CALIFORNIA BEARING RATIO (CBR) AND ROAD PAVEMENT DESIGN

CONTENTS

INTRODUCTION
THE BASIC CBR TEST
THE REASON FOR THE CBR TEST
ROAD PAVEMENT DESIGN  MANUALS AND PUBLICATIONS USING CBR VALUES
SUPER SINGLE TYRE LOADING
TABLE OF CBR's FOR COMMONLY FOUND SUB-GRADE CONDITIONS
CBR VALUES IN RELATION TO SITE CONDITIONS AT THE TIME OF CONSTRUCTION
OTHER METHODS OF DETERMINING CBR VALUES

INTRODUCTION

The California Bearing Ratio, believe it or not, was developed by The California State Highways Department.
It is in essence a simple penetration test developed to evaluate the strength of road subgrades.

THE BASIC CBR TEST

This consists of causing a plunger of standard area to penetrate a soil sample, (this can be in the laboratory or on site). The force (load) required to cause the penetration is plotted against measured penetration, the readings noted at regular time intervals.
This information is plotted on a standard graph, and the plot of the test data will establish the CBR result of the soil tested.
The test is fully covered in :-
BS 1377 : Soils for civil engineering purposes:Part 4,Compaction related tests.

THE REASON FOR THE CBR TEST

It sounds complicated, but the basis behind it is quite simple.
We are determining the resistance of the subgrade, (i.e. the layer of naturally occurring material upon which the road is built), to deformation under the load from vehicle wheels.
Even more simply put, ''How strong is the ground upon which we are going to build the road''.

THE CBR TEST IS A WAY OF PUTTING A FIGURE ON THIS INHERENT STRENGTH, THE TEST IS DONE IN A STANDARD MANNER SO WE ARE ABLE TO COMPARE THE STRENGTHS OF DIFFERENT SUBGRADE MATERIALS, AND WE ARE ABLE TO USE THESE FIGURES AS A MEANS OF DESIGNING THE ROAD PAVEMENT REQUIRED FOR A PARTICULAR STRENGTH OF SUBGRADE.

The stronger the subgrade (the higher the CBR reading ) the less thick it is necessary to design and construct the road pavement, this gives a considerable cost saving.
Conversely if CBR testing indicates the subgrade is weak (a low CBR reading) we must construct a suitable thicker road pavement to spread the wheel load over a greater area of the weak subgrade in order that  the weak subgrade material is not deformed, causing the road pavement to fail.

ROAD PAVEMENT DESIGN  MANUALS AND PUBLICATIONS USING CBR VALUES

The CBR in spite of its limited accuracy still remains the most generally accepted method of determining subgrade strength, and as such this information, along with information on traffic flows and traffic growth is used to design road pavements.

The "Transport and Road Research Laboratory Report 1132: The Structural Design of Bituminous Roads",

is the current basic design document for road pavements involving highly trafficked roads i.e. mainly motorway and trunk roads.
Recently published excellent documents on road foundation/design and including CBR information are :-

D.Tp. DESIGN MANUAL HD 25/94, ROAD FOUNDATIONS (There is a draft 25/06 currently being circulated.)

D.Tp. DESIGN MANUAL HD 26/06, ROAD PAVEMENT DESIGN.

These manuals are included in :-
Volume 7 of the D.Tp. DESIGN MANUAL FOR ROAD AND BRIDGES, and you should to have a copy of "Volume 7" in your office library.

To keep up to date with current Highway Design Guides (HD's) please refer to the Standards for Highways website, 
where the relevant documents can be downloaded.

It is still possible to use  :-
Road Note 29 : A Guide to the Structural Design of Pavements for New Roads", for lesser trafficked roads, providing all parties agree to the use of the standard.

Also some authorities have their own planning / design documents giving minimum highway pavement construction requirements for housing/industrial estate roads in relation to CBR results.

Links to a selection of such Local Authority Websites, press -----------> HERE

It is impossible to summarise the mentioned documents in limited space, but you will find in them, graphs relating sub-base and roadbase thickness to CBR values and cumulative traffic (in million standard axles, m.s.a.'s).
Also information on other methods of obtaining CBR results which differ to the basic test described above are included in some of these publications.

