N. K Bhasin, Scientist
Panchanan Oli, Scientist
Narinder Krishan, Scientist
Jagdish Chander, Scientist
Ashwani Kumar, Junior Scientific Assistant
Central Road Research Institute, New Delhi

ABSTRACT

Due to non-availability of hard stone within economical distances in desert areas, particularly in Central Rajasthan, road construction has been posing serious problems, especially in rural areas. However, a calcareous material locally known as 'Dhandla' is found in abundance under an overburden of 1.5 to 2.5 metres. Dhandla, being quite soft material, gets completely crushed under the road roller. Moreover, it is quite problematic to compact dhandla with road roller over the non-cohesive desert sand subgrades. As such, it has not found favour with the road construction agencies.

With a view to evolve low-cost specifications for the construction of rural roads using the locally available low grade materials to the maximum possible extent, a study has been undertaken at this Institute. It has been found that the bearing capacity of dhandla was considerably improved when compacted manually and could be effectively used in the lower layer of low-volume roads.

The details of the work carried out on a section of a rural road in Central Rajasthan are presented. The subsequent performance of the test track for a period of 5 years and the economic aspects are also discussed.

Introduction

For effective implementation of developmental programmes in the country, providing adequate network of roads in rural areas is, more or less, an essential necessity. The traditional specifications being followed for rural road construction are provision of stone soling or WBM (OS) in the sub-base, followed by a layer of nous surfacing. Due to non-availability of hard stone within economical distances, especially in desert areas, the cost of road construction is abnormally high. It is, therefore, felt that efforts should be made to find the scientific usage of locally available low grade road materials to the maximum possible extent, resulting in considerable economy. Use of low grade road materials to the maximum possible extent, resulting in considerable economy. Use of low grade road materials has, however, not found favour with the construction agencies. The same set of specifications are being adopted for rural road construction in desert areas, as is being followed in other parts of the country.

It is a common observation in desert areas that on account of the non-cohesive nature of desert sand sub-grade, the sub-base material has the tendency to sink into the subgrade resulting in deformations of various shapes and sizes. These deformations are subsequently reflected in the road surface causing either immobility or hampering the vehicular traffic. It is, there-fore, considered an essential pre-requisite to provide adequate support to the pavement for better performance. This could be achieved either by stabilising the desert sand sub grade with additives or by providing additional thickness in the sub base. Such treatments would result in increased cost of road construction. The Central and Western parts of Rajasthan have, at present, quite thin density of population where the rate of growth in the volume of traffic on rural roads is not expected to rise at a fast rate in the near future. It is quite imperative, therefore, to utilise the locally available road materials to the maximum possible extent.

A calcareous material locally known as 'dhandla' is found in abundance in Central Rajasthan, under an overburden of desert sand varying from 1.5 to 2.5 metres or so. It has been observed that dhandla could not be effectively compacted with road rollers over the loose and dry sand subgrade. The compaction plants often get bogged down in the process. If the dhandla layer(s) are not compacted properly, the road surfaces show different types of distress due to differential settlement.

There are many factors that govern the design, construction and performance of a road. These are (i) subgrade soil and its engineering properties, (ii) topography and climatic conditions prevalent in the area, (iii) type and volume of traffic likely to use the road, (iv) ground water table and drainage and (v) selection of proper road materials for construction including exercising the needed quality control. With a view of suing low grade materials for rural road construction in desert areas, experimental test tracks were laid on a section of a rural road, namely Lachhadsar-Momasar road in district Churu of Rajasthan. The selected site of the test tracks is shown in Fig. 1. It may be mentioned here that for selecting the site for the experimental tracks, due weightage was given to various factors briefly listed above.

The paper sets forth the materials used, details of the test tracks including construction procedure and performance of the test sections for a period of five years. Also discussed are the cost analysis of experimental sections vis-a-vis the cost of road construction by adoping conventional specifications.

