Soil Stabilization Technology

Negus are sole distributors for Soil Tek ES in Jamaica

Overview

Road performance can be enhanced with chemical stabilizers, which include a wide variety of compounds that act on the physical and mechanical characteristics of soil. With nanotechnology, it is possible to combine the properties of conventional formulations with the capacity of nanoparticles for molecular level interaction due to their specific large surface area, resulting in economic and environmental benefits

SoilTek® is described as a nanotechnological chemical compound that is able to modify the physical and chemical characteristics of soil (capillarity, expansiveness and permeability), controlling and reducing the effects caused by water, such as water erosion and percentage of swelling in expansive soils. It also increases simple compressive strength and relative bearing capacity. Besides stabilization of roads, this product has also been used to stabilize hard shoulders, bases and sub-bases, parking and maneuvering areas and setting of slopes.

The innovative aspect of this product is the homogeneous incorporation and distribution of zerovalent nanoiron in soil cement mortar, thereby modifying the geochemical conditions during the setting process to achieve better mechanical performance. The nanoiron is dosed into the product in the form of a stable suspension in a submicron polymer matrix, which in turn acts as a binder and sealant, increasing the impermeability and molecular cohesion of the soil.

Mechinisms in Soil Stabilization and Technical Performance

There are several soil stabilizers based on synthetic polymers available in the market. The main concern with them is useful life, which depends on resistance of the formulation to environment, mainly sun and rain. To solve this problem, soilTek-ESTM achieves immediate stabilization due to the same mechanisms to those of conventional stabilizers such as Rovene 4045, but then provides a quite longer useful life due to the nanotechnology involved in the product. This performance is due to a formulation based on a matrix of synthetic copolymers which, besides known conventional effects such as waterproofing and binder, also acts as a carrier for our stabilized nanoiron (nanoFeTM), to homogenously deploy these nanoparticles into the treated soil. Thus, the initial behavior of soilTek-ESTM will be seen as equivalent to that of other chemical stabilizers, but the long-term performance will be enhanced by the interaction of nanoFeTM with natural occurring micro particles from the soil (such as montmorillonites) and components of the cement (mainly portlandites). Through physicochemical interactions with nanoFeTM, these components are stabilized at microscopic level throughout the whole mass of treated soil. This long-term stabilization of hydrophilic micro constituents interrupts the natural cycle of soils, which expand with water (muddy soil) and contract in dry conditions (dust generation). At the same time, these mechanisms, which attach iron to treated soil components, immobilize nanoFeTM, thus preventing leakage or lixiviation away from the treated soil.

Technically, the iron nanoparticles react with the natural colloids of the cement (portlandites and tobermorites), generating a hard and resistant matrix, even with very low concentrations of cement: 2.5 to 4%, depending on the hydrogeological characteristics of the native soil and the type and intensity of stress to which the road is subjected. For their part, the polymer chains provide a self-binding capacity by increasing the dynamic support force. This process reduces the specific surface area of soils due to the agglomeration and setting effect of the fine particles, which will be coated with submicronic water- repellent particles supplied by the polymer. This effect occurs both at depth and in the surface layers, resulting in greater structural impermeability and positively influencing sensitivity to water, the main problem in the maintenance and stabilization of any type of soil with a high content of active fine particles. The lower residual moisture content enables achieving a much higher dry density than that of the native soil, which should be strengthened through appropriate mechanical compaction. The reduction in capillarity of the treated unit ensures control over soil expansiveness, leading to swelling values lower than the limits established by the National Road Directorate in Argentina.

