3. Highway Drainage.pdf

4.1 Introduction and important of highway drainage
 Effects of Moisture
 Moisture has the following effects on a highway
i. The bearing capacity of different layers of soil is reduced as a result of which the
pavement may not be able to take the design loads
ii. The strength of various layers of pavement is reduces hence roadway may
undergo permanent deformation
iii. The water flowing over the slopes may cause erosion
iv. Seepage of water along the roads may cause slope instability leading to landslides
v. There is a chance of slippage of vehicles running at high speed
vi. Rain water takes away with it the precious road material away
highway drainage is a set of engineering structures and techniques which ensures water
is kept as far away from the surface i.e. ground water, seepage and infiltration is kept as
low as possible and rain water is drained as soon as possible
Highway drainage ensures the following
 The flow of water which takes place for short period of time which shall not damage
the road surface
 Seepage and other sources of ground water are be kept far away from the surface of
the road
 The highest level of ground water is at least 1.2m below the top of road surface
 In water logged areas it is ensured that special precautions are taken

4.2. Causes of moisture variation in subgrade soil
 There various sources through which water may be
introduced to the road surface some of them are as follows
 1. By free water
 Seepage from higher ground
 Penetration through pavement
 Transfer from shoulder and pavement edges
 2. By Ground water
 Rise and fall of water
 Capillary rise from lower soil level
 Transfer of water vapor through soil

4.3. Requirements of good drainage system
 In general a good drainage shall fulfill the following
requirements
 i.The surface water from the carriageway and shoulder
should be effectively drained off without allowing it to
percolate through subgrade
 ii.The surface water from adjacent land and natural
drainages should be prevented from entering the
roadway
 iii.The side drain should have sufficient capacity and
slope to carry away the water coming to the surface

4.4. Classification of highway drainage system
 Surface Drainage system
 The components which removes water from road and road side ground is called surface
drainage
 Sub-Surface Drainage
 The components which removes water Below the road surface is called the sub surface
drainage system
 Cross Drainage Structure
 When a stream has to cross the road way, special precautions have to be taken in order to
cross the stream the structure thus formed is called cross drainage structure
 Some if the common cross drainage structures are bridge , culvert , cause way etc
 Energy dissipating structures
 Water flowing along the side drains have tremendous energy due to their Flow and might
erode the canal away
 Those structures which are constructed in order to dissipate the energy carried by the
water is like falls road rapids etc are called energy dissipating structures

4.5 Surface drainage system
 The removal of water from the road surface and road side pavement is called
surface drainage
 It contain two components
T
ransversedrainage
 It isusedto provide thecontinuityof natural water lines,intercepted by theroad. It is
designedto avoid theflooding of theplatform and surrounding areas.

• Longitudinal channels
– Ditches along side of road to collect surface water after
run‐off
L03‐4 EG611CE Transportation
Engg.
Dr. Pradeep K. Shrestha
8

 The combination of these are used in the following
configrations
 A. Drainage In Plain Highway/Roads
 The water from shoulder and the pavement is collected by
the side drains
 The side drains are provided at the base of the shoulder or
at the base of the embankement as per the nature of the
cross section of the road
 Sometimes when the space is restricted and open drains are
not safe to administered , under such circumstances
covered drain closed by layers of pervious sand and gravel
are used

 B. Drainage In Urban streets
 In case of urban streets due to the presence of footpath
, dividing island , other feature and lack of land
underground drains have to be provided
 Water from pavement and footpath is collected by the
kerb or gutter outlet and is deposited into the catch
pits suitably placed
 The water is laid to main sewer by means of the sewer
pipe

 C. Drainage in Hill roads
 In hill roads side drains are provided in the form of triangular
drains
 Along with this there is additional water collected due to the
presence of huge slopes
 If this water is not drained properly then the slippage of the road
surface ultimately leading to the blockage may take place
therefore the water flowing from the slopes is intervened by
means of catch drains
 Apart from this the water coming from the rivers which
bring about a huge amount of water in the rainy seasons
are a major problems
 Therefore proper cross drainage structure shall be
established

4.6. Design of surface drainage
 The design process involves the following steps
(i) Hydrological analysis :
 The main objective of this phase is the estimation of the maximum
quantity of water expected to reach the drainage system.
 When rain falls on the ground some portion is infiltrated along the
ground, some is evaporated while the rest of it flows above the ground,
this portion is called surface runoff
 The main objective of surface drainage is to drain off this surface runoff
 There are a no of formulas which utilize the catchment area and the
nature of soil, in order to calculate the amount of runoff
 The most popular is the rational formula
lity
Impermeabi
of
t
Coefficien
C
Ha.
in
Area
A
mm/hr
in
rainfall
i
360
*
*




