Introduction

Among the major communication systems, road transport is the most important facility. It contributes to the economic, industrial, social, and cultural development of a region or a nation. It helps primarily in linking production and consumption centres. As a result, raw and finished goods are utilized in distant places from their centres of production. The growth of population has created need for better and economical vehicle operation which requires good highways having proper geometric design, pavement condition, and maintenance. Road transport plays a significant role in India’s economy, carrying 80 % of land transport demand. The total length of national highway network is about 65,000 km. This accounts for less than 2 % of total road network, but carries over 40 % of road traffic. Presently, the pavements of national as well as state highways have not properly used the soil stabilization techniques. Hence, it is proposed to study the engineering properties of soil in sub-base as well as subgrade layers along the existing highways and select a few soil stabilization methods to improve the engineering properties. Priority is given to stress deformation response and permeability characteristics of stabilized soil, as these are the important factors influencing the performance of highway pavement. Chemical soil stabilization technique is one of the important methods recently used. Chemical as soil stabilizers has been used to improve the strength due to low cost and relatively wide applicability compared to standard stabilizers. The use of chemical as stabilizer has been rarely subjected to technical developments and is presently carried out using empirical guidelines based on previous experience. Therefore, it becomes a primary priority to study and determine the effects of chemicals on the strength of different soils. In the present study, laterite soil was stabilized with chemical and shear tests are performed to study the effects of chemical modification on the soil properties. In a developing country like India, due to rapidly growing traffic volume, engineers have to come out with some solutions for better performance of roads keeping in mind cost effectiveness and sustainability of road network development. Cost effective roads are very vital for economical growth in any country. There is an urgent need to identify the new materials, improve the road construction techniques to expand the road network. When poor quality of soil is available at the construction site, the best option is to modify the properties of soil, so that it meets the pavement design requirements. This has led to the development of soil stabilization techniques. Since the nature and properties of natural soil vary widely, a suitable stabilization technique has to be adopted for a particular situation after considering the soil properties. Soil stabilization by mechanical or chemical means is widely adopted. Stabilization of soils is an effective method of improving the properties of soil and pavement system performance. The objective of any stabilization technique is to increase the strength and stiffness of soil, decrease plasticity index (PI) and to improve both workability of soil and constructability of pavement. In order to stabilize the soils for improving their strength and durability, certain chemical additives both organic and inorganic, have also be used in previous research works. Recently, a chemical named Zycosoil has emerged as a new soil stabilizer, which is used to improve the stability of soil layers for pavement structures.

Objective of the Study

In the present investigation, an attempt is made to study the behaviour of laterite soil with and without addition of the chemical, Zycosoil. The chemical when added to water and mixed with soil alters the engineering properties depending upon the type of soil and dosage of chemical. These chemicals are liquid additives, which act on the soil to reduce the voids between soil particles and minimize absorbed water in the soil for maximum compaction. In this present investigation, the effectiveness of Zycosoil in stabilizing the laterite soils of South Canara districts is investigated through laboratory experiments. Some of the important geotechnical properties, their improvement with different percentages of chemical additions are derived. The important properties such as index properties, compaction characteristics, Unconfined compressive strength (UCS), California bearing ratio (CBR) values, and fatigue behaviour were studied. Pavement analysis was conducted for low volume roads with chemically treated soil using KENPAVE software.

Chemical Stabilization

Extensive research has been conducted studying the application of traditional stabilizing additives such as lime, cement, and fly ash (Santoni et al. 2001). However, little research has been documented pertaining to the use of commercial non-traditional stabilization additives such as emulsions, acids, lignin derivatives, enzymes, tree resins, silicates etc. Stabilization process may include the blending of different types of soils or other materials like lime, cement etc. to achieve a desired gradation; or mixing of commercially available additives like chemical, enzyme etc. to alter the texture or plasticity, or act as a binder for cementation of the soil. The different uses of soil pose different requirements of mechanical strength and a resistance to environmental forces, controlling method to be used for the stabilization. Stabilization is being used for a variety of engineering works, the most common application being in the construction of road and air-field pavements, where the main objective is to increase the strength and to reduce the construction cost by making best use of locally available materials. In other words, stabilization includes compaction, preconsolidation, drainage, and many other such processes. A cement material or a chemical is added to a natural soil for the purpose of stabilization. The decreasing availability and increasing cost of construction materials and uncertain economic climates force engineers to consider more economical methods for building roads.

Miller and Azad (2000) conducted an experiment to evaluate the effectiveness of cement kiln dust (CKD) as a soil stabilizer. The study revealed UCS value of soil increases with the addition of CKD. Increase in UCS was inversely proportional to the PI of the untreated soil. Significant PI reductions were occurred with CKD treatment, particularly for soils with high PI.

