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Performance of two cast-in-place box culvertsunder high embankments
Olajide Samuel Oshati, Arun J. Valsangkar, and Allison B. Schriver
Abstract: Two cast-in-place reinforced concrete box culverts (Longs Creek and McBean Brook culverts) with similargeometry were instrumented to monitor earth pressures. The Longs Creek structure is a 7.30 m wide by 4.53 m highdouble-cell box under 14.10 m of fill, constructed using the positive projection installation (PPI) method. The McBeanBrook structure is a 6.75 m wide by 4.65 m high double-cell box culvert under 25.10 m of fill, constructed using theinduced trench installation (ITI) method. Earth pressures were monitored as the fill height above the culvert was increasedfor both structures. Data from the McBean Brook culvert at a fill height of 14.40 m are compared with the data from theLongs Creek culvert. The similar geometries of these two prototype culverts provide a unique opportunity to compare theirperformance based on the two different installation methods. For a comparable height of soil cover, measured vertical earthpressures on the roofs were 0.94 and 0.56 times the overburden pressures for the Longs Creek (PPI) and the McBeanBrook (ITI) culverts, respectively. The average measured lateral earth pressure was 0.32 times the overburden pressure atthe Longs Creek culvert (PPI), and 0.46 times the overburden pressure at the McBean Brook culvert (ITI). Data from theMcBean Brook culvert pertaining to base contact pressure are also presented, but could not be compared directly with theLongs Creek culvert, as base pressures were not measured. Measured pressures at the base of the McBean Brook culvertshow the presence of drag forces developed on the sidewalls of box culverts. Results from centrifuge model testing arealso presented and compared with the field-measured pressures.
Key words: induced trench, earth pressure, box culvert, soil arching, centrifuge testing, high embankments.
Résumé : Deux ponceaux rectangulaires coulés sur place et renforcés (ponceaux Longs Creek et McBean Brook), qui ontune géométrie similaire, ont été instrumentés afin de mesurer les pressions des terres. La structure Longs Creek est unponceau rectangulaire a double cellule de 7,30 m de largeur par 4,53 m de hauteur, placé sous 14,10 m de remblai,et construit a l’aide de la méthode d’installation par projection positive (IPP). La structure McBean est un ponceaurectangulaire a double cellule de 6,75 m de largeur par 4,65 m de hauteur, placé sous 25,10 m de remblai, construit avec laméthode d’installation de tranchée induite (ITI). Les pressions des terres sont suivies depuis que la hauteur du remblai surle ponceau a été augmentée pour les deux structures. Les données du ponceau McBean Brook a une hauteur de remblai de14,40 m sont comparées aux données du ponceau Longs Creek. La géométrie similaire de ces deux prototypes de ponceauxoffre une opportunité unique de comparer les performances selon les différentes méthodes d’installation. Pour une hauteurde remblai comparable, les pressions des terres verticales mesurées sur les toits étaient de 0,94 et 0,56 fois les pressionsdes terres pour le ponceau Longs Creek (IPP) et le ponceau McBean Brook (ITI), respectivement. La pression des terreslatérale moyenne était de 0,32 fois la pression des terres au ponceau Longs Creek (IPP), et 0,46 fois la pression des terresau ponceau McBean Brook (ITI). Les données du ponceau McBean Brook en lien avec la pression de contact a la basesont aussi présentées mais n’ont pas pu être comparées directement avec le ponceau Longs Creek, puisque les pressions ala base n’y ont pas été mesurées. Les pressions mesurées a la base du ponceau McBean Brook démontrent la présence deforces de traînée développées sur les murs de côté des ponceaux rectangulaires. Des résultats obtenus a partir d’essais encentrifuge sont aussi présentés et comparés aux mesures de pression sur le terrain.
Mots-clés : tranchée induite, pression des terres, ponceau rectangulaire, effet d’arche, essai en centrifuge, diguesélevées.
[Traduit par la Rédaction]
Introduction
Modern highway construction and development sometimesrequires construction of high embankments (fill height abovethe culvert top, H � 10 m) over underground rigid and flexiblestructures. The high embankment fills usually impose signifi-
cant earth loads on the underground structures. Rigid culverts,either circular or box, have been a common design choice forhigh embankment fills. The use of precast or cast-in-place boxculverts under embankments has increased over the years,owing to its structural competence and ease of construction. Indesigning these underground box structures, several factors are
Received 9 February 2012. Accepted 22 August 2012. Published at www.nrcresearchpress.com/cgj on 19 November 2012.
O.S. Oshati, A.J. Valsangkar, and A.B. Schriver. Department of Civil Engineering, University of New Brunswick, Head Hall, 17Dineen 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: 1331–1346 (2012) Published by NRC Research Pressdoi:10.1139/t2012-094
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considered: the site condition, fill height, rigidity of the cul-vert, material properties, bedding, available time, cost, andhydraulic and structural requirements.
When high embankment fills are required over rigid culverts,most jurisdictions favour two main installation methods: thepositive projection installation (PPI) and the induced trench in-stallation (ITI) methods. In the ITI method, a zone of compress-ible material (hay, sawdust, peat, shredded rubber tires, etc.) isplaced above the culvert. The concept behind the ITI method is toreduce the vertical earth pressure, not only by the inclusion oflower density fill material (compressible layer), but also mainlyfrom inducing positive arching (i.e., column of soil directly abovethe culvert settles more relative to the adjacent soil) from differ-ential settling initiated by the compressible fill material. In ITIs,earth load from the column of soil directly above the culvert ispartially supported by the shear forces developed on the soilinterface with adjacent soil columns, thereby resulting in some ofthe load being redistributed to the adjacent fill. On the other hand,the PPI does not include a compressible zone within the embank-ment and there is no load reduction on the culvert roof. Instead,negative soil arching is experienced when using the PPI method,where the column of soil directly above the culvert settles lessrelative to the adjacent soil. In the PPI, the earth load from theadjacent soil columns is supported by shear forces developed onthe soil interface with the middle soil column, thereby causingsome of the load to be redistributed to the middle soil column.
