design manual for low volume roads

7/28/2019 Design Manual for Low Volume Roads

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)('(5$/'(02&5$7,&5(38%/,&2)

(7+,23,$152$'6$87+25,740 then this should be increased to 7.0m and shoulders reduced to 1.0m.3. Parking lanes and footpaths may be required.4. On hairpin stacks the minimum radius may be reduced to a minimum of 15m.5. Length not to exceed 200m and relief gradients required (


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Table B.4.5: Geometric design standards for unpaved DC4(1) (AADT 150-300)

Design Element Unit Flat Rolling Mountain Escarpment Populated areas

Design speed km/hr 70 60 50 25 50

Road width m 7.0(3) 7.0(3) 7.0 7.0 7.0(2,3)

Min stopping sight

distance

m 125 105 75 28 70

Min horizontal radius m 245 175 110 23(4) 110

Max desirable gradient % 4 6 6 6 4

Max gradient % 6 9 9 9 6

Max. super-elevation % 6 6 6 6 6

Min crest vertical curve K 34 19 11 6 11

Min sag vertical curve m 4.8 3.5 2.2 1.3 2.2

Normal cross-fall(5) % 6 6 6 6 6

Notes:1. If there are more than 80 large vehicles then DC5 should be used.2. Parking lanes and footpaths may be required.

3. If the number of large vehicles >40 then this should be increased to 7.5m.4. On hairpin stacks the minimum radius may be reduced to a minimum of 15m.5. Cross fall can be reduced to 4% where warranted (eg poor gravel (for safety), low rainfall).

Table B.4.6: Geometric design standards for paved DC3(1) (AADT 75-150)



Width of runningsurface

m 6.0 6.0 6.0 6.0 6.0

Width of shoulders m 1.0 1.0 0.5 0.5 1.0(2)

Total width m 8.0 8.0 7.0 7.0 8.0

Min stopping sightdistance

m 110 90 70 25 65

Min horizontal radiusfor SE=4%

m 195 135 85 20(3) 85


m 170 120 75 18(3) NA


m 150 105 70 16(3) NA


Maximum gradient % 7 10 12(4,5) 12(4,5) 6

Min crest vertical curve K 21 12 7 2 7Normal cross-fall % 4 4 4 4 4

Minimum sag verticalcurve

m 4.8 3.5 2.2 1.3 2.2

Normal cross-fall % 3 3 3 3 3

Shoulder cross-fall % 6 6 3 3 6

Notes:1. If there are more than 30 large vehicles then DC4 should be used.2. Parking lanes and footpaths may be required.3. On hairpin stacks the minimum radius may be reduced to a minimum of 15m.4. Length not to exceed 200m and relief gradients required (


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Table B.4.7: Geometric design standards for unpaved DC3(1) (AADT 75-150)



Road width m 7.0 7.0 6.5 6.5 7.0(2)

Min stopping sight

distance

m 125 105 75 28 70

Min horizontal radius m 245 175 110 23(4) 110

Max desirablegradient

% 4 6 6 6 4



Min crest verticalcurve

K 34 19 11 3 11

Minimum sag verticalcurve

K 4.8 3.5 2.2 1.3 2.2

Normal cross-fall(3) % 6 6 6 6 6

Notes:1. If the number of large vehicles is >30, then DC4 should be used.2. Parking lanes and footpaths may be required.3. Cross fall can be reduced to 4% where warranted (eg poor gravel (for safety), low rainfall).4. On hairpin stacks the minimum radius may be reduced to a minimum of 15m.

Table B.4.8: Geometric design standards for DC2 paved(1) (AADT 25-75)

Design Element Unit Flat Rolling Mountain EscarpmentPopulated

areas


Width of running surface m 3.3 3.3 3.3 3.3 3.3

Width of shoulders m 1.5 1.5 1.0 1.0 1.5(2)

Total width m 6.3 6.3 5.3 5.3 6.3

Min stopping sight distance m 85 70 50 17 65

Min horizontal radius forSE=4%

m 135 85 50 15(3) 85


m 120 75 45 15(3) NA


m 105 70 40 15(3) NA


Max gradient % 7 10 12(4) 15(4) 6Max. super-elevation % 6 6 6 6 6


Minimum sag vertical curve K 3.5 2.2 1.3 0.7 2.2


Shoulder cross-fall % 6 6 3 3 6

Notes:1. If the number of large vehicles >20 then DC3 should be used.2. Parking lanes and footpaths may be required.3. On hairpin stacks the minimum radius may be reduced to a minimum of 13m.4. Length not to exceed 200m and relief gradients required (


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B - Chapter 4 - 15

Table B.4.9: Geometric design standards for DC2(1, 2)unpaved (AADT 25-75)

Design Element Unit Flat Rolling Mountain EscarpmentPopulated

areas


Road width(5) m 6.0 6.0 6.0 6.0 6.0(3)

Min stopping sight distance m 95 75 55 20 70Min horizontal radius m 175 110 70 15(4) 110





Minimum sag vertical curve K 3.5 2.2 1.3 0.7 2.2


Notes:1. If the number of large vehicles is >20 then DC3 should be used.2. If the number of large vehicles is


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Table B.4.11: Minimum standards for basic access

Characteristic Minimum requirements

Radius of horizontal curvature 12m absolute but up to 20m depending on expected vehicles

Vertical curvatureK value for crestsK value for sags

2.50.6

Maximum gradientsOpen to all vehiclesOpen only to cars and pick-ups

14%16%

Minimum stopping sight distance Flat and Rolling terrainMountainousEscarpments

50m35m20m

For classes of road with the higher design speeds, adverse cross fall should be removed for curves withlow radii as indicated in Table B.4.12.

Table B.4.12: Adverse cross-fall to be removed if radii are less than shown

Design speed (km/h)Minimum radii (m)

Paved Unpaved


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4.4 Design-by-eye

The design-by-eye method is best suited to rehabilitation or upgrading projects where a road alignmentalready exists and is the preferred method for developing a design for a track or undesignated road undera community roads programme where a walking track is being improved to enable it to carry occasionalvehicles. Nevertheless, considerable experience and skill is needed to carry out the design-by-eye methodand the approach should only be used under the guidance and supervision of an experienced engineer.

4.5 Typical Cross Sections

Typical cross sections for a range of conditions are shown in Figures B.4.2 to B.4.13. They include: Roads on flat terrain; Roads on rolling terrain; Roads on mountainous terrain; Roads on escarpments; Roads through populated areas; Roads on expansive soils.

Slope dimensions for the various conditions are summarised in Table B.4.15.

Table B.4.15: Slope dimensions for cross-sections (ratios are vertical:horizontal)

Material Height of slope (m)Side slope

Back slopeSafety

classificationCut Fill

Earth(1)

0.0-1.0 1:4 1:4 1:3 Recoverable

1.0-2.0 1:3 1:3 1:2Not

recoverable

>2.0 1:2 1:2 1:1.5 Critical

Rock Any height Dependant on costs Critical

Expansiveclays(2)

0-2.0 n/a 1:6 Recoverable

>2.0 n/a 1:4

Notes:1. See Cross Section2. Certain soils may be unstable at slopes of 1:2. Geotechnical advice required.3. The drainage ditch should be moved away from the embankment

The detailed cross-sections to scale are given in the Standard Detail Drawings (2011).

PART B: DESIGN STANDARDS FOR LOW VOLUME ROADSPART B: DESIGN STANDARDS FOR LOW VOLUME ROADS


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B - Chapter 4 - 18

FigureB.4.2:Typic

alcrosssection,DC14,FlatTerrain,Unpaved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m)

3.3

5.0

5.5

6.0

B

Shoulderwidth(m)

0.6

0.5

0.75

0.75

C

MinCrossfall/Ca

mber(%)

4

4

4

4

D

Backslopeofditch(v:hratio)

SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

DepthofSideditch(m)

Varies

H

Crownheight(m

)

0.35

0.3

5

0.5

0.5

J

Clearedwidth(m

)

15

20

20

20

Notes:1

.

Sectionnotdrawntoscale;

2.

V-shapeisthestandardshap

eofthedrainageditchconstructedbymo

torortowedgrader;

3.

Trapezoidaldrainsarecomm

onlyusedandaremucheasiertodigandcleanusinglabour-intensivemethods.T

heminimumrecommendedwidthis400m

mandthetypicalcross-

sectionisshownbelow

!

6400

1

2

1-3,1'

4.

Rectangulardrainsneedtob

elinedwithrock,brickstonemasonryorconcretetomaintaintheirshape;

5.

Moredetailonsidedrainsis

providedinPartD,Section5.4.4.



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B - Chapter 4 - 19

0

FigureB.4.3:Typ

icalcrosssection,DC14,Flat

Terrain,Paved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m),minimum

3.3

6.0

6.5

B

Shoulderwidth(m)

1.5

1.0

1.25

B1

ShoulderCrossfall(%)

6

6

6

C

Crossfall/Cambe

r(%)

3

3

3

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

Depthofsideditch(m)

Varies

H

Crownheight(m

)

0.7

5

0.75

0.75

J

Clearedwidth(m

)

20

20

20

Notes:1

.


