Bioenzymes are organic degradable materials, currently introduced as soil improvement additives. In this experimental study, three types of bioenzymes from three different countries were used to improve Universiti Kebangsaan Malaysia (UKM) soil. UKM soil has properties quite similar to soils recommended as suitable by bioenzyme suppliers. The effect of the three bioenzymes on Atterberg limits, compaction characteristics, and unconfined compressive strength was studied. Controlled untreated and treated samples for two dosages at curing times up to three months were prepared and tested after completion of the curing period. Some results showed little improvement in compaction characteristics, and unconfined compressive strength, but no notable improvement was noticed in Atterberg limits. X-ray diffraction (XRD), X-ray fluorescence (XRF), and field emission scanning electron microscopy (FESEM) tests were conducted for untreated and treated soil samples after two months of curing. XRD and XRF did not show any change in mineralogy and chemical composition between controlled untreated samples and samples treated with the three bioenzymes. However, the FESEM images revealed a denser packing of particles for soil samples treated with two of the bioenzymes.
In the construction industry, maintaining a balance between performance and cost, while at the same time satisfying environmental regulations, has become a challenge for building material manufacturers, design engineers, and contractors. This challenge has led to identification and use of new construction materials and techniques. Geotechnical engineering projects are closely related to economic and environmental issues; therefore, improving sustainability of materials used in these projects may help in attaining overall sustainable development [
Recently, bioenzymes have been introduced for soil stabilization, especially in highway projects. They are organic materials, which are supplied as a concentrated liquid. It is claimed by bioenzyme manufacturers that their products are effective, environmentally friendly (nontoxic), cost-effective, and convenient to use. They are generally extracted by the fermentation of vegetables and sugar canes; thus they are degradable; that is, they easily break down and dissolve with time. They are supplied in liquid form and are easily soluble in water, which is used for soil compaction. This saves time and costs normally consumed by the mixing of traditional solid stabilizers with soil. Kestler [
Enzymes are biological catalysts present in all living organisms. They are obtained from plants and animals, including microorganisms, by extraction using suitable solvent. Kestler [
Scholen [
Enzyme manufacturers and suppliers claim that enzymes, when used in soil stabilization, can enhance the wetting and bonding properties of the soil particles. The enzymes make the soil more workable, which can be compacted more heavily. Furthermore, the enzymes enhance the chemical bonding of soil particles, which aids in combining them. Thus, a more durable soil structure is built that is more resistant to weathering, traffic, and water infiltration.
Strength tests have shown a considerable increase in strength for soils treated with bioenzymes [
Soil treated with PermaZyme 11-X showed very modest or no improvement in stiffness, freeze-thaw, leaching, and wet-dry tests [
Soil investigation practices are sometimes criticized for the extensive time needed and high cost involved. However, larger losses can be saved, which may arise if proper soil investigation prior to field application is bypassed. If a stabilizer is not effective in controlled laboratory conditions, then it is likely that it cannot produce the desired results in the field. As discussed previously, the experimental studies conducted to evaluate enzyme’s suitability as a soil stabilizer have revealed dissimilar results. Thus, this experimental study was aimed at evaluating the suitability of three commercial enzymes. The experimental studies conducted thus far used optimum moisture content (OMC) of untreated soil to prepare soil samples treated with enzymes. However, in this study, the effect of enzymes on compaction characteristics was examined. For this purpose, a revised protocol for sample preparation was adopted as recommended by Rauch et al. [
Three types of bioenzymes, DZ-1X (Boron Innovations Pvt., Ltd., India), EarthZyme (Cypher Environmental Ltd., Canada), and TerraZyme (Nature Plus, Inc., USA), were selected for this study and designated as E-I, E-II, and E-III, respectively. EarthZyme and TerraZyme suppliers provided the material safety data sheet (MSDS), whereas the supplier of DZ-1X did not provide MSDS, though several requests were made. However, some of the properties of the DZ-1X enzyme were determined in the laboratory, and the information contained in MSDS for EarthZyme and TerraZyme is presented in Table
Physical and chemical properties of enzymes.