Materials Used

It has been stated earlier that, in addition to the desert sand, a calcareous material 'dhandla' was available locally. However, stone metal had to be transported over long distances. Representative samples of desert sand, dhandla and stone metal were brought to the laboratory for evaluating their engineering properties. The test results of these materials are given in Table 1

The general preactice that was normally followed by construction agencies for road construction in desert areas was, that the road material in the lower layer was laid on the subgrade without imparting adequate compaction because the compaction plants, very often, get bogged due to the underlying loose and dry desert sand subgrade. In case of experimental test tracks, however, the top 25-30 cm of the subgrade was watered and compacted by a 35 HP tractor, giving 10-12 passes. IT was observed that adequate compaction of the subgrade could be achieved, to about 95 per cent of the maximum laboratory density, by addition of water and subsequent compaction by the tractor.

Table 1
Test results of road materials

S. No.	MaterialAtterberg limits	Sieve analysis-per cent Passing IS sieve						Moisture density relationship				Soaked CBR Value	Aggregate impact value
		Liquid Limit (%)	Plasticity Index (%)	10 mm	4.75 mm	2.36 mm	600 microns	75	Maximum dry density	Optimum moisture content (%)	(%)	Dry (%)	Wet
1.	Desert sand	Non-plastic	-	-	-	100	90	9	1.65	7	8	-	-
2.	Dhandla	Non-plastic	60	42	30	19	13	9	1.90	11	25	65	70
3.	Stone metal	-	-	-	-	-	-	-	-	-	-	18	19

Fig. 1 Map of Churu District showing loaction of test track on Lachadsar-Momasar road

3.3 After preparing the subgrade, requisite quantities of dhandla for the subbase layer mixed with needed amounts of water were spread, hand packed and compacted manually by hand-rammers. It was observed that the dhandla layer was compacted to 95 per cent of the maximum laboratory density, in the manner described above. For laying the base course, required quantities of dhandla mixed with moisture upto optimum moisture content, were spread to proper thickness over hand-rammed dhandla sub-base layer. The base layer could be conveniently and effectively compacted by power roller to the desired degree of densification. In order to facilitate adhension between base and bituminous surface, a layer of hard stone at the rate of 0.3 cu.m/10 sq. m was provided in some test sections. All the 10 test sections were topped with 2 cm thick premix carpet laid as per the convential practice of the State. The field density for the subgrade, sub-base and base layers for different test sections as obtained at the time of construction are given in Table 2.

Performance

It is well known that a road undergoes a gradual time-dependent deterioration depending upon the pavement structure and the type and volume of traffic, besides other considerations. The damages caused by the traffic may be in the form of ravelling, cracking, pot holing, rutting, settlement, etc. In order to know the type and intensity of traffic using the experimental test tracks, traffic counts were made before and after the construction of these sections. The traffic counts were recorded during the performance observations every year. The year-wise traffic volume on the experimental test sections is given in Table 3.

Table 2

Field densities of various layers of the different test sections at the time of construction

	Dry bulk density (gm/cc) for the various test sections
Description of the layer	1	2	3	4	5	6	7	8	9	10
Desert sand subgrade	1.58	1.57	1.58	1.56	1.59	1.60	1.60	1.60	1.58	1.59
Hand-rammed dhandla subbase	1.81	1.81	1.80	1.81	1.81	1.82	1.81	1.81	1.81	1.82
Roller-compacted dhandla base	1.84	1.86	1.82	1.83	1.81	1.82	1.83	1.82	1.82	1.82

Table 3

Year-wise traffic count on the test sections
Vehicle type	Average daily traffic for the year
Heavy-Commercial Vehicles, i.e. busses, trucks. etc.	4	4	5	9	8	12
Other vehicles like tractors, jeeps, cars, pneumatic-tyred animal-drawn carts etc.	30	32	40	80	90	100

It may be seen from Table 3, that the total traffic volume comprising heavy commercial vehicles and other light vehicles remained, more or less, the same for the first 3 years. However, it was observed that the daily traffic volume gradually increased to 12 heavy vehicles and 100 other light vehicles during the last 3 years; pneumatic using the rural roads in desert areas. A typical pneumatic tyred camel-drawn cart is shown in Photo 1.

