ground settlement considerations for the design of long culverts beneath high embankments on clay

8/10/2019 Ground Settlement Considerations for the Design of Long Culverts Beneath High Embankments on Clay

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ABSTRACT:

This paper describes a case record that involves a 3 km long upgrading along a major highway inOntario, Canada. The associated improvements to the horizontal and vertical highway alignmentsrequire the construction of a new higher and wider embankment on top of and adjacent to theexisting highway embankment. The new embankment, up to about 11 m high, needs to beconstructed with long (85 m and 136 m) culverts to allow stream flow and fish migration beneaththe highway in order to provide a hydraulic connection to existing creeks. The rockfill embankmentwould be founded on soft, compressible clay that will settle with time. This required that theculverts be designed and constructed to account for large anticipated time-dependent settlements inorder to achieve a suitable long-term hydraulic profile and performance of the culverts.

The paper summarizes the geotechnical investigation carried out, the ground conditionsencountered, the settlement analysis performed and the proposed alternative design constructionsubgrade profiles along the culvert so that suitable hydraulic grades would be achieved throughoutthe service life of the culverts while they undergo settlement.

1 INTRODUCTION

The design and construction of long culverts beneath high embankments need to take into accountthe anticipated long-term ground settlement profile along the alignment of the culverts if theculverts are to perform satisfactorily, from a hydraulic perspective, throughout their design servicelife. In most cases, the settlement profile along the culvert is dish-shaped with the maximum

settlement taking place near the centreline of the highway alignment. Special culvert design andconstruction methods are required if suitable hydraulic grades are to be provided throughout thedesign life of the culvert. Rutledge and Gould (1973) describe the movement of articulatedconduits beneath earth dams and this experience and knowledge can be applied to long culvertsbeneath highway embankments.

This paper describes a case record that involves a 3 km long upgrading along a major highway inOntario, Canada. The associated improvements to the horizontal and vertical highway alignmentsrequire the construction of a new higher and wider embankment on top of and adjacent to the

Ground Settlement Considerations for the Design of Long

Culverts Beneath High Embankments on Clay

SAIHI Faten1and BECKER Dennis

2

1Institut Suprieur des Technologies de lEnvironnement, de lUrbanisme et du Btiment, Tunisia

2Principal, Golder Associates Ltd., Calgary, Alberta, Canada

International Conference on Geotechnical Engineering. 2008


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existing highway embankment. The new embankment, up to about 11 m high, needs to beconstructed with long (85 m and 136 m) culverts to allow stream flow and fish migration beneaththe highway to provide a hydraulic connection to existing creeks. The rockfill embankment wouldbe founded on soft, compressible clay that will settle with time. This required that the culverts bedesigned and constructed to account for large anticipated time-dependent settlements in order toachieve a suitable long-term hydraulic profile and performance of the culverts.

2 INVESTIGATION PROCEDURES

A detailed geotechnical investigation was carried out as part of the final design for the highwayimprovement works to obtain information on the ground conditions to better assess the likely long-term settlement of the ground beneath the embankments. The investigation consisted of sampledboreholes, piezo-cone penetration tests (CPT), field vane tests and laboratory tests on selected soilsamples. Five (5) boreholes and a CPT were advanced along and in the general vicinity of theproposed 136 m long culvert alignment (Area 2), and three (3) boreholes and a CPT were advancedalong and in the general vicinity of the proposed 85 m long culvert alignment (Area 7).

The field investigation was carried out using a Bombardier CME 55 auger drill rig, which was alsoused to push the CPTs. The boreholes were drilled and sampled to depths ranging from 10.2 m to16.8 m. In general, all boreholes were advanced to a depth equal to the required height ofembankment fill, or refusal to further penetration by auger. The boreholes were advanced using 208mm outside diameter (O.D.) continuous flight hollow stem augers. Soils samples were obtained atregular intervals of depth, ranging from 0.75 m to 1.5 m, using a 50 mm O.D. split-spoon samplerin accordance with Standard Penetration Test (SPT) procedures. Field vane shear tests were carriedout in clayey deposits. Shelby tube samples were also taken to facilitate specialized laboratorytesting such as oedometer (consolidation) tests. Soil samples underwent detailed visualexamination. Laboratory testing on selected samples from the 2001 investigation included: naturalwater content, Atterberg limits and grain size analyses. All field and laboratory testing wasperformed in general conformance to relevant ASTM Standards.

