Culvert Fitting Manual

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DataBASE Solutions Patrick KariukiStructural Engineer

Highway Culvert Fitting

A Manual for the Site Supervisory Staff


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Universal Culvert Fitting System 2004 Patrick Kariuki

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So what benefits does the software solution offer?

It is accurate and reliable as long as correct data is entered, leading to structuresthat fit perfectly with the embankment slopes. The program logic has been testedfor all possible combinations of flow directions, skew angles, pavement gradients,cross-falls etc.

Efficient. As long as all survey data required is available many culverts can beplotted in a day. It thus enables the engineer to come up with the most efficientand economical culverts within a short time and thus ensure timely instruction tothe contractor.

It is an excellent way to perform quick estimates of culvert lengths. In fact datafor all the culverts can be put into a simple database which can be shared over anetwork by the engineers, surveyors etc and thus reduce the paperwork in todaysnear-paperless office.

Engineers can be free to concentrate on supervision or to support junior staff andnot be tied by too much office work on site.

Cost of archiving of paperwork is reduced. Consequently, staff time spent in filingand retrieving of information is reduced and the ability to produce the right recordat the right time in line with the organisations interests and obligations isenhanced.

Suffice is to say that, like most other professionals have done, engineers working on siteare expected to harness the power of Information and Communication Technology (ICT).For that is where the future lies.


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ACKNOWLEDGEMENT


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DISCLAIMER

THIS DOCUMENT DOES NOT PURPORT TO DESCRIBE FULLY

ALL THE PRINCIPLES OF DESIGN OF CULVERTS NOR IS IT

INTENDED TO BE A SUBSTITUTE TO ANY OTHER

CONTRACTUAL DOCUMENT.

THE MATERIAL HAS BEEN PREPARED IN ACCORDANCE WITH

GENERALLY RECOGNISED ENGINEERING PRINCIPLES AND

PRACTICES AND IS FOR GENERAL INFORMATION ONLY. THIS

INFORMATION SHOULD NOT BE USED WITHOUT FIRST

SECURING COMPETENT ADVICE WITH RESPECT TO ITS

SUITABILITY FOR ANY GENERAL OR SPECIFIC APPLICATION.

ALTHOUGH CARE HAS BEEN TAKEN TO ENSURE THAT ALL

INFORMATION CONTAINED HEREIN IS ACCURATE WITH

RELATION TO EITHER MATERS OF FACT OR ACCEPTED

PRACTICE AT THE TIME, THE AUTHOR ASSUMES NO

RESPONSIBILITY FOR ANY ERRORS IN ORMISINTERPRETATIONS OF SUCH INFORMATION, OR ANY

LOSS OR DAMAGE ARISING FROM OR RELATED TO ITS USE.

ANYONE UTILIZING THIS INFORMATION ASSUMES ALL

LIABILITY ARISING FROM SUCH USE.

No part of this document or software may be reproduced without the authors permission.

System Requirements : Microsoft Office .

All rights reserved.

2004 Patrick Kariuki

Tel 254-02-4445288, 254-0721-759991,


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TABLE OF CONTENTS

PREFACE........................................................................................................................... ii

ACKNOWLEDGEMENT ................................................................................................. iv

DISCLAIMER .................................................................................................................... v

1. CULVERT SPECIFICATION.................................................................................... 8

2. REFERENCE LAYER AND WIDENNING ........................................................... 11

3. OFFSETS AND CAMBERS.................................................................................... 13

4. CHAINAGES AND FINISHED ROAD LEVELS (FRL) ....................................... 15

5. COVER TO CULVERT ........................................................................................... 18

6. APPLICATION TO VARIOUS CULVERT DESIGNS.......................................... 21

7. PLOTTING PROCEDURE. ..................................................................................... 23


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LIST OF FIGURES

Figure 1 : Specification for Offsets and Inverts.................................................................. 8

Figure 2 : Design Convention............................................................................................. 9

Figure 3 : Typical End slope detail at a super elevated road section............................... 11

Figure 4 : Consideration for local embankment widening................................................ 12

Figure 5 : Pavement Cross-section ................................................................................... 13

Figure 6 : Chainages and Levels for skew angle less than 90. ........................................ 15

Figure 7 : Chainages and Levels for skew angle greater than 90.................................... 16

Figure 8 : Entry Order for Road Offsets. .......................................................................... 17

Figure 9 : Levels for all culvert orientation. ..................................................................... 17

Figure 10 : Computation of the fill slopes length for MPCs............................................. 18

Figure 11 : Cover variation for wider culverts.................................................................. 19

Figure 12 : Inlet end Computation of fill slopes lengths ............................................... 20

Figure 13 : Outlet end Computation of cover and fill slopes lengths ............................ 20

Figure 14 : Application for culverts without head beams................................................. 21

Figure 15 : Application for culverts with high head beams.............................................. 22

Figure 16 : Basic data input .............................................................................................. 23Figure 17 : Offsets and Levels .......................................................................................... 23

Figure 18 : Tying with OGL............................................................................................. 24


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8

1. CULVERT SPECIFICATION.

Besides the General Arrangement and Reinforcement Drawings, additional information

necessary to properly fit a culvert on a Highway include:

Chainage location

Orientation (skew angle)

Offset lengths, Left and Right (from the road centre line)

Invert levels, Left and Right

Ground Profile or OGL.

