Chapter 13
Soil and Its Uses
Soil is a medium in which plants are grown for food and fiber—the principal use of soil. It gives mechanical support for plants roots—a necessary function. Soil acts as a decomposer for vegetative and animal remains—a necessity to keep organic materials from continuing to accumulate on the surface. Soil is involved in water and air movement—vital to plant growth. In addition, soil is beautiful. It is an aesthetic resource which is pleasing to many people.
In addition to these general functions of soil, there are many more specific uses of soil other than for producing food and fiber. They range from foundations for structures (roads and buildings) to use in water treatment and waste disposal facilities by municipalities to its use in urban landscapes for lawns, flowers, and vegetable gardens to dams, levees and ponds.
The purpose of this chapter is to discuss the various uses of soils when used for purposes other than for agricultural production. Chapters 3 through 7 provide the properties and characteristics of soil that can be used in its management whether in crop production, in urban landscapes, or for foundations for structures or its many other uses.
Principles of soil management in Chapter 9 are mainly for agricultural production, but they can also be applied to many other uses of soil.
Urban Soils
As population increases there are not only many more people to feed but also many more people to house. As a result, land once in food production is being taken by expanding cities and suburban development. As urbanites populate the communities that include their homes, work places, industry, and infrastructure, the natural soil body will begin to undergo changes to suit each owners needs with only some regard to the consequences of such changes to the soil or the broader environment.
Soils are soils: they are a natural body of matter that once were left only to the forces of nature to evolve into what they are today. An urban soil is one that has been disturbed, modified, or contaminated through human activity and urban/suburban development. They no longer have characteristics like the natural landscape surrounding the urban area. The modifications to the urban soil have brought about significant variability in soil developmental characteristics due to changes in virtually all of the soil-forming factors compared to natural soils. The hydrology of an urban area also changes, resulting in an effect on plant growth. The intensity of these changes will depend on past land use and or site-specific human activity.
Variability and Hydrology
The one common theme of urban soils is the degree of lateral and vertical variability over short distances. While lateral and vertical variability is common in natural soil profiles, as the land begins to become more urbanized there is a need to excavate soil to install underground utilities, foundations, basements, roads, or change elevations to accommodate design features of various structures. As a result soils that are excavated may have material removed and placed at a different location on-site, hauled off site, eroded, backfilled, or hauled in from another location. In addition, human artifacts used in construction are commonly found buried in urban soils and can be a problem when planting shrubs, trees, and so on.
In some cases the natural soil is buried under on-site or off-site materials, while in others it is mixed with soil from a different location or depth. Urban soils are disturbed soils, made up of a mixture of soil materials obtained from one or more locations and placed in layers of varying thicknesses making it difficult to predict their horizon characteristics. The extent of disturbance will depend on the existing soil properties, the intended use and reason for disturbance, and the properties of external soil materials hauled in.
There are many areas of urban land that remain relatively undisturbed and retain their natural soil characteristics. These undisturbed areas are typically represented by urban forests, or areas beyond the influence of the construction zone. These various effects create a "mosaic" of soil conditions, ranging from natural to highly disturbed "anthropogenic" soil profiles.
The hydrology of an area can change considerably under the impact of urban development. This in turn impacts plant growth. Two hydrological aspects that are important are infiltration and overland flow.
Urban areas are characterized by impervious surfaces (roofs, driveways, parking lots), layered subsoils, modified slopes, and land surfaces with or without vegetative cover. Rainwater falling on impervious surfaces may or may not be distributed evenly on the adjacent land surface; instead it may be collected from roof tops and sump pumps and discharged in localized areas on the land surface or may be directly discharged to a storm/sanitary sewer. When the water collected from roof tops and other impervious areas is allowed to run freely on to the adjacent land, water will soak into the soil in an uneven manner (altering the subsurface water movement). Excess water that does not infiltrate the soil is distributed farther down slope from the point of discharge. Water available for infiltration immediately adjacent to the discharge points of the paved areas depends on the slope characteristics and the clay content of the area. In soils where infiltration is rapid, the amount of water entering a soil profile is higher than from a natural rain event. Compared to rural soils, localized urban soils are exposed to more infiltration water than that which would have resulted from natural precipitation in the area.
When the additional water percolates through the soil, it not only impacts water availability to urban vegetation but also impacts future horizon development. Additionally, the excess water can help move soluble chemicals farther down the soil profile and eventually into the groundwater. During short rain events, rainwater usually soaks into the soil plus any overland flow that comes from paved surfaces. During intense storms, the infiltration rate of the soil may not be rapid enough to absorb all the rain; therefore, more overland flow is created from unpaved surfaces.
Restoring soil structure by the addition of organic amendments such as compost and mulches are known to increase infiltration rates by a factor of 6-10 times than that of a similar surface soil without the amendments.
The hydrology of an area can also have an impact on flooding. Modifications can be made to lessen the impact of excess water. In most urban soils, the surface slopes (aspect and steepness) are modified to efficiently remove excess water along the land surface. Slope modifications can change the quantity and the direction of water flow. Often drainage ditches, swales, roof drains, curb, and gutters are designed to carry water rapidly away from areas of human activity immediately after a rain event. Additionally, the rapid removal of water from locations in the upper elevations of a watershed would mean potentially excess water in lower elevations. Adequate precautions must be taken to avoid flooding of low-lying areas.
