HAUNCHING / WIDENING TO RURAL ROAD PAVEMENTS

CONTENTS

DESIGN OF THE ROAD PAVEMENT CONSTRUCTION
EXCAVATION DEPTH
EXISTING ROAD PAVEMENT CONSTRUCTION
WELL GRADED GRANULAR SUB-BASE - (TYPE 1 UNBOUND MIXTURE)
LAYER THICKNESS AND COMPACTION
COMPACTION PLANT
COMPACTION MONITORING
BITUMINOUS BINDER COURSE (BASECOURSE)
FINISHED WIDENING AFTER TWO YEARS TRAFFICKING
CRACKING OF FINISHED WIDENING AFTER TWO YEARS TRAFFICKING

DESIGN OF THE ROAD PAVEMENT CONSTRUCTION
A road haunch repair (haunching) or widening is a road pavement construction just like any other and needs to be designed in in a similar manner to any new road pavement considering such things as strength of the subgrade and numbers and type of traffic.
Two publications that you will find helpful are :-
"A Practical Guide to Haunching", obtainable form the CSS (formerly the County Surveyor's Society).
and
"TRL Report 216 : Road Haunches : A Guide to Re-useable Materials", obtainable from TRL, (Transport Research Laboratory).

EXCAVATION DEPTH The depth of excavation is often governed by the depth of existing construction even if road pavement strength can be achieved with less depth. This is because it is necessary to allow water that may enter the original road pavement to pass to the outside of the road and to drainage systems. If you use a less depth of road construction for the road haunch repair, water may become trapped at the base of the original pavement and cause weakness and this is likely to be in the region of the wheel track.  It is important to have some sort of a drainage system as part of all widening work. Drainage is of major importance in all road construction/maintenance, and should always be included in any works of this nature, however basic the drainage may be.

EXISTING ROAD PAVEMENT CONSTRUCTION Possibly the most difficult part of any road widening, where the existing  road pavement has evolved over the years, is obtaining satisfactory compaction of new materials against this irregular joint. Very particular attention must be paid to the type of material used, placing in thin layers and full compaction before placing the next layer. Granular sub-base and bituminous materials must be well graded with sufficient fines to fill voids when laid in a manner to avoid segregation.

WELL GRADED GRANULAR SUB-BASE  (TYPE 1 UNBOUND MIXTURE) This is normally Type 1 - Sub-base, to Clause 803, (now called Type 1 Unbound Mixture) of the Specification for Highway Works (SHW) in the UK. There are now a number of initiatives to encourage the use of reclaimed and recycled materials as  "Type 1 - Sub-base", and alternatives to it. If you are not familiar with recent amendments to the Specification for Highway Works I suggest you get yourself up to date. It is now possible for "Type 1 - Sub-base" to contain recycled material, or be all recycled material, providing it meets the set engineering criteria set down in the specification.

LAYER THICKNESS AND COMPACTION The construction materials are laid at the appropriate thickness depending on the type of material and the weight and nature of the compaction plant. Unless you are presented with information on the nature and effectiveness of the compaction plant on site, or you know from previous experience the capability of the roller, I suggest you do not allow any granular layer to be laid greater than 100mm, and bituminous material in excess of 75mm., and these suggestions may need to be less to achieve full compaction. Good compaction is even more difficult to achieve when working in confined spaces.

COMPACTION PLANT There are many small rollers of the size shown which incorporate varying degrees of vibration in to their compaction process and are very effective in compacting materials in confined areas such as haunch widenings. All the manufacturers of compaction plant produce booklets to detail the performance of the various models, and these are readily obtainable if you ask. Good information on compaction can be found in the Specification for the reinstatement of Openings in Highways, as well as Clause 802 of the SHW - Volume 1.

COMPACTION MONITORING It is not possible to know if compaction is complete without some form of compaction trial and method specification, or performance testing using a nuclear density meter, sand replacement, or even falling weight penetrometer. In practical terms with experience and good site supervision a method specification of a given layer thickness and a certain number of passes of a roller of a particular weight will ensure adequate compaction. But a quick tip is, if you can still see roller marks the layer is not fully compacted. This is assuming you have not laid material on a soft subgrade, or the material being compacted contains cohesive material that has a high moisture content.

BITUMINOUS BINDER COURSE (BASECOURSE) On this particular site the material was a hot rolled asphalt with 50% content of 20mm. nominal size stone, laid a nominal 60mm. thick.  But a 20mm. Dense Bitumen Macadam with a 190pen. binder for rural roads is another popular choice. A bituminous base was not necessary on this site because of the nature of the road and the traffic it carries. On more heavily trafficked roads the pavement construction needs to be designed to the traffic requirements where ever possible. See EXCAVATION DEPTH, above, with regards to drainage.

BITUMINOUS BINDER COURSE (BASECOURSE) Compacted binder course (basecourse) ready to receive the surface course (wearing course).

FINISHED WIDENING AFTER TWO YEARS TRAFFICKING  I am not looking to make excuses for poor design, materials and workmanship but it is common for slight cracking to occur on this type of work for reasons I have already mentioned above. I repeat, slight cracking. A suitable over-banding or even surface dressing should be able to rectify the problem.  What often happens with this type of work is that the haunching takes place to existing road level with a suitable binder course (basecourse). This is left for two years to allow settlement to take place and then the whole road is surfaced as part of a programme of work.

CRACKING OF FINISHED WIDENING AFTER TWO YEARS TRAFFICKING A close up of surface cracking due to settlement associated with the joint between existing and new road pavement. This will need to be sealed / overbanded with a suitable process to prevent water entering the road pavement and weakening it. This may be a permanent, or temporary treatment prior to an overlay as described above. Whichever reason for the treatment  this banding must be of a suitable skid resistance similar to the rest of the road surface. Be aware of the requirements for over-banding for particular road classifications. Note, there was no "stepping back" into existing road construction on this site where a "vertical" joint was abutted to existing construction. I am in favour of " stepping back" where possible so as to distribute load before it reaches the vertical joint and possibly reduce cracking.

However there are two schools of thought to "stepping back", some believe it has engineering merits others do not. My own experience is that in conjunction with correct compaction procedure in the haunching, stepping back can eliminate or at least reduce subsequent cracking between existing and new construction, and is a worthwhile procedure.

[ Top of Page ]