Forging new generations of engineers

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SOILS

Civil Engineering & ArchitectureTM

Unit 4 - Lesson 4.5 - Activity 4.5.1

Soil Testing

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Soil Testing

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Introduction

• Prior to developing a property for a commercial endeavor, a designer needs to investigate the soil conditions at the site.

• There is no construction material that has both the physical and engineering properties which are more variable than soil. Therefore it is highly desirable to have some information regarding both the surface and subsurface soil conditions.

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Soil – What is it?• Soils 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.

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Why Test the Soil???• The knowledge gained from soil testing allows the

engineer to make estimates for:

Bearing Capacity of the soil Settlements of Foundation ( amount and

rate) Earth Pressure – both lateral and vertical Drainage

These characteristics determine how certain soils behave as a construction material.

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What is Needed to Determine Soil Properties

• Site Plan

• Site Profile (Contour Map)

• Geological Information (USGS Maps)

• Soils Sample Data(obtained through soils analysis/testing)

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• Factors to be taken into consideration when determining the type of the building foundations:• Function of the structure• Loads from the structure• Bearing capacity of foundation material• Total and differential settlement of foundation• Uplift forces acting on the foundation• Costs

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The Soil Bearing Capacity needs to be determined before a foundation system can be designed. This can be determined by:

• Local or State Building Codes

• Uniform Building Code (UBC)

• Soil testing/analysisSite inspection and simple soil testing methods ( surface soils- top 1 foot)

Soil borings taken at proposed foundation locations (subsurface soils)

• Presumptive Bearing Capacity

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Sample Soils

• Soil testing must be done on soil samples that truly represent the the soil at the site.

• Soil sample about 1’-2’ below surface • Boring samples are taken at various locations and

depths below the surface (subsurface) for deep foundations

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Soils Testing• Unified Soils Classification System is a method

for 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 with additional tests for grain size and plasticity required to accurately classify the soil.

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Unified Soil Classification System

• The Unified Soil Classification System is a rapid method used to determine Simple Grain Size Analysis and categorizes the soil as either :

• -Coarse-Grained Soils

• -Fine Grained Soils

• -Highly Organic Soils

•• Only particles sizes smaller than 3” are

considered in the USC System

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• Many soils can be grouped visually with the USGS and additional tests for grain size and plasticity are required to accurately classify the soil

• Coarse-Grained soils described by grain size• Fine-Grained Soils described on the basis of their

plasticity

Coarse vs. Fine Grained Soils

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Sieve Sizes

• Gravels range from 3” down to the size of peas

• Silt and clay can pass through the #200 sieve

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Sieves

#40 sieve ( similar to window screen)

#4 sieve ( ¼” squares, similar to hardware cloth)

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Grain Size(Gradation)

• Gravel Sand

• Boulder Cobbles Silt &

• Coarse Fine Coarse Medium Fine

• Comparable Size:

• >12” 3”–12” ¾”–3” #4 - ¾” #10 - #4 #40-#10 #200-#40 <#200

Clay

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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Gravel Identification

Coarse Gravel from

3” to ¾”, a comparable size – Plum or Lemon

Fine Gravel from #4 to ¾”, a comparable size –Grape or Pea

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Gravel Size

• Coarse gravel Fine gravel

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Sand Identification

Coarse Sand from #4 to #10, a comparable size - Rocksalt

Medium Sand from #10 to #40, a comparable size - Sugar

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Sand Size

• Coarse sand • Medium sand

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Graded Soils• Soils can be defined by how the particle sizes are

distributed within the soils sample.• Well-Graded Soils-have a good representation of all

particle sizes from the largest to smallest• Poorly-Graded Soils- two types

Uniformly-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

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Clean vs. Dirty Graded Soils for Gravel and Sand

• 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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Soil Designations

• The USCS divides soils that have been classified into the major soil categories by letter symbols, such as:

•   S for sand•   G for gravel• M for silt• C for clay• O for organic• Pt for peat

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Grain Shape

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Soil Color• Color can aid in distinguishing soils types• Can vary with moisture content• May indicate the presence of certain chemical or impurities• Dark brown /black may indicate organic material• Gray, olive green indicate inorganic soils• Red or yellow may indicate iron oxides• Gray-blue or gray-yellow indicates poor drainage• White to pink may indicate considerable silica, calcium carbonate, or

aluminum compounds.

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Plasticity

• Plasticity of the soils refers to the varying water content on the consistency of the soil in fine-grained soils.

• This system is sometimes referred to as the ATTERBERG LIMITS, named after the Swedish scientist who developed this test.