Many urban planners will discourage property development in urban flood zones because of a periodic serious risk of being flooded. Land in flood zones is normally near water, often has flat slopes and therefore preferred for construction of homes and offices. Many urbanites have chosen to build on such sites and installed expensive flood control measures. With the hydrology of the higher elevations of the watershed constantly changing due to development, the flood control structures often turn out to be ineffective subjecting the property owner to periodic flooding and loss of life and property. The proximity to surface water on flood plain developments also has a negative impact on surface water quality.
Soil Properties
In addition to the lateral and vertical variation in materials that remain in place in an urban soil environment, the physical, chemical, and biological properties of the soil are significantly altered when soils are disturbed. All these have an impact on urban soil quality and the environment.
Physical Properties
The physical properties of soil that are altered include texture, compaction, temperature, water storage and movement, air, structure and porosity, and slope. The first three listed are discussed in greater detail. Erosion, a function of many of the physical characteristics of soil, is also discussed.
Texture in urban soils will vary significantly in disturbed conditions. The texture of the material placed in the ground will depend on whether it is of natural soil origin or man made (such as ashes, demolition waste or dredged material from a pond or wetland). Sometimes the choice of these materials is based on cost and a specific soil property intended to be altered (drainage, elevation changes, plant growth, etc.). In specific locations where the local soil does not meet a specific engineering need such as physical support (road bed, driveway), water movement or retention may be desired (gravel, sand, or clay may be hauled in). For intensively landscaped areas, soil and amendments rich in organic matter may be hauled in thus modifying soil texture in the surface or subsurface layers.
Compaction in urban soils varies significantly and depends, for example, on the use of heavy equipment and intensive pedestrian or vehicular traffic during and after construction. Compaction is intensified when these activities are carried out on wet soils. Compacted soils will affect root penetration, air movement, water movement, and the amount of stored water available for plant or microbial growth. Such impacts are obvious in areas covered by lawns that are consistently subject to human foot traffic or vehicular traffic. Soil paths get so compacted that the roots have difficulty growing in the soil, and water has difficulty infiltrating the soil causing puddles after a rain event.
Compaction can occur in excavated or unexcavated soils from the weight of construction equipment. Often the soils may be compacted deeper into the profile affecting the structure, porosity and density of the soil. While surface compaction can be undone by tilling to a certain depth, it is more difficult to undo the compaction at deeper soil depths. Such deep compaction can affect urban tree growth, often stunting growth (compared to their counterparts growing in undisturbed forested conditions). One of the causes for stunted growth could be a lack of oxygen or physical restrictions (smaller volume of pores) for root growth.
When soils are compacted deeper in the profile, water penetration can be affected resulting in a build up of a perched water table affecting tree root penetration and oxygen needs of tree roots. Such a soil situation could result in a shallow root system and potential for tipping over under strong wind conditions. In other urban situations, the land adjacent to an existing tree trunk may be covered with additional soil burying the feeder roots deeper and cutting off oxygen supply to the roots.
It may take several years before the subsoil in an urban area returns to its natural density. The uncompaction process will depend on the texture of the compacted soil material, the rooting habits of the vegetation, the frequency of wetting/drying or freeze/thaw cycles, and the extent of additional compacting activities at the surface. Compaction, however, is not of concern in well-managed flower beds and turf grass areas where there is little human traffic after the ornamental plants have been in place. These flower beds may also periodically receive organic matter additions through mulch or compost.
Soil temperature will be impacted by urban development. Concrete surfaces will absorb solar radiation and retain heat that may be later released to the surrounding soils. In addition heat released from human activities tends to make urban environment and urban soils warmer than soils in nonurban areas, thereby, extending the growth cycle. Urban area soils may see fewer frost-free days extending the growing cycle and increasing the decomposition rate of organic matter thereby affecting nutrient flux. Tall structures may have a shading effect on one or more sides of the structure making the soil cooler in areas protected from the sun. Urban effects of shading from buildings is likely to modify moisture and temperature regimes in isolated situations. Application of mulches in landscaping will also have a similar cooling effect impacting organic matter decomposition and nutrient release in the shaded areas.
Erosion in urban areas can be a problem. There is a significant potential for erosion when a bare soil is exposed to wind and rain, especially if the subsoil (with little or no organic matter as binding agents) is exposed to the elements. Erosion at constructions sites could be a serious problem amounting to 100 times more erosion than when the bare A horizon of the same soil is exposed to the elements. Erosion in urban areas is further accelerated due to alteration of slopes when the soil is excavated and placed in a pile with steep side slopes. Precautions should be taken to prevent the soil being washed away and deposited on neighboring property. Often silt fences are placed at construction sites to trap the soil yet allow the water to pass through the fence and prevent any deposits on adjacent land. Excavated bare soil exposed to prolonged dry periods can result in serious dust issues on windy days. It is often recommended that earth-moving activities be coordinated with appropriate weather conditions. If bare soil must be left exposed for prolonged periods of time, all precautions must be taken to somehow bind the soil by hydromulching, spraying water on dry surfaces, or establish a vegetative layer at the surface of the disturbed soil.
Once an urban soil is landscaped, the slopes meticulously modified, and the land surface is covered with vegetation during most times of the year, these soils tend to be less erosive than their natural counterparts under agricultural land use.
Chemical Properties
There is a higher degree of chemical variability in urban soils as compared to rural soils. Two major reasons for such variability are the disturbed/mixed nature of urban soils and the extent of chemical addition from external sources.
