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timely dissemination of this information in an accurate manner to the public.
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“The Right to Information, The Right to Live” “Step Out From the Old to the New”
IS 7500 (2000): Code of Practice for Installation and
Observation of Cross Arms for Measurement of Internal
Vertical Movement in Earth Dams [WRD 16: Hydraulic
Structures Instrumentation]
“!ान $ एक न’ भारत का +नम-ण”
Satyanarayan Gangaram Pitroda
““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee””
“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै”
Bhartṛhari—Nītiśatakam
“Knowledge is such a treasure which cannot be stolen”
“
—
Is 7500:2000
Indian Standard
CODE OF PRACTICE FOR INSTALLATION AND
OBSERVATION OF CROSS ARMS FOR MEASUREMENT
OF INTERNAL VERTICAL MOVEMENT IN EARTH DAMS
(First Revision )
ICS 93.160;17.020
C) BIS 2000
BUREAU OF INDIAN STANDARDS
MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002
September2000 Price Group 8
Hydraulic Structures-Instrumentation Sectional Committee, WRD 16
FOREWORD
This Indian Standard ( First Revision )was adopted bytheBureau of Indian Standards, after the draft finalized by
theHydraulic Structures-Instrumentation Sectional Committee hadbeen approved by the Water Resources Division
Council.
Generally the more important field observations made on the behaviour of earth dams are those of settlement of
embankment and measurements of internal pore pressure. These provide arecord of field performance and furnish
information that would enable future designs to be based on realistic conditions. Vertical movement devices
provide ameans of determining volume change within theembankment andsettlement of foundations, and identify
zonesof excessive anduneven settlement which may leadtostabiIity problem andcracking. The crossarm installation
especially permits the measurement of consolidation within any desired lift or lifts of embankment both during,and
after construction. Consideration governing the choice and location of this instrument is given in IS 7436
( Part 1) :1-993 ‘Guide for types of measurements for structures inriver valley projects and criteria for choice and
location of measuring instruments: Part 1For earth androck-fill dams’.
This standard was first published in 1974. A revision of this standard has been taken up to incorporate certain
changes found necessary in the standard in the light of comments received from the users. The major changes in
this revision are inclusion of paragraphs ongeneral guidance, maintenance and advantages and limitations of the
mechanical type of installation. However, theseprovisions for electrical type of installation shall beconsidered for
inclusion assoon astext for the same isprepared and finalized.
In the formulation of this standard due weightage has been given to international co-ordination among the
standards and practices prevailing indifferent countries inaddition torelating it-tothe practices inthe field inthis
country. As perUSBR Publication ‘Embankment Dam Instrumentation Manual, Jan1987’, mechanical type of cross
arms isnot being used in that country because other instruments that are easier to install and monitor have been
developed.
For the purpose of deciding whether aparticular requirement of this standard iscomplied with, the final value,
observed orcalculated expressing the result of atestoranalysis, shall berounded off inaccordance with IS2: 1960
‘Rules for rounding off numerical values (revised)’. Thenumber of significant places retained in the rounded off
value should be the same asthat of the specified value in this standard.
IS 7500:2000
Indian Standard
CODE OF PRACTICE FOR INSTALLATION AND
OBSERVATION OF CROSS ARMS FOR MEASUREMENT
OF INTERNAL VERTICAL MOVEMENT IN EARTH DAMS
( First Revision)
1 SCOPE of 75 mm steel channel (6.8 kg/m) ‘isfastened with a
U-Bolt normal to the pipe length atthe centre point.
This standard covers the requirements of installation The cross arm anchors the column of telescoping
and observation of cross arms of the mechanical.and pipe tothe embankment (see Fig. I).
electrical types of measurement of internal vertical
movement of earth dams. Vibrating wire settlement 3.1.3 Spacer Section
sensor which are being increasingly used now has
not been covered here. This section isalength of 50 mm nominal steel pipe
that serves asa spacer between the cross mm units.
