Keywords

1 Introduction

Worldwide, population growth at an unprecedented rate is leading to increased demand for new civil infrastructure. The American Society of Civil Engineering has estimated that private–public partnerships on roads alone have now increased to US$67 billion per year in overall investments [1]. There is also insufficient infrastructure in major countries such as India and China due to limits on expansion imposed by geographic boundaries and soil conditions [2]. In Australia, civil infrastructure is increasing due to fewer land limitations than in other major countries, but there are deficiencies in the soil conditions, leading to cracking in ≈50,000 residential dwellings per year [3]. Due to these serviceability concerns, there is a high demand for improving soil conditions through different techniques such as traditional method of mechanical energy (i.e., hammering, vibration or rolling) or by the application of additive-based soil stabilization.

Traditionally, lime has been used in soil stabilization and typically includes hydrated high-calcium lime, dolomite lime, monohydrated dolomite lime and calcite quicklime [4,5,6,7,8]. The physical mechanism of soil stabilization with the addition of lime-based products involves fluctuation of clayey particles from plate-like to rod-like, leading to a greater interlocking between particles. Hence a modified soil matrix, and effectively, as the soil turns drier, it becomes less able to absorb water [9, 10]. Also, the physiochemical progress of the additional lime content in the soil matrix potentially involves the hydration process; thus, when quicklime is added to the soil, a chemical reaction takes place between the quicklime and the available water content in the soil matrix. Subsequently, the hydrated lime reacts with alumina and silica, leading to the formation of C–S–H (calcium–silica–hydrate) and C–A–H (calcium–aluminate–hydrate), which are like Portland cement products. Hence, lime-stabilized soil results in relatively impermeable layers of soil and significant load bearing [11, 12]. Although the usage of lime-based products has positive efficacy in soil stabilization, there can also be drawbacks due to the carbonation process and sulfate attacks, which can have negative economic impacts. The addition of lime product to the soil leads to a change in the chemistry of pore water, thus increasing the pH level and affecting the charge of the clayey particles. Also, the rate of carbonation is potentially higher for soil exposed to the atmospheric environment, which places limitations on the chemical reaction and pozzolanic activities and therefore becomes less effective for the purpose of stabilization [13,14,15]. The addition of lime-based products leads the soil to become more susceptible to sulfate attack which also limits the pozzolanic activities, leading to less effective soil stabilization [16, 17].

Cement products are also used in soil stabilization applications and this process is well known as cement stabilization in the geotechnical engineering field. By mixing cement with the soil material, the cement particles chemically react with the available water content in the soil [18], leading to the formation of crystal particles that bond to each other and with the soil particles, achieving higher compressive strength and stability [19, 20]. Thus, using cement for soil stabilization has been considered effective, but the drawback is the excessive cost and environmental impact during the cement production process.

Fly ash (FA) is also considered as one of the most useful and sustainable additives for soil stabilization applications due to its unique characteristic of acting like a cementitious material in the soil matrix. Although FA is solely incapable of densifying the soil material, it can react chemically to achieve the cementitious compound in combination with only a small amount of activator to improve the strength of the soil. However, the implementation of FA for soil stabilization applications is limited by the water content in the soil material. Thus, in order to achieve the optimum benefit from the addition of FA, the water content in the soil matrix must be at the minimum, and then dewatering is required to maintain the optimum moisture content of the soil material [21]. On the other hand, research has shown that the sulfur content in FA potentially forms expansive soil and, in the long term, leads to reduced strength and durability of the soil [11].

In addition to the common stabilizing approaches outlined above, biotechnological products, such as enzyme-based products, are currently being used as innovative products for improving weak soils. Researchers recently reported on the Eko-Soil enzyme (Eko Enviro Services) for soil stabilization and showed sustainable benefits for the stabilization of expansive subgrades [22]. Pooni et al. [23] used the identical enzyme product from the optimum enzyme content obtained by Rintu et al. [24] and evaluated the hydraulic influence and sustainable benefits of the application to an expansive clay material. The findings revealed that the addition of enzyme resulted in an increase in California Bearing Ratio (CBR) by 58% under soaked conditions due to densification effects. They further showed that the volumetric size of the micropores in enzyme-stabilized samples was drastically decreased, in comparison with raw soil, due to the improved density obtained from enzymatic stabilization [23]. As a result of the high stabilization performance and low costs, enzymatic stabilization using Eko-Soil has been adopted in field applications for unpaved road construction [25].

Here we provide guidelines that demonstrate the methodologies of conducting subgrade soil stabilization for road pavements, typically using the enzyme-based soil stabilization technique. This guideline can be used as a tool for enhancing soil conditions by providing stronger, impervious, and cost-effective subgrade soils under rigid or flexible pavements. In addition, an impervious capping layer over reactive soils can be productive by providing a safe all-weather pavement that is dust resistant and requires minimal maintenance over the long-term life cycle. To ensure these requirements are obtained, we performed the current study to identify the steps that need to be taken by geotechnical practitioners in terms of safety instructions, laboratory investigations and evaluation of site performance in compliance with specifications.