3 GENERAL SITE GEOLOGY AND STRATIGRAPHY

3.1 Site Geology

From published geologic information, the site is located in the physiographic region known asAbitibi Uplands that form the easternmost part of the Canadian Precambrian Shield (Geology ofOntario; OGS Special Volume 4). The terrain is comprised largely of metavolcanic and minormetasedimentary rocks. Bostock (1970) describes the Abitibi Uplands as a rocky landscape,scattered with lakes and large areas that are mantled by deposits from Pleistocene glaciationconsisting of the lacustrine clays and former shorelines of proglacial lakes. Landforms typicallyinclude outwash channels, tills and moraines. The local physiography is generally characterized byvariable overburden materials and an irregular, variable bedrock surface with rock outcrops.

3.2

Site Stratigraphy

In general, the ground conditions encountered consists of very soft to stiff clay, silty clay to clayeysilt with varying structure (i.e. interlayered to varved) ranging in thickness from 6 m to 10 m. Theclayey deposit is underlain by a variable deposit of very loose to compact silt, sand, sand and siltand sand and gravel ranging in thickness from 1 m to 3 m.


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At borehole locations drilled through the existing roadway embankment, compact to dense sandand gravel fill containing occasional cobbles and boulders was encountered to depths ranging from2.7 m to 4.6 m. The embankment fill is generally underlain by the silty clay to clayey silt deposit.However, organic material was found to be located within the upper silty clay stratum immediatelybelow the fill materials.

For the purpose of this paper, varved deposits are layered deposits of clay or silty clay with clayeysilt or silt, where the layers of the respective constituents repeat in a regular pattern and are lessthan about 50 mm thick. Interlayered deposits are similar to varved deposits in composition andstructure; however, the respective layers are greater then 50 mm in thickness. Seams (less than 1mm thick) of fine sand, sandy silt and silt were also typically encountered within the cohesivedeposits. Based on the oedometer, CPT and field vane tests results, the silty clay and clayey siltdeposits encountered have a minimum overconsolidation ratio (OCR) of about 2 and are classifiedas being overconsolidated.

The results of the field vane tests for undrained shear strength are summarized on Figure 1. Basedon the field vane tests, the shear strength of the clayey deposits varies from about 27 kPa to 80 kPa,indicating a firm to stiff consistency. The sensitivity of the deposit, as estimated from the field vanetests, ranges from 2.5 to 7.6 as shown on Figure 2. The majority of the test results range from 3 to

6, implying that the silty clay to clayey silt stratum in this area is medium sensitive to sensitivebased on the classification system provided in Canadian Foundation Engineering Manual (CFEM2006).

Atterberg Limits testing conducted on samples obtained from this stratum show liquid limits (wL)ranging from about 20 to 40 percent and a plasticity index (I P) ranging from about 5 to 40 percent.The lower values of wL and IP are representative of the more silty interlayers, while the highervalues are representative of the more clayey interlayers. The results of the Limits testing classifythe soil in this stratum as inorganic and of low to intermediate plasticity. The natural water contentmeasured in selected samples from this stratum range from about 20 to 50 percent. In general, thewater content is at or higher than the liquid limit, corresponding to a liquidity index of one orhigher. Figures 3, 4 and 5 summarize the results of the water content and Atterberg Limits tests.

Figure 1: Summary of Field Vane Test Results. Figure 2: Summary of Sensitivity.


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The measurements of mass and dimensions that were conducted to estimate the natural bulk unitweight of a single carefully trimmed sample from this stratum produced a value of about 16.5kN/m

3. The specific gravity measured on the sample produced a value of 2.69.

The results of consolidation tests are summarized in Table 1.

Table 1: Results of Consolidation Tests

Borehole(sample)

Elevation(Depth)

(m)

vo(kPa)

p(kPa)

OCR e0 Cr Cs cv(cm2/s)

Area 299-7

245.2(5.0)

30.5 140 4.6 1.5 0.035 0.90* 3.5x10-

Area 799-24

252.3(4.9)

36.2 250 6.9 1.2 0.046 0.39 5x10-

Area 799-24

249.7(7.5)

56.1 132 2.4 1.4 0.064 0.60* 5x10-

Area 799-24

249.1(8.1)

60.3 238 3.9 1.2 0.036 0.33 5x10-

* Note: For stress range of pvo 300 kPa

Where: vo is the effective overburden pressure in kPap is the pre-consolidation overburden pressure in kPaOCR is the overconsolidation ratioe0 is the initial void ratioCr is the recompression indexCs is the compression indexcv is the estimated field coefficient of consolidation in recompression index in cm

2/s

Figure 3: Summary of Atterberg Limits and Water Content.