SECTION 1MECHANICALTYPE Itis2500 mm long for the 3000 mm spacing of cross
arm units (see Fig, 1) in rock-free soils. The spacer
2 REFERENCE pipes are cut totwo 1250 mm length and connected
by pipe couplings when the units are installed in
The following standard contains provision which
rocky soils.
through reference inthis text, constitutes provision
ofthis standard. At thetime of publication theedition NOTE — Whenthemeasurementsofverticalmovement
indicatedwasvalid. All standardsaresubjecttorevision isdesiredatintervalsof 1500 mm the cross arm units
and parties to agreements based on this standard wiIIbe placed.1 500 mm centre tocentre, andthelength
areencouragedtoinvestigate thepossibility ofapplying of the spacer section will be 1000 mm.
themostrecentedition ofthes~andard”indicatedbelow:
3.1.4 Top Section
IS No. 1Title
This pieceof 50mm nominal boresteelpipeofvariable
length extends the column of pipe to the surface of
6!?35:1973 Method for dpterminationof water
the embankment. It iscapped by a 50 mm coupling
level in abo re hole
and apipe plug. The reading scale is attached tothis
3 EQUIPMENT top section while readings are taken. When the
installation iscompleted, the top section should be
3.1 Components of Installation encasedin concrete toreducehorizontal displacement.
An ‘asbuilt’ elevation tothe nearest 2 mm should be
3.1.1 Base Extension established for the pipe coupling on the top of the
pipe (see Fig. 2).
The base pipe extension consist of two lengths of
50 mm nominal bore steel pipe for extending 3.2 Equipment for Observation
2350 mm below the middle of the bottom crossarm
placed onthe foundation excavation line. The lower 3.2.1 Measuring Torpedo
600 mm length of 50 mm pipe inthefoundation serves
Readings of crossarms are taken by lowering from
asatrap to catch debris and is separated from the
the top of the cross arm system, atorpedo attached
upper 1500 mm length by latching plate. The base
toasteelmeasuring tape orcalibrated stainless steel
extension anchors the installation to the foundation
aircraft cable. The torpedo ismachined todimensions
andprovides room tolatch thepawls onthemeasuring
from abrass shaft and contains two wings or pawls
device (seeFig. 1).
whi-ch open or close at opening and closing slots
3.1.2 Cross Arm Unit due to spring action. The maximum dimension of
torpedo atthe wings when fully opened is lessthan
The cross arm unit is a 1000 mm length of 40 mm 50 mm andgreater than 40 mm (see Fig. 3 ).
nominal bore steel pipe towhich a I 000 mm length
1
Is 7500:2000
ELEVATION, TOP OF SYSTEM
—
~$,. a
w
4.
‘1
.,
HEMP
OR OAKUM CROSS ARM
t
HEMP OR OAKUM
=“,
n
II
flREOUIRED EXCAVATION
I ‘ L____
k
l------ -7
I i
w’
II /
/
l..
[ EXCAVATIONOF
ROCK LINE ~
65 i
1t ---- 1-.1
I
I MEASURl Nr3 POINT= 100mm@ MIN HOLE
LATCHl Nr3 PLATE
=
Y1.!
.,
j
&
J
NOTE — As each cross arm unit is placed, determine the elevations of all cross arms installed.
All dimensions in millinvxres.
FIG. 1TYPICAL INSTALLATION OFCROSSARM, MeChaniCal. T)’tw FOR3000 mm SPACING— Gt;NI:RAi.
2
1S 7500:2000
P-’,;,’,$JEXCA
VAT’;, ,
,,
\4fMlNELEvATION cIF \ “ ou-l I
\ EMBANKMENT Ln i
\ BEFORE BEGIN-
I NING EXCAVATION
\ FOR CROSSARM
\
\
\
\,1 1 POWER COMPACTED
‘1
EARTHFILL
\ f
OAKU~ -/ i- I
\
d
I I
0
OAKUM OR I I m
I I N,
I‘--t
‘EM’ ‘AcK’NGl\
I
50mm @ PIPE I
I
\ I I o
In
2 LUQS, I
12mmSQX50mm LONG I 125N
k
R +--
/4-1
HAND COMPACTED
‘+7 50t
EARTHFILL I
I I 00
0
i
IN
200mm + HOLE I
I
BORED TO PROJECTING I
I
p,pE~
(
DEPRESSION AROUND ----1i
PIPE FORMED BY PIPE
,0,,,>,
All dimensionsinmillimt!tres.
