3. Road Dust Suppression: Effect on Unpaved Road Stabilization

3.1 Introduction

A good road (paved or unpaved) requires a suitable foundation which in turn requires stability. A material is stable if it has little or no volume change and resists deformation under repeated or sustained loading conditions whether wet or dry (Transportation Research Board, 1982). The degree of stability is primarily a function of the road material resistance to lateral movement or flow (USDOT, 1979). Different types of road material employ different mechanism for resisting lateral movement. In general, granular soils count on their particle sizes, angularity, and interlocking ability to develop the internal friction required to resist lateral flow. However, in fine-grained soils such as clayey soils, the stability is very much moisture dependent.

There are many varieties of soil available for road construction. Unfortunately, many of the soil deposits do not naturally possess the requisite engineering properties to serve as a good foundation material for roads and highways. As a result, soil-stabilizing additives or admixtures are used to improve the properties of less-desirable road soils (ARBA, 1976). When used these stabilizing agents can improve and maintain soil moisture content, increase soil particle cohesion, and serve as cementing and waterproofing agents (ARBA, 1976; Gow et al., 1961). Unpaved road dust suppressants are considered soil additives because they produce changes in soil characteristics that influence soil stabilization (Gow et al., 1961; Ross, 1988).

For unpaved roads, dust control and road surface stabilization often go hand-in-hand. By controlling the generation of dust by preventing loss of fines, the road surface is stabilized for driving comfort and safety. By stabilizing the road surface, the essential fines which otherwise would be lost in the form of dust are firmly bonded to the coarse road surface material thus preventing road surface deterioration and reducing maintenance cost.

This section of the report examines the effect of the use of road dust suppressants on the stabilization of unpaved road material. The commonly used dust suppressing chemicals: ligninsulfonate (Lignin) and chloride compounds (MgCl₂ and CaCl₂) are evaluated. No specific field-based experiments were performed during this research to quantitatively measure the test sections strength increases as a result of the suppressants application. However, other quantitative laboratory studies on the subject of soil stabilization using lignin and chloride compounds are discussed. The field base methods of applying dust suppressants are also presented.

3.2 Methods of Application

There are two primary methods of incorporating suppressants into road surface soils. The methods are referred to as: 1) surface or topically sprayed and 2) mixed-in-place or in-depth application. Surface or topically sprayed application involves spraying the suppressants on the unpaved road surface after the road surface has been prepared (bladed, shaped with an "A" crown, and compacted). This method of application is simple, fast and cheap (Woods, 1960). Suppressants applied by this method are effective for a short period of time and repeated applications are necessary in a single dust generating season (Hoover, et al., 1973).

Mixed-in-place application involves the addition of the suppressant to the road surface soil in-situ (Woods, 1960). This in-depth treatment is achieved by mechanically mixing the suppressants with the soil using special mixing equipment. The mixing of the suppressants with the aggregate material can occur at the borrow pit and the resulting mixture hauled for placement on the pre-constructed road surface. This process is similar to asphalt placement on paved roads. Another way to achieve the in-depth treatment is to spray the suppressant over the scarified and/or new road material and mix by windrowing from side of the road to the other using the grader. Windrowing the mixture back and forth ensures a thorough mixing of the additive with the soil. As stated by Hoover, et al., (1973), "this method does not only achieve dust palliation but provides improved road surface resulting in reduced maintenance cost from continued suppressant applications and/or aggregate replacement. Furthermore, in-depth stabilization may improve the sub-base or base for further higher-type pavement."

3.3 Factors Influencing Stabilization

Soil is the foundation material for all roads and highways. A stable foundation is key to the durability and longevity of any road. Because most soil deposits for road construction lack the engineering properties required for foundation soil, the subject of soil stabilization has been widely studied. Soil stabilization has been defined as the process of improving certain soil properties (Kezdi, 1979; Mitchell, 1993). Ingles et al., (1973) also defined soil stabilization as the alteration of soil properties to meet specific engineering requirements.