2.

V-shapeisthestandardshap

eofthedrainageditchconstructedbymo

torortowedgrader;

3.

Trapezoidaldrainsarecomm

onlyusedandaremucheasiertodigand

cleanusinglabour-intensivemethods.Theminimumrecommendedwidthis500mm

;

4.

Rectangulardrainsneedtob

elinedwithrock,brickstonemasonryorconcretetomaintaintheirshape;

5.

Moredetailonsidedrainsis

providedinPartD,Section5.4.4.



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FigureB.4.4:Typica

lcrosssection,DC14,Rolling

Terrain,Unpaved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m)

3.3

5.

0

5.5

6.0

B

Shoulderwidth(m)

0.6

0.

5

0.75

0.75

C

MinCrossfall/Ca

mber(%)

4

4

4

4

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

DepthofSideditch(m)

Varies

G

Sideslope(v:hratio)

SeeTableB.4.15

H

Crownheight(m

)

0.35

0.3

5

0.5

0.5

J

Clearedwidth(m

)

15

20

20

20

K

Embankmenttoe(m)

Varies

Notes:1

.

Sectionnotdrawntoscale.



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B - Chapter 4 - 21

0

FigureB.4.5:Typic

alcrosssection,DC14,RollingTerrain,Paved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m)

3.

3

6.0

6.5

B

Shoulderwidth(m)

1.

5

1.0

1.25

B1


6

6

6

C

Crossfall/Cambe

r(%)

3

3

3

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

Depthofsideditch(m)

Varies

G

Sideslope

SeeTableB.4.15

H

Crownheight(m

)

0.7

5

0.75

0.75

J

Clearedwidth(m

)

20

20

20

K

Embankmenttoe(m)

Varies

Notes:1

.




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B - Chapter 4 - 22

5;

/+3

0+/

/+5

"#

FigureB.4.6:Typicalcr

osssection,DC14,MountainousTerrain,Unpaved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m)

3.3

5.

0

5.5

6.0

B

Shoulderwidth(m)

0.6

0.

5

0.5

0.5

C

MinCrossfall/Ca

mber(%)

4

4

4

4

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

DepthofSideditch(m)

0.35

G

Sideslope(v:hratio)

SeeTableB.4.15

J

Clearedwidth(m

)

15

20

20

20

L

Ditchwidth(m)

Varies

Notes:1

.




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B - Chapter 4 - 24

"

%$!

0+/

/+5

/+3

$

$"%$%"

"#

FigureB.4.8:Typicalcrosssection:DC14,Escarpme

ntTerrain,Unpaved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m)

3.3

5.

0

5.5

6.0

B

Shoulderwidth(m)

0.6

0.

5

0.5

0.5

C

MinCrossfall/Ca

mber(%)

4

4

4

4

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

DepthofSideditch(m)

Min0.35

J

Clearedwidth(m

)

15

20

20

20

L

Ditchwidth(m)

Varies

M

Slopeofretainin

gstructure

Varies

Notes:1

.




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B - Chapter 4 - 25

"

%$!

0+/

/+5

/+3

$

$"%$%"

"#

0

FigureB.4.9:Typical

crosssection:DC14,Escarpm

entTerrain,Paved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A

Carriagewidth(m)

3.

3

5.5

6.5

B

Shoulderwidth(m)

1.

0

0.5

0.5

B1

Shouldercrossfa

ll(%)

3

3

3

C

Crossfall/Cambe

r(%)

3

6

3

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

DepthofSideditch(m)

Min0.5

J

Clearedwidth(m

)

20

20

20

L

Ditchwidth(m)

Varies

M

Slopeofretainin

gstructure

Varies

Notes:1

.




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B - Chapter 4 - 26

0+/

$0

/+4

0

$0

/+14

$1

0

3

3

FigureB.4.10Typicalcrosssection,DC14,Populat

edareas,Unpaved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A


3.3

5.

0

5.5

6.0

B

Shoulderwidth(m)

0.6

0.

5

0.75

0.75

C

MinCrossfall/Ca

mber(%)

4

4

4

4

J

Clearedwidth(m

)

15

20

20

20

Notes:1

.

OpenchanneltypeA25cm

thickmortaredstonepitching

OpenchanneltypeB25cm

thickmortaredstonepitching

2.

Wearingcourse

3.

Choiceofopenchanneldependentonlocalconditions

4.

Providelinedchannelsonlyw

heremaintenanceofroadsurfaceandcamberatoriginallevelsisguaranteed.



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B - Chapter 4 - 27

$

#

FigureB.4.13:Typicalcrosssection,DC14,FlatTerrain,Expansivesoils,Paved

Label

DesignCriteria

DesignClasses

DC1

DC

2

DC3

DC4

A


3.

3

6.0

6.5

B

Shoulderwidth(m)

1.

5

1.0

1.25

B1


6

6

6

C

Crossfall/Cambe

r(%)

3

3

3

D


SeeTableB.4.15

E

Sideslopeofdit

ch(v:hratio)

F

Depthofsideditch(m)

Varies

G

Sideslope

Varies

H

Crownheight(m

)

0.7

5

0.75

0.75

J

Clearedwidth(m

)

20

20

20

Notes:1

.




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B - Chapter 5 - 28

For materials specifications see Part D, Section 6.7.

5.1 Subgrades

Subgrades are classified on the basis of the laboratory soaked CBR tests on samples compacted to 97%AASHTO T180 compaction. Samples are soaked for four days or until zero swell is recorded. The subgradestrength for design is assigned to one of six strength classes reflecting the sensitivity of thickness designto subgrade strength. The classes are defined in Table B.5.1.

For the design of earth and gravel roads, if no suitable laboratory is available, the existing subgrade canbe assessed using a DCP at the time of the year that the soil is at its wettest.

Table B.5.1: Subgrade classes

Subgrade Class

Design CBR S2 S3 S4 S5 S6

Range % 3 - 4 5 - 8 9 - 14 15 - 29 30+

No allowance for CBRs below 3% has been made because, from both a technical and economicperspective, it would normally be inappropriate to lay a pavement on soils of such poor bearing capacity.For such materials, special treatment is required (see Section D.6.19.7).

The use of Class S2 soils as direct support for the pavement should be avoided as much as possible.Wherever practicable, such relatively poor soils should be excavated and replaced, or covered with animproved subgrade.

Class S6 covers all subgrade materials having a soaked CBR greater than 30 and which comply with theplasticity requirements for natural sub-base. In such cases, no sub-base is required.

5.1.1 Specifying the design subgrade class

The CBR results obtained from the subgrade soils testing are used to determine which subgrade classshould be specified for design purposes in accordance with Table B.5.1. The variation in results may makeselection unclear. In such cases it is recommended that, firstly, the laboratory test process is checked toensure uniformity (to minimise inherent variation arising from, for example, inconsistent drying out ofspecimens). Secondly, more samples should be tested to build up a more reliable basis for selection.Plotting these results as a cumulative distribution curve (S-curve) in which the y-axis is the percentageof samples less than a given CBR value (x-axis) provides a method of determining a design CBR value(Figure B.5.1).

MATERIALS5.

PART B: DESIGN STANDARDS FOR LOW VOLUME ROADS


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Figure B.5.1: Illustration of CBR strength cumulative distribution

The actual subgrade CBR values used for design depends on the traffic class as shown in Table B.5.2. Forexample, as indicated in the Table, for a design traffic class of LV5 the design CBR value should be thelower 10th percentile (ie the value exceeded by 90% of the CBR measurements).

Table B.5.2: Dependence of design subgrade on design traffic class

Traffic class Design CBR

LV5 (0.5-1.0 mesa) Lower 10-percentile

LV3 and LV4 (0.1-0.5 mesa) Lower 15-percentileLV1 and LV2 (


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B - Chapter 5 - 30

subgrade strength is available. In general, unnecessary working of the subgrade should be avoided andlimited to rolling prior to constructing overlying layers

For the stronger subgrades, especially Class S4 and higher (CBR 9-14% and more) the depth check is toensure that there is no underlying weaker material which could lead to detrimental performance.

It is recommended that the Dynamic Cone Penetrometer (DCP) be used during construction to monitor

the uniformity of subgrade support to the recommended minimum depths given in Table B.5.3.5.1.3 Improved subgrade layers

There are many advantages to improving the CBR strength of the in situ subgrade to a minimum of15% (Subgrade Class S5) by constructing one or more improved layers where necessary. In principle,where a sufficient thickness of improved subgrade is placed, the overall subgrade bearing strength isincreased to that of a higher class and the sub-base thickness may be reduced accordingly. This is oftenan economic advantage as sub-base quality materials are generally more expensive than fill materials,hence the decision whether or not to consider the use of an improved subgrade layer(s) will generallydepend on the respective costs of sub-base and improved subgrade materials.

