Chapter VIChapter VIHIGHWAY DRAINAGE

Tewodros NTewodros N.www.tnigatu.wordpress.com

tedynihe@gmail comtedynihe@gmail.com

I. INTRODUCTION

• Provision of adequate drainage is an essential part of ov s o o d qu d g s ss p opavement design. – Protection of pavement structure – Improves road safety

• Can be categorically studied in three parts:1. Surface Drainage• Drainage on the adjoining land and roadway

fsurface• Side Drainage and Cross Drainage

2 Sub surface Drainage2. Sub-surface Drainage

I. INTRODUCTION

• Effects of water on the pavement t tstructure• Presence of moisture causes:

o reduction in the stability of the soil mass.du o s b y o so sso considerable variation in volume of subgrade in clayey

soils.o Waves and corrugations failure in flexible pavements.o Stripping failure in flexible pavements.

M d f l d to Mud pumping failure in rigid pavements.

II. DESIGN OF SURFACE DRAINAGE SYSTEMS

• Can be divided into three phases:i. Estimation of the quantity of water that can i. Estimation of the quantity of water that can

reach any element of the system.ii. Hydraulic design of each element of the y g

system. iii. Comparison of alternative systems and p y

materials• Criteria-Lowest annual cost alternative

II. DESIGN OF SURFACE DRAINAGE SYSTEMSSYSTEMS

1. Rainfall Intensity

• Runoff is obtained by considering expected sever storm. Return period of 5 10 20 25 50 and 100 Return period of 5, 10, 20, 25, 50, and 100

years• Quantity of runoff depends on intensity and duration.• Duration= Time of Concentration• Duration= Time of Concentration• The time required for water from the remotest place

to reach a specific point on the drainage system.• T +T• =T1+T2

• T1= over land flow time• T2= time of flow in the longitudinal drain

II. DESIGN OF SURFACE DRAINAGE Y ESYSTEMS

Source: ERA Manual, 2002

II. DESIGN OF SURFACE DRAINAGE G GSYSTEMS

Source: ERA Manual, 2002

II. DESIGN OF SURFACE DRAINAGE SYSTEMSSYSTEMS

2. Computation of Runoff• Rain water expelled from the road surface

i. Infiltrationii. Runoffiii. Evaporation- insignificant

• Infiltration depends on: • Type and gradation of soilp g• Soil covers, moisture content of the soil• Presence of impervious layers near the surface.

II. DESIGN OF SURFACE DRAINAGE SYSTEMS

• Infiltration contd.• Rate of infiltration on bare soil is less than on a

turfed soil.• Frozen soil is impervious• Rate of infiltration is assumed to be constant

d f d during any specific design storm.

• Runoff depends on: • Nature of the ground, degree of saturation, and

slope of the surfaceR t f ff t th f • Rate of runoff greater on smooth surfaces.

II. DESIGN OF SURFACE DRAINAGE SYSTEMSSYSTEMS

• Rational Formula- accurate way of estimatingff t f 0 5 k 2runoff up to areas of 0.5 km2

CIAQ 00278.0Q

2211

AAACACC • If the water shade

• Q= runoff (m3/sec)

21 AA is made up of different surfaces

Q= runoff (m3/sec)• C=coefficient, representing ratio of runoff to rainfall• I= intensity of rainfall (mm/hr) for a duration equal to the time

of concentration• A= catchment area tributary to the design location, ha

II. DESIGN OF SURFACE DRAINAGE SYSTEMSSYSTEMS

II. DESIGN OF SURFACE DRAINAGE SYSTEMSSYSTEMS

• tc=distance/velocity of flowt th d t • tc is then used to determine the rainfall intensity (I)intensity (I)

II. DESIGN OF SURFACE DRAINAGE SYSTEMS

• This chart can also be alternatively used to determine tto determine tc.

III. DESIGN OF SIDE DITCHES AND OPEN CHANNELSCHANNELS

• The Manning’s Formula• Once the quantity of runoff is known, the design of

ditches and similar structures is based on the principles of open channel flow principles of open channel flow.

• Mannings’s formula assumes steady flow in a uniform channel.

Where:

2/13/21 SRn

V AVQ Where:• V= mean velocity (m/sec)• R= hydraulic radius (m)= Area/wetted perimeter• S=slope of the channel (m/m)• n=Manning’s roughness coefficient

III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS

• The Manning’s Formula

CHANNELS

III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS

• Capacity of a Trapezoidal Channel

III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS

• Examples:

CHANNELS

p1. The maximum quantity of water expected in one of

the open longitudinal drains on clayey soil is 0.9p g y ym3/sec. Design the cross section and longitudinalslope of trapezoidal drain assuming the bottom

dth f th t d l t t b 1 0 dwidth of the trapezoidal section to be 1.0 m andcross slopes to be 1V:1.5H. The allowable velocityof flow in the drain is 1.2 m/sec and Manning’sgroughness coefficient is 0.02.

