1. Soils Investigation Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soils Investigation
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2. Why Test the Soil? Buildings are supported by soil
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Why Test the Soil?
Buildings are supported by soil
Engineering properties of soil are highly variable
Engineers need reliable soil information for structural and site design
Before we can consider designing the building structure or determine the site grading, we need to know the type of soils that are present in the area in which we would like to build. Once we have the information about the type of soil that is present and we know the properties of that particular soil type, we can begin to create a preliminary design.
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3. Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
The knowledge gained from soil investigation allows the engineer to make estimates for:
Bearing Capacity of the soil
Settlement of the Foundation (amount and rate)
Earth Pressure – both lateral and vertical
Drainage characteristics
Wastewater Disposal limitations
Soil testing allows the engineer to determine how the soils will behave as a construction material. Soils classification determines the various physical properties of the soil and provides a correlation to the engineering properties.
The soils underneath a building will influence the type and size of the building foundation. In turn, the foundation may influence the type of building structure that will be erected. The soils conditions can also affect the construction costs.
The bearing capacity is the ability of the soil to carry the load without any failure within the soil. It is also referred to as the stability of the soil. The load-carrying capacity of the soil can vary with the strength of the soils and the method and magnitude in which the loads are applied.
The amount and rate of settlement of the foundation can be estimated for a known soil type, and the building can be designed to accommodate a certain amount of settlement.
Earth pressure refers to the pressure that the earth transmits to the building.
The drainage qualities of the soils can affect the pressure on the foundation with excessive water pressure.
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4. Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
What Is Soil?
Soil is a naturally occurring mixture of minerals, air, water, and organic material. Organic material is made up of dead plant and animal material and tiny living organisms that live in the soil.
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5. Minerals Clay Silt SAND GRAVEL
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Minerals
GRAVEL
SAND
Minerals are produced as the earth’s crust material is weathered and broken down into small pieces. Of most interest when investigating soils are gravel, sand, clay, and silt.
Water and air are present in the voids between the mineral particles.
When investigating soils, we are most interested in the gravel, sand, clay, and silt minerals are produced when. Soil rarely exists as separate components such as sand or gravel.
Soils contain various particle sizes of gravel, sand, silt, and clay, and each contributes to the characteristics of the soil.
Clay
Silt
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6. Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soil Information
Preliminary Information: USDA Web Soil Survey
Soil Testing/Analysis
Previous soils investigations on or near the site
Site inspection and simple soil testing
Soil borings taken at proposed foundation locations
Local Building Department or Other Codes and Regulations
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7. USDA Web Soil Survey http://websoilsurvey.nrcs.usda.gov/
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
USDA Web Soil Survey
A wealth of information is available for approximately 95 percent of the counties in the United States on the Web Soil Survey site.
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8. Soil Boring Log Soils Investigation Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soil Boring Log
Soil testing often includes boring holes into the ground on-site and taking samples of soil. This soil boring log records the location of each sample and gives a description of the soil at each level.
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9. Soil Testing Unified Soils Classification System
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soil Testing
Unified Soils Classification System
(USCS) is a method for identifying and grouping soils
First developed by Casagrande for military construction of airfields
The Unified Soils Classification System is a rapid method for identifying and grouping soils. It was first developed by Casagrande for military construction of airfields.
Many soils can be grouped visually with the USGS. Additional tests for grain size and plasticity are required to accurately classify the soil.
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10. Unified Soil Classification System
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Unified Soil Classification System
Coarse-Grained
Fine-Grained
Highly Organic
Only particles sizes smaller than 3 inches are considered in the USC System.
We will examine soil testing methods that provide information about the size of the soil particles, the percentage of various sizes of soils particles, and the characteristics of the very fine grains within the soil sample. The Unified Soil Classification System is based on the results of a test that determines grain size distribution of the soil sample. Only particles smaller than 3 inches are considered in the USC System. This system is based on the characteristics of the soils that indicate how the soil will behave as a construction material. Many indicators and descriptors for soil exist.
In the USCS, all soils are placed into one of three major categories. They are
 Coarse-grained
Fine-grained
Highly organic
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11. Soil Color Can vary with moisture content
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soil Color
Can vary with moisture content
May indicate the presence of certain chemicals or impurities
Dark brown /black may indicate organic material
Gray, olive green indicates inorganic soils
Red or yellow may indicate iron oxides
Gray-blue or gray-yellow indicates poor drainage
White to pink may indicate silica, calcium carbonate, or aluminum compounds
Color helps in distinguishing between soil types and aids in identifying the particular soil type.
