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Geotechnical Aspects of Pavements Reference Manual
Chapter 7.0 Design Details And Construction Conditions Requiring Special Design Attention (continued)
7.6 Subgrade Improvement And Strengthening
Proper treatment of problem soil conditions and the preparation of the foundation are extremely important to ensure a long-lasting pavement structure that does not require excessive maintenance. Some agencies have recognized certain materials simply do not perform well, and prefer to remove and replace such soils (e.g., a state specification dictating that frost susceptible loess cannot be present in the frost penetration zone). However, in many cases, this is not the most economical or even desirable treatment (e.g., excavation may create disturbance, plus additional problems of removal and disposal). Stabilization provides an alternate method to improve the structural support of the foundation for many of the subgrade conditions presented in the previous section. In all cases, the provision for a uniform soil relative to textural classification, moisture, and density in the upper portion of the subgrade cannot be over-emphasized. This uniformity can be achieved through soil sub-cutting or other stabilization techniques. Stabilization may also be used to improve soil workability, provide a weather resistant work platform, reduce swelling of expansive materials, and mitigate problems associated with frost heave. In this section, alternate stabilization methods will be reviewed, and guidance will be presented for the selection of the most appropriate method.
7.6.1 Objectives of Soil Stabilization
Soils that are highly susceptible to volume and strength changes can cause severe roughness and accelerate the deterioration of the pavement structure in the form of increased cracking and decreased ride quality when combined with truck traffic. Generally, the stiffness (in terms of resilient modulus) of some soils is highly dependent on moisture and stress state (see Section 5.4). In some cases, the subgrade soil can be treated with various materials to improve the strength and stiffness characteristics of the soil. Stabilization of soils is usually performed for three reasons:
A summary of the stabilization methods most commonly used in pavements, the types of soils for which they are most appropriate, and their intended effects on soil properties is presented in Table 7-13.
Mechanical stabilization using thick gravel layers or granular layers in conjunction with geotextiles or geogrids is an effective technique for improving roadway support over soft, wet subgrades. Thick granular layers provide a working platform, but do not provide strengthening of the subgrade. In fact, construction of thick granular layers in some cases results in disturbance of the subgrade due to required construction activities. Thick granular layers are also used to avoid or reduce frost problems by providing a protection to the underlying subgrade layers.
A common practice in several New England and Northwestern states is to use a meter (3.3 ft) or more of gravel beneath the pavement section. The gravel improves drainage of surface infiltration water and provides a weighting action that reduces and results in more uniform heave. Washington State recently reported the successful use of an 0.4 m (18 in.) layer of cap rock beneath the pavement section in severe frost regions (Ulmeyer et al., 2002).
Blending gravel and, more recently, recycled pavement material with poorer quality soils also can provide a working platform. The gravel acts as filler, creating a dryer condition and decreasing the influence of plasticity. However, if saturation conditions return, the gravel blend can take on the same poorer support characteristics of the subgrade.
Geotextiles and geogrids used in combination with quality aggregate minimize disturbance and allow construction equipment access to sites where the soils are normally too weak to support the initial construction work. They also allow compaction of initial lifts on sites where the use of ordinary compaction equipment is very difficult or even impossible. Geotextiles and geogrids reduce the extent of stress on the subgrade and prevent base aggregate from penetrating into the subgrade, thus reducing the thickness of aggregate required to stabilize the subgrade. Geotextiles also act as a separator to prevent subgrade fines from pumping or otherwise migrating up into the base. Geosynthetics have been found to allow for subgrade strength gain over time. However, the primary long-term benefit is preventing aggregate-subgrade mixing, thus maintaining the thickness of the base and subbase. In turn, rehabilitation of the pavement section should only require maintenance of surface pavement layers.
Stabilization with admixtures, such as lime, cement, and asphalt, have been mixed with subgrade soils used for controlling the swelling and frost heave of soils and improving the strength characteristics of unsuitable soils. For admixture stabilization or modification of cohesive soils, hydrated lime is the most widely used. Lime is applicable in clay soils (CH and CL type soils) and in granular soils containing clay binder (GC and SC), while Portland cement is more commonly used in non-plastic soils. Lime reduces the Plasticity Index (PI) and renders a clay soil less sensitive to moisture changes. The use of lime should be considered whenever the PI of the soil is greater than 12. Lime stabilization is used in many areas of the U.S. to obtain a good construction platform in wet weather above highly plastic clays and other fine-grained soils. It is important to note that changing the physical properties of a soil through chemical stabilization can produce a soil that is susceptible to frost heave. Following is a brief description of the characteristics of stabilized soils followed by the treatment procedures. Additional guidance on soil stabilization with admixtures and stabilization with geosynthetics can be obtained from the following resources:
7.6.2 Characteristics of Stabilized Soils
Although mechanical stabilization with thick granular layers or geosynthetics and aggregate subbase provides the potential for strength improvement of the subgrade over time, this is generally not considered in the design of the pavement section, and no increase in structural support is attributed to the geosynthetic. However, the increase in gravel thickness (minus an allowance for rutting) can contribute to the support of the pavement. Alternatively, the aggregate thickness used in conjunction with the geosynthetic is designed to provide an equivalent subgrade modulus, which can be considered in the pavement design, discounting the additional aggregate thickness of the stabilization layer. Geosynthetics also allow more open graded aggregate, thus providing for the potential to drain the subbase into edgedrains and improving its support value.