Hashim et al. (2005) investigated the effect of using rice husk ash and cement on a stabilization of residual soil. Test results showed that both cement and rice husk ash decrease the maximum dry density (MDD) and increase the optimum moisture content (OMC).

Aydogmus et al. (2004) examined some of the mechanical properties when 6 % cement content is added to a typical cohesive soil with and without geogrid reinforcement. The addition of cement to clayey soil reduces noticeably the OMC and marginally the MDD for the same compaction effort. The strength of soil–cement tends to increase in a linear manner with increasing cement content.

Syed et al. (2007) conducted performance studies on soil samples collected from various borings with addition of 3, 4, and 5 % cement. Results indicate that the MDD for the cement stabilized subgrades varied from 105.0 to 126.2 pcf with an average value of 114.0 pcf. They concluded that stabilizing the in situ subgrade soils with small amounts (4 % by weight) of Ordinary Portland Cement (OPC) is a technically viable, cost effective, and speedy way to prepare the subgrades for the reconstruction of the airfield pavements. The UCS of stabilized soils increased with addition of cement with respect to curing days.

Ravi Shankar et al. (2008) reported that the addition of Pond ash to laterite soil improved the strength properties and resistance to moisture susceptibility. It also resulted in the reduction of MDD of blend with slight increase in the OMC.

Sadek et al. (2008) reported that the MDD increases and OMC decreases with the increase in sand and cement additives. The results showed that the additive admixtures altered the engineering properties of tropical peat soils. Higher strength was obtained from samples that had been cured for 14 days compared with 7 days cured samples.

Experimental Program and Test Details

Materials Used

Laterite Soil

Locally available laterite soil is used for the present study. Laterite soil procured from Surathkal, Karnataka State, was tested in the laboratory for the properties like specific gravity of soil solids, grain size distribution, consistency limits, compaction characteristics, UCS, CBR, and co-efficient of permeability values, a summary of which is presented in Table 1.

Table 1 Basic properties of laterite soil
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The grain size analysis results indicate that the soil contains 25.0 % of gravel, 46.0 % of sand, 26.5 % of silt, and 2.50 % of clay. Referring to these results, as per Indian standards (IS: 1498–1970) the soil is classified as clayey sand (SC). The liquid limit (LL) is 54 %, plastic limit (PL) is 28 %, and PI is 26 %. The MDD is found to be 1.68 and 1.91 g/cc and OMC is found to be 18 and 14 %, respectively for light and heavy compaction. The CBR values of laterite soil at soaked condition for light and heavy compaction are found to be 5 and 7 %, respectively. The UCS values of laterite soil for light and heavy compactions are found to be 125 and 168 kPa, respectively. The coefficient of permeability for heavy compaction is 2.31 × 10−04 cm/s.

Chemical Stabilizer

The chemical used for the present investigation to stabilize the laterite soil was Zycosoil manufactured by Zydex Industries, Gujarat, which is a water soluble compound. Zycosoil dissolves in water to form a clear-water solution. It forms a permanent water repellent layer on all types of soils, aggregates and other inorganic road construction materials. The reaction leads to permanent siliconization of the surfaces by converting the water-loving silanol groups to water repellent siloxane bonds. It helps in substantial reduction in soil water infiltration and erosion (Zydex Industries Product Brochure for Zycosoil is obtained from www.zydexindustries.com). The properties of Zycosoil are presented in Table 2.

Table 2 Properties of chemical stabilizer
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Test Details

To assess the suitability of Zycosoil as a soil stabilizer, both natural soil and chemically stabilized soil were tested for engineering properties and strength parameters. The CBR and UCS tests were conducted for different curing periods. In this investigation, an effort is made to analyze the stabilized soil with dosages of 2, 4, 6, and 8 % of the OMC value and for curing period of 0, 1, 2, 4, and 6 weeks. Chemical dosage calculations are explained in Appendix.

Engineering properties like grain size distribution, Atterberg’s limits, OMC, and MDD values were determined as per IS: 2720 (Part 2, 3, 4, 5, 7, 8 and 10). Grain size distribution as explained in Table 1 was obtained from sieve analysis of laterite soil using IS sieves. Liquid limit was found out using Casagrande’s apparatus. Optimum moisture content and MDD were found out for both Standard and Modified Proctor compactions. Permeability characteristics of the soil were determined in falling head permeability method using standard test equipment as per IS: 2720 (part 17)–1986. The UCS tests were conducted on treated soil samples for both standard and modified proctor densities. As per IS: 2720 (Part 16–1987) California bearing ratio tests were conducted for both samples in soaked and unsoaked conditions. Unconfined compressive strength and CBR characteristics were checked for chemically treated soils with different curing periods.