The soil properties of concern that require improvement include, but are not limited to, strength, durability, permeability, and small volume changes. The soil property of strength, measured in terms of the shearing strength of the soil, has been noted to govern the ability of the soil deposit to support an imposing load (McCarthy, 1993). As a result, the shearing strength of soil has become an important design parameter in foundations, roadways, and airfield engineering. Soil stabilization means an increase in shearing strength of the soil corresponding to given engineering requirements (Kezdi, 1979).

In practical terms, the shear strength of a soil is a measure of the soil's strength and stability. Shear strength, like many other soil properties, is influenced by several factors. These factors can be grouped into: 1) compositional factors and 2) environmental factors (Mitchell, 1993).

1. Compositional factors: These factors are said to determine the potential range of values for any given soil property. They are:
   1. type of minerals making up the soil material,
   2. amount of each mineral making up the soil,
   3. type of adsorbed cation,
   4. the soil particles shape and size distribution,
   5. composition of the pore water.
2. Environmental factors: These factors are said to determine the actual value of a given soil property. They include:
   1. water content,
   2. density,
   3. confining pressure,
   4. temperature,
   5. the soil fabric (structure),
   6. availability of water.

The combination of these two groups of factors determines the shear strength of a soil (Mitchell, 1993). The shear strength of a soil is also divided into two components, internal friction and cohesion. These two components according to many researchers influence the stability of unpaved roads (Grow et al., (1961); Public Works, 1990). Mitchell, (1993) gives the relationship between shear strength, internal friction and cohesion as:

Where

For cohesionless soils such as sand c = 0 and shear strength depends only on the normal stress and the angle of internal friction (τ = σ tanΦ). The internal friction according to Rowe, (1962), Mitchell, (1993), and others is in turn influenced by four main factors:

• the sliding resistance between soil particles,
• soil particle rearrangement,
• dilation, and
• particle crushing.

For engineered (aggregate) roads the above factors produce change in internal friction during compaction of the unpaved road surface which increases the shear strength and stability of the road.

For soils with plasticity such as clays, cohesion plays an important contribution to the shear strength and stability of the soil. Two types of cohesion have been described by Mitchell (1993) true cohesion and apparent cohesion.

The proposed sources for true cohesion are:

1. Cementation (chemical bonding between particles),
2. Electrostatic and electromagnetic attraction (prevalent in small-size and closely spaced particles), and
3. Primary valence binding and adhesion.

The proposed sources for apparent cohesion are:

1. Capillary stresses (a combination of water attraction to soil particle surfaces and the surface tension of water) and,
2. Apparent mechanical forces (caused by interlocking rough surfaces) (Mitchell 1993).

The use of chemical additives can increase the shear strength of soils by increasing the internal friction and/or cohesion of the soil (Hemwall, et al., 1962). Unfortunately soils react differently to different admixtures. Hoover, et al., (1960) in a roadbase stabilization study, discovered that the stability of some road soil materials was improved by the addition of certain additives, whereas the stability of others was unaffected.

The interaction between a soil and an additive, among other things, depends on both the soil and the additive physical and chemical properties. Although two soil samples may have the same physical characteristics, i.e. particle size distribution and Atterberg limits, they may posses different chemical properties which in turn may affect their resulting interaction with the same additive. According to Mitchell (1993) the parent material of a soil and its weathering characteristics holds the key to the chemical makeup of that soil. Palmer et al. (1995) states that "a soil's composition, history, current state, and environment are reflected in its fabric and interparticle force system." The fabric and the interparticle forces, therefore, comprise the structure of the soil and the structure of a soil is not permanent. By rearranging the soil particles, changing the size and composition of the particle groups, or changing the size or number of pores spaces in the soil, the fabric (structure) of the soil can be changed (Mitchell, 1993).

For this reason the application of an additive such as lignin may cause soil particle rearrangement to occur because lignin, according to Gow, et al. (1961), can cause dispersion of the clay fraction of some soils. The compositional structure of some soils may also change due to solubility of soil minerals. Applications of some types of additives can increase solubility of soil minerals thus causing soil structure consolidation and cementation (Ross, 1988).

In addition to shear strength, the thickness of the soil layer also affects stability. According to Huang (1993), the stability of paved or unpaved roads depends on the strength of the entire road-layer system (i.e. the subbase, base, and wearing surface course). The following equation given by Huang (1993) shows the relationship between the depth of the soil layer and the amount of load supported at that depth.

where

To achieve optimum stability of unpaved roads, the overall road-layer system should have sufficient depth such that the average vehicle load distribution can be reasonably supported by the unpaved road.

