Technical Note
Engineering Soils Maps
PAUL M. SANTI
Department of Geology and Geological Engineering,
Colorado School of Mines, Golden, CO 80401
JAMIE L. MARTENS
URS Corporation, 10975 El Monte, Suite 100, Overland Park, KS 66211
Key Terms: Engineering Properties, Soils, Hazards,
Mapping
INTRODUCTION
For many applications, ‘engineering soils maps’ may
be preferable to comprehensive engineering geology
maps. Engineering soils maps, which focus on soil
thickness, soil type, and soil engineering properties, may
be more expedient to develop than maps that include
details on rock properties, geomorphology, and geologic
hazards. These details are a critical component of the
input offered by engineering geologists, but for many
smaller sites, they may be summarized as caveats in the
text while the map concentrates on issues of more immediate application to engineering design.
A technique to prepare engineering soils maps has
been developed that is visually simple and uncluttered
and that uses an extensive, standardized database. In this
technique, rose-pie charts provide an overall assessment
of soils hazards and capabilities. Data and base maps
come directly from National Resources Conservation
Service (NRCS, formerly U.S. Department of Agriculture
[USDA]) soil surveys. As many as three levels of six
different hazards or engineering characteristics may be
shown simultaneously, in addition to shorthand notation
that indicates soil thickness and land capability class. The
suggested map scales may be appropriate for developments; linear structures such as pipelines, power lines,
and roads; and regional evaluations of sources of aggregate, clay, or sand. The authors have successfully used
this technique for several years as a teaching tool for
undergraduates (Santi and Gregg, 1999). It should be
recognized that the NRCS maps were developed as a tool
for agronomists, rather than geotechnical engineers or
engineering geologists. Their use is therefore most
beneficial as a tool to plan geotechnical investigations,
not to replace them. Engineering geologists should keep
in mind several other precautions when translating NRCS
soil survey data into geotechnical data:
The maps were compiled on the basis of air photo
interpretation, followed by field checking. As a result,
individual map units may have a percentage of
their area consisting of soils other than the named
deposit.
Units are identified on the basis of composition rather
than genesis. As a result, an individual soil unit may
consist of a durable, stable parent formation as well as
its eroded and re-deposited residuum, which can have
vastly different engineering properties.
Many of the enumerated capability limitations and
hazards can be mitigated by appropriate geotechnical
designs.
EXAMPLES OF PREVIOUS WORK
Engineering soils maps in various configurations are
not uncommon, although they are not usually referred to
by that name. They are often developed by subdividing
and adding detail to the typical ‘Quaternary Alluvium’
(Qal) category from geologic maps. Some methods provide a substantial level of useful engineering information
and are quite successful at delineating various alluvial,
colluvial, residual, and other soil deposits. A few of these
include Genesis-Lithology-Qualifier mapping (Keaton,
1984), a recent system developed by Moore and others
(1999) to delineate and provide thorough descriptions of
various surficial units, and two Canadian systems: the
British Columbia Environmental Land Use Committee
(ELUC) Terrain Classification System and the Northern
Ontario Engineering Geology Terrain Study (Gartner,
1984).
The U.S. Geological Survey has produced numerous
engineering geologic maps that sub-divide soil units and
discuss the expected engineering behavior of each unit
(examples of such maps are Miller and Bryant, 1976;
Lemke and Maughan, 1977; Berg et al., 2000; and
SCAMP, 2000). In addition to sub-dividing soils
deposits, some maps also display engineering properties
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Santi and Martens
Table 1. Summary and method of indicating engineering information available in NRCS (USDA) soil surverys.