Item | DZ-1X | EarthZyme | TerraZyme |
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Water | — | 21.06% | >50% |
Alcohols, C12–C16, ethoxylated | — | — | <30% |
Fermented vegetable extract | — | — | <20% |
Nonionic surfactants | — | 55% | — |
Polysaccharides | — | 2% | — |
Oligosaccharides | — | 3% | — |
Disaccharides | — | 5% | — |
Monosaccharide | — | 8% | — |
Lactic acid | — | 3.5% | — |
Potassium as the chloride | — | 1.2% | — |
Aluminum as the sulphate | — | 0.04% | — |
Magnesium as the sulphate | — | 1.2% | — |
Total |
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Specific gravity | 1.0 | 1.0 to 1.1 | 1.0 to 1.1 |
pH (neat)1 | 4.5 | 3 to 6 | 2.8 to 3.5 |
Boiling point | >100°C | >100°C | >100°C |
Ultimate biodegradability | — | DOC2 reduction >90% after 28 days | — |
Composition | — | A blend of fermented carbohydrates, inorganic salts, and surfactants | — |
Different suppliers often express the recommended application rates by using different terminology and units. However, it would be advantageous to define the following terms, which were suggested by Rauch et al. [ Dilution mass ratio (DMR) is the mass ratio of a concentrated chemical product to water, used to express the product dilution in water prior to soil application. Application mass ratio (AMR) is the mass ratio of a concentrated chemical product to oven-dried material in the treated soil.
In addition, because the recommended doses by the suppliers were very low, the enzymes were diluted in water prior to their application. Suppliers recommended doses, DMR, AMR, and diluted application ratios are given in Table
Recommended dosages, dilution ratios, and diluted application ratios of bioenzymes.
Stabilizer | E-I (DZ-1X) | E-II (EarthZyme) | E-III (TerraZyme) |
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Suppliers recommended dosage | 1 liter per 4.2 m3 | 1 liter per 33 m3 | 1 liter per 25 m3 |
Equivalent dilution mass ratio (DMR) |
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Equivalent application mass ratio (AMR)* |
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Diluted application ratios* | 27 mL per kg of soil | 17 mL per kg of soil | 22 mL per kg of soil |
The selected soil was taken from within the campus of Universiti Kebangsaan Malaysia in Bangi, Selangor, Malaysia. Therefore, it is abbreviated as UKM soil. It is a residual soil classified as CL, using a plasticity chart. The soil for all the tests was collected at once to reduce the chances of heterogeneity while preparing soil samples.
Rauch et al. [
AMR was calculated from the dosage recommended by the product supplier, and concentrated stabilizer product was diluted to the recommended DMR. The soil was mixed with initial moisture content
To explain the abovementioned procedure, consider the determination of MDD and OMC for E-I-D1 (sample prepared with a single dosage of enzyme DZ-1X). Four samples (2 kg each) of oven-dried soil were taken. Moisture loss during sample preparation was estimated as 1%; therefore, the first trial was made with a moisture content of 2% less than the OMC for untreated (UT) soil, that is, 15%. The volume of the diluted enzyme solution (5 mL in 1 litre of water) for a single dosage (D1) was calculated (54 mL for 2 kg). Thus, 246 mL (0.15 × 2000 mL − 54 mL) was added in 2 kg of soil, and the soil was allowed to mellow for 16 h. After the mellowing period, 54 mL of diluted enzyme solution was added in the soil, and, again, the soil was mellowed for 1 h before compaction. Similarly, the other three samples were prepared with moisture contents of 16, 17, and 18%, and MDD and OMC were calculated for two dosages of all three enzymes.
