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Engineering Journal, 50, 3, pp. 17-21, 1967-04-01
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Frost pressures in the tie-back system at the National Arts Centre
excavation
McRostie, G. C.; Schriever, W. R.
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NATIONAL ARTS CENTRE EXCAVATION
by
G. C. McRostie and W. R. Schriever
Reprinted from The Engineering Journal Vol. 50, No. 3, March, 1967
p. 17
Research Paper No. 309
of the
Division of Building Research
Price 10 cents
OTTAWA
April 1967
LE SYSTEME D'ANCRAGE DU BLINDA
AU CENTRE NATIONAL DES A
SOMMAIRE
Les auteurs dCcrivent les effets du gel sur un blindage assujetti par un systkme
prkcontraint d'ancrage. Les dkplacements horizontaux de la paroi des fouilles
ainsi que les modifications affectant la tension iniposCe aux cibles d'ancrage ont
fait l'objet de mesures au cours de I'hiver 1965- 1966. Les r6sultats indiquent qu'en
depit des pressions considCrables engendrCes par le gel, les contraintes subies par
les cibles d'ancrage ont CtC moindres que prCvues cn raison du fluage ayant affect6
le blindage. Les auteurs donnent quelques indications pour la realisation d'autres
installations.
Details of a comp
In the tendons in July 1965 may b to have been partly dissipated by At points of local overstress in the
Building Structures Section,
Division of Building Research, the pretension loads took place prior to National Research Council the frost season and is not related to it. Tendon loads were measured again in March 1966 and were fonnd to be only
& STR 3
Geotechnical engineers frequently face the problem of providing frost pro- tection for temporary excavation brac- ing systems in Canada. T h e prestressed tie-back system of bracing is now com- ing into fairly common use for deep excavations, but no appreciable fund of published experience on the effect of frost on such systems is yet available. When an unusually large excavation in Ottawa was to be exposed for two win- ters, it was decided to use the oppor- tunity to obtain information on frost effects. This paper presents a brief ac- count of the observations made and some suggestions for use elsewhere.
tie-back tendons were smaller than ex- pected, with a maximum of about 0.07 ft. Details of the movements and an air temperature record are shown in Fig. 2. Some data on depth of frost penetra- tion behind the bracing system are shown in Fig. 3. It may be seen that movements did not increase until some days after the start of a cold spell. An estimate of the amount of time available to organize frost protection measures is valuable on jobs where part of the protection is de- layed until the onset of very cold weather.
Observed Tendon Loads
49,000 psi. Assuming elastic conditions, wall movements should have produced stresses higher than the 250,000 psi yield point of the tendons. This drop of ten- sion in the tendons must be attributed chiefly to plastic flow or creep at local high stress concentrations. These would occur at contact points between soldier piles and timber lagging, between lagging and the soil, possibly between bearing plates and wales o r between wales and brackets. These creep movements appear to be of much greater magnitnde and effect than the over-all outward creep movements in the retained soil mass. This would tend to canse an increase in the tendon loads rather than the dis- sipation observed.
The excavation available was that for
the ~ ~
centre,
~ ia building ~ The most interesting result of the pro- ~ ~ l Average Pressl~res complex including a concert hall, a gram was that, when actual tendon loadstheatre and a studio for the performing were measured by calibrated jacking pro- All observations indicated a considerable arts being constructed on confederation cedures, the tendon tensions were only scatter of individual values. This can be Square near the Parliament Buildings by
the Government Of Canada as a centen-
Fig. I . Ge~leral view o f bracer1 iiv~ll of esca\.nfio~l for Nrrtio~lrcl Arts C e ~ l f r e ill
nial project' Some idea of the size of the Ottawa ns seen f r o m wtder Mrrckenzie kin^ Bridze look in^ northwest fowarris excavation involved may be gained by
considering that over six football fields could be accommodated inside the area. A maximum excavation depth of over 50 ft was required, of which the lower part was in rock overlaid by soil up lo 25 ft in depth. This had to be retained along vertical faces.
