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Earth pressures exerted on an induced trenchcast-in-place double-cell rectangular box culvert
Olajide Samuel Oshati, Arun J. Valsangkar, and Allison B. Schriver
Abstract: Earth pressure data from the field instrumentation of a cast-in-place reinforced rectangular box culvert arepresented in this paper. The instrumented culvert is a 2.60 m by 3.60 m double-cell reinforced cast-in-place rectangularbox buried under 25.10 m of fill constructed using the induced trench installation (ITI) method. The average earth pressuremeasured across the roof was 0.42 times the overburden pressure, and an average of 0.52 times the overburden pressurewas measured at mid-height of the culvert on the sidewalls. Base contact pressure under the rectangular box culvert wasalso measured, providing field-based data demonstrating increased base pressure resulting from downward drag forcesdeveloped along the sidewalls of the box culvert. An average increase of 25% from the measured vertical earth pressureson the roof plus the culvert dead load (DL) pressure was calculated at the culvert base. A model culvert was also tested ina geotechnical centrifuge to obtain data on earth pressures at the top, sides, and base of the culvert. The data from thecentrifuge testing were compared with the prototype structure, and the centrifuge test results agreed closely with themeasured field prototype pressures, in spite of the fact that full similitude was not attempted in centrifuge testing.
Key words: induced trench, earth pressure, box culvert, soil arching, centrifuge testing, high embankments.
Résumé : Cet article présente des données de pressions des terres provenant d’instrumentation sur le terrain d’un ponceaurectangulaire renforcé et coulé en place. Le ponceau instrumenté est un ponceau rectangulaire renforcé a double cellule de2,60 m par 3,60 m, coulé en place sous 25,10 m de remblai, et construit avec la méthode d’installation de tranchée induite(ITI). La pression des terres moyenne sur la paroi supérieure était de 0,42 fois la pression des terres, et une moyenne de0,52 fois la pression des terres était mesurée a la mi-hauteur des murs du ponceau. La pression de contact a la base sous leponceau rectangulaire a aussi été mesurée, ce qui permet de démontrer a l’aide de résultats de terrain qu’une augmentationde la pression a la base est causée par les forces de traînée vers le bas développées le long des murs des côtés du ponceaurectangulaire. Une augmentation moyenne de 25 % de la pression des terres verticale mesurée sur la paroi supérieure, enplus de la pression de charge morte du ponceau, a été calculée a la base du ponceau. Un ponceau modèle a aussi été testédans une centrifuge géotechnique afin d’obtenir des données sur les pressions des terres au haut, sur les côtés, et a la basedu ponceau. Les données des essais en centrifuge ont été comparés avec la structure prototype, et les résultats d’essais encentrifuge correspondent bien avec les mesures de pressions sur le prototype de terrain, malgré que les essais en centrifugene tentaient pas d’être totalement similaires au terrain.
Mots-clés : tranchée induite, pression des terres, ponceau rectangulaire, effet d’arche, essai en centrifuge, digues élevées.
[Traduit par la Rédaction]
Introduction
Routing and construction of modern highways often requirehigh embankment fills (fill height above culvert top, H �10 m) crossing over underground rigid and flexible culverts,thereby resulting in significant earth pressures on these struc-tures. In designing these underground structures, several fac-tors need to be considered: the site condition, fill height,rigidity of the culvert, material properties, bedding condition,compaction, available time, cost, and hydraulic and structuralrequirements. Typical embankment loadings coupled withease of construction make rigid culverts (precast or cast-in-place) a preferable choice over flexible culverts.
The induced trench installation (ITI) method is one of thecommon installation techniques used to reduce the load on
rigid concrete culverts. In the ITI method, a zone of compress-ible material (e.g., hay, sawdust, peat, shredded rubber tires,etc.) is placed above the culvert. The concept behind the ITImethod is to reduce the vertical earth pressure, not only by theinclusion of lower-density fill material (compressible layer),but mainly from inducing positive arching (i.e., the column ofsoil directly above the culvert settles more relative to theadjacent soils) from differential settling initiated by the com-pressible fill layer. During this differential settling, some of theearth load from the column of soil directly above the culvert issupported by the shear forces developed on the soil interfacewith adjacent soil columns, thereby causing some of the loadto be redistributed to the adjacent soils.
For almost two decades, the University of New Brunswick(UNB) and the New Brunswick Department of Transportation
Received 21 December 2011. Accepted 20 August 2012. Published at www.nrcresearchpress.com/cgj on 30 October 2012.
O.S. Oshati, A.J. Valsangkar, and A.B. Schriver. Department of Civil Engineering, University of New Brunswick, Head Hall,17 Dineen Drive, P.O. Box 4400, Fredericton, NB E3B 5A3, Canada.
Corresponding author: Olajide Samuel Oshati (e-mail: [email protected]).
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Can. Geotech. J. 49: 1267–1284 (2012) Published by NRC Research Pressdoi:10.1139/t2012-093
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(NBDOT) have been studying the soil–structure interactionpertaining to the ITI method via physical testing, numericalmodelling, and field instrumentation and monitoring. How-ever, most of the previous studies pertain to precast circularpipes installed under high embankments constructed using theITI method. As the practice of using the ITI method withcast-in-place box culverts is not common, the present studywas undertaken to increase knowledge in this area. A 142 mlong, 2.60 m wide by 3.60 m high rigid double-cell rectangularbox culvert installed under 25.10 m of fill using the ITI methodwas instrumented to monitor earth pressures on the roof, sides,and base of the culvert. Centrifuge testing was undertaken tosupplement data from the field instrumentation and monitor-ing. The centrifuge testing results were also compared with theresults from the previous study conducted by McGuigan andValsangkar (2010).