Three geotechnical tests, that is, Atterberg limits, compaction, and unconfined compression strength (UCS), were conducted in this study. The Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (ASTM D 698) was conducted to evaluate any changes brought by enzymes in compaction characteristics, that is, maximum dry density (MDD) and optimum moisture content. For UCS and Atterberg limits tests, initial curing periods of 7, 28, and 56 days were intended. However, it was later decided to cure the samples for 28, 56, and 84 days because no improvement was observed in UCS after 7 days for all of the enzymes and the two dosages used. For each curing time, one Proctor sample (dia. 101.6 mm, height 116.4 mm) was prepared, cured, and trimmed into three samples (dia. 38 mm, height 76–95 mm) just before the unconfined compressive strength test. Three samples were tested, and an average value of the three was recorded. After the test, the whole sample was used for moisture content determination. In total, three untreated (UT) and 18 treated (E series) samples were prepared. Atterberg limits (plastic limit and liquid limit) of the samples were determined after 56 days of curing.
The indices and other properties of UKM soil are shown in Table
Characteristics of UKM soil.
Characteristics | Value/description |
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Plasticity index1 (PI) | 19.5% |
Liquid limit1 (LL) | 42.3% |
Clay fraction2 | 29.6% |
Soil classification3 | CL |
Optimum moisture content (OMC)4 | 16% |
Maximum dry density (MDD)4 | 1.785 gm/cm3 |
pH | 4.05 |
2“Standard Test Method for Particle-Size Analysis of Soils,” ASTM D 422.
3“Plasticity chart,” ASTM D 2487.
4“Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort,” ASTM D 698.
According to Kestler [
Compaction characteristics of UKM soil (untreated) were determined using the standard compaction effort (ASTM D698), and the same procedure was used to identify any change in compaction characteristics due to enzymes. During the preparation of untreated and treated soil samples, an increment of 1% moisture content was chosen so that precise compaction characteristics could be determined. Three important factors that affect the compaction of soil are moisture content, soil type, and compaction effort. For a given soil, as the compaction effort is improved, the MDD is increased and OMC is decreased. The bell-shaped curves with single peak, which were achieved in this study, are typical of clayey soils with liquid limits between 30 and 70 as observed by Lee and Suedkamp [
Compaction curves for untreated (UT) soil samples and soil treated with single dosage of the three different enzymes.
Compaction curves for untreated (UT) soil samples and soil treated with double dosage of the three different enzymes.
The two enzymes E-II and E-III (chemical composition of DZ-1X was not provided by the supplier, but during the dilution of DZ-1X, foaming was formed, showing its surfactant-like behavior) contained nonionic surfactants, yet improvement in compaction was not observed. The reason for this performance could be a very low quantity dose of the enzymes. Therefore, all the treated samples were prepared with optimum moisture content of the control untreated soil samples. The average dry densities of all untreated and treated samples are given in Figure
Average maximum dry density for untreated (UT) and treated (E series) samples.
The moisture content determined at the time of compaction and moisture content at the time of testing (UCS) for all the prepared samples are given in Table
Moisture content of prepared samples.
Sr. number | Enzyme & dosage | One month | Two months | Three months | |||
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1 | UT | 16.0 | 15.7 | 15.9 | 15.8 | 15.7 | 15.1 |
2 | E-I-D1 | 15.3 | 15.2 | 16.2 | 16.0 | 15.9 | 15.9 |
3 | E-I-D2 | 16.0 | 15.8 | 16.2 | 16.1 | 15.9 | 15.5 |
4 | E-II-D1 | 16.5 | 16.1 | 16.1 | 15.9 | 16.3 | 16.2 |
5 | E-II-D2 | 16.4 | 16.2 | 16.1 | 15.7 | 16.3 | 16.2 |
6 | E-III-D1 | 15.7 | 15.6 | 16.1 | 15.9 | 16.4 | 16.0 |
7 | E-III-D2 | 16.7 | 16.6 | 16.1 | 16.1 | 16.1 | 15.8 |
2Moisture content in % at the time of UCS test.
