter. The clay
through which the tunnel is to be driven is stiff and extremely sensitive, known in scientific circles as Leda clay.
R.F.L.
No.
FOR INTERNAL USE
333 DATE May 1961 APPROVED BY
NOTlE
C. B. C. CHECKED BYDIVISION OF BUILDING RESEARCH
'IfE
C
lHlN
][ CAlL
Beaver Construction Company Ltd., Montreal
NATIONAL RESEARCH COUNCIL OF CANADA
,
INSTRUMENTATION"ON THE OTTAWA OUTFALL SEWER TUNNEL-CLAY SECTION
The Division of Building Research, National Research Council, has undertaken to measure the earth pressures that will develop on the Ottawa
outfall sewer tunneL The purpose of this note is to outline the objectives of the
study and to describe in some detail the proposed instrumentation for the infor-mation of those organizations co-operating with the Division.
The primary object of the Divisionis studies is to measure the earth pressures and deformations that will develop about the temporary lining
and the permanent concrete lining. Earth pressures that develop about tunnels
in clay are relatively unknown, with only a few previous attempts having been
made to measure their magnitude and direction. So far as is known, no previous
attempts at suc,h measurements have been made in sensitive clay. Thus the
designer of tunnels in clay has little in the way of factual information upon which
to base his designs. This applies both to the temporary lining used in
construc-tion and to the permanent lining. SUBJECT
PREPARED BY W. J. Eden
NOT FOR PUBLICATION
2
-provide invaluable basic technical information for future construction
opera-ｴｩｯＧｾｳ of this type.
The Ins truments
The Division has decided to rely on the experience gained by the Norwegian Geotechnical Institute with instruments to measure the earth pressure
exerted by the clay on the tunnel. Eight vibrating wire pressure cells built
to NGI specifications were purchased for installation in the tunneL
Each pressure cell consists of a stiff steel circular diaphragm
which is brought into contact with the clay. When earth pressure is exerted on
the diaphragm it will deflect very slightly and stretch a fine wire which is
attached to a pair of posts on the back of the diaphragm. The stretch of the
wire can be calibrated to indicate a change in pressure on the diaphragm.
Briefly, this technique is based upon the prtiriciple that the natural frequency of
vibration of a length of wire varies directly with the tension. The wire is
positioned close to two small permanent magnets fonning the cores of two
electro-magnets. To measure the tension in the wire, the wires are plucked by sending
a pulse of current through the electromagnets. The vibrations of the wire
excite an alternating current in the permanent magnets, the frequency of which
corresponds to the ｦｲ･ｱｾ｟･ｮ｣ｹ of vibrations in the wire. This induced alterriating
current is amplified and its frequency determined with electronic instruments.
Such a system has many advantages. Because the wire and the
steel diaphragm are made of nearly identical materials, the cells are insensitive
to temperature changes. Since the frequency is the only measurement made,
the system is also insensitive to the length of the lead wire. Because the
dia-phragm is deflected due to difference in pressure between. the two sides of the diaphragm, the pressure on one side must be known to measure the pressure on
the otlher. Thus, it is necessary to maintain the inside of the cell. at a constant
pressure, preferably atmospheric. Special provisions are necessary for the
Ie ads to the cells.
The earth pressure cell will measure the total pressure acting on
its diaphragm. It cannot distinguish between pressure that is due to the pore
water in the clay and that which is due to effective pressure of clay particles. Hence it is also necessary to install several piezometers to determine the water pressure regime around the tunnel.
. Since the earth pressure cells will be attached to the temporary
steel lining, and depend on the lining as a reaction, some indication of the
structutal behaviour of the liner rings is necessary. In order to investigate
this, it is proposed to measure the diameter of the tunnel as accurately as
3
-studs will be placed in a ring adjacent to the earth pres sure cell installation
to serve as reference points for these measurements. Some consideration
is being given to the installation of reference points for accurate strain readings
on the liner plate as well in order to determine hoop stresses. These
measure-ments are relatively easy to make and will be repeated at two or three other locations in the tunnel.
This installation is complicated by the fact that the temporary liner
plates are segmental. It has been decided to adopt the following procedure to
make the installation so that no undue delay will result to the contractor. Installation Procedure
1. The Division will obtain from the contractor three complete liner
rings. Two of these rings will be equipped with reference points for strain and
diametrical measurements. The other ring will be equipped with "patches and
collars" for the water pressure and earth pressure gauges. These rings will
be coded and returned to the contractor for installation.