The improvement of subgrade or unbound aggregate by application of a stabilizing agent is intended to cause the improvements outlined above (i.e., construction platform, subgrade strengthening, and control of moisture). These improvements arise from several important mechanisms that must be considered and understood by the pavement designer. Admixtures used as subgrade stabilizing agents may fill or partially fill the voids between the soil particles. This reduces the permeability of the soil by increasing the tortuosity of the pathways for water to migrate through the soil. Reduction of permeability may be relied upon to create a waterproof surface to protect underlying, water sensitive soils from the intrusion of surface water. This mechanism must be accompanied by other aspects of the geometric design into a comprehensive system. The reduction of void spaces may also tend to change the volume change under shear from a contractive to a dilative condition. The admixture type stabilizing agent also acts by binding the particles of soil together, adding cohesive shear strength and increasing the difficulty with which particles can move into a denser packing under load. Particle binding serves to reduce swelling by resisting the tendency of particles to move apart. The particles may be bound together by the action of the stabilizing agent itself (as in the case of asphalt cement), or may be cemented by chemical reaction between the soil and stabilizing agent (as in the case of lime or Portland cement). Additional improvement can arise from other chemical-physical reactions that affect the soil fabric (typically by flocculation) or the soil chemistry (typically by cation exchange). The down side of admixtures is that they require up front lab testing to confirm their performance and very good field control to obtain a uniform, long lasting product, as outlined later in this section. There are also issues of dust control and weather dependency, with some methods that should be carefully considered in the selection of these methods.
The zone that may be selected for improvement depends upon a number of factors. Among these are the depth of soft soil, anticipated traffic loads, the importance of the transportation network, constructability, and the drainage characteristics of the geometric design and the underlying soil. When only a thin zone and/or short roadway length is subject to improvement, removal and replacement will usually be the preferred alternative by most agencies, unless a suitable replacement soil is not economically available. Note that in this context, the use of the qualitative term "thin" is intentional, as the thickness of the zone can be described as thick or thin, based primarily on the project economics of the earthwork requirements and the depth of influence for the vehicle loads.
7.6.3 Thick Granular Layers
Many agencies have found that a thick granular layer is an important feature in pavement design and performance. Thick granular layers provide several benefits, including increased load-bearing capacity, frost protection, and improved drainage. While the composition of this layer takes many forms, the underlying strategy of each is to achieve desired pavement performance through improved foundation characteristics. The following sections describe the benefits of thick granular layers, typical characteristics, and considerations for the design and construction of granular embankments.
Objectives of Thick Granular Layers
Thick granular layers have been used in design for structural, drainage, and geometric reasons. Many times, a granular layer is used to provide uniformity and support as a construction platform. In areas with large quantities of readily accessible, good quality aggregates, a thick granular layer may be used as an alternative to soil stabilization. Whatever the reason, thick granular layers aim to improve the natural soil foundation. By doing this, many agencies are recognizing that the proper way to account for weak, poorly draining soils is through foundation improvement, as opposed to increasing the pavement layer thicknesses. The following is a list of objectives and benefits of thick granular layers:
Characteristics of Thick Granular Layers
Thick granular layers have been incorporated in pavement design in several ways. They can be referred to as fills or embankments, an improved or prepared subgrade, and select or preferred borrow. Occasionally, a thick granular layer is used as the pavement subbase. The two most important characteristics for all of these layers are material properties and thickness. While geometric requirements (e.g., vertical profile) and improved surface runoff can be achieved by embankments constructed of any soil type, the most beneficial effects are produced through utilization of good quality, granular materials. Several methods are used to characterize the strength and stiffness of granular materials, including the California Bearing Ratio (CBR) and resilient modulus testing. In addition, several types of field plate load tests have been used to determine the composite reaction of the embankment and soil combination. In general, materials with CBR values of 20% or greater are used, corresponding to resilient moduli of approximately 120 MPa (17,500 psi). These are typically sand or granular materials, or coarse-grained materials with limited fines, corresponding to AASHTO A-1 and A-2 (GW, GP, SW and SP) soils.
Aggregate gradation and particle shape are other important properties. Typically, embankment materials are dense-graded, with a maximum top-size aggregate that varies depending on the height of the embankment. Many times, the lowest embankment layer may contain cobbles or aggregates of 100 - 200 mm (4 - 8 in.) in diameter. Granular layers placed close to the embankment surface have gradations, including maximum size aggregates, similar to subbase material specifications. Although dense-graded aggregate layers do not provide efficient drainage relative to open-graded materials, a marginal degree of subsurface seepage can be achieved by limiting the fines content to less than 10%. The type of granular material used is normally a function of material availability and cost. Pit-run gravels and crushed stone materials are the most common. The high shear strength of crushed stone is more desirable than rounded, gravelly materials; however, the use of crushed materials may not always be economically feasible.
The thicknesses of granular layers vary, depending upon their intended use. Granular layers 150 - 300 mm (6 - 12 in.) thick may be used to provide uniformity of support, or act as a construction platform for paving of asphalt and concrete layers. To increase the composite subgrade design values (i.e., combination of granular layer over natural soil), it is usually necessary to place a minimum of 0.5 - 1.5 m (1½ -- 5 ft) of embankment material, depending on the strength of the granular material relative to that of the underlying soil. Likewise, granular fills placed for frost protection may also range from 0.5 - 1.5 m (1½ - 5 ft). In most cases, embankments greater than 2 m (6½ ft) thick have diminishing effects in terms of strength, frost protection, and drainage. Granular embankments greater than 2 - 3 m (6½ - 10 ft) thick are usually constructed for purposes of geometric design.
Considerations for Pavement Structural Design
The use of a thick granular layer presents an interesting situation for design. The placement of a granular layer of substantial thickness over a comparatively weak underlying soil forms, essentially, non-homogeneous subgrade in the vertical direction. Pavement design requires a single subgrade design value, for example CBR, resilient modulus, or k-value. This is generally determined through laboratory or field tests, when the soil mass in the zone of influence of vehicle loads is of the same type, or exhibits similar properties. In the case of a non-homogeneous subgrade, the composite reaction of the embankment and soil combination can vary from that of the natural soil to that of the granular layer. Most commonly, the composite reaction is a value somewhere between the two extremes, dependent upon the relative difference in moduli between the soil and embankment, and the thicknesses of the granular layer. The actual composite subgrade response is not known until the embankment layer is placed in the field, and it may be different once the upper pavement layers are placed.