2 Pavements

The safety and lifespan of pavement structures significantly depend on the condition of the subgrade soil, because of the significant economic impact of frequent repairing [26] if mechanical properties are weakened. Some naturally occurring soils are suitable for compaction, forming a homogeneous material that is capable of supporting commercial and residential infrastructure. For instance, weathered rock extracted from the ground is generally a suitable fill material for infrastructure, typically when used with cementitious products for stabilization applications [27]. However, there are other materials that are not suitable as fill materials because of factors such as the in-situ conditions, environmental conditions, and applied loadings during the construction timeframe. The following aspects are important to consider when selecting a fill material from the economic viewpoint.

  • Clay material that has a high plasticity index (PI), and is therefore a reactive soil, must be considered under strict moisture and density control when selected as a fill material.

  • After completion of compaction, there can be large particles within the soil that potentially limits trenching or drilling of piers excavation, footings, services, and driving of piles.

  • Over-wet soil materials, which can be found mainly in low-lying areas, have the potential to dry out sufficiently within a shorter time during the project lifespan.

  • Large-size individual graded rock fill material has limitations on the breaking mechanism during the compaction process, which leads to higher porosity within the soil that subsequently creates pathways for fine particles to migrate within the soil matrix.

  • The salinity of the soil, in relation to aggressive chemical products or unwanted (polluted) soils.

  • Soil carbonation leads to the occurrence of acidic products.

  • The soil material has a PI between 6 and 20 with even graduation of particle size from 20 mm minus.

There are soil types that cannot be used as fill material, due to high contamination or undesirable performance of the soil material, and must be removed or transported and used elsewhere. Unsuitable fill materials may include:

  • Organic soils that contain severely root-affected material and peat, where these soils are more likely to be the top soils.

  • Soil materials contaminated through past site usage, which may contain toxic substances or soluble compounds harmful to water supply or agriculture.

  • Soil materials containing substances that can be dissolved or leached out in the presence of moisture (e.g., gypsum), or are susceptible to volume change or loss of strength when disturbed and exposed to moisture (e.g., sandstone), unless these matters are specifically addressed in the design.

  • Silts or related materials that have the deleterious engineering properties of silt.

  • Soil materials with properties that are unsuitable for forming structural fill.

  • Fill that contains wood, metal, plastic, boulders or other deleterious material.

Furthermore, for road pavements, it is also important that high attention be given to the shoulders abutting the pavement. Special treatments must be considered to ensure that microbial metabolism and activities of all the substrates are minimal under the permeable pavement, because they can lead to an increase in the total organic carbon content and incremental water content, which, potentially, lower the performance of the structure [28, 29].

Attention must be given to any erosion, which is the process by which the soil is worn away by water or wind and sediment is produced. Some soils are more susceptible to erosion than others, depending on their mechanical, chemical, and physical properties and the terrain [19, 30]. Effectively, erosion can be increased by several factors, including in high rainfall areas at the points where the overland flow is concentrated; where roadside activities such as vehicular traffic and maintenance practices increase the potential for erosion and sediment production or where any road construction interrupts the natural topography or drainage flows. When the runoff discharges turbid water into waterways, it can cause serious environmental harm, by reducing the sunlight penetrating the waterway [29], which can affect the growth of plant life and reduce the capacity of visual predators (e.g., fish and birds). Moreover, road dust can have a significant detrimental effect on the environment, affecting adjacent crops, waterways, buildings, vehicle amenity, aesthetics and human health by aggravating respiratory illness and road safety, through poor visibility and affecting driver behavior [31].

The designed pavement for a road mainly depends on the type of road. Rural road types are typically categorized as sealed, unsealed and stabilized. The sealed rural road has a flexible pavement that is designed, as per geotechnical recommendation, to include the addition of a top layer of bituminous concrete, asphalt or bituminous spray seal. For example, sealed roads are usually formed by excavating and preparing the existing ground (i.e., base) before placing crushed rock layers and a wearing course. For urban roads or rural roads of significance, underground drainage, footpaths, kerbing, and traffic management devices may also be considered [6, 31]. On the other hand, for unsealed rural roads, the top surface of the road has no bituminous layers but consists only of granular material (usually local gravel) or imported quarry product [32]. Within Australia, there are numerous safety, economic, social and environmental shortcomings regarding access for communities, and the extent of such shortcomings is largely dependent upon the characteristics of each individual road’s construction and traffic volumes. Significant regular maintenance is required to ensure surface conditions do not change until the geometry and surface can be improved to a safe acceptable level by the construction of an all-weather sealed road [9, 33].