Among the chemical properties altered are pH; ionic concentrations of the soil solution; nutrient balance and fertilizer additions; organic matter content; addition of air pollutants from cars, industries and the burning of fossil fuels; and possibly contaminants released to the soil from human activity and manufacturing industries. Adding to the alteration of chemical properties is foreign matter such as concrete or construction materials, ashes, pesticides, soil amendments, and the disposal of human wastes via septic systems.
Urban environments receive higher concentration of atmospheric chemicals in the form of dry or wet atmospheric deposition. The sources of these chemicals are burning of fossil fuels, emissions from industry, and exhaust from automobiles. Atmospheric temperature inversions in urban environments often keep these chemicals suspended over the urban areas. Heavy metals such as lead, copper, nickel, and air pollutants such as sulfur and nitrogen compounds are often present in these deposits. These chemicals accumulate in the surface horizons of urban soils.
The pH of urban soils is subject to many factors. Many of the air pollutants may lower the soil pH in soils that have a low buffering capacity. The acidic conditions promote leaching of bases in urban soils. Heavy metals present in urban soils could be released at lower pH and absorbed by the plant. In extreme situations some of the heavy metals can reach toxic concentrations for plants and animals. Soil pH can vary between localized areas managed for turf grass such as transportation and commercial corridors and those around residences. Soils in contact with or close proximity of building and paving materials rich in calcium tend to exhibit a higher pH. Often lime additions are practiced in humid regions to maintain an adequate soil pH. The higher pH limits heavy metals from coming into solution; therefore, they remain locked into the soil matrix and reduce the risk to plants and animals.
The levels of plant nutrients in urban surface soils are generally found to be adequate because in most instances a certain amount of topsoil is placed on disturbed land. In intensively managed landscapes where fertilizer, compost, and organic mulches are added on a regular basis, nutrient levels may be at higher than normal levels compared to rural soils. In some urban soils repeated fertilizer additions containing a disproportionate amount of one nutrient can often create an imbalance in nutrient availability to plants. The overapplication of even a balanced nutrient source, while helping vegetation to grow faster, may have a negative impact on surface and ground water. Excess phosphorus in urban lakes and streams has been traced to excessive fertilization of lawns with phosphorus. This impacts biodiversity in the receiving water. Evidence shows that overfertilization with nitrogen combined with irrigation (practiced often in urban environments) may lead to flushing of nitrogen into the groundwater.
Addition of soluble salts is also a factor in urban soils. Irrigation water, excessive use of inorganic fertilizers, salts used for deicing paved surfaces (streets, driveways, and sidewalks) in cold climates, water softeners, and so on, have an impact on the soluble salt content of the soil water and the vegetation growing at that location. In areas where the salt concentration is high, salt tolerant plants gain a competitive edge. Salt concentration will also impact the types of microorganisms that will be active in the soil.
Heavy metals in urban soils come from various sources. Atmospheric additions, original rock (copper, mercury, lead, zinc), and past industrial/mining activities (cadmium, nickel, copper, lead) serve as potential sources of heavy metals in the soil. Lead in particular is common in urban soils as a result of automobile exhaust when lead was part of gasoline, and lead paint used in the exterior and interior of old buildings (prior to 1978). Urban fires and dredged marine sediments also add metal contaminants to local soils. Presence of metals does not preclude plant growth, only in situations where toxic levels accumulate will there be no vegetative growth.
Extraneous chemicals can be a problem at times for urban soils. Modern industrial societies have developed new organic and inorganic compounds for various industrial, agricultural, and manufacturing applications. Some of these chemicals are inert and harmless while many of these can be extremely toxic to humans and other organisms. Some of these chemicals are not biodegradable. Many have accidentally made their way into our environment (soil, water, air, and plant and animal life). The intensity of human activity and use of these chemicals by humans in various applications leads to increased concentrations of such chemicals in urban soil environments than in rural environments.
The soil is often expected to miraculously decompose, alter, treat, detoxify some or all of these chemicals and somehow make the soil environment unharmed and yet have little impact on surface and groundwater. Once these materials enter the soil, however, they affect all forms of organisms in one way or the other and impact the overall ecological cycle in these environments. Each soil organism has a certain threshold concentration below which its activities are minimally affected. Once these limits are exceeded, ecological damage is enhanced.
Biological Properties
The organic matter content of urban soils is highly variable and will depend on several factors including intensity of landscape management; additions of organic amendments; degree of soil disturbance; extent of organic matter contributions from vegetation; and rate of organic matter decomposition. Litter and organic matter decomposition rate is dependent on the soil environmental conditions, microbial and invertebrate population present, and the source/type of organic matter. Typically, organic matter decomposition rates are faster in urban soils than in rural soils. This is attributed to the higher temperatures and adequate amounts of nitrogen contributed from fertilizer additions as well as atmospheric deposition. Other studies have shown that due to the higher ozone concentration in urban air, the vegetative matter exposed to ozone is subject to slower decomposition rates.
Soil organisms in urban soils are impacted by soil conditions. The consequence of changes in soil physical and chemical properties in urban areas means that the habitat for native species may have changed allowing non-native species of plants and animals to become dominant. The fragmented nature of urban landscapes reduces the habitat available for any given animal and plant species allowing native species to be displaced and replaced by nonnative species. Many non-native species are planted in urban areas that may perform well under disturbed soil conditions while the native species may not be able to do as well under the disturbed soil conditions.
Sometimes these plant species are introduced via seeds present in soil or compost material hauled in from external sources. At other times plant species are part of a landscaping plan. Unless the plant can handle the physical and chemical conditions of the soil, it may not thrive. The interaction of soil, plants, insects, and microorganisms among each other can determine the soil nutrient flux under the new environment.