Fatigue Test

The fatigue tests were conducted on Repeated Load Testing Machine shown in Fig. 1. All experiments were conducted on specimens cured for predetermined period. The loading level in the present study was taken as a fraction of the UCS value of their respective specimen at the same condition of chemical dosage. The soil specimens having standard dosage of chemical with varying curing period were tested for repeated loading with 50, 40, and 20 % of their UCS values.

Fig. 1
figure 1

Fatigue testing machine

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Test Procedure

  • The cylindrical specimen (38 mm diameter and 75 mm height) was mounted on the loading frame and the deflection sensing transducers (Linear Variable Deflection Transducer—LVDT) were set to read the deformation of the specimen. The load cell and sample are arranged as shown in Fig. 2.

    Fig. 2
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    Sample arrangement

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  • In the control unit through the dedicated software, the selected loading stress level, frequency of loading, and the type of wave form were fed into the loading device.

  • The loading system and the data acquisition system were switched on simultaneously and the process of fatigue load application on the test specimen was initiated.

  • The repeated loading, at the designated excitation level (i.e. at the selected stress level and frequency) was continued till the failure of the test specimen.

  • The data acquisition system continuously record the vertical deformation of the test specimen with cycles of loading until the failure and the output is saved in a result file.

  • The failure pattern of the test specimen is visually observed.

Results and Discussion

The chemical dosages used for stabilizing the laterite soil are dosage 1, 2, 3, and 4 for different curing periods of 0, 1, 2, 4, and 6 weeks. Effect of dosages on index properties, strength, and permeability characteristics of laterite soil during the curing period are studied.

Effect on Consistency Limits

As the percentage of chemical increases, there is an improvement in Atterberg’s limits of soil as in Table 3. For pavement construction, soil with lesser LL and PI values are considered as its good characteristics. For untreated soil, LL, PL and PI values were 54, 28, and 26 %, respectively and for further addition of chemical dosage, LL and PI values were found to be decreasing. This is due to the chemical reaction causing substantial reduction in soil water infiltration and chemically treated soils do not allow absorption of water, resulting in reduced plasticity.

Table 3 Atterberg’s limits
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Effect on Compaction

The effects of chemical dosage on MDD and OMC of laterite soil for light and heavy compaction immediately after mixing are presented in Table 4. As chemical dosage increases, the MDD increases and OMC decreases for both light and heavy compactions. While adding the chemical to the soil, it reduces the voids between the soil particles and minimizes the adsorbed water in the soil for maximum compaction.

Table 4 Compaction test results
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Effect on Permeability

Permeability tests were carried out on laterite soil with different chemical dosages and the test results are tabulated in Table 5. The test results indicate that, as the dosage increases from zero to four, there is a considerable decrease in permeability. The chemical reaction leads to permanent siliconization of the surfaces by converting the water-loving silanol groups to water repellent siloxane bonds. But the test results indicate that there is not much variation in the co-efficient of permeability beyond chemical dosage 2.

Table 5 Permeability test results
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Effect on UCS

The samples are cured for 0, 1, 2, 4, and 6 weeks for the chemically treated samples and the results are tabulated in Table 6. As the dosage increases, UCS value increases up to certain level and beyond that it marginally decreases. The UCS values increases up to 4 weeks curing period and further curing marginally affects the UCS values for both light and heavy compaction. The dosage 2 is found to be optimum for all the cases, and beyond that it will be marginal. The chemical reacts with the soil particles and makes the surfaces water proof permanently and stiffen the soil to increase its strength.

Table 6 UCS test results
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Effect on CBR

California bearing ratio tests were conducted for both unsoaked and soaked condition with curing periods of 0, 1, 2, 3, 4, and 6 weeks. Since maximum UCS value was obtained for treated soil with dosage 2, CBR test was conducted for the same dosage. The results are tabulated in Table 7. The unsoaked and soaked CBR values of untreated soils are 14.4 and 7.0 %, respectively. There is a considerable increase in the load bearing capacity of the soil with the increase in curing period. The soaked CBR value is found to be 104.0 % after 4 weeks of curing. Since for the stabilized soils, the CBR test results obtained are not realistic (165, 118 % etc.), this method is not suggested for any chemically treated soil.

