DRAINAGE (CONTROL OF WATER) IN ROAD PAVEMENT CONSTRUCTION AND MAINTENANCE

CONTENTS

INTRODUCTION
SUBGRADE DRAINAGE
EARTHWORKS CONSOLIDATION
FROST HEAVE 
SUB-BASE DRAINAGE
WATER DAMAGE TO A ROAD PAVEMENT (FAILURE DUE TO HYDRAULIC PRESSURE)
WATER DAMAGE TO A ROAD PAVEMENT (FAILURE DUE TO BINDER STRIPPING)
ROAD PAVEMENT STRUCTURAL FAILURE
DTP DESIGN MANUAL HD 25/94
DRAINAGE 0F POROUS SURFACE COURSES
WATER TABLE LEVEL

INTRODUCTION

DRAINAGE is VERY! VERY! VERY! IMPORTANT, both in relation to road pavement construction and maintenance.

You MUST keep the water-table low to prevent the moisture content of the subgrade increasing, and hence decreasing the subgrade strength, measured by the CBR value, on which the road pavement was designed.
If this is not done by the use of french drains, or even open ditches adjacent to the highway, the road pavement will weaken and fail.

The water-table of naturally occurring ground will rise and fall from winter to summer, bear this in mind when designing the road, and design for the highest water table (weakest subgrade) conditions. 
Good drainage will help to keep the water table (and strength) of the road pavement in equilibrium.

Also the road pavement itself must be constructed so that it will drain in the event of a failure of the integrity of the surfacing layers, i.e. if water is able to enter the road pavement there must be a path for it to exit.
The internal drainage function of a road pavement is usually performed by the GSB layer, this itself MUST be drained in some way. 
In my younger days this function was provided by a french drain on the lower side channel and periodically piped to an outside drain or open ditch, but this practice seems to have become redundant.

Water below the road pavement must be kept low and not be allowed to rise up into the construction layers, (and water CAN flow upwards, by capillary action).

IF WATER IS ABLE TO ENTER THE ROAD PAVEMENT, FOR WHAT EVER REASON, THE DESIGN OF THE ROAD PAVEMENT SHOULD BE SUCH THAT THE WATER HAS A WAY OUT, THIS IS USUALLY THROUGH A SUB-BASE LAYER THAT IS DRAINED TO AN INSTALLED DRAINAGE SYSTEM.
IF WATER CANNOT FIND A PATH OUT OUT THE ROAD PAVEMENT FAILURE OF THE HIGHWAY WILL BE PREMATURE AND SWIFT.

SUBGRADE DRAINAGE

The subgrade is the layer of naturally occurring material the road is built upon, OR It can refer to the imported fill material that has been used to create an embankment upon which the road pavement is constructed.

The strength of the subgrade or the material constituting the subgrade is commonly measured using the CBR test.

The strength of the subgrade is an important factor influencing the thickness of the road pavement design.

Where the subgrade is weak, i.e. a low CBR, this will usually be associated with clay subgrades and/or soils that have a high clay content, it will be necessary to have a capping layer over the subgrade to increase the strength before the actual road pavement thickness is designed.

The strength of the subgrade can be maintained or even increased by good drainage, and bad drainage may weaken an existing good subgrade.

EARTHWORKS CONSOLIDATION

Consolidation is the compression of soil under steady pressure due to the expulsion of water from the voids.
So, you MUST remember soil can consolidate after it has been fully compacted, especially if the soil has a high moisture content.
Consolidation may take many years depending on the drainage conditions of the embankment / subgrade / foundation. 
If the load is removed from the soil, water may return to the voids causing swelling, and heave of any road built in this situation.

FROST HEAVE 

The term frost heave is referring to the phenomena where a road will actually "heave", i.e. rise up above its normal level due to the action of frost.
It is uncommon in this country because of the relative mildness of the winters, and the good engineering practice that has been taking place for many years, i.e. our design standards have specified that no frost susceptible material is included in the top 450mm. of any road pavement, also good drainage to prevent water availability.

Where frost heave does take place the process is such that the freezing takes place from the surface downwards, "sucking" water up from lower levels, laying down subsequent layers of ice causing the road to expand upwards, i.e."heave".
For frost heave to take place you need road construction material that absorbs water and water for it to absorb, so good drainage helps prevent frost heave.

SUB-BASE DRAINAGE

The sub-base also acts as a drainage layer, 

BUT IT CAN ONLY DO SO IF YOU INCORPORATE A DRAINAGE SYSTEM (HOWEVER BASIC) TO REMOVE THE WATER FROM THE SUB-BASE.

Sub-base is also a load spreading layer, especially during construction, carrying site traffic and protecting the subgrade, to an extent, (as long as conditions are not too wet).

WATER DAMAGE TO A ROAD PAVEMENT (FAILURE DUE TO HYDRAULIC PRESSURE)

Once water has entered a road pavement, water damage is initially caused by hydraulic pressure,
i.e. vehicles passing over the road pavement impart considerable sudden pressure on the water present in the road pavement, this pressure forces the water further into the road fabric and breaks it up, this process can be very rapid once it begins.

Water that has entered the road pavement and is subject to the process of freezing (expansion) and thawing during the winter also brings about the swift failure of the road pavement.

Eventually the water will descend to the subgrade layer below the road pavement and weaken this layer thus lowering the CBR of the subgrade which the road pavement design was based upon, and deep seated failure of the road will begin.

