Term paper

TRANSPORTATION ENGG.

Topic

DRAINAGE SYSTEM IN HIGHWAYS

SUBMITTED TO SUBMITTED BY

Mr.Amit Kumar Dhir Daulat chauhan

Roll no.13

Course: civil engg.

ACKNOWLEDGEMENT

History of all great works into witness that no great work was ever done

without either active or passive support of a person ‘surrounding and one’s

close quarters. thus is it not hard to conclude how active assistance from

senior could positively impact the execution of a project .I am highly thankful

to our learned faculty Mr Amit Kumar Dhir for his active guidance throughout the

completion of term paper .

CONTENTS

1. Introduction

2. Importance of highway drainage

3. Surface drainage

4. Design of surface drainage

5. Cross drainage

6. Sub surface drainage

7. Design of sub surface drainage

8. Design of filter material

9. Drainage of slopes and erosion control

INTRODUCTION

Highway drainage is the process of removing and controlling excess surface and sub-surface

water within the right way. This includes interception and diversion of water from the road

surface and sub-grade. The installation of suitable surface and sub-surface drainage system is

an essential part of highway design and construction.

During rain, part of the rain water flow on surface and part of it percolates through the soil

mass as gravitational water until it reaches the ground water below the water table. Removal

and diversion of surface water from the roadway and adjoining land is termed as surface

drainage, while the removal of excess soil-water from the sub-grade is termed as sub-surface

water, some water is retained in the pores of the soil mass and drained off by the normal

gravitational method and this water is termed as held water.

IMPORTANCE OF HIGHWAY DRAINAGE

An increase in moisture content causes in strength or stability of a soil mass; the variation in

soil strength with moisture content also depend on the soil type and the mode of stress

application. Highway drainage is important because of the following reasons:

1. Excess moisture in soil sub-grade causes considerable of its stability. The pavement is

likely to fail due to sub-grade failure.

2. Increase in the moisture cause reduction in strength of many pavement materials like

stabilized soil and water bound macadam.

3. In some clayey soil variation in moisture content causes considerable variation in

volume of sub-grade. This sometimes contributes to pavement failure.

4. One of the most important causes of pavement failure by the formation of waves and

corrugation in flexible pavements is due to poor drainage.

5. Sustained contact of water with bituminous pavements causes failure due stripping

bitumen from the aggregates like loosening or detachment of some of the bituminous

pavement layer and formation of pot holes.

6. The prime cause of failures in rigid pavements by mud pumping is due the presence of

water in fine sub-grade soil.

7. Excess water on shoulders and pavement edge causes considerable damage.

8. Excess moisture causes increase in weight and thus increase in stress and simultaneous

reduction in strength in soil mass. This is one of the main reasons of failure of earth

slope and embankment foundations.

9. In place where freezing temperatures are prevalent in winter, the presence of water in

sub-grade and a continuous supply of water from the ground water can cause

considerable damage to the pavement due to in frost action.

10. Erosion of soil from top of un-surface roads and slopes of embankment, cut and hill side

is also due to surface water.

SURFACE DRAINAGE

The surface water is to be collected and then disposed off. The water is first collected in

longitudinal drains, generally in side drains and then the water is disposed off at the nearest

stream, valley or water course. Cross drainage structures like culverts and small bridges may be

necessary for the disposal of surface water from the road side drains. For the preparation of

surface drainage, following should keep in mind.

1. COLLECTION OF SURFACE WATER

The water from the pavement surface is removed by providing the camber or cross

slope to the pavement. The rate of this cross slope is decided on the type of pavement

surface and amount rainfall.

In rural highways, the water which is drained prom the pavement surface has also to

drain across the shoulders before it is lead to the side drains. Hence the shoulders of

these roads are constructed with suitable cross slopes so that the water is drained

across the shoulders to the side drains. These side drains of rural roads are generally

open (kutcha) drains of trapezoidal shape, cut to suitable cross-section and longitudinal

slopes. These sides are provided parallel to the road alignment and hence these are also

known as longitudinal drains. In embankments the longitudinal drains are provided on

one or both sides beyond the toe; in cutting, drains are installed on either side of the

formation.

In urban roads because of the limitation of land width and also due to the presence of

foot path, diving island and other road facilities, it is necessary to provide underground

longitudinal drains. Water drained from the pavement surface can be carried forward in

the longitudinal direction between the kerb and the pavement for short distances. This

water may be collected in catch pits at suitable intervals and lead through underground

pipes.

Drainage of surface water is all the more important in hill roads. Apart from the

drainage of water from the road formation, the efficient diversion and disposal of water

flowing down the hill slope across the road and from numerous cross streams is an

important part of hill road construction. If drainage system in hill road is not adequate

and efficient, it will result in complex maintenance problems.