To account for non-homogenous subgrades in pavement structural design, it is recommended to characterize the individual material properties by traditional means, such as resilient modulus or CBR testing, and to compare these results to field tests performed over the constructed embankment layers, as well as the completed pavement section. Analytical models, such as elastic layer programs, can be used to make theoretical predictions of composite subgrade response, and these predictions can then be verified by field testing. Some agencies use in-situ plate load tests to verify that a minimum composite subgrade modulus has been achieved. Deflection devices, including the Falling Weight Deflectometer (FWD), can be used for testing over the compacted embankment layer and over the constructed pavement surface.
It is advisable to use caution when selecting a design subgrade value for a non-homogenous subgrade. Experience has shown that a good-quality embankment layer must be of significant height, say 1 m (3 ft) or more, before the composite subgrade reaction begins to resemble that of the granular layer. This means that, for granular layers up to 1 m (3 ft) in height, the composite reaction can be much less than that of the embankment layer itself. If too high a subgrade design value is selected, the pavement will be under-designed. Granular layers less than 0.5 m (1.6 ft) thick have minimal impact on the composite subgrade reaction, when loaded under the completed pavement section.
7.6.4 Geotextiles and Geogrids
Geosynthetics are a class of geomaterials that are used to improve soil conditions for a number of applications. They consist of manufactured polymeric materials used in contact with soil materials or pavements as an integral part of a man-made system (after ASTM D4439). The most common applications in general use are in pavement systems for both paved and unpaved roadways, for reinforcing embankments and foundation soils, for creating barriers to water flow in liners and cutoffs, and for improving drainage. The generic term "geosynthetic" is often used to cover a wide range of different materials, including geotextiles, geogrids, and geomembranes. Combinations of these materials in layered systems are usually called geocomposites.
Geotextile and geogrid materials are the most commonly used geosynthetics in transportation, although certainly others are sometimes used. This generality is more accurate when only the pavement itself (not including the adjoining fill or cut slopes, retaining walls, abutments, or drainage facilities) is considered. Table 7-14 provides a list of transportation applications for specific basic functions of the geosynthetic. Each of these functional classes, while potentially related by the specific application being proposed, refers to an individual mechanism for the improvement of the soil subgrade. Stabilization, as reviewed in this section, is a combination of the separation, filtration, and reinforcement functions. Drainage can also play a role.
The separation function prevents the subgrade and the subbase from intermixing, which would most likely occur during construction and in-service due to pumping of the subgrade. The filtration function is required because soils requiring stabilization are usually wet and saturated. By acting as a filter, the geotextile retains the subgrade without clogging, while allowing water from the subgrade to pass up into the subbase, thus allowing destabilizing pore pressure to dissipate and promote strength gain due to consolidation. If the subbase is dirty (contains high fines), it may be desirable to use a thick, nonwoven geotextile, which will allow for drainage in its plane (i.e., in this case, pore water pressure dissipates through the plane of the geotextile).
Geotextiles and geogrids also provide some level of reinforcement by laterally restraining the base or subbase and improving the bearing capacity of the system, thus decreasing shear stresses on the subgrade. Soft, weak subgrade soils provide very little lateral restraint, so when the aggregate moves or shoves laterally, ruts develop on the aggregate surface and also in the subgrade. A geogrid with good interlocking capabilities or a geotextile with good frictional capabilities can provide tensile resistance to lateral aggregate movement. The geosynthetic also increases the system bearing capacity by forcing the potential bearing surface under the wheel load to develop along alternate, longer mobilization paths and, thus, higher shear strength surfaces.
Geotextiles serve best as separators, filters and, in the case of nonwoven geotextiles, drainage layers, while geogrids are better at reinforcing. Geogrids, as with geotextiles, prevent the subbase from penetrating the subgrade, but they do not prevent the subgrade from pumping into the base. When geogrids are used, either the subbase has to be designed as a separator or a geotextile must be used in conjunction with the geogrid, either separately or as a geocomposite.
As defined by AASHTO M288, geotextiles or geogrids in conjunction with an appropriately designed thickness of subbase aggregate provide stabilization for soft, wet subgrades with a CBR of less than 3 (a resilient modulus less than 30 MPa (4500 psi)). Table 7-15 provides subgrade conditions that are considered to be the most appropriate for geosynthetic use. These are conditions where the subgrade will not support conventional construction without substantial rutting. Engineers have compiled over 20+ years of successful use for this application in these types of conditions. Geosynthetics do not provide improvements for expansive soils, and use in stabilization for subgrade conditions that are better than those defined in Table 7-15 is questionable. However, geosynthetics may still provide a valuable function as separators for any subgrade containing large amounts of fines or as base reinforcement, even with competent subgrades, as discussed in Section 7.2.
Separation is a viable function, for soils that are seasonally weak (e.g., from spring thaw) or for high fines content soils, which are susceptible to pumping. This is especially the case for permeable base applications, as covered in Section 7.2. A greater range of geotextile applicability is recognized in the M288 specification (AASHTO, 1997). With a CBR ≥ 3, the geotextile application is identified as separation. By simply maintaining the integrity of the subbase and base layers over the life of the pavement, the serviceability of the roadway section will be extended, and substantial cost benefits can be realized. Research is ongoing to quantify the cost-benefit life cycle ratio of using geosynthetics in permanent roadway systems. Initial work by Al-Qadi, 1997 indicates that the use a geosynthetic separator may increase the number of allowable design vehicles (ESALs) by a factor of two. Considering the cost of a geosynthetic is generally $1.25/m2, while the cost of a modern pavement section is on the order of $25/m2, the life extension of the roadway section will more than make up for the cost of the geosynthetic. In addition, as previously indicated, the geosynthetic maintains the integrity of the base such that rehabilitation should only require surface pavement restoration. The ability of a geosynthetic to prevent premature failure and reduce long-term maintenance costs provides extremely low-cost performance insurance.
