1 CHAPTER 1: INTRODUCTION 1.1 General Road transport is an only means of transport that offers itself to the whole community alike. It is accepted fact that of all the modes the transportation, road transport is the nearest to the people. India is an agricultural country. There are about 5.76 lakhs villages in India. The creation of better access in rural areas is a must for the developing countries like India. In India the roads have been classified as national highway, state highway, major districts road, other district road and village roads. Based on the funds available these roads are constructed with a variety of material and varying construction specification. The Rural roads are constructed with local material and are either paved or unpaved. The deterioration of these roads is governed by the behavior of the road material and lack of maintenance activities. This results in rutting, pot holes, corrugation etc. which creates a bad shape and poses problems for the engineers. Further, insufficient drainage, change in weather condition, increased traffic density and poorly graded material are the reasons for the deterioration which results in higher cost of maintenance for the repair process. Since the availability of funds is limited, improved and cost effective techniques are to be thought of to make the pavement problem free. With over 75% of the population of the country living in the villages, the developments in urban centers alone do not indicate the overall development of the country. Only with the improvement in the transportation facilities in rural areas, there could be faster development of the rural centers. The inputs of the agricultural and other commodities could reach the rural population easily and similarly the products can be sold at the nearest marketing centers for more remunerative price resulting in faster economic
2 growth and decreased wastages. With improved facilities for education, health care and other social needs in the villages, the urge for the migration to urban centers decreases, thus shall be in balanced development as a country as a whole. These roads are constructed with a variety of material and various constructions specification. The pavements are designed as flexible pavements and rigid pavements. The structured components of flexible pavements are consisting of sub grade, sub base, base course and wearing courses. The sub grade is an integral part in the pavements construction. It supports the pavements from beneath. Hence sub grade soil properties are very important for the design of pavement. These pavements are damaged at a shorter period due to poor quality of the soil. Hence the study of the engineering behavior of the different types of soils is extremely important to the civil engineer. The strength of the soil to withstand loads, under different site conditions therefore becomes an important for the design of flexible pavements. It has been thought that a soil largely containing small, particles will differ in its engineering behavior from a soil mainly containing relatively larger particles. In reality, soil contains different particles viz. gravel, sand, clay and silt. Pure sands and pure clays behave altogether differently in the field. The sandy granular particles are responsible for giving strength and hardness to the soil but lacked cohesion and binding power between the grains. Soil containing such sandy particles is thus liable to be washed away easily and its slopes will be very unstable. On the other hand, clay soils containing finer particles do possess sufficient binding force between the grains but lack in shear strength, particularly, when they get saturated. The presence of water is thus harmful to clays, whereas it provides apparent cohesion and strength in sands. It can thus, be appreciated
3 that s sands and clays do behave almost opposite to each other, and it is possible to obtain the good properties by blending these 2 material (Robert W.Day, 1994) Gravels, sands and silts are cohesion less materials that exist in deposits ranging from a state of loose to dense and course to fine. Most deposits however are in a medium to fairly dense state. These materials can have cohesion from clay minerals in the fine sand and silt fill that may present. Silts and clays are of particular interest in pavement construction because they tend to be most troublesome in terms of strength and settlements. The clay particle has a high affinity for water, and individual particles may absorb 100+ times the particle volume. The presence (or) absence (during drying of water can produce very large volume and strength changes. Clay particles also have very strong into particle attractive forces, which account in part for the very high strength of a dry lump (Joseph E. Bowles). 1.2 Sub grade Subgrade can be defined as a compacted layer, generally of naturally occurring local soil, just beneath the pavement crust, providing a suitable foundation for the pavement. The subgrade in embankment is compacted in two layers, usually to a higher standard than the lower part of the embankment The subgrade, whether in cutting or in embankment, should be well compacted to utilize its full strength and to economize on the overall pavement thickness. The current MORTH Specifications require that the subgrade should be compacted to 100% MDD achieved by the Modified Proctor Test (IS 2720-Part 8). For both major roads and rural roads the material used for subgrade construction should have a dry unit weight of not less than 16.5kN/m 3. Subgrade soil is a gathering or deposit of earth material, derived naturally from the breakdown of rocks or decay of undergrowth