Background
The induced trench method was originally proposed forrigid pipes under high embankment fill load. It includes a zoneof compressible material (e.g., hay, sawdust, peat, shreddedrubber tire, etc.) placed above the culvert. Figure 1 shows thebasic features of box culverts under negative projection, pos-itive projection, and an induced trench embankment installa-tion. The concept of the ITI method originated from Marstonin the early 1900s (Marston and Anderson 1913; Marston1930). In subsequent years, further studies were carried outand modifications were made to the ITI method by Spangler(Spangler 1950; Spangler and Handy 1973). The ITI methodattempts to simulate the soil–structure interaction mechanismassociated with the trench installation method, where the col-umn of soil directly above the culvert settles more relative tothe adjacent soils (positive arching), thereby reducing the loadon the culvert.
Limitations of the ITI theory have been reported by Sladenand Oswell (1988) and Scarino (2003). Kang et al. (2007a,2007b) reported that for polyvinyl chloride (PVC) and con-crete pipes, the base contact pressures would be higher thanthe pressure at the crown plus weight of the culvert. McAffeeand Valsangkar (2008) observed from numerical modellingthat the average vertical pressure calculated at the bottom of abox culvert installed using the ITI method was 9% higher thanthe vertical loads on top of the culvert plus the weight of theculvert and 30% higher for positive projection installations(PPI) owing to downward drag forces developed on the side-walls of box culverts. Kang et al. (2008) and McGuigan andValsangkar (2010) also identified and confirmed the need toaccount for significant downward drag forces generated alongthe sidewalls of box culverts, which in turn leads to an increasein base contact pressure.
The data pertaining to the long-term performance of in-duced trench culverts were recently presented by Hansen et al.(2007). Inspection of 50 ITIs with service life varying from 16to 8 years indicated that all these installations have performedsatisfactorily. McAffee (2005) reported on a twin culvert in-stalled using the ITI method in Baddeck, Nova Scotia, whichhas performed satisfactorily over a period of 35 years. Thesedocumented studies show that the ITI method is a viableoption when considering both short- and long-term behaviour.
Previous box culvert research
The majority of research on box culverts has focused onPPI, with limited research on ITI and even less information onfield instrumentation of box culverts using the ITI method.Vaslestad et al. (1993) instrumented and monitored three pro-totype culverts, one of which was a 2.0 m wide by 2.55 m highbox culvert (Bc/Hc � 0.78) at Hallumsdalen, where Bc is thewidth of the culvert and Hc is the height of the culvert. Thestudy represents one of the few field studies on box culvertsinstalled under an induced trench constructed embankment.Expanded polystyrene (EPS) was used as the compressiblelayer. The Hallumsdalen study focused mainly on verticalearth pressure on the box culvert roof and the long-termbehaviour of induced trench installed culverts. Earth pressureon the top slab was measured using hydraulic pressure cellsand was compared with the overburden pressure. The lateralearth pressures and base contact pressures were not measuredin this study. Upon completion of the embankment at a fillheight of 10.8 m, the measured earth pressure was 63% of theoverburden pressure. Their 3 year research showed no signif-icant increase in vertical earth pressure on the culvert afterconstruction.
Bourque (2002) used the UNB geotechnical centrifuge tomeasure vertical and horizontal soil pressures acting on twininduced trench box culverts. Bourque (2002) also performedparametric studies using numerical modelling to address theeffect of culvert spacing, width of the compressible layer,culvert geometry, and backfill type. Bourque (2002) observedthat the horizontal pressure for ITI was higher than verticalpressure for both single and twin culvert installation. How-ever, Bourque’s study did not address base pressures and alsoinvestigated only a model culvert with Bc/Hc � 1. Resultsfrom his work were later incorporated into a detailed study byMcGuigan and Valsangkar (2011), where the issue of basepressure was addressed.
MacLeod (2003) investigated the earth pressures aroundinduced trench conduits using centrifuge testing and numericalmodelling and compared the results with prototype resultsreported in the literature. MacLeod (2003) also investigatedthe effect of variables such as conduit shape (circular and box),backfill material, compressible zone width, compressible zonethickness, compressible zone stiffness, and compressible zonelocation on the performance of induced trench conduits. Re-sults from the centrifuge tests performed by MacLeod (2003)were incorporated in a later study by McGuigan and Valsangkar(2010).
Kim and Yoo (2005) used finite element modelling toanalyze different geometric configurations and backfill mate-rial properties for a concrete box culvert installed using the ITImethod. The study reported that the preferred width of thecompressible layer should not exceed 1.5Bc and that the ratioof the thickness of the compressible layer to the height of theculvert should not be greater than 1.5. Kim and Yoo (2005)observed that the maximum load reduction rate is achievedwhen the compressible layer is placed directly on top of theculvert, and they concluded that the ITI method could reducethe soil–structure interaction factor (Fe). The study focused onthe beneficial load reduction that can be achieved from the ITImethod and did not address lateral earth pressure or base
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contact pressure. Kim and Yoo (2005) also examined onlysquare-shaped culverts.
McAffee (2005) used centrifuge tests to simulate differentH/Bc ratios of field prototype structures installed using the ITImethod, for both single and twin box culverts, and comparedthe centrifuge test results with PPI. Results from his studyconfirmed significant reduction in vertical pressure for eachcondition, but increased lateral pressures on the sidewalls.McAffee (2005) observed that the compressibility, width, andheight of the compressible layer are important factors to theload reduction. McAffee (2005) also conducted tests consid-ering different compressible zone configurations. The study,however, did not include base contact pressures and consid-ered only square-shaped culverts.
