For developing a good and durable road network in black cotton soil areas, the nature of soils shall be properly understood. Black cotton soils absorb water heavily, swell, become soft and lose strength. Black cotton soils are easily compressible when wet and possesses a tendency to heave during wet condition. BC soils shrink in volume and develop cracks during summer. They are characterised by extreme hardness and cracks when dry. The stability and performance of the pavements are greatly influenced by the sub grade and embankment as they serve as foundations for pavements. On such soils suitable construction practices and sophisticated methods of design are to be adopted. In the present paper, reasons for poor condition of roads in B.C soils and measures to be taken for construction and improvement of roads on BC soils are presented.
Following are some of the important reasons for poor condition of roads in BC soil terrain.
- Nature of BC soils
- Poor drainage facilities
- Use of gravelly soil in base and sub-base
- Improper estimate preparations
- Plying of overloaded vehicles and iron wheeled tractors trolleys/carts
- Damage of roads during collection of materials
- Forming roads on canal banks and tank bunds
Nature of Black Cotton SoilsRoads running in black cotton soils are known for bad condition and unpredictable behavior for which the nature of the soil contributes to some extent. Nature of blackcotton soils is discussed here.
- BC soils absorb water heavily, swell, become soft, lose strength, easily compressible and has atendency to heave during wet condition.
- BC soils shrink in volume and develop cracks during summer. They are chracterised by extreme hardness and cracks when dry.
- Soils are called highly expansive when Free Swell Index exceeds 50%. Such soils undergo volumetric changes leading to pavement distortion, cracking and general unevenness due to seasonal wetting and drying.
- BC soils produce a CBR value of 2 to 5% if compacted efficiently
- Buffer layer: Providing a non expansive layer of 0.6 to 1m thick prevents ingress of water into expansive soil layer and counteracts swelling. It also reduces the harmful effects of heaving and reduces the stress on expansive layer.
- Blanket course: A blanket course of at least 225 mm thick composed of coarse/medium sand or non plastic moorum with PI less than 6% shall be provided for full width of formation over expansive sub–grade.
- Drainage: Appropriate surface drainage and sub–surface system to prevent ingress and retention of water in the pavement structure.
- Thick BT surfacing: Desirably, 40mm thick BT surfacing shall be provided to prevent ingress of water.
Poor Drainage FacilitiesFor Indian roads drainage is a badly neglected aspect. Special publication of Indian Roads Congress, IRC: SP 42 clearly gives guidelines for providing surface and sub surface drainage systems. The main objective of road drainage is to prevent early damage of the pavement due to entry of excess of water and preventing saturation up to a depth of 1 meter belowthe top of sub grade
Various types of damages arising due to inadequate drainage are listed below.
- Reduction in bearing capacity of sub grade soil
- Pavement failures like potholes, rutting, waviness and corrugation in flexible pavement
- Reduction in strength of many pavement materials like stabilised soil, WBM and BT surfacing
- Damages to shoulder and pavement edges from surface water
- Considerable erosion of soilfrom sub strata, slopes, cut and hill side due to surface water.
- Ingress of water through berms and porous BT surfacing
- Seepage water flowing across the pavement structure
- High water table conditions
- Capillary rise of water through embankment and sub grade soils
- Absorption of water during curing of subsequent layers
Adequate arrangements to cater for both surface drainage and sub surface drainage are essential to prevent flooding of roads, weakening of road structure, formation of potholes, stripping of bitumen etc. The aim of good design should, therefore, be to remove the surface water efficiently and to keep water level well below pavement. Pavements are damaged more by water than from the effect of large volumes of traffic. The results of AASHTO road test have clearly indicated that the rate of serviceability loss of pavement where sub grade soil is saturated was 10 to 40 times higher as compared to those on dry sub grade soils.
In the above table, observed heights of capillary rise in various soils are given. It indicates that the chance of ground water entering into the road crust is very high in BC soil embankment and sub grade.
Use of Gravelly Soil in Base and Sub-baseThe biggest contributor of road damages in the southern states is the use of highly adhesive and plastic gravelly soil in sub base and base layers. Even today, many highway engineers strongly believe that adhesive quality is very much required for sub base and blindage materials. A look into the definitions and specifications prove that this belief is totally wrong.