Recent publications (Katona and Vittes 1982; Kang et al.2008; McAffee and Valsangkar 2008; McGuigan andValsangkar 2010; Oshati et al. 2012) have indicated the pres-ence of drag forces developed on sidewalls of box culverts.These drag forces develop due to the shear forces generatedfrom differential settling between the soil column and thesidewalls of the culvert. As a result, the base pressure isgreater than the vertical pressure on the roof plus the pressureexerted by the dead load (DL) of the culvert.
The University of New Brunswick (UNB) and the NewBrunswick Department of Transportation (NBDOT) have beenstudying the soil–structure interaction of rigid circular and boxculverts using physical testing, numerical modelling, and fieldinstrumentation and monitoring for over 10 years. However,most of the previous studies undertaken at UNB pertain toprecast circular pipes installed under high embankments con-structed using the ITI method. Recently, Oshati et al. (2012)published results from the field instrumentation of an inducedtrench double-cell rectangular box culvert, providing field-based evidence of increased base pressure. This paper focuseson field instrumentation and monitoring of two similar cast-in-place double-cell rectangular box culverts approximately45 km apart: Longs Creek and McBean Brook culverts. TheLongs Creek box culvert was constructed and installed usingthe PPI method, while the McBean Brook culvert was con-structed and installed using the ITI method.
The latest edition of the American Association of StateHighway and Transportation Officials (AASHTO) AASHTOLRFD bridge design specifications (AASHTO 2010) providesequations for calculating the unfactored earth load acting onbox culverts for embankment and trench installation. Equa-tions [1] and [2] present the AASHTO (2010) equation fordetermining the unfactored load on top of box culverts in anembankment installation (PPI).
[1] WE � FeγsBcH
where WE is the total unfactored earth load,
[2]Fe � 1 � 0.20 H/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 culvert roof. The term Fe representsindirectly the negative arching factor to account for positiveprojecting installation. The proposed equations for Fe aresemi-empirical in nature. Equation [2] implies that the factorFe increases linearly with ratio H/Bc, but reaches a limitingvalue of 1.15 at an H/Bc ratio of 0.75 for compacted fill and 1.4at an H/Bc ratio of 2 for an uncompacted fill. The combinedterm (�s, Bc, H) in eq. [1] represents the weight of the soilprism above the culvert, per unit length of the culvert.
AASHTO (2010) provides no guidelines as to the method orprocedure for determining the earth load exerted on box cul-verts installed under an induced trench embankment. How-ever, AASHTO (2010) recognizes the ITI method as one of theacceptable methods of installation and suggests the use ofaccepted tests methods, soil–structure interaction analyses orprevious experience to determine the load.
The purpose of the current study is to compare earth pres-sure data from two instrumented field installations of similarbox culverts constructed using the PPI and ITI methods. Thestudy presents earth pressures measured on the roof and oneside of the culvert for both field installations, along with thebase contact pressures measured for the ITI. Centrifuge modeltest results for the PPI and the ITI are also presented. Theeffect of the H/Bc ratio for the different field installationsreported in literature is compared with the AASHTO recom-mended soil–structure interaction factor (Fe).
Box culvert research
General backgroundResearch related to earth loads on underground structures
has been ongoing for decades. The pioneering contributions toearth load on buried structures and installation methods can beattributed to Marston (Marston and Anderson 1913; Marstonet al. 1917; Marston 1930). Early contributions from Schlick(1920, 1932, 1952) and Spangler (Spangler 1933, 1950, 1958;Spangler and Handy 1973) also paved the way to understand-ing the mechanism behind load transfer in buried structures.The majority of the research done on box culverts has focussedon the PPI method with limited research on the ITI construc-tion. Figure 1 shows a simplified arching mechanism of apositive projection embankment installation (negative arching)and an induced trench installation (positive arching).
Previous field instrumentation and monitoringPenman et al. (1975) reported on a summary of work done
by Binger in 1948 as part of works done on the 1942 PanamaCanal third locks project on a 2.70 m wide by 3.30 m high(Bc/Hc � 0.82) reinforced concrete culvert. The culvert wasinstalled under 15 m of compacted sandstone fill embankment.The pressure measuring gauges installed on the roof of the
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culvert were protected by a 0.60 m layer of hand-compactedclay. Pressure measured on the roof of the culvert at the centreof the rockfill embankment was 415 kPa at the end of con-struction, which corresponded to 180% of the overburdenpressure (230 kPa) estimated from the field density tests.
Katona and Vittes (1982) reported the measured earth pres-sure data from an instrumented 1.22 m by 1.22 m (Bc/Hc � 1)box culvert constructed by the Kentucky Department of Trans-portation. The culvert was founded on a dense granular bedwithin bedrock. Eight Carlson earth pressure cells were in-stalled around the buried culvert. A total of 23 m of silt (MH,as classified according to ASTM 2006) embankment fill wasplaced above the culvert roof. The reported earth pressuremeasured on the roof and average base pressures were approx-imately 148% and 161% of the overburden pressure, respec-tively. The approximate average measured lateral pressureswere 35% and 18% of the overburden pressure on the right andleft side of the culvert, respectively. Katona and Vittes (1982)reported the presence of shear traction on sidewalls, whichproduce significant downward force that must be accountedfor with respect to increased pressure on the bottom slab.
Tadros et al. (1989) instrumented and tested a cast-in-placereinforced double-cell concrete box culvert. The 30.50 m long,3.66 m by 3.66 m (Bc/Hc � 1) culvert was instrumented tomeasure soil pressure, strains, deflections, and settlementsaround the culvert. The sidefill was compacted as the embank-ment was raised; after reaching a permanent fill height of2.59 m above the roof of the culvert, an additional 1.07 msurcharge was placed for a total soil cover of 3.66 m. The earthpressures were monitored using 28 vibrating wire earth pressurecells, mounted in steel boxes. The earth pressure measurementswere supplemented with six vibrating wire piezometers and 40vibrating wire strain gauges to measure hydrostatic boundarywater pressure, moments, and thrusts. Soil pressure measured
from the study showed that both magnitude and distribution ofpressure seemed to be strongly influenced by compaction. Earthpressure measured under a permanent soil cover of 2.59 m,80 days after completion of the embankment at the top wasapproximately 72 kPa on an average, corresponding to 144% ofthe overburden pressure. The approximate average coefficient ofmeasured lateral earth pressure on the sidewalls was 0.57.