The design of the geosynthetic for stabilization is completed using the design-by-function approach in conjunction with AASHTO M288, in the steps from FHWA HI-95-038 outlined below. A key feature of this method is the assumption that the structural pavement design is not modified at all in the procedure. The pavement design proceeds exactly according to standard procedures, as if the geosynthetic was not present. The geosynthetic instead replaces additional unbound material that might be placed to support construction operations, and replaces no part of the pavement section itself. However, this unbound layer will provide some additional support. If the soil has a CBR of less than 3, and the aggregate thickness is determined based on a low rutting criteria in the following steps, the support for the composite system is theoretically equivalent to a CBR = 3 (resilient modulus of 30 Mpa (4500 psi)). As with thick aggregate fill used for stabilization, the support value should be confirmed though field testing using, for example, a plate load test or FWD test to verify that a minimum composite subgrade modulus has been achieved. Note that the FHWA procedure is controlled by soil CBR, as measured using ASTM C4429.
Figure 7-21. Thickness design curves with geosynthetics for a) single and b) dual wheel loads (after USFS, 1977, and FHWA NHI-95-038, 1998).
Field installation procedures introduce a number of special concerns; the AASHTO M288 standard includes a guide specification for geotextile construction. FHWA HI-905-038 (Holtz et al. 1998) recommends that this specification be modified to suit local conditions and contractors and provides example specifications. Concerns and criteria for field installation include, for example, the seam lap and sewing requirements, and construction sequencing and quality control.
7.6.5 Admixture Stabilization
As previously indicated in Section 7.6.1, there are a variety of admixtures that can be mixed with the subgrade to improve its performance. The various admixture types are shown in Table 7-16, along with initial guidance for evaluating the appropriate application of these methods. Following is a general overview of each method, followed by a generalized outline for determining the optimum admixture content requirements. Design details for each specific method are contained in Appendix F.
Table 7-16. Guide for selection of admixture stabilization method(s) (Austroads, 1998).
Lime treatment or modification consists of the application of 1 - 3% hydrated lime to aid drying of the soil and permit compaction. As such, it is useful in the construction of a "working platform" to expedite construction. Lime modification may also be considered to condition a soil for follow-on stabilization with cement or asphalt. Lime treatment of subgrade soils is intended to expedite construction, and no reduction in the required pavement thickness should be made.
Lime may also be used to treat expansive soils, as discussed in Section 7.3. Expansive soils as defined for pavement purposes are those that exhibit swell in excess of 3%. Expansion is characterized by heaving of a pavement or road when water is imbibed in the clay minerals. The plasticity characteristics of a soil often are a good indicator of the swell potential, as indicated in Table 7-17. If it has been determined that a soil has potential for excessive swell, lime treatment may be appropriate. Lime will reduce swell in an expansive soil to greater or lesser degrees, depending on the activity of the clay minerals present. The amount of lime to be added is the minimum amount that will reduce swell to acceptable limits. Procedures for conducting swell tests are indicated in the ASTM D 1883 CBR test and detailed in ASTM D 4546.
The depth to which lime should be incorporated into the soil is generally limited by the construction equipment used. However, 0.6 - 1 m (2 - 3 ft) generally is the maximum depth that can be treated directly without removal of the soil.
Lime or pozzolanic stabilization of soils improves the strength characteristics and changes the chemical composition of some soils. The strength of fine-grained soils can be significantly improved with lime stabilization, while the strength of coarse-grained soils is usually moderately improved. Lime has been found most effective in improving workability and reducing swelling potential with highly plastic clay soils containing montmorillonite, illite, and kaolinite. Lime is also used to reduce the water content of wet soils during field compaction. In treating certain soils with lime, some soils are produced that are subject to fatigue cracking.
Lime stabilization has been found to be an effective method to reduce the volume change potential of many soils. However, lime treatment of soils can convert the soil that shows negligible to moderate frost heave into a soil that is highly susceptible to frost heave, acquiring characteristics more typically associated with silts. It has been reported that this adverse effect has been caused by an insufficient curing period. Adequate curing is also important if the strength characteristics of the soil are to be improved.
The most common varieties of lime for soil stabilization are hydrated lime [Ca(OH)2], quicklime [CaO], and the dolomitic variations of these high-calcium limes [Ca(OH)2×MgO and CaO×MgO]. While hydrated lime remains the most commonly used lime stabilization admixture in the U.S., use of the more caustic quicklime has grown steadily over the past two decades. Lime is usually produced by calcining2 limestone or dolomite, although some lime-typically of more variable and poorer quality-is also produced as a byproduct of other chemical processes.
For lime stabilization of clay (or highly plastic) soils, the lime content should be from 3 - 8% of the dry weight of the soil, and the cured mass should have an unconfined compressive strength of at least 0.34 MPa (50 psi) within 28 days. The optimum lime content should be determined with the use of unconfined compressive strength and the Atterberg limits tests on laboratory lime-soil mixtures molded at varying percentages of lime. As discussed later in this section, pH can be used to determine the initial, near optimum lime content value. The pozzolanic strength gain in clay soils depends on the specific chemistry of the soil - e.g., whether it can provide sufficient silica and alumina minerals to support the pozzolanic reactions. Plasticity is a rough indicator of reactivity. A plasticity index of about 10 is commonly taken as the lower limit for suitability of inorganic clays for lime stabilization. The lime-stabilized subgrade layer should be compacted to a minimum density of 95%, as defined by AASHTO T99.