Definition of sand and gravel as per Code IS: 1490 – 1987: Sand and gravel are defined as Cohesion less aggregates of angular, subangular, sub-rounded, rounded, flaky or flat fragments of more or less unaltered rocks or minerals. According to the system, gravel is the fraction of the material between 80mm IS sieve size and 4.75mm IS sieve size. Sand is a fraction of the material between 4.75mm IS sieve size and 75 micron IS sieve size.
Andhra Pradesh Standard Specification 138 for moorum: The moorum shall be composed of well graded coarse siliceous and gritty to touch and free from dirt and deleterious matter. Material passing 75 micron shall not exceed 10%. Liquid limit shall not exceed 20% and Plasticity index shall not exceed 6% for sub base, filler material in surface treated WBM roads and backing for revetment. The above values are 33% and 6to 9% respectively for filler material in WBM roads.
Fourth revision of Ministry of Shipping, Road Transport and Highways (MOSRT&H) specifications for road and bridge works Clause 401 and IRC: 37 – 2001, specified the following requirements for the materials to be used in sub-base.
- Material in mix passing 425 micron sieve shall have liquid limit less than 25% and Plasticity index less than 6%.
- Material passing 75 micron sieve shall be less than 10%.
- Material in the mix shall satisfy specified grading
- Material in mix shall have 10% fines value more than 50 KN
MOSRT&H 404.2.7. Binding material: Binding material to be used for water bound macadam as a filler material meant for preventing raveling, shall comprise a suitable material approved by the Engineer having a Plasticity Index (PI) value of less than 6
501.2.2: Coarse aggregates for BT courses: Coarse aggregates shall consist of crushed rock, crushed gravel or other hard material retained on 2.36mm sieve. They shall be clean, hard, and durable, of cubical shape, free from dust and soft or friable matter, or other deleterious matter. Where crushed gravel is proposed for use as coarse aggregate, not less than 90% by weight of the crushed material retained on the 4.75mm sieve shall have at least two fractured faces.
All the above requirements clearly indicate that if the locally available gravel is to be used in sub-base or in base or as coarse aggregates for bituminous surfacing, the gravel must be rocky type material not soil type material.
Materials available in various approved gravel quarries throughout Andhra Pradesh are red earth, clayey gravel or clayey sand but not gravel or moorum as per the stipulations of Standard Specifications. R&B (Roads & Buildings) Soil Testing Cell, Vijayawada and R&B Road Research Station, Hyderabad are conducting tests on soils for the past 35 years. Number of test results on samples from different Districts of Andhra Pradesh are obtained and enclosed for perusal. Some of the test results fromvarious Districts are enclosed for perusal. As Executive Engineer Roads and Buildings Quality Control division, Vijayawada, the author conducted tests on gravel samples from various gravel quarries in Andhra Pradesh and their results also produced values in the range shown in Table 2.
What does the gravel test results shown in the Table-2 indicate?
- The material available in our gravel quarries is not gravel and it may be called as red earth only.
- It possesses all bad qualities of BC soils such as water absorption, softening, high plastic nature, expansiveness and easy compressibility etc., but to a lesser extent.
- It produces high Maximum Dry Density and useful for use in sub grade and shoulder construction.
- Blending gravel with stone dust, sand or flyash will reduce the harmful effects of gravel to some extent but it is very difficult in the field for proper blending.
- Blending may be resorted to only for very low traffic roads and no gravel shall be allowed for high traffic roads in the road crust.
Improper Estimate Preparations
Both the black cotton soil and redearth absorbs water and becomes soft and compressible. For these roads, there is huge chance of water entering the embankment, sub grade, shoulder, sub-base and base and most of the water entering the pavement is likely to be absorbed by black cotton soil and red earth. If the blindage gravel in WBM layers is saturated lot of pot holes form on the bituminous surface. If the so called gravel base or sub grade are soaked sinking type of failures take place.
Plying of Overloaded Vehicles and Iron Wheeled Tractor, Trolleys & Carts
Plying of overloaded vehicles is prevalent in this area and the resultant high vehicle damage factor causes damage to all categories of roads. Plying of iron wheeled
Damage of the Roads During Collections of MaterialsVehicles conveying construction materials to the road site carry very heavy loads, inducing high vehicle damage factor, damage the sub grade, sub base and base layers. This type of damage is predominant when Water Bound Macadam layers are constructed. Strict control regarding over loading of vehicles shall be exercised and collection of materials shall not be allowed during rainy season. During rainy season, sub grade soils are saturated and are in a plastic state. Plying of heavy loaded vehicles at such times cause irrepaerable damage to the roads.