5.1.4 Dealing with poor subgrade soils

Methods of design and treatment for problem soils are described in Part D Section 6.19.

5.2 Pavement Materials

The material code and characteristics of the material types for both paved and unpaved roads aredescribed in Table B.5.4.



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Table B.5.4: Pavement material types and abbreviated nominal specificationsused in the paved and unpaved catalogue of designs

Code Material Abbreviated Specifications

G80 Natural gravel

Min. CBR: 80% @ 98/100% AASHTO T180 and 4 days soakingMax. Swell: 0.2%

Max. Size and grading: Max size 37.5mm, grading as specified.PI: < 6 or as otherwise specified (material specific).

G65 Natural gravel

Min. CBR: 65% @ 98/100% AASHTO T180 and 4 days soakingMax. Swell: 0.2%Max. Size and grading: Max size 37.5mm, grading as specifiedPI: < 6 or as otherwise specified (material specific)

G55 Natural gravel

Min. CBR: 55% @ 98/100% AASHTO T180 and 4 days soakingMax. Swell: 0.2%Max. Size and grading: Max size 37.5mm, grading as specifiedPI: < 6 or as otherwise specified (material specific)

G45 Natural gravelMin. CBR: 45% @ 98/100% AASHTO T180 and 4 days soakingMax. Swell: 0.2%Max. Size and grading: Max size 37.5mm, grading as specifiedPI: < 6 or as otherwise specified (material specific)

G30 Natural gravel

Min. CBR: 30% @ 95/97% AASHTO T180 & highest anticipated moisturecontentMax. Swell: 1.0% @ 100% AASHTO T180Max. Size and grading: Max size 63mm or 2/3 layer thicknessPI: < 12 or as otherwise specified (material specific)

G25 Natural gravel

Min. CBR: 30% @ 95/97% AASHTO T180 & highest anticipated moisturecontentMax. Swell: 1.0% @ 100% AASHTO T180Max. Size and grading: Max sixe 63mm or 2/3 layer thickness.PI:


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5.2.1 Materials requirements for roadbase

A wide range of materials including lateritic, calcareous and quartzitic gravels, river gravels and othertransported and residual gravels, or granular materials resulting from weathering of rocks can be usedsuccessfully as road base materials.

Particle size distribution: The grading envelopes to be used for road base are shown in Table B.5.6.Envelope A varies depending whether the nominal maximum particle size is 37.5mm, 20mm or 10mm. Arequirement of five to ten per cent retained on successive sieves may be specified at higher traffic (>0.3mesa) to prevent excessive loss in stability. Envelope C extends the upper limit of envelope B to allowthe use of sandy materials, but its use is not permitted in wet climates. Envelope D is similar to a gravelwearing course specification, and is used for very low traffic volumes. The grading is specified only interms of the grading modulus (GM) and can be used in both wet and dry climates.

Table B.5.5: Particle size distribution for natural gravel base

Test Sievesize

Per cent by mass of total aggregate passing test sieve

Envelope ANominal maximum particle size Envelope B Envelope C

37.5mm 20mm 10mm50mm 100 100

37.5mm 80-100 100 80-100

20mm 55-95 80-100 100 55-100

10mm 40-80 55-85 60-100 40-100

5mm 30-65 30-65 45-80 30-80

2.36mm 20-50 20-50 35-75 20-70 20-100

1.18mm - - - - -

425m 8-30 12-30 12-45 8-45 8-80

300m - - - - -

75m 5-20 5-20 5-20 5-20 5-30

Envelope D1.65 < GM < 2.65

Strength and plasticity: The strength requirement varies depending on the traffic level and climateas outlined in the Catalogue of Structures (Chapter B.6). The soaked CBR test is used to specify theminimum road base material strength.

The plasticity requirement also varies depending on the traffic level and climate as shown in Tables B.5.7and B.5.8. A maximum plasticity index of 6 has been retained for higher traffic levels and also on weakersubgrades. For designs in dry environments the plasticity modulus for each traffic and subgrade class

can be increased depending on the crown height and whether unsealed or sealed shoulders are used asdescribed in Part D, Section 6.17.2 and Figure D.6.22.



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Table B.5.6: Plasticity requirements for natural gravel road base materials

Subgrade

class4

Property

Traffic class (Mesas)

LV1 LV2 LV3 LV4 LV5


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Table B.5.7: Guidelines for the selection of lateritic gravel road base materials

Subgrade

class

Property

Traffic class (Mesas)

LV1 LV2 LV3 LV4 LV5


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Cinder gravels: Cinder gravels have been used successfully as road base on experimental sectionsconstructed in the late 1970s (TRL, 1987). From these trials it was concluded that with careful selection,cinder gravels can be used for lightly trafficked paved roads in accordance with the requirements of thepavement design chart 2 (Table B.6.7)

5.2.2 Material requirements for sub-base

Strength requirements: A minimum CBR of 30% is required at the highest anticipated moisture content

when compacted to the specified field density, usually a minimum of 95% (preferably 97% wherepracticable) AASHTO T180 compaction.

Under conditions of good drainage and when the water table is not near the ground surface, the fieldmoisture content under a sealed pavement will be equal to or less than the optimum moisture contentin the AASHTO T180 compaction test. In such conditions, the sub-base material should be tested in thelaboratory in an unsaturated state.

If the road base allows water to drain into the lower layers, as may occur with unsealed shoulders andunder conditions of poor surface maintenance where the road base is pervious, saturation of the sub-base is likely. In these circumstances the bearing capacity should be determined on samples soaked inwater for a period of four days. The test should be conducted on samples prepared at the density and

moisture content likely to be achieved in the field.

Particle size distribution and plasticity requirements: In order to achieve the required bearing capacity,and for uniform support to be provided to the upper pavement, limits on soil plasticity and particle sizedistribution may be required. Materials which meet the recommendations of Tables B.5.9 and B.5.10 willusually be found to have adequate bearing capacity.

Table B.5.8: Typical particle size distribution for sub-bases

Sieve Size (mm)Per cent by mass of total

aggregate passing test sieve

50 100

37.5 80 10020 60 80

5 30 100

1.18 17 75

0.3 9 50

0.075 5 - 25

Table B.5.9: Plasticity characteristics for granular sub-bases

Climate Liquid Limit Plasticity Index Linear Shrinkage

Moist tropical and wet tropical (N


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B - Chapter 5 - 36

An alternative to using linear shrinkage and the shrinkage product is to use the plasticity index and theassociated plasticity product. For the range of materials likely to be used for gravel wearing course, theplasticity index can be assumed to be 2 x linear shrinkage. The linear shrinkage (shrinkage product) isrecommended as it is based on one relatively simple test which has good precision limits in the shrinkageranges of acceptable gravel wearing course material.

Figure B.5.2: Material quality zones

The characteristics of materials in each zone are as follows:A: Materials in this area generally perform satisfactorily but are finely graded and particularly prone

to erosion. They should be avoided if possible, especially on steep grades and sections with steepcross-falls and super-elevations. Roads constructed from these materials require frequent periodiclabour intensive maintenance over short lengths and have high gravel losses due to erosion.

B: These materials generally lack cohesion and are highly susceptible to the formation of loosematerial (ravelling) and corrugations. Regular maintenance is necessary if these materials are usedand the road roughness is to be restricted to reasonable levels.

C: Materials in this zone generally comprise fine, gap-graded gravels lacking adequate cohesion,resulting in ravelling and the production of loose material.

D: Materials with a shrinkage product in excess of 365 tend to be slippery when wet.E: Materials in this zone perform well in general, provided the oversize material is restricted to the

recommended limits.

Gravel loss: Gravel loss is the single most important reason why gravel roads are expensive in wholelife cost terms and often unsustainable, especially when traffic levels increase. Reducing gravel loss byselecting better quality gravels or modifying the properties of poorer quality materials is one way ofreducing long term costs. Gravel losses (gravel loss in mm/year/100vpd) are determined in relation to

the quality of the gravel wearing course (Table B.5.11).



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Table B.5.10: Typical standardised gravel loss

Material Quality Zone1 Material Quality Typical gravel loss (mm/yr/100vpd)

Zone A Satisfactory 20

Zone B Poor 45

Zone C Poor 45

Zone D Marginal 30Zone E Good 10

Notes:1. See Figure B.5.2

The gravel losses shown in Table B.5.11 probably hold only for the first phase of the deterioration cyclelasting possibly two or three years. Beyond that period, as the wearing course is reduced in thickness,other developments, such as the formation of ruts, will also affect the loss of gravel material. However,the rates of gravel loss given in the Table can be used as an aid to the planning for regravelling in thefuture. A more accurate indication of gravel loss for a particular section of road can be obtained fromperiodic measurement of the gravel layer thickness.