Typical effects of lime stabilization on the engineering properties of a variety of natural soils are shown in Table 7-18 and Figure 7-22. These are the result of several chemical processes that occur after mixing the lime with the soil. Hydration of the lime absorbs water from the soil and causes an immediate drying effect. The addition of lime also introduces calcium (Ca+2) and magnesium (Mg+2) cations that exchange with the more active sodium (Na+) and potassium (K+) cations in the natural soil water chemistry; this cation exchange reduces the plasticity of the soil, which, in most cases, corresponds to a reduced swell and shrinkage potential, diminished susceptibility to strength loss with moisture, and improved workability. The changes in the soil-water chemistry also lead to agglomeration of particles and a coarsening of the soil gradation; plastic clay soils become more like silt or sand in texture after the addition of lime. These drying, plasticity reduction, and texture effects all occur very rapidly (usually with 1 hour after addition of lime), provided there is thorough mixing of the lime and the soil.
Figure 7-22. Effect of lime content on engineering properties of a CH clay (from Rollings and Rollings, 1996; from data reported by McCallister and Petry, 1990).
When soils are treated properly with lime, it has been observed that the lime-soil mixture may be subject to durability problems, the cyclic freezing and thawing of the soil. The durability of lime stabilization on swell potential and strength may be adversely affected by environmental influences:
Soils classified as CH, CL, MH, ML, SC, and GC with a plasticity index greater than 12 and with 10% passing the 0.425 mm (No. 40) sieve are potentially suitable for stabilization with lime. Lime-flyash stabilization is applicable to a broader range of soils because the cementing action of the material is less dependent on the fines contained within the soil. However, long-term durability studies of pavements with lime-flyash stabilization are rather limited.
Hydrated lime, in powder form or mixed with water as a slurry, is used most often for stabilization.
Portland cement is widely used for stabilizing low-plasticity clays, sandy soils, and granular soils to improve the engineering properties of strength and stiffness. Increasing the cement content increases the quality of the mixture. At low cement contents, the product is generally termed cement-modified soil. A cement-modified soil has improved properties of reduced plasticity or expansive characteristics and reduced frost susceptibility. At higher cement contents, the end product is termed soil-cement or cement-treated base, subbase, or subgrade.
For soils to be stabilized with cement, proper mixing requires that the soil have a PI of less than 20% and a minimum of 45% passing the 0.425 mm (No. 40) sieve. However, highly plastic clays that have been pretreated with lime or flyash are sometimes suitable for subsequent treatment with Portland cement. For cement stabilization of granular and/or nonplastic soils, the cement content should be 3 - 10% of the dry weight of the soil, and the cured material should have an unconfined compressive strength of at least 1 MPa (150 psi) within 7 days. The Portland cement should meet the minimum requirements of AASHTO M 85. The cement-stabilized subgrade should be compacted to a minimum density of 95% as defined by AASHTO M 134.
Several different types of cement have been used successfully for stabilization of soils. Type I normal Portland cement and Type IA air-entraining cements were used extensively in the past, and produced about the same results. At the present time, Type II cement has largely replaced Type I cement as greater sulfate resistance is obtained, while the cost is often the same. High early strength cement (Type III) has been found to give a higher strength in some soils. Type III cement has a finer particle size and a different compound composition than do the other cement types. Chemical and physical property specifications for Portland cement can be found in ASTM C 150.
The presence of organic matter and/or sulfates may have a deleterious effect on soil cement. Tests are available for detection of these materials and should be conducted if their presence is suspected.
Stabilization with Lime-Flyash (LF) and Lime-Cement-Flyash (LCF)
Stabilization of coarse-grained soils having little or no fines can often be accomplished by the use of LF or LCF combinations. Flyash, also termed coal ash, is a mineral residual from the combustion of pulverized coal. It contains silicon and aluminum compounds that, when mixed with lime and water, forms a hardened cementitious mass capable of obtaining high compressive strengths. Lime and flyash in combination can often be used successfully in stabilizing granular materials, since the flyash provides an agent with which the lime can react. Thus LF or LCF stabilization is often appropriate for base and subbase course materials.
Flyash is classified according to the type of coal from which the ash was derived. Class C flyash is derived from the burning of lignite or subbituminous coal and is often referred to as "high lime" ash because it contains a high percentage of lime. Class C flyash is self-reactive or cementitious in the presence of water, in addition to being pozzolanic. Class F flyash is derived from the burning of anthracite or bituminous coal and is sometimes referred to as "low lime" ash. It requires the addition of lime to form a pozzolanic reaction. To be acceptable quality, flyash used for stabilization must meet the requirements indicated in ASTM C 593.
Design with LF is somewhat different from stabilization with lime or cement. For a given combination of materials (aggregate, flyash, and lime), a number of factors can be varied in the mix design process, such as percentage of lime-flyash, the moisture content, and the ratio of lime to flyash. It is generally recognized that engineering characteristics such as strength and durability are directly related to the quality of the matrix material. The matrix material is that part consisting of flyash, lime, and minus No. 4 aggregate fines. Basically, higher strength and improved durability are achievable when the matrix material is able to "float" the coarse aggregate particles. In effect, the fine size particles overfill the void spaces between the coarse aggregate particles. For each coarse aggregate material, there is a quantity of matrix required to effectively fill the available void spaces and to "float" the coarse aggregate particles. The quantity of matrix required for maximum dry density of the total mixture is referred to as the optimum fines content. In LF mixtures, it is recommended that the quantity of matrix be approximately 2% above the optimum fines content. At the recommended fines content, the strength development is also influenced by the ratio of lime to flyash. Adjustment of the lime-flyash ratio will yield different values of strength and durability properties.