4 that can be excavated readily with power equipment in the field or disintegrated by gentle reflex means in the laboratory. The supporting soil below pavement and its special under course is called sub grade. Without interruption soil beneath the pavement is called natural sub grade. Compacted sub grade is the soil compacted by inhibited movement of heavy compactors. Following are the desirable Property of Subgrade Soil: Stability, Incompressibility, Permanency of strength, Minimum changes in volume and stability under adverse conditions of weather and ground water, superior drainage, and Ease of compaction. 1.3 Soil Compaction Soil compaction is one of the most critical components in the construction of roads, airfields, embankments, and foundations. The durability and stability of a structure are related to the achievement of proper soil compaction. Structural failure of roads and airfields and the damage caused by foundation settlement can often be traced back to the failure to achieve proper soil compaction. Compaction is the process of mechanically densifying a soil. Densification is accomplished by pressing the soil particles together into a close state of contact with air being expelled from the soil mass in the process. Compaction, as used here, implies dynamic compaction or densification by the application of moving loads to the soil mass. This is in contrast to the consolidation process for fine-grained soil in which the soil is gradually made denser as a result of the application of a static load. With relation to compaction, the density of a soil is normally expressed in terms of dry density or dry unit weight.
5 1.3.1 Advantages of soil compaction Certain advantages resulting from soil compaction have made it a standard procedure in the construction of earth structures, such as embankments, subgrades, and bases for road and airfield pavements. No other construction process that is applied to natural soils produces so marked a change in their physical properties at so low a cost as compaction (when it is properly controlled to produce the desired results). 1.3.2 Properties affected by soil compaction Principal soil properties affected by compaction include - Settlement, Shearing resistance, Movement of water and Volume change. Compaction does not improve the desirable properties of all soils to the same degree. In certain cases, the engineer must carefully consider the effect of compaction on these properties. For example, with certain soils the desire to hold volume change to a minimum may be more important than just an increase in shearing resistance. Settlement: A principal advantage resulting from the compaction of soils used in embankments is that it reduces settlement that might be caused by consolidation of the soil within the body of the embankment. This is true because compaction and consolidation both bring about a closer arrangement of soil particles. Densification by compaction prevents later consolidation and settlement of an embankment. This does not necessarily mean that the embankment will be free of settlement; its weight may cause consolidation of compressible soil layers that form the embankment foundation.
6 Shearing resistance: Increasing density by compaction usually increases shearing resistance. This effect is highly desirable in that it may allow the use of a thinner pavement structure over a compacted subgrade or the use of steeper side slopes for an embankment than would otherwise be possible. For the same density, the highest strengths are frequently obtained by using greater compactive efforts with water contents somewhat below OMC. Largescale experiments have indicated that the unconfined compressive strength of clayey sand could be doubled by compaction, within the range of practical field compaction procedures. Movement of water When soil particles are forced together by compaction, both the number of voids contained in the soil mass and the size of the individual void spaces are reduced. This change in voids has an obvious effect on the movement of water through the soil. One effect is to reduce the permeability, thus reducing the seepage of water. Similarly, if the compaction is accomplished with proper moisture control, the movement of capillary water is minimized. This reduces the tendency for the soil to take up water and suffer later reductions in shearing resistance. Volume change Change in volume (shrinkage and swelling) is an important soil property, which is critical when soils are used as subgrades for roads and airfield pavements. Volume change is generally not a great concern in relation to compaction except for clay soils where compaction does have a marked influence. For these soils, the greater the density, the greater the potential volume change due to swelling, unless the soil is restrained. An