Growing Plants in Urban Landscapes
To obtain optimum growth of plants in the urban landscape, proper management practices are just as important as with the production of food and fiber. The ideal situation is to establish and maintain the correct combination of all soil factors necessary to optimize the growth of plants.
The physical condition of the soil is the starting point. Starting with and keeping the soil in good physical condition means that it will be easy to till and that seed germination will be optimum or that transplants will resume growth quickly. The rate at which water soaks into the soil and the water-holding capacity of the soil will be optimal. Root growth and soil-air interchange will benefit from good physical condition of the soil. Applying and incorporating organic materials is one way to assure good physical condition.
Next the chemical characteristics of the soil need to be in proper balance, which means maintaining soil fertility levels and the correct pH. Start with soil pH—the acidity or alkalinity of the soil. Many plants can achieve optimum growth over a wide range of soil pH. Some plant species, however, prefer an acid soil condition while some will perform best in an alkaline soil condition. Apply limestone to correct the pH of soil if it is too low, or use sulfur for a soil that is too alkaline. Limestone and sulfur needs can be determined by soil tests.
Next assure a proper balance and supply of plant nutrients. If needed, apply fertilizer. It can provide the plant nutrients necessary for good growth provided it includes those needed to balance the nutrient levels in the soil and to provide the nutrients needed by plants. Fertilizer may contain only one nutrient or any combination of the 14 elements required for growth (other than C, H, and O).
Nutrients other than nitrogen normally need to be applied prior to planting or transplanting because they normally move very little in the soil after application. An exception is potassium in sandy soils. High nitrogen-use plants, such as grasses, may need a relatively large amount applied at seeding or sodding while for low nitrogen-use plants, such as flowers and shrubs, nitrogen can be applied minimally at planting and top dressed as the season progresses. Apply plant nutrients according to soil tests.
Soil amendments such as limestone and organic materials should be applied prior to planting. It would be convenient to apply these at the same time that fertilizer is applied. Amendments and fertilizer should be incorporated into the soil before planting or transplanting.
The biological characteristics of the soil are important for good growth. Organic matter and humus need to be maintained at an adequate level so that maximum benefits result. Materials that serve this purpose are peat moss, compost, manure (preferably treated and partially decomposed), or almost any type of plant residue (leaves, grass clippings, etc.).Organic materials need to be incorporated prior to seeding, sodding, or transplanting.
As with any crop in agricultural production, proper selection of a variety or species is desirable, whether it is trees, grass, shrubs, or flowers. Water needs should be considered. For example, in the low-rainfall area of the Western United States, use species of grass that have a low water requirement such as Buffalo grass. In the case of trees, shrubs, and ornamentals, a Xeriscape approach is often desirable.
Keep in mind that the needs of soils used in urban landscapes are the same as those in agricultural production. Chapter 9 on Soil Management gives a much more complete discussion on soil management, most of which applies to turf, trees, ornamentals, vegetable gardens, and shrubs.
There are many aspects of landscape soils and plants and their management that often require professional assistance in soil preparation and in planting. This is particularly true for turf planted as sod and for trees and shrubs. For a long-term use situation, it is often better to ask for professional assistance to assure long-term success.
Predicting soil behavior of an urban soil is difficult because the physical, chemical, and biological changes brought about by one landowner to suit their own needs may not serve the needs of a new landowner. These changes in land use or modification of soil conditions are not always accurately recorded and transmitted to the next landowner. Therefore, it is critical that a prospective landowner should exercise due diligence in evaluating soil conditions that meet the needs of a specific project.
Engineering Uses
Soil is a source of material that has a wide array of uses for engineering purposes. Some examples include fill for dams and levees; foundation material for roads, runways, and buildings; aggregate (sand and gravel) for making concrete; clay for sealing the bottoms and sides of ponds, canals, and solid waste landfills; cover material over tanks, utility lines, tunnels, culverts, and conduits (sewers, drains, pipes for water, oil, and gas); and a porous medium for treating liquid wastes.
Soil has been used to make homes like the adobes of the Southwest and the sod houses of pioneers settling on the American prairies. Soils support enormous loads, both inanimate (roads and buildings) and animate (people, animals, and plants).
To the engineer, soil is any surficial material of the earth that is unconsolidated enough to be dug with a spade. Soil mechanics is the field of engineering devoted to the use of soil as a building material. In the vocabulary of engineers, soil includes both the soil of the soil scientist and any loose substratum that may be present. The concepts of the soil as considered by the scientist and the engineer are merged in this section.
An advantage of using soil for engineering purposes is that there is so much of it, and it may be already on the site, which avoids the expense of hauling in other material. Another advantage is that soil can be so readily shaped into almost any desired form. Depending on how it is manipulated, soil can allow the passage of water through it or it can be made almost impermeable.
There are also some disadvantages for using soil in engineering. Soil is extremely variable, both geographically and over time. Cycles of wetting and drying as well as freezing and thawing change the engineering properties of soil. Unlike known types of steel or wood, soil is not a uniform material for which reliable strengths can be computed. Stable dry loam may be adjacent to unstable wet clay in a lowland. During a rainy period, the dry loam may also become wet and unstable. In winter both soils may freeze and heave in such a way as to crack pavements and basement walls, especially where the moisture content is high. Table 13.1 compares the suitability of three soils for various engineering uses. Even though these soils are usually found next to each other in the landscape, their suitability for different engineering applications varies widely.