Kang et al. (2008) reported the results of numerical analysison box culverts under high fill and highlighted the effects offrictional forces developed on sidewalls of box culverts on thebase contact pressure. Kang et al. (2008) proposed a preferredcompressible zone geometry and formulas for the earth loadreduction rate for the proposed compressible zone geometryand the PPI soil–structure interaction factor accounting fordownward drag forces developed on box culvert sidewalls.Kang et al. (2008) also found that the American Association ofState Highway and Transportation Officials (AASHTO)(2004) design equation used for embankment installationcould be unconservative.
McGuigan and Valsangkar (2010) presented the results ofparametric studies on a single box culvert using centrifugetesting and numerical modelling to evaluate the pressuresacting on the top, sides, and base of box culverts. McGuiganand Valsangkar (2010) also recommend practical optimal pre-ferred compressible zone geometry, along with physical evi-dence for the increased base pressure caused by downwarddrag forces developed on the sidewalls of box culverts. Resultsof the study compared ITI with PPI and checked the effect ofthe compressible zone stiffness. In their study, McGuigan andValsangkar (2010) observed that with a preferred compress-ible zone width of 1.2Bc, a thickness of 0.5Hc, and an unyield-ing foundation, a 78% reduction in vertical pressure on theroof was achieved compared with the PPI, and a 47% reduc-tion was achieved for the base contact pressure compared with
the PPI. A yielding foundation, on the other hand, achieved a78% and 35% reduction in roof and base pressures, respec-tively, compared with the PPI. In agreement with McAffee(2005), McGuigan and Valsangkar (2010) observed that com-pressible zone stiffness played an important role in load re-duction achieved on the roof and base of culverts. However,the study by McGuigan and Valsangkar (2010) was limited toa fill height of 12 m above the culvert roof and Bc/Hc � 1.
In 2011, McGuigan and Valsangkar (2011) used centrifugetesting and numerical modelling to evaluate culvert spacing(clear distance between culverts) and compressible zone ge-ometry for twin positive projecting and induced trench boxculverts. In their study, McGuigan and Valsangkar (2011)observed that twin culverts installed using the PPI methodgave lower earth pressures than single culverts. A singlecompressible zone spanning both culverts was proposed forculverts spaced at 0.5Bc and 1Bc, while for a spacing of 1.5Bc,two individual compressible zones of 1.2Bc width were pre-ferred. Earth pressures on the roof of twin induced trenchculverts were observed to be higher than the correspondingpressures on the roof of single culverts. Lower lateral pres-sures were observed for twin culverts compared with single culverts,and lower base pressures were observed for twin culverts with 0.5Bcspacing than for single culverts. McGuigan and Valsangkar (2011)concluded that lower pressures correspond to the smallestspacing (0.5Bc). The detailed parametric study, however, didnot consider double-cell rectangular box culverts.
In the latest edition of the AASHTO LRFD bridge designspecifications (AASHTO 2010), the ITI method is recognizedas one of the acceptable methods of installation. However,there are no guidelines as to the method or procedure todetermine the earth load exerted on box culverts installedusing the ITI method. Instead, AASHTO (2010) suggests theuse of accepted test methods, soil–structure interaction analy-ses or previous experience to determine the load on the culvert.AASHTO (2010), however, provides equations for calculatingthe unfactored earth load acting on box culverts for embank-ment and trench installations. Equations [1] and [2] present theAASHTO (2010) equation for determining the unfactored loadon the roof of box culverts in an embankment installation(PPI).
Fig. 1. Simplified features of embankment installation types: (a) negative projection, (b) positive projection, and (c) induced trench.
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Fig. 2. McBean Brook induced trench installation design (a) plan, (b) transverse section (all dimensions in metres), and (c) cross section.CL, centre line.
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[1] WE � FeγsBcH
where WE is the total unfactored earth load,
[2]Fe � 1 � 0.20H/Bc
Fe � 1.15 for compacted fill on culvert sidesFe � 1.40 for uncompacted fill on culvert sides
Fe is the soil–structure interaction factor, �s is the unit weightof the fill soil, Bc is the outside width of the culvert, and H isthe fill height above the top of the culvert.
McBean Brook culvertThe McBean Brook culvert is a cast-in-place reinforced
double-cell rectangular box culvert buried under 25.10 m offill. It is part of an upgrade of the new Route 8 Nashwaak–Marysville bypass in New Brunswick, Canada. The 142 mlong culvert has internal dimensions of 2600 mm width,3600 mm height for each cell, a 600 mm thick base slab,outside and inside wall thicknesses of 550 and 450 mm,respectively, and a roof thickness of 450 mm. The culvert wasdesigned to support a maximum uniform pressure of 400 kPaat the base.
The culvert was formed and cast in sections, starting withthe base slab, followed by the walls, and finally the roof. Theexisting ground was excavated to an approximate depth of
2900 mm; the culvert was then founded on a 500 mm thickclass “A” backfill compacted to 95% standard Proctor maxi-mum dry density. The sides and roof were also backfilled withclass A fill compacted to 95% standard Proctor maximum drydensity up to a height of 500 mm above the roof of the culvert.A 2500 mm thick, 8000 mm wide compressible fill layer wasplaced over the 500 mm thick class A backfill. The compress-ible layer was then backfilled with borrow “A” material to afinal height of 25.10 m above the roof of the culvert. Trans-verse and longitudinal sections through the culvert installationare shown in Fig. 2.
Class A backfill is a well-graded granular material withuncoated particles, free of clay lumps and other deleteriousmaterials, with no more than 10% retained on the 100 mmsieve and less than 10% passing the 75 �m sieve. Borrow Amaterial, however, consists of soil and rock particles with lessthan 25% passing the 75 �m sieve and contains no organic ordeleterious substances.