3.4 Ligninsulfonate (Lignin)

Various researches who have studied lignin as a soil additive have all concluded that lignin is primarily a cementing agent (Landon et al., 1983; Ingles et al., 1973; Woods, 1960). The natural cementing sugars that bind the wood fibers together also appear to perform the same fundamental function when combined with soil particles. The resulting lignin-treated unpaved road is one that exhibits concrete-like qualities of a hardened surface that gains strength over time. The cementing effect helps with stabilization by increasing the true cohesion between soil particles. Lignin has also been shown to posses the property of hygroscopicity, which may also contribute to soil strength by retarding evaporation (Gow et al., 1961).

Adding lignin to clayey soils increases soil stabilization by causing dispersion of the clay fraction (Gow, et al., 1961; Davidson, et al., 1960). As stated by Gow, et al., (1961) "dispersion of the clay fraction benefits stability of the soil-aggregate mix by:

1. plugging voids and consequently improving watertightness and reducing frost susceptibility,
2. eliminating soft spots caused by local concentrations of binder soil,
3. filling voids with fines thus increasing density, and
4. increasing the effective surface area of the binder fraction which results in greater contribution to strength."

Woods, (1960) also explains that lignin acts as a clay dispersant, making the soil more plastic at lower moisture content which, after compaction, leads to denser, firmer road surface. For this reason, fines or clay are an important component of the road surface material and a prerequisite for successful road surface stabilization with lignin.

Lignin like the other dust suppressants (soil additives) is introduced into the road surface layer for dust-control purposes. Lignin is water soluble and therefore during wet conditions leaches into the underlying base and subbase layers. When this occurs, the presence of lignin in the underlying layers can increase shear strength thus benefiting the overall stabilization of the unpaved road (Sultan, 1976; Apodaca et al., 1990).

The solubility of lignin is also considered a disadvantage because the surface binding action of the lignin may be reduced or completely destroyed by heavy rain (Langdon et al., 1983; Addo et al., 1995). Lignin is also corrosive to aluminum and its alloys because caustic compounds are used in the extraction process (Compendium 12, 1980). As reported by Schotte (1988), the corrosive and solubility effects of lignin can be reduced by the addition of calcium carbonate slurry to the lignin-soil mixture. Adding bichromate to lignin in a chrome-lignin-soil stabilization study revealed that the mixture formed a gel and acted as a waterproofing agent (Hough, 1951). Lignin has successfully been used to treat unpaved roads in Europe, Canada, and United States since the 1920s. It promotes stabilization and consolidation of roadway mixtures (Harmon, 1957; La Touche, 1959). The degree of stabilization has varied from study to study; some researchers have reported notable strength increases and others have reported no strength gains. Section 3.6 summaries the results of some of the soil stabilization studies done.

3.5 Chloride Compounds (CaCl₂ and MgCl₂)

Chloride compounds are probably the most widely used dust suppressant (additive) on unpaved roads. They also produce changes in soil that influences stabilization (Gow, et al., 1961; Ross, 1988; Compendium 12, 1980). The soil stabilization is generally attributed to the salt's hygroscopic and deliquescent properties, giving the soil the ability to resist drying out, and maintaining the soil at a semi-moist state. The salts may aid in the compaction of some soils by lubricating the soil particles and reducing friction between the particles (Gow, et al., 1961; Ross, 1988). The additional lubrication provided by the salts over and above water alone results in higher compactive densities without increasing compactive efforts.

Another benefit associated with the use of chloride compounds is that they introduce a divalent cation into the soil. This may affect the clay fraction of the soil by reducing spacing between the particles and thereby increasing flocculation (Mitchell, 1993). Increasing flocculation results in shear strength increases thus stabilizing the soil (Mitchell, 1993). Like lignin, CaCl2 and MgCl₂ additives also cause dispersion of clay in soil-aggregate mixture. The benefit is that when salts leaching out because of rain or a high water table, the clay may disperse and fill the voids, thus retarding further leaching. The recrystallization of these salts in the pore spaces also makes them effective road material stabilizers (Squier, 1974).