Table 7 CBR test results
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Effect on Fatigue Life

The experimental studies on fatigue life of laterite soil with and without addition of Zycosoil for different curing period subjected to cyclic loading with constant amplitude applied at a frequency of 1 Hz were conducted. The untreated soil samples were found to be so weak that they could not withstand even 20 % stress level for one cycle. For 6 weeks cured untreated samples also, fatigue life was found to be one cycle for all the stress levels. The fatigue lives of treated samples (dosage two) for different curing periods are presented in Table 8, which shows the enhancement in the fatigue behaviour of soil with chemical treatment.

Table 8 Fatigue test results for optimum usage of chemicals
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S–N Curve

The most basic information about the fatigue behaviour of specimens is represented by its S–N curves. S denotes stress amplitude and N denotes the number of stress cycles to complete fracture. In general, S–N curves represent progressive structural deterioration and gradual breaking of bonds. Figure 3 shows the S–N curve obtained by plotting stress level vs. fatigue cycle for chemical stabilized laterite soil samples with different curing periods. Fatigue life is found to be the lowest for soil samples without curing and it increases with increase in curing period. Also it is the maximum for minimum stress level (20 %) and decreases with increase in stress level.

Fig. 3
figure 3

S–N curve for different curing periods

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KENPAVE Software

KENPAVE, a computer package for pavement analysis and design (Yang 2004) is specifically used for the design of both rigid and flexible pavements. It offers extensive features that can be used to design the pavement subjected to different conditions. It performs the analysis based on stiffness matrix method.

This package can be used to analyse pavements, considering different types of loads and stresses likely to be induced over the pavement. It is being widely used now-a-days, and it was found that the results obtained from KENPAVE are well with those obtained from other conventional methods.

Pavement Design Catalogues suggested by IRC: SP: 72-2007 (for low volume roads) provides 35 cases with five different subgrade CBR values and seven traffic conditions. Out of these, cumulative ESAL 30,000–100,000 (five classes) were considered. Generally in a single layer, 75 mm thick Water Bound Macadam (WBM) is provided in practical cases, depending on size of aggregates and for effective compaction. Considering this fact, in this study, the WBM thickness is limited to 75 mm for single layer. Wherever the thickness of WBM is given as 100 mm, only 75 mm thick WBM is laid and the remaining 25 mm is added to GSB layer. This is done as per the suggestion by IRC: SP: 72-2007.

Allowable stresses and vertical displacements at every layer interface of each case were determined using KENPAVE. From the analysis, it is observed that the costly WBM layer can be replaced by Zycosoil treated laterite soil with minimum 4 weeks curing period (100 % CBR) of same thickness. Since providing a bituminous surfacing on the treated soil is difficult, minimum 75 mm thick bituminous treated WBM layer is suggested as surface course. The remaining WBM layers, below the surface course are replaced by chemically treated soil and the suggested modifications to pavement design catalogue is presented in Table 9. For cases which are not using WBM as base material, usage of chemically treated soil is not recommended.

Table 9 Suggested modifications to pavement design catalogue for IRC: SP: 72-2007
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Conclusions

Based on the tests conducted the following conclusions have been drawn.

  1. 1.

    The consistency tests conducted on chemically treated laterite soil shows that the PI value decreases as the chemical dosage increases.

  2. 2.

    The standard and modified proctor compaction tests conducted on treated laterite soil indicate that, as the dosage increases the MDD increases and OMC decreases. At optimal dosage of two, the MDD was 1.74 g/cc and OMC 16.5 % for light compaction; similarly, the MDD was 1.99 g/cc and OMC 12.5 % for heavy compaction.

  3. 3.

    At optimum chemical percentage (dosage two) the UCS value after 4 weeks curing period was found to be 636 and 788 kPa for light and heavy compactions, respectively, whereas for untreated soil, the values were 125 and 168 kPa. The UCS value increases gradually up to 4 weeks period of curing and beyond this there is a decrease.

  4. 4.

    The test results indicate that there is a continuous improvement in the CBR value with the higher curing period. For 4 weeks of curing period, the soaked CBR value increases to 104 from 7 % (in the case of untreated soil).

  5. 5.

    The co-efficient of permeability decreases as the dosage of the chemical increases.

  6. 6.

    The treated soil samples show a tremendous improvement in fatigue life. Untreated samples were failing in a single cycle, whereas the minimum fatigue life value found for treated samples was 58 (for the maximum critical case of treated sample, i.e. without curing subjected to maximum stress level of 50 %). For treated soil samples, fatigue life is found to be increasing with increase in curing period. After curing treated soil samples for 6 weeks, they attained about 95, 102, and 248 % increase in fatigue life for 0.20, 0.40, and 0.50 stress levels.

  7. 7.

    KENPAVE analysis shows that in low volume roads, costly WBM material for base courses can be replaced by treated soil with minimum 4 weeks curing periods.