Liquid limit and plastic limit were determined after 56 days of curing. Marasteanu et al. [
Plasticity indices for untreated (UT) and treated (E series) samples.
Mgangira [
The Standard Test Method for Unconfined Compressive Strength of Cohesive Soil (ASTM D 2166) was used to evaluate the ultimate compressive strength of untreated and treated soil samples. Untreated control samples were also prepared and cured for three curing periods. After the curing period, the sample was unsealed and trimmed into three samples of the required size, and all three samples were weighed before testing. The three samples were then tested on a strain controlled machine, and the average of the three ultimate compressive stresses was taken as the final value. The tested samples were then placed in an oven for moisture content determination.
The results are shown in Figures
Variations in UCS for untreated (UT) and treated (single dosage) soil samples.
Variations in UCS for untreated (UT) and treated (double dosage) soil samples.
Controlled untreated samples were prepared to account for any strength gain due to thixotropy or aging. Some of the clayey soils were reported to reduce their unconfined compression strength when tested after remoulding without any change in moisture content. The main reason behind this loss was the destruction of clay particle structure, which developed from the original sedimentation process. However, when remoulded samples are held for some time, keeping the moisture content unchanged, they may gradually gain strength [
Unconfined compression tests on different soils treated with PermaZyme 11-X were also carried out by Peng et al. [
Willie and Norman [
Comparison of XRF results for untreated (UT) soil samples and soils treated with three enzymes.
Formula | UT | E-I-D2 | E-II-D2 | E-III-D2 |
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Concentration (%) | Concentration (%) | Concentration (%) | Concentration (%) | |
SiO2 | 49.07 | 51.42 | 46.17 | 48.74 |
Al2O3 | 28.89 | 29.84 | 30.35 | 29.80 |
Fe2O3 | 9.07 | 9.20 | 9.78 | 9.70 |
TiO2 | 1.59 | 1.55 | 1.60 | 1.61 |
K2O | 0.48 | 0.55 | 0.57 | 0.57 |
MgO | 0.40 | 0.44 | 0.45 | 0.44 |
ZrO2 | 0.14 | 0.16 | 0.12 | 0.14 |
SO3 | 0.08 | 0.12 | 0.08 | 0.09 |
V2O5 | 0.04 | 0.04 | 0.04 | 0.04 |
CaO | 0.03 | 0.04 | 0.08 | 0.07 |
Cr2O3 | 0.01 | 0.01 | 0.02 | 0.01 |
Comparison of XRD results for untreated (UT) soil samples and soils treated with three enzymes.
FESEM results: (a) UT, (b) E-I-D2, (c) E-II-D2, and (d) E-III-D2.
In this experimental study, the effects of three enzymes on Atterberg limits, compaction characteristics, and unconfined compressive strength were evaluated. A Standard Proctor test was carried out to examine any change in optimum moisture content and maximum dry density with two doses of all three enzymes. The same test was conducted to prepare control untreated soil samples and soil samples treated with two doses of three enzymes for three curing periods (28, 56, and 84 days). The Atterberg limits test was carried out on untreated and treated soil samples after 56 days of curing. XRD, XRF, and FESEM were conducted to identify if any chemical change had occurred.
It was found that the three enzymes did not produce any comprehensible improvement in the three tests conducted, that is, Atterberg limits, compaction, and unconfined compression tests. Little improvement, in some cases, could be related to the hypothesis that the enzymes did not produce any chemical change, and they only prevented moisture absorption to bring the particles closer. Therefore, when selecting an invalidated stabilizer, it is imperative to check its suitability before using it on larger scale. It is hoped that this study will be beneficial for designers, contractors, and constructors when choosing bioenzymes as a soil stabilizer.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the support of the Ministry of Education, Malaysia, through Universiti Kebangsaan Malaysia, which provided funding for the project under Code DPP-2014-047, and the Centre for Research & Instrumentation Management (CRIM) for XRD, XRF, and FESEM tests.