Soil stabilization is considered as an efficient method to ensure soil behavior is within the required shear strength, permeability and compressibility parameters. Various methods of soil stabilization have been implemented throughout the history of subgrade soil stabilization, including mechanical and chemical methods [11, 34]. Soil stabilization through mechanical methods involves changing the soil mixture by degrading and densifying the soil using compaction with heavy rollers, rammers, and vibrational equipment and may sometimes involve blasting techniques for superior stability. Mechanical stabilization methods can be costly due to the requirement for labor and specialized equipment, so soil stabilization using chemical additives is becoming more common, using academic and practical engineering applications to ensure soil stabilization and densification are obtained by mixing minerals or biological additives [35]. In addition, chemical additives have the potential to reduce the timeframe of the construction by using available construction equipment, which is beneficial from the economic aspect. Stabilized road pavements are constructed with one or more layers/courses mixed with an additive to bind the pavement material [36, 37]. The preferred option for a conventional pavement is using the in-situ subgrade soils and gravels as a subgrade. This is especially important for saving natural resources, particularly where deficiencies in the existing in-situ gravels or clays can be rectified by importing more suitable gravels or clays and mixing them with a stabilizer additive to construct an unsealed pavement that is an environmental friendly, cost-effective, impervious and strong road [38].

3 Construction Commencement

This section covers the specific working steps that need to be taken before commencing the earthworks and soil stabilization process.

3.1 Construction Site Fencing

In the initial stage of construction, it is highly important that fencing around the perimeter of the construction site is installed before any earthwork commences. Fencing installation is one of the safest methods for identifying the boundaries of the working zone because permission to enter the construction site is then only given to authorized users, thereby preventing the public from entering the site and disturbing the work performance while ensuring their protection.

3.2 Drainage, Erosion and Sedimentation Control

Before conducting the stabilization works, precautions must be taken to ensure the earthworks will not cause siltation or erosion of adjoining lands, streams or watercourses. Drainage, erosion and sedimentation controls should be installed before the natural surface is disturbed. Sedimentation basins, stream diversion or other works may be appropriate in some environments or topographies. Careful planning is required to ensure both erosion and sedimentation controls are effective by minimizing the area of disturbance and through progressive revegetation or redevelopment of the site. Wherever water may tend to accumulate, provision for temporary drainage should be made by the contractor and care should be taken to guard against scour during any part of the construction. All temporary provisions for drainage should be installed to the satisfaction of the superintendent before stabilization and/or pavement materials are placed. The cost of temporary drainage, unless otherwise directed to be retained for use as catch or shoulder drains, is the responsibility of the contractor. The location of each drainage line is determined on geotechnical advice and with the approval of the superintendent.

3.3 Site Clearing

The site must be cleared (to the minimum extent required for the work) of all trees, stumps and other materials unsuitable for incorporation in the works. The roots of all trees and debris, such as old foundations, and buried pipelines are removed to sufficient depth to prevent any inconvenience during subsequent excavation or foundation work. The resulting excavations should be backfilled and compacted to the same standard as required for subsequent filling operations. Disposal of cleared combustible material may have to be off-site if clean air or bushfire regulations prevent on-site burning.

3.4 Stripping

The area in which fill is to be placed and the area from which the cut is to be removed are stripped of all vegetation and of such soils that may be unsuitable for incorporation into fills, subject to density, moisture or other specified controls. Topsoil may need to be stripped either as unsuitable material or as required for subsequent revegetation. Extreme care needs to be considered to ensure that materials that will inhibit or prevent the satisfactory placement of subsequent fill layers are not allowed to remain in the foundations of the fills. Geotechnical assessment of the depth and quality of topsoil or vegetable cover of the underlying soils and of the quality and depth of the proposed fill may obviate the need for such stripping in some circumstances. All stripped materials should be deposited in temporary stockpiles or permanent dumps in locations available for subsequent re-use if required and where there is no possibility of the material being unintentionally covered by or incorporated in the earthworks.

3.5 Slope Preparation

Where a fill abuts sloping ground, benches should be cut progressively with each lift as appropriate. It is unlikely that slopes flatter than 8:1 (horizontal to vertical) gradient will require benching. The benches should be shaped to provide free drainage. The boundary of cut-and-fill areas requires special consideration. All topsoil and other compressible materials should be stripped prior to benching into the natural material of the cut zone. The depth of the cut can vary depending upon the natural slope of the ground, the nature and proposed end use of the fill and the equipment being used.

3.6 Foundation Preparation

The ground surface exposed after stripping should be shaped to assist drainage and be compacted to the same requirements as for the overlying layers of fill. The surface exposed upon completion of excavation works may also require preparation prior to the fill placement proceeding. This will typically be the case when the subsequent fill to be placed is for pavement construction or the base material of a project. In such circumstances, it is necessary to loosen the exposed excavation surface to a certain depth (depending on the soil conditions), then moisture-condition and compact this loosened material. The depth to which this loosening is carried out should not exceed that of the compacted soil layer above it. The degree of compaction achieved should be consistent with the required subsequent filling operations unless design advice has been obtained. In such cases a working platform generally of granular material, end-dumped and spread in sufficient depth to allow the passage of earthmoving equipment with minimal surface deflection, can provide a suitable foundation for subsequent filling. Localized springs or seepages in the foundation area, detected during site investigation for the work, should be noted and considered in the design. If such problems are not detected before the work progresses, it is critical they be assessed so that measures such as subsoil or rock rubble drains can be designed for incorporation in the works.