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Figure 6 summarizes the values of overconsolidation ratio (OCR) in the two studied areas based onthe results of the oedometer test and the interpretation of OCR based on correlations with theresults of the field vane tests. The following correlation relating field vane shear strength toprconsolidation pressure (Mesri, 1975) was employed.

su = 0.22 p (1)

where suis the in-situ measured undrained shear strength.

Figure 4: Summary of Plasticity Index.

Figure 5: Summary of Liquidity Index.


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4

CULVERT SETTLMENT ASSESSMENT

The alignment of the existing creeks, in relation to the proposed new highway the two areas, resultsin a requirement for culverts that are longer than the shortest possible (i.e. perpendicular) distancefrom one side of the new embankment to the other. For example, at Area 2, although the new 11 mhigh embankment will be about 50 m wide, maintaining the current alignment of the creek

necessitates a culvert length of 136 m. At area 7, a culvert length of 85 m is required.

At the site of the culverts replacements, relatively low embankments support the current highwayalignment. The existing embankments are approximately 3m high with side slope profiles varyingfrom 2H (horizontal):1V (vertical) to 3H:1V. The new overall embankment was designed withsimilar side slopes.

4.1 Estimated Settlement

Analysis was carried out to assess the ground settlement to be induced by the construction of thenew rockfill embankment. The analysis was based on the results of the site geotechnicalinvestigations.

The settlement analyses were performed using the commercially available program UNISETTLE(v3.0) produced by Unisoft Limited. The location of the existing highway embankments relative tothe proposed new alignment requires that the new embankments be constructed overtop of theexisting ones. As such, the geometry and loading of the existing highway was included in thesettlement analysis. At Area 2, the obliqueness of the angle between the proposed culvert and newroadway alignments (i.e. the two are not at right angles) and the varying terrain result in largevariations in the thickness of required embankment rockfill, in the existing ground surfaceelevation and in the geometry of the existing highway embankment. The effect of these variationson the estimated settlement along the culvert centreline was accounted for in the analysis byanalyzing several different cross-sections based on the available survey information.

The overconsolidation ratio (OCR) profile required in the analysis was established using the resultsof the oedometer tests as well as correlations with the results of the field vane tests. The followingcorrelation relating field vane shear strength to pre-consolidation pressure (Mesri, 1975) wasemployed.

su= 0.22 p (1)

where suis the in-situ measured undrained shear strength in kPa.

A summary plot showing the estimated values of OCR and the design profile of OCR versus depthused in the analysis for the clayey stratum are shown on Figure 6. The OCR results from thesetests were consistent with the lower values estimated from the correlation with the field vane shearstrength. Therefore, the design line follows the lower bound of the trend provided by the scatter ofall vane strength correlations. The following additional parameters were used in the analysis.

Table 2: Parameters used in the settlement analyses

Area Initial VoidRatio

e0

RecompressionIndex

Cr

CompressionIndex

Cs

Thickness ofClayey Stratum

(m)

Unit Weight

(kN/m3)

2 1.5 0.035 0.75 8 17.6

7 1.5 0.035 0.75 9 17.6


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Figure 6: Summary of Overconsolidation Ratio

The immediate settlement of the silt, sand and gravel layers underlying the clayey deposits wasmodelled assuming an estimated elastic compression modulus of 6 MPa. The rockfill for the newembankment construction and the sand and gravel fill of the existing highway embankments wereboth assumed to have a unit weight of 18 kN/m3.

The results of the settlement analysis are presented on Figures 7 and 8 showing the estimatedsettlement profile along the centreline of the proposed 136 m and the 85 m long culverts,respectively. The calculated settlements presented in these figures are due to the loading imposed

on the compressible clayey and silt and sand foundation soils by the construction of the newembankments. As shown on Figures 7 and 8, the maximum ground settlement along the proposedculvert alignments is estimated to about 0.34 m in Area 2 and about 0.61 m in Area 7.