Vaslestad et al. (1993) instrumented and monitored threeprototype culverts, one of which was a 2.0 m wide by 2.25 mhigh box culvert (Bc/Hc � 0.78). The study represents one ofthe few field studies on box culverts installed using the ITImethod. Expanded polystyrene was used as the compressiblelayer. The study focused mainly on the vertical earth pressureexerted on the box culvert roof, and the long-term behaviour ofinduced trench installed culverts. Earth pressure on the topslab was measured using hydraulic pressure cells and wascompared with the overburden pressure. The lateral earthpressures and base contact pressures were not measured in thestudy. Upon completion of the embankment at a fill height of10.80 m, the measured earth pressure on the roof was 63% ofthe overburden pressure. The 3 year research showed nosignificant increase in vertical earth pressure on the culvertafter construction.
Yang et al. (1999) instrumented two sections of a 7.0 mwide by 3.9 m high double-cell box culvert (Bc/Hc � 1.79), tomeasure earth pressure distribution on the roof and sidewall.Geokon model 4810 vibrating wire hydraulic type soil pres-sure cells were used. The culvert was founded on 600 mm ofwell-graded crushed gravel. Two sections were instrumented:section A was buried under 19 m of fill and section B under11.7 m of fill. Data collected over 600 days showed that withH/Bc � 0.5, soil–structure interaction coefficient (Fe) rangedfrom 0.73 to 1.85 and with H/Bc � 0.5, the soil–structureinteraction coefficient ranged from 0.8 to 2.0. The recorded
Fig. 1. Simplified arching mechanism of a PPI and an ITI.
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weighted average vertical pressures were about 30% and 20%greater than the AASHTO (1996) recommended pressures forH/Bc � 0.5 and H/Bc � 0.5, respectively. The largest lateralpressure was measured at the base of the wall. Results from thestudy suggest that the pressures calculated using equationsgiven in AASHTO (1996) did not match field observations.
Bennett et al. (2005) instrumented two sections of a 3.0 mwide by 2.4 m high cast-in-place double-cell reinforced con-crete box culvert (Bc/Hc � 1.25). Vibrating wire pressure cellswere used to determine the earth pressures on the roof. Straingauges mounted on the wall and roof were used to determinethe axial force in the wall and bending moment in the roof,which was then used to obtain the pressure on the roof as thefill height increased. Soil–structure interaction factors werealso predicted using results from the pressure cells and straingauges. Findings from the pressure cells that had good corre-lations with the strain gauges in the wall and roof suggest thatsoil–structure interaction factor did not change with the em-bankment height, with an observed mean of 1.51. Bennettet al. (2005) also observed that the pressure on the roof of aculvert installed using the PPI method was significantlygreater than the soil overburden pressure on the culvert.
Sun et al. (2011) monitored earth pressures around aninduced trench cast-in-place box culvert using geofoam as thecompressible fill. The culvert was 3.35 m wide and 3.73 mhigh (Bc/Hc � 0.90), founded on an unyielding bed, with fillheight of approximately 15.90 m above the culvert roof. Sunet al. (2011) monitored three sections: the first section had a0.61 m thick geofoam with a width of 3.35 m (Bc), the secondsection had 0.61 m thick geofoam with a width of 4.88 m(1.5Bc), and the third section was used as the control withouta geofoam compressible layer. Instrumentation consisted ofthree strain gauges, 12 pressure cells, and two inverted settle-ment plates. The upper half of the embankment consisted oflimestone rock mixed with red residual clay and the lower halfwas compacted red residual clay. On the culvert roof, forsections with geofoam width 1.5Bc and 1Bc, Sun et al. (2011)measured an average pressure of 8.9% and 11.2%, respec-tively, of the pressures measured at the section without geo-foam. Average measured pressure on the sidewall was 13.6%of the pressure measured on the culvert roof at the sectionwithout geofoam. Sun et al. (2011) reported that measuredlateral pressures at the section with a geofoam width of 1Bcwere similar to the vertical pressure, but 41.37 kPa larger thanthe vertical pressure at the section with a geofoam width of1.5Bc. On the basis of the projected linear relationship betweengeofoam and the logarithm of time, Sun et al. (2011) con-cluded that the earth pressure on box culverts will remainstable for the design life using the ITI method. However, basecontact pressure was not measured in the 5 year study.
Oshati et al. (2012) published results of earth pressuremeasurements around an induced trench constructed rectangu-lar box culvert. The instrumented culvert was 6.75 m wide and4.65 m high (Bc/Hc � 1.54), under 25 m of embankment fill.Base pressures measured across two instrumented sections ofthe culvert showed an increase of 25% from the measuredpressure on the roof plus dead weight pressure of the culvert.The increased base pressure was attributed to downward dragforces developed on the sidewalls of box culverts installedusing the ITI method. Average earth pressures reported were
42% and 52% of the overburden pressure at the roof and sidesof the culvert, respectively.
Previous model tests and numerical analysisDasgupta and Sengupta (1991) studied the behaviour of
reinforced box culverts, using a large-scale model box culvertbackfilled with sand. Deflection, pressure, and strain were mea-sured on the 1200 mm by 1200 mm (Bc/Hc � 1) box culvertfounded on sand. The culvert was installed under 2.40 m of sandembankment fill. Twelve deflecting diaphragm pressure cellswere placed around the culvert for pressure measurement pur-poses. The measured pressures showed a linear relationship withthe fill height at the top and bottom of the culvert. The rate ofpressure increase measured for the top slab was greater than thebottom slab; after 2 m of fill the rate of change of pressure on thewall decreased. Nonuniform pressure distribution was observedat the top and bottom slab with the higher pressures at the stifferend walls. The pressure distribution on the walls displayed aparabolic distribution with the maximum value at about 0.3 timesthe height of the wall from the base of the culvert. The totalvertical pressure measured in the study was the same on both thetop slab and the bottom raft, and was greater than the weight ofsoil directly on the slab.