4 Laboratory Test Investigations

This section elaborates on the required laboratory tests for evaluating the soil condition and required stabilization measures prior to construction.

4.1 Enzyme Stabilization

An enzyme is a biological macromolecule that catalyzes biological reactions, found in ribozymes (catalytically active RNA molecules) and some proteins (protein enzymes that have the ability to catalyze specific biochemical reactions), which are capable of initiating a biochemical reaction. The process involves changes (both formation and breakage) in chemical bonds. This method creates an interaction between the enzyme and bacterial strains within the soil product and is a replication of the natural construction of termite mounds [23]. Enzyme soil stabilization is based on reducing surface tension in soil particles through an ionic reaction, hence incrementing the soil compaction conditions. After the absorbed water is reduced through the compaction efforts, the soil particles agglomerate and as a result of the relative movement between particles, the surface area is reduced and less absorbed water can be held, which in turn reduces the swelling capacity. Figure 1 shows clay particles when entrapped with the enzyme strains.

Fig. 1
A schematic representation indicates the negative clay particle entrapped in the positive enzyme strain.

Representation of the clay particle entrapped by the enzyme strains

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4.2 Laboratory Results Illustrating the Enzymatic Soil Stabilization Mechanism

The behavior of the enzyme has been established through various studies. Laboratory results have indicated that the addition of enzyme causes water content reduction, as shown by different critical tests including FTIR (Fourier-transform infrared spectroscopy), SEM (scanning electron microscope) and microtomography (μ-CT). The addition of the enzyme to the soil has shown a marginal reduction in the intensity of the interlamellar water region due to the reduced affinity by the enzyme product (Fig. 2) [23, 39]. Porosity analysis and clay microstructure via SEM images indicate that enzyme-stabilized soil samples potentially show reduced permeability and increased mechanical strength as ingress of water is restricted and density is enhanced through clay aggregation [3]. Pooni et al. [40] showed by μ-CT analysis that the addition of enzyme can reduce the porosity from 2.67 to 1.44% (i.e., 46.07% reduction in pore volume). Effectively, the enzyme mechanism is increasing the density with decreased affinity for water.

Fig. 2
An F T I R of transmittance in percentage versus wavelength in centimeters power minus 1 for control, 7.5 percent F A, and 7.5 percent F A 1 percent E. Two-Mu-C T analyses of raw and stabilized soils. Two S E M images of raw and stabilized soils indicate numerous small voids, large voids, and flaky and aggregated clay minerals.

Laboratory results for the mechanism of enzyme-treated soil. a FTIR evaluating the enzyme base material in the soil and densification determination; b μ-CT analysis of the compactness of treated soil compared with control soil material; c SEM images demonstrating the lower void content and clay aggregation with the addition of enzyme product (Right) compared with control soil material (Left)

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4.3 Testing Techniques

Subject to the scale of the project, difficult conditions may be expected, and it is not envisaged to relax the test frequencies specified herein; in some cases, more frequent testing may be required. These testing frequencies relate to acceptance on a ‘not one to fail’ basis.

In order to obtain optimized performance of the stabilized road, it is recommended to estimate the performance of the stabilized soil through a comprehensive test plan in the laboratory prior to the field application. This is mainly due to the performance of the stabilized soil (i.e., treated road pavement), which is governed by the in-situ soil type and its condition. Figure 3 shows the recommended laboratory tests that can be conducted in the application of enzymes to stabilize pavements. The proposed tests will facilitate determination of the suitability of in-situ soil in stabilizing the pavement, as well as obtaining the optimal amounts of enzymes that will result in the expected road performance. It is to be noted that oven-drying is not allowed in any of the tests prescribed in this plan. The soil needs to be air-dried, or lime could be used as a drying agent prior to testing.

Fig. 3
A flow diagram depicts the stage 1 reference tests on in-situ soil, stage 2 mechanical tests on in-situ soil plus enzyme, and stage 3 optional mechanical tests depending on the need on in-situ soil plus enzyme plus cement. Stage 1 includes gradation, chemical, Atterberg, compaction, and permeability tests.

Typical soil stabilization using different testing techniques at different stages

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The proposed test methods can be conducted in three stages. In stage 1, initial tests are proposed to obtain the description and physical (and chemical) properties of the raw soils (possibly in-situ/natural soil). These tests include gradation, mineralogy, chemical, Atterberg, compaction and permeability tests. Having obtained the description and properties of the raw soils, samples are prepared at maximum dry density and optimum moisture content and left to cure for 72 h before mechanical testing (Stage 2 in Fig. 3). Should the natural soils be over-saturated, then add 3% lime mixture to the soil, and allowed to dry for 3 days, repeat the procedure until the natural soils are dried. This procedure can be used in the field to obtain the desired result. Furthermore, every mechanical test can be repeated at 4 and 7 days after curing to characterize the time-dependent strength gain of stabilized soils. Such mechanical tests will be helpful to assess the performance of the pavement material in accordance with relevant road standards. In addition to the tests noted in Stage 2, a permeability test of the stabilized samples is also recommended to determine whether the stabilization has improved the permeability of the mix. If the material is shown to be unsuitable, the strength of the stabilized mix could be improved with the addition of cement. The proposed series of mechanical tests on the stable mix with cement and enzymes will be able to ascertain the suitability of in-situ soil as a pavement material.