Generally, asphalt-stabilized soils are used for base and subbase construction. Use of asphalt as a stabilizing agent produces different effects, depending on the soil, and may be divided into three major groups: 1) sand-bitumen, which produces strength in cohesionless soils, such as clean sands, or acts as a binder or cementing agent, 2) soil-bitumen, which stabilizes the moisture content of cohesive fine-grained soils, and 3) sand-gravel bitumen, which provides cohesive strength and waterproofs pit-run gravelly soils with inherent frictional strength. The durability of bitumen-stabilized mixtures generally can be assessed by measurement of their water absorption characteristics. Treatment of soils containing fines in excess of 20% is not recommended.
Stabilization of soils and aggregates with asphalt differs greatly from cement and lime stabilization. The basic mechanism involved in asphalt stabilization of fine-grained soils is a waterproofing phenomenon. Soil particles or soil agglomerates are coated with asphalt that prevents or slows the penetration of water that could normally result in a decrease in soil strength. In addition, asphalt stabilization can improve durability characteristics by making the soil resistant to the detrimental effects of water, such as volume. In noncohesive materials, such as sands and gravel, crushed gravel, and crushed stone, two basic mechanisms are active: waterproofing and adhesion. The asphalt coating on the cohesionless materials provides a membrane that prevents or hinders the penetration of water and thereby reduces the tendency of the material to lose strength in the presence of water. The second mechanism has been identified as adhesion. The aggregate particles adhere to the asphalt and the asphalt acts as a binder or cement. The cementing effect thus increases shear strength by increasing cohesion. Criteria for design of bituminous-stabilized soils and aggregates are based almost entirely on stability and gradation requirements. Freeze-thaw and wet-dry durability tests are not applicable for asphalt-stabilized mixtures.
There are three basic types of bituminous-stabilized soils, including:
Bituminous stabilization is generally accomplished using asphalt cement, cutback asphalt, or asphalt emulsions. The type of bitumen to be used depends upon the type of soil to be stabilized, method of construction, and weather conditions. In frost areas, the use of tar as a binder should be avoided because of its high temperature susceptibility. Asphalts are affected to a lesser extent by temperature changes, but a grade of asphalt suitable to the prevailing climate should be selected. As a general rule, the most satisfactory results are obtained when the most viscous liquid asphalt that can be readily mixed into the soil is used. For higher quality mixes in which a central plant is used, viscosity-grade asphalt cements should be used. Much bituminous stabilization is performed in-place, with the bitumen being applied directly on the soil or soil aggregate system, and the mixing and compaction operations being conducted immediately thereafter. For this type of construction, liquid asphalts, i.e., cutbacks and emulsions, are used. Emulsions are preferred over cutbacks because of energy constraints and pollution control efforts. The specific type and grade of bitumen will depend on the characteristics of the aggregate, the type of construction equipment, and the climatic conditions. Generally, the following types of bituminous materials will be used for the soil gradation indicated:
The simplest type of bituminous stabilization is the application of liquid asphalt to the surface of an unbound aggregate road. For this type of operation, the slow- and medium-curing liquid asphalts SC-70, SC-250, MC-70, and MC-250 are used.
The recommended soil gradations for subgrade materials and base or subbase course materials are shown in Tables 7-19 and 7-20, respectively.
Stabilization with Lime-Cement and Lime-Bitumen
The advantage of using combination stabilizers is that one of the stabilizers in the combination compensates for the lack of effectiveness of the other in treating a particular aspect or characteristic of a given soil. For instance, in clay areas devoid of base material, lime has been used jointly with other stabilizers, notably Portland cement or asphalt, to provide acceptable base courses. Since Portland cement or asphalt cannot be mixed successfully with plastic clays, the lime is added first to reduce the plasticity of the clay. While such stabilization practice might be more costly than the conventional single stabilizer methods, it may still prove to be economical in areas where base aggregate costs are high. Two combination stabilizers are considered in this section: lime-cement and lime-asphalt.
Design of admixtures takes on a similar process regardless of the admixture type. The following steps are generally followed and are generic to lime, cement, L-FA and L-C-FA, or asphalt admixtures.
Step 1. Classify soil to be stabilized.
Step 2. Prepare trial mixes with varying % content.
Step 3. Develop moisture-density relationship for initial design.
Step 4. Prepare triplicate samples and cure specimens at target density.
Step 5. Determine index strength.
Step 6. Determine resilient modulus for optimum percent admixture.
Step 7. Conduct freeze-thaw tests (Regional as required).
Step 8. Select % to achieve minimum design strength and F-T durability.
Step 9. Add 0.5 - 1% to compensate for non-uniform mixing.
Appendix F provides specific design requirements and design step details for each type of admixture reviewed in this section. Additional design and construction information can also be obtained from industry publications including:
7.6.6 Soil Encapsulation
Soil encapsulation is a foundation improvement technique that has been used to protect moisture sensitive soils from large variations in moisture content. The concept of soil encapsulation is to keep the fine-grained soils at or slightly below optimum moisture content, where the strength of these soils can support heavier trucks and traffic. This technique has been used by a number of states (e.g., Texas and Wyoming) on selected projects to improve the foundations of higher volume roadways. It is more commonly used as a technique in Europe and in foundation or subbase layers for low-volume roadways, where the import of higher quality paving materials is restricted from a cost standpoint. More than 100 projects have been identified around the world, usually reporting success in controlling expansive soils (Steinberg, 1998).
Fine-grained soils can provide adequate bearing strengths for use as structural layers in pavements and embankments, as long as the moisture content remains below the optimum moisture content. However, increases in moisture content above the optimum value can cause a significant reduction in the stiffness (i.e., resilient modulus) and strength of fine-grained materials and soils. Increased moisture content in fine-grained soils below pavements occurs over time, especially in areas subject to frost penetration and freeze-thaw cycles. Thus, fine-grained soils cannot be used as a base or subbase layer unless the soils are protected from any increase in moisture.