• Plastic Limit-lowest moisture content at which soils can be rolled into 1/8” dia.thread with out breaking

• Liquid Limit- minimum moisture content at which soil will flow when a small shear or cutting force is applied

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Soils GroupsSoil Type Gradation Liquid Limit

Gravel- G

Sand-S

Silt-M

Clay-C

Organic O

Peat- Pt

Well Graded- W

Poorly Graded - P

LL over 50- H

LL under 50 -L

These letters are then used in combination to form the soils groups

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Determining the Silt or Clay Characteristics of Fine Grained Soil

• In addition to the Sieve Analysis the following tests will be done to determine of the Fine Grained soils

• Dry Strength- crushing characteristics• Dilatancy- reaction to shaking• Toughness- consistency near plastic limit

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Unified Soil Classification System

Start here

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Equipment for Testing

• Sample of soil about the size of a coffee can and collected below the top soil horizon (about 1 to 2 feet).  

• 2 mason jars with tops   • 5 gallon pail • Water• Mixing bowl• A 2’ x 2’ metal pan and rubber mallet• Sieve collection, ASTM No 4 and ASTM No.

40Bottom sieve pan and cover• Triple-beam balance or Digital Scale

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Testing

USCS SYSTEM SEIVE ANALYSIS

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• Weigh each sieve separately

Weigh mason jars for later use

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Soils Sampling• Take a representative

soil sample from the property

• A shovelful or coffee can of soil should be enough

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•Soil sample should be dry

•Pulverize the soil with a rubber mallet

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•Continue to Pulverize the soil with your hands

•Be sure that all small and large clumps are completely broken apart.

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Stack sieves with Number 4 on top, then the Number 40 sieve below that, and then finally the bottom pan.

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Place soil in Number 4 sieve, place cover on top and shake vigorously.

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Shaking the Soil Sieves

•Shake with up and downward motion

or / and

• Shake with side to and side motion

•Shake vigorously.

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• Weigh the sample material and the sieve.

• Subtract the weight of the sieve to obtain weight of soil. In this case , the weight of the particles is the Gravel fraction in the soil sample

• Do this for each sieve.

Weighing of each Sieve with Sample Particles

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#4 Sieve with Gravel

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#4O Sieve with Sand

• Record and subtract the weight of the No.40 sieve only (i.e., without the soil sample). The difference is the coarse and medium grained SAND fraction in the soil sample.

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Bottom pan with Fines

• Weigh the bottom pan and its contents. Record and subtract the weight of the empty pan. The difference is the fine SAND, SILT, and CLAY fraction of the soil sample.

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Relative Particle Sizes

#40 sieve#4 sieve

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• Coarse-grained soil, also called 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 granular soil.

• If the soil is predominantly coarse-grained, identify the soil sample as being gravel or sand by estimating whether 50% or more, by weight, of the coarse grains (GRAVEL and SAND) are larger or smaller than the No 4 sieve size.

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Sieve Analysis ResultsUSCS Classification for Coarse Grained Soils

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USCS Classification for Fine Grained Soils

• Fine-grained soils, also called cohesive soils contain greater than 50% SILT and CLAY particles.

• It is classified further into either SILT or CLAY and them determined if it is high or low plasticity by estimating its dry strength (crushing characteristics), dilatancy (reaction to shaking), and toughness (consistency near the plastic limit)

• Individual fine- grains are not visible to the naked eye

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Determining the Fine Grained Sand Fraction of Soil

• Place the contents of the bottom pan from the sieve test in a Mason jar.

• Weigh and Record results• Fill jar with water so that I• it is ¾ full• Shake jar with lid on and let

the fine sand settle 5 to 10 seconds

• Drain off

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• Determining the Fine Grained - • Sand, Silt and Clay fraction of the soil sample

•Pour off the dirty water, be careful not to spill any of the soils at the bottom of jar

•Repeat the shake, settle and drain off procedure several times

•Repeat until water is clear

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Determining the weight of Silt and Clay in Soil Sample

• Place the Mason jar, without top, and the contents into the oven and dry.

• Weigh the jar and contents. Record the weight and subtract the weight of the empty jar. The difference is the fine grained SAND fraction in the soil sample.