7 expansive clay soil should be compacted at moisture content at which swelling will not exceed 3 percent. Although the conditions corresponding to a minimum swell and minimum shrinkage may not be exactly the same, soils in which volume change is a factor generally may be compacted so that these effects are minimized. The effect of swelling on bearing capacity is important and is evaluated by the standard method used by the US Army Corps of Engineers in preparing samples for the CBR test. 1.3.3 CBR and its influence on flexible pavement design The CBR test was originally developed by O.J. Porter for the California Highway Department during the 1920s. It is a load-deformation test performed in the laboratory or the field, whose results are then used with an empirical design chart to determine the thickness of flexible pavement, base, and other layers for a given vehicle loading. Though the test originated in California, the California Department of Transportation and most other highway agencies have since abandoned the CBR method of pavement design. In the 1940s, the US Army Corps of Engineers (USACE) adopted the CBR method of design for flexible airfield pavements. The USACE and USAF design practice for surfaced and unsurfaced airfields is still based upon CBR today (US Army, 2001). The CBR determination may be performed either in the laboratory, typically with a recomputed sample, or in the field. Because of typical logistics and time constraints with the laboratory test, the field CBR is more typically used by the military for design of contingency roads and airfields. The thickness of different elements comprising a pavement is determined by CBR values. The CBR test is a small scale penetration test in which a cylindrical plunger of 3 in2 (5 cm in diameter) cross-section is penetrated into a soil mass (i.e., sub-grade material) at the rate of 0.05 in. per minute (1.25 mm/minute).
8 Observations are taken between the penetrations resistances (Called the test load) versus the penetration of plunger. The penetration resistance of the plunger into a standard sample of crushed stone for the corresponding penetration is called standard load. The California bearing ratio, abbreviated as CBR is defined as the ratio of the test load to the standard load, expressed as percentage for a given penetration of the plunger. CBR = (Test load/standard load) 100 In most cases, CBR decreases as the penetration increases. The ratio at 2.5 mm penetration is used as the CBR. In some case, the ratio at 5 mm may be greater than that at 2.5 mm. If this occurs, the ratio at 5 mm should be used. The CBR is a measure of resistance of a material to penetration of standard plunger under controlled density and moisture conditions. The test procedure should be strictly adhered if high degree of reproducibility is desired. The CBR test may be conducted in re-molded or undisturbed specimen in the laboratory. The test is simple and has been extensively investigated for field correlations of flexible pavement thickness requirement. 1.4 Organization of thesis Chapter 1 discusses an overview of road transport and its importance. It also deals with subgrade soil and the factors governing the properties of subgrade. In addition the soil compaction, its advantages and the properties affected by compaction such as Settlement, Shearing resistance, Movement of water and Volume change are discussed in this chapter. A description on importance of CBR in flexible pavement design also included in this chapter. Chapter 2 gives the description of mechanics of stabilization and different types of stabilizations like mechanical, chemical and bioenzymatic stabilization etc. The effects of
9 enzymatic stabilization are discussed in detail. Detailed review of literature on bioenzymatic stabilization in and around the world done by various researchers are discussed elaborately in this chapter and findings from the literature also summarized. The objectives of present research work are also given at the end of this chapter. Chapter 3 summarizes the details of experimental investigations and theoretical investigations. Under experimental investigations part, a complete description of various laboratory tests done on selected soil samples for the present research work like grain size distribution analysis, index properties of soil and strength properties like compaction, unconfined compressive strength and California Bearing Ratio is given. Procedure and methodology of field test to study the performance of pavement subsurface layer by non destructive testing method called Benkelman beam deflection measurement is given in detail. The different dosage of bioenzyme used in the present research work and its calculation also given in this chapter. Under theoretical investigation part, methodology of performing simple linear regression analysis, multiple linear regression analysis and Artificial Neural Network are described. Chapter 4 discusses the results of various laboratory tests conducted on different soil samples in detail. It also discusses the results of theoretical investigations. Chapter 5 summarizes the work done and discusses the salient conclusions drawn from the present research work.