In order to proceed with testing using the additive/s for stabilization, initially samples should be taken from the field for each different soil type that is observed. In the case of a road, samples should be taken every 300 m, or less if there appears to be inconsistency within the sampled area. Moisture content within the sample bore is also a good measure for indicating a change in soil characteristics. In a remote area, geotechnical maps will assist in the location of samples if required. The initial tests are conducted to evaluate the mechanical strength of the soils. Figure 3 shows the types of tests that need to be conducted when enzyme is used as an additive for stabilization. Soil stabilization enhances an in-situ soil to support loads well in excess of those normally possible under natural conditions. Geotechnical analysis of soil samples collected from prospective stabilized sites must be carried out.

In all practical applications where moisture enters a pavement section, the permeability of the designed pavement mix must be tested in the laboratory to ensure specification requirements are either met or exceeded. It is recommended that the soil or soil with additives being considered for use at the construction site is tested to ensure that it conforms with the ideal specifications when using Eko-Soil. These specifications include the following.

  1. 1.

    Minimum 18% non-granular cohesive fines passing the 0.075 mm screen or a PI ≥ 6%. These fines will react best with Eko-Soil.

    Test method: ASTM D-11, D-422 [41], AS1289.3.6.3 or similar particle size analysis.

  2. 2.

    PI of 6–15 is the ideal soil condition when using Eko-Soil.

    Test method: ASTM D-4318 [42] Atterberg Limits AS1289.3.9.1 [43].

  3. 3.

    pH 4.5–8.5 is the most satisfactory. Soils with low pH may be amended with crushed limestone. Alkaline soils may be treated with a cheap acid such as sodium acid sulfate. Eko-Soil is classified as a naturally produced weak acid with pH 4–5.

Rintu et al. [44] used Eko-Soil to evaluate the effectiveness of different contents of Eko-Soil enzyme on fine-grained field soil. The authors found that the optimum enzyme stabilization is achieved with a diluted additive of 1% to the weight of soil in a diluted mass ratio in water of 1:500. The performance at an optimum dosage showed an increase in CBR value for the tested soil by 500% in comparison with the control and a reduction in water demand and improved dry density.

5 Field Procedure to be Followed

To assess the quality of materials and workmanship provided on a project, regular inspections and testing are required in the field at suitable time intervals. However, site construction should not rely on test results alone; where good supervision is essential for inspection measures such as test rolling. Such inspections should be carried out by experienced and knowledgeable users in earthworks.

Having conducted laboratory tests as outlined in Sect. 4, careful measures should be put in place in the field to apply the stabilization. Enzyme stabilization will prove less than effective if the following minimum procedures are not followed.

  • Ensure a soil analysis is undertaken on the subject soils and proposed additives before treatment begins.

  • During initial preparation, it may be necessary to add further plain water (Fig. 4a) to bring the soil close to optimum moisture level for best compaction, or to aerate and allow it to dry. Proper engineering supervision is essential (Fig. 4b).

    Fig. 4
    Four photographs. Pouring water into the broken soil, ripping and shaping an existing road base, a mixing machine and water tanker adding stabilizer, and a 16-ton smooth drum roller compacting the surface.

    a For an optimum moisture content mix water lightly through the broken soil. b Ripping and shaping an existing road base. c Adding stabilizer using a mixing machine and water tanker in tandem. d Compaction by 16-tonne vibrating roller

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  • Add the appropriate quantity of enzyme additive to the water truck after the truck is filled with water; as the quantity of enzyme depends on the in-situ soil type and its condition. The recommended tests proposed in Fig. 3 will facilitate the designer determining such optimized amounts of enzyme.

  • After the soil/gravel and any other cementitious additives is mixed, the enzyme-treated water is dispensed by the tanker and the material thoroughly mixed using a purpose-built roto-mill (Fig. 4c). In most cases, adequate mixing can be achieved using a single pass.

  • The mixed material should then be spread and shaped before compaction by rolling in maximum layers of 250 mm. For fill depths, refer to engineering plans.

  • The rollers must make enough passes to ensure adequate compaction is taking place. A vibrating pad foot roller is most effective for this application. On the final passes, when the tire tracks no longer show, the vibrator should be turned off to prevent excessive surface cracking caused by rapid drying. The top surface should be rolled until it shows a uniform sealed appearance. The final stages of rolling should be performed by a 16-tonne smooth drum roller (Fig. 4d), followed by a pneumatic (rubber) roller to assist in drainage and preventing ponding of water on the surface.