FIG. 2TYPICAL INSTALLATIONOFCROSSARM, MECHANICAL TYPE FOR3000 mm SPAC]NG—”DETA]LNEAR ToP
3.2.2 Reading Scale and Adaptor 3.2.3 Water Level Indicator
Any suitable device as described in IS 6935. The
The details of reading scale with adaptor used for
measuring .devices should be kept clean and free of
lowering thetorpedo are given in Fig. 4. If torpedo
grit. It is suggested that each type of instrument
isattached to calibrated stainless steel aircraft cable,
be disassembled, insofar aspracticable, andcleaned
reading scale may be suitably modified for aircraft
following completion of readings. The tape should
cable.
be carefully inspected for kinks and breaks.
3
IS 7500:2000
QSTEEL lTAPE !F
Ii
TAPE EYE
10mm YOKE
00
w
tml’ A
1
—
—35mm d 1: w
-3EL-. “
I
READING %
Dfie,Tin.& ! ~_-PAWL S
FuallluN - 0
1
s
PAWLS 1
LOTIii
FIG. 3MEASURING TORPEDO
4 METHOD OF LOCATING A COLUMN OF sections anchored tothe embankment by horizontal
CROSS ARMS cross arms. The base extension is installed at the
foundation level while the cross arm and spacer
Prior to installation of the testequipment, permanent
sections areplaced alternatively, one above the other
instrument benchmarks and targets should be
up to the top of the dam progressively as the
established ontheabutments ofthedam. These targets
construction proceeds. To prevent the ingress of
should be visible and accessible throughout
soil all telescopic joints should be protected with
constructionoftheentireembankment. Fromthecontrol
oakum or hemp packing held in-place by hessian
targets of especially established targets on opposite
wired on to the pipes. The cross arms a~e installed
abutments, line can beextended which will passover
progressively astheembankment isplaced. Settlement
and will intersect at the centre of the installation.
readings are obtained by means of the specially
From thissystem of intersecting baselines, temporary
designed measuring torpedo. The torpedo islowered
straddle hubsmay beestablished at any embankment
into the pipe barrel by means of a steel tape of the
elevation. When chalk lines are stretched between
cable soattached that the upper edge of the pawls of
the straddle hubs, the points of intersection will be
thetorpedo corresponds to zero points of graduation
the centre.ofthe installation.
ofthetapeorthecable. While making ameasurement
the torpedo is lowered to a point a few centimetres
5 DESCRIPTION OF INSTALLATION
below theelevation oftheuppermost measuring point,
The installation comprises a series of.alternate then lifted until the extended pawls engage the lower
telescopic 40 mm diameter and 50 mm diameter pipe end of the inside 40 mm diameter pipe.
4
—
AfMlrSTVERNIERSCALE
F~ ZERO ATTHISPOINT
(LEvELPOINT)
\
1-
SCALE STRAPIR* SUPPORT
BRASSSTRIP
#-
F-
r 64 mm STANO
PIPE
19mm 51AN0AR0
[ COUPLING \
‘oRpEoO\
r
-.
u
40mm STANDARD ENLARGED L)
F * AOAPTGRFORSOmmPtPE
PIPESINSERT $
sOmm* STANDARD I
PIPE -
[
HEXAGONAL NUT
WELOEO
e u
\
THUM8 SCREW
z
NOTES
1 Recommend a 15-kg spring scale. grapple hook. tape clamp handle anda 10-kg tension ontape wheit taking settlement readings.
2 Usedeviceasshown during construction. Thereafter cut device to I I20 mm length and thread bottom endof 50-mm pipe to tit top of installation. ..
N
o
FIG. 4 READINGSCALEwtrri ADAWOR 0
0
I
. .... I