Where
A
i
C
Q

Run-off Coefficient
15
• It isthe fraction of rainfall that becomes runoff
• Depends on
• Characteristics of soil
• Shapeof drainagearea
• Existing moisture conditions
• Slope of watershed
• Amountof impervioussoil
• Land use
• Duration and intensity of rainfall

16.2 Values of Runoff Coefficients, C
RunoffCoefficient(AASHTO)
Table
Type of Surface Coefficient, C*
RuralAreas
Concrete sheet asphalt pavement
Asphalt macadam pavement
0.8-0.9
0.6-0.8
0.4-0.6
0.2-0.9
Gravel roadways or shoulder
0.5-0.7
0.1-0.4
0.1-0.3
0.2-0.4
Bare earth
Steep grassed areas
Turf meadows
Forested areas
Cultivated fields
s
(2:1)

Run-off Coefficient
18
• When adrainage area hasdistinct parts with
different Cvalues
Usethe weighted average
n

C Ai
C1A1C 1A1 C An n i 1
n
i
C  
A1 A1  An
 i
A
i1

Watershed(Drainage) Area
19
The drainage area is the area of land that contributes to the runoff
at the point where the channel capacity is to be determined. i.e.
combined areaofall
surfacesthat drain to agiven intake or culvertinlet
•
20
DrainageAreais determined from:
• Topographicmaps
• Aerial photos
• Digital elevation models
• Drainagemaps
• Fieldreviews

Rainfall
21
•
Three properties of rainfall
– Therate of fall, known asintensity;
known as
– Thelength of time foragiven intensity,
duration;
– The probable number of years that will elapse
before a given combination of intensity and
duration will be repeated, known as frequency
(return period)

DesignPeriod
22
Overdesign (longer return period) is costly and Under
design (shorter return period) maybeinadequate
•
Factors usually considered in making this decision
include,
– the importance of thehighway,
– the volume of traffic on thehighway, and
– the population density of thearea.

RainfallIntensity
23
• Averageintensity for aselected frequency and
duration over drainage area for duration of storm
Basedon values of :
• time of concentration
• recurrence interval or designfrequency

Design RainFall(IRCSP13method)
• Therainfall intensity is,
11 
T
I  F
tc 1
Where,
• F=theof rainfall in cmdropped by severeststorm overa
periodofThours
tc =time of concentration ( Time for water to flow from mostdistantpoint
in drainageareato the draininlet ) in hours
• 0.385

 0.87L3
tc   

H

Where, ‘L’is the distance from the critical point to the
culvert in km, and ‘H’ is the fall in level from the critical
point to culvert inm

TimeofConcentration(tc)
• Depends on:
–Sizeand shapeof drainage area
– Typeof surface
– Slope of drainagearea
– Rainfall intensity
– Whether flow is entirely overland or whethersome is
channelized

(i) Hydraulic Design
 This phase involves the process of determining the size of the drain
based on the slope , amount of runoff and the nature of drainage
material.
 It utilized manning's formula
 The steps involved are as follows
(i) Determine the hydraulic radius using permissible velocity V
(ii) Determine the required area using
P
A
Radius
Hydraulic
R
catchment
of
*
*
1 2
/
1
3
/
2




Area
A
S
R
A
n
Q
2
/
3
2
/
1 






S
nV
R
V
Q
A 

(iii) Determine the perimeter using
(iv) Determine the dimensions solving
(v) Determine the actual velocity(V’) from area calcualted
by rounded off dimensions
(iv) Check For critical depth using d >V2 /2g if ok the design
is ok else change dimesnions and review
R
A
P 
)
..(
..........
1
2
)
.........(
..........
2
2
ii
d
m
B
P
i
Sd
Bd
A






4.7. Subsurface drainage system
 Subsoil(Sub Surface) drainage can be achieved through the following
 A. Lowering of water table
 Water table must be at least 1.2m away from the road surface
 If depth is more than that it is ok else it is shall be lowered
 The best way is to keep the top surface of the road above the natural
the natural ground till the depth is satisfactory
 For relatively Granular (Permeable soils) Water table can be lowered by
the construction of longituditional drains filled with filter sand
 The depth of these drains depends upon the type of drain the spacing
of drains and the type of soils
 If soil is relatively less permeable the lowering of ground water may not
be adequate at the center therefore additional transverse drains may
have to be provided in order to effectively drain off the water and thus
to lower the water table

