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Plastic as a Soil Stabilzer

Published on Sep 27, 2018


Soil stabilization is the permanent physical and chemical alteration of soils to enhance their physical properties. Stabilization can increase the shear strength of a soil and control the shrink-swell properties of a soil, thus improving the load-bearing capacity of a sub-grade to support pavements and foundations. Stabilization can be used to treat a wide range of sub-grade materials from expansive clays to granular materials. Stabilization can be achieved with a variety of chemical additives including lime, fly ash, and Portland cement, as well as by-products such as lime-kiln dust and cement-kiln dust. These are the existing techniques to improve soil stabilization. This paper presents the details of studies conducted on the possible use of waste plastic for soil stabilization. We think that the addition of plastic strips into the soil will be a innovative technique to improve the shear strength, tensile strength and California bearing ratio (CBR) value of the soil in an economic way.


Soil stabilization can be done in many ways. But the stabilization using waste plastic fibers is an economic method since the stabilizer used here is waste plastic materials, which are easily available. A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic plastics of high molecular mass, but they often contain other substances. They are usually synthetic, most commonly derived from petrochemicals, but many are partially natural.

Plastic as a Soil Stabilizer

In the investigation the waste plastic materials has been chosen as the reinforcement material and it was randomly included in to the clayey soils with different plasticity indexes at five different percentages of fiber content (0%, 1%,2%, 3%, 4%) by weight of raw soil. The use of plastic fibers in unreinforced soil tremendously increases the CBR value, Shear Strength, Resistance to desiccation cracking, reduces Consolidation and Swelling.

Improved Properties Of Soil By Using Plastic As Soil Stabiliser

i) CBR Value

ii) Increased shear strength

iii) Reduction in consolidation settlement

iv) Reduction in swelling

v) Reduction in cracks

vi) Avoids disposal problems of plastic

CBR Value

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of soil. To find out the change of CBR value of the fiber reinforced soil with respect to unreinforced soil, CBR test is conducted. In this study, in order to find the optimum percentage of fiber content for the stabilized peat soil that would provide the maximum strength, peat soil samples at their natural water content were mixed with different percentages of cement and polypropylene fibers and were cured in air for a period of 90 days and then CBR test was performed on them. The samples examined for this purpose were prepared by adding 5, 15 and 25% cement and 0.1, 0.15, 0.2 and 0.5% polypropylene fibers. The sample which showed the maximum value of CBR after 90 days of curing was chosen as the optimum percentage of polypropylene fibers for further evaluation of strength of the stabilized peat soil.

Effect of stabilization on CBR value

The results of CBR tests for stabilized peat soil samples with cement and polypropylene fibers after air curing for 90 days .The CBR value of undisturbed peat soil is 0.785%. With the addition of 50% cement, it increased to 34% for unsoaked condition and 30% for the soaked condition. With the addition of 0.15% polypropylene fibers with 50% cement, this increased to 38% and 35% for unsoaked and soaked conditions. The results indicate that as cement amount in the mixture is increased, the CBR values also increase and addition of polypropylene fibers causes a further increase of the CBR values.

Increased Shear Strength

This experimental work has been performed to investigate the influence of Plasticity Index and percentage of waste plastic materials on the shear strength of waste plastic materials on the shear strength of unsaturated clayey soils. For this purpose, clayey soils with different plasticity Indexes were used and mixed with different percentage of waste materials to investigate the shear strength parameters of unreinforced and reinforced samples in terms of direct shear test. In order to determine the shear strength parameters (C and φ) of unreinforced and reinforced samples, a series of shear box tests at vertical normal stresses of 100-300 KPa and strain rate of 0.2% mm/min were carried out in accordance with ASTMD 3080.shear stresses were recorded as a function of horizontal displacement up to total displacement of 17 mm to observe the post failure behavior as well. Verification tests were also performed in order to examine the repeatability of the experiments.

Effects of stabilization on shear strength

Soil can be non-liner variation because the reinforcement materials exhibited a distribution with In general angle of internal friction increased with fiber content. The variation of with percentages of fiber contents leads to a conclusion that the behavior of the fiber included. The shear strength of fiber reinforced soil is improved due to the addition of the waste plastic fibers and it is a non linear function. Up to a critical fiber content shear strength increased considerably and later small reduction is observed. However shear values are greater than unreinforced soil.