As there had been local cases of bracing failure as a result of frost pres- sures, it was decided to be conservative and to enclose and heat the full length of the braced wall except for a short test section, as is shown in Fig. 1. Obser- vations were made on the exposed test section during the winter of 1965-66. These yielded results that can add use- fully to the few published facts on frost pressures and may aid in the assessment of the need for frost protection require- ments at similar projects.
u
-
-
Confederntio~l Squnre a~trl P n r l i n ~ ~ i e ~ l f Hill. Arroiv points to e x p e r i ~ n e ~ l f n l secfiorl where heating and i~~sulnfirig bla~iket ivere o~rlifred.
Observed Horizontal Moveme~its Measnred movements due to horizontal "frost heave" of the upper ends of the
D A l L Y T E M P E R A T U R E V A R I A T I O N S AVERAGE DAlLY M A X . O F L A S T 10 Y E A R S DAILY M A X . 1966 V E R A G E DAILY M I N . F L A S T 10 Y E A R S I L - D A I L Y M I N . 1 9 6 6 I I I I I I I I
DEC.1965 JAN.1966 F E B M A R . APR. MAY J U N E JULY AUG.
T l M F I N DAYS F R E E Z I N G I N D E X AT O T T 1965-1966- 1347 DEGREE D 2 0 YEAR AVERAGE- 1 9 3 0 DEGREE D 2 0 YEAR M A X I M U M (1958 2 2 0 0 DEGREE D
Fig. 2 . Air lerrlpernirtre varintiorls and lower lie-back rnovernents.
expected to be typical owing to the varying amounts of creep dissipation. The average pressures can be said to fall within the broad range of 300 to 600 psf for maximum values. This range is between 50 and 100 per cent of the average level of active soil pressure expected in the design, which was inten- tionally conservative owing to variations in site and exposure conditions. Frost pressure distribution would appear to be best approximated by an assumption of uniform distribution throughout the height of the bracing, rather than a trape- zoidal or triangular distribution com- monly used for soil pressures.
Factors Affecting Pressures When attempting to generalize upon the results of observations made at the Na- tional Arts Centre, at least four factors should be considered-climate, soil and water conditions, fit of structural ele- ments and other conditions affecting creep. Naturally the variations in sever- ity of weather at different locations and during tlitlerent years in the same loca-
tion must be considered. Freezing index values for Ottawa have been inserted in Fig. 2 to enable comparisons to be made.
Figure 4 gives a generalized soil pro- file at the test section. This was chosen from the available sections because its silt and till soils were known to be strongly frost susceptible. Smaller frost movements might be expected in dif- ferent soil and groundwater environ- ments. Variations in the dissipation of pretension loads are also to be antici- pated. In a situation where all structural elements happen to be in good contact with each other, pretension stress levels might remain higher before the start of the freezing season.
Finally, the amount of creep that may occur in any system, and hence the amount of stress reduction, is bound to vary with details of the installation pro- cedures and bracing design. This might result in higher maximum stress levels since there is good experimental evidence that frost pressures are increased by retlucetl anlounts of yield in the confin- ing memhers.1-4
Observation Tech~iiques Further observation programs appear to be needed to evaluate the effects of the variables involved. A brief description of the observation techniques used at the National Arts Centre and the prob- lems encountered with them may there- fore be helpful in planning future pro- grams.
Wall movements were measured using simple horizontal scale targets and a surveying theodolite. Estimates of tendon loads from wall movements would have depended on elastic stress strain behav- ior in the structural elements and the anchorage in the underlying rock, neither of which was actually elastic. Load esti- mates would also have depended on an assumption of effective tendon lengths. These were somewhat indeterminate ow- ing to unknown amounts of yield in the weathered rock zone above sound rock. Load determinations based on wall movements were therefore not thought to be really useful.