WATER DAMAGE TO A ROAD PAVEMENT (FAILURE DUE TO BINDER STRIPPING)

Most aggregates have a greater affinity for water than they do bitumen, and with the presence of water and movement of the aggregate it is quite possible for the binder film on the aggregate particle to be broken and water to come in to contact with the aggregate surface.

Once the integrity of the binder layer has been broken it will depend upon the chemical nature of the aggregate particle and the viscosity of the binder as to how long it will be before stripping of the aggregate particles becomes an engineering problem.

Depending on the viscosity of the binder and the thickness of the binder film surrounding the aggregate the stripping of the bitumen will occur hardly at all, fairly slowly or quite quite quickly.

This can be compared to a film of highly viscous 50pen. grade bitumen enhanced with cellulose or mineral fibres which will create a thick stable coating of bitumen around the aggregate particle,
to
a low viscosity, highly workable, low temperature, hand-lay cutback material for emergency patching or very low stress sites.

The first example will be highly resistant to stripping and is the binder coating used on most stone mastic asphalts, and other Thin Surfacing surface courses (wearing courses), many of these surfacing materials actually designed to allow the passage of water through the material matrix.
However, because of their exposed position in the road pavement structure even these binder systems can break down over time, (and the access of oxygen to the internal matrix of the materials will increase the rate of oxidation of the bitumen).

The second example strips quite readily, and in most cases should only be regarded as a temporary measure before being removed and a more stable bituminous material used to replace it. 

There are many degrees of stripping between the two examples I quote, and it is a fact that some aggregate sources are more prone to stripping than others.

But, I do not want to cause panic amongst those less familiar with bituminous mixtures, it is unlikely that you will have significant stripping with any material that has a 200pen. grade bitumen or above (meaning stiffer i.e. 100pen., 70pen., 50pen.), with a sensible bitumen content, combined with a reasonably proportioned, dense, aggregate grading.  

Most of the dense roadbases and basecourse, and close graded wearing courses in BS 4987 (BS EN 13108:Part1 from 01-01-08), coupled with the appropriated grade of binder will give you these properties.

Hot rolled asphalts found specified in BS 594 (BS EN 13108:Part 4 from 01-01-08) in general have higher binder contents of stiffer grade bitumens and as such are very unlikely to incur binder stripping.

ROAD PAVEMENT STRUCTURAL FAILURE

This is when the pavement has failed, maybe not completely, but in a major way.

The pavement is no longer able to absorb and transmit the wheel loading through the fabric of the road without causing fairly rapid further deterioration of the road pavement.

The layers making up the road pavement have failed for various reasons, but one of the most common is poor drainage, either by :-
Lack of adequate drainage provision in the original road pavement design,
Lack of maintenance of the drainage so that it no longer functions in a correct manner and the water table has risen thus weakening the road pavement,
Failure of the impervious nature of the surface course (wearing course), or the binder course (basecourse) where the surface course is a porous textured material, thus allowing the passage of surface water in to the road pavement matrix.

DTP DESIGN MANUAL HD 25/94 - ROAD FOUNDATIONS - CBR - CAPPING - DRAINAGE

This standard forms part of the :-
DESIGN MANUAL FOR ROADS AND BRIDGES : VOLUME 7 - PAVEMENT DESIGN AND MAINTENANCE : SECTION 2 - PAVEMENT DESIGN AND CONSTRUCTION : PART 2 - HD 25/94 - FOUNDATIONS

It is a very useful standard, describing the requirements for a road foundation, the ways of testing for subgrade strength, the strengths needed, and the ways of creating a stronger subgrade prior to road pavement construction.

It includes useful tables on subgrade CBR's and required capping layers, along with the drainage requirements relating to foundations.

DRAINAGE 0F POROUS SURFACE COURSES

It has to be understood that in some circumstances surface course (wearing course) materials are deliberately designed to be porous so that "spray" from vehicle wheels will be substantially reduced, improving visibility in wet conditions.
The porosity of the interconnecting voids of the surface course matrix can be verified by a "porosity test".

This test is a measure of the porosity of Porous Asphalt or other open textured bituminous mixture, i.e. how quickly the water will drain through it, and the test is usually performed shortly after laying to check that the material/surface complies with the specification.

The method of the test is to be found in BS DD 229 : 1996.

It will also be performed from time to time to to test that the interconnecting voids which allow drainage have not become clogged.

The test is described in,
Clause 938:Porous Asphalt Surface Course, of Volume 1, of the Specification for Highway Works.

There is an average minimum requirement of 0.12s(-1).

WATER TABLE LEVEL

You MUST keep the water-table low to prevent the moisture content of the SUB-GRADE increasing, and hence decreasing the CBR value on which the road pavement was designed.
If this is not done by the use of french drains, or even open ditches the road will weaken and fail.

The water-table of naturally occurring ground will rise and fall from winter to summer, bear this in mind when designing the road, and design for the highest water table conditions.

Water below the road pavement must be kept low and not be allowed to rise up into the constuction layers, (and water CAN flow upwards, by capillary action).

The fact that road drainage needs periodic maintenance just like an other part of the road network must not be ignored, and it is a lot less expensive than reconstructing failed road pavement.

Water that enters the road pavement from the surface MUST have a drainage path out. 

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