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 the induced trench was higherthan vertical pressure for both single and twin culvert installation.However, Bourque’s study did not address base pressures andalso only investigated a model culvert of ratio Bc/Hc � 1. Resultsfrom Bourque’s study were later incorporated into a detailedstudy by McGuigan and Valsangkar (2011) where the issue ofincreased base pressure was addressed.
MacLeod (2003) investigated earth pressures around in-duced 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).
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 the PPI method. Results fromMcAffee’s study confirmed significant reduction in verticalpressure on the roof and increased lateral pressures on thesidewalls for each condition studied. McAffee (2005) ob-served that the compressibility, width, and height of the com-pressible layer were important factors for load reduction inunderground conduits. McAffee (2005) also conducted teststhat considered different compressible zone configurations.The study, however, did not investigate base contact pressuresand only considered square-shaped culverts.
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Li and Qubain (2004) used the finite element method (FEM)to model a concrete box culvert to evaluate the effects offoundation yielding on design load. Yielding, partially yield-ing, and unyielding foundations were analysed and comparedwith classical and conventional methods. Plane strain FEMwas used to analyse a 3.35 m wide by 3.81 m high (Bc/Hc �0.88) precast concrete box culvert under 11.58 m of embank-ment fill. A two-dimensional FEM program SIGMA/W wasused. Li and Qubain (2004) reported that the vertical earthpressure measured from the unyielding foundation was 1.30times the nominal earth load on average, while for the yieldingfoundation, measured vertical earth pressure was approxi-mately equal to the nominal earth load. Li and Qubain (2004)observed that the partially yielding foundations showed simi-lar pressure distribution to the unyielding foundation. Li andQubain (2004) also observed that with yielding foundations,resulting moment in the middle section of the top slab issignificantly reduced and end moments consequently wereincreased slightly. The reported average lateral pressure for theunyielding foundation was 0.27 times the nominal verticalpressure, and 0.41 times the nominal vertical earth pressurefor the yielding foundation. Li and Qubain (2004) reported thatearth pressure predictions based on AASHTO (1998) couldunderestimate the magnitude of soil pressure for unyieldingfoundations due to the lack of consideration of the site con-dition in the specified upper limit. In conclusion, Li andQubain (2004) suggest that the conventional design proposedby AASHTO (1998) may still be conservative with the as-sumption of uniform pressure distribution for unyieldingfoundations.
Kim and Yoo (2005) used finite element modelling toanalyse 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 the
culvert and concluded that the ITI method could reducethe soil–structure interaction factor. The study focused on thebeneficial load reduction that can be achieved from the ITImethod, and did not address lateral earth pressure or base contactpressure. Kim and Yoo (2005) also only examined square-shaped culverts.
Kang et al. (2008) reported on the results from numericalanalysis of a box culvert under high fill, and highlighted theeffects of frictional forces developed on sidewalls of box culverts
Fig. 2. Longs Creek culvert instrumentation locations (data from Take and Valsangkar 1999).
Fig. 3. Longs Creek culvert with lateral earth measuring contactcell installed.
Table 1. Nuclear density results around Longs Creek box culvert.
Averagedensity (kg/m3)
Average unitweight (kN/m3)
Standarddeviation (kN/m3)
No. ofobservations
2180.42 21.38 0.18 122151.42 21.1 0.44 122142.5 21.01 0.6 20
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on the base contact pressure. Kang et al. (2008) proposed apreferred compressible zone geometry, formulas for earth loadreduction rate for the proposed compressible zone geometry,and a PPI soil–structure interaction factor accounting fordownward drag forces developed on the sidewalls of the boxculvert. Kang et al. (2008) also found that the AASHTO(2004) design equation used for calculating earth loads forembankment installation could be unconservative.
McGuigan and Valsangkar (2010) presented the results ofparametric studies on a single box culvert using centrifugetests and numerical modelling to evaluate the pressures actingon the top, sides, and base of a box culvert. McGuigan andValsangkar (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 investigated the effectof the compressible zone stiffness on earth pressures exertedon the culvert. In their study, McGuigan and Valsangkar(2010) observed that with a preferred compressible zone widthof 1.2Bc and a thickness of 0.5Hc, a 78% reduction in verticalpressure on the roof was achieved for an unyielding founda-tion when compared with the PPI and a 47% reduction in basecontact pressure compared with PPI. The yielding foundationon the other hand achieved a 78% and 35% reduction inaverage roof and base pressures, respectively, compared withthe PPI. In agreement with McAffee (2005), McGuigan andValsangkar (2010) observed that compressible zone stiffnessplays an important role in the load reduction achieved on theroof and base of culverts. However, the study by McGuiganand Valsangkar (2010) was limited to a fill height of 12 m anda ratio 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. McGuigan and Valsangkar (2011) observed that twinculverts installed using the PPI method gave lower pressuresthan single culverts. A single compressible zone spanning bothculverts was proposed for culverts spaced at 0.5Bc and 1Bc,while for a spacing of 1.5Bc, two individual compressiblezones of 1.2Bc width were found to give optimum earthpressures. Earth pressures on the roof of twin induced trenchculvert were observed to be higher than the correspondingpressures on the roof of single culverts. Lower lateral pres-sures were observed for twin culverts compared to singleculverts and lower base pressures were observed for twinculverts with 0.5Bc spacing than for single culverts. McGuiganand Valsangkar (2011) concluded that lower pressures corre-spond to the smallest spacing (0.5Bc).