The soil encapsulation concept, sometimes referred to as membrane encapsulated soil layer (MESL), is a method for maintaining the moisture content of the soil at the desired level by encapsulating the soil in waterproof membranes. The waterproof membranes prevent water from infiltrating the moisture sensitive material. The resilient modulus measured at or below optimum conditions remains relatively constant over the design life of the pavement.
The prepared subgrade is normally sprayed with an asphalt emulsion before the bottom membrane of polyethylene is placed. This asphalt emulsion provides added waterproofing protection in the event the membrane is punctured during construction operations, and acts as an adhesive for the membrane to be placed in windy conditions. The first layer of soil is placed in sufficient thickness such that the construction equipment will not displace the underlying material. The completed soil embankment is also sprayed with an asphalt emulsion before placement of the top membrane. To form a complete encapsulation, the bottom membrane is brought up the sides and wrapped around the top, for an excavated section, or the top membrane is draped over the sides, for an embankment situation. The top of the membrane is sprayed with the same asphalt emulsion and covered with a thin layer of clean sand to blot the asphalt and to provide added protection against puncture by the construction equipment used to place the upper paving layers.
The reliability of this method to maintain the resilient modulus and strength of the foundation soil over long periods of time is unknown. More importantly, roadway maintenance and the installation of utilities in areas over time limit the use of this technique. Thus, this improvement technique is not suggested unless there is no other option available.
If this technique is used, the pavement designer should be cautioned regarding the use of the environmental effects model (EICM) to predict changes in moisture over time. Special design computations will be needed to restrict the change in moisture content of the MESL over time. The resilient modulus used in design for the MESL should be held constant over the design life of the pavement. The designer should also remember that any utilities placed after pavement construction could make that assumption invalid.
7.6.7 Lightweight Fill
When constructing pavements on soft soils, there is always a concern for settlement. For deeper deposits where shallow surface stabilization may not be effective, thicker granular aggregate as discussed in Section 7.3, may be effective for control deformation under wheel load, but would increase the concern for settlement. An alternate to replacement with aggregate would be to use lightweight fill.
The compacted unit density of most soil deposits consisting of sands, silts, or clays ranges from about 1,800 - 2,200 kg/m3 (112 - 137 lbs/ft3) Lightweight fill materials are available from the lower end of this range down to 12 kg/m3 ( 0.75 lbs/ft3). In many cases, the use of lighter weight materials on soft soils will likely result in both reduced settlement and increased stability. The worldwide interest and use of lightweight fill materials has led to the recent publication by the Permanent International Association of Road Congresses (PIARC) of an authoritative reference "Lightweight Filling Materials" in 1997.
Many types of lightweight fill materials have been used for roadway construction. Some of the more common lightweight fills are listed in Table 7-21. There is a wide range in density of the lightweight fill materials, but all have a density less than conventional soils. Additional information on the composition and sources of the lightweight fill materials listed in Table 7-21 can be found in FHWA NHI-04-001 Ground Improvement Methods technical summaries.
Some lightweight fill materials have been used for decades, while others are relatively recent developments. Wood fiber has been used for many years by timber companies for roadways crossing peat bogs and low-lying land, as well as for repair of slide zones.
The steel-making companies have produced slag since the start of the iron and steel making industry. Initially, the slag were stockpiled as waste materials, but beginning around 1950, the slag were crushed, graded, and sold for fill materials.
Geofoam is a generic term used to describe any foam material used in a geotechnical application. Geofoam includes expanded polystyrene (EPS), extruded polystyrene (XPS), and glassfoam (cellular glass). Geofoam was initially developed for insulation material to prevent frost from penetrating soils. The initial use for this purpose was in Scandinavia and North America in the early 1960s. In 1972, the use of geofoam was extended as a lightweight fill for a project in Norway.
The technique of using pumping equipment to inject foaming agents into concrete was developed in the late 1930s. Little is known about the early uses of this product. However, the U.S. Army Corps of Engineers used foamed concrete as a tunnel lining and annular fill. This product is generally job-produced as a cement/water slurry with preformed foam blended for accurate control and immediate placement.
Shredded tires and tire bales are a relatively recent source of lightweight fill materials. The availability of this material is increasing each year, and its use as a lightweight fill is further promoted by the need to dispose of tires. In most locations, the tires are stockpiled, but they are unsightly and present a serious fire and health hazard. Shredded tires have been used for lightweight fill in the United States and in other countries since the mid 1980s. More than 85 fills using shredded tires as a lightweight fill have been constructed in the United States. In 1995, three tire shred fills with a thickness greater than 8 m (26 ft) experienced an unexpected internal heating reaction. As a result, FHWA issued an Interim Guideline to minimize internal heating of tire shred fills in 1997, limiting tire shred layers to 3 m (9.8 ft).
Expanded shale lightweight aggregate has been used for decades to produce aggregate for concrete and masonry units. Beginning in about 1980, lightweight aggregates have also been used for geotechnical purposes. Completed projects include the Port of Albany, New York marine terminal, where lightweight fill was used behind a bulkhead to reduce the lateral pressures on the steel sheeting. Other projects include construction of roadways over soft ground. The existing high-density soils were partially removed and replaced with lightweight aggregate to reduce settlement. Other projects have included improvement of slope stability by reduction of the gravitational driving force of the soil in the slope and replacement with a lightweight fill.
Waste products from coal burning include flyash and boiler slag. Both of these materials have been used in roadway construction. One of the first documented uses of flyash in an engineered highway embankment occurred in England in 1950. Trial embankments led to the acceptance of flyash fills, and other roadway projects were constructed in other European countries. In 1965, a flyash roadway embankment was constructed in Illinois. In 1984, a project survey found that flyash was used in the construction of 33 embankments and 31 area fills. Boiler slag has been used for backfill since the early 1970s. Many state highway department specifications allow the use of boiler slag as an acceptable fine or coarse aggregate.