The ideal curing time for a 250-mm pavement depth would normally be 72 h. Light traffic may be permitted, as soon as tyre tracks are not visible from the surface. Light rain or high humidity will increase curing time. Application of enzyme is not to be undertaken during rain unless otherwise approved by the superintendent. Once field stabilization is completed, tests (Fig. 5) can be performed to ascertain field efficiency.

Fig. 5
A flow diagram depicts the various testing techniques. It includes C B R tests, U C S tests, shear tests, resilient modulus tests, and permeability tests.

Different testing techniques that can be conducted on the basis of field samples. CBR, California bearing ratio; UCS, unconfined compressive strength

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5.1 Unconfined Compressive Strength

Unconfined compressive strength (UCS) is an effective test in ensuring pavement strength is within the range of 0.5 and 1.5 MPa or if the UCS value exceeds such a range the pavement is classified as a rigid type and therefore subjected to cracking. To assess the quality of the material through mechanical testing methods for areas of a minimum of 1500 m2, such as subdivisions, large industrial lots, road embankments, etc., the following requirements must be considered in corresponding different testing locations within the field.

Not less than:

  • 1 test per layer or 250 mm thickness per material type per 2500 m2 or

  • 1 test per 500 m3 distributed reasonably evenly throughout full depth and area or

  • 3 tests per visit, whichever requires the most tests.

  • Confined operations: 1 test per 2 layers per 50 m2.

To ensure that enzyme and cementitious modified materials (clays mixed with local gravels, cement, and enzyme) are achieving desired strengths in the order of 0.5–1.5 MPa (at 7 days), it is recommended that a minimum of two molded samples of these materials be re-compacted in the laboratory and allowed to cure for 7 days for the purpose of obtaining UCS values.

5.2 Deflection Test

Field deflection testing can be used to evaluate the strength of the soil. This test is highly recommended when working with large platforms or pavements (Table 1).

Table 1 Results of deflection testing of both treated and untreated pavement sections: Harvey Norman/Ikea site, Springvale, November 2008
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5.3 Plasticity Index

The PI needs to be 6–15 for a minimum of 18% non-granular cohesive fines passing the 75-micron sieve.

5.4 Maximum Density/Optimum Moisture

This test will determine the amount of enzyme required to obtain maximum results. The test method follows ASTM D-1557 [45] modified proctor. Typical enzyme rate (based on Eko-Soil) from field experience can be 1–1.5 L of the enzyme to 30 m3 of compacted pavement in general. Moisture content to achieve maximum compaction should be 1–2% below optimum. In the field, the moisture content is determined by hand squeezing of the mixed material. If it crumbles, then add more water and retest. If it has an excess of water, allow drying.

5.5 Bearing Strength or CBR

The bearing strength, or CBR, is an effective test for determining the bearing strength of the soil. However, the laboratory CBR may not conform to or replicate the field bearing strength because the compacted CBR samples must be allowed air dry for 72 h before submerging in water. CBR tests should be undertaken on cementitious modified materials for each 2nd day’s production or every 2500 m3 whichever is the lesser. Desired CBR values of cementitious modified materials should be >15%.

5.6 Permeability

With the addition of enzymes, the reduction in moisture directly affects the design of the structural section of the pavement. Reductions of up to 100-fold are achieved. The test method can be performed in accordance with ASTM D–5084 [46]. The representative values of relative permeability of the different types of soils are shown in Table 2. To ensure that the cementitious materials maintain the desired permeability of <5 × 10–8 m/s, preferably <5 × 10–9 m/s, permeability testing should be undertaken on cementitious modified materials for every 4th day’s production or every 5000 m3 whichever is lesser. The permeability for the enzyme treated soil is approximately 10–8–10–11 m/s.

Table 2 Representative soil permeability for various soil types (obtained from AS1547 [47])
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5.7 Field Density

Methods for the determination of field dry density are as described in Sects. 5.8–5.11 below.

5.8 Direct Density Test

In order to perform the direct density test, various standards describe specific methods of conducting the test and evaluating the analysis including: AS1289.5.3.1 [48], AS1289.5.3.2 [49], AS1289.5.8.1 [50] and AS1289.5.3.5 [51].

5.9 Indirect Density Test

This test provides an empirical measure of achieved density by measuring another engineering property, principally shear strength, and may be used to further validate density; however, the method of direct testing will govern acceptance.

5.9.1 Establishment of a Reference Density for Calculation of Relative Compaction

To permit relative compaction to be calculated, it is necessary to establish a laboratory reference density. Procedures for establishing such reference densities have been developed empirically over many years and standardized with test procedures of AS1289.5.1.1 [52], AS1289.5.2.1 [53], AS1289.5.7.1 [54], and AS1289.5.5.1 [55].

5.9.2 Sample Selection for Reference Density

For routine “compaction” testing, the sample for determination of laboratory reference density should comprise either the material recovered from the field density determination, (see AS1289.5.3.1 [48]) or from the volume of material considered in the field density.