• Determine the weight of SILT and CLAY in the soil sample

• Determine the percentage of Silt and Clay in the soil sample

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Dilatancy Test• Dilatancy is the reaction to wet shaking

• Prepare a pat of moist soils approximately 3/4 in3 in size

• Add water, if necessary, to make soil soft but not sticky, like soft putty

• Place pat in open palm of hand

• Shake horizontally, striking with other hand several times

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Dilatancy Test • Observe rapidity of water rising to the surface when

sample is shaken and when the sample is squeezed between fingers

• Speed of appearance during shaking and disappearance when squeezed assists in identifying the fines in the sample

• If sample become soft and glossy when shaking and then become hard, dull and forms crack when squeezed between the fingers , it has a “reaction to shaking,” or dilatancy”

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Dilatancy Test Results

• Very clean sands give the quickest and most distinct reaction ( S)

• Rapid reaction, sample is silt ( M)

• No reaction indicates clay ( C)

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Toughness Test Effect of Water on Fine-Grained Soils

• A method of testing the plasticity of the soil is referred to as the Toughness Test.

• Water is added to the “fines” portion of the soils sample ( those particles that passed the No. 40 sieve) and the sample is then rolled into a 3mm ( 1/8”) diameter thread.

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Toughness Test

• Mixing about a 100 cm3

( 1/3 cup of the soil that passed through the No. 40 sieve) with water. Mixture should be the consistency of putty

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Toughness (Thread Test)• Roll out specimen on a smooth

surface until it is approx. 1/8” in diameter.

• Fold and re-roll thread repeatedly

• Soil samples moisture content will gradually be reduced

• Thread breaks and sample becomes lumped

• Continue until lumps crumble

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Toughness Test Results

• The toughness of the thread that forms when the soil is rolled will identity the fines as either silt or clay.

• Inability to form thread or very weak thread indicates a plastic silt (ML)

• Weak spongy thread indicates silt (either organic or inorganic) with a high liquid limit and a low low-plasticity (MH)

• Firm thread indicates low-plasticity clay (CL)• Tough thread indicates highly plastic clay ( CH)

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Dry Strength• Form a moist pat of

approximately 2”-3” in diameter and ½” thick

• Allow to dry with low heat • The crushing characteristic of

the soil sample will be evaluated after this sample is completely dry.

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Dry Strength Test and Results

Dry strength is a measure of the character and quality of colloidal fraction of soil.

•Break dry pat sample between thumb and index finger

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Dry Strength Test Results

• Breakage Easy – Silt

• Breakage Difficult - Inorganic Clays of low to medium plasticity

• Breakage Impossible- Inorganic Clays of high plasticity

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Soils Test Results for Fine Grained Soils

Soils Type

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Soils Types 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

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Soil Bearing CapacitiesSoil Type Allowable Bearing

(lb/ft 2)Drainage

BEDROCK 4,000 to 12,000 PoorGRAVELS 3,000 GoodGRAVELS w/ FINES 3,000 Good SAND 2,000 GoodSAND W/ FINES 2,000 GoodSILT 1,500 MediumCLAYS 1,500 MediumORGANICS 0 to 400 Poor

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Soil Preparation

• What is the site has poor soils, is there anything that can be done?

• If the soils at the site are not suitable to carry the foundation loads, the site may have to be prepared using some of the following methods

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Soils Preparations for foundations if soil is undesirable

• Compaction of the soil- increase the density of the soils and therefore increases the bearing capacity of the soil. Compaction of the soils is down by rolling the soils

• Bring in “Engineered Fill”, must remove old soils first• Blasting to remove rock or soil

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Soil Types and Properties

Source: U.S. Army Corps of Engineers. (1953). The unified soil classification system. U.S. Army Technical Memorandum, No.3-357.

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Soil Description•Color: Dark Brown to White

•Grain size, including estimated maximum grain size is an estimated percent by weight of fines (material passing the No. 200 sieve) :

Coarse-grained soils, maximum particle size 2”, estimating 60 percent gravel, 36 percent sand,

and 4 percent passing the #200 seive,

•Gradation: Poorly-graded, insufficient fine gravel

•Grain shape : Gravel particles well rounded

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Soils Description (continued)

• Plasticity: Nonplastic

• Predominant type: Predominantly gravel

• Secondary components: Considerable sand and trace amounts of nonplastic fines(silt)

• Classification symbol : GP ( Poorly Graded Gravel and sandy gravel mixture with little or no fines)

• Other remarks, such as organic, chemical, or metallic content; compactness; consistency; cohesiveness near PL; dry strength and source

• No dry strength, dense in undisturbed states

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References: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.

GGuthrie, P. (2003). Architects portable handbook: First step rules of thumb for building design. NY: McGraw-Hill.

Unified Soil Classification http://www.hydro.unr.edu/homepages/benson/classes/hydro/uscs.html August 2004

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References (continued):U.S. Army Corps of Engineers. (1953). The Unified Soil Classification System. U.S. Army Technical Memorandum, No.3-357.

US Army Unified Soil Classification System. [On-line]. Available: http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/5-472/apb.pdf (August 5, 2004)

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