Table 13.1 Suitability or limitation rating for soils of the Clarion-Nicollet-Webster association
| Clarion | Nicollet | Webster | |
| USDA classification | Fine-loamy, mixed, mesic Typic Hapludolls |
Fine-loamy, mixed, mesic Aquic Hapludolls |
Fine-loamy, mixed, mesic Typic Haplaquolls |
| Shrink-swell potential Roadfill | Low Good | Moderate Fair (wetness and low strength) |
Moderate Fair (low strength, wetness, and shrink-swell) |
| Embankments, dikes, and levees | Severe (piping) | Moderate (piping) | Severe (wetness) |
| Dwellings with basements | Slight | Moderate (wetness) | Severe (wetness) |
| Septic tank absorption field | Slight | Severe (wetness) | Severe (wetness) |
| Sewage lagoon area | Moderate (slope and seepage) | Severe (wetness) | Severe (wetness) |
| Sanitary landfill area | Slight | Severe (wetness) | Severe (wetness) |
| Daily cover for landfill | Good | Fair (wetness) | Poor (wetness) |
Source: Nelson, G. D. 1990. Soil Survey of Murray County, Minnesota. USDA-Soil Conservation Service, U.S. Government Printing Office, Washington, DC. Reasons for limitations are given in parentheses
Engineering Properties of Soils
Preceding chapters have discussed soil physical properties from the perspective of factors relating to crop production. Soil mechanics also deals with physical properties as they relate to the use of soil as a building material. Some physical properties like particle size distribution and bulk density are important to both soil scientists and engineers. Even so, engineers have developed different soil classification systems specifically for engineering applications (Fig. 13.1).
Figure 13.1 Soil classification systems used by engineers (AASHTO and USC) have different ranges for particle size distributions than the USDA system.
The Unified Soil Classification (USC) System was developed during World War II for the construction of military airfields and has subsequently been modified for use in foundation engineering. The American Association of State Highway and Transportation Officials (AASHTO) System is widely used by state transportation departments and the Federal Highway Administration for the design and construction of transportation lines. Both the USC and AASHTO classification systems include several tests in addition to particle size distribution.
Two standardized tests are often completed to test a soil's suitability as a building material. These tests are called the Atterberg limits, or the liquid and plastic limits. At a high water content, a soil possesses the properties of a liquid. As it dries, it acts more like a plastic, then like a semisolid, and finally like a solid when it is dry. The liquid and plastic limit tests are completed to identify the moisture content at which a soil changes from the consistency of a liquid to a plastic (liquid limit) and from a plastic to a semisolid (plastic limit).
The Atterberg limits help engineers decide whether the soil material under consideration is suitable for their project or whether a different soil is needed. Liquid and plastic limit values are used with particle size information and other tests to classify soils in the USC and AASHTO systems.
Some of the soil characteristics that should be known before decisions about engineering uses are made are given in Table 13.2. Many soil survey reports include tables showing the suitability of the different soils for various engineering purposes. The American Society for Testing and Materials (ASTM) publishes official methods for measuring engineering properties of soils. It is important to understand the soils of an area by studying soil survey maps and reports, talking with soil scientists and engineers, and making the proper measurements.
Table 13.2 Characteristics of soils for engineering purposes
| Kinds of Information about soils | Comments |
| Soil texture | The inorganic (mineral) part of soils is a mixture of sand, silt, clay, and coarse fragments, even including boulders. the USDA, USC, and AASHTO systems are compared in Figure 13.1. |
| Kinds of clay | Clay species vary in degree of shrink-swell potential and other activity. |
| Depth to bedrock | Very shallow soils are usually unsuitable for excavation for basements, ditches along roads, or utility lines. |
| Kinds of surficial bedrock | Bedrock may be very hard (granite) or porous (sandstone, shale). |
| Soil density | Soil horizons range in density from porous to cemented. The denser soils have the higher bearing capacities and rate of transmission of vibrations (sound may travel twice as fast through dense soil as through air). |
| Content of rock fragments | Soil with many rock fragments is usually difficult to excavate and compact uniformly. |
| Erodibility | Many sandy soils are susceptible to wind erosion. Silty soil gully easily. Some clay soils are subject to piping, which is subsurface erosion by spontaneous tunneling. |
| Surface geology | The lay of the land affects land use. Proportions of steep and level land vary as well as soil pattern (linear, circular) |
| Soil pH (reaction) | Degree of acidity or alkalinity influences soil behavior physically, chemically, and biologically. To stabilize soil, engineers sometimes add hydrated lime to it, which raises the pH into the alkaline range. |
| Salinity | High salt content of soil affects its stability and that of vegetative cover. |
| Corrosivity | Soils differ in capacity to corrode buried pipes and tanks. Wet, acid soils are usually very corrosive |
| Depth to seasonal water table | Soils with a seasonally high water table provide inadequate support to roads and structures. Frost action is most severe in wet soils. |
| Plasticity | Clayey soils are commonly quite plastic and, with wetting, become fluid like a thick liquid. The "Plastic index" is the range of percent moisture content in which a soil is plastic. |
| Content of organic matter | To support growth of protective sod, a soil layer containing organic matter is needed. Otherwise, organic soils, paritcularly peats and mucks, are usually removed in engineering projects. |
Roads, Residences, and Structures
Nearly all roads and small buildings are placed on soils, many of which are soft in wet seasons. In some areas subject to seasonal frosts, the soil heaves (lifts) and dirt roads may become impassable in certain seasons. Modern engineers find that naturally well-drained, very sandy, and gravelly soils provide the most trouble-free bases for roads and building foundations. On clay soils, a blanket of sand and gravel is placed over the clay before pavement is laid, and ditches are dug on either side to drain away storm and seepage waters (Fig. 13.2).