2. DESIGN OF SURFACE DRAINAGE SYSTEM

The design of surface drainage system is divided in two phases:

A. Hydrologic Analysis

B. Hydraulic Analysis

A. HYDOLOGIC ANALYSIS: - The main logic of hydrologic analysis is to estimate the

maximum quantity of water excepted to reach the element of the drainage system

under consideration. A portion of the precipitation during the rain fall infiltrate into

the ground as ground water and small portion get evaporated. The remaining

portion of water which flow, over the surface is termed as run-off. Various factor

affecting the run-off are rate of rain fall, type of soil and moisture condition,

topography of area, type of ground cover like vegetation etc. the surface drainage is

to be designed to drain away the surface run-off water. The details of rain fall in the

area including intensity duration and frequency of occurrence of storm are to be

collected. Next the run-off and maximum rate of run-off for the area under

consideration is determined uses any of the accepted approaches. It is also

necessary to find the drainage area where is likely to flow in.

Rational formula is widely used to estimate the peak run-off water for highway

drainage.

Q = Ci Ad

Q= run-off, meter cube/second

C= run-off coefficient

I= intensity of rain fall mm/second

Ad= drainage area in 1000 meter square.

B. HYDROULIC ANALYSIS: - Once the design run-off Q is determined, the next step is

the hydraulic design of drains. The sides drains are partially filled culverts are

designed based on the principles of flow through open channels.

If Q is the quantity of surface water to be removed by side drain and V is the allowable velocity

of flow on the side drain, the area of cross section A of the channel is found from the relation:

Q = AV

DATA FOR DRAINAGE DESIGN

The following data are to be collected for the design of road side drain:

1. Total road land and width of land from where water is expected to flow on the

stretch of the side drain.

2. Run-off coefficient of different types of surface in the drainage area and their

respective areas.

3. Distance from farthest point in the drainage area to the inlet of the side drain along

the steepest gradient and the average value of the slope.

4. Type of soil of the side drain, roughness coefficient allowable velocity of flow in

the drain.

5. Rain fall data including average intensity and frequency of recurrence of flood.

DESIGN STEPS

Simplified steps for the design of longitudinal drains of a highway to drain off the surface water

are given below:

1. The frequency of return period such as 10 years, 25 years etc. is decided based on the

finances available and desired margin of safety, for the design of drainage system.

2. The values of coefficient of run-off from drainage area are found and the weighted value

is computed.

3. Inlet time T1 for the flow of storm water from the farthest point in the drainage area to

drain inlet along the steepest path of flow is estimated from the distance, slope of the

ground and type of cover.

4. Time of flow along the longitudinal drain T2 is determined for the estimated length of

longitudinal drain L upto the nearest cross drainage, and for the allowable velocity of

flow V in the drain i.e. T2=L/V.

5. The total time T for the inlet flow and along the drain is taken as the time of

concentration or the design value of rain fall duration, T = T1 +T2.

6. From the rain fall intensity-duration-frequency curves the rain fall intensity is found in

mm/sec. corresponding to duration T and frequency of return period.

7. The total area of drainage Ad is found in units of 1000 meter square.

8. The run-off quantity Q is computed = C i Ad.

9. The cross sectional area of flow A of the drain is calculated = Q/V, where V is the

allowable speed of flow in the drain.

10. The required depth of flow in the drain is calculated for a convenient bottom width and

the side slope of the drain. The actual depth of the open channel drain may be increased

slightly to give a free board. The hydraulic mean radius of flow R is determined.

11. The required longitudinal slope S of the drain is calculated using manning’s formula

adopting suitable value of roughness coefficient n.

2. CROSS DRAINAGE

Whenever streams have to cross the roadway, facility for drainage is to be provided. Also often

the water from the side drain is taken across by these cross drains in order to divert the water

away from the road, to a water course or valley. The cross drainage structures commonly in use

are culverts and small bridges. When a small stream crosses a road with linear water way less

than amount six meter, the cross drainage structure provided is called culvert; for higher value

of linear waterway, the structure is called bridge.

The common types of culverts in use are:-

1. Slab culvert

2. Box culvert

3. Arch culvert

4. Pipe culvert.

In slab culverts RCC slab is placed over abutments made of masonry and the span is generally

limited to 3 meter. Box culvert of square or rectangular shape is made of RCC. Arch culvert is

generally built using brick or stone masonry, plain cement concrete may also be used. Pipe

culvert of minimum diameter 75 cm and made of steel or prefabricated RCC is used when the

discharge is low.