For cement-modified stabilized materials, including enzyme, the reference density may vary with time but the laboratory compaction should still be carried out on material that has been mixed and compacted by on-site purpose-built machinery. The density tested and re-compacted in the laboratory must be conducted as soon as practicable to ensure minimum curing has occurred. For granular materials, including pavement base and sub-base materials that have been manufactured from a hard rock source under controlled conditions, consideration may be given to providing an assigned value as further discussed within the procedures of AS1289.5.4.2 [56].

6 Construction Applications

This section describes various applications of enzymatic soil stabilization.

6.1 Flood Mitigation

Enzyme-stabilized pavements in the field are constructed in a similar manner to conventional stabilization methods, using water tankers, motor graders with rippers, and a 16-tonne vibrating steel roller. Additives should be verified for weight/square meter to ensure that the quantity of additive meets specifications. For speed of construction and to ensure a homogeneous mix, self-propelling and towing mixing machines are recommended. Both types of machines can be fitted with a computer water feed system and in a recent project that we were involved in, 7000 m2 of 300-mm deep stabilization were completed in an 8-h day. Should rock or cement be required as part of the geotechnical design as an addition to the pavement mix, this should be placed and mixed before the stabilization process.

Cross fall should be at least 3% and side drains should be cut into the pavement (Fig. 6). Just as the pavement is losing its plasticity, a skim of one-size crushed rock (10 mm) should be rolled into the top layer to provide a skid-resistant surface. Light traffic can be allowed to traverse the pavement as soon as the tyre marks of the stabilization equipment disappear. Full curing is achieved in 72 h after which the road may be opened to all traffic.

Fig. 6
An illustration of a typical cross-section of road pavement indicates stabilized natural soil and existing pavement, non-stabilized natural soil, wearing course, water flow, and a stabilized surface.

Typical cross-section of road pavement subject to flooding

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Stabilized pavements subject to inundation must be tested in the laboratory to ensure permeability either achieves or exceeds specification. Enzymes, or an enzyme blended with up to 3% cement, in both laboratory and field testing, can achieve the required specification but will have an advantage over clay pavements due to their residual tensile strength, which prevents cracking of the pavement once the water recedes; hence, stabilized pavements must envelop the road or dam surfaces.

6.2 Embankments and Slopes

Enzymatic soil stabilization can be used successfully in the stabilization of embankments and slopes. Preliminary work is presently underway on problems with some highly plastic soils (e.g., colloid clays that are highly expansive). These soils contain fatty clays and fine gravel or sands and silts with a PI ranging from 45 to 80.

In making calculations for enzymatic stabilization application for these types of soils, the ratio of cohesive fines is different from the standard gradation specification. Normal gradation is calculated on 18–30% cohesive fines. Therefore, correlating these expansive clay results in a greater number of cohesive materials being present. The enzymatic composition works on the molecules within these expansive clays but not the other materials present. The standard application of 1 L per 30 m3 of compacted pavement must be increased to compensate for the increased amounts of cohesive fines. The standard rate for high amounts of expansive soil is calculated using the same ratio as for a standard application. As an example, the rate of application for soils containing 48% cohesive fines would be 1 L per 15 m3 of compacted stabilized slope pavement. However, this has to be determined from adequate tests as explained previously.

Recommended thicknesses for slopes are:

  • Up to 30 degrees, 150 mm (Fig. 7)

    Fig. 7
    A schematic representation of erosion simulation equipment indicates soil samples inclined at an angle of 45 degrees, water, and soil sample collection.

    Erosion simulation equipment used in modified version of the tests [57]

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  • 30–50 degrees, 150–225 mm

  • 50–65 degrees, 250–350 mm

  • ≥65 degrees must be used in conjunction with soil nails.

Compaction has always been difficult in slope construction. In recent years, new mechanical attachments have been introduced for compaction of trenches, slopes, and embankments. Compaction load is applied by the backhoe. A filtration membrane, mostly likely sand, must be placed between the slope and the pavement. Weep holes are placed in the pavement at locations as directed by a civil engineer experienced in such methods. Recent compaction equipment introduced into slope stabilization enhances the ability to compact slopes at much steeper angles than was previously possible.

6.3 Soil Nailing

Soil nails have been used for many years in slope stabilization. The normal soil nail is constructed using concrete and steel reinforcement, which can be an expensive additional cost in embankment stabilization projects.

It has been observed that concrete soil nails will catch water on their upper surface, allowing water to penetrate around the soil nail, loosening adjacent soil and aggregate materials and resulting in loss of support of surrounding materials.

Enzymatic-stabilized soil nails are economic alternatives. As with concrete soil nails, they can be pre-manufactured or constructed on-site, and similarly, it is recommended that enzymatic soil nails be reinforced with vinyl-coated reinforcement rods, which should be ≈30% below the bottom of the soil nail for anchoring into the in-situ material. Enzymatic soil nails are recommended to be pre-formed and pressed into pre-bored holes that are slightly undersized. It is also recommended that enzymatic soil nails be placed on the slope after being dampened with a 1:10,000 mist of enzymatic composition and water.