Color may also indicate the presence of certain chemicals or impurities. Color often varies with the soil's moisture content.
Colors in general become darker as the moisture content increases and lighter as the soil dries. Some fine-grained soils with dark, drab shades of brown or gray (including almost black) contain organic colloidal matter. In contrast, clean, bright shades of gray, olive green, brown, red, yellow, and white are associated with inorganic soils. Gray-blue or gray-and-yellow mottled colors frequently result from poor drainage. Red, yellow, and yellowish-brown colors result from the presence of iron oxides. White to pink may indicate considerable silica, calcium carbonate, or aluminum compounds.
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12. Coarse vs. Fine-Grained Soils
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Coarse vs. Fine-Grained Soils
Coarse-Grained Soils described by grain size
Fine-Grained Soils described on the basis of their plasticity
Many soils can be grouped visually with the USGS. Additional tests for grain size and plasticity are required to accurately classify the soil.
The coarse-grained soils consist mostly of soils that have grains that are visible to the naked eye and are described by grain size.
The fine-grained soils consist mostly of particles that are not individually visible to the naked eye and are described on the basis of their plasticity.
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13. Sieve Analysis Gravels range from 3 inches down to the size of peas
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Sieve Analysis
Gravels range from 3 inches down to the size of peas
Soils are usually a mixture of gravel, sand, silt, and clay. Gravels range from about 3 inches down to the size of peas. Sands start just below this size and decrease until the individual grains are just distinguishable by the naked eye. The eye can normally see individual grains about 0.07 millimeter in size, or about the size of the #200 sieve. Silt and clay particles, which are smaller than sands, are indistinguishable as individual particles.
A sieve analysis can be used to separate the particles into different grain sizes. This test consists of passing soil samples through a set of sieves that have openings of varying sizes.
Silt and clay can pass through the #200 sieve
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14. Sieves #4 sieve (¼ in. squares, similar to hardware cloth)
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Sieves
#4 sieve (¼ in. squares, similar to hardware cloth)
In our testing, we will use only the #4 sieve (which has a openings of ¼ inch) and the #40 sieve (which has an opening similar to that of a window screen).
We will perform additional testing to determine the silt and clay content of the fine particles that pass through the #40 screen.
#40 sieve (similar to window screen)
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15. Grain Size (Gradation) Clay
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Baseball sized
Grain Size
(Gradation)
Pea sized
Rock salt sized
Sugar sized
Gravel Sand
Boulder Cobbles Silt &
Coarse Fine Coarse Medium Fine
>12” 12” ” ¾” # # # #200 <#200
Clay
There are many different soil types, such as rock, gravel, sand, clay, and silt. The soils that we encounter are a heterogeneous mixture of these basic types.
The gradation of the soil particles ranges from boulders, which are over 12 inches in diameter, to silt and clay particles that are so small that they cannot be individually distinguished by the naked eye.
Gravels are separated into either coarse gravel or fine gravel with the 3/4-inch sieve as the dividing line.
Sands are divided into coarse, medium, or fine with the #10 and #40 sieves acting as the dividing lines.
[click] Gravels range in size from about 3 inches (baseball sized) to [click] about a quarter of an inch, pea sized (#4 sieve).
Course Sands range from pea sized to [click] rock salt sized (#10 sieve)
Medium sands range from rock salt sized to sugar sized (#40 sieve).
Fine sand grains are smaller than sugar grains.
NOTE: Particles finer than fine sand (#200 sieve) can not be seen by the naked eye at a distance of 8 inches.
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16. Gravel and Sand Graded Soils
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Gravel and Sand
Graded Soils
Coarse-Grained Soils can be defined by how the particle sizes are distributed within the soil sample.
Well-Graded Soils provide a good representation of all particle sizes from the largest to smallest.
Soils are described by their gradation characteristics.
A well-graded soil is defined as having a good representation of all particle sizes from the largest to the smallest.
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17. Gravel and Sand Graded Soils
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Gravel and Sand
Graded Soils
Coarse-Grained Soils can be defined by how the particle sizes are distributed within the soil sample.
