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Geotechnical Aspects of Pavements Reference Manual
Chapter 7.0 Design Details And Construction Conditions Requiring Special Design Attention (continued)
7.5 Subgrade Conditions Requiring Special Design Attention
Considering variables such as soil type or mineralogy along a length of roadway, the geology (soil genesis and deposition method) and groundwater and flow properties make each project unique with respect to subgrade conditions. It is not surprising that certain conditions will exist that are not conducive to support, or even construction, of pavement systems. This section provides an overview of subgrade conditions that require special design attention. These subsurface conditions are often regional in nature and have usually been identified as problematic by the agency. Several foundation problems, such as collapsible or highly compressible soils, expansive or swelling soils, subsurface water and saturated soils, and frost-susceptible soils, occur extensively across the U.S. and are not specific to one region. For example, frost heave occurs in over half of the states in the U.S. and damage may be most severe in the central states, where many more frost cycles occur than in the most-northern states. Identification of these widely variable problematic subgrade conditions are also reviewed in this section, along with design and construction alternatives to achieve an adequate foundation on which to build the pavement structure.
Most of the subgrade conditions presented in this section can be anticipated through a complete exploration program, as described in Chapter 4, and mitigated or at least minimized via well-conceived designs. By identifying such subgrade issues in the design stage, or even the potential for such problems along an alignment, alternative designs can be established. Alternate designs can then be placed in the bid documents with indicators clearly identified that show where these alternatives should be considered, and then implemented if and where such conditions are encountered. When these special subgrade conditions are not recognized in design, they are often identified during construction, usually resulting in claims and overruns. However, identifying problems in construction is still somewhat fortunate, considering the impact such problems may have on the pavement performance. If the soil conditions described in this section go undetected, there typically is decreased serviceability, usually resulting in premature localized rehabilitation or, not uncommon, reconstruction of the pavement within the first few years of the pavement performance period.
7.5.1 Problematic Soil Types
Obviously, a pavement is to be constructed on whatever material and condition is naturally occurring. The strength and stability of some soils can present problems during construction and certainly can affect the long-term performance of the pavement during its service life. In order to properly discuss these potential problems, it is necessary to define some terms as they relate to problematic mineralogy (Sowers, 1979). Some of the terms are true geological terminology, while some are local or regional terminology. The terms may describe a particular material or condition, but all are problematic and care must be taken when constructing pavements in regions containing these materials.
Adobe. Sandy clays of medium plasticity found in the semiarid regions of the southwestern U.S. These soils have been used for centuries to make sun-dried brick. The name is also applied to some highly plastic clays of the West, which swell significantly when wet.
Bentonite. Highly plastic clay, usually montmorillonite, resulting from the decomposition of volcanic ash. It may be hard when dry, but swells considerably when wet.
Buckshot clay. Applied to clays of the southern and southwestern United States. Cracks into small, hard, relatively uniform sized lumps on drying. Dry lumps will degrade upon wetting (e.g., after they have been used as fill). These soils also tend to swell when wet.
Caliche. A silt or sand of the semiarid areas of the southwestern United States that is cemented with calcium carbonate. The calcium carbonate is deposited by the evaporation of water brought to the ground surface by capillary action. The consistency of caliche varies from soft rock to firm soil.
Coquina. A soft, porous limestone made up largely of shells, coral, and fossils cemented together. Very friable, and breaks down during construction.
Gumbo. A fine-grained, highly plastic clay of the Mississippi Valley. It has a sticky, greasy feel, highly expansive, and forms large shrinkage cracks on drying.
Kaolin. A white or pink clay of low plasticity. It is composed largely of minerals of the kaolinite family.
Loam. A surface soil that may be described as a sandy silt of low plasticity or a silty sand that is well suited to tilling. It applies to soils within the uppermost horizons and should not be used to describe deep deposits of parent material. Loam-type soils are typically sensitive to moisture, easily disturbed in construction, and frost susceptible.
Loess. A deposit of relatively uniform, windblown silt. It has a loose structure, with numerous rootholes that produce vertical cleavage and high vertical permeability. It consists of angular to subrounded quartz and feldspar particles cemented with calcium carbonate or iron oxide. Upon saturation, it becomes soft and compressible because of the loss of cementing. Loess altered by weathering in a humid climate often becomes more dense and somewhat plastic (loess loam). Loess is also highly frost susceptible.
Marine clay. Clays deposited in a marine environment, which, if later uplifted, tend to be extra sensitive due to salt leaching, dramatically losing strength when disturbed.
Marl. A water-deposited sand, silt, or clay containing calcium carbonate. Marls are often light to dark gray or greenish in color and sometimes contain colloidal organic matter. They are often indurated into soft rock.