4.2 The experimental test sections were inspected every year after the construction, observing carefully in regard to type and degree of distress that might have developed in the form of potholes, ravelling, settlement, cracking, rut depth, etc. A general view of the experimental test sections, after 5 years of service, is shown in Photo 2. Based on the surface conditions of the test tracks, the sections were rated into the following three categories.

'Good'- section free from formation of phtholes,ravelling settlement and surface ireguarities.

'Fair'- sections where formation of potoles, cracking, ravelling and settlement was of the order of 7-10 per cent of the surface area.

'Poor'- section where formation of potoles, cracking, ravelling and settlement, etc., was more than 10 per cent of the surface area.

4.3 The data on the pavement crust, field densities of various layers and the rated performance of the test sections for a period of 5 years are given in Table 4. It may be seen from the Table that all the 10 test sections remained in 'good' condition for the first 3 years. It may further be seen that the sections where a layer of hard stone grafting had been provided i.e. test sections no. 4, 5, 7, 8 and 10, have successfully stood the test of time for a period of 4 years. However, those sections where the bituminous surface was alid directly over the dhandla base i.e., section no. 1,2,6 & 9, showed a certain amount of distress by way of pothole formation, ravelling, surface cracks and settlement, etc., during the last 2 years and could be categorised as 'fair' when compared to those sections described earlier. Test section no. 3 where a WBM layer had been provided also developed distress to the same degree as other 'fair' sections. The condition of dhandla base layer in test section no. 7 under the bituminous surfacing is shown in Photo 3.

Cost Analysis

The cost of experimental test sections laid for different thickness is given in Table 5. it may be seen that the cost of road construction by adopting conventional specifications (in the case of section no. 3) was Rs.

Table 4
Showing total thickness, field densities of various layers and performance of test sections over a period of 5 years

Test Section No.	Thickness of the road crust (cm)	Dry bulk density (gm/cc) of different layer Rated performance of test sections during
		Subgrade	Hand-rammed Sub-base	Roller compacted dhandla Base	1979	1980	1981	1982	1983
1.	20.0	1.59	1.82	1.86	Good	Good	Good	Fair	Fair
2.	25.0	1.60	1.83	1.84	Good	Good	Good	Fair	Fair
3.	22.5	1.61	1.83	1.86	Good	Good	Good	Fair	Fair
4.*	25.0	1.61	1.84	1.87	Good	Good	Good	Good	Good
5.*	25.0	1.60	1.83	1.86	Good	Good	Good	Good	Good
6.	25.0	1.61	1.84	1.87	Good	Good	Good	Fair	Fair
7.*	20.0	1.61	1.83	1.85	Good	Good	Good	Good	Fair
8.*	15.0	1.62	1.84	1.86	Good	Good	Good	Good	Fair
9.	15.0	1.62	1.84	1.86	Good	Good	Good	Fair	Fair
10.*	10.0	1.60	1.84	1.86	Good	Good	Good	Fair	Fair

* Test sections where stone grafting had been provided over the dhandla base.

58,000/- per km. The cost of subgrade preparation and that of the bituminous surface has not been included, being common for all the test sections. It may further be seen that the cost of rural road construction in desert areas by using locally available low grade materials could be drastically reduced to Rs. 36,000/- for a crust of 15 cm and to Rs. 42,000/- for a pavement crust of 20 cm. In other words, a saving of the order of 28 to 38 per cent could be effected by using the low grade materials.