Poorly-Graded Soils
Uniformly Graded – Soil particles are nearly the same size.
Gap Graded – Contains both large and small particles, but the graduation continuity is broken by the absence of some particle sizes
Poorly-graded soils are divided into two types.
A uniformly graded soil consists primarily of particles of nearly the same size.
A gap-graded soil contains both large and small particles, but the gradation continuity is broken by the absence of some particle sizes.
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18. Gravel and Sand Clean and Dirty
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Gravel and Sand
Clean and Dirty
Clean soil contains little or no fines (<5%).
Dirty soil contains an appreciable amount of fines (>12%).
Silty
Clayey
A gravel or sand soil type can be classified as “clean” or “dirty,” depending on the percentage of fines within the soil sample.
If a soil is identified as being “clean,” it contains little or no fines (<5%).
If a soil is identified as being “dirty,” it contains an appreciable amount of fines ( >12%).
Silty fines
Clayey
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19. Clay and Silt Plasticity
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Clay and Silt
Plasticity
Fine-grained soils are classified by the plasticity of the soil.
Plasticity refers to the consistency of fine-grained soils as the water content varies.
The plasticity of the soil is based on the effect of varying water content on fine-grained soils.
The plastic limit refers to the lowest moisture content at which soils can be rolled into 1/8 inch diameter threads without breaking.
The liquid limit is the minimum moisture content at which soil will flow when a small shear or cutting force is applied.
These mean little unless they are used as indexed to the significant properties of the soil.
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20. Soils Groups Soil Type Gradation Plasticity Gravel – G Sand – S
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soils Groups
Soil Type
Gradation
Plasticity
Gravel – G
Sand – S
Silt – M
Clay – C
Organic – O
Peat – Pt
Well Graded – W
Poorly Graded – P
High Plasticity – H
Low Plasticity – L
The letter symbols are derived from the soil fractions of gravel, sand, silt, clay, organic, or peat and from the relative gradation of the soil particles and the value of the liquid limit. These letters are then used in combination to form the soils groups.
These letters are used in combination to indicate soil classifications.
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21. USCS Classification for Coarse-Grained Soils
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Sieve Analysis Results
USCS Classification for Coarse-Grained Soils
Coarse-grained soil (granular soil) has more than half of the soil grains visible to the naked eye
If the percentage of GRAVEL and SAND is greater than 50% of sample, then the soil is a course-grained or granular soil.
SAND if more than half of the coarse grains are smaller than #4 sieve
GRAVEL if more than half of the coarse grains are larger than #4 sieve
After you have sieved the soil, you must determine whether the soil is coarse or fine-grained.
A coarse-grained soil, also called a granular soil, has more than half of the soil grains visible to the naked eye. If the percentage of GRAVEL and SAND is greater than 50%, then the soil is a course-grained soil.
If the soil is predominantly coarse-grained, identify the soil sample as being gravel or a sand by estimating whether 50% or more, by weight, of the coarse grains (GRAVEL and SAND) are larger or smaller than the #4 sieve size.
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22. USCS Classification for Fine-Grained Soils
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Sieve Analysis Results
USCS Classification for Fine-Grained Soils
Fine-grained soils (cohesive soils) contain greater than 50% SILT and CLAY particles.
In addition to the sieve analysis, the following tests will be performed to determine the plasticity characteristics of the fine-grained soils (in lieu of LL and PL).
Dry Strength – crushing characteristics
Dilatancy – reaction to shaking
Toughness – consistency near plastic limit
Fine-grained soils, also called cohesive soils, contain greater than 50% SILT and CLAY particles.
The fine-grained soils are classified further into either SILT or CLAY. Next it is determined whether the sample is high or low plasticity by estimating its dry strength (crushing characteristics), dilatancy (reaction to shaking), and toughness (consistency near the plastic limit).
The individual fine grains are not visible to the naked eye.
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23. Soils Test Results for Fine-Grained Soils
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soils Test Results for Fine-Grained Soils
FINE GRAINED SOILS
Dry Crushing Strength
Dilatancy
Toughness
Soils Type
None to slight
Rapid
Low ML
Medium to high
None to very slow CL
Slight to medium
Slow to none
Medium MH
High to very high
None
High CH
Highly Organic Soils
Identifiable by color, odor, spongy feel, and/or fibrous texture
OL, OH,
and Pt
This is a summary of the tests performed on the fine-grained portion of the soil sample. In this chart the L and H that appear after the M (silt) or C (clay) indicate soils of high plasticity (H) or low plasticity (L).