2. When 'station 325
+
75 is reached, the contractor will install thespecial lining ring for the pressure measurements according to the coding
instructions shown on Fig. 1. Immediately adjacent to this ring, he will
install a ring containing the reference studs according to the code instructions. These rings should req uire no special precautions on the part of the contractor
other than to install them in the proper ｳ･ｱｵ･ｾ｣･ conforming with the code
numbers. They should be bolted to neighbouring rings in the usual manner.
The third ring containing reference studs should be the last ring installed prior to the first "test period" as outlined in "Clause 1.39" of the "Specifications for Contract No.3 Outfall Sewer Tunnel" near station 327
+
00. This ring will also serve for some of the measurements to be made by the con-sulting engineers to determine whether it will be necessary to use compressed air in the remainder of the tunnel work.3. Mter the test ring has been installed at station 325
+
75, theDivision should be notified. At the first opportunity, so that the work of the
contractor is not held up, the Division will begin the installation of the earth pressure cells and piezometers. This may be done during the first "off-shift"
period during the contractor's schedule. It is anticipated it will require
approxi-mately 8 hours by the staff of the Division to complete the installation.
Eight earth pressure cells and four piezometers will be installed. The eight pressure cells will be located at the top, bottom, mid-point on each
side and at the eighth points in the liner segment. The piezometers will be
4
-points of the liner will be accessible and unaffected by train loads, it will be neces sary for the rail service track in the tunnel to bridge the test ring.
4. The test ring for the earth pressure measurements will be
modified by cutting out a rectangular patch about 12 by 16 inches in each segment
of the liner. The patch will be equipped with flanges which can be bolted to the
remainder of the segment. A 6-inch diameter steel collar will be welded in
the patch to receive the earth pressure cell. There will be four small i-inch
diameter holes fitted with plugs which will be used for grouting after the
instru-ments are installed. On four of the patches a i-inch diameter hole with a
threaded plug will be left for the piezometers.
When received by the contractor., this liner ring will have the
patches bolted in place. The collars will be sealed at each end with temporary
plugs. The ring should be installed in the usual manner, but it is hoped there
will be no large void at any point between the liner ring and the clay.
5. The first operation in installing the instruments will be to complete
the lead-in system. This consists of a 16 conductor communications cable
en-cased in a i-inch diameter polythene pipe. The reading station will be in'the
power house near the collar of the shaft. Thus the lead-in cable and casing
must be strung from the power house down the manway in the shaft, thence along the tunnel, through the air lock, along the tunnel to a junction box located
at station 325
+
75. The contractor has provided a 2i-inch conduit through theair lock for the cable casing. Inside the tunnel, the cable casing will be fastened
to the roof with wire stirrups. To prevent air leakage through the conduit in
the air lock, a sealing flange will be installed around the cable casing. The
installation of the lead-in system can be best done during an off-shift period and could be done just prior to the installation of the test ring.
6. After the ring is in place, the patches will be removed. The clay
face thus exposed will be smoothed and any grout will be removed. The patch
will then be repositioned on the liner. The temporary seal on the collar will
be removed. A length of Shelby tube will be pushed through the collar until its
cut ting edge enters undisturbed clay. A special trimming tool will be inserted
in the rube and the clay surface carefully trimmed so that it will conform with
the face of the earth pressure cell. Next the earth pressure cell with its
mounting shaft will be placed in the Shelby tube and pushed by hand until it is
in contact with the clay face. An
no"
ring and its retaining ring will be positionedaround the outside of the pressure cell. The correct spacers will be placed on
the mounting shaft.,. The b.a.ck plate will be placed on the collar and bolted into
place. The positioning nuts will then be started in the backplate. At this stage,
the leads for the cell will be taken through the junction box and connected to the
lead-in cable. The reading instrument will be hooked to the cell circuit and by
means of the positioning nuts the cell will be pressed against the clay to a pre-determined level of prestress.
5
-The piezometers will be connected by means of a 3/8-inch
diameter polythene tube to a Bourdon gauge. The four Bourdon gauges will
be positioned near the roof of the tunnel.