Culvert Design Slope = 0.680%Culvert Skew(Orientation) = 79

Figure 1 : Specification for Offsets and Inverts

The culvert ORIENTATION is usually specified in terms of SKEW angle in degrees. The

specification should follow some design convention such as the one illustrated in Fig 2

below. Facing the Destination, the orientation is measured clockwise. Thus an orthogonal

culvert will be 90 degrees orientation and not 0 degrees. The idea is to avoid confusion,

for instance if orientation is specified as 20 degrees, it is not clear whether the designer

means 70 or 110 degrees without a sketch aid.

L H S Length:(m)

885.980

12.1 7.8

886.116

R o a d C L R H S Length:(m)

L H S Invert: R H S Invert:

FRL =891.67 103+214.00

4 . 4


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There may be a temptation to want to construct a standard culvert or to try and adopt

one whose construction details already exist. The culvert orientation should be one that

allows the culvert to naturally fit into existing or new channels while keeping the length

reasonable. The existence of obstacles suck as rock out crops should be considered as

well.

Figure 2 : Design Convention.

The DESTINATION refers to the road end point or town e.g. Mombasa. All references to

do with invert levels and offset lengths and skew angles should be referenced in this

direction. In deed these references should normally be specified as either RIGHT or

LEFT with respect to the destination.

The culvert CHAINAGE is the chainage along the road centre line at which the culvert

centre line intersects it.


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From the OGL profile taken at the culvert location the direction of flow is determined as

either LEFT or RIGHT. A culvert whose outfall is to the right is considered a RIGHT

DISCHARGING culvert and vice versa.

The Offset Lengths specified should refer to the distances from the road centreline along

the culvert centreline to the end of the culvert. The sum of right and left offset should

equal to the total length of the culvert. Additionally the DESIGN SLOPE should be the

slope ALONG the length of the culvert.


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2. REFERENCE LAYER AND WIDENNING

The REFERENCE layer is the layer against which the covers to the culvert at both the

inlet and outlet end are based. It is selected for convenience and should ideally extend

over the entire road cross section. In fact it may even be imaginary and its position will

determine the PAVEMENT thickness above the reference pavement Layer.

The user should know the total required cover range and thus determine what minimum

or maximum need to be provided below the reference layer.

Figure 3 : Typical End slope detail at a super elevated or cambered road section

At locations for guardrail installation, where widening should normally be allowed for,

the Design Reference Layer should preferably be that on which the guardrails will be

installed and which probably extend over the entire road cross section. Guardrails will

normally be installed at sharp bends or at high fill locations. Widening may also be

required for various other reasons and before specifying a culvert for construction this

information should be confirmed first.


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Figure 4 : Consideration for local embankment widening

The FILL SLOPE is expressed as a ratio (1:n). The value (n), should be as specified in

the appropriate road cross section. Note that this may be different for high and low fills

normally determined by the difference between Finished Road Level (FRL) and Original

Ground Level (OGL) at the road centreline.


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3. OFFSETS AND RATES OF CROSSFALL

The figure below illustrates the design convention for offset lengths and the road

crossfalls as used in this program. Note that OFFSET 1 may consist of several lanes. At

super elevation run off sections the crossfall rate for the shoulder input should be that

corresponding to the chainage at OFFSET 2. This applies, of course, to non-orthogonal

culverts, which cross various chainages. Strictly speaking the outer edge of Offset 1 is not

necessarily where the shoulder begins but rather where the crossfall rate changes.

Figure 5 : Pavement Cross-section

On a straight road section, where the road is always in camber, the crossfall is taken as

positive for both the carriageway and shoulders as illustrated in the figure. Therefore all

crossfalls about the road centreline rising above the horizontal plane are taken as negative

(super elevations crossfalls on the rising side of the road).


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The crossfall for shoulders is always positive and will always be specified in the relevant

road drawings. Normally the value is the greater of the specified minimum and the

crossfall of adjacent lane if in the same sense of fall.

Two offsets are allowed for on either side of the centre line to allow for different crossfall

rates for the shoulders and the carriageway. The program takes into account possible

asymmetry of the road cross-section such as at locations with additional lanes (such as

acceleration /deceleration lanes) or local widening for guardrail installation.