Field construction and instrumentationThe two sites selected for this study are approximately
45 km apart: the Longs Creek double-cell rectangular boxculvert (Bc/Hc � 1.61) was installed under 14.10 m of fill in apositive projection embankment while the McBean Brookdouble-cell rectangular box culvert (Bc/Hc � 1.45) was in-stalled under 25.10 m of fill in an induced trench embankment.The two sites were selected for their similarities in culvert size,shape, site condition, Bc/Hc ratio, and construction materialsused. The comparison was based on measured data at approx-imately 14.10 m of fill above the roof of both culverts, whichgives a comparable H/Bc ratio. The section titled “McBeanBrook culvert� provides details on the typical dimensions forthe two cast-in-place reinforced rectangular boxes.
Longs Creek culvertThe Longs Creek culvert is a 193.50 m long cast-in-place
double-cell reinforced rectangular concrete box with a height of
Fig. 4. Transverse section through McBean Brook induced trench culvert (all dimensions in metres; from Oshati et al. 2012).
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4.53 m and a base width of 7.70 m installed under 14.10 m of fill.The culvert was part of the re-alignment of the Trans-CanadaHighway passing through New Brunswick, Canada. Constructionwas done in two time periods, the first period running from Julyto October 1997 and the second from May to August 1998. TheLongs Creek culvert was designed and constructed as a positiveprojecting culvert. The Longs Creek culvert instrumentation con-sists of 13 pneumatic-type contact pressure cells (Petur ModelTP-100P), nine settlement plates, one pneumatic piezometer, andtwo standpipes to check the accuracy of the piezometer data
(Take and Valsangkar 1999). Nine pressure cells were placedwithin the soil mass with sand bedding to measure verticalearth pressure and four pressure cells were mounted on thewall to measure lateral earth pressures. Settlement plates werelocated close to each pressure cell.
The vertical earth pressure measuring cells were positionedto enable comparison of the measured pressures around therelatively stiff concrete culvert with the measured pressurewithin the fill (Fig. 2). The locations of lateral earth pressuremeasuring cells are shown in Fig. 3. A nuclear density gauge
Fig. 5. (a) McBean Brook culvert dimensions and instrument location (from Oshati et al. 2012); (b) typical dimensions of Longs Creekconcrete box culvert (all dimensions in metres).
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was used to observe the field density of the backfill material(Table 1). Field-corrected standard Proctor dry density mea-surements on eight fill materials used around the site averaged2085 kg/m3. The culvert was backfilled with Class “A” back-fill as shown in Fig. 2, and the rest of the embankment wasbuilt with Borrow “A” material. The specifications of thesematerials are presented in the next subsection. The groundwa-ter table was below the base of the culvert during the moni-toring period.
McBean Brook culvertThe McBean Brook culvert is a cast-in-place reinforced
double-cell rectangular box buried under 25.10 m of fill. Theconstruction of this culvert was part of an upgrade of the newroute 8 Nashwaak–Marysville bypass in New Brunswick,Canada. The 142 m long culvert has a base width of 7.35 mand height of 4.65 m. The existing ground was excavated to adepth of 2900 mm; the culvert was then founded on a 500 mmthick class A backfill compacted to 95% standard Proctormaximum dry density. The sides and top of the McBean Brookculvert were also backfilled with class A fill compacted to 95%standard Proctor maximum dry density up to a height of500 mm above the roof of the culvert. Class A backfill is awell-graded granular material with not more than 10% re-tained on a 100 mm sieve and not more than 10% passing a75 �m sieve. Earlier testing done at the UNB gave an angle ofinternal friction of 40° and an effective cohesion of 0(McGuigan and Valsangkar 2011). A 2500 mm thick by8000 mm wide compressible fill layer was placed above theculvert, on top of the class A backfill. The compressible layerwas then backfilled with borrow A material to a final height of25.10 m above the roof of the culvert (Fig. 4). Borrow Amaterial is any soil or rock particles with no more than 25%passing a 75 �m sieve and contains no organic or deleterious
materials. Due to significant variation in gradation of borrowA material, the angle of internal friction is assumed to be 30°with an effective cohesion of 0 (McGuigan and Valsangkar2011). Construction work at the McBean Brook was com-pleted in two phases. Phase 1 started in January 2010 andended in October 2010 with an approximate fill height of 4 mabove the roof of the culvert. Phase 2 of the constructioncommenced in April 2011 and was completed in July 2011,when an embankment fill height of 25.10 m above the culvertroof was reached. The groundwater table was below the baseof the culvert (elevation of 52.9 m) for most of the initialmonitoring period, but was 0.60 m above the base slab (ele-vation of 54.6 m) as the water was diverted through the culvertbefore the embankment was raised to the final height.
To measure earth pressures around the culvert, a total of32 Geokon vibrating wire earth pressure cells (model 4800;Geokon Inc., Lebanon, New Hampshire) were used. The pressurecells are made with de-aired hydraulic oil stored in circularstainless steel plates with a diameter of 230 mm, thickness of6 mm, and an aspect ratio of 38.3. Figure 5 shows the locationsof the pressure cells around the prototype culvert. Two differ-ent sections, A and B, 8 m apart along the length of the culvertwere selected for redundancy in earth pressure measurements.Both sections were under the full embankment height and eachsection had 16 pressure cells installed around the culvert. Atotal of 10 pressure cells were installed at the base and roof ofeach section of the culvert to measure the base contact androof vertical earth pressures. To measure lateral pressures atotal of six pressure cells were installed on the sides in astaggered fashion; for each section, the north and south sideshad three pressure cells each.
Prior to field installation, 17 pressure cells were ran-domly selected from each pressure range and calibrated in
Fig. 6. Centrifuge model set-up (all dimensions in millimetres; from Oshati et al. 2012). EPS, expanded polystyrene.
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the laboratory. The cells were not calibrated to their maximumcapacity as the maximum pressure applied was just over 400 kPa,which is approximately one-fifth of the cell with the highestcapacity used. However, the calibration proved that the cells wereworking within the maximum range of pressures expected at thetop, sides, and base of the culvert. For the purpose of this study,earth pressure readings at a fill height of 14.40 m are used forcomparison with the Longs Creek culvert.