7. To fill any void spaces behind the patches after the instruments
are installed, a cement or clay gel grout will be pumped through the grout holes
left in the patches. Four grout holes have been left in each patch so that complete
filling can be achieved.
8. To protect the instruments, steel mesh guards will be installed
extending from the flanges of the liner plate so that the patches will be covered.
9. The instruments can be removed at any time up until the concrete
lining is ｰｬ｡｣･ｾＬＺ should malfunctions develop. Since this is the first attempt
on the part of the Division at such an installation, it is probable that the experience gained from this installation will suggest improvements in the measuring
tech-niques. Should the installation at station 325
+
75 prove unsatisfactory, it isplanned to remove all the instruments from this station and reinstall them near
station 352
+
00 at the centre of the tunnel project. Because of the distancefrom the shaft, a different lead-in system will be required for the second installation.
10. Excepting the four piezometers installed behind the patches
in the test ring, all piezometers will be installed from the surface. The location
of these piezOIlleters is shown in Figure 2. These piezometers consist of a
porous filter point pushed into position by "E" drill rods. A 3/8-inch polythene
tube leads from the filter tip to the surface through the dr ill rods. The water
level inside the tube is determined by an electrical feeler gauge.
11. ' Thecpress"ur...es' olLtlle: e.arth press.ure cells ,will be measured:by
ｲ･｡､ｩｮｧｳＺｴ｡ｫｾｮＭｩｮＺＮｴｨ･ powerh.ou'se near the ｾｏＧｬｬ｡ｲＢｯﾣＮｴｨ･ shaft. '
No
｡Ｚｩｩ｣･ｳｳﾷＮｴｯｾｴｨ･tunnelLi's. requiI"tid ;for:these r,eadtngs. The piezometer readings, strain gauge and diametrical measurements must be taken inside the tunnel, and therefore
require acces s to the tunnel. No bulky equipment is reqUIred for this work, and
hence it should be possible to take these readings without interference to the
tunnel operations. Initially, the readings may be taken at approximately weekly
intervals, with the time interval between readings increasing to approximately monthly intervals after a state of temporary equilibrium has been achieved about the tunnel.
Use of the Results.
The initial results will be of use in determining whether it is
necessary to use air pressure to assist in driving the tunnel. The final results
6
-five years will be required before a state of final equilibrium is achieved in the
soil around the tunnel. The Division anticipates publishing the results of the
measurements in technical journals with the approval of the civic authorities. Such publications will include descriptions of the instruments and techniques used in obtaining measurements, the measurements themselves and interpre-tation of the results with reference to theoretical treatments of earth pressures. No attempt will be made by the Division of discuss the desigIkof the tunnel or the method of tunnelling used.
Acknowledgments
The proposed installation has been made possible with the co-opera-tion of the City of Ottawa, Department of Planning and Works, DeLeuw Cather and Company of Canada Limited and G. C. McRostie and Associates Ltd. , consulting engineers, Ottawa, and Beaver Construction Company Limited, Montreal, the general contractors for the project.
APPENDIX A
e
REQUffiED WORK BY THE'CONT,RACTOR - BEAVER CONSTRUCTION CO.LTD.
The following list outlines the steps requiring special attention
by the general contractor to facilitate the installation of the instrum.ents. At
the date of writing (16 May 1961) some of this work has already been completed... as indicated by an asterisk.
1. Supply three complete liner rings to the Division of Building Research.
*
2. Install a 2i-inch diameter conduit through the air lock for the lead-in
casing to the instrum.ents.
*
3. Install the test ring for the earth pressure cells at station 325
+
75 inaccordance with the code num.bers shown on Fig. 2.
4. Install a test ring with diameter Teierence points immediately adjacent to
the first test ring in accordance with the code num.bers shown on Fig. 3.
5. Bridge the test ring at 325
+
75 for the service rail tracks so that accesscan be gained to the bottom segments of the liner ring.
6. Notify the Division of Building Research when the test ring has been
installed so that mutually satisfactory arrangements can be made to complete the installation of instruments.
7. Grant access to the tunnel to staff of the Division so that they can make the
installation and conduct readings from time to time in accordance with the regulations of the Ontario Department of Labour.