The use of CaCl₂ and MgCl₂ significantly increases the surface tension of water molecules between soil particles (Hillel, 1980). The increased pore water surface tension causes an increase in apparent cohesion of the soil resulting in overall soil strength gains (Shepard et al., 1991).

Another major effect that chloride compounds have on soil stabilization is that they reduce vapor pressure in the soil structure. At lower vapor pressure, soils maintain a higher moisture content (Ross, 1988; Shepard, et al., 1991). The higher moisture content increases apparent cohesion and maintaining the moisture content is essential for maintaining unpaved road surface stability. A higher moisture content, along with other factors, prevents raveling and degradation of the road surface. For chloride compounds to be effective in attracting and holding moisture, the relative humidity which in turn is temperature dependent, must be above 29 to 40 percent (Langdon, et al., 1983, Ross, 1988; Shepard, et al., 1991).

CaCl₂ and MgCl₂ additives also depress the freezing point of aqueous solutions in relation to their concentration. As reported in Woods, (1960), a 30 percent CaCl₂ solution freezes at approximately -60F while a 22 percent MgCl₂ solution freezes at approximately -27F. In chloride treated unpaved roads this property minimizes frost leave and reduces freeze-thaw cycles, thus reducing maintenance cost (Woods, 1960; Ingles, et al., 1973).

The main disadvantages with the use of chloride compounds are that they are:

• water soluble and easily washed away by rain and may require more than one application in a single season to maintain their effectiveness.
• Corrosive to most metals - their corrosiveness depends on the air temperature, humidity and concentration.

Like lignin, mixed stabilization results have been reported on CaCl₂ and MgCl₂.

3.6 Summary of Research Studies

Laboratory methods as well as onsite testing have been done to quantify soil stabilization using chemical additives. In one such study, Lane, et al., (1984) used laboratory methods to measure soil cohesion increases resulting from the addition of some commercially available dust suppressants (additives). The laboratory methods included the unconfined compression (UC) test and a modified wet sieve analysis test. The UC (ASTM test No. C-39) was used to quantify the soil-additive cohesion strength gains under different sample-drying conditions. The modified wet sieving analysis (ASTM test No. C-117) was used as an indicator of the dust suppressant's ability to resist washout during intense rainfall and thunderstorms because that ability is critical to the longevity of the stabilized road surface. The additives tested include an emulsified petroleum residue, a processed chemical derived from petroleum residue, and calcium ligninsulfonate. The soil material used was classified as cohesionless. Because road surface moisture conditions may vary over time, the specimens were made at moisture contents of 4, 6, and 8 percent by weight. The suppressant manufacturers' recommended addition of 6 percent by weight was used. The testing was performed at three sample-drying conditions, 24-hour air-dried, 24-hour bag cured, and immediate sample testing.

Figure 3.1 shows the resulting cohesive strength measured for the 24-hour air-dried test condition. The results indicate that each additive tested varies in cohesive strength with a range of 4-55 psi. The calcium ligninsulfonate at each of the initial aggregate moisture content (4, 6, and 8 percent) showed a higher cohesive strength than the petroleum-based additives. Meanwhile the petroleum based additives resisted water striping better than the lignin under air-dry conditions. The researchers concluded that the initial moisture content of the road material mix is critical to the success of the soil stabilization effort.

The Quebec Department of Roads conducted laboratory tests comparing the engineering properties of lignin-treated aggregate with that of raw aggregate and clay-mixed gravel (Hurtubise, 1953). The bearing capacity of the aggregate treated with 1.2 percent lignin was higher than that of the raw aggregate soil and clay-mixed aggregate. Cohesive strength increased with the addition of 2 percent lignin. The strength increase was also found to be nearly linearly proportional to the amount of lignin used. Water absorption tests indicated that water absorption through capillary action was reduced substantially. Moisture density relationship tests showed that an increase in the amount of lignin added to the soil increased the density and reduced the optimum moisture content.

Davidson, et al., (1957), in a similar study confirmed that lignin admixtures indeed do improve some engineering properties related to stability of soils. They also reported that the strength of lignin-treated soil increases rapidly with an increase in the length of air curing.