Muck or mud. An extremely soft, slimy silt or organic silt found on river and lake bottoms. The terms indicate an extremely soft consistency rather than any particular type of soil. Muck implies organic matter.
Peat. A naturally occurring highly organic substance derived primarily from plant materials (ASTM D 5715). Peats are dark brown or black, loose (void ratio may be 5 to 10), and extremely compressible. When dried, they will float. Peat bogs often emit quantities of inflammable methane gas. These soils will experience significant short-term and long-term settlement, even under light loads, and are often moisture sensitive, losing significant strength when wet. They are easily disturbed under construction activities. Peat containing a high degree of easily identifiable fibers is often called fibrous peat for geotechnical applications. Peat containing highly decomposed fibers and a significant highly organic soil component is often called amorphous peat.
Quicksand. Refers to a condition, not a soil type. Gravels, sands, and silts become "quick" when an upward flow of groundwater and/or gas takes place to such a degree that the particles are lifted.
Saprolites. Soils developed from in-situ weathering of rocks. Relic joints from the parent rock often control the weathered soils' strength, permeability, and stability. Fragments may appear sound, but prove to be weak. Identifying the transition of soil to weathered rock to sound rock is difficult, often resulting in claims.
Shale. Indurated, fine grained, sedimentary rocks, such as mudstones, siltstone, and claystone, which are highly variable and troublesome. Some are hard and stable, while others are soft and degrade into clay soon after exposure to the atmosphere or during the design life of the structure. Clays developed from shale are often highly plastic.
Sulfate. A mineral compound characterized by the sulfate radical SO4, which may be contained in soil. It creates significant expansion problems in lime-stabilized soil and, in some cases, distress in concrete.
Sulfide. A mineral compound characterized by the linkage of sulfur with a metal, such as lead or iron, creating galena and pyrite, respectively.
Till. A mixture of sand, gravel, silt, and clay produced by the plowing action of glaciers. The name boulder clay is often given such soils, particularly in Canada and England. The characteristics of glacial till vary depending on the sediments and bedrock eroded. The tills in New England are typically coarser and less plastic that those from the Midwest. The tills in the Northeast tend to be broadly graded and often unstable under water action. The complex nature of their deposition creates a highly unpredictable material.
Topsoils. Surface soils that support plant life. They usually contain considerable organic matter. These soils tend to settle over time as organic matter continues to degrade. They are often moisture sensitive, losing significant strength when wet, and are easily disturbed under construction activities.
Tuff. The name applied to deposits of volcanic ash. In humid climates or in areas in which ash falls into bodies of water, the tuff becomes cemented into a soft, porous rock.
Varved clays. Sedimentary deposits consisting of alternate thin layers of silt and clay. Ordinarily, each pair of silt and clay layers is from 3 - 13 mm (1/8 - 1/2 in.) thick. They are the result of deposition in lakes during periods of alternating high and low water in the inflowing streams, and are often formed in glacial lakes. These deposits have a much higher horizontal than vertical permeability, with the horizontal seams holding water. They are often sensitive, and will lose strength when remolded.
7.5.2 Compressible Soils
Effect of Compressible Soils on Pavement Performance
Highly compressible (very weak) soils are susceptible to large settlements and deformations with time that can have a detrimental effect on pavement performance. Highly compressible soils are very low density, saturated soils, usually silts, clays, and organic alluvium or wind blown deposits and peats. If these compressible soils are not treated properly, large surface depressions with random cracking can develop. The surface depressions can allow water to pond on the pavement's surface and more readily infiltrate the pavement structure, compounding a severe problem. More importantly, the ponding of water will create a safety hazard to the traveling public during wet weather.
Treatments for Compressible Soils
The selection of a particular technique depends on the depth of the weak soil, and the difference between the in-situ conditions and the minimum compaction or strength requirements to limit the amount of anticipated settlement to a permissible value that will not adversely affect pavement performance. When constructing roadways in areas with deep deposits of highly compressible layers, the specific soil properties must be examined to calculate the estimated settlement. Under these conditions, a geotechnical investigation and detailed settlement analysis must be completed prior to the pavement design. When existing subgrade soils do not meet minimum compaction requirements and are susceptible to large settlements over time, consider the following alternatives:
7.5.3 Collapsible Soils
As with highly compressible soils, collapsible soils can lead to significant localized subsidence of the pavement. Collapsible soils are very low density silt type soils, usually alluvium or wind blown (loess) deposits, and are susceptible to sudden decreases in volume when wetted. Often their unstable structure has been cemented by clay binders or other deposits, which will dissolve on saturation, allowing a dramatic decrease in volume (Rollings and Rollings, 1996). Native subgrades of collapsible soils should be soaked with water prior to construction and rolled with heavy compaction equipment. In some cases, residual soils may also be collapsible due to leaching of colloidal and soluble materials. Figure 7-17 provides a method of identifying the potential for collapsible soils. Other local methods for identification may be available. Collapsible soils can also be created in fills when sand type soils are compacted on the dry side of optimum moisture. Meniscus forces between particles can create a soil fabric susceptible to collapse.