The FHWA NHI-04-001 provides an overview of the more common lightweight fill materials that have been used for geotechnical applications in highway construction. Typical geotechnical engineering parameters that are important for design are provided. In addition, design and construction considerations unique to each of these lightweight fill materials are presented. This information can be used for preliminary planning purposes. The technical summary also presents guidelines for preparation of specifications along with suggested construction control procedures. Four case histories are also presented to demonstrate the effectiveness of lightweight fills for specific situations. Approximate costs for the various lightweight fill materials are also presented.
With regard to pavement design, if a minimum of 1 m (3 ft) of good quality gravel type fill is placed between the pavement structure and the lightweight materials as a cover, then the lightweight material will have little impact on pavement design, even for the more compressible tire and geofoam materials. However, if a thinner cover must be used, the support value for these materials must be determined. Lab tests can be used, as discussed in Chapter 5, especially for the granular type materials. The ideal method is to perform field resilient modulus tests on placed material (i.e., on cover soils after placement over the lightweight material(s)), especially for the bulkier materials, such as tires and geofoam.
7.6.8 Deep Foundations and Other Foundation Improvement Methods (from Elias et al., 2004)
In some cases, the extent (area and depth) of poor subgrade conditions are too large for surface stabilization or removal. In extreme cases, the soils may be too week to support the roadway embankment (even for embankments that only consist of the pavement structure). In these cases, other deep ground improvement methods, such as deep foundations, may be required. Ground improvement technologies are geotechnical construction methods used to alter and improve poor ground conditions so that embankment and structure construction can meet project performance requirements where soil replacement is not feasible for environmental or technical reasons, or it is too costly.
Ground improvement has one or more than one of the following main functions:
There are three strategies available to accomplish the above functions representing different approaches. The first method is to increase the shear strength, density, and/or decrease the compressibility of the foundation soil. The second method is to utilize a lightweight fill embankment to reduce significantly the applied load to the foundation, and the third method is to transfer loads to a more competent deeper layer.
The selection of candidate ground improvement methods for any specific project follows a sequential process. The steps in the process include a sequence of evaluations that proceed from simple to more detailed, allowing a best method to emerge. The process is described as follows:
State-of-the-art design and construction methods and/or references are provided in each of the FHWA NHI-04-001 Ground Improvement Methods technical summaries to form the basis of a final design. The success of any ground improvement method is predicated on the implementation of a QA/QC program to verify that the desired foundation improvement level has been reached. These programs incorporate a combination of construction observations, in-situ testing and laboratory testing to evaluate the treated soil in the field. Details are provided in each technical summary contained in the FHWA NHI-04-001.
Recycling, in principal, is a very powerful and often political concept. While the benefits of recycling including conservation of aggregate and binders and preservation of the environment, it requires serious consideration. The long-term performance of recycled materials in pavements and, in come cases the environmental impact, must be carefully evaluated to avoid costly performance and maintenance issues. In this section, the evaluation requirements for recycled materials will be reviewed. There are two forms of recycling in pavements: 1) reuse of the pavement materials themselves and 2) the use of recycled waste materials for subgrade stabilization or as a substitute for aggregate.
7.7.1 Pavement Recycling
The method of recycling the pavement will, in most cases, depend on whether the surface pavement has an AC or PCC surface pavement. In either case, the material could be rubblized, or, in some cases, processed (e.g., sieving, stockpiling, and reusing the reclaimed asphalt pavement (RCP) materials or recycled concrete materials (RCM) plus the aggregate base). Both pavement types can also be rubblized in place and compacted. This procedure is known as rubblize and roll for PCC pavements and full-depth reclamation for AC pavements. For AC pavement materials, there are also several other methods, including hot mix asphalt recycling, hot in-place recycling, and cold in-place recycling, all of which produce a bound product, which is beyond the scope of this manual.
The design requirements for RCP aggregates are essentially the same as natural aggregates. The strength of the material must be determined using the methods outlined in Chapter 5 and Section 7.3, and an assessment must be made of the drainage characteristics, as discussed in Section 7.2. With full-depth reclamation, all of the asphalt pavement sections and a predetermined amount of underlying materials are treated with recycling agents to produce a stabilized base course, and is well covered in FHWA-SA-98-042 (Kandhal, and Mallick, 1997) . The advantages of this process are establishing high production rate and maintaining the geometry of the pavement or shoulder reconstruction. The primary drawbacks are aggregate size, depth limitation and depth control, and need for specialized equipment. With the sizing, RAP can often only be effectively screened down to a maximum size of 50 mm (2 in.). If a significant amount of contaminated base course (i.e., containing significant amount of fines) is removed with the asphalt, the hydraulic properties of the aggregate could also be poor.
Again, the design requirements for RCM aggregates are essentially the same as natural aggregates. Recycled concrete has been used by a number of states as base materials since the 1980s. However, several states have identified three significant issues, including:
As a result, these states are now primarily using the recycled concrete, mixed with natural soils, as embankment fill.
7.7.2 Recycled Waste Materials
A number of recycled waste materials have been used in permanent construction, practically all of which where covered in Section 7.6.7 since they have a lighter weight than conventional aggregate. Other applications not reviewed in Section 7.6.7 include the use of recycled materials as a replacement for base materials (e.g., slag and bottom ash) and, in some cases (e.g., glass and tire shreds) drainage aggregate. As indicated in Section 7.6.7, the materials must be evaluated with respect to the same property requirements as the material they will replace. The pavement support value (e.g., resilient modulus or CBR) should be determine based on lab tests reviewed in Chapter 5. Field trails using FWD tests to confirm the as constructed properties are also recommended. Durability is a critical issue with many of these materials, and, obviously, an assessment of environmental issues must be made.