RE-INTRODUCTION OF RUBBLE AS A BASE COURSE MATERIAL & ITS APPROPRIATENESS

FOR THE RURAL ROAD CONSTRUCTION ( PROGRAMME), IN PARTICULAR

By: Eng. S.D.S. Deshapriya -RDA

ABC is a widely used road material for the construction of road bases. It has become more

popular due to its easiness for handling. However it shows that comparatively large stones

such as rubble (9”) can be used very effectively for the construction of road bases.

Accordingly the rubble is proposed here for constructing the road bases together with ABC

only as the filling material for covering the interstitial voids there in.

In the technical aspect, it gives more bearing capacity than ABC material.

Since it is big in particle size, more area involves, hence it can withstand more load & transfer

more load to the ground with less settlement. As a result of this, rubble layer characterizes

higher B.C. value.

Since rubble is big in particle size, more energy can be absorbed before crushing. Hence it is

a big advantage to achieve stronger bases by using rubble than ABC material.

Other point is the less cost involved. It is cheaper material than ABC & mainly due to this

reason the construction cost is reduced remarkably. Hence the rubble bases will be very

attractive for constructing roads. Stronger base at a lesser cost is the main significance there

in.

Even though, rubble has been used in traditional Mecadom construction earlier, It is stressed

here that this proposal is different to it, due to involving well graded aggregate in place of

uniformly graded aggregates which used in Mecadom construction. Hence the failures which

had experienced in traditional method due to unfilled voids, lack of compaction ,lack of

compatibility & consistency in particle size etc. is avoided here by introduction of well graded

ABC (GABC).

After facing problems as above, with Macadam construction, authorities have changed straight

away to the present (well graded) ABC method. In the circumstances. This proposal lies, in mid-

way between traditional Macadum method & present( well graded)ABC laying operation in

order to economize the construction work. Accordingly the ample savings which involved in

this method can be imagined merely by assuming that the present (well graded) ABC layer is

replaced as much as by rubbles (9”) which is cheaper in the market.

In the light of the government rapid rural road improvement programme, benefit of the above

proposal will be much highlighted. Base strengthening has also been seen incorporated there

in as a measure of strengthening & upgrading rural roads. In the process of above road

programme, financial difficulties which face by the government could be reduced considerably

by introducing rubble laying in place of (well) graded ABC to the rural roads. In addition to this

rubble laying which followed by a cheaper surfacing on top will further reduce the cost. Finally,

being able to construct a more road length at a lesser cost, will be the main point of attraction.

It will be a direct benefit to the villagers & rural folk to have a good road network. Length of

road section which has to be cut off now due to the limited funding could be avoided

completely by this method & many villages in remote areas could be able to link within the

available budget. It will be a big boost to their lively hood as far as the rural falk is concerned.

In addition to the above effect, some more advantages are also involved in this proposal, in

the public point of view. One is that it will give an opportunity to have a very intensive public

participation from the village level.

In a situation where the quality rock is not available in the village ABC material will have to be

transported at a higher cost. Instead of it nearby rock in the village could be used for supplying

Rubble, since it requires only less quality parameters compared to GABC. This will be further

justified due to the less traffic volume& less speed in general which experienced in rural roads,

compared to the national roads. Responding to this situation, many available rocks could be

used for this rural roads uplifting programme very effectively. It can be organized as a village

centered labor intensive programme to supply rubble from the village itself.

Since the main role is played by the Rubble here, filling material will not be much important as

far as the quality is concerned. Hence it is an added advantage that the quality of the filling

also can be varied through a large range without affecting to the final quality of base layer.

Accordingly the various filling materials of which available in the village can further be

considered, including insitu soil gravel, weathered rock peaces etc. under the technical

guidance. Accordingly people will be benefited in two ways by participating to this construction

& earning the wages in addition to processing a good road to their village.

In addition to the wide public participation, being able to use the available machine fleet for

this operation is the encouraging factor there in.

Base Construction can be started by laying rubble on the existing road or compacted sub grade.