Figure 13.2 Use of sand and gravel provides a stable base for a structure on potentially unstable ground.
Water, so essential for plant growth, is often undesirable in soils on which structures are placed. The long life of a road or building foundation depends on maintaining soil conditions that permit it to behave like a compact, well-drained sand or gravel.
Roadside soils also perform special functions unrelated to their engineering uses. Certain organisms in soil can metabolize various components of automobile exhaust. Embankments of soil along major highways absorb much of the sound of traffic, thereby reducing noise pollution in the area.
The high capacity of sand and gravel to support weight arises from the particle-to-particle contact without lubrication of silt and clay between them. Gravel made of strong and stable quartzite is long lasting under the stress of heavy traffic on pavement above the gravel bed. Concrete pavement itself contains as much as 50% gravel and 25% sand.
Dams and Levees
Soil is the most readily available and least expensive material for the building of dams and levees. Numerous reservoirs, lakes, and ponds back up behind dams largely made of soil. Many cities located in floodplains are defended against overwhelming floods by miles (kilometers) of levees built of soil.
Such earthworks need to have two qualities: stability and impermeability. Four kinds of soil materials are needed to achieve these properties in a dam or levee: (1) a clay core and blanket are compacted and kept moist as a seal against leakage of water; (2) a sandy mass is added, surrounding the clay, to drain water away; (3) a layer of stone and rubble (riprap) is piled on the surface exposed to moving water such as waves and river currents; and (4) a good loam is used to cover remaining surfaces of the earthwork to support the growth of protective vegetation (Fig. 13.3).
Figure 13.3 Various kinds of soil materials are used in construction of an earthwork.
Sand is the most easily excavated and transported construction material because it is loose and does not become sticky upon wetting or hard upon drying. Bagged sand is used in emergency enlargement of levees during exceptional floods. Because reservoirs and river floodplains tend to gradually fill with sediment washed in from upstream, this material often must be removed from the reservoir or channel (by dredging) every 20–50 years or so. The dredged sediment can be used to build artificial islands or protected mounds on the floodplains. It can be returned to the farmland but usually at great expense. Reducing erosion is therefore doubly important for both soil conservation and reducing sediment buildup in water bodies.
Ponds and Canals
Bottoms and sides of ponds and canals commonly need to be sealed to prevent leakage. Various kinds of lining materials may be used that are less expensive and less bulky than concrete. If clay is available locally, it may be mixed with bentonite, a special type of swelling clay, to make a tight clay liner. Fabric liners containing plastic or rubber or coated with asphalt may be used in the absence of clay.
Underground Structures and Lines
Earth sheltering of homes and burial of utility lines (Fig. 13.4) and tunnels protect structures and facilities from unfavorable temperatures that take place at or near the surface of the ground. However, some precautions should be followed.
Figure 13.4 Two examples of earth sheltered installations.
Underground installations must be designed to support the great weight of soil cover. The strain on buried structures from the weight of overburden may be heightened in wet seasons by expansion of clays in the soil. Seepage of water into cracks in buried structures may be a recurrent problem. Growth of deposits of calcite or iron oxides in cracks may gradually shatter concrete below ground.
Concrete and metal pipes and tanks are subject to corrosion in some moist soils (Fig. 13.5). Tiny electric circuits may develop spontaneously between the soil and iron pipes in such a manner as to literally bore minute holes in the pipes by dissolving the metal. In regions of permafrost, water mains must be insulated against freezing both from above and below.
Figure 13.5 Cross-section of a buried pipe and the effect of severe corrosion in a wetland position.
Municipal Waste
Waste Treatment
Human activities generate liquid and solid wastes that require treatment and/or containment. Liquid wastes include residential and industrial wastewaters. Solid wastes include household and office wastes that consist of yard waste, paper, plastic, glass, and metal, and industrial wastes that include manufacturing by-products such as sludges from paper mills. Many types of liquid and solid wastes can be disposed of in soil.
Soil is a porous medium with enormous internal surface area populated by microorganisms that are capable of decomposing biodegradable materials. Because of this characteristic with microorganisms, when biodegradable wastes are disposed of in soil, they are broken down and transformed mostly into water, carbon dioxide, and other gases.
Nondecomposable wastes such as rock, metal, glass, plaster, and plastic remain buried or stored in the soil. Care must be taken to make sure that liquid and gaseous contaminants derived from wastes decomposing in soils do not contaminate groundwater, lakes, and streams or come to the surface in unacceptable amounts.
Wastewaters and Biosolids
In urban areas, residential and industrial wastewaters are discharged into sewers and are treated at a wastewater treatment plant. The treated wastewater or effluent is typically discharged to a river or lake (or the effluent can be applied to farmland). One of the by-products generated at wastewater treatment plants is sewage sludge, now called biosolids. Biosolids can be dewatered and handled as a solid (10% or more solids) or as a liquid typically over 90% water. Biosolids contain plant nutrients and the organic remains of treated wastewater. Biosolids are typically spread on agricultural land as a source of plant nutrients and carbon for soil organic matter. Biosolids application is not without environmental or health risk; weed seeds, human pathogens, odors, and industrial contaminants may be present in improperly treated biosolids.