 B. Control of Seepage flow
 Where ground slope as well as the impervious layer is
slopping towards the road surface the seepage from higher
grounds is likely to reach the road surface the seepage is
likely to damage the road and reduce he strength of the
road
 If the depth of seepage gets lower then 60 to 90 cm from
the road surface then it has to be intervened to keep the
seepage fairly below the ground level
 This is achieved by introducing a perforated drain pipe in
between the slope and the road
 The pipe is covered by a layer of filter material like coarse
gravel sand etc
 The top of the pipe is sealed by a clay seal so that the
outside water may not enter the drain pipe

 C. Control of Capillary rise
 If capillary rise water is fairly near the road surface
then it may affect the strength and durability of the
road
 Under such conditions, steps should be taken to arrest
the capillary rise of water
 For sub grade soil is permeable the lowering of water
table is more proffered but for impermeable soils the
drainage is too costly therefore a capillary cut off is
proffered

 1. By providing a layer of granular material during the construction of
the road embankment
• The thickness of the granular capillary cut off shall be sufficiently
higher than the anticipated capillary rise
• The capillary rise is intervened by the granular layer and cannot
reach beyond it

2. By inserting a impermeable layer in the place of
granular layer for eg a layer of bitumen

4.8 Cross Drainage Structure
 They are structures formes at the junction of road
embankement and natural drain in order to drain the
water flow efficiently
 The different type of cross drainage structures used in
highway are as follows
 (i)Culvert
 (ii) Causeway
 (iii) Bridge

Culvert
 It is a conduit placed under the embankment to carry
water. It is of following types
 Pipe culvert
This type of culvert is used when stream carries low
discharge and the cover over the drain is sufficiently high
It consists of a RCC Pipe laid over a concrete base of
15cm provided such that there is a minimum cover of
50cm above the top of pipe
The pipe size ranges from 60cm to 3m and are available in
langths of 2.5 to 3m

 Slab Culvert
This kind of culvert has two stone masonry
abutements on either side over which a slab is
provided
The abutements are made by stone masonry in places
where stone is available abundantly and from brick
masonry in other places
These type of culvert span from 2.5 to 3m and can
economically discharge moderate flows

 Box culvert:
If the soil is not suitable for the individual fottings of a
slab culvert then a complete RCC box is provided in the
form of culvert this is called box culvert.
This does not require a high cover and can even be aligned
with the road surface.
This kind of culvert contains a complete RCC box of
minimum size 60X60cm and can be upto 3mX3m
This kind of culvert has grater resistance to damage due to
debris

 Arch Culvert
This kind of culvert is made when huge amount of loads
are occurs
It contains two masonry abutements over which a arch is
provided, deck is provided over the arch
The arch is generally made RCC or Stone masonry
The span of arch is limited to 3m

Causeway
 They are submersible bridges i.e. are inundated in
when heavy flow occurs
 These are provided in less important roads where
depth of flow does not exceed 1.5m.
 When flood occurs traffic is stopped at both the ends
and continued when flood recedes
 They are of two types
 Low level type which can run only during dry
conditions
 High level type which can pass a certain magnitude of
flood

4.9 Erosion control and Energy Dissipation
 At the outlet and other critical points the velocity of water
is higher than the non-scouring velocity, which causes
erosion under normal conditions and therefore special
precaution shall be applied they are
 Lining Of drains and Ditch checks
 In order to reduce erosion the slopes are covered with turf
and bottom with gravel or cobble of desired size
 If the flow is too high then the turf can be replaced by
stone masonry,RCC or Lining based on the flow and budget
availability
 Simillarly if the grade of the bottom is too high for ordinary
measures then series of steps are formed these are called
ditch checks

 Road Rapids
 These are structures provided at the inlet and outlet of
cross drainage on order to dissipate energy
 It contains three parts inlet outlet and stilling basin
 Fall or Drop Structure
 They are provide in hill slopes where the bed slope
of the existing drainage is too high
 They are provided at the inlet and the outlet of the
cross drainage

 Miscellaneous Erosion Control Measures
 The flow of water over the shoulders and hill slopes causes erosion.
 Erosion also occurs on the surface of earthen roads
 This may be tackled through
(i) Vegetation
 Vegetation performs protective as well as aesthetical functions.
 It is dealt under the topics of road side arboriculture or bioengineering
 Turfing is the most economical method
 In this process turfs are transferred to critical sites which stabilizes the slopes and
control erosion
(ii) Slope pitching
 Slopes can also be stabilized by pitching
 Pitching can be done by following methods
 Dry Stone pitching
 Gabion crate pitching
 Cocnrete lining
 Retaining wall

3. Highway Drainage.pdf

In this document

More Related Content

What's hot

Similar to 3. Highway Drainage.pdf

More from Er. Bam Bhandari

Recently uploaded

3. Highway Drainage.pdf