Palmer, et al., (1995), in a low-volume road study used laboratory methods to evaluate the strength and density modification of unpaved road soils because of chemical additives. The additives tested included lignin, CaCl₂ and MgCl₂ at different concentrations. Three different road soil materials with different soil classifications were used. The test procedures were designed to find changes in soil characteristics. The soil cohesion and density changes as affected by additive concentrations were evaluated. Moisture and density relationships using ASTM standard D698 were performed to measure the optimum moisture contents and dry densities of the test specimens. Unconfined compression (UC) tests were also performed to measure the cohesive strength changes. The tests were performed under wet conditions (immediately after the specimens were formed) and after seven-day air-dried conditions. The test results were evaluated based on measured changes in dry density and UC. The results were given as a comparison between water only as the additive, and lignin, CaCl₂, and MgCl₂ as the additive.

The results of the cohesion and density measurements were mixed for all three additives at the different concentration tested. The lignin additive, for example, increased the dry density of some samples at certain moisture contents over the compaction with water alone. On the other hand, lower dry densities than the water only testing were measured for some soil samples at lower and higher moisture contents. The chloride compounds, for the most part, showed decreased dry densities when compared to soils compacted with only water at lower initial concentrations but showed increased dry densities as concentrations increased.

UC tests performed on wet specimens showed lower cohesive strengths than that of water-only specimens for all three additives. The seven-day air-cured samples exhibited large strength increases for the lignin-treated specimens at all concentration levels. Changes in UC strength however, were not as consistent for specimens formed with CaCl₂ and MgCl₂ additives. Figures 3.2 and 3.3 illustrate results of some of the measurements performed.

For each of the soils tested, lignin provided the greatest increase in strength as determined by the UC tests. Palmer, et al. (1995) concluded by noting, "Because each soil may react differently to the application of a particular additive, each soil should be tested with the additives being considered for purposes of dust control or stabilization. This testing can be done more economically in laboratories than in the field. Laboratory test results can be used to recommend additive choices, additive concentrations, and application methods that have the best chance of improving the stability of an unpaved road's surface."

Other research studies have evaluated the relative effectiveness of dust suppressants (i.e. lignin, CaCl₂ and MgCl₂ ) in controlling the generation and emission of fugitive dust from unpaved roads (Hoover, et al., 1973 and 1981; Squier, 1974; Addo and Sanders, 1995). Although most of these studies were aimed at dust control, they serve as a surrogate measure of unpaved road surface stabilization. The studies attempted to measure traffic-generated fugitive dust and aggregate pullout from the surface of unpaved roads, which is a fair measure of the stability of unpaved road surface.

3.7 Summary

Natural soils rarely possess the necessary engineering properties for road construction. Thus, adding chemicals to soil to improve the road soil properties, termed "soil stabilization," has become a common practice in construction of both paved and unpaved roads. For unpaved roads, the application of dust suppressants for the purpose of controlling fugitive dust generation has been noted to produce changes in the road soil characteristics that influence soil stabilization. With an improved road base and a stabilized road surface, the loss of road surface fines in the form of dust is reduced which in turn prevents the road surface from deteriorating and eventually reduces maintenance costs.

A stabilized unpaved road surface can serve as a base for a high type road surface such as "chip and seal" suitable for high traffic volume roads. Usually three to four successive dust suppressant treatments may stabilize the road surface enough to receive a higher type surfacing. In general, there are two methods of incorporating suppressants into the road surface soil: the surface or topically sprayed application method and the mixed-in-place or in-depth application method. The mixed-in-place application is relatively time consuming and therefore more costly compared to the topically sprayed application method. However, the mixed-in-place application is the preferred method because its stabilization effect is more pronounced.

Many factors influence soil stabilization. The most notable factors are the physical and chemical properties of the soil and the chemical additive. The stabilization effect of a soil additive is measured in terms of the increase in shear strength of the soil-additive mixture. Lignin as a soil additive causes dispersion of the clay fraction of some soils resulting in the shear strength increase of the soil. The application of some salt additives may cause the solubility of some soil minerals, thus causing soil structure consolidation and cementation which leads to shear strength increases.