Some biosolids from industrial wastewaters contain trace amounts of metals such as cadmium, chromium, lead, and zinc that may contaminate soil and limit the use of biosolids on agricultural land. Dried or composted biosolids (as low as 20% water content) are sold as a fertilizer. It can be a good soil amendment in the same manner as animal manure.
In rural areas, septic tanks and soil absorption systems or drainfields are used to treat residential wastewaters (Fig. 13.6). Wastewater is discharged from a house into a septic tank buried beneath the soil surface, where solids are degraded and the wastewater undergoes primary treatment. The wastewater is then discharged to a gravel bed and the soil beneath the bed is used for final treatment and removal of contaminants. The texture and structure of soil beneath the drainfield and the rate of water discharged determine the degree of treatment. While most pathogens in the wastewater are removed, there is a risk of nitrogen leaching into the groundwater. When soils contain too much clay or gravel or the depth to bedrock or groundwater is too shallow, an artificial soil mound of sandy or local soil material is constructed above the original soil surface. These types of systems remove some pathogens and nitrogen compounds from the wastewater.
Figure 13.6 Residential wastewaters generated in rural homes are recycled by soil absorption of septic tank effluent.
Industrial wastewaters vary in composition depending on the process. Some waste waters, such as those generated at canneries, are often spray-irrigated on agricultural land after primary treatment (Fig. 13.7). Many other industrial wastewaters, if not discharged to wastewater treatment plants, are disposed of in stabilization ponds or absorption ponds. The by-products of industrial wastewater treatment such as biosolids or pond bottom sludge may be land-applied if they contain adequate plant nutrients and a minimal amount of industrial contaminants. Stabilization ponds contain impermeable liners and rely on treatment within the pond and evaporation. Wastewater in absorption ponds is treated as it infiltrates through the soil beneath the ponds. Some industries generate hazardous wastes that must be treated and disposed of at special handling facilities. Researchers are currently developing alternatives to these methods of wastewater treatment, including discharge to constructed wetlands.
Figure 13.7 Irrigation of farmland is being tried on a limited basis as a means of disposal for wastewater generated by some small industries such as canneries.
Solid Wastes
Solid wastes are typically buried in an engineered landfill that is made up of several modules that are filled in sequence over a period of several years. A compacted clay and/or a synthetic liner at the bottom of each module forms a seal to prevent infiltration of liquids that might contaminate the soil and groundwater beneath the landfill.
Precipitation that comes in contact with the solid waste and the liquid generated by the decomposition of solid waste is called leachate. Leachate is collected in pipes leading to a containment tank and then treated at a wastewater treatment plant. Solids placed in a cell (small area of the landfill) are compacted and covered with soil daily. The soil on the waste serves several purposes: traps odors, keeps paper and plastic from blowing away, traps contaminants in leachate. When the module is full, it is covered with a clay and/or a plastic cap to minimize the infiltration of precipitation. Several feet of rooting soil is placed as part of the final landfill cover to establish vegetation. Vents are installed in the landfill to allow gas to escape from slowly decomposing waste (Fig. 13.8). Landfill gases that typically contain 65% methane and 35% carbon dioxide are either burned on-site or converted to useable electricity.
Figure 13.8 Cross-section of a landfill cell when filled.
Soil and groundwater quality beneath and surrounding a landfill are intensively monitored to prevent environmental contamination. The contaminants of most concern are trace metals, nitrogen compounds, and organic solvents. Due to the large tracts of land needed and the potential for contamination, many municipalities are using alternatives to landfills, such as incineration, composting, and recycling.
Disturbed or Contaminated Lands
Disturbance of land, either by natural processes or human activity, and the contamination of land are often of concern to engineers. Disturbed and contaminated lands result in a situation where soil quality may be impacted, the growth of plants may be severely limited, or plants may not grow at all. Reclamation and remediation of a disturbed area is achieved by restoring it to a productive state. This may include artificial land forming and the application of soil amendments and/or fertilizer to be followed by reestablishment of vegetative cover or enhancement of the soil environment to promote degradation of contaminants.
Naturally Disturbed Land
Soil may be disturbed or made unproductive by natural events including landslides; floods that deposit sediment on lowlands; sand dune and dust invasions; blowdown of trees with consequent exposure of soil; and burial of land under fresh lava flows and volcanic ash falls.
Artificially Disturbed Land
Most artificial disturbance of land is caused by human activities, including (1) mechanical strip mining for coal, oil shale, or metallic ore; (2) hydraulic mining of soil material for gold or phosphorus-bearing minerals used in fertilizer; (3) concentration of liquid and solid wastes (including mine tailings) on limited acreage; (4) contamination of soil areas with oil brine around oil wells and with toxic materials near chemical plants such as smelters and oil refineries; (5) contamination of soil beneath and surrounding leaking underground storage tanks; (6) sterilization with residues of agricultural chemicals in low spots in farmland; (7) quarrying; (8) construction (using cut-and-fill operations) in landscapes for development of residential and commercial buildings as well as roads and other facilities in urban areas; (9) operation of vehicles on fragile soils in deserts and tundra areas; (10) overgrazing of rangelands; and (11) overcultivation of croplands.
Many disturbed lands are left in the form of deep holes, mudholes, drifted sand, or mountains of overburden that are steeply sloping and where runoff is rapid. If this is the case, land forming is the first step in reclamation.