Note that while the skew angle is specified in degrees in way of data input, all

trigonometric functions on computers are by default based on radian units.

Thus for all possible theoretical skew angles (0-180 degrees) the approximate length of

the culvert is given by:

L = ProjectedLength/(cos(SkewAngle-0.5pi))

Where the Skew angle is in radians and

The projected length is the sum of

Thickness of Left Headwall (if any)

Length of Left Fill slope (if any)

Left Offset 2

Right Offset 2

Length of Right Fill slope (if any)

Thickness of Right Headwall (if any)


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4. CHAINAGES AND FINISHED ROAD LEVELS (FRL)

Fig 6 below illustrates how the design chainages and the corresponding levels are be

derived depending on the direction of flow for skew angles less than 90 degrees.

The levels here are FRLs i.e the Final Finished Road Levels as allowance is made for the

pavement thickness down to the Reference Layer in the program.

The LEAD LEVEL is the level corresponding to the inlet side at the intersection of the

road edge and the culvert centreline in plan. The CHECK LEVEL is similar to the Lead

Level and in addition it is the level against which the culvert cover at the outlet is

computed.

Figure 6 : Chainages and Levels for skew angle less than 90.

For non-orthogonal culverts, centreline levels are generated automatically by specifying

that the road at the culvert location is on grade (Y or N) and entering the appropriate


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grade (as a percentage). In case of culverts at vertical curve locations the levels are

entered manually in the appropriate column in Fig 8, but these too can be programmed to

be computed from the parameters that define the curve.

The grade is considered NEGATIVE if the road is falling towards the destination and

POSITIVE if rising.

Fig 8 shows the relationship between the Offsets and FRLs. Offset distances should

always be entered from Left to Right in the program from top to bottom in column 1 of

the table. Note that, in this example, the reference layer is the Cement Stabilised Sub-

base Layer (CSSB) and Pavement Thickness above is 325mm.

The Levels for Culverts on Grade in the figure are not used unless the culvert is ONGRADE in which case the GRADE value is used to compute the corresponding levels.

Manual Levels should correspond to the respective Chainages.

Figure 7 : Chainages and Levels for skew angle greater than 90.


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Fig 7 above illustrates how the design chainages and the corresponding levels are derived

depending on the direction of flow for skew angles greater than 90 degrees. It can be

observed that levels are interchanged depending on whether the skew angle is less of

greater than 90.

Figure 8 : Entry Order for Road Offsets.

The table below summarizes the relationships between Flow Direction, Lead and Check

Levels for all possible culvert orientations.

Figure 9 : Levels for all culvert orientation.

Offsets L>>Rat top ofCSSB

DistancesOrthogonal to

Road CLEffective

Chainages

FRL for culverts onVertical curve (If

Applicable)

FRL forculverts On

Grade Design Levels at

top of CSSB Change in

Level from CL

EffectiveLevel at

Culvert CL

9.673 1.247 103215.247 891.678 891.353 0.366 890.987

3.65 0.709 103214.709 891.674 891.349 0.208 891.1410 0.000 103214.000 891.670 891.345 0.000 891.345

3.65 -0.709 103213.291 891.666 891.341 -0.208 891.549

6.413 -1.880 103212.120 891.659 891.334 -0.058 891.392


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5. COVER TO CULVERT

The importance of cover on a road culvert cannot be over-emphasised. In deed, structural

failures of many metal pipe culverts can be attributed to inadequate cover especially

those installed on emergency or temporary basis where design considerations are

overlooked.

For a given side slope, the greater the cover the longer the culvert. The cover to culvert is

entered by the user for the LEADSIDE (inlet). The cover at the CHECKSIDE (outlet) iscomputed as a function of the Check Level, the culvert slope, length, and side slope.

The covers should be checked against the specified minimum or maximum. For Metal

Culverts such as pipes and arches, the minimum and maximum fill heights will normally

be specified by the manufacturer.

Figure 10 : Computation of the fill slopes length for MPCs.

For tight fitting culverts, the minimum cover can easily be compromised depending on

the gradient of the road, the flow direction and culvert slope. This is more so for wide


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culverts where the cover may significantly vary over the width of the culvert as illustrated

in figure 10a. Clearly, the minimum cover on the program for such a culvert should be

that corresponding to the point where the cover is minimum and not at the centre line.

Figure 11 : Cover variation for wider culverts.

The CULVERT HEIGHT is the height from the culvert invert to the top of the culvert

roof. For a box culvert this may be approximated as the sum of opening height and the

top slab thickness. For a metal pipe culvert this could just be taken as the norminal height

of the culvert.

The thickness of the culvert HEADWALL (if any) is measured orthogonal to road

centreline and not along the culvert centreline.