If pavement systems are to be constructed over collapsible soils, special remedial measures may be required to prevent large-scale cracking and differential settlement. To avoid problems, collapse must be induced prior to construction. Methods include:
Figure 7-17. Guide to collapsible soil behavior (Rollings and Rollings, 1996).
7.5.4 Swelling Soils
Effect of Swelling Soils on Pavement Performance
Swelling or expansive soils are susceptible to volume change (shrink and swell) with seasonal fluctuations in moisture content. The magnitude of this volume change is dependent on the type of soil (shrink-swell potential) and its change in moisture content. A loss of moisture will cause the soil to shrink, while an increase in moisture will cause it to expand or swell. This volume change of clay type soils can result in longitudinal cracks near the pavement's edge and significant surface roughness (varying swells and depressions) along the pavement's length.
Expansive soils are a very significant problem in many parts of the United States (see Figure 7-18) and are responsible for the application of premature maintenance and rehabilitation activities on many miles of roadway each year. Expansive soils are especially a problem when deep cuts are made in a dense (over-consolidated) clay soil.
Figure 7-18. Estimated location of swelling soils (from Witczak, 1972).
Identification of Swelling Soils
Various techniques and procedures exist for identifying potentially expansive soils. AASHTO T 258 can be used to identify soils and conditions that are susceptible to swell. Two of the more commonly used documents are listed below:
Clay mineralogy and the availability of water are the key factors in determining the degree to which a swelling problem may exist at a given site. Different clay minerals exhibit greater or lesser degrees of swell potential based on their specific chemistry. Montmorillonitic clays tend to exhibit very high swell potentials due to the particle chemistry, whereas illitic clays tend to exhibit very low swell potentials. Identification of clay minerals through chemical or microscopic means may be used as a method of identifying the presence of high swell potential in soils. The soil fabric will also influence the swell potential, as aggregated particles will tend to exhibit higher swell than dispersed particles, and flocculated higher than deflocculated. Generally, the finer-grained and more plastic the soil, the higher the swell potential the soil will exhibit.
The identification of swelling soils in the subgrade is a key component of the geotechnical investigation for the roadway. Soils at shallow depths beneath the proposed pavement elevation are generally sampled as part of the investigation, and their swell potential may be identified in a number of ways. Index testing is a common method for identifying swell potential. Laboratory testing to obtain the plastic and liquid limits and/or the shrinkage limit will usually be conducted. The soil activity (ASTM D 4318), defined as the ratio of the plasticity index to the percentage of the soil by weight finer than 0.002 mm (0.08 mils) is also used as an index property for swell potential, since clay minerals of higher activity exhibit higher swell. Activity calculation requires measurement of gradation using hydrometer methods, which is not typical in geotechnical investigations for pavement design in many states. In addition to index testing, agency practice in regions where swelling soils are a common problem may include swell testing (e.g., ASTM D 4546), for natural or compacted soil samples. Such testing generally includes measurement of the change in height (or volume) of a sample exposed to light loading similar to that expected in the field and then allowed free access to water.
Treatment for Swelling Soils
When expansive soils are encountered along a project in environments and areas where significant moisture fluctuations in the subgrade are expected, consideration should be given to the following alternatives to minimize future volume change potential of the expansive soil:
AASHTO 1993 (Appendix C) provides procedures and graphs to predict the direct effect of swelling soils on serviceability loss and is treated with respect to the differential effects on the longitudinal profile of the road surface. If the swelling is anticipated to be relatively uniform, then the procedures do not apply.
7.5.5 Subsurface Water
It is important to identify any saturated soil strata, the depth to groundwater, and subsurface water flow between soil strata. Subsurface water is especially important to recognize and identify in the transition areas between cut and fill segments. If allowed to saturate unbound base/subbase materials and subgrade soils, subsurface water can significantly decrease the strength and stiffness of these materials. Reductions in strength can result in premature surface depressions, rutting, or cracking. Seasonal moisture flow through selected soil strata can also significantly magnify the effects of differential volume change in expansive soils. Cut areas are particularly critical for subsurface water.
Treatments for Subsurface Water
When saturated soils or subsurface water are encountered, consideration should be given to the following alternatives for improving the foundation or supporting subgrade:
7.5.6 Frost-Susceptible Soils
Effect of Frost Action on Pavement Performance
Frost action can cause differential heaving, surface roughness and cracking, blocked drainage, and a reduction in bearing capacity during thaw periods. These effects range from slight to severe, depending on types and uniformity of subsoil, regional climatic conditions (i.e., depth of freeze), and the availability of water.