ConstructionConstruction work at McBean Brook started in January
2010 with site preparation work, after which construction ofthe first 20 m length of the box culvert began in February2010. Stream diversion followed in the spring, before theremaining 122 m length of the box culvert was constructed.Construction of the box culvert started with preparation and
Fig. 3. McBean Brook cast-in-place reinforced culvert: (a) concrete placement, (b) trench excavation, (c) placement of hay by hand, and(d) completed embankment.
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compaction of the bed to 95% standard Proctor maximum drydensity, after which the base slab was formed, poured, cured,and then stripped. This was followed by the construction of thewalls and roof. Prior to the completion of the entire culvertconstruction, backfilling started around the completed sectionsof the culvert (downstream). Figure 3a shows the method usedfor concrete placement.
Upon the completion of the culvert construction, the brookwas diverted through the culvert in September 2010, afterwhich fill was placed and compacted in lifts. A vibratory rollerwas used to compact the backfill on the sides of the culvertusing 305 mm lifts. A plate compactor and 200 mm thick liftswere used on the culvert roof until a minimum fill cover of 1 mwas reached. After this stage, the fill was raised and compacted to aheight of 3 m above the culvert roof. A 2500 mm deep by 8000 mmwide fill was then carefully excavated from the roof of the culvert(Fig. 3b), leaving 500 mm of fill on top of the culvert. As theexcavation was being made, the installation of the compressiblelayer (hay) began. The hay was placed loosely by hand along theentire excavated length of the culvert (Fig. 3c). After the com-pressible zone was installed, the backfilling and compactioncontinued, adding an extra metre of fill over the compressiblelayer. A total height of 4 m of fill and hay above the top of theculvert was reached in October 2010 before constructionstopped. Construction resumed in April 2011, and the embank-
ment was raised to the final height of 25.10 m fill above theculvert by July 2011 (Fig. 3d).
Field instrumentationTo measure earth pressures around the culvert, a total of 32
Geokon vibrating wire earth pressure cells (EPC) were used(model 4800; Geokon Inc., Lebanon, New Hampshire). Thepressure cells are made with de-aired hydraulic oil stored incircular stainless steel plates with a diameter of 230 mm,thickness of 6 mm, and an aspect ratio of 38.3. The pressurecell converts oil pressure to an electrical signal via a hydraulicmedium before being transmitted to a readout unit. TheGeokon EPC was selected based on past research experience atUNB (e.g., Parker et al. 2008; McGuigan and Valsangkar2011). Figure 4 shows the locations of the pressure cellsaround the culvert. For the purpose of this study, two differentsections, A and B, 8 m apart approximately mid-length of theculvert, were instrumented for redundancy of earth pressuremeasurements. The two sections were under the full embank-ment height (25.10 m), and each section had 16 pressure cellsinstalled around it.
Ten pressure cells were installed under the base slab tomeasure the base pressures. Prior to the pouring of the baseslab, 10 sequentially spaced small excavations, approximately
Fig. 4. McBean Brook culvert and instrument location (all dimensions in metres).
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600 mm by 900 mm, were made across the proposed base afterthe culvert bedding had been prepared and compacted. Theexcavations were lined with geotextile before approximately200 mm of sand was hand-compacted in layers to install thepressure cell. The pressure cells were then placed on thelevelled sand and then covered with approximately 200 mm ofhand-compacted sand before being wrapped at the top withgeotextile. Special care was taken to avoid soil particles largerthan 8 mm around the cell, and efforts were also made tocompact the soil to be as stiff as the cell to allow for moreaccurate registration of the earth pressures. A total of 10pressure cells were installed on the roof of the culvert tomeasure the vertical earth pressures, using the same procedureas was used for the base pressure cell installation (Fig. 5a).
A total of six pressure cells were installed on the sidewallsfor each section, in a staggered fashion, to measure lateralearth pressures. The north and south sides had three pressurecells each. The installation sequence for the side pressure cellswas different from that of the base pressure cells. Excavationwas made at the desired location after being backfilled andcompacted above the target elevation. The excavation was
then lined with a geotextile and filled in with layers of hand-compacted sand. A thin strip about the size of the plate andstem of the pressure cell was then excavated for the pressurecell. Care was taken to make sure the cell plates were vertical(culvert facing) to measure the lateral pressure effectively.Figure 5b shows the installation method for a side pressurecell.
Three different capacity ranges were selected for the pres-sure cells: 2 MPa for the measurement of base contact pres-sures, 1 MPa for the measurement of lateral earth pressures,and 700 kPa for the measurement of vertical earth pressures onthe roof. These ranges were selected based on previous expe-rience, which indicated unexpected excessive stress concen-trations under culverts (McGuigan and Valsangkar 2011).Prior to field installation, a total of 17 pressure cells wereselected randomly from each capacity range and calibrated inthe laboratory. The calibration was done in a steel box housingusing a pressure bag to apply uniform pressure. Readings fromthe pressure cells were collected using a readout unit, and theresults were plotted as a function of applied pressure. Theresults from the calibration tests carried out in the laboratorywere within �10% of the factory calibration test results. Thispercentage difference is considered acceptable based on thesize of the box used to calibrate pressure cells at the UNBlaboratories. The pressure cells were not calibrated to theirmaximum capacity, as the maximum pressure applied was just
Fig. 5. Earth pressure cell installation on (a) base and roof and (b) side.
Fig. 6. Laboratory calibration setup. Fig. 7. University of New Brunswick geotechnical centrifuge.