Centrifuge testingAll centrifuge model tests were done using a 1.6 m radius,
geotechnical centrifuge with the ability to accelerate a 100 kgpayload at 200 times earth’s gravitational force. To model thefield prototype geometries, a scaled aluminum model culvert76 mm by 52.4 mm by 195 mm in length was used. The modelculvert housed two pressure transducers that were used tomeasure earth pressures. The transducers used were KYOWABEC-A-500 kPa soil pressure cells, with a capacity of 500 kPaand a high stiffness to reduce localized soil arching around thecell. The pressure-measuring face of the transducers have adiameter of 23 mm. In total, the two transducers measure
pressures over 60% of the combined culvert width for both thetop and base orientation, and for the sides the average pressureover 44% of the culvert height.
The transducers were calibrated prior to centrifuge testing.The calibration was done for the top and side transducers usinga rigid foundation and rigid side support, with the rigid foun-dation thickness equal to the height of culvert (Hc). Theoverburden pressure for the top (�H) and at mid-height on theside (�Hs) were determined, where � is the unit weight ofthe fill, H is equal to the fill height above the culvert top, andHs is the fill height from mid-height of the culvert side to thetop of the fill. McGuigan and Valsangkar (2010) recom-mended a separate calibration for transducers located at thebase of model culverts due to less localized arching aroundthese transducers. Therefore, separate calibration was com-pleted for transducers located at the culvert base. Rigid sup-ports were used on the sides of the culvert calibrating thetransducers at the base of the culvert. For all calibrations thecentrifuge was gradually ramped up to 50 times earth’s grav-itational field (50g). All calibrations were done in the box toaccount for all boundary effects. The theoretical overburden
Fig. 7. Average vertical earth pressure measured: (a) transducer A (b) transducer B (adapted from Oshati et al. 2012).
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pressure was calculated based on the fill height, density of thesand, and gravity level. Upon calibration several tests weredone for both the PPI and the ITI, both constructed on ayielding foundation. Vertical, lateral, and base pressures weremeasured for both conditions. General setup for the centrifugeconfiguration is shown in Fig. 6.
Silica sand was used as backfill and the yielding base layer.Expanded polystyrene (EPS) was used as the compressible fillmaterial. The H/Hc ratio for all tests was maintained at 5.38,with an H/Bc ratio of 3.71, where Bc is the outside width of theculvert. A gravitational acceleration of 50g represents approx-imately 14.10 m of fill on a culvert with a Bc/Hc ratio of 1.45.The manual air pluviation method as described by Ueno(1998) was used to prepare models. This technique has beenused by other researchers in the past (e.g., Bourque 2002;
McAffee and Valsangkar 2008; McGuigan 2010) and wasfound to produce uniform backfill beds. A constant drop heightof sand was maintained throughout the sample preparation tomaintain a relatively constant density. After each test, thedensity of the sand used was determined, providing an averagedensity of 1541 kg/m3. The strong box used in this study hasinternal dimensions of 265 mm in width, 195 mm in length,and 495 mm in depth. Figures 7 through 9 show the averagepressures measured in centrifuge tests on the roof, sides, andbase of the culvert for each installation method. The figurespresent the centrifuge test results as a plot of the prototype fillheight against measured pressure as recorded during testing.Figure 7 presents the data pertaining to the vertical earthpressures measured on the roof of the model culvert. It is seenfrom this figure that the vertical earth pressures on the roof are
Fig. 8. Average lateral earth pressure measured: (a) transducer A (b) transducer B (adapted from Oshati et al. 2012).
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significantly reduced in ITI culverts when compared with PPIculverts. The negative arching in the case of PPI results in thevertical pressure on the roof being 1.12 to 1.23 times theoverburden pressure. In comparison, the earth pressures onthe roof for the ITI model culvert recorded 0.21 to 0.24 timesthe overburden pressure due to positive arching. The datapresented in Fig. 7 also demonstrate that load cells A and Bwere giving consistent readings. Measured lateral earth pres-sures in the centrifuge model tests are presented in Fig. 8. Thedata indicate that the lateral earth pressures exerted on thesidewalls for ITI model culverts are higher than those forthe model PPI culverts. Similar findings have been alreadyreported by Bourque (2002), MacLeod (2003), McAffee andValsangkar (2008), and McGuigan and Valsangkar (2010).The data pertaining to the base contact pressures from thecentrifuge model testing are presented in Fig. 9. The measuredvalues are compared with the overburden vertical pressure onthe roof plus the pressures resulting from the DL of the culvert.Comparison of vertical pressures on the roof of 1.12 to 1.23times the overburden (Fig. 7) plus DL with those at the base of1.56 to 1.77 times the overburden pressure plus the pressure
from the DL of culvert (Fig. 9) for PPI model culverts dem-onstrates that the pressures at the base are increased due todrag loads mobilized on the sidewalls. Similar observationscan also be made for the model ITI culverts.
Comparison of field and centrifuge dataresults
Earth pressure data from two similar geometry culverts arecompared for performance under high fill embankments. Thetwo culverts were installed using two different installationmethods (ITI and PPI). Field-measured pressures are presentedand compared with model culvert pressures. The comparisonswere done at similar fill heights above the roof of the culverts.
The earth pressure measured on the roof of the LongsCreek culvert was 0.94 times the theoretical overburden earthpressure. For the McBean Brook culvert, 0.56 times the the-oretical overburden earth pressure was measured on the roof ofthe culvert. The average pressure measured on the roof of apositive projecting culvert in the centrifuge model culvert was1.17 times the overburden pressure, which agrees closely with
Fig. 9. Average base contact pressure measured: (a) transducer A (b) transducer B (adapted from Oshati et al. 2012).