Material requirements for gravel roads in rural areas: Table B.5.12 shows the recommended specificationsfor materials for unsealed rural roads

Table B.5.11: Recommended material specifications(1,3)for unsealed rural roads

Maximum size (mm)Oversize index (I

o)a

Shrinkage product (Sp)b (2)

Grading coefficient (Gc)c (2)

Soaked CBR (at 95 per cent Mod AASHTO)Treton impact value (%)(4)

37.5 5 %

100 - 365 (max. of 240 preferable)16 - 34 15 %20 65

a Io = Oversize index (percent retained on 37.5 mm sieve

b Sp = Linear shrinkage x percent passing 0.425 mm sievec Gc = (Percentage passing 26.5 mm - percentage passing 2.0 mm) x percentage passing4.75 mm)/100

Notes:1. Specifications should be applicable after placement and compaction2. The Grading Coefficient and Shrinkage Product must be based on a conventional particle size distribution determination

which must be normalised for 100% passing the 37.5 mm screen.3. Only representative material samples are to be tested.4. The Treton Impact Value (TIV) limits exclude those materials that are too hard to be broken with a grid roller (TIV < 20%)

or too soft to resist excessive crushing under traffic (TIV > 65%).

Material requirements for gravel roads in urban areas: The specifications in Table B.5.13 arerecommended for unsealed roads in areas where there is a significant number of dwellings and localbusinesses. In comparison with the limits for rural roads, the limits for the oversize index have been reducedto eliminate stones whilst the shrinkage product has been reduced to a maximum of 240 to reduce thedust as far as practically possible. This lower limit reduces the probability of having unacceptable dustfrom about 70% to 40%.



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Table B.5.12: Recommended material specifications for unsealed urban roads

Maximum size (mm)

Oversize index (Io)

Shrinkage product (Sp)

Grading coefficient (Gc)

Soaked CBR (at 95 per cent Mod AASHTO)

Treton impact value (%)

37.5

0

100 - 240

16 - 34

15 %

20 65

5.2.4 Material Improvement

Obtaining materials that comply with the necessary grading and plasticity specifications for a gravelwearing course can be difficult. Many of the natural gravels tend to be coarsely graded and relatively nonplastic and the use of such materials results in very high roughness levels and high rates of gravel loss inservice and, in the final analysis, very high life-cycle costs.

In order to achieve suitable wearing course properties a suitable Particle Size Distribution (PSD) can beobtained by breaking down oversized material to a maximum size of 50 mm or smaller. Atterberg limitsmay be modified by granular/mechanical stabilisation (blending) with other materials. These material

improvement measures are discussed in Part D, Section 6.7.6.



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6.1 Design traffic classes

For structural pavement design, five traffic classes have been defined as shown in Table B.6.1. If theestimate of cumulative traffic is close to the boundaries of a traffic class, then the basic traffic data and

forecasts should be re-evaluated and sensitivity analyses carried out to ensure that the choice of trafficclass is appropriate. If there is any doubt about the accuracy of the traffic estimates the next higher trafficclass should be selected for the design.

Table B.6.1: Traffic classes for flexible pavement design

Traffic range(mesas)

LV1 LV2 LV3 LV4 LV5/T2(1)

< 0.01 0.01 0.1 0.1 0.3 0.3 0.5 0.5 1.0

LV5/T2 is the transition traffic zone between low-volume and high-volume roads with the former trafficclass (LV5) applying to the lower boundary of the traffic range and the latter traffic class (T2) applying tothe upper boundary.

6.2 Engineered natural surfaces

The following design standards are recommended for Engineered Natural Surfaces (ENS) in the DC1design class carrying < 25 vpd. The details of the cross-section are given in Chapter B.4 but shownschematically in Figure B.6.1 for convenience. Further supporting information is given in Section 6.15 ofPart D.

Figure B.6.1: Cross-section details ENS

The crown height of the earth road should be at least 35 cm above the bed of the drain. Where the topography allows, wide, shallow longitudinal drainage for earth roads are preferred.

They minimise erosion, and will not block as easily as narrow ditches. The ditches grass over intime, binding the soil surface and further slowing down the speed of water, both of which act to

prevent or reduce erosion. The surface of earth roads should be graded and compacted to provide a durable and levelrunning surface for traffic and the road surface should have a minimum camber of 4% to ensurewater runs off the surface and into the side drains.

Areas where there are specific problems (usually due to water or to the poor condition of thesubgrade) may be treated in isolation by localised replacement of subgrade, gravelling, installationof culverts, raising the roadway or by installing other drainage measures. This is the basis of a spotimprovement approach.

Water should be drained away from the carriageway side drains by installing lead off (mitre) drains,to divert the flow into open space.

PAVEMENT DESIGN6.



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6.3 Natural gravel roads

A gravel road consists of a wearing course and a structural layer (base) which covers the in situ material.The minimum thickness of the structural layer is maintained in service by providing a wearing coursethroughout the design life of the road.

To achieve adequate external drainage, the road must also be raised above the level of existing ground

such that the crown of the road is maintained at a minimum height (hmin) above the table drain inverts.Cross sections are shown in detail in Chapter B.4 and shown here schematically for convenience (FigureB.6.2).

Figure B.6.2 Typical gravel road cross section in flat terrain.

The minimum height is dependent on the climate and road design class as shown in Table B.6.2.

Table B.6.2: Required minimum height (hmin) between road crownand invert level of drain in relation to climate

Road Class

Climate

Wet (N < 4) Dry (N > 4)

hmin (mm) hmin (mm)DC-1 350 250

DC-2 400 450

DC-3 500 300

DC-4 350 400

Gravel roads are divided into two broad categories for design purposes namely major and minor gravelroads. Gravel roads in classes DC3 and DC4 are defined as major gravel roads, minor gravel roads areclasses DC1 and DC2, except where the number of heavy vehicles exceeds about 10 per day. Majorgravel roads are engineered to a higher specification.

6.3.1 Major gravel roads

For major gravel roads the approach is as follows: The sub-grade should be prepared in the same way as for a low volume sealed road. It is assumed that the wearing course will be replaced at intervals related to the expected annual

gravel loss and before the structural layer is exposed to traffic and itself begins to wear away; The geometry and drainage are upgraded to acceptable minimum levels during construction.

This may require the introduction of a fill layer between the compacted in situ sub-grade and thewearing course.

Major gravel roads are likely to incur high maintenance costs in some circumstances namely; When the quality of the gravel is poor. Where no sources of gravel are available within a reasonable haul distance.



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On road gradients greater than about 6%. In areas of high and intense rainfall.

In these circumstances spot improvements will almost certainly be justified, and, in some cases, it mayprove to be more economical to build a fully paved road at the outset.The structural design procedure for major gravel roads: The design procedure consists of the followingsteps:

Determine the traffic volume and traffic loading (Section B.3.4). Determine the strength of the sub-grade at the appropriate moisture condition (Section B.5.1.1). Establish the quality of the gravel that is to be used (Section B.5.2.3). If only very poor gravel is

available, blending with another gravel or soil to improve its properties may be an option (SectionB.5.2.5).

Determine the thickness of gravel base that is necessary to avoid excessive compressive stressesin the sub-grade from Tables B.6.3 (a), (b) and (c).

Calculate the thickness of the wearing course based on the expected rate of gravel loss and arealistic choice of the frequency of re-gravelling.

Table B.6.3 (a): Gravel base thickness for major gravel roads strong gravel (G45)

SubgradeStrength Class

CBR (%)

Traffic Classes (mesas)

LV1(


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The following points should be noted: The thicknesses required increases considerably if the gravel is weak hence stronger gravels should

generally be used if they are available at reasonable cost. On relatively weak subgrades (S2 and S3), the use of strong gravels (G45) should be avoided

because of the poor balance of such pavements. Instead, the use of an improved subgrade layershould be considered (Section B.5.1.4).

Where the available gravel is not homogeneous, it will be necessary to substitute a particular

class of gravel with one or more different classes of gravel of appropriate thickness. The followingconversion factors may be used for this purpose.G45 = 1.5 x G15G30 = 1.3 x G15

Thus, a 200mm layer of G45 material could be substituted with a 300 mm layer of G15 material.

For effective compaction of the gravel layer, it is necessary to restrict the loose thickness of gravel to amaximum lift of about 200 mm. Thus, any of the gravel layers that require a compacted thickness of morethan 150 mm must be compacted in more than one 200 mm lift.

Determination of wearing course thickness: The wearing course thickness depends on the annual gravelloss and the number of years between re-gravelling operations. The predicted annual gravel loss is givenin Table B.6.4.

Table B.6.4: Typical gravel loss

Material Quality Zone(1) Description of Material QualityTypical gravel loss (mm/

yr/100vpd)

Zone A Satisfactory 20

Zone B Poor 45

Zone C Poor 45

Zone D Marginal 30

Zone E Good 10

Note:1. See Figure B.5.2

The rates of gravel loss increase significantly on gradients greater than about 6% and in areas of high andintense rainfall. On some gradients, the increase could be greater than 50% depending on the steepnessof the gradient and material quality. Spot improvements should be considered on these sections.