An alternating pattern of rows should be used. The spacing of the nails and the rows should be approximately 1.5 times the diameter of the soil nails. A civil engineer experienced in slope stabilization should design the use of soil nails. Construction and replacement of soil nails should be performed in favorable construction climatic conditions by avoiding freezing and wet weather conditions.

6.4 Enzymatic Composition Blocks (Hollow Blocks/Cement Blocks) and Bricks

The construction of cemented crushed shell blocks can be achieved with the aid of enzymatic composition. A reduction of ≈5% in the use of water will be realized and a reduction of mold breakage rate of between 35 and 50% can be achieved. This is an economical saving for any manufacturing process.

Enzymatic-stabilized soils have been used to construct solid construction blocks designed for the construction of interior walls and low retaining walls. They were manufactured using small, fractured gravels and cohesive fines within standard gradation specifications, and have proved to be exceptionally strong, durable, and easily constructed with a hand-operated lever-style press and a steel mold.

More efficient methods are used to manufacture enzymatic-stabilized blocks/bricks, where facilities are available. Hydraulic power units have the capacity to produce 6000–8000 blocks per shift per machine.

Mortar for joining the enzymatic-stabilized blocks and bricks should be a combination of the makeup of the blocks/bricks (without the larger aggregate) and applied as normal mortar to all connecting surfaces. Curing times of the mortar joints will be in the order of 24 h. The utilization of enzyme is currently under investigation for unfired brick fabrication.

6.5 Working Platform

A typical example of soil stabilization by utilizing enzyme product in the field was undertaken on an area of 65,000 m2 for a whitegoods site in Australia (Fig. 8). The pavement was 150 mm of in-situ fill material and 150 mm of recycled material recovered from demolished site buildings. The testing was conducted in the laboratory and the field as recommended here. The compaction density of the subgrade achieved results >100% while the strength of the subgrade, in field testing on this site achieved CBRs of ≈80% and permeability of 10–14 m/s.

Fig. 8
A photograph of soil stabilization by utilizing enzymes on an area of 65,000 square meters at a white goods site in Australia.

Using enzyme as an additive for soil stabilization of a construction site consisting of 65,000 m2

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The process for working platforms also applies to roads, large industrial/commercial building sites, and most civil infrastructure sites. They are useful for subgrade improvement of over-reactive clays to stabilize the subgrade moisture and to limit differential movement in the subgrade. Figure 9 shows the comparisons of road pavement based on unstabilized and stabilized base in the same locality.

Fig. 9
Two photographs of unstabilized and stabilized bases on road pavement.

a Cracked seal: base not stabilized. b Good seal: enzyme-stabilized base

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6.6 Reclaimed Materials

Testing performed for the Hong Kong University recommended that the sludge retrieved from the Hong Kong Harbour be used as a working platform for the Hong Kong Housing Department. The material was delivered to the site and dried by using 3% lime mixed into the sludge and left to dry for 3 days. A pavement mixture consisting of 150-mm of dried reclaimed material, 150 mm of 19-mm of recycled concrete, 3% cement (due to the variation of soil consistency) and 1% of enzyme stabilizer produced a CBR of 80%.

6.7 Water Containment

Ponds were constructed at Warrnambool Airfield. The method of construction was to build a base with an enzyme additive, build up the pavement mix for the walls then stabilize the existing base. The stabilized material was then moved from the top of the pond walls to the base on the 1 in-3 slope using vibrating rollers.

7 Conclusions

Based on the procedures and criteria from different standards and guidelines, the utilization of additives for soil stabilization is more effective when compared with the mechanical methods traditionally used for soil stabilization. Soil stabilization through enzymatic bonding, although highly dependent on the soil material, requires soil materials to biochemically react with the additives in order to obtain an effective stabilization. An excellent understating of the topography and geology of the construction site is imperative.

Consequently, different mechanical and chemical composition tests must be undertaken on the soil types available from the site for each and every combination of chemical additives that will potentially be used for stabilization of those materials. Specifically, when using enzyme products, the soil properties need to be carefully evaluated before any stabilization commences. Once the laboratory results are evaluated, fieldwork can be performed based on the optimum additive content determined. Subsequently, to maintain the safety of infrastructure, comprehensive construction management of the site must be considered before conducting any construction work. Thus, monitoring the field test in accordance with available guidelines, standards, and contractor management protocols is equally important and the field tests are undertaken regularly throughout the lifespan of the construction work to ensure that construction is performed using the highest quality standards and with minimum geotechnical issues.

Stabilization is a science, so a suitably qualified engineer must design and sign off the pavement stabilization construction to ensure the pavement has met or exceeded standards of permeability, strength and density. It is to be noted that the information provided in this document is for guidance only and should not be used without required tests and suitable evaluations as detailed herein.