In order to assure the uniform laying, simple control bar can be fixed to the dump truck. It will

be followed by a Front end bucketed truck so that the ABC can be laid on rubble layer to

cover all undulations & to make the surface smooth enough to travel along. Finally the

compaction is achieved by using the vibratory rollers while adding any extra ABC material as

required for a smooth finish. Due to the effect of vibration, will push rubble to the final stable

position & orientation which surrounded by the well compacted ABC particles. Suitability to

village conditions is another point as considered when using rubble.

Most of the village roads are not wide enough, hence subjected to erosions & most roads pass

through paddy fields which goes under water frequently. In such a situation, compared to ABC,

rubble stands well ahead by withstanding against erosion, inundation & wash away. Especially

the road built on rubble base since it provides a having foundation, will be passable even after

subjecting to a flood till it repairs whereas the no signs of road remains after a flood which

built on ABC base. In such a situation road with ABC base will be a complete reconstruction

using heavy budget but the remaining strong rubble base after the flood needs only an overlay

on it. Accordingly the less secondary cost is very significant in rubble base as far as the life

cycle cost is concerned. Hence many of public grievances for restoring the roads after floods &

delaying repair work due to lack of funds can be minimized & be attended at a low budget.

Easier to handle during rainy season is another factor towards positive direction.. Mostly the

prepared sub base surface become muddy during the rainy period & laying & compaction of

ABC on it will not be practical. If the work is also urgent, in such a situation construction using

Rubble would be easy due to the fact that bigger stones will not be buried (embedded

completely) into mud fully , hence the construction can be continued smoothly. Further the

rubble which embedded so, will cause to improve the sub-grade condition as well against

settlements in sub grade. (There in the rubble will act as a strengthening measure to the

underneath weak soil.) This has to be considered in a situation of a high bearing capacity (BC)

which processed generally by the rubble base resulting to impose more resistance against

settlement.

Hence the cumulative effect of above two will work successfully against settlement.

Hence rubble base will well answer to the settlements which occur frequently in road sections

nearby paddy fields in rural roads. With the less maintenance budget also will be able to survive

the roads in passable condition for years of which built on rubble base. This will cause the

Rubble laying to promote, as to be the lasting solution in general, for the settlements of which

observed along roads.

However the benefit of using rubble bases is not limited to the rural roads. It can be widely

used for National roads as well. Ultimately, country will be processing a very strong rubble

base (foundation) all over at a lesser capital cost to receive any overlay in future. When the

base is strong, it is a secondary decision to be taken for the overlay, either for DBST, AC etc. It

will avoid future settlements & the need for reconstructions from the bottom.

Theoretically a rubble base course needs less compaction since it transfer loads mainly by

means of interlocking. Other factor which attributed for less compaction is only the filling

material of ABC requires compaction whereas Rubble denies. Hence it would be very effective

in areas where the compaction is difficult to achieve.

Since it will withstand against washing away & erosion, even during construction stage exposed

ruble layer will withstand against any such adverse situation. Further the rubble bases has

shown long lasting & satisfactory performance in hilly terrains than with ABC material. Hence

recommended for these areas.

As far as the large volume which acquired by the rubble (being the main constituent) is

concerned, energy required for crushing to ABC particles & compacting again into dense form

as in the present practice is completely eliminated due to the intervention of these stones

here . Hence with energy saving point of view, use of rubble as an alternative method is an

environmental friendly solution.

When more thickness is required depending on the site conditions, then rubble would be more

economical & stronger due to the load distribution improves significantly.

Instead of 9” rubble, even when the road is constructed using material billow 9” size (say 4”-

6”) also `will increase B.C. due to the particle size is still larger than ABC. Hence particle size 4”-

6” etc. also can be applied very effectively. In such a situation, even the layer thickness lesser

than 200mm. also can be applied for satisfactory performance of base course so that the

improved B.C. will compensate for lesser layer thicknesses.

As stated earlier, it is an added advantage to relax the quality of filling material in presence of

rubble. Accordingly, when using rubble, filling material can be varied through a large range

without affecting to the final quality of base course. Reintroduction of rubble as a base course

material is the main concern in this article. However in addition to this, filling materials are

also to be searched for in a large range here as an extension to the above exercise. Accordingly

various filling materials are proposed here including insitu soil, gravel, Contaminated ABC, ABC

mixed soil or with excessive fines weathered rock salty clay etc. For further researching.

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