Figure 7: Estimated Settlement along Culvert Centreline, Area 2


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In Area 2, based on a field coefficient of consolidation (c v) of 3.5x10

-3cm2/s, it is estimated thatabout 65 percent of the primary consolidation component of the settlement will be complete inabout 6 months and about 95 percent will be complete in about 1.6 years. For the secondary (creep)consolidation component of the settlement, it is calculated (based on an estimated coefficient ofsecondary consolidation c of 0.32 percent/log cycle time) that approximately 25 mm of

settlement will occur over each log cycle of time. Following completion of primary consolidation,about 25 mm of creep settlement is expected to occur within about 15 years.

Figure 8: Estimated Settlement along Culvert Centreline, Area 7

In Area 7, based on a field coefficient of consolidation (cv) of 5x10-3

cm2/s, it is estimated that

about 70 percent of the primary consolidation component of the settlement will be complete inabout 6 months and about 95 percent will be complete in about 1.4 years. For the secondary(creep) consolidation component of the settlement, it is calculated (based on an estimatedcoefficient of secondary consolidation cof 0.45 percent/log cycle time) that approximately 40mm of settlement will occur over each log cycle of time. Following completion of primaryconsolidation, about 40 mm of creep settlement is expected to occur within about 15 years.

4.2 Design Culvert Invert Elevation during Construction

The geometry of the proposed embankments and the presence of the existing roadway cause a non-uniform loading on the subsoils that result in uneven settlement along the culvert centreline. Giventhat the culverts are to provide a pathway for stream flow and fish migration from one side of thenew highway embankment to the other, careful consideration is required regarding the initial grade

and final location of the culvert invert elevation during construction. The culvert invert should beplaced such that positive flow is assured following completion of construction and after post-construction settlements have occurred. Table 3 lists the required upstream and downstream culvertinvert/bottom elevations as provided by the client.

As a result of the long culverts and relatively small change in grade between the upstream anddownstream sides, maintaining a positive downstream gradient at all times (i.e. during constructionand following post-construction settlements) may not be possible. Two alternative construction


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profiles of culvert invert were considered as follows. Figures 9 and 10 show schematically thedifferent culvert installation alternatives in Areas 2 and 7, respectively.

Table 3: Required Upstream and Downstream Culvert Invert/Bottom Elevations

Location

(Culvert Length)

Culvert Station Required Invert/Bottom

Elevation(m)

Overall Change in

Invert Elevation andAverage Culvert Grade

Area 2(136 m)

9+932 248.15 -10+068 247.80 0.35 m / 0.26%

Area 7(85 m)

9+958.5 256.10 -10+043.5 255.25 0.85 m / 1.0%

If a positive (or zero) downstream gradient is to be maintained during construction (Alternative 1),it is estimated that some ponding (i.e. possible accumulation of standing water) will occur over thecentral sections of the culverts at the completion of the long-term settlements (Figures 9 and 10).

If a positive downstream gradient is to be achieved over the entire length of each culvert followingthe completion of the long-term settlements (Alternative 2), the culvert invert must be initially

placed during construction such that some upstream ponding will occur (Figures 9 and 10).

Tables 4, 5, 6 and 7 summarize possible culvert invert elevations for the two different alternatives(for each culvert location) based on the results of the settlement analysis. The impact of each optionon the local hydraulics of the stream system is also identified.

Table 4: Area 2 Alternative 1

Station Distance FromLeft /Upstream

End (m)

Initial Construction Final ConditionElevation

(m)Interval Grade

(%)Elevation

(m)Interval Grade

(%)9+932 0.0 248.150 - 248.126 -

9+956.5 24.5 248.150 0.00 248.044 -0.339+987.5 55.5 248.150 0.00 247.811 -0.7510+018.5 86.5 248.016 -0.43 247.933* 0.3910+049.5 117.5 247.882 -0.43 247.869 -0.2110+068 136.0 247.800 -0.44 247.800 -0.37

Notes - ve implies downstream gradient* Approximately 0.12 m deep pool (maximum) is calculated to occur over a 31 mlong section of culvert at end of long-term settlement



End (m)


(m)Interval Grade

(%)Elevation

(m)Interval Grade

(%)

9+932 0.0 248.150 - 248.126 -9+956.5 24.5 248.150 0.00 248.044 -0.339+987.5 55.5 248.350* 0.65 248.011 -0.1110+018.5 86.5 248.016 -1.08 247.933 -0.2510+049.5 117.5 247.882 -0.43 247.869 -0.2110+068 136.0 247.800 -0.44 247.800 -0.37

Notes - ve implies downstream gradient * Approximately 0.20 m deep upstream ponding iscalculated to occur at initial construction.


