SITE INVESTIGATION WITH TRIAL HOLES

CONTENTS

AREAS OF FAILURE
THE BASICS WHEN TAKING A TRIAL HOLE
DO NOT IGNORE THE OBVIOUS - BE OBSERVANT
ASK THE QUESTIONS - LOOK FOR THE ANSWERS
REFERENCE PUBLICATIONS

AREAS OF FAILURE
When you have areas of failure in a road surface it is necessary to investigate the cause of the failure to decide the course of action to effect a permanent repair.

Excavating a trial hole is an excellent way of finding out just what the road pavement consists of, or does not consist of, as the case may be.

It will also indicate the nature of the failure, i.e. failure of the materials making up the road pavement, lack of construction thickness, high water table/poor drainage, etc..


THE BASICS WHEN TAKING A TRIAL HOLE
It is good practice to record all the trial hole details.

Record the thickness of the various pavment layers, i.e. wearing course, basecourse, roadbase, sub-base.

If you are sufficiently knowledgeable identify the various materials, e.g. hot rolled asphalt wearing course with precoats, or close graded macadam wearing course, etc..



If you are not sure what the materials are take cut-outs and send them to a good materials laboratory for analysis.
A good laboratory should be able to tell you what the materials are and if they conform to the relevant specification.

Record the equilibrium water table level.


DO NOT IGNORE THE OBVIOUS - BE OBSERVANT
If possible be on site when the trial hole is excavated and observe the procedure, it is amazing what you can learn just from using your eyes.

How hard was it for the workman to jack hammer through the various layers.
If it was easy the materials in the varying pavement layers are not that strong.
If it was "hard going" through the various layers the pavement is strong, and the materials themselves are unlikely to be the cause of any failure.
This may suggest that the road pavement was under designed, or not constructed to the design, i.e. not sufficient thickness of material or layers to spread the load widely enough before it acted upon the subgrade.

ASK THE QUESTIONS - LOOK FOR THE ANSWERS  :-

Were the materials well compacted, i.e. voids in the materials were not visible.

Were the pavement layers observed those of the specified thickness quoted in the design document.

Was the sub-base material in a "clean" condition or was it contaminated with clay, thus weakening its ability to spread load.

Was there water in the sub-base layer, i.e. indicating a lack of good drainage.

And, if you excavate to the subgrade level, what condition is it in, meaning if it is a granular material is it waterlogged, or if it is a clay material has it become soft and lost its bearing capacity.
Drainage of the road pavement may have been omitted, or drainage that was present at the time of construction has become "broken" by other works in the road, or may have silted up over time and is no longer performing its very important function.

I think you may be beginning to understand what an excellent site investigation process a simple trial hole is, I hope so.
Information gained from trial holes can be confirmed by the quicker and less damaging process of coring.

For information on coring, press ----------------------> HERE

REFERENCE PUBLICATIONS 

For further information on investigating the structural failure of road pavements I suggest you read,

Highway Design Note (HD) 29, Structural Assessment Methods
and
Highway Design Note (HD) 30, Structural Assessment Procedure

of the

Design Manual for Roads and Bridges : Volume 7, Pavement Design and Maintenance

These design notes give comprehensive information on the examination of road pavements for structural failure and are recommended reading,
as are all five sections of Volume 7, as they embrace practically all sectors of knowledge relating to pavement design and maintenance.

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