Disturbed soils are not necessarily lost to agriculture. At one site in Wisconsin, a sandy clay loam subsoil was needed by a foundry company for making molds in which to pour and cool molten metal. They first removed the topsoil, and then removed the needed subsoil. The fertile loam topsoil was returned to the site, and it now produces high-quality crops just as it did before the excavation (Fig. 13.9).
Figure 13.9 Cross-section of parts of two fields. To the left of the post, the soil is undisturbed. To the right, the topsoil was removed and saved, and then the desired subsoil was removed. The original topsoil was replaced, thereby permitting crop production.
Reclamation and Remediation Procedures
Confinement of Objectionable Substances
Containment of waste-water and biosolids at wastewater treatment plants is necessary to prevent illegal and ecologically damaging spills from flowing into adjacent waters. Fertile agricultural soil itself becomes a polluting waste material if allowed to wash into streams and lakes. In this sense, the reason for preventing soil erosion is to confine the soil by keeping it in place. Brine at oil wells is now commonly pumped into deep layers in the ground, where it can be confined, instead of being poured on the land.
Agricultural chemicals, especially pesticides, should be confined above-ground, where they may be safely used or disposed, to avoid contamination of the groundwater and the resulting pollution of drinking water.
As with biosolids, fly ash from coal-burning electric power plants, if free of heavy metal contaminants, may be spread on farmland as a fertilizer or soil conditioner. If heavy metal contamination is present, those substances must first be removed or the materials must be disposed of in an approved hazardous waste handling facility or landfill.
Removal of Harmful Substances from Soil
Excess salt in soil can be leached downward if there is an adequate supply of freshwater and proper drainage underground or through conduits and ditches to dispose of the salty effluent. It is important that the soils be sufficiently permeable to allow the movement of water through them.
Where chunks of iron sulfide (pyrite and marcasite minerals) in mine tailings yield acid effluent, removal and proper disposal of the sulfide and the neutralization of acid with lime may be implemented.
When the soil beneath underground storage tanks has been contaminated with petroleum products such as gasoline, the soil surrounding the tank may be remediated by using several techniques. The soil is most often excavated and hauled away from the site and spread on agricultural land or placed in a landfill. Many of the contaminants evaporate or are degraded by soil microorganisms. At some sites, the soil is treated in place by enhancing the soil environment for degradation of contaminants (bioremediation). This technique involves injecting air into the soil, which drives some of the contaminants to the surface where they evaporate, while providing an environment more suitable for certain contaminant-degrading microorganisms. Researchers are currently trying to identify soil microorganisms that effectively degrade contaminants and could be introduced into an area where the soil is contaminated. If the groundwater beneath these sites is contaminated, it must also be remediated.
Land Forming
Shaping the land by construction of grassed diversion terraces and waterways can spread runoff and conduct water safely from sloping farmland. Where runoff is caught in holding ponds, the water can be used for irrigation or can simply be allowed to percolate to the water table. Properly maintained terraces can successfully subdivide long slopes into a sequence of short ones, thereby reducing both runoff and soil erosion.
Strip-mined land may be smoothed to slopes that are no steeper than the original ones. Correct stockpiling of topsoil and subsoil during the initial phase of mining makes it possible to restore the topsoil cover.
Some of the sediment that is dredged each year from streams, canals, and catch basins for the benefit of navigation and aquatic life may be useful on agricultural fields. Careful attention must be given to texture, organic matter content, pH, and other physical and chemical characteristics of the dredged materials (spoils).
Some abandoned quarries are filled with soil in such a way as to make them useful for cropping or other purposes. In landscapes with a fairly high water table, standing water is used as a cover for disturbed land. Roadside excavations may become recreational lakes and ponds or sources of irrigation water.
Interim Protection of the Soil Surface
In disturbed land, the soil surface normally needs protection. At some construction sites where disturbance of soil continues for months or even years, mulches of straw or sheets of special fabrics have been used to cover the bare soil until final cover by buildings, pavements, and lawns has been completed.
Establishment of Vegetative Cover
To reestablish vegetation, it is just as desirable to have a good seedbed prepared at the disturbed soil site as it is in a field to be planted to crops. In many cases, this may be difficult because of the nature of the soil material. In most cases, soil amendments and fertilizers may need to be applied. Disturbed soils may be high in some essential elements, but nitrogen is usually quite low.
Reestablishment of native species of vegetative cover by seeding and irrigation has been successful on land that was strip-mined for coal in the Four Corners area of the southwestern United States. In Ohio and Illinois, agricultural crops are being grown today on some prosperous farms that were inactivated for several years by strip-mining operations that ended with reconstruction of the landscape and its soils.
Researchers are currently trying to develop metal-scavenging plants called hyperaccumulators that remove metals from contaminated soil, especially in mined areas, and store them in their leaves and stems. The leaves and stems are harvested and the metals recycled.
Shelterbelts of trees and shrubs illustrate discontinuous establishment of vegetation for the protection of adjacent cropland from wind erosion. The trees reduce the wind speed, thereby reducing the amount of soil detachment and often creating a microclimate that reduces evaporation and increases yields.
Roadside pits from which construction materials have been removed may be stabilized by cut-and-fill operations and subsequent revegetation with trees and/or herbaceous cover. To avoid the need for reclamation, every effort should be made to avoid contamination of the soil and to maintain it in acceptable form. To do this requires knowledge and dedication by those who use the land.
Economic considerations have sometimes led to misuse of the land; thus, government regulations have been necessary to protect the rights of citizens and to ensure productive and beautiful land for future generations.