Forming Roads on Canal Banks and Tank Bunds
Figure. 4, canal slopes in stable condition when water is flowing. During drawdown condition water force on slopes gets removed allowing pore pressures and swelling pressures push the soil in to the canal.
Failure of canal slopes during draw down condition: When the canal runs full the water force exerts pressure on the slopes. After saturation of soil in the slopes pore pressure develops and the water force is countered resulting in the equilibrium state. During draw down condition the water force acting on the slopes is suddenly removed and there is no opposing force to the force developed in soil of slopes. It results in the pushing of soil slices into the canal. This causes damage to the road formation. Sudden shutdown of canals causes extensive damage to road and canal slopes. During Ognee Cyclone several Irrigation canals were shut down resulting in extensive damages to R&B roads and canal slopes.
Recommendations for Improvement of Roads in BC Soil Areas
- Drainage layer in the form of Granular Sub Base with crushed aggregates, up to the formation edge shall be provided in all new road constructions. In widening works GSB as a drainage layer shall be extended up to the edge of formation
- Base widths shall be more than BT surfacing width by 150mm to 200mm on either side to prevent accumulation of water below BT surfacing and to protect BT edges.
- Multi layer base constructions shall be restricted to places where required machinery is not available. Wet Mix Macadam or Crusher Run Macadam is cheaper than WBM at many places.
- Use of natural gravel, which is highly plastic, shall not be allowed in sub base or base layers.
- Earthwork excavations near the toe of formation shall be prohibited. Only selected earth required as per clause 305 of MOSRT&H Specifications for road and bridge works shall be allowed duly ignoring the cost criteria.
- Shoulder and sub grade are the most neglected items in the road construction. Equal importance shall be given to these items as specified in the above mentioned clause.
- For recently done roads, drainage chutes 300mm×450mm with drainage material using HBG crushed aggregates confirming to type A grading of table 300-4 shall be provided from the bottom edge of sub base to the edge of formation. They shall be spaced at 25m on both sides.
- Necessary steps shall be taken to prevent the unauthorized plying of iron wheeled vehicles and overloaded vehicles.
For improving the road connectivity to the pilgrim town of Amaravathi in Guntur District on the occasion of “Kalachakra Initiation - January 2006,” several R&B roads are widened and improved to facilitate smooth flow of traffic.
In all, 10 roads for a length of 62.28 Km are widened and strengthened and 4 roads for a length of 24.16 Km are improved in less than 6 months time. The main reason for the early completion is the use of fully mechanized Granular Sub Base and Wet Mix Macadam. If the old specifications are adopted, it would have taken 2 years time.
The existing roads are with 150 to 200mm thick gravel base (done with gravelly soil or red earth), 200mm to 250mm thick WBM with gravelly soil blind age and a thin bituminous surfacing. Such roads are widened with the provisions shown in the figure 5.
The remedial measures implemented in Kala Chakra works are:
- Providing GSB with HBG crushed aggregates as sub base up to the edge of formation.
- Providing Wet Mix Macadam in base layers.
- Providing suitable soils for the shoulders.
- Improving road geometrics wherever necessary.
- Badly damaged stretches are recycled by fully picking the crust, removing the gravelly soil, sectioning the picked metal, rolling, applying stone dust as screenings and binder. By this way the sunken roads are converted as new WBM surfaced roads. Further, GSB is proposed in low lying stretches and overall WMM followed by BM and SDBC. This process was adopted for strengthening and improving two problematic roads namely Tadikonda Kantheru road and Mangalagiri and Neerukonda roads in Kala Chakra works.
- Ministry of Road Transport & Highways Specifications for Road and Bridge Works – 2001.
- Ministry of Rural Development Specifications for Rural Roads- 2004.
- Bhavanna Rao D.V (2005)," Adverse effects of using natural gravel in sub base, base and Water Bound Macadam.” Indian Road Congress’s Indian Highways –February (2005).