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over 400 kPa, which is approximately one-fifth of the highest-capacity cell used. However, the calibration proved that thecells were working within the maximum range of pressuresexpected at the top, sides, and base of the culvert. The labo-ratory calibration setup is shown in Fig. 6. Zero-pressurereadings were obtained for each pressure cell in the fieldbefore installation. After installation, the pressure cell wireswere channelled through 3/4 in. (1 in. � 25.4 mm) PVCconduit pipes to provide extra protection to the cable and thenthrough 4 in. PVC conduits further down the length of theculvert to the outlet wall. A readout station for data collectionwas built at the head wall.
Centrifuge testingAll centrifuge tests were done using a 1.6 m radius geotech-
nical centrifuge with the ability to accelerate a 100 kg payloadat 200 times Earth’s gravitational acceleration. Figure 7 showsthe basic features of the centrifuge. To model the field proto-type geometry, a scaled aluminum model culvert — 76 mm by52.4 mm by 195 mm in length — was used. The model culverthoused two pressure transducers, which were used to measurevertical, lateral, and base pressures. The transducers used wereKyowa BEC-A-500 kPa soil pressure cells, with a capacity of500 kPa and a high stiffness to reduce localized soil archingaround the cells.
The pressure-measuring face of the transducer has adiameter of 23 mm. Each face therefore gives an averagepressure over 60% of the entire culvert width. In total, thetwo transducers measure pressure over 60% of the com-bined width for both the top and base orientations. For thesides it measures the average pressure over 44% of theculvert height. Prior to centrifuge testing, the transducerswere calibrated. The calibration was done for the top andside transducers using a rigid foundation and rigid side
support, with rigid foundation thickness equal to the heightof the culvert (Hc). The overburden pressure for the top(�H) and mid-height of the culvert side (�Hs) were deter-mined, where � is the unit weight of the fill, H is the fillheight above the culvert top, and Hs is the fill height fromthe mid-height of the culvert side to the top of the fill.McGuigan and Valsangkar (2010) recommended a separatecalibration for transducers located at the base of model cul-verts, because of less localized arching around these transduc-ers. Therefore, separate calibration was done for transducerslocated at the culvert base. Rigid supports were used on thesides of the culvert to prevent bearing capacity failure whilecalibrating the transducers at the base of the culvert. For allcalibrations the centrifuge was gradually ramped up to approx-imately 89 times Earth’s gravitational acceleration (89g). Allcalibrations were done in the strong box used for centrifugetesting to account for all boundary effects. The theoreticaloverburden pressure was calculated based on the fill height,density of the sand, and the gravitational acceleration level.Upon calibration of the pressure transducers, several tests wereperformed for both PPI and the ITI, both constructed on ayielding foundation. Vertical, lateral, and base pressures weremeasured for both conditions. The general setup for the cen-trifuge configuration is shown in Fig. 8.
Silica sand was used as backfill and the yielding base layer.EPS was used as the compressible layer. The H/Hc ratio for alltests was maintained at 5.38, with an H/Bc ratio of 3.71. Alltests were performed at a maximum gravitational accelerationlevel of approximately 89g. A model culvert of 76 mm by52.4 mm with a fill height (H) of 281.7 mm accelerated to 89times the gravitational acceleration is equivalent to a 6.75 mby 4.65 m prototype culvert under 25 m of fill. A 5.6 mm thicklayer of silica sand was placed between the top of the culvert
Fig. 8. Centrifuge model setup (all dimensions in millimetres).
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and the base of the compressible layer, representing 500 mmof backfill in the prototype design. The manual air pluviationmethod of sand placement suggested by Ueno (1998) was usedto prepare the models. This technique has been used by otherresearchers in the past (Bourque 2002; McAffee and Valsangkar2008; McGuigan 2010) and was found to produce uniformbackfill beds. A constant drop height of sand was maintainedthroughout the sample preparation to maintain a relativelyconstant density. After each test, the density of the sand usedwas determined, and an average density of 1541 kg·m–3 wasfound. The strong box used in this study was 265 mm wide,195 mm long, and 495 mm deep (internal dimensions).
Centrifuge test results
As previously stated, centrifuge testing was done at a max-imum acceleration of 89g, which corresponds to 25 m of fillabove a 6.75 m by 4.65 m double-cell rectangular prototypebox culvert. The Kyowa pressure transducers were used tomeasure the average pressure over 60% of the model double-cell box culvert for the top and bottom. The pressures from thetwo transducers on each box of the model culvert were aver-aged for an overall average pressure at the top and the base.Owing to the high gravitational level used, extra safety pre-cautions were taken, and hence only two spins were done for
Fig. 9. Average vertical earth pressure measured by (a) transducer A and (b) transducer B, as defined in Fig. 8.
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each test configuration to ensure reproducibility, and threespins for the calibration. The results from the centrifuge testingare expressed in terms of the overburden pressure (�H) at the topof the culvert. An average pressure of 1.16�H was measuredacross the roof of culverts with PPI configuration, while anaverage pressure of 0.21�H was measured across the roof ofculverts with the ITI configuration. The average lateral pressuremeasured on culverts with the PPI configuration was 0.36�Hs,while for culverts configured with the ITI, an average lateralpressure of 0.54�Hs was measured. This increase in lateral pres-sure for ITI is as expected, as the load reduction from the top isredistributed to the sides and the base. At the base of the culvert,an average pressure of 1.61(�H � DL) was measured for the PPI,
while for the ITI condition, the average pressure measured at thebase was 0.70(�H � DL), where DL is the dead load pressure ofthe culvert. Figures 9 through 11 show the average pressuresmeasured during the centrifuge tests.
The results from the present series of tests are in closeagreement with the results from the previous study usingsingle culverts by McGuigan and Valsangkar (2010). Table 1shows a detailed comparison between centrifuge tests donefor similar prototype fill heights and a comparison withthe numerical model by McGuigan and Valsangkar (2010)using a finite difference analysis program called “FastLagrangian Analysis of Continua” (FLAC). Results from thecentrifuge testing suggest that the width (Bc) of the culvert,
Fig. 10. Average lateral earth pressure measured by (a) transducer A and (b) transducer B.