Fig 12 below illustrates how the length of fill slopes are derived at the INLET end of the

culvert. Fig 13 illustrates how the culvert cover and fill slope lengths are derived at the

OUTLET end of the culvert. Note that the culvert is assumed to be straight and of

uniform height over its length.


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Figure 12 : Inlet end Computation of f ill slopes lengths

Figure 13 : Outlet end Computation of cover and fill slopes lengths.


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6. APPLICATION TO VARIOUS CULVERT DESIGNS

Irrespective of the form of design of the culvert inlet and outlet structures, the program

concept can still be applied by manipulating the various parameters. Some examples are

briefly illustrated below.

Culverts without head beams

A culvert such as shown in figure 14 below has no head walls and the Headwall thickness

Ht is taken as 0.

Figure 14 : Application for culverts without head beams

Precast Culverts

In the sizing of pre-cast culverts, the calculated total culvert length will most likely not be

divisible into exact number of units. Without the need for cutting, this problem can be

easily overcome by insitu casting of the ends of the culverts together with the inlet and

outlet structure including the aprons which will normally be in in-situ concrete.


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Culverts with high Head Beam

For culvert with headwalls extending right to the top of the pavement, the fill is directly

supported by the head beam and there is thus no sloping fill. The non-existent fill slope

can be considered to have a slope of 1 to 0, meaning its vertical.

Figure 15 : Application for culverts with high head beams


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7. PLOTTING PROCEDURE.

The suggested general culvert fitting procedure is as follows:

1 Enter all relevant geometric, pavement and culvert data. Determine skew

angle from the survey plan or OGL taken at the proposed culvert location

Figure 16 : Basic data input

2 Determine if culvert is situated on normal width of road or on a widened

section. Enter the applicable offsets orthogonal to the road centreline. The

order of entry of the offsets is crucial for culverts at asymmetrical road

sections, and should always be entered from Left to Right down column 1 of

Fig 17 starting at the top. The program calculates the distances orthogonal to

road Centreline and the effective chainages.

Figure 17 : Offsets and Levels

1.0 Geometric Data: 1.1 Pavement and Culvert Data

Ref. Layer (Top of:) CSSB

KM: 103+214.00 FRL at CL (M) 891.670

Shoulder, L 2.50%

Pav. thk. above

CSSB (M) 0.325Carriageway, L -5.71% Culvert Skew () 79

Carriageway, R 5.71% Fill Side Slopes 1: 2Shoulder, R 5.71% Outfall, Left/Right Left

On Grade, Y/N y Slab Height (M) 4.4Grade (If appl.) 0.61% Head wall thk (M) 0.30

C r o s s f a l l s

2.0 Chainages and Pavement Levels:

Offsets L>>Rat top ofCSSB

DistancesOrthogonalto Road CL

EffectiveChainages

FRL for culverts onVertical curve(If Applicable)

FRL forculverts On

Grade Design Levelsat top of CSSB

Change inLevel from

CL

EffectiveLevel at

Culvert CLCulvert

End side

9.673 1.247 103215.247 891.678 891.353 0.366 890.987 Right 3.65 0.709 103214.709 891.674 891.349 0.208 891.141

0 0.000 103214.000 891.670 891.345 0.000 891.345

3.65 -0.709 103213.291 891.666 891.341 -0.208 891.549

6.413 -1.880 103212.120 891.659 891.334 -0.058 891.392 Left


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3 If the culvert is located on a vertical curve enter Finished road levels at the

corresponding chainages. If you entered Y for On grade, these levels will

not be used in the computations, instead appropriate figures will be generated

from the grade value entered. Tip: For Orthogonal culvert just enter Y for

on grade and 0 for grade value.

4 From the culvert OGL profile determine the most suitable value for the culvert

slope that will let the culvert invert tie naturally with the ground surface

profile and enter this as a percentage e.g 2.5 for 2.5%. Note that a minimum

slope of 2% is normally specified to enhance self-cleaning.

5 Enter a trial culvert cover at the inlet (say minimum specified cover). Checkthat the culvert cover, which the program calculates for the outlet, is within

specified minimum and maximum. Note that this cover only relates to the

levels at the culvert centre line in consideration as entered in step 3 above.

For a wide culvert, additional checks may be necessary at each extreme

end on either side of the culvert centreline in addition to the culvert

centreline (see figure 11).

Figure 18 : Tying with OGL

6 Check that the output inverts and offsets tie well with the OGL profile.

Normally this will not be the case at first trial. If the culvert is IN THE AIR,

enter a bigger value for inlet cover and vice versa if the culvert is BURRIED.

By playing around with the two parameters namely the inlet cover and


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culver slope an appropriately balanced design is soon arrived at. Note that

these two are the only parameters that can be manipulated without

compromising covers or altering the geometric design of the road. The other

option is of course to change the culvert size and/or orientation.

7 Confirm all entered design data by ticking against each figure, probably on a

hardcopy printout.

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