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28
<br />SOIL SURVEY
<br />potential of the soil. Soil texture, plasticity and in-place
<br />density, potential frost action, soil wetness, and depth to a
<br />seasonal high water table were also considered. Soil wet-
<br />ness and 'depth to a seasonal high water table indicate
<br />potential difficulty in providing adequate ,drainage for
<br />basements, lawns, and gardens. Depth to bedrock, slope,
<br />and large stones in or on the soil are also important con-
<br />siderations in the choice of sites for these structures and
<br />were considered in determining the ratings. Susceptibility
<br />to flooding is a serious hazard.
<br />Local roads and streets referred to in table 6 have an
<br />all-weather surface that can carry light to medium traffic
<br />all year. They consist of a subgrade of the underlying soil
<br />material; a base of gravel, crushed rock fragments, or soil
<br />material stabilized with lime or cement; and a flexible or
<br />rigid surface, commonly asphalt or concrete. The roads
<br />are graded with soil material at hand, and most cuts and
<br />fills are less than 6 feet deep.
<br />The load supporting capacity and the stability of the
<br />soil as well as the quantity and workability of fill material
<br />available are important in design and construction of
<br />roads and ,streets. The classifications of the soil and the
<br />soil texture, density, shrink-swell potential, and potential
<br />frost action are indicators of the traffic supporting capaci-
<br />ty used in making the ratings. Soil wetness, flooding,
<br />slope, depth to hard rock or very compact layers, and con-
<br />tent of large stones affect stability and ease of excava-
<br />tion.
<br />Sanitary Facilities
<br />Favorable soil properties and site features are needed
<br />for proper functioning of septic tank absorption fields,
<br />sewage lagoons, and sanitary landfills. The nature of the
<br />soil is important in selecting sites for these facilities and
<br />in identifying limiting soil properties and site features to
<br />be considered in design and installation. Also, those soil
<br />properties that affect ease of excavation or installation of
<br />these facilities will be of interest to contractors and local
<br />officials. Table 7 shows the degree and kind of limitations
<br />of each soil for such uses and for use of the soil as daily
<br />cover for landfills. It is important to observe local or-
<br />dinances and regulations.
<br />If the degree of soil limitation is expressed as alight,
<br />soils are generally favorable for the specified use and
<br />limitations ,are minor and easily overcome; if moderate,
<br />soil properties or site features are unfavorable for the
<br />specified use, but limitations can be overcome by special
<br />planning and design; and if severe, soil properties or site
<br />features are so unfavorable or difficult to overcome that
<br />major soil reclamation, special design, or intensive main-
<br />tenance is required.
<br />Septic tank absorption fields are subsurface systems of
<br />file or perforated pipe that distribute effluent from a sep-
<br />tic tank into the natural soil. Only the soil horizons
<br />between depths of 18 and 72 inches are evaluated for this
<br />use. The soil properties and site features considered are
<br />those that affect the absorption of the effluent and those
<br />that affect the construction of the system.
<br />Properties and features that affect absorption of 1
<br />effluent are permeability, depth to seasonal high wa
<br />table, depth to bedrock, and susceptibility to floodi
<br />Stones, boulders, and shallowness to bedrock interf~
<br />with installation. Excessive slope can cause late
<br />seepage and surfacing of the effluent. Also, soil erns:
<br />and soil slippage are hazards if absorption fields are
<br />stalled on sloping soils.
<br />In some soils, loose sand and gravel or fractur
<br />bedrock is less than 4 feet below the file lines. In the
<br />soils the absorption field does not adequately filter the
<br />fluent, and ground water in the area may be c~
<br />taminated.
<br />On many of the soils that have moderate or sevE
<br />limitations for use as septic tank absorption fields,
<br />system to lower the seasonal water table could be
<br />stalled or the size of the absorption field could be
<br />creased so that performance is satisfactory.
<br />Sewage lagoons are shallow ponds constructed to he
<br />sewage while aerobic bacteria decompose the solid a
<br />liquid wastes. Lagoons have a nearly level floor and c
<br />slopes or embankments of compacted sail material. Aer
<br />bic lagoons generally are designed to hold sewage wits
<br />a depth of 2 to b feet. Nearly impervious. soil material f
<br />the lagoon floor and sides is required to minimize seeps,
<br />and contamination of ground water. Soils that are ve
<br />high in organic matter and those that have cobble
<br />stones, or boulders are not suitable. Unless the soil h
<br />very slow permeability, contamination of ground water
<br />a hazard where the seasonal high water table is above t~
<br />level of the lagoon floor. In soils where the water table
<br />seasonally high, seepage of ground water into the lago~
<br />can seriously reduce the lagoon's capacity for liqu
<br />waste. Slope, depth to bedrock, and susceptibility to floo
<br />ing also affect the suitability of sites for sewage lagoon
<br />or the cost of construction. Shear strength and perme
<br />bility of compacted soils affect the .performance of er
<br />bankments.
<br />Sanitary landfill is a method of disposing of sol
<br />waste by placing refuse in successive layers either in e:
<br />cavated trenches or on the surface of the soil. The wasi
<br />is spread, compacted, and covered daily with thin layer
<br />of soil. Landfill areas are subject to heavy vehicular tra
<br />fie. Risk of polluting ground water and trafficability a
<br />feet the suitability of a soil for this use. The best soi
<br />have a loamy or silty texture, have moderate to slo~
<br />permeability, are deep to a seasonal water table, and ar
<br />not subject to flooding. Clayey soils are likely to be stick
<br />and difficult to spread. Sandy or gravelly soils general]
<br />have rapid permeability, which might allow noxioc
<br />liquids to contaminate ground. water. $oil wetness can t
<br />a limitation, because operating heavy equipment on a wE
<br />soil is difficult. Seepage into the refuse increases the ris
<br />of .pollution of ground water.
<br />In the trench type of landfill, ease of excavation al,
<br />affects the suitability of a soil for this purpose, so the sa
<br />must be deep to bedrock and free of large stones an
<br />boulders. Where the seasonal water table is high, Ovate
<br />seeps into trenches and causes problems in filling.
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