III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS

• Examples:

CHANNELS

2. The surface water from road side is drained to thelongitudinal side drain from across one half a bituminouspavement surface of total width 7.0 m, shoulder andpavement surface of total width 7.0 m, shoulder andadjoining land of width 8.0 m one side of the drain. On theother side of the longitudinal drain, water flows across fromreserved land with grass and 2% cross slope towards thereserved land with grass and 2% cross slope towards theside drain, the width of this strip of land being 25 m. The runoff coefficients of the pavement, shoulder and reserve landwith grass surface are 0 8 0 25 and 035 respectively Thewith grass surface are 0.8, 0.25, and 035 respectively. Thelength of the stretch of land parallel to the road from wherewater is expected to flow to the side drain is about 400 m.Estimate the quantity of run off flowing in the drain assumingEstimate the quantity of run-off flowing in the drain assuming25 years period of frequency.

IV. SUBSURFACE DRAINAGE

1. Lowering of Water Table

• Highest level of water table should be below the subgrade. • Practically 1.0 to 1.2 m below subgrade

• Relatively permeable soil-• Longitudinal drains are mainly used

• Impermeable soils-T d b dd • Transverse drains may be necessary in addition to longitudinal drains

IV. SUBSURFACE DRAINAGE

1. Lowering of Water Table

Fig. Symmetrical longitudinal drains used to lower the groundwater table and to collect water infiltrating the pavement.

IV SUBSURFACE DRAINAGE• Lowering of Water Table

IV. SUBSURFACE DRAINAGE

Longitudinal DrainDrain

T Transverse Drains

F L f t t bl Fig. Lowering of water table using Transverse Drains (Plan View)

IV. SUBSURFACE DRAINAGE

2. Seepage Control

• If seepage zone is at a depth less than 0.6 to 0.9 m below subgrade level, g ,• Use longitudinal pipe drain in trench

with filter material to intercept the with filter material to intercept the seepage flow.

• This phenomenon can be explained • This phenomenon can be explained using figures.

IV. SUBSURFACE DRAINAGE

2. Seepage Control

Fig. Longitudinal interceptor drain used to cut off seepage and lower the groundwater table. p g g

IV. SUBSURFACE DRAINAGE

2. Seepage ControlGroundwater seeps through the slope where the water Groundwater seeps through the slope where the water table intersects the land slope, andGroundwater flows beneath the pavement while also entering the pavement foundation materials.

Fig. (A) Illustration of ground water flow along a sloping impervious layer toward a roadway.

IV. SUBSURFACE DRAINAGE

2. Seepage Control

Fig. (B) Illustration of interceptor drain on the drawdown of the groundwater table the groundwater table.

IV. SUBSURFACE DRAINAGE

2. Seepage Control

Fig. Longitudinal collector drain used to remove water seeping into the pavement structural

t section.

IV. SUBSURFACE DRAINAGE

3. Control of Capillary Rise

• Capillary rise can be controlled by Using a layer of granular material of suitable thickness.

U f ff Using a layer of impermeable capillary cutoff. Capillary water should not rise above the thickness of the

granular layerg y

Granular material

Capillary rise

Highest water tableFig. Granular capillary

cutoff

IV. SUBSURFACE DRAINAGE

3. Control of Capillary Rise

• Bituminous layer or other geo-textiles can be used as an impermeable layer.

Impermeable layer

Capillary rise

Highest water table

Fig. Impermeable layer capillary cutoff

IV. SUBSURFACE DRAINAGE

4. Design of Filter Material

• Proper filter material should be used for: Subsurface drainage system and backfilling the

drainage trenches and drainage trenches and

Criteria: Permeability and Piping Permeability and Piping

515 foundationofD

filterofDPermeability criteria

15 foundationofD

515 filterofD

Piping criteria585

foundationofD

Piping criteria

IV. SUBSURFACE DRAINAGE

4. Design of Filter Material D =size of perforation in drain pipe

10

DP=size of perforation in drain pipe D85 Filter = 2DP

10080

60

2DP○ The area between the two red curves

h

rcen

t ss

ing

60

40

Foundation soil

Filter Material

represents the filler material.

Per

pas

1.00.0

20

0. 0.0 0.001.

5D15

○ ○5D85

Particle size (mm), log scale0

1 1 10

Fig. Design of Filter Material

• References:R s1. Highway Engineering, 7th Ed. Paul H. Wright and Karen

K. Dixon. Wiley (2004)2. Highway Engineering, 8th Ed. S.K. Khanna and C.E.G.

Justo. (2001)

Tha k Y !Thank You!Y