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24. Clay and Silt Plasticity PI = LL - PL
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Clay and Silt
Plasticity
In lieu of dry strength, dilatancy, and toughness, ATTERBERG LIMITS can be used to classify fine- grained soils.
Plastic Limit (PL) – lowest moisture content at which soils can be rolled into 1/8 in. dia. thread without breaking
Liquid Limit (LL) – minimum moisture content at which soil will flow when a small shear or cutting force is applied
Plastic Index (PI) – difference between the LL and PL
PI = LL - PL
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25. USCS Soil Classification Chart
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
USCS Soil Classification Chart
This chart can be used to classify soils using the Unified Soil Classification System.
A grain size (sieve) analysis is needed to distinguish coarse grained soils from fine grained soils. If more than half of the soil, for example, is coarse grained, you would use the left half of the chart. [click]
Then the percentage of gravel is compared to the percentage of sand. Let’s assume that the percentage of sand is greater than the percentage of gravel [click]. We can then concentrate on the portion of the chart that describes sand.
From this point in the graph, you will follow the path indicating the percentage of fines for the soil sample. If more than 12% of the soil sample is fines (and therefore passes the No. 200 sieve) you will need to find the liquid limit (LL) and Plastic limit (PL) and calculate the plastici index (PI). Then consult the plasticity chart (shown on the next slide).
Let’s assume that there is more than 12% fines in our fictitious soil sample. The LL = 77 and the PL = 19.
What is the Plastic Index ? [PI = LL – PL = 77 – 19 = 58]
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26. Plasticity Chart Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Plasticity Chart
The plasticity chart can help distinguish among the fine-grained classifications of clay and silt. Once the LL and PI are determined, a point is plotted on the chart. The classification is indicated by the region in which the point falls on the chart.
[click] Using the LL = 77 and PI = 58 [click], we can classify the fines as a highly plastic clay.
[click] Therefore the soil would be classified as a Clayey Sand or SC as indicated on the chart
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27. Soil Classifications for Foundations
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Soil Classifications for Foundations
Sand and gravel – Best
Medium to hard clays – Good
Soft clay and silt – Poor
Organic silts and clays – Undesirable
Peat – No Good/Avoid
Identifying the soils type is a major factor in determining the type of foundations than can be used. Generally, the better the soils, the higher the bearing capacity and the smaller the footing size. As the soils range from good to poor, the cost of the foundation system will increase.
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28. Allowable Bearing (lb/ft2)
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Estimated Soil Bearing Capacities
Soil Type
Allowable Bearing (lb/ft2)
Drainage
BEDROCK
4,000 to 12,000
Poor
GRAVELS
3,000
Good
GRAVELS w/ FINES
SAND
2,000
SAND W/ FINES
SILT
1,500
Medium
CLAYS
ORGANICS
0 to 400
The allowable soils bearing capacities are noted for different types of soils. These are commonly used values and may be referred to as the presumptive soil bearing capacity. Also noted on the chart are the drainage characteristics of the soil types.
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29. Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Resources
Sowers, G.F. (1979). Introductory soil mechanics and foundations: geotechnical engineering. New York, NY: Macmillan Publishing.
Lindeburg, M. R. (1994). Civil engineering reference manual (9th ed.). Belmont, CA: Professional Publications, Inc.
Guthrie, P. (2003). Architects portable handbook: First step rules of thumb for building design. NY: McGraw-Hill.
Unified soil classification. Retrieved August 5, 2004,from classes/hydro/uscs.html
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30. Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Resources
U.S. Army Corps of Engineers. (1953). The unified soil classification system. U.S. Army Technical Memorandum, No
US Army unified soil classification system. Retrieved August 5, 2004, from bin/atdl.dll/fm/5- 472/apb.pdf
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31. Image Sources Fox, A. (photographer). University of South Carolina
Soils Investigation
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Image Sources
Fox, A. (photographer). University of South Carolina
U.S. Army Corp of Engineers. Technical Manual FM5-410 Soils Engineering
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