Hazard or
Engineering
Property
Slope
Flooding
Low strength
Shrink-swell
Stony soil
Depth to rock
Permeability
Roadfill quality
Topsoil quality
Construction sand
quality
Construction gravel
quality
Soil thickness
Land capability
class
Liquid limit,
plasticity index
Method of Data Summary in Engineering Soils Maps
Location of information
in Soil Survey
Table 11: Building Site
Development
Table 11: Building Site
Development
Table 17: Soil Water Features
Table 11: Building Site
Development
Table 11: Building Site
Development
Table 16: Physical and Chemical
Properties of the Soils
Table 11: Building Site
Development
Table 11: Building Site
Development
Table 16: Physical and Chemical
Properties of the Soils
Table 13: Construction Materials
(see soil survey text for
specific properties)
Table 13: Construction Materials
(see soil survey text for
specific properties)
Table 13: Construction Materials
Open Pie Slice
Entirely Filled Slice
Low slope
Moderate
Severe
Hazard is absent
Moderate
Severe
Hazard is absent
—
Severe
Hazard is absent
Moderate
Severe
Not stony
Moderate
Severe
Hazard is absent
—
Severe (,60 in. of soil)
,0.06 in./h
Good
0.06–0.2 in./h
or 0.2–0.6 in./h
Fair
0.6–2.0 in./h
or 2.0–6.0 in./h
Poor
Good
Fair
Poor
Acceptable
—
Excess fines
Table 13: Construction Materials
Acceptable
—
Excess fines
Table 15: Engineering Index
Properties
Table 17: Soil and Water
Features
Table 6: Categories are
summarized in soil survey
text section entitled ‘‘Use and
Management of the Soils’’
Table 15: Engineering Index
Properties
When ‘‘depth to unweathered bedrock’’ is noted, use this value as a minimum
value of soil thickness (e.g., 35 in.).
If not given, express soil thickness as greater than the deepest unit described
(e.g., .65 in.).
e ¼ erosion; w ¼ water; s ¼ shallow, droughty, or stony soil; I ¼ few limitations;
II ¼ moderate limitations; III–VIII ¼ severe limitations (e.g., IIe or VIw)
Give ranges for either: thickest layer or layer with worst properties.
on the map (Mulder and Hillen, 1990; Dearman, 1991).
Rockaway and Lutzen (1970) and Lutzen and Rockaway
(1971) use three-dimensional block diagrams, similar to
those included in soil surveys, to show the geomorphic
relationships between the engineering geologic units
delineated on their maps.
Thomas (1991) uses pie-chart graphs to illustrate
relative gravel, sand, and silt/clay content across a site,
similar to the use of pie charts in this study. He suggests
that other engineering parameters could be similarly
graphed (and displayed in cross-section format), including strength, plasticity, permeability, or Standard Penetration Test (SPT) friction ratio.
None of these methods makes extensive use of USDA
Soil Surveys, which contain a wealth of engineering data,
especially for the upper few feet of soil and sediment
units. Soil surveys also contain 1:24,000 air photo
overlay maps on which various soil types are delineated.
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Half-Filled Slice
An overview of the type of engineering information
available in soil surveys and potential applications to
geotechnical studies is provided in Hasan (1994).
Prokopovich (1984) illustrates the grouping of soil survey
units to delineate geologic and geomorphic units such as
‘‘Holocene Deltaic Alluvium,’’ ‘‘Intermediate Flood-Plain
Terrace,’’ and ‘‘Recent or Historic Lake Beds.’’ Bergstrom
and others (1976) include summary tables of soil survey
information and use this information to develop geology
maps for construction and waste disposal.
ENGINEERING SOILS MAPS
The purpose of an engineering soils map is to extract
generalized engineering data from an NRCS soil survey
and display it such that a large area can be judged for hazards or engineering properties. Such maps can be used to:
Environmental & Engineering Geoscience, Vol. IX, No. 2, May 2003, pp. 179–183
Engineering Soils Maps
Plan a geotechnical exploration program for a proposed
project
Select zones within a larger area that minimize one or
several hazards
Identify zones with particular desirable characteristics,
such as thick soils or low-permeability soils
Group materials by land capability class or hazard
type.
Many natural hazards, such as sinkholes, faults, or
landslides, are not shown on soil survey maps and
would not be adequately recognized in an engineering
soils map.