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Figure 9: Culvert Installation Schematic, Area 2.



End (m)


(m)Interval Grade

(%)Elevation

(m)Interval Grade

(%)9+958.5 0.0 256.100 - 256.081 -9+972.4 13.9 256.056 -0.32 255.944 -0.999+995.0 36.5 255.984 -0.32 255.375 -2.5210+010.3 51.8 255.752 -1.52 255.538* 1.07

10+034.4 75.9 255.388 -1.51 255.333 -0.8510+043.5 85.0 255.250 -1.52 255.244 -0.98

Notes - ve implies downstream gradient* Approximately 0.16 m deep pool (maximum) is calculated to occur over a 15 mlong section of culvert at end of long-term settlement



End (m)


(m)Interval Grade

(%)Elevation

(m)Interval Grade

(%)9+958.5 0.0 256.100 - 256.081 -

9+972.4 13.9 256.100 -0.00 255.988 -0.679+995.0 36.5 256.200* 0.44 255.591 -1.7610+010.3 51.8 255.752 -2.93 255.538 -0.3510+034.4 75.9 255.388 -1.51 255.333 -0.8510+043.5 85.0 255.250 -1.52 255.244 -0.98

Notes - ve implies downstream gradient* Approximately 0.10 m deep upstream ponding is calculated to occur at initialconstruction


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Figure 10: Culvert installation schematic, Area 7

If Alternative 1 is adopted as the method of construction (i.e. maintain a zero or downstreamgradient over all sections of culvert during construction), potential problems associated with thepossibility of standing water (or very shallow areas) over a central section of the culvert could bepartially mitigated by decreasing the thickness of the granular substrate over this section followingthe completion of all settlements. A localized thinning of the substrate would decrease the invertelevation as required and could reinstate a downstream gradient over the affected section.

If Alternative 2 is adopted as the method of construction (i.e. maintain a downstream gradient overall sections of culvert at end of long-term consolidation settlement), potential problems associatedwith initial upstream ponding are expected to be relatively short-lived. Based on the oedometer(consolidation) test data obtained at the sites, it is estimated that 0.10 m to 0.20 m of consolidationsettlement should occur in about one month or less following the placement of the embankment fill.

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CONCLUDING REMARKS

This paper presents a case record that identifies and discusses the importance of considering long-term settlements in the design and construction of long culverts beneath high embankments onclay. Special culvert design and construction methods are required if suitable hydraulic grades andperformance are to be provided throughout the design life of the culvert. Two alternativeconstruction culvert invert profiles are identified to accommodate anticipated long-term settlementprofiles. The hydraulic advantages and limitations of each alternative are also discussed.

The project will need to be completed and the culvert performance observed over time before thesuccess of the settlement mitigation measures described in this paper can be assessed. This may bethe topic of a future paper as a supplement to the current paper.


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REFERENCES

Bostock, H.S. 1970. Physiographic subdivisions of Canada; in Geology and Economic Minerals of Canada,

Geological Survey of Canada, Economic Geology Report No. 1: 11-30.

Canadian Foundation Engineering Manual (CFEM). 2006. Fourth Edition. Edited by D.E. Becker and I.D.

Moore. Published by the Canadian Geotechnical Society through Bi-Tech Publishers, Richmond, BC.

Geology of Ontario.1991. Ontario Geological Society, Special Volume 4, Part 1. Eds. P.C. Thurston, H.R.

Williams, R.H. Sutcliffe and G.M. Stott. Ministry of Northern Development and Mines, Ontario.

Handbook of Steel Drainage & Highway Construction Products. 1984. First Canadian Edition. American Iron

and Steel Institute, Washington, D.C.

Mesri, G. 1975. Discussion on new design procedure for stability of soft clays. ASCE Journal of the

Geotechnical Engineering Division, 101 (GT4): 409-412.

Rutledge, P.C. and Gould, J.P. 1973. Movements of Articulated Conduits Under Earth Dams on

Compressible Foundations, In: Embankment Dam Engineering Casagrande Volume. Eds. Hirschfeld,

R.C. and Poulos, S.J. John Wiley & Sons, New York.

ground settlement considerations for the design of long culverts beneath high embankments on clay

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