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along with the compressible zone geometry and the fill prop-erties, have significant influence on the redistribution of loadto the side and base of the culvert. It can be postulated that awider culvert will result in increased net drag forces on thesidewalls of box culverts, particularly for positive projectioninstallations.
Effect of aspect ratio of culvertsFor comparison purposes, results from the present centri-
fuge tests are compared with the results from previous testsconducted at the UNB by McGuigan and Valsangkar (2010).The tests done previously by McGuigan and Valsangkar(2010) were completed at a maximum gravitational accelera-
tion of 70g, on a 38 mm by 38 mm model culvert (Bc/Hc � 1)using a model fill height (H) of 152 mm with H/Hc � 4.0.Table 1 shows a comparison of the results from previous testswith the results from the present study at similar prototypeheight. In terms of aspect ratio, earth pressure comparison wasmade between Bc/Hc � 1 and Bc/Hc � 1. Results from thepresent study suggest that the Bc/Hc ratio influences the pres-sure distribution around box culverts. For model culverts con-structed using the PPI method, the average vertical pressureson the roof and lateral earth pressure measured were lowerwhen Bc/Hc � 1 than when Bc/Hc � 1 (i.e., wider culvertsresult in lower vertical roof and lateral earth pressures). Incomparison, the average base contact pressure measured was
Fig. 11. Average base contact pressure measured by (a) transducer A and (b) transducer B.
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significantly higher for Bc/Hc � 1 when compared with Bc/Hc � 1(i.e., wider culverts result in higher base pressures). Forculverts constructed using the ITI method, the vertical pres-sures on the roof and lateral earth pressure measured wereobserved to be higher for Bc/Hc � 1 compared with Bc/Hc � 1(i.e., wider culverts result in higher vertical and lateral pres-sures). In comparison, the base contact pressure measured waslower or similar for Bc/Hc � 1compared with Bc/Hc � 1 (i.e.,wider culverts result in lower base pressures).
When a culvert is installed using the PPI method, it isknown that the weight of the column of soil above the culvertwill exceed the theoretical overburden weight, owing to neg-ative arching. This fact, in addition to the development ofdownward drag forces on the sidewalls of box culverts, makesthe assumption of the base contact pressure (sum of the roofoverburden pressure plus culvert DL pressure) in a PPI un-conservative. This is particularly of concern when dealing witha culvert with Bc/Hc � 1. McAffee and Valsangkar (2008) andKang et al. (2008) also observed similar outcomes of basepressure being greater than the overburden pressure plus thedead load pressure of the culvert, for culverts installed usingthe PPI method. Results from centrifuge testing from thepresent study show an average increase of 61% in the basepressure compared with the theoretical overburden plus thedead weight of the culvert for the PPI.
To further emphasize the need to account for the down-ward drag forces developed on the sidewalls in the designof box culverts, results from the centrifuge tests are pre-sented in Table 2. The results are expressed as a fraction of theoverburden (�H). It should be noted that the increase in basepressure is not only present in the ITI, but also present in PPI,especially when dealing with a relatively wide culvert (Bc/Hc �1). In the current study, at a prototype fill height of H � 20 m andBc/Hc � 1, centrifuge testing results show a 28.4%–39.1% in-crease in base contact pressure from the weight of the soil prismabove the culvert plus the dead load pressure of the culvert (�H �DL) for an ITI and a 1.83%–5.07% increase in base contact
pressure for a PPI. Comparing the results from a fill height H �20 m with those from a fill height H � 25 m reveals nosignificant increase in base contact pressure. The ITI showsan increase of 23.3%–30.4%, and the PPI shows an increase of0%–2.0%in base contact pressure from the weight of the soil prismabove the culvert plus the dead load pressure of the culvert(�H � DL). The percentage increase of the measured base pressurefrom the theoretical pressure of (�H � DL) when an extra 5 m of fillwas added did not show any significant change. The PPI resultsagree with the findings by Kang et al. (2008), implying that the netdownward frictional forces on sidewalls of box culverts do notdiffer much, even though the magnitude of the frictional forcesincrease with increased fill height.
Table 1. Earth pressure comparison at different aspect ratios and fill heights.
Gravity level 70g* 30g** FLAC*Model culvert size (mm) 38�38 52.4�76 38�38Model fill height, H (mm) 152 281.7 152H/Hc (model) 4 5.38 4H/Bc (model) 4 3.71 4Bc/Hc (model) 1 1.45 1Prototype fill, Hp (m) 10.64 8.45 10.64Density, � (kg·m–3) 1486 1541 1486EPS width Bc 1.19Bc 1.20Bc
EPS thickness 0.66Hc 0.54Hc 0.5Hc
H/Hc (prototype) 4 5.37 4Top pressure PPI 1.33�H 1.20�H —Top pressure ITI 0.24�H 0.28�H 0.28�HSide pressure PPI 0.43�Hs 0.36�Hs —Side pressure ITI 0.46�Hs 0.50�Hs 0.47�Hs
Base pressure PPI 1.13(�H�DL) 1.75(�H�DL) —Base pressure ITI 0.75(�H�DL) 0.70(�H�DL) 0.73(�H�DL)
Note: FLAC, Fast Lagrangian Analysis of Continua.*McGuigan and Valsangkar (2010).**Present study.
Table 2. Comparison of base contact pressures.