Based on the functionality described, the nanoformulated compound enables direct use of the native soil and a substantial reduction in the addition of Portland cement. The product's fluidity facilitates its application through spraying, as well as the work to stabilize the soil layer using simple machinery, which reduces total work time by 1,500 to 2,000 m2/day, depending on local conditions and the type of native soil. From an ecological point of view, the product helps reduce the environmental impact generated by road construction because it is an iron-based compound (incorporated in the form of nanoparticles), which is the fourth most abundant element in the earth's crust and is usually found in natural soils in a much higher proportion than that contained in the dosage of the stabilizer [1]

The reactions produced by the combining of the nanotechnological gels of the hydraulic binders and during the setting process, result in the following benefits:

• Increased resistance to simple compression.
• Increased bending strength.
• Increased impermeability.
• Lower swell percentage.
• Reduced capillarity.

Compressive strengths developed in specimens containing nano-particles after 28 days are in every case higher than that of control concrete specimens (without nano-particles): compared to a control specimen with compressive strength of 92.3 MPa, the influence of nanoiron improves the value to 119 MPa. Indirect tensile strength of samples containing nano admixtures is also higher than that of control concrete sample, with nanoiron (NF in the figures below) showing the highest increase. Nano particles can also reduce the water absorption of concrete samples. The values of the water absorption (in Percent) in all the concrete samples with nano-particles were less than 0.4 of water absorption of control specimens [2]

Swelling characteristics on Type HBR A.7-5 (16) soil (AASHTO Soil Classification System - from AASHTO M 145 or ASTM D3282) were assessed comparing natural soil (3.65% swelling), to soil + lime (1.25% swelling) and soil + lime + SoilTek-ESTM (0.8% swelling).

Table 1 below shows the results of compressive strength tests conducted on Type HBR A.6 (9) soil.

Table 1: Compressive strength of soil with and without SoilTekTM.

Quantitative data show that even with minimum dosage of cement (4%), soilTek-ESTM achieve (in 15 days) the same strength that conventional soil cement with a 50 % increase in cement (from 4% to 6%). If maximum strength is required, a 6% cement dosage is recommended, to achieve a 58% increase in strength by the addition of soilTek-ESTM. If cost of cement is an issue and a lower strength is admissible, with only 4% cement, a 64% increase in strength can be achieved with soilTek-ESTM.

Economics

A study assessing the economic feasibility of different natural soil stabilization options, as alternatives to costly conventional pavement, was conducted [3]. The assessment used a Life-Cycle Cost Analysis (LCCA) based on Net Present Value (NPV), depending on the periodic maintenance tasks required by each alternative. The options that were considered include imported commercial products and a locally manufactured stabilizer in the form of a nanoiron suspension in a copolymer matrix. As a control, the standard maintenance practice for unpaved roads, limited to shaping the surface with a motor grader, was also assessed. The analysis period was six years, based on available data on the performance of the nanoformulated alternative. A summary of the results of the study is shown in Table 2, which shows the relative NPV of the various options considered. It can be seen that conventional methods are between 1.4 and 2.1 times higher than using nanochemical stabilization.

Table 2: Comparative cost of various stabilization methods.

The study showed that a thorough analysis of the real cost of the technological alternatives cannot be limited to a simple consideration of initial investments, since this approach neglects the real cost over the total useful life of the road project.

It was pointed out that the nanotechnological alternative requires significantly less work time than with conventional procedures, such as asphalt. This reduces slowdowns and traffic interferences. Putting aside important issues such as noise level and the longer time needed to get to work, and only considering air pollution from engines while vehicles are waiting for an alternative lane to open, raises the issue of exhaust from internal combustion vehicles that generate CO2, CO, NOx, SOx, unburned hydrocarbons and particulate matter. All these compounds in the air have well-known effects on health, such as toxicity, irritations, and allergies, as well as disagreeable sensations from smoke and odors. They also produce undesirable environmental impacts such as acid rain. Particulate materials smaller than 10 microns, in turn, remain in suspension for up to 10 days, allowing them to be dispersed over large areas, increasing the influence of their impact far beyond their localized emission. If these elements are incorporated into the analysis and the difference between conventional technologies and nanochemical stabilization is quantified just from the point of view of less construction time required, it is quickly understood that the intangible benefits of this option can be highly significant.