Table 5

Cost analysis of different section of experimental test tracks laid on Lachhadsar-Momasar road

		Cost in Rs. (for 50 m)
(a)	Specification No. 1 (20 cm)
(i)	Hand-rammed dhandla sub-base layer	768
(ii)	Roller-compacted dhandla base course	656
(iii)	Over-head charges like premium, Supervision, etc.     Cost for 3 m wide road per km.	243 1,694 Say Rs. 1,700 34,000
(b)	Specification No. 2 (25 cm)
(i)	Hand-rammed dhandla sub-base layer	898
(ii)	Roller-compacted dhandla base course	706
(iii)	Over-head charges like premium, Supervision, etc.     Cost for 3 m wide road per km.	406 2,010 say Rs. 2,000 40,000
(c)	Specification No. 3 (22.5)
(i)	Hand-rammed dhandla sub-base layer	337
(ii)	Roller-compacted dhandla base course	84
(iii)	Water bound macadam base	1,135
(iv)	Overhead charges like premium, supervision, etc.   Cost for 3 m wide road per km.	590 2,909 Say Rs. 2,900 58,000
(d)	Specification No. 4 (25 cm)
(i)	Hand-rammed dhandla sub-base layer	898
(ii)	Roller-compacted dhandla base with a layer of stone grafting	1,115
(iii)	Overhead charges like premium, supervision, etc.     Cost for 3 m wide road per km. Rs.	510 2,513 2,500 50,000
(e)	Specification No. 5 (25 cm)
(i)	Hand-rammed dhandla sub-base layer	581
(ii)	Roller-compacted dhandla base with a layer of stone grafting	1,256
	Overhead charges like premium, supervision, etc.     Cost for 3 m wide road per km. Rs.	467 2,304 Say Rs. 2,300 46,000
(f)	Specification No. 6 (25 cm)
(i)	Hand-rammed dhandla sub-base layer	786
(ii)	Roller-compacted dhandla base with a layer of stone grafting	847
(iii)	Overhead charges like premium, supervision, etc.     Cost for 3 m wide road per km. Rs.	416 2,049 2,050 41,000
(g)	Specification No. 7 (25 cm)
(i)	Hand-rammed dhandla sub-base layer	786
(ii)	Roller-compacted dhandla base with a layer of stone grafting	974
(iii)	Overhead charges like premium, supervision, etc.     Cost for 3 m wide road per km. Rs.	447 2,207 Say Rs.2,200 44,000
(h)	Specification No. 8 (25 cm)
(i)	Hand-rammed dhandla sub-base layer	673
(ii)	Roller-compacted dhandla base with a layer of stone grafting	833
(iii)	Overhead charges like premium, supervision, etc.     Cost for 3 m wide road per km. Rs.	381 1,887 Say Rs. 1,900 38,000
(j)	Specification No. 9 (15 cm)
(i)	Hand-rammed dhandla subbase layer	673
(iii)	Roller-compacted dhandla base	424
(iii)	Overhead charges like premium, Supervision, etc.     Cost for 3 m wide road per km. Rs.	278 1,375 Say Rs. 1,400 28,000
(k)	Specification No. 1 (10 cm)
(i)	Hand-rammed dhandla subbase layer	449
(ii)	Roller-compacted dhandla base	692
(iii)	Overhead charges like premium, Supervision, etc.     Cost for 3 m wide road per km. Rs.	291 1,432 Say Rs. 1,450 29,000

Note: The cost of preparation of subgrade and that of premix carpet surface course has not been included, being common for all the specification.

Concluding Remarks

As a result of the study, the following broad conclusions could be arrived at:-

• Locally available low grade materials, like dhandla could be effectively and economically used for rural road construcution in desert areas.
• Substaintial economy could be achieved by the use of low grade materials for rural road construction.
• In order to provide a stable subbase for the upper crust, the low grade materials should be compacted manually to the required state of densification, before laying the upper course.
• The most predominst vehicle-types on rural roads in desert areas are the pneumatic-tyred camel-carts. These vehicles have lesser damage causing capacity to the road surface and crust. In view of escalating prices, the road crust with lower thickness could be provided, for the present. These roads could be upgraded in the form of stage development as and when required.

Acknowledgements

This study has been conducted as part of the research programme of the Central Road Research Institute, New Delhi. The authours are grateful to Dr. M. P. Dhir, Director, for permission to publish the paper. The authors are thankful to Dr. N. B. Lal for his guidance and valuable suggestions throughout the study. The authors are also thankful to the Chief Engineer and concerned officers of the Rajasthan P.W.D. for providing the necessary facilities and cooperation for the construction of experimental test tracks.