Base contact pressure(ratio to �H)
20 m at 70g 25 m at 89g
Low High Low High
ITIMeasured overburden (�H) 0.21 0.22 0.21 0.22Calculated dead load (DL) 0.36 0.36 0.39 0.39�H�DL 0.57 0.58 0.6 0.61Measured base pressure 0.74 0.79 0.75 0.79Theoretical base pressure 1 1 1 1
PPIMeasured overburden (�H) 1.16 1.17 1.16 1.17Calculated dead load (DL) 0.36 0.36 0.39 0.39�H�DL 1.52 1.53 1.55 1.56Measured base pressure 1.56 1.59 1.54 1.58Theoretical base pressure 1 1 1 1
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For a model box culvert buried under 25 m of fill, centrifugetests performed at UNB indicated that the base pressure forboth the PPI and the ITI were higher than the overburdenpressure (�H) on the roof plus the dead load (DL) pressure.Table 2 shows a comprehensive comparison of the base con-tact pressure results for two centrifuge tests at different gravitylevels representing different fill heights.
Field-measured pressuresEarth pressures were monitored and recorded around the
culvert as the embankment height was increased. The site wasinspected constantly to note stockpiling of fill and otherconstruction-related activities that would affect earth pressureson the culvert. Figure 12 shows earth pressures measured on
the roof at sections A and B as the fill height increased. Atsection A, the pressure on the roof ranged from 186.8 to267.3 kPa, with an average pressure of 218.5 kPa (0.41�H,0.46�H=). Similarly at section B, the pressure on the roofranged from 157.6 to 222.8 kPa, with an average pressure of186 kPa (0.35�H, 0.39�H=). The term �H represents theoverburden pressure on the roof using the unit weight of fillalone. The term �H= represents the overburden pressure on theroof accounting for the unit weight of the compressible fillmaterial. Based on the research conducted by McAffee andValsangkar (2004), 21 kN·m–3 was used for the unit weight ofthe fill material and 0.53 kN·m–3 as the unit weight of hay. Theearth pressure readings from A16 and B12 were excluded fromfurther analysis, as the two cells started recording significantly
Fig. 12. Vertical earth pressure on top of McBean Brook culvert in (a) section A and (b) section B, as defined in Fig. 2a. (Instrumentlocations as given in Fig. 4.)
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different pressures compared with other cells along the samesection, immediately after the induced trench was installed(elevation at 61 m), as seen in Fig. 12. The two cells (A16 andB12) are believed to have been disturbed during the installa-tion of the compressible layer. The range of pressures mea-sured on the roof was expected, as roof slab sections above thewalls should record relatively higher earth pressures than theother slab sections. At an elevation of approximately 76 m,there was a noticeable increase in the measured earth pres-sures, which can be explained by a sudden change in fill liftthickness before compaction. On average, section A recordedslightly higher vertical earth pressures on the roof than section B.
Lateral earth pressures were also measured as fill heightincreased (Fig. 13). Lateral pressures measured on the north
side of section A ranged from 118.1 to 381.4 kPa, with anaverage of 231.1 kPa (0.40�Hs, 0.44�H=s), while lateral pres-sures on the north side for section B ranged from 146.7 to391.2 kPa with an average of 246.3 kPa (0.43�Hs, 0.47�H=s).In comparison, lateral pressures measured on the south sideranged from 147.9 to 454.1 kPa, with an average of 277.7 kPa(0.48�Hs, 0.53�H=s) for section A, while for section B, mea-sured lateral pressures ranged from 249.9 to 499.1 kPa, with anaverage of 340 kPa (0.59�Hs, 0.65�H=s). Although the mea-sured average lateral pressures seem reasonable, it can be seenfrom Fig. 13b that the south side of the culvert shows anunconventional pressure distribution. The measured lateralpressures were observed to decrease with depth rather thanincreasing with depth. The pressures on the lower section on
Fig. 13. McBean Brook culvert lateral earth pressure on (a) north side and (b) south side. (Sections A and B as defined in Fig. 2a andinstrument locations as given in Fig. 4.)
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the south wall (B6, A7, and A9) were observed to havedecreased when the brook was diverted through the culvert.However, subsequent pressure increases were measured onhigher sections of the south sidewall (B8, B10, and A11).Erosion of material around the lower part of the culvert isbelieved to have loosened the fill, which resulted in localizedarching. A relatively low pressure was measured around pres-sure cell B12 on the roof of the culvert wall, and a relativelyhigh pressure was measured around pressure cell B10, whichis on the wall perpendicular to B12. This can also be attributedto some localized arching around pressure cell B12 as therouting of the cables was within close proximity of the cellenroute to the readout box. The localized arching around B12on the roof appears to be significant, resulting in an increased
lateral pressure in the adjacent sidefill. Averaging the coeffi-cient of lateral earth pressure from both sections A and Byielded a coefficient of 0.41 for the north side and 0.55 for thesouth side.
The base contact pressures measured at section A rangedfrom 243.3 to 336.6 kPa, with an average of 288.9 kPa(0.53[�H � DL], 0.58[�H= � DL]), while at section B, contactpressures ranged from 233.2 to 373.1 kPa, with an average of294.4 kPa (0.52[�H � DL], 0.57[�H= � DL]). Figure 14shows the base contact pressure as the embankment heightincreased. Results from the field monitoring using averagevalues of measured base pressure show an increase of 25.1%in the base contact pressure from the average measured pres-
Fig. 14. McBean Brook culvert base contact pressure in (a) section A and (b) section B, as defined in Fig. 2a. (Instrument locations asgiven in Fig. 4.)
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sure on the roof plus the average DL pressure of the culvert,and in terms of load, an increase of 37.5% was calculated fromthe weighted averages.