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the AASHTO (2010) embankment installation (PPI) unfac-tored earth pressure equation for culverts installed with com-pacted sidefills. The measured vertical earth pressure on theroof of the Longs Creek culvert presented in Fig. 10 is from asingle pressure cell (PC-20540) located on the roof. The pres-sure cell (PC-20540) was positioned on a less stiff portion ofthe roof. The earth pressure measured over the roof of the rigidbox culvert is not uniform and the earth pressures measured onthe stiffer wall section tend to be higher than earth pressuresmeasured in a relatively less stiff section (Li and Qubain 2004;Dasgupta and Sengupta 1991; Katona and Vittes 1982; Oshatiet al. 2012). In addition to the issue of location of the pressurecell in a less stiff section of the roof, the earth pressures on theroof were also influenced by the yielding foundation at the LongsCreek site. The effect of a yielding foundation on the earthpressures is further discussed in the following paragraph. Foreach analysis, fill unit weight of 21kN/m3 and a fill height ofapproximately 14.10 m were used.
Figure 10 shows the results of the vertical earth pressuresmeasured on the roof of the McBean Brook culvert (ITI),Longs Creek culvert (PPI), computed AASHTO (2010)recommended design pressure for PPI, and centrifuge-measured vertical earth pressures for a PPI. It should be notedthat AASHTO (2010) equations are applicable to PPI culverts,but the data from ITI culvert are also presented in this figurefor comparison. The validity of earth pressure measure-ments on the roof of the Longs Creek culvert (PPI) wasconfirmed with settlements measured within a similar hor-izontal plane in the embankment, and evidence of theabsence of differential settlement between the soil columnabove the culvert and adjacent soil columns was observed(Take and Valsangkar 1999; Valsangkar 2000). Valsangkar(2000) attributed the absence of negative arching to a rela-tively yielding foundation underneath the Longs Creek cul-vert. Therefore, the vertical earth pressure on the roof of theLongs Creek culvert installed within a positive projection
embankment was approximately equal to the overburden pres-sure of the embankment.
The average measured lateral earth pressure was 0.34 timesthe measured overburden earth pressure adjacent to the side-wall and 0.32 times the calculated overburden earth pressure atthe Longs Creek (PPI) site. The difference between the mea-sured and calculated earth pressures is negligible as there wasneither positive nor negative arching occurring at the LongCreek site. The average measured lateral earth pressure at theMcBean Brook site was 0.82 times the measured earth pres-sure on the roof and 0.46 times the calculated earth pressure onthe roof. This significant difference in estimated lateral earthpressures is anticipated as the measured vertical earth pres-sures are significantly reduced due to positive arching, therebyresulting in a higher percentage when measured lateral pres-sures are expressed as a ratio of measured vertical pressures.Average measured lateral earth pressures from centrifuge test-ing were 0.34 times the calculated overburden earth pressurefor the PPI and 0.52 times the calculated overburden earthpressure for the ITI. Lateral earth pressures measured on theculvert sidewalls for the McBean and Longs Creek culverts arepresented in Fig. 11. The plot shows the earth pressuresmeasured on the side from the top of the culvert downwards.The average centrifuge measured pressure for PPI agreesclosely with the average pressure measured by the four lateralearth measuring cells at the Longs Creek culvert site. Thelateral earth pressures measured on the two north sections atthe McBean Brook culvert site show that the lateral earthpressures increase linearly with depth. For the Longs Creekculvert, the maximum and minimum coefficients of lateralearth pressure were 0.46 and 0.25, respectively. The estimatedcoefficient of at-rest earth pressure based on the angle ofinternal friction of 40° for class A material is 0.35. The lateralearth pressure distribution for the Longs Creek culvert wallwas approximately parabolic, similar to the findings byDasgupta and Sengupta (1991). Average lateral earth pressure at
Fig. 10. Comparison of average vertical earth pressures across culvert roofs.
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the McBean Brook culvert over two instrumented sections was140 kPa, which corresponds to a 49% increase from the averagemeasured lateral earth pressures at the Longs Creek (PPI) culvertsite. This percentage increase is also close to the results from thecentrifuge models tested, where ITI models registered a 50%increase in lateral earth pressure when compared with PPI. Theobservation of increased lateral earth pressures for ITI culverts inrelation to PPI culverts is consistent with earlier research done atUNB. The positive arching in the case of ITI culverts reducesvertical earth pressures on the roof, but also results in additionallateral stresses on the sidewalls that translates into drag forcesacting downwards on the sidewalls.
Base contact pressure was not monitored at the Longs Creekculvert site. The results presented in Fig. 12 are from the mea-sured base pressures at the McBean Brook culvert, the LongsCreek culvert measured vertical earth pressure on the roof plusapproximated DL pressure of the culvert, the AASHTO (2010)unfactored vertical load plus approximated culvert DL pres-sure, and the centrifuge measured base pressures for a modelLongs Creek culvert installation. The McBean Brook culvertmeasured average base pressure (221.2 kPa) was 0.66 timesthe calculated overburden (fill only) plus the DL pressure ofthe culvert and 0.72 times the measured roof pressure plus theDL pressure at the Longs Creek (PPI) site. These results were
Fig. 11. Comparison of lateral earth pressures.
Fig. 12. Comparison of base contact pressure.
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compared with AASHTO (2010) predicted pressures for thesame scenario. Based on the reasonable agreement betweenthe vertical and lateral earth pressures measured by the cen-trifuge with the actual measured pressures for the Longs Creekculvert, the centrifuge base pressure result is assumed to beclose to the average expected pressure at the site. Both thecompacted and uncompacted sidefill for AASHTO (2010)predictions were lower than the centrifuge average measuredpressure, suggesting the need to account for downward dragforces developed on the sidewalls of box culverts.