Re-gravelling should take place before the sub-base is exposed. The re-gravelling frequency, R, is typicallyin the range 5 - 8 years. This decreases considerably if poor quality gravels have to be used. For example,if the gravel quality is in zones B or C, the loss rate will be 45mm per year per 100vpd. Therefore a classDC4 gravel road carrying 200vpd will lose 90mm per year and require re-gravelling every two years

The wearing course thickness = R x GL

R = regravelling frequency in yearsGL = annual gravel loss.

6.3.2 Minor gravel roads

The approach to the design of minor gravel roads is as follows: The design chart (Table B.6.5) is based on the AADT (not the cumulative esas) of the road and

assumes the traffic includes approximately 30% of vehicles of classes 3 and above (as defined inTable B.3.1);

The subgrade materials need not necessarily comply with the requirements of a low volumesealed road;

A nominal wearing course thickness of 150 mm of G15 is assumed for all road classes and sub-grade conditions with the sub-base thickness being influenced by the sub-grade class;



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Drainage, but not necessarily geometry, is upgraded to acceptable minimum levels duringconstruction. As for Class DC3 and DC4 roads, this can be achieved by building up the formationto an appropriate height to achieve the hmin requirements given in Table B.6.2.

The recommended sub-base thicknesses and wearing course material strengths for different sub-grade and traffic conditions are shown in Table B.6.5.

Table B.6.5: Design Chart for minor gravel roads

Subgrade StrengthClass CBR (%)

Traffic Classes (AADT)

DC1/DC2(1)

(< 75)

S2 (3-4)150 WC

200 G15 (2)

S3 & S4 (5-14) 150 WC

S5 (15-29) Earth Road

Notes:1. If more than 10 heavy vehicles per day, design as a major gravel road2. If a G30 material is available the thickness can be reduced to 150 mm

6.4 Surfacing options and design standards for paved roads

The types of surfacing options and a rational procedure for selecting appropriate surface options iscontained in Part D, Chapter 7, along with the advantages and potential concerns regarding each option.Some surface options are not appropriate for the higher traffic categories and are marked accordingly.

6.4.1 Bituminous surfaced roads

The design standards for paved roads with a bituminous surface assume a flexible pavement with agranular base/sub-base. Table B.5.5 shows the material types for the various structural layers used in thecatalogues. For sub-bases, G30 and G25 materials are both suitable but G30 is preferred.

The design charts for roads with bituminous road surfaces are shown in Tables B.6.6 and B.6.7. The use

of the charts is described as follows.

Climatic zones N < 4(a) Where the total sealed surface is 8 m or less, use Pavement Design Chart 1 (Table B.6.6). No road

base materials adjustments are allowed.(b) Where the total sealed surface is 8 m or more, use Pavement Design Chart 2 (Table B.6.7). The limit

on the plasticity modulus of the road base may be increased by 20 per cent.(c) (Where the total sealed surface is less than 8m but the pavement is on an embankment in excess of

1.2 m in height, use Pavement Design Chart 2 (Table B.6.7). The limit on the plasticity modulus ofthe road base may be increased by 20 per cent.

(d) If the engineer deems that other risk factors (eg poor maintenance and/or construction quality) aretoo high, then Pavement Design Chart 1 should be used.

Climatic zones N > 4Use Pavement Design Chart 2 (Table B.6.7).(a) Where the total sealed surface is less than 8 metres, the limit on the plasticity modulus of the road

base may be increased by 40%.(b) Where the total sealed surface is over 8 metres and when the pavement is on an embankment in

excess of 1.2 metres in height, the plasticity modulus of the road base may be increased by up to40% and the plasticity index by 3 units.

Once the quality of the available materials and haul distances are known, the flow chart shown in FigureD.6.22 of Part D and the design charts can be used to review the most economical cross-section and



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pavement; this involves assessment of design traffic class, design period, cross-section and otherenvironmental and design considerations.

When the project is located close to the border between the two climatic zones, the lower N-value shouldbe used to reduce risks.

When the design is close to the borderline between two traffic design classes, and in the absence of more

reliable data, the next highest design class should be used.

It may be more economical to use a wider cross-section in the seasonal tropical and wet climate zone andthen use Pavement Design Chart 2 rather than to design a narrow cross-section and a pavement usingPavement Design Chart 1.

The design charts do not cater for weak subgrades (CBR < 3%) and other problem soils. Design guidancefor these conditions is given in Part D, Section 6.19.2.

Table B.6.6: Bituminous Pavement Design Chart 1

SG CBRLV1 LV2 LV3 LV4 LV5

30%) 150 G45 150G65 175 G65 175 G65 200 G80

Table B.6.7: Bituminous Pavement Design Chart 2


30%) 150 G45 150 G45 150 G55 150 G55 175 G65

6.4.2 Non bituminous surfaced roads

Table B.6.8 lists the non-bituminous pavement (NBP) options with their respective design charts.



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Table B.6.8: Non-bituminous pavement surfacing options

NBP Option Code Ref. Table

Water-bound and Dry-bound Macadams WBM and DBM B.6.10

Hand-Packed Stone HPS B.6.11

Stone Setts or Pav SSP and MSSP B.6.12

Cobblestone/ Dressed Stone CS, DS & MCS, MDS B.6.12Fired Clay Brick CB, MCB B.6.12

Non reinforced Concrete NRC B.6.13

Ultra-thin Reinforced Concrete UTRCP B.6.14

In Tables B.6.10 to B.6.14, unbound gravel material is used for capping, subbase and road base. In manycases the specifications for the strength of these materials is flexible and, depending on the materialsavailable, substitutions can be made. It is indicated in the Tables where substitutions are allowed andwhere they are restricted. Table B.6.9 defines the allowable substitutions. Table B.6.9 is used by simplytaking the ratio of thicknesses of the material to be used and the material designated in the thicknessdesigns in Tables B.6.10 to B.6.14 and scaling the thickness given in the Tables appropriately. For example,

if the thickness of a G45 material is given as 150mm in the Tables and a G80 material was more readilyavailable the thickness required becomes:150 x 65/80 = 122mm

Table B.6.9: Substitution of pavement layer material

MaterialDesignation

Material CBR(%)

Required thickness(mm)

G15 15 100

G30 30 90

G45 45 80

G65 65 70

G80 80 65

Water-bound and Dry-bound Macadam (WBM and DBM)A Macadam layer consists of a stone skeleton of single sized coarse aggregate in which the voids are filledwith finer material. The stone skeleton, because it is a single size large material, contains considerablevoids which are filled by fine aggregate which is washed or slushed into the coarse skeleton withwater. Dry-bound macadam is a similar technique to the original WBM, however instead of water anddeadweight compaction being used in the consolidation of fine material, a small vibrating roller is used.WBM or DBM are commonly used as layers within a sealed flexible pavement, but in the appropriatecircumstances may be used as an unsealed option with a suitably cohesive material being used as thefines component. The WBM or DBM may be constructed as a low cost, initial surface to be later sealed

and upgraded in a stage construction strategy.

WBM is suitable for labour based construction and should provide a relatively high quality surface layersimilar to a good quality natural gravel surface. However, like gravel, it is worn away by traffic and rainfalland therefore requires similar maintenance.

The structural designs for WBM are similar to those required for a gravel road as shown in Table B.6.10with the WBM itself acting as the wearing course. A capping layers and a sub-base are required asindicated but thicknesses can be reduced if stronger material is available.



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Table B.6.10: Thickness designs for WBM pavements


30%)150 WBM 150 WBM 150 WBM

NA NANOTE NOTE NOTE

Notes:1. The capping layer of G15 material and the subbase layer of G30 material can be reduced in thickness if stronger

material is available (Table B.6.9)2. On sub-grade > 15%, the material should be scarified and re-compacted to ensure the depth of material of in situ CBR

>15% is in agreement with the recommendations in Figure D.6.7 and Table D.6.7.

Hand-Packed Stone (HPS)HPS paving consists of a layer of large broken stone pieces (typically 150 to 300mm thick) tightly packedtogether and wedged in place with smaller stone chips rammed by hand into the joints using hammersand steel rods. The remaining voids are filled with sand or gravel. A degree of interlock is achieved andhas been assumed in the designs shown in Table B.6.11. The structures also require a capping layer whenthe subgrade is weak and a conventional sub-base of G30 material or stronger.

The HPS is normally bedded on a thin layer of sand (SBL). An edge restraint or kerb constructed, forexample, of large or mortared stones improves durability and lateral stability.