I J n f o ~ t ~ ~ n a t e l y , job rontli~ions pre- vented the installation of load cells be-
1
I
Fig. 3. Location o f observation points, frost penetration and effect o f o e e p in rerlucing tendon stresses.
neath the upper tendon anchors, al- though this was recognized as the most desirable method of load measurement.5 It was therefore decided to measure the tendon loads by other means, first by measuring the strain in representative wires and second by a jacking procedure. As all the tendons had been ten- sioned before the beginning of the study it was necessary, in order to measure the strain due to load, to cut some representative wires. The gauges used were narrow ( ] / I 0 in. wide) foil-type electrical-resistance strain gauges. A total of four wires in four tendons were in- strumented in this way, but the scheme was only moderately successful because the strains varied too much from wire to wire, as determined from laboratory tests. It was not permissible to cut more than one wire per tendon and no aver- ages could be determined.
In view of the lack of success with the strain gauges, it was decided to use an additional method. This consisted of exerting a force, by means of two 30- ton hydraulic jacks inserted hetween the pair of I2RI' 89 wales and the 2%-in. hearing plate (Fig. 4), until the firs1
separation occurred between the wales cal result of a jacking test. The breaks and the plate or between the tendon and in the load-displacement curve were not the wall. The separation was measured always as clearly discernible as one would by means of four dial gauges set upon have hoped, probably due to the disturb- the bearing plate or by a single dial ing effects of general deformation taking gauge o n the tcndon. Fig. 5 shows a typi- place during jacking. The range of ten-
Date Jan. 4 5 Feb. 1 1 Feb. 10 Feb. 15 Mar. 16 18 June 23 Table I
Observed Telldoll Loads (In Kips)
A B C D E F 5 6 77 Re~~rarks Loads determined by wire cutting Change of load determined by re-reading gauges on uncut wires Loads determined by jacking and nieasurcments at base plate Loads dctermincd by jacking and measurements on tcndons
- --
Fig. 4 . Soil conditions and wall section.
don loads given in Table I indicates this at another site. clusion that it would be preferable to use
spread of results. a retaining system that could be readily
Instrumentation for observing frost of Pressures adjusted. The adjustments can maintain penetration involved the use of removable Large and uncontrollable variations in pressures and their distribution at those clear plastic tubes containing a colored frost pressures were observed, and will levels were chosen in the original design. liquid that turned white when frozen.6 As probably occur as a general rule. The Fortunately, one economical method of tubes of insufficient length were installed, fact that both frost and earth pressures allowing adjustments has already been penetrations greater than 2 feet could can be increased o r decreased by pre- developed; others may exist o r be de- not be observed. Those shown o n Fig. 3 venting o r permitting outward movement veloped in the future.
were estimated from similar installations of a bracing system leads to the con- The method known to the writers contains as its essential elements two
Fig. 5 . Displrtcemerrts at tendorz B durirlg jockitlg test.
I
,
I
?strong plates separated by two o r more high-strength threaded bolts. T h e dis- tance between the plates and hence the tie rod or tendon loads can be altered
8 0
-I
/
8 04
/
by tightening or loosening the bolts. A( A ) M E A S U R E D AT P L A T E . ( B ) AT TENDON. common torque wrench can be used to AVERAGE OF 4 D I A L GAUGES S I N G L E D I A L GAUGE control the bolt-turning forces with suf- ficient accuracy and these forces in turn control the tie rod loads. It has not been found difficult o r uneconomical to
2
4 0o have each bolt and hence each tie rod
checked and, if necessary, adjusted a t regular intervals during the progress of construction. Such a system would offer advantages over the fixed upper anchor system actually used at the National
2 0 4 0 6 0 20 4 0 6 0 Arts Centre and will be considered for
4. E. Hoekstra
measured by cali- Ice Segregation, Highway Research 6. R. Gandahl. Deterrnin
ainst base plates, Board, Special Report No. 40, 1959. Ground Frost Line by
ng system that is capable of Conference, National Research Council holm, 1963.