Summary and discussionsA reinforced double-cell rectangular concrete box culvert in-
stalled using the ITI method was instrumented to measure earthpressures around the culvert. The earth pressures were monitoreduntil a final embankment height of 25.10 m above the culvert roofwas reached. The measured pressures were compared with aphysical model tested in a centrifuge. Across the roof of theprototype structure, an average pressure of 202.2 kPa was mea-sured across the two instrumented sections, corresponding to0.38�H and 0.42�H=. The weighted average load measured onthe roof was 1301.1 kN/m. The average vertical earth pressure onthe roof of the culvert was higher in the field than observed in thecentrifuge tests. This is not surprising, as the centrifuge test wasconducted in a controlled environment. The average lateral earthpressure measured on the north side of the culvert was 238.7 kPa,with a coefficient of lateral earth pressure of 0.45, while for thesouth side an average lateral pressure of 309.1 kPa, correspondingto a coefficient of lateral earth pressure of 0.61,was measured.The highest earth pressure on the culvert was measured on thesidewall and resulted from construction-related nonhomogene-
ities. The average base contact pressure measured was 291.6 kPa,which corresponds to 0.52(�H � DL) and 0.58(�H= � DL). Thepressures measured in the field agreed closely with those of themodel culvert tested on the centrifuge for the sides and base.
Across the roof of the culvert, the centrifuge test predictedan average pressure of 0.21�H, which overpredicts the posi-tive arching pressure reduction on the roof by 17% in acontrolled environment for a uniform sand overburden. Thelateral pressure predicted from the centrifuge test ranged from0.49�Hs to 0.60�Hs, which is in good agreement with theaverage measured lateral earth pressure of 0.48�Hs. The basecontact pressure predicted by the centrifuge ranged from0.56(�H � DL) to 0.79(�H � DL), which compares well withthe field-measured average of 0.52(�H � DL). The basecontact pressure measured in the field was 25% higher than thepressure measured on the roof of the culvert plus the pressuredue to the dead weight of the culvert. The percentage increasein base pressure, from the overburden pressure plus DL pres-sure, also agrees with the centrifuge-indicated increase. Thisincrease further shows the influence of drag load that devel-oped on the sidewalls of box culverts. In terms of load, a37.5% increase from the load on top of the culvert plus the DLof the culvert was calculated. Despite the increased basepressure, the measured pressure at the base of an induced
Fig. 15. Measured and assumed design earth pressures on McBean Brook culvert.
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trench installation is still less than the pressure at the base ofa culvert with an embankment installation, as shown by cen-trifuge testing. Figure 15 shows a summary of the field-measured pressures and design pressures used.
To achieve optimum load reduction in the field, proper careshould be taken when the compressible layer is being installed,and sufficient time should be given for an embankment toequalize after construction (before paving). Table 3 shows acomparison between the pressures and loads predicted byKang et al. (2008), AASHTO (2010), and centrifuge tests forthe roof and base of a culvert the size of the one installed atMcBean Brook using the PPI method. The predictions of roofpressures and load reasonably agree, but base contact pres-sures calculated using AASHTO (2010) appear to be relativelyunconservative for Bc/Hc � 1 culverts. Compared with theembankment installation (PPI) unfactored load calculated ac-cording to AASHTO (2010), the measured McBean Brookloads achieved approximately 68% load reduction on the roofand 51% at the base.
ConclusionsA case history of a cast-in-place rectangular box culvert
installed within an induced trench embankment was presented.Average vertical earth pressure measured on the roof of theculvert was 0.42 times the overburden pressure. Average lat-eral earth pressure was measured to be 0.52 times the over-burden pressure at mid-height of the culvert. Contact pressureat the base of the culvert was successfully measured, indicat-ing an increase from the overburden pressure plus pressure dueto culvert dead load.
Field-based evidence of downward drag forces developedalong the sidewalls of box culverts installed in an inducedtrench embankment was presented. The average measuredload at the base of the culvert increased by 37.5% from theaverage load measured on the roof plus the average load dueto the dead weight of the culvert.
Presence of water channels (high water table) on the sides ofculverts can affect the bedding conditions and the pressuredistribution on the sides of culverts. In this study, the highestsingle earth pressure was measured on the culvert sidewall.The high lateral pressure measured was due to erosion-relatedlocalized nonhomogeneity.
Direct comparison was made between a rectangular boxculvert under an induced trench embankment in the field anda controlled laboratory centrifuge model test. Centrifuge testresults agreed closely with field-measured pressures, showingthat centrifuge testing can be used to evaluate expected earthpressures on culverts.
Results from short-term monitoring (2 years) and otherresearch at UNB indicate that the induced trench installationmethod is a viable option for box culverts under high embank-ment fills, regardless of the increased base pressure from dragforces developed on the exterior walls of the culvert. Thisresearch provides insight into soil–structure interaction relatedto induced trench installed box culverts.
AcknowledgementsFinancial support for this research was provided by the New
Brunswick Department of Transportation (NBDOT) Canada.The help and assistance provided by the NBDOT personnelduring the field work is gratefully acknowledged.
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Hansen, P., Miller, L., Valsangkar, A.J., Bourque, S., and MacLeod,T. 2007. Performance of induced trench culverts in New Bruns-wick. In Proceedings of the Annual Conference of the Transpor-tation Association of Canada, Saskatoon, Sask., 14–17 October2007. TAC, Ottawa, Ont.
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Table 3. Predicted pressures and load comparison on the roof andbase of a PPI.
Roof (overburden)Base(overburden � DL)
Pressure(kPa)
Load(kNm)
Pressure(kPa)
Load(kN/m)
Centrifuge PPI 610.9 4123.4 897.7 6059.1Kang et al. (2008) 676.9 4569.3 964.7 6511.8AASHTO (2010) 605.6 4087.9 638.2 4307.5
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