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Table B.6.11: Thicknesses designs for Hand Packed Stone (HPS) pavement (mm)


30%)

150 HPS 200 HPS 200 HPS 250 HPS

NA50 SBL 50 SBL 50 SBL 50 SBL

NOTE NOTE NOTE NOTE

Notes:1. The capping layer of G15 material and the subbase layer of G30 material can be reduced in thickness if stronger

material is available (Table B.6.9)2. On sub-grades > 15%, the material should be scarified and re-compacted to ensure the depth of material of in situ CBR

>15% is in agreement with the recommendations in Table B.5.3

Stone Sett or Pav Pavements (SSP or MSSP).Stone sett surfacing or Pav consists of a layer of roughly cubic (100mm) stone setts laid on a bed ofsand or fine aggregate within mortared stone or concrete edge restraints. The individual stones shouldhave at least one face that is fairly smooth to be the upper or surface face when placed. Each stone settis adjusted with a small (masons) hammer and then tapped into position to the level of the surroundingstones. Sand or fine aggregate is brushed into the spaces between the stones and the layer is thencompacted with a roller. Suitable structural designs are shown in Table B.6.12.



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Table B.6.12: Thicknesses designs for various discrete element surfacings (mm)


30%)

100 SSP 100 SSP 100 SSP 100 SSP 100 SSP

25 SBL 25 SBL 25 SBL 25 SBL 25 SBL

125 G65 150 G80 150 G80 150 GBO 175 G80

NOTE NOTE NOTE NOTE NOTENotes:

1. The capping layer of G15 material and the subbase layer of G30 material can be reduced in thickness if strongermaterial is available (Table B.6.9)

2. The capping layer can be G10 provided it is laid 7% thicker3. The road base layers (G65 and G80) must not be weaker4. The subbase layers can be material stronger than G30 and laid to reduced thickness as shown in Table B.6.95. On sub-grades > 15%, the material should be scarified and re-compacted to ensure the depth of material of in situ CBR

>15% is in agreement with the recommendations in Table B.5.3.

Cobblestone or Dressed Stone Pavement (CS, DS, MCS or MDS)Cobble or Dressed Stone surfacing consists of a layer of roughly rectangular dressed stone laid on a bedof sand or fine aggregate within mortared stone or concrete edge restraints. The individual stones shouldhave at least one face that is fairly smooth, to be the upper or surface face when placed. Each stone isadjusted with a small (masons) hammer and then tapped into position to the level of the surroundingstones. Sand or fine aggregates is brushed into the spaces between the stones and the layer thencompacted with a roller. Cobble stones are generally 150 mm thick and dressed stones generally 150-200mm thick. These options are suited to homogeneous rock types that have inherent orthogonal stresspatterns (such as granite) that allow for easy break of the fresh rock into the required shapes by labourbased means.

The thickness designs are given in Table B.6.12 except that the thickness of the cobblestone is generally150mm instead of 100mm shown in the Table.



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Fired Clay Brick PavementFired Clay Bricks are the product of firing molded blocks of silty clay. The surfacing consists of a layer ofedge-on engineering quality bricks within mortar bedded and jointed edge restraints, or kerbs, on eachside of the pavement. The thickness designs are as shown in Table B.6.12 for LV1 and LV2. Fired clay bricksurfacings are not suitable for traffic classes above LV2.

Mortared options

In some circumstances (eg on slopes in high rainfall areas and volume susceptible sub-grade) it may beadvantageous to use mortared options. This can be done with Hand-packed Stone, Stone Setts (or Pav),Cobblestone (or Dressed Stone), and Fired Clay Brick pavements. The construction procedure is largelythe same as for the un-mortared options except that cement mortar is used instead of sand for beddingand joint filling. The behaviour of mortared pavements is different to that of sand-bedded pavementsand is more analogous to a rigid pavement than a flexible one. There is, however, little formal guidanceon mortared option, although empirical evidence indicates that inter-block cracking may occur. For thisreason the option is currently only recommended for the lightest traffic divisions up to LV2 (Tables B.6.12)until further locally relevant evidence is available.

Non Reinforced Concrete (NRC)The non-reinforced cement concrete option for LVRs involves casting slabs of 4.0 to 5.0 metres in lengthbetween formwork with load transfer dowels between them. In some cases, where continuity of trafficdemands it, these slabs may be half carriageway width.

Table B.6.13: Thicknesses (mm) - Non-Reinforced Concrete Pavement (NRC)


30%) 150 NRC 150 NRC 160 NRC 170 NRC 180 NRC

Notes:1. Cube strength = 30 MPa at 28 days2. On sub-grades > 30%, the material should be scarified and re-compacted to ensure the depth of material of in situ CBR

>30% is in agreement with the recommendations in Table B.5.3

Ultra-thin Reinforced Concrete Pavement (UTRCP)An Ultra-thin Reinforced Concrete Pavement (UTRCP) option has been developed in South Africa for a

low maintenance surfacing suitable for LVRs. A thin (50mm) layer of reinforced concrete is used as a rigidstructural surfacing over a good sub-base layer comprising well compacted good quality material, thetop 150mm of which should have an effective CBR of 80%. The pavement layers below a UTRCP slabmust contribute significantly to the strength of the pavement as a whole.

It should be emphasised that the formal design approach for this option is still under development andthat its use within an Ethiopian LVR road environment should be undertaken with caution.Areas where the use of UTRCP can be considered include:

Surfacing of a new road or the rehabilitation/upgrading of an existing road; All traffic and road classes from low-volume urban streets to inlays, to provincial roads where

typical traffic volumes are below 2 000 vehicles per day with less than 5% heavy vehicles (at thisstage);



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Areas of steep grades and stop/start heavy traffic; Areas where regular maintenance is unlikely.

The concrete is only 50mm thick and therefore tolerances are critical. The success of the UTRCP process istherefore dependent on attention to detail. This applies not only to the concrete layer (concrete strength,thickness, placing, curing) but also to the placing, supporting and joining of the steel mesh panels, aswell as the tolerances of the layer supporting the UTRCP. The need for meticulous monitoring and control

during construction cannot be over-emphasised. Competent site staff must be intensively involved in allthe processes associated with and control of all the construction activities.

Table B.6.14: Ultra-Thin Reinforced Concrete Pavement (UTRCP) Design

SG CBR%Traffic(2)

Low Medium High

S2 (3-4%)50 RC(1)

150 G80200 G30

50 RC150 G80250 G30

50 RC150 G80350 G30

S3 (5-7%)50 RC

150 G80

125 G30

50 RC150 G80

150 G30

50 RC150 G80

200 G30

S4 (8-14%)50 RC

150 G80

50 RC150 G80100G30

50 RC150 G80150 G30

S5 (15-29%)50 RC

100 G8050 RC

125 G8050 RC

150 G80

S6 (>30%)50 RC

75 G8050 RC

100 G8050 RC

100 G80

Notes:1. Concrete must have a 28-day cube strength of 30MPa2. The currently suggested traffic divisions are

L A 30kN wheel load division suggested for urban streets,M A 40kN wheel load division for bus routes, andH A 60kN wheel load division for provincial roads carrying up to 2000 vpd (10% heavy)



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The maximum water flow in a watercourse can be estimated using the following methods: Direct observation of the size of watercourse, erosion and debris on the banks, history and local

knowledge; The Rational Method. The SCS method (USA Soils Conservation Services, TR-55)

A combination of these methods should be used to provide the maximum level of reliability.

7.1 Size of watercourse

Watercourses enlarge to a size sufficient to accommodate the maximum water flow. The cross-sectionalarea of the water course is measured and a cross-sectional area of apertures of the structure provided thatis equal to that of the water course. If the return period of the maximum flow is much longer than that forwhich the structure is being designed, the typical high water level can be estimated from lateral erosionon the banks or debris caught in the branches of trees. The cross-sectional area of the water course tothis level is calculated and a structure provided with cross-sectional area of apertures equal to this area.

Future high water levels can also be estimated from recorded history, including measurements taken inthe watercourse or from the recollections of local residents.

7.2 The Rational Method

The flow of water in a channel, q, is calculated from the following equation.

q = 0.278 x C x I x A (m3/s) Equation B.7.1Where:

C = the catchment runoff coefficientI = the intensity of the rainfall (mm/hour)A = the area of the catchment (km2)

7.2.1 Catchment runoff coefficient, C

C is obtained from Table B.7.1 and Table B.7.2.

Table B.7.1: Runoff coefficient: Humid catchment

AverageGround Slope

Soil Permeability

Very low(rock & hard clay)

Low(clay loam)

Medium(sandy loam)

High(sand & gravel)

Flat 0-1% 0.55 0.40 0.20 0.05

Gentle 1-4% 0.75 0.55 0.35 0.20

Rolling 4-10% 0.85 0.65 0.45 0.30Steep >10% 0.95 0.75 0.55 0.40

DRAINAGE AND EROSION CONTROL7.