8. Provide access to the power house so that readings can be taken from a
·e
APPENDIX B
ｒｅｑｏｭｾｗｑｒｋＭ BY THE DIVISION OF BUILDING RESEARCH
The"-following list is a summary of the work required by the Division of Building Research to carry out the installation of the instruments. At date of writing, (16 May 1961), several of the items listed have been com-pleted as indicated by an asterisk.
1. Acquire and calibrate eight earth pressure cells from Geonor A/s, Oslo,
Norway.
*
2. Design, procure and prepare the accessory equipment including collars,
Shelby tubes, mounting shafts, spacer s, positionipg nuts,
back-up plates and temporary seals for the earth pressure cells.
*
3. Prepare the water pressure piezometers and accessories, and the diameter
reference pins and measuring system.
*
4. Prepare the three liner rings received from the general contractor for the
instruments. On two of the liner rings, reference pins will be
installed and the rings coded. On the third ring, rectangular
patches will be cut into which the collars for the earth pressure
cell will be welded. Provision will be made to bolt the patches
back on the liner rings and to stop up any openings left in the
patches.
*
5. Procure and prepare the lead-in cable and casing.
6. Make provisions in the "power house" for a 110-volt a-c outlet and a suitable space for switching terminal board and reading instrument.
7. From the surface over station 325
+
75 install piezometers to measure thewater pressure regime around the tunnel.
8. Immediately after the installation of the test ring at station 325
+
75, installthe lead-in cable, casing, junction boxes, air seal flange at conduit in air-lock, and switching terminal.
9. When the test ring at 325
+
75 is in place, install the earth pressure cellsand piezometers in the test ring. Complete the lead-in system,
seal the voids behind the patches by grouting and install protective guards over the instruments.
e
e
e
#3 . #1
ｾｒｅｆＮ BOLT (BRASS STUD)
Ｑｾ
OffSf.I
ｆｾＮＬＮ Nt" __ - - - - - . . . . ｾｾｲＺ＾ＬＭ --..I.,
.. , / ... I> q" ,/ ... I , ｾ / / • " Ｎｳ［Ｎｾ / " G( / \ ｾ$' /
\
%-ｾ I \ ｾ o I \ ｾ/
\..
ｾｉ
+
\tl ｾ Ｍｬｾ I L - - - _ . . I ｬＡｩｾ I I 101 \ J \ II ...!
ｾ \ / ｾ'"
ｾ \ ' \ I ｾ ',p , /'% "
/ ...
ｾ"
/ ｾ ｾ"
.
/ / ｾ ....ol'...ｾｾt
,/
...-
.---
ＢＢｾＧＢＧｾ I., 1>1 - - - - I)ｴＧＱｾ S''''ｾＧ
#5 . #7 #3 CEll NO. 103 #4 #2 i #1 J CELL NO. 101ｾ
14 PI - NO OFFSET ｓｾｾ ｟Ｍｾ｟｟ I., A ｏｾｩ _ - - TOP --... -'.... ｾＬＬｾ ｾＮａＮ OF LINERS ASSEMBLEi)'... ｯｬＧＯＬｪ［ｾ ･Ｌｾ / " ｾq,'
//
"
\ ｾ /,
/ \ / \/
\ { \ I \ I I t - . '----' \ I \ I \ I \ I \ / \ / ｾ '6> , I ｾ ｾ \ / ｾ... , , " 0 Ｌｰｾ " / ｾ ＧｓＭｾ '... / ｾｾＢ ｾ ... , / S" O,ot-",.. ... _ • _ - , , / 'l" $","1"--r-
ｾ \'"
' . P/- DOUBLE OffSt:ｾ
CELL NO. lOSI #5
I
#7 Jo CELL NO. 107(a) TEST RING FOR PRESSURE MEASUREMENTS
AT STATION 325+75
(b) REFERENCE RING TO BE INSTALLED
NEXT TO TEST RING AT STATION ＳＲｾＫＷＵ
NOTE:
REFERENCE RING ROTATED 2 BOLT HOLES TO TEST RING. NUMBERS VALID FOR FACING DIRECTION OF CHAINAGE. NUMBERS PAINTED ON INSIDE OF LINER PLATES IN DARK FIGURES ON WHITE BACKGROUND.
FIGURE I
x -
SURFACE SETTLEMENT GAUGE _10 - PIEZOMETERPIEZOMETER NO. DEPTH LOCATION
I 37' ON It 2 52,5' ON