SUPER SINGLE TYRE LOADING

Attention must be paid to the different, and increased, wheel loading that is applied to a road pavement by "super single" tyres that are rapidly replacing the twin wheel configuration on the axles of heavy goods vehicles. 
The effect of super single tyres is to dramatically increase the loading on a road pavement and consideration must be paid to this factor in the road design and the choice of road pavement materials. 
The properties of load bearing and durability must be considered together not in isolation.
Although this paragraph is not directly related to CBR, it is related to road pavement design, which is related to the strength of the subgrade.

TABLE OF CBR's FOR COMMONLY FOUND SUB-GRADE CONDITIONS

This table is  only for guidance, you should refer to the appropriate design document for specific information.

CBR VALUES IN RELATION TO SITE CONDITIONS AT THE TIME OF CONSTRUCTION

CBR values "on site" may not bear any relationship to the CBR values employed in the road design, due to softening from wet weather and trafficking from site vehicles.

This is of course true for any design method you employ if the soil conditions at the time of construction are different to the soil conditions upon which you based your design.
It could be some time before the properties of the soil revert back to their original engineering condition, and by this time failure could have occurred.

"Capping layers" have been introduced to help solve the problem of sub-grades wetting up and losing strength during construction by protecting the subgrade from the worst of the damage caused by site traffic.

The opposite is also true, if CBR values are taken on site after the sub-grade has been exposed and dry weather has caused the moisture content of the soil to decrease, increasing soil stiffness, the CBR value will be higher than natural moisture content, this is an incorrect value for design purposes and if accepted will cause a serious under design of the road pavement.

Natural soil moisture content, after drainage, is the correct moisture content for determining CBR values for highway design purposes because in the course of time natural soil moisture conditions will be re-established.

Good drainage is an essential part of road construction to allow the optimum strength/CBR to be obtained, and maintained, from the soil foundation, whether it be in-situ soil or imported fill.

It of course follows that the drainage must be kept operating efficiently during the life of the road to prevent the strength/CBR decreasing through weakening of the foundation by a rising water table. 

For further information on Road Pavement Drainage, press ----------> HERE

OTHER METHODS OF DETERMINING CBR VALUES

"Mexe Penetrometer"

There is on the market a robust hand held penetrometer (designed by the Army) which will "indicate" CBR measurement quite accurately on site.
It is simple to use and will produce good results relating to on site CBR , many results can be achieved very quickly, the number of results able to be taken helps off-set the lesser accuracy of this device.
Indirectly it can also be used to check cohesive soil compaction, i.e. poorly compacted soil will have a low resistance to penetration, and so show as a low CBR value.
But this equipment is only effective to a relatively shallow depth, approx. 300 cms., even with the extension piece.
It is a good piece of equipment to have access to, and relatively speaking for quality laboratory equipment, low cost, about £850. (With attachments it will also test road hardness for surface dressing design.)

Dynamic Cone Penetrometer (Falling weight penetrometer)

The Dynamic Cone Penetrometer is a long steel rod with a standard size hardened steel cone at the penetrating end.
Its length is approximately two metres, but it can be extended by extension rods.
At upper end of the steel bar is a captive weight surrounding the rod, this weight is able to fall freely through a given drop height to achieve a standard amount of penetrative effort at every drop.
By measuring the penetration of the cone against number of drops of the weight it is possible to plot resistance to penetration and indirectly the strength / compaction of the layer being tested.
This equipment is very useful for testing fill compaction, especially in trench reinstatement, performed with care.
It is possible to obtain an approximate CBR figure from the results of using this equipment by applying a simple formula. 
It is a guide figure but still useful to know.

I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception.

For further information regarding various hand operated pieces of soil testing equipment, press ----------------> HERE

Soil Classification Tests (Liquid Limit and Plastic Limit)

By performing the Liquid Limit test and the Plastic Limit test it is possible to determine the Plasticity Index, and from tables it is then possible to find CBR values from tables in the relevant road design documents, e.g. Road Note 29.

Note, the CBR values obtained in this way will refer to optimum soil moisture conditions, and may not reflect actual on site conditions at the time of construction.

For further information on Road Pavement Design, press ----------> HERE


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