In order to assess the effect of random fiber inclusion on consolidation settlement, swelling and hydraulic conductivity, oedometer tests were conducted according to ASTM D2435-96. In the current investigation all samples were prepared using the same dry density and molding moisture content equal to 70% of the liquid limit.

Consolidation settlements results

Effects of random fiber inclusion on consolidation settlement of soil samples were evaluated as function of fiber length, content and consolidation pressure. Prior to the fiber inclusion, consolidation settlement of unreinforced soil sample was determined. A Constant pressure, increasing the fiber contents from 1 to 8% resulted in reducing consolidation settlement of the samples. This is a common trend with all fiber lengths examined. Maximum and minimum consolidation settlements of 7.5 and 2.6 mm were respectively measured for the unreinforced sample and the sample reinforced by 8% fibers having 5mm length . This shows a reduction in consolidation settlement of approximately 25%.

Reduction In Swelling

Oedometer was used for swelling saturated on molding; they showed no affinity for further water absorption after flooding the oedometer water bath. Therefore, they did not exhibit much free swelling in order to be able to assess the effects of fiber inclusions on this characteristic. Therefore, volume changes during the unloading stage of the consolidation tests were measured and used as an indication of the possible effects of fiber inclusion on swellings. The swellings presented were measured after unloading the maximum consolidation pressure of 200kPa.

Effects of swelling test

It can be seen that by increasing the fiber content, the amount of swellings decreased. The unreinforced sample produced the highest swelling of about 3.4mm. This was reduced to approximately 1.5mm for the sample reinforced with 8% fibers having 5mm length which is a substantial reduction in swelling. For constant fiber contents, an increase in the fiber length from 5 to 10mm resulted in a slight increase in swelling. As a whole, however, the increase in the fiber length did not have a significant effect on swelling reduction. This was particularly true when the fiber contents remained constant. It can therefore be concluded that with the increase in fiber contents and lengths, the soil/fiber surface interactions were increased. This resulted in a matrix that binds soil particles and effectively resists tensile stresses produced due t swelling. Resistance to swelling is mainly attributed to cohesion at the soil/fiber interfaces.

Desiccation Cracks

Oedometer rings were used to investigate the effects of random fiber inclusion on desiccation cracking of the soil. After molding, confining rings containing the specimen were placed in open air in the laboratory at a temperature of about 30°C. Samples were regularly weighed and when no changes in three consecutive measurements were observed, they were considered completely dried. Then, samples were used for observational examination of the extent of cracking.

Effects of desiccation cracks

Observational examination of samples after desiccation showed that by increasing the fiber contents and lengths, the extent and depth of cracks were significantly reduced. It can be seen that extensive, deep and wide cracks were formed in the unreinforced sample. The reinforced sample, however, has mainly experienced separation from the metal ring with no visible sign of cracks forming within the sample. This clearly shows the effectiveness of random fiber inclusion in resisting and reducing desiccation cracking which is of paramount importance in surface cracking of clay covers used in landfills. Therefore, it can be concluded that random fiber inclusion seems to be a practical and effective method of increasing tensile strength of the clayey soils to resist volumetric changes.

Reduce Environmental Hazards

The most important point is the environmental concern regarding the effects of waste plastic in soil and the problems and threats that is related with their excessive usage and disposal. This gives an effective solution to waste treatment with the advent of soil reinforcement.


Plastic is one of the major toxic pollutants of our time. Being a non-biodegradable substance, composed of toxic chemicals, plastic pollutes earth, air and water. Beside all these ill effects we here suggested one method which drastically change the view by which the people are concerned it today. Here without affecting the normal texture of the soil we are stabilizing it with the fiber format of the plastic.


1. Carol J. Miller and Sami Rifai, (2004), “Fiber Reinforcement for Waste Containment Soil Liners”, (ASCE) Journal,(1-5).

2. S. A. Naeini and S. M. Sadjadi ,(2008) ,” Effect of Waste Plastic Materials on Shear Strength of Unsaturated Clays”, EJGE Journal, Vol 13, Bund k,(1-12).

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