Under the conditions examined, the McBean Brook culvertITI instrumentation appears to record significantly lower earthpressures on the roof for a fill height of 14 m compared withthe Longs Creek culvert PPI. The significant increase in lateralearth pressure exhibited with depth at the McBean Brook ITIwas not observed at the Longs Creek PPI. The average basecontact pressure measured beneath the McBean Brook culvertwas 221 kPa, which shows a 10% increase from the measuredearth pressure on the roof plus the culvert DL pressure, indi-cating the presence of downward drag forces on the sidewalls.Though the base contact pressure was not measured for theLongs Creek positive projecting rectangular box culvert, cen-trifuge testing data for base contact pressure showed that thebase pressure is in excess of the pressure on the roof of theculvert plus the DL pressure. Similar observations of increasedbase pressure have been reported in literature (Katona andVittes 1982; Kang et al. 2008; Oshati et al. 2012). Centrifugeresults show an increase ranging from 0%–4% and 16%–31%in base contact pressure from the overburden pressure plus thedead weight for the PPI and ITI configurations, respectively.Results from the centrifuge testing in the current study suggestthe need for field instrumentation of box culverts (Bc � Hc)under high fills installed using PPI to measure base contactpressure.
Results from field measurements of earth pressures showthat earth pressure on box culverts is influenced by the H/Bcratio along with compaction effort. Measured pressures fromthe Longs Creek culvert and the McBean Brook culvert aspresented in Tables 2 and 3 show that the soil–structureinteraction factor (Fe) either reduces or reaches a constantvalue as the H/Bc ratio increases. For PPI culverts, the value ofFe increases or stays constant until an H/Bc ratio of approxi-
mately 0.60 is reached. For ITI culverts, the factor Fe will beless than one due to positive arching. From Table 3 it is notedthat factor Fe did not reach a minimum limiting value at theMcBean Brook site. More field data are needed to arrive at anappropriate equation for Fe as a function of the H/Bc ratio.
Figure 13 shows the variation of Fe with different H/Bcratios for PPI culverts and comparison with the AASHTO
Table 2. Calculated Fe on Longs Creek culvert roof.
H (m)Measuredpressure (kPa) H/Bc Fe
1.75 32 0.24 0.872.95 57 0.4 0.923.47 81 0.47 1.113.57 79 0.49 1.054.42 94 0.6 1.015.38 116 0.74 1.037.7 161 1 18.72 178 1.19 0.9712.85 258 1.76 0.9612.85 266 1.76 0.9913.89 271 1.9 0.9314.15 279 1.94 0.94
Table 3. Calculated Fe on the roof of two instrumented sections ofMcBean Brook culvert.
H (m)Section A – Measuredpressure (kPa) H/Bc Fe
0.18 6.83 0.03 1.830.4 6.83 0.06 0.811 26.13 0.15 1.243 62.94 0.44 13.3 55.45 0.49 0.83.8 73 0.56 0.914 96.5 0.59 1.156.29 123.97 0.93 0.948.52 141.54 1.26 0.7910.43 153.2 1.54 0.712.13 162.3 1.8 0.6412.96 165.99 1.92 0.6114.43 172.54 2.14 0.5718.03 184.14 2.67 0.4918.66 198.77 2.76 0.5118.81 199.44 2.79 0.519.78 204.88 2.93 0.4921.61 215.66 3.2 0.4823.56 228.15 3.36 0.4625.08 235.33 3.71 0.4525.08 230.83 3.71 0.4425.08 226.07 3.71 0.43
H (m)Section B – Measuredpressure (kPa) H/Bc Fe
0.18 6.92 0.03 1.850.4 6.92 0.06 0.821 29.33 0.15 1.43 65.14 0.44 1.033.3 47.95 0.49 0.693.8 61.64 0.56 0.774 97.54 0.59 1.166.29 123.17 0.93 0.938.52 144.56 1.26 0.8110.43 155.9 1.54 0.7112.13 162.82 1.8 0.6412.96 164.31 1.92 0.614.43 168.46 2.14 0.5618.03 179.05 2.67 0.4718.66 184.77 2.76 0.4718.81 183.51 2.79 0.4619.78 185.72 2.93 0.4521.61 192.56 3.2 0.4222.68 199.54 3.36 0.4225.08 203.07 3.71 0.3925.08 200.83 3.71 0.3825.08 201.1 3.71 0.38
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(2010) values. This figure includes the data from the presentresearch as well as the data reported in the literature. It can benoted from this figure that AASHTO (2010) predicted valuescould be unconservative and therefore these equations shouldbe used with caution. The available data for PPI culverts showthat factor Fe reaches a limiting value for the H/Bc ratio ofabout 5.
ConclusionA relatively yielding foundation can significantly reduce
vertical earth pressure on culverts, as was the case for theLongs Creek culvert (PPI). Average vertical earth pressure onthe roof of the McBean Brook culvert (ITI) at a fill height ofapproximately 14.4 m was 170 kPa, which corresponds to a39% reduction in vertical earth pressure (270 kPa) measured atthe Longs Creek culvert (PPI). Average lateral earth pressure(139.6 kPa) measured at the McBean Brook culvert (ITI)compared with the average measured pressure (93.7 kPa) atthe Longs Creek culvert (PPI) showed an increase of 49%.Base contact pressures measured at the McBean Brook culvert(ITI) indicate that downward drag forces are developed on thesidewalls of box culverts. Centrifuge testing results confirmedthe presence of increased base pressure from the overburdenpressure plus the DL pressure of box culverts.
The results from this study suggest that the soil–structureinteraction factor (Fe) reduces as the ratio H/Bc increases, butthe relationship is dependent on the rate of settlement, whichcan be influenced by several construction factors (compaction,bedding, installation method). Findings from the present studysuggest that the AASHTO (2010) unfactored vertical earthload equations for embankment installations (PPI) appearsto be a reasonable estimate for yielding foundations, pro-vided necessary accommodations for an uneven pressuredistribution on the roof are considered. There is a need torecognise and account for the presence of downward drag
forces along sidewalls of box culverts, which subsequentlyincreases the base contact pressure. Results from this studyshow that centrifuge testing can also be used for prelimi-nary evaluation of culverts installed under high fill embank-ment, and that the ITI method provides substantial costsavings from the reduction of earth pressures on the roofand base of box culverts.
Acknowledgement
Financial support for this research was provided by NewBrunswick Department of Transportation (NBDOT) Canada.
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