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Table B.7.2: Runoff coefficient: Semi-arid catchment

AverageGround Slope

Soil Permeability

Very low(rock & hard clay)

Low(clay loam)

Medium(sandy loam)

High(sand & gravel)

Flat 0-1% 0.75 0.40 0.05 0.05

Gentle 1-4% 0.85 0.55 0.20 0.05Rolling 4-10% 0.95 0.70 0.30 0.05

Steep >10% 1.00 0.80 0.50 0.10

7.2.2 Rainfall intensity, I (mm/hour)

The intensity of rainfall (I) is obtained from the Intensity-Duration-Frequency charts in Annex A. The stormduration is equal to the Time of Concentration (Tc). Tc is the time taken for water to flow from the farthestextremity of the catchment to the crossing site.

Tc = Distance from farthest extremity (m) / Velocity of flow (m/s) Equation B.7.2

The velocity of flow depends on the catchment characteristics and slope of the watercourse. It isestimated from Figure B.7.1.

The storm design return period is taken from Table B.7.3. If the route is of strategic importance, or ifthe alternative route in the event of a drainage failure is more than an additional 75km or if there is noalternative route, Table B.7.4 should be used.

Figure B.7.1: Velocity of flow



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Table B.7.3: Storm design return period (years)

Structure typeGeometric design standard

DC4 DC3 DC2 DC1

Gutters and inlets 2 2 2 1

Side ditches 10 5 5 2

Ford 10 5 5 2Drift 10 5 5 2

Culvert diameter 2m 25 15 10 5

Gabion abutment bridge 25 20 15 -

Short span bridge (50m 100 100 50 -

Table B.7.4: Storm design return period (years) for severe risk situations

Structure typeGeometric design standard

DC4 DC3 DC2 DC1

Gutters and inlets 5 5 5 2

Side ditches 15 10 10 5

Ford 15 10 10 5

Drift 15 10 10 5

Culvert diameter 2m 50 25 20 10

Gabion abutment bridge 50 25 20 -

Short span bridge (50m 100 100 100 -

7.2.3 Catchment area, A (km2)

The area of the drainage catchment should be estimated from a map or an aerial photograph.In the Rational Method it is assumed that the intensity of the rainfall is the same over the entire catchmentarea. The consequence of applying the method to large catchments is an over-estimate of the flow andtherefore a conservative design.

A simple modification can be made to take into account the spatial variation of rainfall intensity across alarger catchment. The effective area of the catchment is reduced by multiplying by the areal reductionfactor (ARL) given by the following equation:



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ARL = 1 - 0.04 x t-1/3 x A1/2 Equation B.7.4

Where,t = storm duration in hoursA = catchment area in km2

7.3 The SCS method

The SCS method for calculating rates of runoff requires much of the same basic data as the RationalMethod namely catchment area, a runoff factor, time of concentration, and rainfall. However the SCSmethod also considers the time distribution of the rainfall, the initial rainfall losses to interception andstorage, and an infiltration rate that decreases during the course of a storm. It is therefore potentiallymore accurate than the Rational Method and is applicable when the catchment area is larger than 50hectares.

7.3.1 Catchment area

The catchment area is determined from topographic maps and field surveys. For large catchment areasit might be necessary to divide the area into sub-catchment areas to account for major land use changes.

7.3.2 Rainfall

The SCS method is based on a 24-hour storm event. The characteristics of storms are defined in termsof the relationship between the percentage of the total storm rainfall that has fallen as a function oftime. Three basic types of storm are defined for three levels of maximum intensity, Type I being the leastintense and Type III being the most intense. In Ethiopia a Type II distribution is used (see ERAs DrainageDesign Manual 2002 or the revised version when available).

A relationship between accumulated rainfall and accumulated runoff was derived by SCS for numeroushydrologic and vegetative cover conditions. The storm data included total amount of rainfall in a calendarday but not its distribution with respect to time. The SCS runoff equation is therefore a method ofestimating direct runoff from 24-hour or 1-day storm rainfall.

The equation is:Q = (P - I

a)2 / [(P - I

a) + S] Equation B.7.5

Where:Q = accumulated direct runoff, mm.P = accumulated rainfall (ie, the potential maximum runoff), mmIa

= initial abstraction including surface storage, interception, and infiltration priorto runoff, mm.

S = potential maximum retention, mm.

S is related to the soil and cover conditions of the catchment area through the Curve Numbers, CN,described below.

S = 25.4(1000/CN 10) Equation B.7.6The relationship between I

aand S was found to be;

Ia = 0.2S = 50.8.(100/CN-1) Equation B.7.7Substituting into Equation B.7.5,

Q = [P 50.8(100/CN - 1)]2/[P + 203.2(100/CN - 1)] Equation B.7.8

Figure B.7.2 shows a graphical solution which enables Q, the direct runoff from a storm, to be obtainedif the total rainfall and catchment area curve number are known.



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Figure B.7.2: Relationship between Precipitation, Direct Runoff and CN

7.3.3 Runoff and Curve Numbers

The physical catchment area characteristics affecting the relationship between rainfall and runoff (ie theCN values) are land use, land treatment, soil types, and land slope.

Land use is the catchment area cover and it includes agricultural characteristics, type of vegetation,water surfaces, roads and roofs. Land treatment applies mainly to agricultural land use, and it includesmechanical practices such as contouring or terracing and management practices such as rotation ofcrops. The SCS method uses a combination of soil conditions and land-use to assign a runoff factor toan area. These runoff factors or curve numbers (CN), indicate the runoff potential of an area. The higherthe CN, the higher is the runoff potential.

Soils are divided soils into four hydrologic groups (Groups A, B, C, and D) based on infiltration rates(Table B.7.5). These groups are described in detail in the ERA Drainage Design Manual.

Table B.7.5: Hydrological characteristic soil groups

Soil Group General Description

A Well drained, sandy High infiltration, low runoff

B Sandy loam, low plasticity

C Clay loam, medium plasticity

D Highly plastic clay Low infiltration, high run off

Runoff curve numbers also vary with the antecedent soil moisture conditions, defined as the amount ofrainfall occurring in a selected period preceding a given storm. In general, the greater the antecedentrainfall, the more direct runoff there is from a given storm. A five-day period is used as the minimum forestimating antecedent moisture conditions.



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Table B.7.6 gives runoff curve numbers for various land uses. (NB: More comprehensive tables are givenin the ERA Drainage Design Manual). This Table is based on an average antecedent moisture condition(ie soils that are neither very wet nor very dry when the design storm begins). Table B.7.7 gives conversionfactors to convert average curve numbers to wet and dry curve numbers. The recommended antecedentmoisture conditions (AMC) in Ethiopia are shown in Table B.7.8.

Table B.7.6: Runoff Curve Numbers (CN)

Land use A B C D

Cultivated landWithout conservation treatment 72 81 88 91

With conservation treatment 62 71 78 81

Pasture land Poor condition 68 79 86 89

Good condition 39 61 74 80

Meadow 30 58 71 78

Wood or forestThin stand, poor cover, no mulch 45 66 77 83

Good cover 25 55 70 77

Open spaces, lawns,parks

Good condition, grass cover >75% of

area

39 61 74 80

Fair condition, grass on 50-75% 49 69 79 84

Urban districts

Commercial and business areas, 85%impervious

89 92 94 95

Industrial districts, 70% impervious 81 88 91 93

Residential

Average lot size Average % impervious

< 0.05 hectares 65 77 85 90

0.1 hectares 38 61 75 83

0.2 hectares 25 54 70 80

0.4 hectares 20 51 68 79

0.8 hectares 12 46 65 77Paved roads with curbs and storm drains, paved parking areas, roofs. 98 98 98 98

Gravel roads 76 85 89 91

Earth roads 72 82 87 89

Open water 0 0 0 0



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Table B.7.7: Conversion from average to wet and dry antecedent moisture conditions

CN values

Average conditions Dry Wet

95 87 98

90 78 96

85 70 9480 63 91

75 57 88

70 51 85

65 45 82

60 40 78

55 35 74

50 31 70

45 26 65

40 22 60

35 18 55

30 15 50

Table B.7.8: Antecedent moisture conditions

Region(1) Antecedent moisture conditions

D Dry

B Wet

All other regions Average

Bahir Dar area Although in region A, use Wet

Notes:1. See Appendix B.1 for regional map

7.3.4 Time of concentration

The next step in the SCS Method is to determine the Time of Concentration. This is the time it takes waterto flow from the edge of the catchment area to the point of interest. It is a combination of three values;

A sheet flow,B shallow concentrated flow, andC open channel flow.

The type that occurs is a function of the conveyance system and is determined by field inspection. It isoften a combination of these so that the total travel time is the sum of the time taken for the water to passthrough all of the segments of the catchment.

Travel time is the ratio of flow length to flow velocity:T = L/(3600V) Equation B.7.9

Where:T = travel time, hrL = flow length, mV = average velocity, m/s3600 = conversion factor from seconds to hours.