Various types of bridges are in use; the choice is based on several considerations including the

span. RCC and steel bridges are commonly constructed these days.

On less important roads, in order to reduce the construction cost of cross drainage structures,

some time submersible bridges or cause ways are constructed. During the floods the water will

flow over the road. The total period interruption to traffic has however to be as low as possible,

not exceeding about 15 days in a year.

SUB-SURFACE DRAIN

Change in moisture content of sub-grade are caused by fluctuations in ground water table

seepage flow, percolation of rain water and movement of capillary water and even water

vapour. In sub-surface drainage of highways, it is attempted to keep the variation of moisture in

sub-grade soil to a minimum. However only the gravitational water is drained by the usual

drainage systems.

1. LOWERING OF WATER TABLE

The highest level of water table should be fairly below the level of sub grade, in order that the

sub grade and pavements layers are not subjected to excessive moisture. From practical

considerations it is suggested that the water table should be kept at least 1.0 to 1.2 meter

below the sub grade. In place where water table is high (almost at ground level at times) the

best remedy is to take the road formation on embankment of height not less than 1.0 to 1.2

meter. When the formation is to be at or below the general ground level, it would be necessary

to lower the water table.

If the soil is relatively permeable, it may be possible to lower the high water table merely

construction of longitudinal drainage trenches with drain pipe and filter sand. The depth of the

trench would on the required lowering of water table, distance between the drainage trenches

and soil type. If the soil is relatively less permeable, the lowering of ground water level may not

be adequate at the center of the pavement or in between the two longitudinal drainage

trenches. Hence in addition, transverse drainage may have to provide in order to effectively

drain off the water and thus lower the water table up to the level of transverse drains.

CONTROL OF SEEPAGE FLOW

When the general ground and impervious strata below are slopping, seepage flow is likely to

exist. If the seepage zone is at depth less than 0.6 to 0.9 meter from the sub grade level,

longitudinal pipe drain in trench filled with filler material and clay seal may be constructed to

intercept the seepage flow.

CONTROL OF CAPILLARY RISE

If the water reaches the sub grade due to capillary rise is likely to be detrimental, it is possible

to solve the problem by arresting the capillary rise instead of lowering the water table. The

capillary rise may be checked either by capillary cut-off of any one of the following two types:-

1. A layer of granular material of suitable thickness is provided during the construction of

embankment, between the sub grade and the highest level of sub surface water table.

The thickness of the granular capillary cut-off layer should be sufficiently higher than the

anticipated capillary rise with in the granular layer so that the capillary water cannot rise

above the cutoff layer.

2. Another method of providing capillary cut-off is by inserting an impermeable or

bituminous layer in the place of granular blanket.

DESIGN OF SUBSURFACE DRAINAGE SYSTEM

The size of spacing of subsurface drainage system would depend on the quantity of water to be

drained off, the type of soil and type drains. Mostly this is decided based on experience and

other practical considerations. However, proper filter material should be used for back filling

the drainage trenches and also for use in all subsurface drainage system.

DESIGN OF FILTER MATERIAL

The filter material used in subsurface drain should be designed to have sufficient permeability

offering negligible resistance to the flow. The filter material should also be designed to resist

the flowing of the fine foundation soil resulting in problem like piping. Hence the grain size

distribution of filter material is decided based on these two criteria of permeability and piping.

The procedure for design of filter is briefly discussed below:-

1. On a grain size distribution chart plot the grain size distribution curve for the foundation

soil.

2. Find the value of D15 size of foundation material and plot a point of particle size 5D15 of

foundation to represent the lower limit of D15 size of filter. This to fulfill the

permeability condition given by:- ( D15 of filter / D15 of foundation ) should be > 5.

3. To fulfill the condition to prevent piping :- ( D15 of filter / D85 of foundation ) should be

less than ( < ) 5, hence plot a point to represent the upper limits of D15 size of filter

given by 5D85 of foundation.

4. Find the size of perforation in the drain pipe or the gap in the open joints pipes and let

this be = Dp. Plot a point to represent D85 size of filter given by the size 2Dp.

DRAINAGE OF SLOPES AND EROSION CONTROL

Drainage of slopes and embankment, cutting and hill side are of utmost important to prevent

instability of slopes and slides. Soaking of the slope causes stress and reduction in strength.

Hence an efficient network of surface drainage system consisting of interception drains and

sloping drains to keep the slopes properly drained is very useful for stability. The sloping drains

may be provided with lining or pitching or may be filled with gravel. The water from the sloping

drains is collected in the catch pits ant diverted across through the culverts at suitable intervals.