Table 1 summarizes the various sources of engineering
information in typical soil surveys, as well as how this
information may be consolidated into simple ratings.
Recent soil surveys share a common list of tables and
common text layout, so the location of the information
noted in Table 1 may be consistently applied. For example, information on slope steepness is expressed in
several tables within the soil survey, namely tables 9, 11,
12, and 14. In the interest of consistency and referring to
the fewest number of tables possible, the authors suggest
obtaining slope information exclusively from soil survey
table 11. The soil survey suggests that slope hazards are
either ‘moderate’ or ‘severe.’ A third category, that of
‘absent,’ is implicit when slope hazards are not noted in
the soil survey.
Figure 1. Proposed method of displaying engineering soils information
from NRCS soil surveys. Two or three hazard levels may be
simultaneously displayed for up to six hazards. In each case, the
hazard or engineering property is displayed so that higher hazard levels
correspond to filled pie-chart slices. The example chart describes a 35in.-thick soil with severe erosion limitations, severe depth to rock
(shallow rock) and low strength conditions, and moderate slope and
shrink-swell conditions.
Figure 2. Example engineering soils map of a portion of Iron County, Missouri. Base map is from USDA (1991).
Environmental & Engineering Geoscience, Vol. IX, No. 2, May 2003, pp. 179–183
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Santi and Martens
Figure 3. Example use of a block diagram from a soil survey (USDA, 1991) to show both geomorphic relationships and engineering characteristics of
soil units.
Figure 1 details proposed methods of displaying
engineering soils information, using rose-pie charts.
These charts display up to six simultaneous hazards or
engineering properties at two (present/not present) or
three (high/moderate/low or absent) levels. For example,
the slope hazard may be shown as absent (pie slice is
open), moderate (slice is half filled) or severe (slice is
entirely filled). Each hazard or engineering property is
displayed such that higher hazard levels correspond to
filled pie-chart slices.
Any combination of categories may be displayed
within the charts, depending on the intended use of the
map, but all are developed from the same rose-pie format,
so only an abbreviated key is necessary to interpret the
maps.
Because engineering soils maps created directly on the
1:24,000-scale orthophoto maps included in soil surveys
may be cluttered, the authors recommend mapping at
1:12,000 scale or larger. The inherent inaccuracy of these
maps is exaggerated when expanding the base map, so
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contacts between units should be considered approximate, at best. Alternatively, the 1:24,000-scale base map
could be retained, and similar units could be combined or
inset maps used to reduce clutter.
Figure 2 is an example engineering soils map using
data exclusively from a published soil survey (USDA,
1991), representing a site in eastern Missouri. Using
Figure 2, one may quickly identify areas susceptible to
flooding, areas with particularly thin or thick soils, or
areas underlain by expansive soils. At the scale shown
(originally created at 1:12,000), the area covered is
roughly 3 km by 1.5 km, which may be appropriate for
planning investigations for housing developments; linear
structures such as pipelines, power lines, and roads; and
regional evaluations of sources of aggregate, clay, or
sand.
Figure 3 is an example use of a block diagram
contained in a soil survey (USDA, 1991) to show both
geomorphic relationships and engineering characteristics
of soil units. In this example, the same rose-pie charts
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Engineering Soils Maps
used in Figures 1 and 2 are overlain on the block
diagram.
CONCLUSIONS
The purpose of an engineering soils map is to extract
preliminary engineering data from an NRCS (USDA) soil
survey and display it such that a large area can be judged
for hazards or engineering properties. The NRCS soil
surveys comprise an extensive, standardized database and
include 1:24,000 air photo overlay maps on which
various soil types are delineated. Rose-pie charts may be
developed from this database to display up to three levels
of six hazards simultaneously. With some loss of
accuracy, the map scale may be doubled to 1:12,000, to
cover an area that may be appropriate for developments;
linear structures such as pipelines, power lines, and roads;
and regional evaluations of sources of aggregate, clay, or
sand.
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