One effect of frost action on pavements is frost heaving caused by crystallization of ice lenses in voids of soils containing fine particles. As shown in Figure 7-19, three conditions must be present to cause frost heaving and associated frost action problems:
If these conditions occur uniformly, heaving will be uniform; otherwise, differential heaving will occur, causing surface irregularities, roughness, and ultimately cracking of the pavement surface.
Figure 7-19. Elements of frost heave.
A second effect of frost action is thaw weakening. The bearing capacity may be reduced substantially during mid-winter thawing periods, and subsequent frost heaving is usually more severe because water is more readily available to the freezing zone. In more-southerly areas of the frost zone, several cycles of freeze and thaw may occur during a winter season and cause more damage than one longer period of freezing in more-northerly areas. Spring thaws normally produce a loss of bearing capacity to well below summer and fall values, followed by a gradual recovery over a period of weeks or months. Water is also often trapped above frozen soil during the thaw, which occurs from the top down, creating the potential for long-term saturated conditions in pavement layers.
Identification of Frost-Susceptible Soils
Frost-susceptible soils have been classified into four general groups. Table 7-12 provides a summary of the typical soils in each of these four groups based on the amount of fines (material passing the 0.075 mm (No. 200) sieve. Figure 7-20 graphically displays the expected average rate of frost heave for the different soil groups based on portion of soil finer than 0.02 mm (0.8 mils).
Little to no frost action occurs in clean, free draining sands, gravels, crushed rock, and similar granular materials, under normal freezing conditions. The large void space permits water to freeze in-place without segregation into ice lenses. Conversely, silts are highly frost-susceptible. The condition of relatively small voids, high capillary potential/action, and relatively good permeability of these soils accounts for this characteristic.
Figure 7-20. Average rate of heave versus % fines for natural soil gradations (Kaplar, 1974).
Clays are cohesive and, although their potential capillary action is high, their capillary rate is low. Although frost heaving can occur in clay soils, it is not as severe as for silts, since the impervious nature of the clays makes passage of water slow. The supporting capacity of clays must be reduced greatly during thaws, even in the absence of significant heave. Thawing usually takes place from the top downward, leading to very high moisture contents in the upper strata.
A groundwater level within 1.5 m (5 ft) of the proposed subgrade elevation is an indication that sufficient water will exist for ice formation. Homogeneous clay subgrade soils also contain sufficient moisture for ice formation, even with depth to groundwater in excess of 3 m (10 ft). However, the magnitude of influence will be highly dependent on the depth of the freezing front (i.e., frost depth penetration). For deep frost penetration, groundwater at even a greater depth could have an influence on heave.
Identification of Frost-Susceptible Conditions
The most distinguishing factor for identifying a pavement frost hazard condition is water supply. For frost susceptible soils within the frost zone, the frost hazard may be rated as high or low, according to the following conditions. An unknown rating may be appropriate when conditions for both high and low ratings occur and cannot be resolved, or when little or no information is available. The inclusion of a frost hazard rating in the site evaluation documentation verifies that an evaluation of frost action has been attempted and has not been overlooked. When the rating is unknown, a decision to include frost action mitigation measures in a design will be based more upon the unacceptable nature of frost damage than the probability of occurrence.
The conditions associated with a high frost hazard potential include:
The conditions associated with a low frost hazard potential include:
Treatment for Frost Action
When frost-susceptible soils are encountered, consideration should be given to the following alternatives for improving the foundation or supporting subgrade:
Pavement design for frost action often determines the required overall thickness of flexible pavements and the need for additional select material beneath both rigid and flexible pavements. Three design approaches have been used for pavement in seasonal frost areas:
AASHTO 1993 (Appendix C) provides procedures and graphs to predict the direct effect of frost heave on serviceability loss and is treated with respect to the differential effects on the longitudinal profile of the road surface. If the frost is anticipated to be relatively uniform, then the procedures do not apply.
For the most part, local frost-resistant design approaches have been developed from experience, rather than by application of some rigorous theoretical computational method. A more rigorous method is available in the NCHRP 1-37A design procedure to reduce the effects of seasonal freezing and thawing to acceptable limits, as discussed in Chapter 6. The Enhanced Integrated Climatic Model is used to determine the maximum frost depth for the pavement system at a particular location. Various combinations of layer thicknesses and material types can be evaluated in terms of their impact on the maximum frost depth and total amount of base and select materials necessary to protect the frost susceptible soils from freezing.
Problematic soils can be treated using a variety of methods or a combination thereof. Improvement techniques that can be used to improve the strength and reduce the climatic variation of the foundation on pavement performance include:
Details for most of these stabilization methods will be reviewed in the next section.