Sheet flowSheet flow is flow over plane surfaces. It usually occurs in the headwater of streams. With sheet flow,the friction value (Mannings n) is an effective roughness coefficient that includes the effect of raindrop



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impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and erosion andtransportation of sediment. These n values are for very shallow flow depths of about 0.03m or so. TableB.7.9 gives Mannings n values for sheet flow for various surface conditions.

Table B.7.9: Mannings roughness coefficients for sheet flow

Surface n1

Smooth surfaces: concrete, asphalt, gravel or bare soil 0.011Fallow (no residue) 0.05

Cultivated soils

Residue cover < 20% 0.06

Residue cover > 20% 0.17

Grasses

Short grass 0.15

Dense grass 0.24

Range 0.13

Woods(1)

Light underbrush 0.4

Dense underbrush 0.8

Note:1. Consider cover to a height of 30mm. This is the only part of the cover that will affect sheet flow

For sheet flow of less than 100 metres Mannings kinematic solution should be used to compute the traveltime T,

T = [0.091 (n.L)0.8.8/ (P2_)0.5S0.4] Equation B.7.10Where:

T = travel time, hrn = Mannings roughness coefficient (Table B.7.9)

L = flow length, mP2

= 2-year, 24-hour rainfall, mmS = slope of hydraulic grade line (land slope), m/m

Shallow Concentrated FlowAfter a maximum of 100 metres, sheet flow usually becomes shallow concentrated flow. The averagevelocity for this flow can be determined from the following equations in which average velocity is afunction of watercourse slope and type of channel.Unpaved

V = 4.918 (S)0.5

PavedV = 6.196 (S)0.5

Where:

V = average velocity, m/sS = slope of hydraulic grade line (watercourse slope), m/m

After determining average velocity, the travel time for the shallow concentrated flow segment is calculatedfrom Equation B.7.9.

Open Channel flowOpen channels are assumed to begin where surveyed cross section information has been obtained,where channels are visible on aerial photographs, or where blue lines (indicating streams) appear onEthiopian Mapping Authority topographic maps (1:50,000). Average flow velocity is usually determinedfor bank-full elevation. Mannings equation or water surface profile information can be used to estimate



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average flow velocity. When the channel section and roughness coefficient (Mannings n) are available,then the velocity can be computed using the Manning Equation.

V = (R2/3. S1/2)/n Equation B.7.11Where:

V = average velocity, m/sR = hydraulic radius, m (equal to a/P

w)

a = cross sectional flow area, m2

Pw = wetted perimeter, mS = slope of the hydraulic grade line, m/mn = Mannings roughness coefficient (Table B.7.10)

After the average velocity is computed, the travel time for the segment can be calculated using EquationB.7.9.

Reservoir or LakeSometimes it is necessary to compute a time of concentration for a catchment area having a relativelylarge body of water in the flow path. The travel time is computed using the equation:

Vw

= (g.Dm)0.5 Equation B.7.12

Where:V

w

= the wave velocity across the water, m/s

g = 9.81 m/s2D

m= mean depth of lake or reservoir, m

This equation only deals with the travel time across the lake, not the time at the inflow or outflow channels.The times for these are generally very much longer and must be added to the travel time across the lake(see ERAs Drainage Design Manual). Equation B.7.12 can be used for swamps with much open water, butwhere the vegetation or debris is relatively thick (less than about 25% open water), Mannings equationis more appropriate.

7.3.5 Steps in the SCS procedure

The steps in using the SCS method are as follows:1. Ethiopia has been divided into five regions based on rainfall characteristics (Annex A). Determine

which region is appropriate.2. Determine the catchment area, A, and its soil and land use characteristics.3. Determine the curve runoff number, CN, from Table B.7.6 and any adjustment based on the likely

antecedent soil moisture conditions (Tables B.7.7 and B.7.8).4. Calculate the value of Ia from equation B.7.7.5. Choose the appropriate design storm recurrence frequency. This is based on the class of road and

the drainage structure being designed.6. For the recurrence frequency chosen, determine the 24-hour rainfall (P) for the appropriate rainfall

region from Figure B.7.3.7. Determine the direct runoff (Q) for the rainfall (P) and curve number (CN) obtained in steps 3 and 5

from Figure B.7.28. The catchment must be divided into uniform areas for the purpose of determining the Time of

Concentration, Tc. The flow lengths for sheet flow, shallow concentrated flow, and channel flowmust be determined and the relevant equations in Section B.7.3.4 used to calculate the total time ofconcentration.

9. At this stage the following data have been obtained,Ia

the initial abstraction based on the curve number CN (step 4)P the design storm precipitation (step 6)Q the accumulated direct runoff (step 7)T

c the Time of Concentration (step 8)

The next step is to determine the unit peak discharge, Qu and this is done using Figure B.7.4, thevalue of Ia/P and the Time of Concentration.

10. The final step is to compute the actual peak discharge from the unit value as follows,Design peak discharge = Q

ux Q x A



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Where Q is in mm and A is in units of 100 hectares.

Region

Frequency Interval (years)

2 5 10 25 50 100

24-hour depth in mm

A1, A4 60 79 93 113 127 142

A2, A3 52 67 79 95 107 118B, C 65 84 98 118 132 147

D 67 89 105 127 144 161

Lake Tana 74 106 131 163 187 211

Figure B.7.3: 24 hour depth-frequency curves



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Figure B.7.4: Unit peak discharge (Type II rainfall)

7.4 Design of culverts

A fall of 3 - 5% should be allowed on culverts to ensure that water flows without depositing silt andother debris. All pipes should have a minimum diameter of 600mm to ensure that they can be manuallycleaned.

7.4.1 Nomograph method for culvert sizing

The required size of a culvert opening is estimated using the nomographs in Figure B.7.5 to Figure B.7.7.These figures apply to culverts with inlet control where there is no restriction to the downstream flow ofthe water.

In flat terrain, where there is a high risk of silting, a factor of safety of 2 should be allowed in the designof the culvert.

7.4.2 Correlation with successful practice

If a high proportion of structures along a road or in a region have been in operation for a number of years

without overtopping, it is reasonable to assume that the relationship between catchment area, catchmentcharacteristics, rainfall intensity and maximum water flow used in their design is valid. The design of newculverts can be based on simply the catchment area using the same relationships.

7.4.3 Design of drifts and fords

Drifts and fords are designed for water to flow over the running surface. It is not expected that vehiclescan use them at all times. The following criteria should be considered when designing drifts:

The level of the drift should be as close as possible to the existing river bed level. The normal depth of water should be a maximum of 150mm and the maximum 5 year flow should

be 6m3/second on the drift to allow traffic to pass. Approach ramps should have a maximum gradient of 10% (7% for roads with large numbers of

heavy trucks).



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Figure B.7.6: Headwater depth and capacity for corrugated metal pipeculverts with inlet control (Adapted from FHWA, 1998)



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Figure B.7.7: Headwater depth and capacity for concrete pipeculverts with inlet control (Adapted from FHWA, 1998)



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Figure B.7.8: Headwater depth and capacity for concrete boxculverts with inlet control (Adapted from FHWA, 1998)



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Table B.7.10: Roughness coefficient (n) for drains

Material in the drain Roughness coefficient

Sand, loam, fine gravel, volcanic ash 0.022

Stiff clay 0.020

Course gravel 0.025

Conglomerate, hard shale, soft rock 0.040Hard rock 0.040

Masonry 0.025

Concrete 0.017

Side drains (as well as the road itself) should have a minimum longitudinal gradient of 0.5%, except oncrest and sag curves. Reduction of the side drain gradient in the lower reaches of a long length of drainshould be avoided in order to prevent siltation.

7.5.3 Erosion control in the side drain

Limiting values for the velocity of flow to prevent scour in excavated drains are given in Table B.7.11.



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Table B.7.11: Permissible flow velocities (m/sec) in excavated ditch drains

Soil type Clear water Water carrying fine silt Water carrying sand and fine gravel

Fine sand 0.45 0.75 0.45

Sandy loam 0.55 0.75 0.6

Silty loam 0.6 0.9 0.6

Good loam 0.75 1.05 0.7Lined withestablishedgrass on goodsoil

1.7 1.7 1.7

Lined withbunchedgrasses(exposed soilbetween plants)

1.1 1.1 1.1

Volcanic ash 0.75 1.05 0.6

Fine gravel 0.75 1.5 1.15Stiff clay 1.15 1.5 0.9

Graded loam tocobbles

1.15 1.5 1.5

Graded silt tocobbles

1.2 1.7 1.5

Alluvial silts(non colloidal)

0.6 1.05 0.6

Alluvial silts(colloidal)

1.15 1.50 0.9

Coarse gravel 1.2 1.85 2.0

Cobbles andshingles

1.5 1.7 2.0

Shales 1.85 1.85 1.5

Rock Negligible scour at all velocities

Drain erosion is controlled by building scour checks or lining the drain.

Scour checks reduce the speed of water and help prevent it from eroding the road structure. The scourcheck acts as a small dam. When the scour check is naturally silted up on

design manual for low volume roads

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